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Design of mechanical machines and machines
Typology: Schemes and Mind Maps
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A clutch is a machine member used to connect a driving shaft to a driven shaft so that the driven shaft may be started or stopped at will, without stopping the driving shaft. The use of a clutch is mostly found in automobiles. A little consideration will show that in order to change gears or to stop the vehicle, it is required that the driven shaft should stop, but the engine should continue to run. It is, therefore, necessary that the driven shaft should be disengaged from the driving shaft. The engagement and disengagement of the shafts is obtained by means of a clutch which is operated by a lever.
Positive Clutch In this type of clutch, the engaging clutch surfaces interlock to produce rigid joint they are suitable for situations requiring simple & rapid disconnection, although they must be connected while shaft are stationary & unloaded. The jaw may be square jaw type or spiral jaw type. They are designed empirically by considering compressive strength of the material used.
a. Simple design , b. No heat generated, C. No slip , d. compact, e. low cost.
a. One pair of friction surface only. b. The tendency to grab.
Friction Clutches work on the basis of the frictional forces developed between the two or more surfaces in contact. Friction clutches are usually over the jaw clutches due to their better performance. There is a slip in friction clutch.
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. Fig1.4. Cone clutch.
α = Semi-angle of the cone (also called face angle of the cone) or angle of the friction surface with the axis of the clutch,
b = Width of the friction surfaces (also known as face width or cone face). Consider a small ring of radius r and thickness dr as shown in Fig. 24.7. Let dl is the length of ring of the friction surface, such that, dl = dr cosec α ∴ Area of ring = 2π r. dl = 2π r.dr cosec α We shall now consider the following two cases :
1. When there is a uniform pressure, and 2. When there is a uniform wear. 1. Considering uniform pressure We know that the normal force acting on the ring, δWn = Normal pressure × Area of ring = Pn × 2π r.dr cosec α and the axial force acting on the ring, δW = Horizontal component of δWn (i.e. in the direction of W) = δWn × sin α = pn × 2π r.dr cosec α × sin α = 2π × pn.r.dr ∴ Total axial load transmitted to the clutch or the axial spring force required,
Fif1.8. Friction surfaces as a frustrum of a
Conclusion The cone clutch, Figure 1.9, can be considered as the general case of a disk clutch having a cone angle of 90°. Due to the increased frictional area and the wedging action of the parts, cone clutches convey a larger torque than disk clutches with the identical outside diameters and actuating forces. Practically, a cone clutch can have no more than one friction interface: hence, N = 1. Cone clutches are often used in low-speed applications. They could also be employed as cone brakes with some slight modifications. Figure 1.9. Cone clutch.
