AUXILIARY COMPONENTS

Transcription

1 CHAPTER 9 AUXILIARY COMPONENTS Fluid Power Circuits and Controls, John S.Cundiff, 2001 INTRODUCTION Discussion of the hardware needed to connect the various components (pumps, actuators, valves, accumulators, filters, oil coolers) in a circuit. Two key issues in fluid power are P (pressure) Q (flow). 1

2 INTRODUCTION Two key issues in the design of the lines between components are sizing for the recommended maximum fluid velocity (related to Q). selection of the pressure rating (P). Conductors must have higher a working pressure rating than the maximum expected pressure in the system. RESERVOIR The reservoir has four functions. It separates entrained air. Dwell time in the reservoir provides opportunity for air bubbles in the fluid to rise to the top and burst on the surface. It dissipates pressure pulses. Circuits with several actuators and random actuation of these actuators can have significant pressure pulses in the return line. It provides cooling. Heat exchange from the reservoir to the surroundings. It traps contaminant. If the reservoir has to trap contaminant, the filtration is not working correctly. 2

3 RESERVOIR CONSTRUCTION Recommended reservoir capacity is one to three times the pump output. Machines are often less than three times the pump flow, but should never be less than one times the total pump flow. Reservoir must be sized to hold all fluid from the cylinders when they are fully retracted. When all cylinders are fully extended, the level of fluid in the reservoir must still be above the suction line to the pump. RESERVOIR CONSTRUCTION Refer Fig 9.1 A reservoir with maximum surface area per unit volume gives the best heat exchange. 3

4 RESERVOIR CONSTRUCTION Refer Fig 9.2 The reservoir location should be chosen such that the distance h between the fluid level and the pump inlet is as great as possible. A larger h reduces the potential for cavitations. There should be good air circulation across all exterior surfaces. RESERVOIR CONSTRUCTION Some modular units have pumps mounted on the top of the reservoir. The pump must provide enough suction to lift fluid into the pump. 4

5 RESERVOIR CONSTRUCTION Both inlet and return lines should be submerged. If the return flow jets across the surface of the fluid, air will be entrained in fluid. RESERVOIR CONSTRUCTION Pump inlet should be some distance above the bottom of the reservoir to minimize the potential for ingestion of contaminants that have settled on the bottom. A strainer should always be used at the inlet. The drain plug should be located such that all oil can be drained from the reservoir. 5

6 RESERVOIR CONSTRUCTION Two baffle patterns. Thorough mixing of oil in larger reservoirs prevents temperature gradients and improves heat exchange. RESERVOIR CONSTRUCTION Pressurize the reservoir using a bladder, if it is not possible to provide a large enough h to completely fill the pump at design operating speed. 6

7 RESERVOIR CONSTRUCTION A pressurized reservoir eliminates the exchange of air into the reservoir. Eliminates a pathway for dirt ingress Eliminates the entrance of water vapor into the reservoir where water vapor can condense on the interior surface and drop water into the oil. There are three types of lines used for pressurized fluid : Pipe Seamless tubing Hose 7

8 Pipe Pipe and tubing are both rigid conductors. Normal sizes of American standard pipe and pipe fittings are defined by ANSI standard B36.10, There are four schedules Double extra heavy The outside diameter is held constant for all schedules of a given nominal size, because the threads cut into the OD must always fit those tapped into a mating port or fitting. The wall thickness increases to provide a higher pressure rating. Pressure ratings for steel pipe are given in the table. (Refer Table 9.2) 8

9 Pipes and pipe fittings have tapered threads. These threads seal by a metal-to-metal contact between the threads in the mating parts. 9

10 NPT threads engage mating threads on their flanks. This design leaves a small spiral groove along the thread tips, which must be filled with sealant. NPTF threads seal by tightening until the thread crests are crushed enough that the flanks make full contact. Hydraulic tubing Tubing is either seamless carbon steel or seamless stainless steel. Stainless steel tubing is for applications in which the exterior surface is attacked by the surrounding chemical environment (Or) the fluid itself is corrosive and the inside is subject to attack. Hydraulic tubing is specified by its outside diameter (OD). It has a thin wall compared to pipe. Methods other than threading have been developed to connect tubing. 10

11 Three main types of fittings to connect tubing Flared tubing fittings Flareless tubing fittings Welded tubing fittings Flared tubing fittings Three piece type is the most widely used for hydraulic circuits. A sleeve is placed on the tubing before it is flared. When flare nut is tightened sleeve absorbs the twisting friction produced by the nut so that only axial forces are exerted against the flared tube. 11

12 The standard flare angle for hydraulic tubing is 37 o. Making the flare too narrow is a common fabrication error. The correct amount of extension of the fare above the sleeve is shown. (Refer Fig 9.10) Flareless tubing fittings Tubing wall thickness ca be increased to produce tubing with a higher pressure rating. Flaring is more difficult as wall thickness increases, as a result flareless fittings were developed. 12

13 There are a number of different designs All the different designs have some means whereby a ferrule is pressed against the tubing and actually bites into the surface. Once this ferrule is seated, it cannot be removed. Flareless fittings will leak if under-tightened or overtightened. It is best to moderately tighten, check for leaks and then tighten in 37 o increments until the leak is sealed. Flareless fittings are not recommended for tubing below a certain wall thickness. 13

14 Welded Fittings Welded fittings are used for the most severe applications. Of the three types, the cost for welded fittings is the highest Selection of hydraulic tubing involves choosing the correct material and then determining the size, OD and wall thickness. Pressure rating for hydraulic tubing and fittings are given in SAE Standard J1065 Jul95. The design pressure data is based on severity of service rating A. This rating uses a design factor of 4, meaning that the burst pressure to working pressure ratio is 4:1 14

15 Example of severe service is the following application. A press is used to form bladders for truck brakes. Sheets of reinforced elastomer material are placed in the mold and the press closes. A combination of pressure and temperature is used to form the part. The procedure calls for a bump cycle where pressure is cycled for several cycles. Operating temperature is 320 o F. The pressure valve for the press circuit is set on 2000 psi and 0.75 OD carbon steel tubing has been selected. Find the wall thickness required. 15

16 Solution: The derating factor for B service is 0.67 (Refer Table 9.4) and for 320 o F, the derating factor is The combined factor is 0.67 x 0.99 = Tubing selected must have a design pressure rating of 2000 psi = 3015 psi From Table A9.1, 0.75-in. C-1010 steel tubing is selected with a in. wall thickness has a design pressure rating of 3050 psi. The burst pressure rating is 4 x 3050 psi = 12,200 psi; thus the overall design factor for this application is 12,200 psi = 6.1 2,000 psi 16

17 Referencing Table 9.3, the tubing has an in. wall thickness; thus either flared or flareless fittings can be used. Hydraulic Hose Hydraulic hose can be divided into two categories: Fabric-reinforced hose Wire-reinforced hose 17

18 Fabric-reinforced hose Fabric reinforced hose has a plastic (or rubber) inner tube covered by one or more layers of woven fabric. The outer surface is protected by a rubber or plastic covering. Wire-reinforced hose. Wire-reinforced hose has a synthetic rubber inner tube, one or more layers of wire reinforcement, and a synthetic rubber outer coating to protect the wire. Two types of reinforcement: Woven Spiral bound 18

19 Pressure ratings for hydraulic hose are given in SAE Standard J517 May97. The design pressure rating decreases as diameter increases. There are several techniques for attaching the hose end to the hose. Barbed nipples with bolted clamps. Reusable fittings with screw-in nipples. Barbed nipples with bolted clamps 19

20 Reusable fittings with screw-in nipples Advantages of reusable fitting: Being able to salvage the fitting when the hose is replaced Potential for changing the nipple to obtain a different fitting on the end of the hose. Large hoses have large fittings, and it is often difficult to get the right size wrench in position to tighten the fitting. Split flange fittings were developed so that assembly can be done by tightening four small bolts. (Refer Fig 9.17) 20

21 FLUID VELOCITY IN CONDUCTORS Pressure drop per foot of tubing is plotted, for a viscosity of 100 SUS and flowing in a straight 0.5-in. ID tube or hose. Pressure drop in the hose is slightly higher, because the friction factor for elastomer inner tube > factor for steel surface of the tubing. FLUID VELOCITY IN CONDUCTORS Importance of maintaining oil temperature and therefore viscosity, in the desired range is shown. Pressure drop is plotted versus fluid velocity for viscosities 100 and 400 SUS. 21

22 FLUID VELOCITY IN CONDUCTORS These viscosities were chosen because they represent the viscosity range of typical hydraulic oil where the system is started at 0 o F ambient temperature versus starting at 100 o F. Pressure drop in the lines decreases as the system comes up to the operating temperature. FLUID VELOCITY IN CONDUCTORS It is recommended that the lines be sized such that the following velocities are not exceeded: Pressure line 15 ft/s Return line 10 ft/s Suction line 4 ft/s 22

23 FLUID VELOCITY IN CONDUCTORS The cost for the conductors (pipe, tubing or hose) increases significantly as size increases. A one-inch fittings cost about eight times more than 0.5-in. fittings. Selection of a maximum velocity for pressure lines of 15 ft/s is a trade-off between fixed cost for the conductor and higher operating costs due to higher pressure drop in the lines. FLUID VELOCITY IN CONDUCTORS Referring to Fig 9.18, pressure drops increases more rapidly as velocity increases above 15 ft/s. Referring to Fig 9.19, pressure drop in the lines represents a conversion of hydraulic energy to heat energy, and thus increases operating cost. Return lines are low pressure lines and thus less expensive. Costs less to increase return line size and achieve lower velocity. 23

24 Options for Connecting Components There are a wide variety of fittings used to connect tubing, hose and pipe. A dash numbering system has been developed to facilitate the selection of needed fittings. Dash number is the number of sixteenths of an inch in the nominal size. Data for selected tubing and hose (Table 9.6) Options for Connecting Components 24

25 Options for Connecting Components Tubing with a 0.5-in. OD has a -8 number. Nominal 0.5-in. hose has a -8 number. The real advantage comes in the selection of fittings. A -8 tube fitting mates with a -8 adapter, which mates with a -8 hose end. The ID is approx. constant through the connection,which minimizes pressure drop. (Refer Fig 9.20) Options for Connecting Components Many tubing and hose connections for mobile and stationary applications are made with 37 o F flare fittings. It is recommended that pumps, motors and valves be purchased with straightthread O-ring ports. An adapter is screwed into this port as shown. (Fig 9.21) 25

26 Options for Connecting Components O-ring fits into a shallow groove machined into the surface of the port. The O-ring is compressed as the adaptor is tightened. The threads do not seal. Sealing is done by the O-ring. Options for Connecting Components Fittings are available that can swivel 360 o. (Fig 9.22) These fittings are more expensive. Sometimes they are needed to prevent a hose from twisting as it moves to follow an actuator. 26

27 End of Chapter 9 THANK YOU 27