1. Clutches (Kopling tidak tetap)
Rini Dharmastiti
Jurusan Teknik Mesin FT UGM
Jl. Grafika no.2 Yogyakarta
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Introduction
Clutches and brakes are essentially the same device
Each provides a frictional, magnetic, hydraulic or mechanical connection between two elements.
Clutches: if both connected elements can rotate; primary function is to transmit power on an intermittent basis.
Brakes: if one element rotates and the other is fixed; absorbs the kinetic energy of moving bodies and controls their motions.
Machine Elements II - Rini D - TM FT UGM

3. Classification of clutches and brakes
Method of engagement: mechanical, pneumatic & hydraulic, electrical, automatic
Positive contact: square jaw, spiral jaw, toothed
Friction: axial, radial, cone
Ocerrunning: roller, sprag, spring wound
Magnetic: magnetic particle, hysterisis, eddy current
Fluid coupling: dry fluid, hydraulic
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Positive contact clutches Transmit power from the driving shaft to the driven shaft by means of jaws or teeth
+ transmit large torque with no slip
+ develop very little heat, because they do not depend upon friction
+ generally lighter
+ less costly than a friction clutches of similar torque capacity
Can not be engaged at high speeds, max 60 rpm for jaw clutches, and 300 rpm for toothed clutches
Shock accompanies engagement at any speed
Require some relative motion in order to engage when both driving and driven shafts are at rest
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6. Positive contact clutches
Although not as widely used as friction clutches, do find important use in such applications as automotive transmissions, presses, household appliances
A synchromesh clutch in a manual automotive transmission: combined with a friction-type clutch, which drags the two elements to nearly the same velocity before the jaws or teeth engages. Automotive transmission typically use helical gears, for quiet operation. The helical gears can not be easily shifted in and out of engagement in manual transmission because of their helix angle. Each gear has a synchromesh clutch connecting it to its shaft.
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7. Positive contact clutches
The force acting on the jaw depends upon the horsepower and speed that the clutch is to transmit.
T = torque (in-lb), hp = horse power to be transmitted, n = revolutions per minute
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9. Positive contact clutches
The average force, assumed to be acting at the center of each jaw:
F = average force (lb), k= number of jaws on one member, ro and ri = outside and inside jaw radius (inch)
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10. Positive contact clutches
The shearing stress (lb/inch2), acting on one jaw is:
t = thinkness of jaw (inch),  = angle subtended by one jaw in degrees, 2(ro + ri )/2 represents the average circumference of the jaws.
Machine Elements II - Rini D - TM FT UGM

11. Positive contact clutches
The bearing stress (lb/inch2), can be found from:
t = thinkness of jaw (inch), b = length of jaw
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Friction clutches Transmit power from the driving shaft to the driven shaft by means of plates, disks or cones
+because they can slip relative to each other, there is very little shock during engagement
+can be used for high speed engagement applications
Do slip
Do wear out (requiring replacement of friction material)
Heat is developed (might require external cooling)
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13. Friction clutches : disk clutches
For higher torque capacity, disk clutches are often made with multiple disks to increase the number of friction surfaces, but the heat of friction generated can become the limiting factor on its capacity
Multiple clutches are more difficult to cool, so are appropriate for high load, low speed applications
For high speed dynamic load, fewer friction surfaces are better
Friction clutches may be operated either dry or wet. Oil: reduces the coefficient of friction, increases the heat transfer
Friction coefficients of clutch/brake material combination typically range from 0.05 in oil to 0.60 dry
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14. Friction clutches : disk clutches
Automatic transmissions for automobiles and trucks contain many wet clutches and brakes operating in oil that is circulated out of the transmission for cooling.
Manual transmissions for off-road vehicles, such as motorcycles, use sealed, oil filled, multidisk wet clutches to protect the friction surfaces from dust, water and dirt.
Manual transmission automobiles and trucks, typically use single disk, dry clutches
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Clutch materials
Materials for the structural parts of clutches: typically made of gray cast iron or steel.
The friction surfaces are usually lined with material having a good coefficient of friction and sufficient compressive strength and temperature resistance: asbestos fiber, but no longer used in many applications because of its danger as a carcinogen.
Linings may be molded, woven, sintered or of solid material
Molded linings: use polymeric resins to bind a variety of powdered filled or fibrous materials.
Brass or zinc chips are sometimes added to improve heat conduction and wear resistance
Woven material: use long asbestos fibers
Sintered metals provide high temperature resistance and compressive strength than molded or woven materials.
Materials: cork, wood and cast iron are sometimes uses as lining as well
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16. Clutch and brake material properties
Maximum Temperature, ºF
Coefficient of friction, f
Maximum allowable pressure, psi
Metal on metal
0.25
200 – 250
Wood on metal
200
0.2 – 0.3
50 – 90
Leather on metal
150 – 200
0.3 – 0.4
15 – 40
Molded blocks
500 – 600
0.25 – 0.5
100 – 150
Asbestos on metal in oil
500
0.35 – 0.45
50 – 150
Sintered metal on cast iron in oil
450
0.2
400
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17. Uniform pressure and uniform wear
The pressure between the clutch surfaces can approach a uniform distribution over the surface if the disks are flexible enough.
The wear will be greater at larger diameters because wear is proportional to pressure times velocity (pV) and the velocity increases linearly with radius.
As the disks wear preferentially toward the outside, the loss of material will change the pressure distribution to a non-uniform one and the clutch will approach a uniform wear condition of pV=constant.
A flexible clutch may be close to a uniform pressure condition when new, but will tend toward a uniform wear condition with use.
A rigid clutch will more rapidly approach the uniform wear condition when use.
The uniform wear assumptions gives a more conservative clutch rating, because clutch capacities are lower than the uniform pressure assumption
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18. Other types of clutches
Cone clutches : another types of axial clutch, being able to transmit a larger torque than disc clutches with the same outside diameter and actuating force, because increased frictional area and the wedging action that takes place.
Cone clutches: low speed applications
Hydraulic or pneumatic clutch can minimize shock and vibration
Centrifugal clutches: engage automatically when the shaft speed exceeds a certain magnitude. Friction elements are thrown radially outward against the inside of a cylindrical drum to engage the clutch.
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