Plate Clutch
Axial or Plate Clutch
The Disk Clutch is used to transmit torque from and input to and output shaft via circular plates (disks). One of each pair of plates is coated with a friction lining material. The force applied to the friction element may by manual, electrical, pneumatic, hydraulic or inertial via a mechanical linkage. As the input and output shafts are fixed the clutch should include a method of allowing the driving plates to move axially against the driven plates which are fixed to the driven shaft (or vice versa).
A selection of disk clutch plate materials with their important properties
Desirable Properties for friction materials/linings for clutches
• The two materials in contact must have a high coefficient of friction.
• The materials in contact must resist wear effects, such as scoring, galling, and ablation.
• The friction value should be constant over a range of temperatures and pressures
• The materials should be resistant to the environment (moisture, dust, pressure)
• The materials should possess good thermal properties, high heat capacity, good thermal conductivity, withstands high temperatures
• Able to withstand high contact pressures
• Good shear strength to transferred friction forces to structure
• Should be safe to use and acceptable for the environment
Note: Brake materials from asbestos are not safe and are no longer acceptable for use in brakes and clutches
Important properties of materials used for brakes and clutches
Note: These properties are very general in nature and should not be used for detail design. Suppliers information should be used for important work. Asbestos should not be used for new or replacement equipment.
|
Material Combination |
Coefficient of Friction |
Temp.(max) |
Pressure (Max) |
|
|
Wet |
Dry |
Deg.C |
MPa |
|
|
Cast Iron/Cast Iron |
0,05 |
0,15-0,20 |
300 |
0,8 |
|
Cast Iron/Steel |
0,06 |
0,15-0,20 |
300 |
0,8-1,3 |
|
Hard Steel/Hard Steel |
0,05 |
0,15-0,20 |
300 |
0,7 |
|
Wood/Cast Iron-steel |
0,16 |
0,2-0,35 |
150 |
0,6 |
|
Leather/Cast Iron-steel |
0,12-0,15 |
0,3-0,5 |
100 |
0,25 |
|
Cork/Cast Iron- Steel |
0,15-0,25 |
0,3-0,5 |
100 |
0,1 |
|
Felt/Cast Iron- Steel |
0,18 |
0,22 |
140 |
0,06 |
|
Woven Asbestos/Cast Iron- Steel |
0,1-0,2 |
0,3-0,6 |
250 |
0,7 |
|
Moulded Asbestos/Cast Iron- Steel |
0,08-0,12 |
0,2-0,5 |
250 |
1,0 |
|
Impregnated Asbestos/Cast Iron- Steel |
0,12 |
0,32 |
350 |
1.0 |
|
Carbon-graphite/Cast Iron- Steel |
0,05-0,1 |
0,25 |
500 |
2.1 |
|
Kelvar/Cast Iron- Steel |
0,05-0,1 |
0,35 |
325 |
3,0 |
F = Applied Force (N)
P = Clutch transmitted Power kW
M = Torque (Nm)
F = Actuating Force (N)
μ = Coefficient of Friction.
r = Radius of clutch ring thickness dr (m)
r i, r o = Inner, Outer radius of clutch plate. (m)
n = Rotational Speed (RPM)
p = Pressure for friction surface(N/m2)
p max = Maximum pressure for friction surface(N/m2)
Theory
There are two operating conditions applicable to clutch plates
• Uniform wear.. Applicable for practical clutch assemblies after period of operation
• Uniform pressure.. Applicable for new clutch plates.
The plate clutch is used in automotive and industrial service to connect and disconnect the transmission of rotation / torque / power.
To determine the torque transmitted it is necessary to make an assumption about the pressure distribution over the friction surfaces. For perfectly aligned new surfaces, it could be assumed that the pressure is uniformly distributed over the entire surface. However once the system has had some use, a better assumption is that the rate of wear is uniform over the friction surfaces. As a first approximation it can be assumed that the wear rate is proportional to the product of the velocity of sliding and the pressure. Since the sliding velocity is proportional to the radius r to the annular element dr, the following can be written:
wear = k p r, since the wear is constant for the entire face, the maximum pressure will occur at the inner radius, ri hence wear = k pmax ri
Eliminating wear and the constant, k, gives the pressure at any radius, p
p = pmax ri/r
The total force Fn which must be exerted by the actuating spring, is found by multiplying the element area 2 x 3.142 x r x dr, by the pressure and integrating over the surface. This gives
Fn = 2 x 3.142 x pmax ri(ro - ri)
The torque is found by multiplying the force on the element by the coefficient of friction, f, and the radius, and integrating over the area. This gives:
T = 3.142 x f x pmax x ri ( ro2 - ri2) = 0.5 f (ro + ri)Fn
Single plate clutches have lining on both sides of the plate. Multiple plate disc clutches have friction linings on both sides of alternate plates. The above gives the torque for a single face, thus this quantity must be multiplied by the number of friction faces to find the torque for the entire clutch.
Uniform wear
The wear (W)at any location on a cluthch is assumed to be proportional to the pressure intensity (p)and the associated relative velocity (v)of the local ring of contact.
The torque capacity of a clutch plate is the integral of the friction force (μ F) x Radius (r)
Uniform pressure
When considering the capacity of a disk clutch plate subject uniform pressure, every point on the clutch plate face is subject to the the maximum design pressure for the friction material. This condition applies mainly to new clutch plates
The torque capacity of a clutch plate is the integral of the friction force (μ F) x Radius (r)
Multiple Plate
The above theory relates to single plate clutches. For multiple plates in contact the torque values should be multiplied by the number of contacting plates
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