Clutches and brkesSelect any 3 position random motion out of real world and design a mechanism using Specified moving pivots Specified fixed points selected intelligently.pdf
- 2. Introduction • Mechanically Same Device • Frictional, magnetic, hydraulic or mechanical connection • Used in Production machinery • Clutches – Allow high inertia load to start • Emergency disconnecting devices/production machines – Clutch Brake Combination • Fail Safe, stops the load in case of power failure –heavy trucks, train • Clutch engagement for short time/Brake for large time • Brakes absorb more energy as compared to clutch
- 3. Types of Brakes and Clutches • Means of Actuation – Mechanical – Pneumatic – Hydraulic – Electrical • Means of Energy Transfer • Character of Engagement
- 5. Positive Contact Clutches • Toothed or Serrated Clutch • Not useful for Brakes • Engaged at low relative velocities (60rpm for jaw and 300 rpm for toothed) • Can transmit large torque with no slip • Sometimes engaged with friction type clutch
- 6. Friction Clutches and Brakes • Two or more surfaces with a normal force • Cylindrical/Conical • Flat and perpendicular to axis of rotation • One is metal surface other some friction material • Cylindrical with normal force in radial direction or conical • One is cast or surface lining • For heavy torque multiple plates • Dry/Wet (heat transfer), Oil bath • Most of automobiles, oil dipped (Also protect from dirt, and good heat transfer) • m 0.05 oil to 0.6 dry
- 7. Overrunning Clutches • One-way clutches • Relative velocity of two elements • Reverse Motion – Locks up • Lifts • Indexing Mechanism • Sprag Mechanism • Roller Clutch • Rear Hub o bicycle
- 8. Centrifugal Clutches • Engage on increase of shaft speed • Friction Elements thrown racially outward against the inside of drum cylinder • Couple IC Engine to Drive Train • Go-Carts, Chain Saws, overload protection
- 9. Magnetic Clutches • Friction Clutches Electromagnetically operated • Rapid Response time, ease control, smooth start/stop
- 10. Magnetic-particle clutches and Brakes • No direct frictional contact b/w clutch disk and housing • Gap filled with fine ferrous powder • Coil energized- powder particles form chains along magnetic field flux lines • Torque depends upon current
- 11. Magnetic Hysteresis Clutches and Brakes • No mechanical contact/no Friction • Rotor – drag cup dragged along (braked) by magnetic field set up by field coil. • Winding machines • Torque is controllable • Long life •
- 13. Fluid Couplings • Transmit torque through a fluid typically oil • Impeller/set of blades turned by input shaft imparts angular momentum to the oil surrounds it • Turbine/runner with similar blades attached to output shaft • Some slip • Smooth • Heat transfer • Never totally decouple • Use in -- marine and industrial machine drives
- 14. Clutch/Brake Selection and Specs • Catalogues • Procedures for selection and specs • Suggest service factors • Standard factors • Particular applications
- 15. Service Factors • Apply adequate service factors • Different safety factors and service factors • Based upon extensive testing field service experience • Slip, overheat • Inertia, overloading motor • MOI Service Condition Service Factor Light load pickup, no shock during operation 1.0 X Rated Torque Medium load pickup, medium shock during operation 0.5 X Rated Torque Heavy load pickup, medium shock during operation 0.3 X Rated Torque Sudden load pickup, heavy shock during operation 0.2 X Rated Torque Ratchet Clutch Service Factors
- 16. Clutch Location and Materials • Clutch Location – High speed – High torque • Clutch and Brake Materials – Disks or Drums • Gray CI, Steel – Friction Surfaces – Good COF, Compressive strength, high Temperature • Asbestos Fiber brake/clutch lining – Carcinogen • Lining – molded, woven, sintered, solid • Molded – polymeric resins, powdered, fibrous material • Brass or zinc chips – improve heat wear resistance reduce scoring of drums and disks • Woven- long asbestos firers • Sintered – high temp resistance, compressive strength • Wood, cast iron lining
- 17. Disk Clutches • Two disks - high friction • Normal force – Mechanically, hydraulically, electromagnetically • Uniform pressure – if disks flexible • Wear pV • pV=Const • uniform wear more conservative
- 18. Uniform Pressure
- 19. Uniform Wear Maximum torque for any outside radius ro will be obtained when the inside radius is:
- 20. • uniform-wear assumption gives a lower torque capacity for the clutch than does the uniform- pressure assumption. • The higher initial wear at the larger radii shifts the center of pressure radially inward, giving a smaller moment arm for the resultant friction force. • Clutches are usually designed based on uniform wear. They will have a greater capacity when new but will end up close to the predicted design capacity after they are worn in
- 24. Disk Brakes • No Lining covering entire circumference to avoid overheat • Ventilation • N is at least 2 • Automobiles • Good Controllability • Linearity
- 25. Drum Brakes • Apply Friction material to the circumference of a cylinder (ext/int) • Brake Shoe + Brake Drum • Brakes/Clutches • Shoe/Drum • Band Brake • Short Shoe • Long Shoe
- 26. Short-Shoe External Drum Brakes •Self Energizing cFf adds to aFa-increase the braking torque •Self –deenergizing - Reverse rotation- cFf •Self Locking mc≥b Angle of contact < 45o distributed force between shoe and drum to be uniform, and it can be replaced by the concentrated force Fn in the center of the contact area
- 27. • This self-energizing -potential advantage – reduces the force compared to a disk brake • Drum brakes typically have two shoes, one of which can be made self-energizing in each direction, or both in one direction. • SELF-LOCKING: If the shoe touches the drum, it will grab and lock. Not a desired condition except in so-called backstopping applications, - overrunning clutches; hoists
- 28. • Problem: Determine a suitable size and required force for an axial disk clutch. • Given: The clutch must pass 7.5 hp at 1725 rpm with a service factor of 2. • Assumptions: Use a uniform-wear model. Assume a single dry disk with a molded lining. • service factor of 2 requires derating the clutch by that factor, so we will design for 15 hp instead of 7.5. pmax = 225 psi and μ = 0.35. • The clutch specification is then a single disk with 3.6-in outside diameter and 2-in inside diameter, a molded lining with μ dry ≥ 0.35, and an actuating force ≥ 1108 lb. EX: 17.1
- 29. Ex 17.2 • For the drum-brake arrangement shown in, determine the ratio c / r that will give a self- energizing ratio Fn / Fa of 2. Also find the c / r ratio that will cause self-locking • Given The dimensions are a = b = 6, r = 5. • Assumptions Coefficient of friction μ = 0.35. • For self-locking to begin, Fa becomes zero, making Fn / Fa = ∞ and Fa / Fn = 0. The second of these ratios will need to be used to avoid division by zero. • Form the c / r ratio for self-locking with the given brake geometry. • Note that these ratios are specific to the dimensions of the brake. The length a was set equal to b in this example in order to eliminate the effect of the lever arm ratio a / b, which further reduces the application force Fa required for any normal force Fn
- 30. Long Shoe External Drum Brakes • Angle of contact >45 • Real shoe brake not; rigid • Assume; Drum velocity const, wear friction work pbsin sin P=Ksin K=p/Sin =pmaxSin /Sinmax
- 31. Long Shoe External Drum Brakes
- 32. Long Shoe External Drum Brakes
- 33. Controlling input Torque-FlyWheels • Large Oscillations due to torque variations • Average Torques – Small • Sizing of motor
- 34. Controlling input Torque-FlyWheels • Large Oscillations due to torque variations • Average Torques – Small • Sizing of motor
- 35. • Torque Peak Values + 341.7 lb-in, -166.4 lb-in • Average Torque – 70.2 lb-in
- 36. • Torque Peak Values + 341.7 lb-in, -166.4 lb-in • Average Torque – 70.2 lb-in
- 37. Flywheel Energy
- 38. Sizing flywheel • K is coefficient of fluctuation
- 39. Sizing flywheel • K is coefficient of fluctuation
- 40. Energy Considerations and Temp Rise