Power on the farm is generally transmitted by three methods: 1. Belt Drive 2. Chain and Sprocket Drive 3. Gear Drive.
Method # 1. Belt Drive:
Belt is a flexible material, placed around two pulleys, having a certain amount of tension to transmit the power from one pulley to another. When two parallel shafts rotate in same direction or opposite direction or opposite direction at some distance, belt drive is recommended. The grip between at some distance, belts are obtained due to friction between the two surfaces.
Belts used in agricultural machines are usually of following two types:
i. Flat Belt
ii. V-Belt
i. Flat Belt:
(a) Leather,
(b) Rubber,
(c) Canvas, and
(d) Woven Cotton.
Leather belt is very durable and reliable for general works. For prolonged life, leather belt should be kept dry otherwise moisture may damage the glued joints. It should be free from grease dirt, oil and should not be subjected to intense heat.
It should be cleaned occasionally by washing with soap and oil and be allowed to dry. The use of mineral oil is injurious for this belt. To prevent slippage of leather belts, commercially available adhesives may be used to improve grip between the belt and the pulley.
Rubber belt is usually cheaper than the leather belt. Its thickness is uniform and can withstand high heat also. It has the disadvantage that it stretches itself during process of operation. It should be free from oil and grease. The foundation of rubber belting consists of cotton ducking specially treated with a rubber compound. In case it gets covered with dirt, it may be washed with soap and water.
Canvas belt is made up of layers of canvas, stitched together through the centre as well as along the edge. It is very conveniently used on portable machines. It has got the disadvantage that it stretches itself while in operation. Crude oil makes a good dressing and keeps the belt soft and pliable.
Woven cotton belt is a cheap kind of belt. It also stretches much while in operation. It is preferred on portable machines. It is treated with special material to withstand friction, dirt and atmospheric conditions.
Flat belts are available in various widths, ranging from 2.5 to 50 cm. Drives may be open, crossed or quarter turn. Quarter turn drives run only in one direction. Small pieces of belts are joined together to make a long belt. Belt pieces are cross belt joined either by fastening, stitching or cementing.
ii. V-Belt:
V-belts are so named because of their shape. They run over pulley, having V shaped groove, less slippage is encountered with V-belt as compared with flat belt. V-belt drive is used where less space is available and the transmission distance is short. Usually V-belts are made of cords of nylon or other material embedded in rubber, the whole being encased in fabric, often cotton duck impregnated with rubber.
The belt section has a usual angle of 40°. The groove angle varies from 32° in small pulleys to 40° in larger pulleys. V-belts have trapezoidal cross section. V-belt friction between the sides of the belt and the sides of the sheave grooves.
The contact area per unit belt width is greater than that of flat belts.
V-belt is very popular in agricultural machinery because of the following features:
(a) Speed variation is obtained easily
(b) Direction of shaft rotation can be reversed by crossing the belt
(c) No lubrication is required
(d) Alignment is not difficult
(e) It is smooth running
(f) A single belt can be used to drive several units on a machine by passing over several sheaves and
(g) It works as a safety clutch due to slippage on overload.
Belt Pulley:
Belt pulley is the essential part of belt drive unit. The system consists of two pulleys, one is driving pulley and the other is the driven pulley. For flat belts, flat faced cylindrical pulleys are selected and V-belts run on V-grooved pulleys. Pulleys are made of cast iron, wood or steel. Cast iron pulleys are very popular due to their greater strength and freedom from joints and are suited to all class of works. They are fitted on shafts either with set screws, clamps or keys.
Wooden pulleys are usually of the split type to facilitate placing on the shaft. They are fitted on the shafts by means of keys or clamps. Pressed steel pulleys are also used. They are lighter and stronger than cast iron pulleys but they are more expensive. In some cases, paper pulleys are also used. These are made of several layers of suitable paper elements, pressed on an iron hub, with iron plates. Paper pulleys are generally made in small sizes and are used on electric motors, generators etc.
Speed Cones:
Speed cones are pulleys, having several steps of different diameters. The belts move on these steps. These steps have got different diameters to have different speed reductions according to the requirement.
Tightening of Belt:
There are three different devices for adjusting tension of the belts:
(a) Idler Pulley,
(b) Slide Rail, and
(c) Tilting Plate.
(a) Idler Pulley:
Idler pulley increases the angle of contact on the pulley and maintains the desired tension as the belt stretches. Centre to centre distance between the pulleys are not altered at all.
(b) Slide Rail:
In this case, the pulley along with the power unit is shifted over the slide rails which are fixed permanently on a suitable foundation. Two big screws, provided on the slide rail are used for shifting purpose.
(c) Tilting Plate:
Uniform tension is obtained in the belt by fixing the power unit on a suitably pivoted support.
Velocity Ratio:
The belt transmits power by virtue of difference in tension in the belt between the points at which it enters and leaves the sheave. The difference in tension is developed through friction.
If two pulleys (one driving pulley and other driven pulley) are connected by a belt so that rotation of one causes rotation of the other, then
π x N1 x D1 = π x N2 x D2 (if there is no slippage)
Or, N1/N2 = D2/D1 = Velocity ratio
Where,
N1 = number of revolutions of driving pulley
N2 = number of revolutions of driven pulley
D1 = diameter of the driving pulley
D2 = diameter of the driven pulley
Power Transmitted by Belt:
If a pulley A drives a pulley B by means of a belt, it is necessary that the belt should be stretched tightly over the pulleys and thus, provides an initial tension say T.
When rotation commences, the tension on the tight side would be increased to T1 and that on the slack side will be reduced to T2 kg, thus giving an effective belt pull of (T1 – T2) kg. If the speed of the belt is V metre/min,
Relation between Tensions:
The ratio between the belt tensions on the tight and slack side is given by:
T1/T2 = eµƟ
Where,
µ = coefficient of friction
Ɵ = angle of lap of the belt over the pulley in radians and
e = base of Napierian log = 2.728
Length of Belt:
Where,
D1 = diameter of the driving pulley
D2 = diameter of the driven pulley and
C = distance between the centers of two pulleys
Method # 2. Chain and Sprocket Drive:
Chain and sprocket drive is used where two parallel shafts are spaced apart at short distance. A chain drive system needs an endless chain and a sprocket. It is a positive drive and is very reliable method.
The chains are of three types:
i. Roller Chain,
ii. Hook Link Chain, and
iii. Pintle Chain.
All these chains move over a toothed wheel called Sprocket.
i. Roller Chain:
It is usually for higher speeds and higher loads. It is made of high grade steel. The components are hardened and polished.
ii. Hook Link Chain:
It is used for external drive such as potato digger, manure spreader etc. It is usually made of malleable iron. It is preferred where power requirement is low and speed is also low.
iii. Pintle Chain:
Pintle chain is used for heavy duty, slow speed work in any exposed atmosphere. It is made of malleable links, held together by suitable pins.
Method # 3. Gear Drive:
Gears are used to transmit power at uniform angular velocity from one shaft to another. When two shafts are close to each other in a compact form, gear drive is recommended.
It is used with different aims such as:
i. To change the speed of rotation,
ii. To change the direction of rotation, and
iii. To change the direction of shafting.
The common gears used for different jobs are:
i. Spur Gear
ii. Worm Gear
iii. Bevel Gear
iv. Helical Gear and
v. Spiral Gear
i. Spur Gear:
Spur gears are used for transmitting power from one shaft to another shaft when two shafts are parallel to each other. It has the teeth, cut parallel to the axis of the shaft. The internal spur gear has the teeth on the inside surface of the rim and the external spur gear has teeth on the outside surface of the rim.
ii. Worm Gear:
Worm gear is used when two shafts are at right angles but not intersecting to each other. The worm is a form of screw thread, cut on a shaft. It meshes with the worm wheel. It is used when high velocity ratio is required in a limited space, such as cream separator, hand chaff cutter etc.
iii. Bevel Gear:
It is used where the axis of the two shafts are at right angles and can intersect each other. When the power has to turn a corner, bevel gears are used. Plain bevel gears have straight teeth while spiral bevel gears have twisted teeth.
iv. Helical Gear:
It is used for transmitting power between parallel shafts. The teeth are inclined to the axis of the wheel which helps in meshing more than two teeth at a time and remain in contact longer than straight teeth.
v. Spiral Gear:
When two shafts are placed at an angle, spiral gear is used. It also connects shafts, placed in different planes.