9.0 : CLUTCHES

9.6 SINGLE FRICTION DISK CLUTCH

Single friction disk clutch consists of two disks or flanges or plates (shown in Fig. 9.6.1). One of the disks (friction disk) is lined with friction material. It is also called as ‘Single plate friction clutch’.

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Animation 9.6.1 Operation of single friction disk clutch

Fig. 9.6.1 Single friction disk clutch

One disk ‘driving disk’ is fastened to the driving shaft. The driven disk is free to move along the driven shaft due to splined connection. Both the shafts are coaxial. During disengagement of the clutch, a contact lever keeps the driven disk away from the driving disk. To engage the clutch, the contact lever is gradually released. Then a spring provides an actuating force to the driven disk forcing it to move towards the driving disk and finally makes contact with it. The driven disk starts rotating at low speed due to the friction between the disks. When the contact lever is fully released, the spring provides the required axial force to press the driven disk against the driver disk, the friction force between them increases, and the driven disk attains the speed of the driver disk. Torque is transmitted by means of frictional force between these plates.

The friction clutches are classified as two-plane disks or multiple-lane disks depending upon the number of friction surfaces. Based on the shape of the friction lining, they are classified as disk clutches, cone clutches or expanding shoe clutches. Friction clutches permit smooth engagement at any speed. In the event of over loads, the friction clutches slip momentarily, safeguarding the machine or mechanism against breakage.

Torque transmission capacity of single friction disk clutch

A two-plane disk friction clutch is shown in Fig. 9.6.2.

Fig. 9.6.2 Notations of single disk friction clutch

F = total actuating force (axial force) (N)
Mt = Torque transmitted by friction (N-mm)
m = Coefficient of friction between friction disks
Consider an elemental ring of radius, r and radial thickness dr. For this ring, the cross-sectional area of the element,

Torque transmission capacity of old and new disk clutches

There are two criteria to obtain the torque transmission capacity of friction clutches, viz., uniform pressure and uniform wear.

Torque transmission under uniform pressure

This theory is applicable to new clutches. In new clutches employing a number of springs, the pressure can be assumed as uniformly distributed over the entire surface area of the friction disk. With this assumption, the intensity of pressure between disks, p is regarded as constant. From Eq. (9.1) and (9.2)

The above equation is valid for a single pair of mating disk surfaces.

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 (μp) and the rubbing velocity (2πrn ) 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 α pr ..............................................................................................(9.6)

According to this assumption,
pr = Constant .......................................................................................(9.7)
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 = pa.r ............................... ...............................................................(9.8)
Where pa is the pressure at the inner edge of plate, which is also the maximum pressure. From equation (9.1) and (9.2)

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.

From Eq. (9.11), 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 (m);
(b) Increasing the intensity of pressure (p) between disks; and
(c) Increasing the mean radius of friction disk (R + r)/2.

Delayed starting of drafting in ring spinning

The drafting rollers are driven through gear trains. When the ring spinning machine is started, the spindles lag behind the drafting rollers due to slackness of tape. The tape takes time to build up sufficient tension ratio around the spindle wharves to turn the spindle. Meanwhile, the front roller has started delivering the fibre strand. This creates slackness on the yarn that leads to yarn breaks. To avoid this, the starting of drafting rollers must be delayed synchronizing with the start of spindles. A gear (A) mounted on the main shaft drives a gear (B) that is loosely mounted on an intermediate shaft ( Fig. 9.6.3). A friction disk can be moved along the intermediate shaft by means of piston actuated by compressed air in a cylinder. When the machine is started, this disk is kept away from the gear, B (clutch is disengaged); the gear B revolves freely without transmitting motion to the intermediate shaft, the drive to the front drafting roller is disconnected. After a pre-set time delay, the clutch is engaged, the gear B and the intermediate shaft revolve together, driving the drafting rollers.

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Animation 9.6.3 Delayed starting of drafting rollers

Fig. 9.6.3 Disk clutch for delayed starting of drafting on ring spinning machine

 
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