MECHANICAL DRIVES 1 KEY FASTENERS LEARNING ACTIVITY PACKET BB502-XD02AEN

Transcription

1 MECHANICAL DRIVES 1 LEARNING ACTIVITY PACKET KEY FASTENERS BB502-XD02AEN

2 LEARNING ACTIVITY PACKET 2 KEY FASTENERS INTRODUCTION In the previous LAP you learned how to mount and operate the motor, but you didn t connect it to anything. In this LAP you will learn how a key fastener connects the motor shaft to other devices. On the job, you will encounter key fasteners in almost every rotating machine application. In this LAP you will use a key fastener to connect the motor to a prony brake. The prony brake is a device that is used to place a load on a motor. It was chosen for two reasons: it is a simple device to connect to the motor because it does not require much alignment and it allows you to learn about motor torque and power. In later LAPs the prony brake will also be used to load the mechanical transmission system to demonstrate the effects of real world loads on the system. ITEMS NEEDED Amatrol Supplied 950-ME1 Mechanical Drives 1 Learning System School Supplied Bench Vise Hacksaw Files FIRST EDITION, LAP 2, REV. C Amatrol, AMNET, CIMSOFT, MCL, MINI-CIM, IST, ITC, VEST, and Technovate are trademarks or registered trademarks of Amatrol, Inc. All other brand and product names are trademarks or registered trademarks of their respective companies. Copyright 2012 by AMATROL, INC. All rights Reserved. No part of this publication may be reproduced, translated, or transmitted in any form or by any means, electronic, optical, mechanical, or magnetic, including but not limited to photographing, photocopying, recording or any information storage and retrieval system, without written permission of the copyright owner. Amatrol,Inc., 2400 Centennial Blvd., Jeffersonville, IN USA, Ph , FAX

3 TABLE OF CONTENTS SEGMENT 1 KEYSEAT FASTENERS OBJECTIVE 1 Describe the function and operation of a key fastener OBJECTIVE 2 Describe the construction of six types of keys and give an application of each OBJECTIVE 3 Describe how keys and keyseats are specifi ed SKILL 1 Select a key size for a given application SEGMENT 2 KEY ASSEMBLY OBJECTIVE 4 Describe how to measure the actual size of a key and keyseat OBJECTIVE 5 Describe six types of set screws SKILL 2 Measure the actual size of a key and keyseat given a sample SKILL 3 Cut and fi le key stock to fi t a keyseat OBJECTIVE 6 Describe how to assemble a hub to a shaft using a key SKILL 4 Assemble a hub to a shaft using a key fastener SEGMENT 3 TORQUE AND POWER MEASUREMENT OBJECTIVE 7 Describe two methods of loading a mechanical drive system SKILL 5 Use a prony brake to measure shaft torque OBJECTIVE 8 Describe how to calculate rotary mechanical power SKILL 6 Calculate rotary mechanical power SKILL 7 Convert between U.S. Customary and S.I. units of motor power SEGMENT 4 MECHANICAL EFFICIENCY OBJECTIVE 9 Describe how to calculate mechanical effi ciency and explain its importance SKILL 8 Calculate mechanical effi ciency OBJECTIVE 10 Describe two methods of measuring shaft torque and give an application of each OBJECTIVE 11 Describe three methods of measuring electric motor current SKILL 9 Measure electric motor current 3

4 SEGMENT 1 KEYSEAT FASTENERS OBJECTIVE 1 DESCRIBE THE FUNCTION AND OPERATION OF A KEY FASTENER A key fastener is used to secure a shaft to other devices such as couplings, sheaves, and gears, as shown in figure 1. Its job is to make sure that the drive shaft and the driven component are locked together and do not slip on each other. COUPLING SHEAV EAR KEY FASTENER Figure 1. Key Fastener Applications 4

5 A key fastener consists of up to three parts, as shown in figure 2: Key Keyseat - Shaft Keyseat - Hub A key is simply a piece of metal that is snugly fitted between two grooves, which are machined in a shaft and the hub of a component to which it is to be connected. The groove in the drive shaft is called a keyseat. The groove in the hub is also called a keyseat or sometimes a keyway. KEY KEYSEAT: SHAFT KEYSEAT: HUB Figure 2. Parts of a Key Fastener 5

6 In many cases, hubs have one or more set screws that can apply extra force to the key to lock the hub in place, as shown in figure 3. SET SCREW SHEAVE KEY SHAFT HUB Figure 3. Key Fastener and Set Screw 6

7 OBJECTIVE 2 DESCRIBE THE CONSTRUCTION OF SIX TYPES OF KEYS AND GIVE AN APPLICATION OF EACH There are many types of keys used in industry, depending on the application. Each key has a certain shape which is designed based on either ease of use, cost, or its ability to hold a high load. Six of these types, as shown in figure 4, are: Square - The square key is the most widely used because it is easy to make and provides a strong connection. Its cross section is square and its length can either be tapered or parallel. The parallel type is the most commonly used. The tapered type is used when there is a need to use the wedging action of the key to keep it in place. Rectangular - This key looks like a square key except that its width is greater than a square key. It can also be a parallel or tapered type. The rectangular key has a greater shear strength because of its larger cross-sectional area. Rectangular keys are preferred for shaft sizes of greater than 6-1/2 inches. For smaller sizes, a square key is preferred. SQUARE RECTANGLE GIB HEAD WOODRUFF SADDLE OFFSET OR Figure 4. Key Fastener Shapes 7

8 Gib head - The gib head key is a tapered square key with a head on it. The head provides a way to easily remove the key if only one side of the assembly is accessible. A tapered key without a head is called a plain taper key. Woodruff - The woodruff key is shaped like a half moon. It is often used in light duty applications, because it gives more holding strength (more shear area) without requiring a large portion of the shaft to have a key seat machined in it. It is also used with tapered shafts because it reduces the tendency of the key to tip when a load is applied. Saddle - The saddle key is used when there is no keyseat of any kind. These are used in light duty applications. Offset - The offset or step key is type of square key that has a different width on one side of the key. This allows the key to connect a coupling hub and shaft which have different keyseat sizes. It is also used for repair and salvage of keyseats that have become larger through wear. 8

9 OBJECTIVE 3 DESCRIBE HOW KEYS AND KEYSEATS ARE SPECIFIED Keys are made from standard stock sizes, which are available from machine parts suppliers. The key stock is cut to the length needed and the burrs are filed off. A key and keyseat are specified, as shown in figure 5, by the following features: Nominal Width Nominal Height Width Tolerance Height Tolerance Length Material Type KEY HUB WIDTH HEIGHT WIDTH HEIGHT WIDTH HEIGHT SHAFT Figure 5. Key and Keyseat Features 9

10 Nominal Width and Height of Keys and Keyseats The nominal width is the width of both the key stock and the keyseat, without accounting for tolerance. Key stock is available in a variety of standard widths. It is usually matched to a certain shaft size with a table like the one shown in figure 6. The keyseats are machined into the shaft and hub, usually by the manufacturer of the equipment. However, many drive components can be ordered with no keyseat to allow the user to machine the keyseat themselves. This allows the user to select any type or size keyseat desired. NOMINAL SHAFT DIAMETER Over To (Incl.) Width, W 5/16 7/16 9/16 7/8 1-1/4 1-3/8 1-3/4 2-1/4 2-3/4 3-1/4 3-3/4 4-1/2 5-1/2 6-1/2 7-1/2 9 7/16 9/16 7/8 1-1/4 1-3/ /4 2-3/4 3-1/4 3-3/4 4-1/2 5-1/2 6-1/2 7-1/ /32 1/8 3/16 1/4 5/16 3/8 1/2 5/8 3/4 7/ /4 1-11/2 3-3/ /2 NOMINAL KEY SIZE NOMINAL KEYSEAT DEPTH Height, H H/2 Square Rectangular Square Rectangular 3/32 1/8 3/16 1/4 5/16 3/8 1/2 5/8 3/4 7/ /4 1-1/2 1-3/ /2... 3/32 1/8 3/16 1/4 1/4 3/8 7/16 1/2 5/8 3/4 7/ /2* 1-1/ /64 1/16 3/32 1/8 5/32 3/16 5/16 3/8 7/16 1/2 5/8 3/4 7/ /4... 3/64 1/16 3/32 1/8 1/8 3/16 7/32 1/4 5/16 3/8 7/16 1/2 All dimensions are given in inches. For larger shaft sizes, see ANSI Standard. Square keys preferred for shaft diameters above heavy line; rectangular keys, below. * Some key standards show 1-1/4 inches; preferred height is 1-1/2 inches. Figure 6. Key Size Versus Shaft Diameter 3/4 3/4 7/8 10

11 The nominal key height is the height of the key stock, without accounting for tolerance. For a square key, the nominal height is the same as the nominal width. The height sizes available are therefore determined by the nominal width. The nominal keyseat height, however, is not the same as the key height. The nominal keyseat height is normally chosen to be 1/2 the key height because the key must extend into the keyseats of both the hub and the shaft, as shown in figure 7. KEY HEIGHT KEYSEAT HEIGHT Figure 7. Keyseat Height vs. Key Height Width and Height Tolerances The width and height tolerances are the allowable variations of the width and height dimensions of the key and keyseat. The exact variations of the width and height dimensions are very important. The key and keyseats should be of sizes so that the key is neither too loose nor too tight. If the key width is too small, making a loose fit, it will shear off. If either the key width or the key height are too large, making a tight fit, stress cracks can occur in the shaft or hub and cause it to fail. The tolerances of the key and keyseat can have one of two types of fits as defined by ANSI: either class 1 or class 2. Class 1 is a looser fit than class 2. In a normal application, the fit you used with a key should be a class 1 fit so it will be the only fit discussed. A class 1 fit is a type of clearance fit called a sliding fit. This means that there is a slight clearance between the key and the keyseat (typically to inches clearance) but the clearance should not be detectable by touch. The key should be able to be pushed into the keyseat with your thumb. 11

12 The class 1 tolerances for square keys and keyseats for various sizes of shafts are given in the following table in figure 8. This table, as well as the one for a class 2 fit, can be found in the Machinery s Handbook. You should make sure that the key stock you choose and the actual dimensions of the keyseats meet the tolerance specifications shown in this table for class 1 fit before you attempt to assemble them. Type of Key Square Rectangular KEY WIDTH SIDE FIT TOP AND BOTTOM FIT Over 1/2 3/ /2 2-1/2... 1/2 3/ / To (Incl.) 1/2 3/ /2 2-1/2 3-1/2 1/2 3/ / Width Tolerance Key Keyseat Fit Range* Key Class I Fit for Parallel Keys CL CL CL CL CL CL CL CL CL CL CL CL CL CL All dimensions are given in inches. * Limits of variation. CL = Clearance; INT = Interference To (Incl.) 3-1/2 inch Square and 7-inch Rectangular key widths. Figure 8. ANSI Standard Class 1 Fits for Parallel Keys Depth Tolerance Shaft Key- Seat Hub Key- Seat Fit Range* CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL CL 12

13 Key tolerances are specified as either undersize, oversize, or over/undersize. Undersize key stock fits the tolerance specification shown for a class 1 fit. It has a zero upper tolerance. For example, a typical undersize tolerance is to inches. This tolerance type makes sure that there is always some clearance. It is what you should normally use. The undersize tolerance is also called a negative tolerance or minus tolerance. An oversize key tolerance means that the lower tolerance is zero. A typical oversize tolerance is to inches. Oversize key stock is used when the keyseats are worn and have therefore become larger than the normal specification allows. The oversize key allows the shaft to still be used. Another term used for this tolerance type is a plus tolerance. The third tolerance specification, over/undersize, means that there is both an upper and lower tolerance. A typical example is 5 to inches. This tolerance is used when you want a tighter fit than normal fit. An example of an application is with a reversing motor. Keyseats are usually machined into the shaft and hub with class 1 tolerance, so that there is a clearance fit. Many times the manufacturer of the shaft or hub machines the keyseat, so your only job is to select the key stock tolerance. 13

14 Key and Keyseat Length Another feature that must be specified is the length of the key. The length of a key for an application is usually determined during the initial design stage. While the nominal length of the key is important, it is not a critical dimension that requires a close tolerance. The general guideline is to make the key long enough to fit flush on one side of the hub and a little shorter than the length of the keyseat on the shaft, as shown in figure 9. This assures that the key cannot slide around in the keyseat. Keys can be purchased having various lengths. However, keys are usually cut to length from longer lengths of key stock. A typical stock length is 12 inches. KEYSEAT KEY KEY LENGTH K S KEYSEAT LENGTH Figure 9. Length of Key Key Stock Material Type Keys are purposely chosen to be of a softer material than the shaft so that they will shear first if the shaft is overloaded. Common key materials include: Cold rolled steel, e.g. C1018 Zinc-Plated cold rolled steel High carbon steel, e.g. C1095 Brass The most common material is cold rolled steel. This material may be zinc plated for corrosion resistance. Stainless steel, typically 316 or 18-8, is also used for the same reason. In marine applications, brass is also used. For higher load applications, where tighter tolerances and higher strength are needed, a high carbon steel can be used. This steel is often annealed to make it easier to machine and has a tighter size tolerance. 14

15 SKILL 1 SELECT A KEY SIZE FOR A GIVEN APPLICATION Procedure Overview In this procedure, you will be given various scenarios and challenged to select the size and type of key stock that best fits the application. You will first be given an example. Then you will do it yourself. 1. Examine the following application information, select the type and size of key for the application. Given Information: Application: Shaft to coupling connection for a hydraulic system Shaft diameter: inches Shaft keyseat length: inches Hub keyseat length: inches Find: FEATURE Key Type Nominal Width Nominal Height Width Tolerance Height Tolerance Length Key Material SPECIFICATION The solution is as follows: The key type for most applications having a shaft diameter below 6-1/2 inches should be square. 15

16 Key Type: Square The nominal key width and height can be determined by either checking the shaft keyseat width and depth or by looking in the table of figure 6. Since the shaft keyseat size is not given, use the table. It shows that the width for a key for a shaft size of between 7/16 and 9/16 inches should be 1/8 inches. It also shows that the height should be 1/8 inches as well. You know this because it is a square key. Nominal Key Width: 1/8 inches Nominal Key Height: 1/8 inches The tolerance on the width and height can be determined from the table in figure 8. A key that has a width and height of 1/8 inches has a width tolerance of, inches and a height tolerance of, inches. Key Width Tolerance:, inches Key Height tolerance:, inches The length should be determined by the length of the keyseat in the shaft and the hub. The length of the key should be at least as long as the length of the keyseat in the hub and slightly shorter than the keyseat in the shaft. Therefore, a length of 1-1/4 inches is an acceptable length. Length: 1-1/4 (inches) The key material is determined from the application of the system. Because this key will be used to connect a shaft and hub for a hydraulic system, high forces can be assumed. Therefore, a high carbon steel key should be used. Key Material: High Carbon Steel 16

17 2. Select the size and type of key given the following information. Given Information: Application: Shaft to coupling connection for an ore crusher used in mining. Shaft diameter: 8.5 inches Shaft keyseat length: inches Hub keyseat length: inches Find: FEATURE Key Type Nominal Width Nominal Height Width Tolerance Height Tolerance Length Key Material SPECIFICATION The solution is as follows: In this case, the shaft size is above 6-1/2 inches, so you should use a rectangular key. Key Type: Rectangular The nominal key width and height can be determined by either checking the shaft keyseat width and depth or by looking in the table of figure 6. Since the shaft keyseat size is not given, use the table. It shows that the width for a key for a shaft size of between 7-1/2 and 9 inches should be 2 inches. It also shows that the height should be 1.5 inches. 17

18 Nominal Key Width: 2 inches Nominal Key Height: 1.5 inches The width and height tolerance are obtained from the table in figure 8. The width tolerance for a rectangular key having a width between 1.5 and 3 inches is, inches. The tolerance for the height is, inches. Width Tolerance:, inches Height Tolerance:, inches The length of the key should be longer than the length of the keyseat in the hub and shorter than the keyseat in the shaft. Therefore, a length of is selected. Key Length: (inches) The material for the key should be high carbon steel because this is a heavy duty application. Key Material: High Carbon Steel 3. Select the size and type of key given the following information. Given Information: Application: Shaft to gear connection Shaft diameter: 3.0 inches Shaft keyseat length: inches Hub keyseat length: 2.0 inches Find: FEATURE Key Type Nominal Width Nominal Height Width Tolerance Height Tolerance Length Key Material SPECIFICATION The solution is as follows: The key should be 3/4 inch square, having a width tolerance of inch, a height tolerance of inch, and a length of 2.0 inches. The key should be made from high carbon steel. 18

19 4. Select the size and type of key given the following information. Given Information: Application: Shaft connected to a belt drive to operate an exhaust fan. Shaft diameter: 5/8 inch Shaft keyseat length: 1.25 inches Hub keyseat length: 1.0 inches Find: FEATURE Key Type Nominal Width Nominal Height Width Tolerance Height Tolerance Length Key Material SPECIFICATION The solution is as follows: The key should be 3/16 inch square, having a width tolerance of inch, a height tolerance of inch, and a length of 1-1/8 inches. The key should be made from cold rolled steel. 19

20 5. Select the size and type of key given the following information. Given Information: Application: Shaft coupled to a pump used to supply cooling fluid to a CNC Milling Machine. Shaft diameter: 3/8 inch Shaft keyseat length: 1 inch Hub keyseat length: 3/4 inch Find: FEATURE Key Type Nominal Width Nominal Height Width Tolerance Height Tolerance Length Key Material SPECIFICATION The solution is as follows: The key should be 3/32 square, having a width tolerance of inch, a height tolerance of inch, and a length of 7/8 inch. The key should be made from cold rolled steel. 20

21 SEGMENT 1 SELF REVIEW 1. A key fastener consists of up to three parts which are the key, keyseat -shaft, and. 2. A key is a piece of metal that fits snugly between two. 3. A(n) key is a tapered square key with a head on it. 4. A(n) key is shaped like a half moon. 5. A key is made from which is cut to the length needed and the burs filed off. 21

22 SEGMENT 2 KEY ASSEMBLY OBJECTIVE 4 DESCRIBE HOW TO MEASURE THE ACTUAL SIZE OF A KEY AND KEYSEAT Selecting the right size for a key is very important. It requires the keyseat to be accurately measured and the right size key stock to be selected. Three measuring tools that are used to measure the key and keyseat are: Dial Caliper Micrometers Rule Dial Caliper The dial caliper has the ability to measure the inside width of a keyseat and the depth of the keyseat, as shown in figure 10. These measurements allow you to determine the width and height of the key. HEIGHT (DEPTH) WIDTH Figure 10. Measuring the Width and Height of a Keyseat 22

23 0 1 2 Micrometer Either the micrometer or the caliper can be used to measure both the width and height of the key stock used to make the key. The micrometer is a more accurate measuring device than the dial caliper and is commonly used for this application. A key stock which is purchased from a supplier has a specific tolerance. For example, square key stock (e.g. zinc-plated, cold-drawn C1018 steel key stock) is typically sold with a tolerance of , Therefore, it is usually only necessary to verify that the nominal size of the key stock is correct. The tolerance of the key stock has already been chosen. HEIGHT KEY WIDTH KEY HEIGHT Figure 11. Micrometer Used to Measure Width and Height of Key 23

24 Rule A rule is used to measure the length of a keyseat. This measurement cannot be easily obtained using the micrometer or the dial caliper. Because the length is not critical, a rule is the easiest and quickest method of determining the keyseats approximate length Figure 12. Rule Used to Measure Length of Keyseat 24

25 OBJECTIVE 5 DESCRIBE SIX TYPES OF SET SCREWS A set screw is a threaded fastener used to hold components together. These fasteners generally do not have a head. There are various types of set screws available. Six of these types are: Cup Point Flat Point Dog Point Oval Point Cone Point Soft Tipped Cup Point Cup point set screws have a dished out area on its tip. This cup bites into the shaft for maximum locking strength. Figure 13. Cup Point Set Screw 25

26 Flat Point Flat point set screws are used because they offer the least amount of shaft deformation. They are typically used on frequently dismantled components. Figure 14. Flat Point Set Screw Dog Point Dog point set screws have a point that fits into a hole in the shaft. This provides not only locking strength, but also provides precise locating of the components in reference to each other. Figure 15. Dog Point 26

27 Oval Point Oval point set screws do not create excessive indentations in the shaft. However, they are best used when the set screw will contact the shaft at an angle. Figure 16. Oval Point Cone Point Cone point set screws are used for permanent mounting of components on shafts. The point bites into the shaft to create a high axial and torsional locking strength. Figure 17. Cone Point Set Screw 27

28 Soft Tipped Soft tipped set screws have a different material on the point. This material conforms to the shape of the shaft. This provides adequate locking strength for many applications. However, it prevents damaging or scarring of soft shafts. Typical materials are nylon and brass. Figure 18. Soft Tipped Set Screw 28

29 SKILL 2 MEASURE THE ACTUAL SIZE OF A KEY AND KEYSEAT GIVEN A SAMPLE Procedure Overview In this procedure, you will measure the size of a key and keyseat. You will then compare your measurements to the specifications for the key and keyseat to determine if they are in tolerance. 1. Obtain the following items from the tool crib. Drum Brake Hub Motor Dial Caliper 6-inch Rule 2. Perform the following substeps to measure the width, depth, and length of the keyseat on the motor shaft. A. Use the inside jaws of the dial caliper to measure the width of the keyseat in the shaft, as shown in figure 19. Shaft Keyseat Width: (inches) The width of the keyseat should be approximately 3/16-inch (0.187 inches). WIDTH Figure 19. Measuring the Width of the Keyseat in the Shaft 29

30 B. Use the dial caliper to measure the depth of the keyseat, as shown in figure 20. This is called the depth gauge of the dial caliper. The distance that the rod of the caliper extends into the keyseat is the same as the opening of the jaws. This is also shown in figure 20. Shaft Keyseat Depth: (in/mm) The depth of the keyseat should be approximately 3/32-inch (0.094 inches). C. Use a 6-inch rule to measure the length of the keyseat in the shaft. HEIGHT (DEPTH) DEPTH GAUGE Figure 20. Measuring the Depth of the Keyseat in the Shaft Shaft Keyseat Length: (inches) NOTE Some keyseats taper off at the end of the keyseat. Measure only the usable length of the keyseat. The length of the keyseat should be approximately 1-3/4 inches. 30

31 3. Perform the following substeps to determine the width, depth, and length of the keyseat in the brake drum hub. A. Use the inside jaws of the dial caliper to measure the width of the keyseat in the brake drum hub, as shown in figure 21. Brake Drum Hub Keyseat Width: (inches) The width of the keyseat should be approximately 3/16-inches (0.187 inches). BRAKE DRUM HUB KEYSEAT WIDTH Figure 21. Measurement of Keyseat Width 31

32 B. Use the dial caliper to measure the thickness of the hub wall, as shown in figure 22. Hub Wall Thickness: (inches) BRAKE DRUM HUB HUB WALL THICKNESS Figure 22. Measurement of Hub Wall Thickness 32

33 C. Use the dial caliper to measure the thickness from the outside of the hub wall to the bottom of the keyseat, as shown in figure 23. Hub Wall to Keyseat: (inches) BOTTOM OF KEYSEAT THICKNESS TO MEASURE Figure 23. Measurement from the Outside of the Hub Wall to the Bottom of the Keyseat 33

34 D. Calculate the depth of the hub s keyseat by subtracting the Hub Wall to Keyseat measurement from the Hub Wall Thickness measurement. Hub Keyseat Depth = (inches) The depth of the keyseat should be 3/32-inch (0.094 inches). E. Use a 6-inch rule to measure the length of the keyseat in the hub. Shaft Keyseat Length: (inches) The length of the keyseat is approximately 1½ inches. 4. Perform the following substeps to select the nominal size for a key. A. Select the nominal width of the key. The width should be the same width as the keyseat width in the motor shaft and brake drum hub. Key Width: (inches) Therefore, the key width is 3/16-inch. B. Select the nominal height of the key. The height of the key is determined by adding the depth of the shaft keyseat to the depth of the hub keyseat. Key Height: (inches) This is a 3/16-inch square key. C. Determine the length of the key. The length of the key should be at least as long as the keyseat in the hub and slightly shorter than the length of the keyseat in the shaft. Key Length: (inches) The key length should be at least 1 inch. 34

35 SKILL 3 CUT AND FILE KEY STOCK TO FIT A KEYSEAT Procedure Overview In this procedure, you will learn how to cut key stock to the correct length and then use a file to prepare the key for assembly into a keyseat. 1. You will need the following items: 3/16-inch square key stock Hacksaw Flat File- Single Cut, Smooth Bench Vise with Soft Jaw Caps 0-1 inch Micrometer 6-inch Rule 2. Using the micrometer, measure the width and thickness of the square key stock to verify that it is 3/16-inch key stock. Key Stock Width: (in/mm) Key Stock Height: (in/mm) 3. Use the 6-inch rule to measure 1 inch of the key stock. Use a pencil to mark this measurement, as shown in figure 24. Figure 24. Measuring 1 Inch of Key Stock 35

36 4. Perform the following substeps to cut the key stock to the required length. A. Place the key stock in the bench vise, as shown in figure 25. NOTE Make sure that the soft jaw caps are in place to avoid marring the surface of the key stock. KEY BENCH VISE Figure 25. Key Stock Placed in the Bench Vise B. Use the hacksaw to cut off the 1 inch of required key stock. C. Remove the keystock from the bench vise and return it to stock. D. Retain the 1-inch piece for the next step. 36

37 5. Perform the following substeps to prepare the key for use. A. Place the 1-inch key in the vise, as shown in figure 26. KEY BENCH VISE Figure 26. Key Placed in the Bench vise B. Use the flat file to deburr the area where the cut was made. 37

38 C. Use the flat file to lightly chamfer the edges of the key, as shown in figure 27. You will need to turn the key over to chamfer both sides of the key. The key is ready for assembly at this time. WARNING Chamfering means to only remove the sharp corners from the key. Do not round off the edges of the key. This will reduce the holding power of the key and create a potential hazard. Figure 27. Chamfering the Edges of the Key 6. Return all tools to the tool crib. The key you made will be used in the next skill. 38

39 OBJECTIVE 6 DESCRIBE HOW TO ASSEMBLE A HUB TO A SHAFT USING A KEY FASTENER Assembling a shaft and hub with a key fastener is a very easy task if the components are sized correctly. The steps are as follows: Step 1 - Check to see if the hub has a set screw hole drilled in its side, as shown in figure 28. A set screw is sometimes used to provide extra holding force on the key to hold it in position. If there is a set screw, make sure to back it out so that it is not extending into the shaft hole. Figure 28. Set Screw and Hub Step 2 - Clean the shaft keyseat and the hub keyseat with a wire brush to make sure that no dirt or burrs are in the keyseats. Step 3 - Slide the key onto the keyseat of the shaft. The key should fit into the keyseat without forcing it. If it is too tight, take it out and measure it to see which part is out of tolerance. You can either replace the key, machine the keyseat, or sand the key. Sanding the key is usually not recommended because it is hard to do it evenly. If you choose to sand it, use a belt sander, not a grinder. Also, check the key for play when it is in the keyseat by wiggling it. There should be no play. If there is, replace the key. Step 4 - Remove the key from the shaft keyseat and insert it into the hub keyseat. It also should slide in without forcing it and have no play. 39

40 Step 5 - Remove the key from the hub and insert into the shaft keyseat. Line it up flush with the end of the shaft, as shown in figure 29. KEY MOTOR SHAFT Figure 29. Key Positioned on Shaft Step 6 - Pick up the hub in your hand and line it up in front of the shaft so that the hub s keyseat is in line with the key on the shaft, as shown in figure 30. KEY HUB KEYSEAT MOTOR Figure 30. Aligning the Shaft and Hub 40

41 Step 7 - Then slide the hub onto the shaft until the end of the hub is flush with the end of the shaft, as shown in figure 31. The hub should slide without using tools. If it doesn t, pull it off and check the dimensions. HUB FLUSH WITH SHAFT Figure 31. Hub Slides onto Shaft Step 8 - Tighten the set screw onto the key. Sometimes you may use two set screws, as shown in figure 32, to keep the first one from backing out when the shaft is turning. MOTOR SET SCREW ACCESS HOLE MOTOR SHAFT FIRST SET SCREW SECOND SET SCREW KEY Figure Set Screw Key Assembly 41

42 Hub Removal The best way to remove a hub from a shaft is to use a bearing puller, as shown in figure 33. This unit pushes on the end of the shaft while it pulls on the hub. This method will remove the hub without damaging the components. KEY MOTOR COUPLING PULLER Figure 33. Removing a Coupling Hub with a Bearing Puller Another method of removing a hub is to use a key punch and soft hammer to tap the key out. The coupling can then be removed by hand. In no case, however, should the coupling itself be knocked out by a hammer. This will destroy the coupling. 42

43 SKILL 4 ASSEMBLE A HUB TO A SHAFT USING A KEY FASTENER Procedure Overview In this procedure, you will assemble onto the motor shaft a hub called a brake drum, which is used by the prony brake to load the motor. This will teach you the basic steps of assembling a key fastener. You will apply this process in later LAPs by assembling other types of hubs, including those used in couplings. 1. Perform the following safety checkout to prepare for working with power transmission equipment. Make sure that you are able to answer yes to each item before proceeding. YES/NO SAFETY CHECKOUT Wearing safety glasses Wearing tight fi tting clothes Ties, watches, rings, and other jewelry are removed Long hair is tied up or put in a cap or under shirt Wearing heavy duty shoes Wearing short sleeves or long sleeves are rolled up Floor is not wet 2. Perform a lockout/tagout on the Motor Control Unit s safety switch. 3. Place Shaft Panel 1 on the work station s overhead rack. 4. Perform the following substeps to mount and level the Constant Speed Motor. A. Locate the Constant Speed Motor and place it on the work surface. B. Select four Constant Speed Motor Risers from Shaft Panel 1. C. Make sure that the motor base, risers, and mounting area of the work surface shown in figure 34 are free of dirt, rust, and burrs. 43

44 D. Position the Constant Speed Motor over the set of holes on the 950-ME work surface, as shown in figure 34. The outlines of the other components to be mounted are also shown. MOTOR PRONY BRAKE Figure 34. Location of Components on 950-ME Work Surface E. Place one Constant Speed Motor Riser under each of the motor feet. F. Locate four bolts with the specifications 5/16-18UNC-2A x 1-1/2 Hex Head, along with compatible flat washers, lock washers, and nuts. G. Fasten the motor and risers to the work surface by assembling bolts, washers, and nuts. Use a criss-cross pattern to tighten the bolts. 44

45 H. Check the shaft for run-out. Record below the amount of run-out. Run-out: (in/mm) The run-out should be less than inches. I. Check for motor shaft end float. End Float (in/mm) It should be less than inches. J. Check the level of the motor shaft. Shim the motor feet as needed. Feeler Gage Leaf Thickness (in/mm) Effective Level Length (in/mm) Mounting Bolt Distance (in/mm) Shim Ratio Shim Thickness (in/mm) 5. Locate the components for the prony brake, as shown in figure 35. This unit consists of a brake drum, load unit, and water spray bottle. The spray bottle is used to spray water into the drum to keep it cool while the unit is loaded. The brake drum attaches to the shaft of the motor. Figure 35. Components of Amatrol s Prony Brake 45

46 6. Perform the following substeps to assemble the prony brake drum to the shaft of the motor using a key fastener. WARNING Your instructor must be present for the completion of this skill. This is because you will be using the key you made in an earlier skill. The instructor must be satisfied that the key fits correctly into this application before the motor can be started. A. Locate the two cap screws that hold together the two halves of the brake drum hub, as shown in figure 36. Note that these are not set screws. These screws clamp the two sections of the brake drum together against the key fastener on the shaft to hold the drum hub in place. CAP SCREWS Figure 36. Cap Screws on Hub B. Use a hex key wrench to back out the cap screws so the two halves of the drum hub can be separated enough for the hub assembly to slide over the motor shaft and key. C. Clean the shaft keyseat and the hub keyseat with a wire brush to make sure that no dirt or burs are in the keyseats. D. Obtain the 3/16-inch square key you made in the last skill. 46

47 E. Slide the key into the keyseat of the motor shaft. The key should fit into the keyseat without forcing it. If it is too tight, take it out and measure it to see which part is out of tolerance. Select a correct sized key from the tool crib and continue. F. Check the key for play when it is in the keyseat by wiggling it. There should be no play. If there is play, replace the key. G. Remove the key from the shaft keyseat and insert it into the brake drum keyseat. It also should slide in without forcing it and have no play. H. Remove the key from the brake drum and insert it into the motor shaft keyseat. Line it up flush with the end of the shaft, as shown in figure 37. KEY Figure 37. Key Positioned on Shaft I. Pick up the brake drum in your hand and line it up in front of the shaft so that the drum s keyseat is in line with the key on the shaft. 47

48 J. Then slide the drum onto the shaft until the end of the drum meets the step on the motor shaft, as shown in figure 38. The drum should slide on without using tools. If it doesn t, pull it off and check the dimensions. NOTE You may have to hold the key in place as you do this to prevent it from sliding out of the motor shaft keyseat. Figure 38. Drum Slid Onto Shaft K. Tighten the cap screws with the hex hand wrench so that the key is compressed between the drum hub and the motor shaft. 7. Pull on the drum to see if the drum is securely fastened to the shaft. You should find that it is. 8. Leave the motor and hub set up and proceed directly to the Self Review. In the next segment, you will use this setup. 48

49 SEGMENT 2 SELF REVIEW 1. Checking to see if the hub has a set screw hole drilled in its side is the step to assembling a shaft and a hub using a key fastener. 2. Cleaning the shaft keyseat and the hub keyseat with a wire brush is the step to assembling a shaft and a hub using a key fastener. 3. The length of a keyseat can be measured using a(n). 4. A key stock which is purchased from a supplier has a(n). 5. The depth of the keyseat on the shaft can be measured using a(n). 49

50 SEGMENT 3 TORQUE AND POWER MEASUREMENT OBJECTIVE 7 DESCRIBE TWO METHODS OF LOADING A MECHANICAL DRIVE SYSTEM In some cases, certain mechanical devices are loaded by an external device in order to measure the performance characteristics at various loads. There are two common methods used to load a mechanical drive system: Prony Brake Dynamometer In the next skill, you will use the prony brake to load the motor. This device will be used throughout this module to demonstrate the effects of loads on various types of mechanical drive systems. Prony Brake The prony brake is one device that is used to load a motor. This device also has the ability to tell you how much load is applied to the motor. The prony brake drum, as figure 39 shows, is coupled to the motor shaft and rests inside a canvas friction belt. As the canvas belt is tightened against the brake drum using the wingnut, the load on the motor is increased. This applies a force to the pivot arm. The force is measured by a spring scale that is placed at a specific distance from the center of the motor shaft. This is the radius distance. You can then use the force reading from the scale and the radius distance can then be used to calculate the torque. 50

51 PIVOT ARM WINGNUT RADIUS 6" (15.24cm) PIVOT POINT FORCE SCALE COUNTERWEIGHT CANVAS FRICTION BELT BRAKE DRUM Figure 39. Prony Brake Dynamometer A dynamometer is another type of device that places a load on a motor and measures the amount of power that the motor can produce. Race car builders use dynamometers to tune their engines. SCALE GENERATOR COUPLING DEVICE Figure 40. Typical Dynamometer 51

52 SKILL 5 USE A PRONY BRAKE TO MEASURE SHAFT TORQUE Procedure Overview In this procedure, you will attach the prony brake to the electric motor and load it. This prony brake will used in other skills to demonstrate that the torque output changes as the power is transmitted through various types of mechanical drives. Also, it will allow you to show the power efficiency of mechanical drives. In this skill, you will measure the speed of the motor as its load is increased. This is simply an exercise to reinforce your ability to measure motor speed. However, you will also learn how AC motors react to load changes. 1. Perform the following safety checkout to prepare for working with power transmission equipment. Make sure that you are able to answer yes to each item before proceeding. YES/NO SAFETY CHECKOUT Wearing safety glasses Wearing tight fi tting clothes Ties, watches, rings, and other jewelry are removed Long hair is tied up or put in a cap or under shirt Wearing heavy duty shoes Wearing short sleeves or long sleeves are rolled up Floor is not wet 2. Perform a lockout/tagout on the Motor Control Unit s safety switch. 52

53 3. If you are continuing from the previous Skill, proceed directly to step 4. If not, repeat all steps in Skill 4 to set up the motor, level it, and attach it to the brake drum using the square key. When complete, your setup should appear as shown in figure 41. Figure 41. Setup from Skill 4 53

54 4. Perform the following substeps to mount the prony brake to the work surface and to connect it to the motor. A. Loosen the load nut on top of the prony brake by turning it counterclockwise (looking down on it from above). Continue loosening until the screw threads are flush with the hand knob. PRONY BRAKE CCW DECREASES SETTING LOAD NUT Figure 42. Load Nut on Prony Brake 54

55 B. Position the prony brake on the work surface so that its mounting holes line up the holes on the work surface shown in figure 43. MOTOR PRONY BRAKE Figure 43. Location of Prony Brake C. Make sure that the friction band rests under the drum, as shown in figure 44. Figure 44. Friction Band Positioned Under Brake Hub 55

56 D. Obtain four bolts with the specification 5/16-18UNC-2A x 1-1/4 hex head, along with compatible flat washers, lock washers, and nuts. E. Mount the prony brake to the work surface by assembling the bolts, nut, and washers, as shown in figure 45. Make sure the prony brake belt wraps evenly around the brake drum. Figure 45. Prony Brake Mounted to Work Surface F. Loosen the fasteners on the motor mount and adjust the position of the motor until the prony brake belt is even with the brake drum hub. G. Retighten the motor mounts. 56

57 5. Fill the brake drum with water using the water bottle so that the water level is about 1/4 (0.635 cm ), as shown in figure 46. BLACK TAPE REFLECTIVE TAPE WATER LEVEL 1/4" Figure 46. Brake Drum Water Level and Tape Location 6. Attach a 3-inch strip of black tape and a small piece of reflective tape on the brake drum, as is also shown in figure

58 7. Perform the following substeps to install the mechanical system s guard. WARNING Do not operate the mechanical drive system without the guard in place. Also, do not attempt to open or bypass the guard at any time during operation. Performing any of these actions will create a hazardous situation. A. Locate both halves of the guard, shown in figure 47. Figure 47. Mechanical System s Guard 58

59 B. Place both hatch links in the back position. An example is shown in figure 48. This prevents the latch links from interfering with the placement of the guard halves. LATCH LINK Figure 48. Latch Link in the Back Position C. Place one half of the guard on the work surface, as shown in figure 49. Slide the guard in until the tabs are against the side of the work surface. WORK SURFACE TAB Figure 49. First Half of Guard in Position 59

60 D. Place the other half of the guard on the work surface from the opposite side. Make sure that the interlock tabs, located on the sides of the guard halves, are positioned to the inside of the guard, as shown in figure 50. Also, ensure that the motor s power cord runs through the cut-out in the guard. INTERLOCK TAB TO INSIDE MOTOR CORD Figure 50. Interlock Tab and Motor Cord 60

61 E. Lift the latch handles and rotate the latch link over the top of the strike plate as shown in figure 51. Then, press the latch handles down. This will draw the guard halves together and lock them in position. LATCH HANDLE STRIKE PLATE Figure 51. Latch Operation 61

62 F. Verify that there is a hole in the guard directly over the prony brake. This hole allows you to adjust the prony brake s setting while protecting you from the moving components of the drive system. PRONY BRAKE ADJUSTMENT HOLE Figure 52. Prony Brake Adjustment Hole in Guard 8. Perform the following substeps to start the motor. A. Clear all tools and loose components from the work surface and put them in their proper storage. B. Make sure that the Motor Control Unit s power cord is plugged into a wall outlet. C. Make sure the Motor Power switch is in the OFF or down position. D. Connect the Constant Speed Motor s power cord to the Motor Port. E. Remove the lockout/tagout. F. Turn on the safety switch. The Main Power Indicator on the Motor Control Unit should turn on. G. Make sure that no one is near the motor. H. Turn on the Constant Speed Motor by moving the Motor Power switch to the ON up position. The motor should accelerate to full speed quickly and run at a constant speed. 62

63 9. Perform the following substeps to measure the motor speed at various load settings. A. Observe the reading on the prony brake scale. It should be zero. If not, turn the prony brake s load nut counterclockwise one or more times until it is. B. Measure the speed of the motor using the tachometer. Record your reading in column 2, row 1. This is the unloaded speed of the motor. It should be approximately 1790 RPM. SCALE READING (Ounces) SPEED (RPM) TORQUE (In-ounces) C. While running, increase the load on the prony brake to 4 ounces by turning the load nut clockwise until the scale reads 4 ounces. D. Measure the speed of the motor at this load setting. Record it in column 2, row 2 of the table. E. Repeat substeps C and D for each of the other load settings in the table. As you increase the load on the motor, you should notice that the motor slows down some but still maintains a speed that is close to its unloaded speed. This is a characteristic of AC motors. F. Reduce the load on the motor to zero by turning the load nut counterclockwise until the scale reads zero. 10. Turn off the motor. The motor should coast to a stop. 11. Perform a lockout/tagout. 12. Calculate the torque delivered by the motor for each load setting in the table of step 8. To do this, multiply the scale reading by the radius of the prony brake, which is 6.0 inches (15.24 cm). Record your calculations in column 3 of the table. 13. Leave the setup in place. In the last skill of this LAP, you will use it. 63

64 OBJECTIVE 8 DESCRIBE HOW TO CALCULATE ROTARY MECHANICAL POWER Rotary mechanical power is defined as the rate or speed at which the rotating power transmission system turns the load. Since work is defined as Force x Distance, work in a rotating system is actually torque. This means that the power output at a motor s shaft is found by multiplying the torque by the speed (rate) as follows: Rotary Power = Shaft Torque Shaft Speed TO LOAD MOTOR SPEED (RPM) TORQUE (ft-lb OR N-m) Figure 53. Motor Torque and Speed 64

65 The units of rotary power are expressed in horsepower (Hp) in the U.S. customary system and Kilowatts (kw) in the Systems International (S.I.) system. They are calculated as shown in the following formulas: U.S. Customary Units: where MOTOR POWER FORMULAS P out T S = 5252 P out = Output power (Hp) T = Torque (ft-lb) S = speed (RPM) S.I. Units: where P out T S = 9549 P out = Output power (kw) T = Torque (N-m) S = speed (RPM) 65

66 SKILL 6 CALCULATE ROTARY MECHANICAL POWER Procedure Overview In this procedure, you will calculate the power output of various power transmissions based on given information. 1. Calculate the power delivered by a belt drive system shown in figure 54. P out = (Hp) TORQUE = 25 ft - lbs SPEED = 2400 RPM TORQUE = 50 ft - lbs SPEED = 1200 RPM DRIVER DRIVEN Figure 54. Belt Drive System The solution is as follows: P P P P out out out out T S = = = 5252 = Hp 66

67 2. Calculate the power in kw delivered by the gear drive system shown in figure 55. The formula for converting Hp to kw is: PkW = P Hp P out = (kw) INPUT GEAR TORQUE OUT = 22 ft - lbs SPEED OUT = 550 RPM Figure 55. Gear Drive System It should be approximately 1.72 kw. 3. Calculate the power delivered by the motor given the information shown in figure 56. P out = (Hp) It should be approximately 9.52 Hp. SPEED OUT = 150 RPM SPEED IN = 400 RPM R 1 R 2 WHEEL R 1 = 1.5 in R 2 = 4 in INPUT GEAR F = 1000 lbs. OUTPUT GEAR Figure 56. Electric Motor 67

68 4. Calculate the power in kw at a gear shaft given the following information. Force = 3.75 N Radius Distance = 1.2 m Speed = 1200 RPM P out = (kw) The solution is 0.57 kw. 5. Calculate the maximum torque that can be generated by a motor given the following information. Show all calculations on the data sheet. Round off to the nearest 0.1 N-m. Speed = 1140 RPM P out = 0.25 kw Maximum Torque = (N-m) The solution is 2.1 N-m. 6. Calculate the maximum speed of a motor based on the following information. Show all calculations on the data sheet. Round off to the nearest RPM. Maximum Torque = 425 ft-lb. P out = 100 Hp Maximum Speed = (RPM) The solution is 1236 RPM. 7. Solve the following design problem: Select the best small DC motor from the table based on the following information. Choose the rating closest to your calculated value. However, be careful not to undersize your selection. Torque required = 1.25 N-m Speed required = 1200 RPM STANDARD POWER RATINGS FOR SMALL DC MOTORS Hp 1/20 1/12 1/8 1/6 1/4 1/3 1/2 3/4 1 kw Calculated Power = (kw) Selected Motor s Power = (kw) The solution is kw, kw. 68

69 SKILL 7 CONVERT BETWEEN U.S. CUSTOMARY AND S.I. UNITS OF MOTOR POWER Procedure Overview In this procedure, you will convert motor power values from US customary to SI units, and from SI to US customary units. This is a common task you will do when you work with products made in the United States and other countries. 1. Convert 1.78 KW to PHp. The conversion formula is as follows: P kw = P Hp P Hp = (Hp) It should be approximately 2.39 Hp. 2. Convert the PkW values in the table below to P Hp. MOTOR POWER RATINGS (kw) U.S. CUSTOMARY MOTOR POWER RATINGS (Hp) 69

70 3. Solve the following design problem: Calculate the S.I. output power value (P kw ) of the motor in figure 57. Show all calculations on the data sheet. P kw = (kw) The solution is 1.94 kw. MOTOR SPEED = 1750 RPM TO LOAD MOTOR TORQUE = 7.8 ft-lb Figure 57. Design Problem 70

71 SEGMENT 3 SELF REVIEW 1. On a prony brake, torque is computed by multiplying the load times 2. A prony brake is a device used to apply a to a motor. 3. Work is defined as force times. 4. A dynamometer is used to a mechanical drive system. 5. mechanical power is the rate or speed at which the rotating power transmission system turns a load. 71

72 SEGMENT 4 MECHANICAL EFFICIENCY OBJECTIVE 9 DESCRIBE HOW TO CALCULATE MECHANICAL EFFICIENCY AND EXPLAIN ITS IMPORTANCE One of the problems with power transmission equipment of any kind is that the power output is always less than the power input. This is because there are frictional effects in the transmission that cause some of the power to be lost to heat. The ratio of the Power Out to the Power In is called the Power Efficiency. If it is describing the power lost through a mechanical drive system, it is called the Mechanical Power Efficiency. It can be stated in a formula as follows: FORMULA: MECHANICAL POWER EFFICIENCY Pout Eff = 100 P where Eff = Mechanical Efficiency in % P out = Output Power (Hp or Kw) P in = Input Power (Hp or Kw) in The mechanical power efficiency is important to any machine. The goal of any designer is to make it as high as possible, so that the machine uses as little energy as possible to perform its task. Maintenance technicians also affect the efficiency of the machine by how they align and lubricate it. The mechanical efficiency will also go down as the machine wears. This means that monitoring the efficiency can tell you when a machine needs servicing. The mechanical efficiency of a power transmission can be determined by measuring the shaft speed and torque at the input and the output. In some cases, the power loss may be caused by a loss of speed due to slip in the drive components. In others, the power loss may be caused by lost torque from friction. Lost torque is the most common source of power loss. 72

73 In actual application, measuring the mechanical power at either the input shaft or the output shaft is hard to do because it is not easy to measure the torque. Measuring the torque can be done by either using a torque transducer, an electronic device that attaches to the shaft, or a dynamometer. In most cases, you can more easily monitor the efficiency of the system by measuring the electric power drawn by the motor. If it increases over time, you know that the mechanical drive is losing efficiency. SKILL 8 CALCULATE MECHANICAL EFFICIENCY Procedure Overview In this procedure, you will calculate the mechanical efficiency of a number of power transmissions. 1. Calculate the efficiency of the gear train shown in figure 58 given the following information. Horsepower of Motor = 20 hp Horsepower of Gearbox Output shaft = 17 hp Efficiency = (%) Figure 58. Effi ciency of a Gearbox Connected to a Motor Shaft The solution is 85%. 73

74 2. Calculate the efficiency of the belt drive system shown in figure 59 given the following information. Motor Power = 4 hp Output Shaft Speed = 3500 rpm Output Shaft Torque = 54 in-lb Efficiency = (%) Figure 59. Effi ciency of a Pulley System Connected to a Motor Shaft The solution is 75%. 3. Calculate the efficiency of the chain drive shown in figure 60 given the following information. Motor Shaft Speed = 1750 rpm Motor Shaft Torque = 12 in-lb Output Shaft Speed = 875 rpm Output Shaft Torque = 18 in-lb Efficiency = (%) Figure 60. Effi ciency of a Chain Drive Connected to a Motor Shaft The solution is 75%. 74

75 OBJECTIVE 10 DESCRIBE TWO METHODS OF MEASURING SHAFT TORQUE AND GIVE AN APPLICATION OF EACH Measuring the load on the mechanical drive system is useful because it allows you to determine how the system is operating. A problem in the drive system will often cause a change in the load. For example, excessive tension in a v-belt will cause a higher load. These are two methods you can use to measure the load on a shaft: Current Measurement Torque Transducer Current Measurement Torque is related to the electrical current supplied to the motor. As motor torque increases, so does electrical current. Most motor manufacturers have already tested this relationship and include a graph with the specifications of the motor, like the one in figure 61, that shows the torque vs. current characteristics. By using this graph and measuring input current, it is possible to determine the torque. For example, from the torque vs. current graph in figure 61 you can see that if the measured current is 2 amps, the torque delivered by the motor is approximately 150 in-oz TORQUE (in-oz) CURRENT (AMPS) Figure 61. Torque vs. Current Curve for a Motor 75

76 Current measurement is often used to measure motor torque in the field because the motor is already connected to a load. In a later skill, you will observe the current as load on the motor is increased. This relationship will be used in later LAPs to prove various design formulas and to show the efforts of poor adjustment of the system. Torque Transducer A torque transducer is a device which is directly coupled to the shaft. As the shaft load increases the transducer generates an electrical signal which can be received by an ammeter or a controller. The form of the signal is usually either a ±10 VAC or 4-20 ma signal. This signal is proportional to the torque, as shown in figure VOLTS TORQUE (in - oz) Figure 62. Electrical Signal vs. Torque 76

77 OBJECTIVE 11 DESCRIBE THREE METHODS OF MEASURING ELECTRIC MOTOR CURRENT The efficiency of a mechanical drive system can be monitored by measuring the electric motor s input current. As the efficiency goes down, the motor s current will increase. This shows that the load of the drive has increased. There are three methods by which motor current can be measured: Clamp-on ammeter Hand-held ammeter Built-in ammeter A clamp-on ammeter, shown in figure 63, can be opened and placed around a wire in which you want to measure the current. This is very convenient because it allows you to measure current without disconnecting the circuit and connecting the meter in series. This is particularly important for AC power applications where the current level is often quite high and very dangerous. Figure 63. Clamp-On Ammeter Used to Measure Current 77

78 SKILL 9 MEASURE ELECTRIC MOTOR CURRENT Procedure Overview In this procedure, you will use a built-in ammeter to measure the current to the motor as load is applied using the prony brake. 1. Verify that the motor and prony brake are set up as you did in Skill 5. If the setup has been disassembled, return to Skill 5 and reconnect it. 2. Perform the following safety checkout to prepare for working with power transmission equipment. Make sure that you are able to answer yes to each item before proceeding. YES/NO SAFETY CHECKOUT Wearing safety glasses Wearing tight fi tting clothes Ties, watches, rings, and other jewelry are removed Long hair is tied up or put in a cap or under shirt Wearing heavy duty shoes Wearing short sleeves or long sleeves are rolled up Floor is not wet 3. Verify that the guard is installed. WARNING Do not operate the mechanical drive system without the guard in place. Also, do not attempt to open or bypass the guard at any time during operation. Performing any of these actions will create a hazardous situation. 4. Remove the lockout/tagout. 5. Perform the following substeps to start the motor. A. Observe the reading on the prony brake. It should be zero. If not, turn the prony brake s load nut counterclockwise one or more times until it is. B. Add water to the brake drum if necessary. The water level should be about 1/4 inch (0.635 cm) below the lip of the drum opening. Do not let the drum run dry. C. Turn on the motor and allow it to accelerate to full speed. 78

79 6. With the motor running at zero load, turn on and hold the Meter Read Switch on the Motor control Unit, as shown in figure 64. Record the current reading on the Constant Speed Motor Load Meter in row 1 of the table below. Also record the motor speed. SCALE READING (Ounces) CURRENT (Amps) SPEED (RPM) Figure 64. Meter Read Switch 79