A cone clutch, as shown in fig, was extensively used in automobiles, but now-a-days it has been replaced completely by the disc clutch.

It consists of one pair of friction surface only. In a cone clutch, the driver is keyed to the driving shaft by a sunk key and has an inside conical surface or face which exactly fits into the outside conical surface of the driven. The driven member resting on the feather key in the driven shaft, may be shifted along the shaft by a forked lever provided at B, in order to engage the clutch by bringing the two conical surfaces in contact. Due to the frictional resistance set up at this contact surface, the torque is transmitted from one shaft to another.

In some cases, a spring is placed around the driven shaft in contact with the hub of the driven. This spring holds the clutch faces in contact and maintains the pressure between them, and the forked lever is used only for disengagement of the clutch.

The contact surfaces of the clutch may be metal to metal contact, but more often the driven member is lined with some material like wood, leather, cork or asbestos etc.

The material of the clutch faces (i.e. contact surfaces) depends upon the allowable normal pressure and the coefficient of friction.

There are two operating conditions application to cone clutch plates

• When there is a uniform pressure.
• When there is a uniform wear.

Torque transmission under uniform Wear

This theory is based on the fact that wear is uniformly distributed over the entire surface area of friction disk. This assumption can be used for worn out clutches/old clutches. The axial wear of the friction disk is proportional to frictional work. The work done by the friction is proportional to the frictional force ($\mu p$) and the rubbing velocity ($2\pi rn$ ) where n is the speed of the disk in revolution per minute. When the speed n and the coefficient of friction m are constant for a given configuration, then $$Wear \ \alpha \ pr$$

According to this assumption,

$$pr \ = \ Constant .......$$

When the clutch plate is new and rigid, the wear at the outer radius will be more, which will reduce pressure at the outer edge due to rigid pressure plate. This will change pressure distribution. During running condition, the pressure distribution is adjusted such that the product (pr) is constant. Therefore,

$$p.r = p_a.r...............................$$

Where pa is the pressure at the inner edge of plate, which is also the maximum pressure.

$$\begin{aligned} F &=2 \pi P_{a} \cdot r(R-r) \\ \therefore M_{t} &=\pi \cdot \mu \cdot P_{a} \cdot r \cdot\left(R^{2}-r^{2}\right) \end{aligned}$$

Substituting, $\pi, P_{a}, r$ in above eqution.

The above equation gives the torque transmitting capacity for a single pair of contacting surfaces. The uniform-pressure theory is applicable only when the friction lining is new. When the friction lining is used over a period of time, wear occurs. Therefore, the major portion of the life of friction lining comes under uniform-wear criterion. Hence, in the design of clutches, the uniform wear theory is used.

It is clear that the torque transmitting capacity can be increased by three methods:

(a) Using the friction material with a higher coefficient of friction $(\mu) ;$

(b) Increasing the intensity of pressure $(p)$ between disks; and

(c) Increasing the mean radius of friction disk $(R+r) / 2 .$