Belt

A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.

In a two pulley system, the belt can either drive the pulleys normally in one direction or the belt may be crossed, so that the direction of the driven shaft is reversed. As a source of motion, a conveyor belt is one application where the belt is adapted to carry a load continuously between two points. The belt drive can also be used to change the speed of rotation, either up or down, by using different sized pulleys.

Material used for belt : Leather, rubber, fabric, synthetic polymers

Properties of a belt material

The material from which belt is made of should have high coefficient of friction.
To withstand the tensions created, the belt material should have high tensile strength.
When belt passes through pulley bending stress is induced, to avoid this material should be flexible and should not be rigid.
The material should have water resistance.

Types of belts

1. Flat Belt

2. V-Belt

3. Round Belt

4. Timing Belt

1 Flat belt

Flat belts were widely used in the 19th and early 20th centuries in line shafting to transmit power in factories.They were also used in countless farming, mining, and logging applications, such as bucksaws, sawmills, threshers, silo blowers, conveyors for filling corn cribs or haylofts, balers, water pumps and electrical generators. Flat belts are still used today, although not nearly as much as in the line-shaft era. The flat belt is a simple system of power transmission that was well suited for its day. It can deliver high power at high speeds (373 kW at 51 m/s), in cases of wide belts and large pulleys. But these wide-belt-large-pulley drives are bulky, consuming much space while requiring high tension, leading to high loads, and are poorly suited to close-centers applications, so V-belts have mainly replaced flat belts for short-distance power transmission.

Advantages of flat belts

Flat belt can be used where the distance between pulleys are more.
They have high load carrying capacity.
They can be employed where high operating speed is required.
They produce less noise compared to v-belts.
They can absorb shock loads compared to v-belts.

Disadvantages of flat belts

Due to high load the belt may slip over the pulley.
In long run the belt gets elongated and may slip
High pulley diameter is required.

2. V-Belt

V belts solved the slippage and alignment problem. It is now the basic belt for power transmission. They provide the best combination of traction, speed of movement, load of the bearings, and long service life. They are generally endless, and their general cross-section shape is roughly trapezoidal (hence the name "V"). The "V" shape of the belt tracks in a mating groove in the pulley (or sheave), with the result that the belt cannot slip off. The belt also tends to wedge into the groove as the load increases the greater the load, the greater the wedging action improving torque transmission and making the V-belt an effective solution, needing less width and tension than flat belts.

Advantages and Disadvantages of V-belt Drive over Flat Belt Drive:-

Following are the advantages and disadvantages of the V-belt drive over flat belt drive :-

Advantages:-

1. The V-belt drive gives compactness due to the small distance between centers of pulleys.

2. The drive is positive, because the slip between the belt and the pulley groove is negligible.

3. Since the V-belts are made endless and there is no joint trouble, therefore the drive is smooth.

4. It provides longer life, 3 to 5 years.

5. It can be easily installed and removed.

6. The operation of the belt and pulley is quiet.

7. The belts have the ability to cushion the shock when machines are started.

8. The high velocity ratio (maximum 10) may be obtained.

9. The wedging action of the belt in the groove gives high value of limiting *ratio of tensions. Therefore the power transmitted by V-belts is more than flat belts for the same coefficient of friction, arc of contact and allowable tension in the belts.

10. The V-belt may be operated in either direction, with tight side of the belt at the top or bottom. The center line may be horizontal, vertical or inclined.

Disadvantages:-

1. The V-belt drive cannot be used with large center distances, because of larger weight per unit length.

2. The V-belts are not so durable as flat belts.

3. The construction of pulleys for V-belts is more complicated than pulleys of flat belts.

4. Since the V-belts are subjected to certain amount of creep, therefore these are not suitable for constant speed applications such as synchronous machines and timing devices.

5. The belt life is greatly influenced with temperature changes, improper belt tension and mismatching of belt lengths.

6. The centrifugal tension prevents the use of V-belts at speeds below 5 m/s and above 50 m/s.

3. Round belt

Round belts are a circular cross section belt designed to run in a pulley with a 60 degree V-groove. Round grooves are only suitable for idler pulleys that guide the belt, or when O-ring type belts are used. The V-groove transmits torque through a wedging action, thus increasing friction. Nevertheless, round belts are for use in relatively low torque situations only and may be purchased in various lengths or cut to length and joined, either by a staple, a metallic connector, gluing or welding. Early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great effect.

4.Timing belt

Timing belts also known as toothed, notch, cog, or synchronous belts are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tension, they have no slippage, run at constant speed, and are often used to transfer direct motion for indexing or timing purposes. They are often used instead of chains or gears, so there is less noise and a lubrication bath is not necessary. Camshafts of automobiles, miniature timing systems, and stepper motors often utilize these belts. Timing belts need the least tension of all belts and are among the most efficient. They can bear up to 200 hp at speeds of 16,000 ft/min (4,900 m/min).

Advantages of Timing belt

1. Precision registration and timing with no loss of high torque carrying capability.

2. Minimal vibration.

3. Positive slip proof engagement.

4. Wide speed range, especially important when the entire speed range is developed from a single source.

5. Virtually no elongation (stretching) due to wear.

6. High mechanical efficiency, as much as 98% when properly maintained. By contrast, chain drives are in the 91-98% efficiency range, while V-Belts average in the 93-98% range.

7. Power transmission efficiency is not lost with use.

8. Clean operation, no need for lubrication.

9. Reduced noise.

10. Long, dependable trouble-free service.

11. Excellent abrasion resistance.

12. Rust resistant.

13. Resists chemicals and contaminants.

14. Increased drive design options.

15. Weight savings.

16. Safety issues.

17. Economical operations.

Disadvantages of Timing belt

1. Relatively high purchase cost

2. Need for specially fabricated toothed pulleys.

3. Less protection from overloading, jamming, and vibration due to their continuous tension cords.

4. Lack of clutch action only possible with friction-drive belts, and the fixed lengths, which do not allow length adjustment unlike link V-belts or chains.