A clutch is a device for connecting two parts, such as shafts or a shaft and a pulley. The difference between a coupling and a clutch is that a coupling is used to connect two shafts permanently, while a clutch may ensure easy and quick connection and disconnection of two shafts. Clutches used in lathes are subdivided into several types, such as rigid couplings and disengaging clutches. A. rigid coupling serves for connecting coaxial shafts which are not disengaged in the. process of operation. Fig. 45 shows a rigid coupling, which consists of a solid bushing connecting electric motor and lathe shafts by means of a key. Disengaging clutches are applied in lathes for temporary engagement and disengagement of a shaft and parts connected with it. They are divided into friction clutches and yaw clutches. Friction clutches serve to connect а stationary machine part to transmit the required power. Sometimes friction clutches are intended as safety devices to prevent the breakage of parts in the transmission train.
Fig.45. Rigid Coupling:
1- electric motor shaft; 2- solid bushing; 3- lathe shaft
Fig. 46. Cone Clutch:
1 - toothed wheel; 2 - cone disc; 3 - movable wheel; 4 -eccentric clamp; 5 – spring
Friction clutches may be divided into two groups, according to the direction of the acting force: axial clutches and radial or rim clutches. In axial clutches the contact pressure is applied in a direction parallel to the axis of rotation, while in rim clutches the contact pressure is applied upon a rim in a radial direction. Axial clutches can be subdivided into cone clutches, and combined cone and disc clutches. Fig. 46 shows a cone clutch. By moving a movable wheel the cone disc connected with the toothed wheel by a key may engage the cone located in the movable wheel. Thus the cone disc is pressed against the inside cone of the movable disc, friction necessary for transmitting rotary movement to the movable wheel being created between the two cones. The outside cones are meshed by an eccentric clamp. By turning the eccentric clamp to 90° the friction clutch is disengaged, a spring pressing out the toothed wheel from the friction disc. Radial or rim clutches may be classified as band clutches, block clutches, and as external, internal, and combined internal and external clutches. Jaw clutches consist of two half-clutches – a. fixed one and a movable one which have jaws on their faces. The fixed half-clutch is rigidly fastened on one shaft, the movable one being keyed to another shaft. The shafts are connected through the coupling of jaws on both half-clutches.
The following factors are decisive in selecting the type of clutch to be used: torque, rotative speed, available space, and frequency of operation. When a heavy torque should be transmitted a clutch must have sufficient gripping power, which is usually ensured by multi-disc clutches. For low-speed service cone and rim clutches are used. For high rotative speeds light, compact and internally balanced clutches of the multi-disc type may be applied. Space being limited, multi-disc, twin-cone and double-cone clutches are used because of their greater compactness in comparison with other types of clutches. Single-disc clutches with metal contact surfaces and cone clutches are the most suitable ones for frequent or continuous operation.
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