POWER TRANSMISSION SYSTEMS

Mechanical power can be transmitted from one shaft to another by using four methods Belt drive Rope drive Chain drive Gear drive The shaft from which power is transmitted is called driver shaft and the shaft to which power is transmitted is called driven shaft

BELT DRIVE A belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the same direction, or the belt may be crossed, so that the direction of the shafts is opposite.

TYPES OF BELTS

OPEN BELT DRIVE

It is used with shafts arranged in parallel and to be rotated in same direction. The driver pulley pulls the belt from one side and delivers the same to the other side. Side where tension is more is called tight side and the other side is called slack side

CROSS BELT DRIVE

It used with shafts arranged in parallel and to be rotated in opposite direction. At the point where belts crosses, it rubs against itself and wears. Shafts are placed at a minimum distance of 20b to minimize the wear.

Length of belt

Let tension on the tight side be ‘T1’ and the tension on the slack side be ‘T2’. Let‘θ’ be the angle of lap and let ‘μ’ be the coefficient of friction between the belt and the pulley. Consider an infinitesimal length of the belt PQ which subtend an angle δθ at the centre of the pulley. Let ‘R’ be the reaction between the element and the pulley. Let ‘T’ be tension on the slack side of the element, i.e. at point P and let ‘(T + δT)’ be the tension on the tight side of the element. The tensions T and (T + δT) shall be acting tangential to the pulley and thereby normal to the radii OP and OQ. The friction force shall be equal to ‘μ R’ and its action will be to prevent slipping of the belt. The friction force will act tangentially to the pulley at the point S.

CHAIN DRIVE It consists of an endless chain running over toothed wheel. Toothed wheel is called sprocket.

The smaller sprocket is called pinion and the bigger one is called wheel.

CLASSIFICATION OF CHAINS Based on application we can classify chains as Power transmission chains Pulling chains Used in elevators Hoisting chains Used in lifting loads

TYPES OF POWER TRANSMITTING CHAINS ROLLER CHAIN SILENT OR INVERTED TOOTH CHAIN

1 Outer plate 2 Inner plate 3 Pin 4 Bushing 5 Roller

ROLLER CHAIN It consists of rollers, bushes, pins, inner plates and outer plates. The pin passes centrally through the bush and the roller surrounds the bush. The bush moves freely on the roller and the roller moves freely on the bush.

Bushes are held together by outer plates. Rollers are held together by inner plates. To prevent the lateral movement of the outer plates the pin ends are hammered in the shape of a rivet head.

Inverted tooth chain It is employed when heavier loads are to be transmitted and maximum quietness is desired

GEAR DRIVE

Gear is a term used to generally denote toothed wheels. For transmitting power one gear will be mounted on driving shaft and the other will be mounted on driven shaft. The teeth's of the gears will be meshed with each other The distance between the shafts must be sufficient to enable meshing of the gear tooth.

Gear materials For heavy duty – cast steel and alloy steel For corrosive environment – Non ferrous materials (phosphor bronze, nickel, manganese) The cheapest material – Grey iron

SPUR GEAR In this the teeth will be cut parallel to the shaft Used to transmit power between parallel shafts