Clutches

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Rini Dharmastiti Jurusan Teknik Mesin FT UGM Jl. Grafika no.2 Yogyakarta [email protected]

Machine Elements II - Rini D - TM FT UGM

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

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 Machine Elements II - Rini D - TM FT UGM

Machine Elements II - Rini D - TM FT UGM

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 Machine Elements II - Rini D - TM FT UGM

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. Machine Elements II - Rini D - TM FT UGM

Positive contact clutches

The force acting on the jaw depends upon the horsepower and speed that the clutch is to transmit.

T



hp

.63000

n

T = torque (in-lb), hp = horse power to be transmitted, n = revolutions per minute Machine Elements II - Rini D - TM FT UGM

Machine Elements II - Rini D - TM FT UGM

Positive contact clutches



The average force, assumed to be acting at the center of each jaw:

F



o T



r i

) / 2

F = average force (lb), k= number of jaws on one member, r

o

and r

i

= outside and inside jaw radius (inch)

Machine Elements II - Rini D - TM FT UGM

Positive contact clutches

The shearing stress (lb/inch 2 ), acting on one jaw is:   2  (

r o



T r i

) 2  .

t

.

360 t = thinkness of jaw (inch),  one jaw in degrees, 2  (r o = angle subtended by + r i )/2 represents the average circumference of the jaws.

Machine Elements II - Rini D - TM FT UGM

Positive contact clutches

The bearing stress (lb/inch

2

), can be found from:



b



F

t = thinkness of jaw (inch), b = length of jaw

Machine Elements II - Rini D - TM FT UGM

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) Machine Elements II - Rini D - TM FT UGM

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 Machine Elements II - Rini D - TM FT UGM

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 Machine Elements II - Rini D - TM FT UGM

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 Machine Elements II - Rini D - TM FT UGM

Clutch and brake material properties

Material Metal on metal Wood on metal Leather on metal Molded blocks Asbestos on metal in oil Sintered metal on cast iron in oil Maximum Temperature, ºF Coefficient of friction, f Maximum allowable pressure, psi 500 - 600 200 150 – 200 500 – 600 500 450 0.25

0.2 – 0.3

0.3 – 0.4

0.25 – 0.5

0.35 – 0.45

0.2

200 – 250 50 – 90 15 – 40 100 – 150 50 – 150 400 Machine Elements II - Rini D - TM FT UGM

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 Machine Elements II - Rini D - TM FT UGM

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. Machine Elements II - Rini D - TM FT UGM

Machine Elements II - Rini D - TM FT UGM

Machine Elements II - Rini D - TM FT UGM

Machine Elements II - Rini D - TM FT UGM