Belt and chain drives

The belt drive is the most widely used kind of driving on metal-cutting lathes. The belt drive in lathes is used mainly for transmitting rotation of an electromotor from one pulley to another owing to the tension of the belt. For instance, in some lathes drive to the headstock is provided by belts from a motor, the mounting plate of which is allowed to pivot freely so that the belts are tensioned by the weight of the motor.

Fig. 43. Belt Drive

Pi, P₃ - belt tension

The belt-and-pulley system of transmitting rotary motion is subject to belt slippage and is therefore not positive. Belts are still incorporated in the drive, because of the safety factor inherent in a friction drive and because of the silence, as compared with gearing run at high speeds. Fig. 43 shows a belt drive consisting of two | pulleys A and B, mounted on two parallel shafts which are connected by an endless belt. The pulley A which transmits rotation is called the "driving pulley"* while the pulley В receiving rotation from the pulley A and transmitting it to its shaft is known as "driven pulley."** The friction surface of a driving pulley should be of a

comparatively soft material, while that of the driven pulley is usually made of hard material. Such an arrangement ensures the maintenance of the friction surfaces. Otherwise, if the driven pulley were made of the softer material, its surface would be injured and ruined.

There are two main kinds of belts such as flat belts and V-belts. Flat belts are made of leather, cotton yarn or rubberized cloth. V-belts manufactured of rubberized cloth are of a trapezoidal shape. They are pulled over the pulleys into special grooves, which considerably reduces their slipping off the pulleys during operation. Therefore this kind of belt finds ever increasing application on metal-cutting tools and is tending to replace flat belts. When a chain is applied there is no rolling contact as with gears. It transmits its tension uniformly to every tooth of the chain gear coming into contact with it. In this case there is no one-tooth action as in gears, and no losses due to rolling friction and the shock of transferring the load from the tooth. The link-belt silent chain can be, and should be, run slack. Belts require a tension in excess of the effective working tension, and to that extent there is a loss of power on the pulley of the shaft. Belts run on¹ by inertia and slip and lose power in that way and the less the belt tension is the more it slips, but the link-belt silent chain never slips.

¹. Belts run on - ремни набегают

* American - driver

** American – follower

Дата добавления: 2014-12-27