Some practical considerations

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1 ME 222: Kinematics of Machines and Mechanisms [L9] Practical Considerations Suril V. Shah IIT Jodhpur 1 Some practical considerations Pin Joints versus Sliders and Half Joints Cantilever or Straddle Mount? Short Links Bearing Ratio Commercial Slides Linkages versus Cams Selection of Motors and Drivers Suril V. Shah IIT Jodhpur 2 1

2 Practical considerations Many factors need to be considered to create goodquality designs Not all of them are contained within the applicable theories A great deal of art based on experience is involved in design as well. Suril V. Shah IIT Jodhpur 3 Pin Joints versus Sliders and Half Joints At joint two materials rub together Such a rubbing interface is called a bearing. Proper material and good lubrication are the key to long life of joint. Choice of joint type affects ability to provide good clean lubrication Suril V. Shah IIT Jodhpur 4 2

Revolute (Pin) Joints Simple revolute joint is the clear winner for several

its pure form also called sleeve or journal bearing Geometry of pin-in-hole traps

lubrication in which the parts are separated by a thin film of lubricant

provided at the ends of the hole Suril V.

revolute joints Relatively inexpensive Available pre-lubricated and with end seals.

3 Revolute (Pin) Joints Simple revolute joint is the clear winner for several reasons Easy and inexpensive to design and to build a good quality pin joint In its pure form also called sleeve or journal bearing Geometry of pin-in-hole traps a lubricant film within its interface Promotes a condition called hydrodynamic lubrication in which the parts are separated by a thin film of lubricant Replacement lubricant can be done through radial holes Seals can easily be provided at the ends of the hole Suril V. Shah IIT Jodhpur 5 Ball and roller bearing Available in large variety of sizes for revolute joints Relatively inexpensive Available pre-lubricated and with end seals. Good ability to trap lubricant within the roll cage Relatively high rolling speed of the balls or rollers Promotes long life Suril V. Shah IIT Jodhpur 6 3

4 Spherical Rod End Bearings This has a spherical, sleeve-type bearing Self-aligns to an out of parallel shaft Its body threads onto the link, allowing links to be made from round stock with threaded ends Allow adjustment of link length. Suril V. Shah IIT Jodhpur 7 Revolute joints pivoted to ground Pillow blocks and flange-mount bearings Fitted with rolling or sleeve-type journal bearings. Pillow block: mounting to a surface parallel to the pin axis Flange mount: fasten to surfaces perpendicular to the pin axis. Suril V. Shah IIT Jodhpur 8 4

Prismatic (Slider) Joints Require a carefully machined and straight slot or rod Bearings often must be custom made Lubrication is difficult to maintain as not geometrically captured Lubrication is

5 Prismatic (Slider) Joints Require a carefully machined and straight slot or rod Bearings often must be custom made Lubrication is difficult to maintain as not geometrically captured Lubrication is resupplied either by running the joint in an oil bath or by periodic manual regreasing. An open slot tends to accumulate airborne dirt This can act as a grinding compound when trapped in the lubricant and accelerate wear. Suril V. Shah IIT Jodhpur 9 Higher (Half) Joints Suffer even more acutely from the slider s lubrication problems Two oppositely curved surfaces in line contact, tend to squeeze any lubricant out of the joint. Joint needs to be run in an oil bath for long life. Requires housing the assembly in an expensive oiltight box with seals Suril V. Shah IIT Jodhpur 10 5

An example These joints are used extensively in machinery with great success. Some common examples can be found in an automobile. The windshield wiper mechanism is a pin-jointed linkage.

6 An example These joints are used extensively in machinery with great success. Some common examples can be found in an automobile. The windshield wiper mechanism is a pin-jointed linkage. The pistons in the engine cylinders sliders and bathed in oil. The valves are operated by cam-follower and drowned in oil. You probably change your engine oil fairly frequently. When was the last time you lubricated your windshield wiper? Has this linkage (not the motor) ever failed? Suril V. Shah IIT Jodhpur 11 Cantilever or Straddle Mount? A cantilevered joint has the pin (journal) supported only. It is necessary when a crank must pass over the coupler. Cantilever beam is weaker than a straddle-mounted beam. Straddle mounting can avoid applying a bending moment to the links by keeping the forces in the same plane. Straddle-mounted pin is in double shear a cantilevered pin is in single shear. It is good practice to use straddle-mounted joints wherever possible. Suril V. Shah IIT Jodhpur 12 6

7 Short Links If crank is short it is not possible to provide suitably sized pins or bearings at each of its pivots. The solution is to design the link as an eccentric crank The fixed pivot is placed a distance e from the center of this circle equal to the required crank length. Lubricating large-diameter journal can be difficult. Suril V. Shah IIT Jodhpur 13 Bearing ratio Need for straight-line motion requires extensive use of linear sliding joints. There is a very basic geometrical relationship called bearing ratio, which if ignored leads to several problems. The bearing ratio (BR) is defined as the effective length of the slider over the effective diameter of the bearing: BR = L / D. Suril V. Shah IIT Jodhpur 14 7

8 Effective length is the distance over which the moving slider contacts the stationary guide Effective diameter is the largest distance across the stationary guides in a plane perpendicular to motion A common example of a device with a poor bearing ratio is a drawer in an inexpensive piece of furniture You have probably experienced the sticking and jamming that occurs with such a drawer For smooth operation this ratio should be greater than 1.5, and never less than 1 Suril V. Shah IIT Jodhpur 15 Linkages versus Cams Pin-jointed linkage has all the advantages of revolute joints Cam-follower has all the problems associated with the half joint. Pin-jointed linkage with good bearings is a potentially superior design, and should be the first explored Linkages have relatively large size compared to the output displacement in comparison to Cam. Linkages are relatively difficult to synthesize, and cams are relatively easy to design. Dwells are easy to get with cams, and difficult with linkages. Suril V. Shah IIT Jodhpur 16 8

9 Linkages versus Cams: Comparison Linkages can survive hostile environments, with poor lubrication, whereas cams cannot Linkages have better high-speed dynamic behavior than cams. Linkages are less sensitive to manufacturing errors, and can handle very high loads. Cams can match specified motions better. So the answer is far from clear-cut. It is another design trade-off situation. Suril V. Shah IIT Jodhpur 17 Motors and drivers Need of driver to provide the input motion and energy. Continuous rotary input: Motor is the logical choice. Motors come in a wide variety of types. Most common energy source for a motor is electricity Air and hydraulic motors Suril V. Shah IIT Jodhpur 18 9

10 Electric motors Motors provide continuous rotary output. Stalled momentarily against a load It can not tolerate a full-current, zero-velocity stall for more than a few minutes without overheating. Electric motors are classified by Electrical configuration Function or application Rated power Suril V. Shah IIT Jodhpur 19 Electrical configuration Suril V. Shah IIT Jodhpur 20 10

11 Functional classifications Gear motors Servomotors Stepping motors Rated power Subfractional (> 1/20 HP) Fractional (1/20-1 HP) Integral (> 1 HP) Suril V. Shah IIT Jodhpur 21 DC motors Have electrical configurations: permanent magnet, shunt-wound, series-wound, and compound-wound Rotating armature coils connected to the stationary field coils in parallel (shunt), in series, or in combined series-parallel (compound) Permanent magnets replace the field coils in a PM motor. Each configuration provides different torque-speed characteristics. Torque-speed curve describes how motor will respond to an applied load Suril V. Shah IIT Jodhpur 22 11

12 Permanent Magnet DC Motors Torque varies greatly with speed power = torque x angular velocity. Torque is maximum at stall (zero velocity) This is an advantage when starting heavy loads (unlike IC engine) Suril V. Shah IIT Jodhpur 23 Shunt-wound DC Motors Flatter slope around the rated torque Less speed-sensitive to load variation It stalls very quickly Typically used on fans and blowers. Suril V. Shah IIT Jodhpur 24 12

13 Series-wound DC Motors More speed-sensitive (large slope) Starting torque as high as 800% of fullload No theoretical maximum no-load speed Run away if the load is removed. (Maximum speed 30,000 rpm.) Gives a soft-start Sewing machines and portable grinders Suril V. Shah IIT Jodhpur 25 Compound-wound DC Motors Combination of shunt-wound and serieswound Speed sensitivity is greater than a shuntwound but less than a series-wound It will not run away when unloaded. Has high starting torque and soft-start capability Good choice for cranes and hoists that experience high inertial loads and can suddenly lose the load due to cable failure Suril V. Shah IIT Jodhpur 26 13

14 Speed control DC motors Controller increases and decreases the current against changing load to maintain speed. Speed-controlled (typically PM) DC motors will run from an AC source. Cost of such motor is high. Suril V. Shah IIT Jodhpur 27 AC motors Least expensive way to get rotary motion. Synchronous motors lock on to the AC line frequency and run exactly at synchronous speed. Nonsynchronous motors have a small amount of slip and lag the line frequency by about 3 to 10%. The synchronous motor speed ns = 120f/p. Suril V. Shah IIT Jodhpur 28 14

15 Single-phase motors Shaded pole and permanent split capacitor: starting torque lower than their full-load torque. Split-phase and capacitor-start motors: boost the start torque Suril V. Shah IIT Jodhpur 29 Three-phase motors Differ mainly in their starting torque and speed sensitivity near full-load point. Suril V. Shah IIT Jodhpur 30 15

Shah IIT Jodhpur 31 Servomotors Servomotors are made in both AC and DC configurations Closed-loop-controlled (sensors for

16 Gearmotors Motor s output shaft is connected by gearbox Gearmotor with integral gearbox is available Available in variety of output speeds and power ratings. Suril V. Shah IIT Jodhpur 31 Servomotors Servomotors are made in both AC and DC configurations Closed-loop-controlled (sensors for position/velocity feedback) Precise positioning of the output device Capable of holding a fixed position against a load. Have fast-response Aircraft and guided missiles, numerically controlled machine, robots Suril V. Shah IIT Jodhpur 32 16

Relatively small and have low drive torque capacity Have high holding torque. Expensive and require special controllers Suril V.

17 Stepper Motors Brushless permanent magnet motors. Intermittent motion devices (no continuous rotary motion) They run open loop, without feedback Resolution depends on the number of magnetic strips. Relatively small and have low drive torque capacity Have high holding torque. Expensive and require special controllers Suril V. Shah IIT Jodhpur 33 Air and Hydraulic motors Require a compressed air or hydraulic source. Less energy efficient due conversion of the energy. Air motors used factories where compressed air is available. Cost of all the ancillary equipment is high. Hydraulic motors are found cranes, aircraft, and ships. Suril V. Shah IIT Jodhpur 34 17

Air and Hydraulic Cylinders These are linear actuators (piston in cylinder) Provide a limited stroke, straight-line output Input of either compressed air or hydraulic fluid

Shah IIT Jodhpur 35 Solenoids Electromechanical (AC or DC) linear actuators Share some of the limitations of air cylinders Energy inefficient Very short strokes (about 2 to

18 Air and Hydraulic Cylinders These are linear actuators (piston in cylinder) Provide a limited stroke, straight-line output Input of either compressed air or hydraulic fluid Share the high cost, low efficiency Farm and construction equipment Suril V. Shah IIT Jodhpur 35 Solenoids Electromechanical (AC or DC) linear actuators Share some of the limitations of air cylinders Energy inefficient Very short strokes (about 2 to 3 cm) length Develop a force that varies exponentially over the stroke Inexpensive, reliable, and have very rapid response times. They cannot handle much power. Used in camera shutters, electric door or trunk locking systems in automobiles Suril V. Shah IIT Jodhpur 36 18

19 Thank you Next Class: Dimensional Synthesis IIT Jodhpur 37 19

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