What is belt drive?
A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. 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 (the same if on parallel shafts), or the belt may be crossed, so that the direction of the driven shaft is reversed (the opposite direction to the driver if on parallel shafts).
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.
The amount of power transmitted depends on the following factors:
- The velocity of the belt.
- The tension under which the belt is applied to the pulleys.
- Arc of contact between the belt and small pulley.
- Conditions under which belts are used.
Types of Belt Drive
There are Seven different types of belt drive that can be found and those are:
- Open belt drive
- Closed or crossed belt drive
- Fast and loose cone pulley
- Stepped cone pulley
- Jockey pulley drive
- Quarter turn belt drive
- Compound belt drive
1. Open Belt Drive
The open belt drive is used with shafts arranged parallel and rotating in the same direction. In that case, the driver pulls the belt from one side and delivers it to the other side.
Thus, the tension in the lower side belt will be higher than the upper side belt. The lower side belt is known as the tight side belt while the upper side belt is known as the slack side belt.
When the shafts are too far apart, the bottom of the belt should be the tight side and the upper side should be the slack side. This is so, when the upper side becomes the dull side, it will relax due to its own weight and thus increase the arc of contact.
2. Closed or Crossed Belt Drive
Cross or twisted belt drives are used with rotating shafts in parallel and opposite directions. In this case, the driver pulls the belt from one side and delivers it to the other side.
Thus, the tension at the Bottom Side of the belt will be higher than at the upper side of the belt. The belt is known as the tight side due to high tension, while the belt due to low tension is known as the slack side.
This type of belt drive employs a belt when two parallel shafts have to rotate in the opposite direction. At the junction where the belt crosses, it rubs against itself and wears off. To avoid excessive wear, the shafts should be kept at maximum distance from each other and operated at very low speeds.
A little consideration will show that at a point where the belt crosses, it rubs against each other and there will be excessive wear and tear. To avoid this, the shaft must be placed at a maximum distance of 20 B, where B is the width of the belt and the belt speed must be less than 15 m/s.
3. Fast and loose cone pulley
This type of belt drives is used when the driven or machine shaft is to be started or stopped whenever desired without interfering with the driving shaft. A pulley which is keyed to the machine shaft is called a fast pulley and run at the same speed as that of the machine shaft.
A loose pulley runs freely over the machine shaft and is incapable of transmitting any power. When the driven shaft is required to be stopped, the belt is pushed on to the loose pulley by means of a sliding bar having belt forks.
4. Stepped cone pulley drive
A step or cone pulley drive is used to change the speed of the driven shaft, while the main or driving shaft moves at a constant speed. This is accomplished by shifting the belt from one part of the steps to another.
A step cone pulley is an integral casting consisting of three or different numbers of different sizes of pulleys adjacent to each other, as shown in fig. A set of driven cone pulleys is placed upside down on the drive shaft. An endless belt will be wrapped around a pair of pulleys.
By shifting the belt from one pair of pulleys to another, the speed of the driven shaft can be varied. The diameter of the pulleys driving and operated is such that the same belt will operate when transferred over pairs of different pulleys.
5. Jockey pulley drive.
In an open belt drive arrangement, if the center distance is small, or if the driven pulley is too small, the arc of the belt’s contact with the driven pulley will be very small, which reduces the tension in the belt, or if the belt requires Stress cannot be achieved by other methods, a sluggish pulley, called a jockey pulley, is placed on the slack side of the belt as shown in fig.
This increases the arc of contact and thus stress resulting in an increase in power transmission.
6. Quarter turn belt drive
Quarter turn belt drives are also known as right-angle belt drives. It is used with shafts arranged at right angles and rotating in a certain direction. To prevent the belt from leaving the pulley, the width of the face of the pulley must be greater than or equal to 1.4 b, where b is the width of the belt. If the pulley cannot be arranged or when reversible speed is desired, a quarter-turn belt drive with a guide pulley may be used.
7. Compound belt drive
A compound belt drive is used when power is passed from one shaft to another through multiple shafts.
Factors to consider before using belt drive:
- The shaft must be properly lined to ensure uniform tension in the belt section.
- The pulley should not be too close together so that the arc of contact on the small pulley is as large as possible.
- The pulley must not be far enough to cause heavyweight on the belt shaft, increasing the friction load on the bearings.
- A long belt rotates from side to side, causing the belt to exit the pulley, causing crooked spots in the belt.
- The tight side of the belt should be at the bottom so that whatever sag is present on the loose side will increase the arc of contact on the pulley.
- To achieve good results with flat belts, the maximum distance between the shafts should not exceed 10 m and the minimum should not be less than 3.5 times the diameter of the larger pulleys.
Classification of Belt Drive
The belt drives are usually classified into the following three groups:
- Light drives.
- Medium drives.
- Heavy drives.
Light drives: They are used as agricultural machines and small machine tools to transmit small forces at belt speeds to about 10 m/s.
Medium drive: They are used to transmit medium forces at belt speeds of more than 10 m/s but up to 22 m/s like machine tools.
Heavy drives: They are used in compressors and generators to transmit large forces at belt speeds above 22 m/s.
Types of Belt
There are four different types of belt:
- Round belts. Round belts are generally made of rubber.
- V belts. V belts are arguably the most widely used belts in the industry.
- Flat belts. Flat belts are also used to transmit power from one shaft to another.
- Timing/toothed belts.
There are three basic types of power transmission belting: flat belt, V belt, and synchronous belt. Misalignment is a common cause of premature belt failure. Power transmission belting has been used for more than 200 years. The first belts were flat and ran on flat pulleys.
1. Flat Belt
Flat belts are also used to transfer power from one shaft to another. The flat belt transmits power using the friction output between the belt and the pulley. In a flat belt, the pulleys rotate in the same direction.
They are usually classified as small woven endless belts or high strength flat belts. Woven endless belts are particularly useful, where minimal vibration is required on the driven pulley due to the semi-elastic material used in construction.
High-strength flat belts are often useful because they eliminate the need for high belt tension used to hold the pulleys, which in turn reduces the load on the shaft bearings.
The material used for high strength flat belts is a viscous resistant abrasive compound.
2. V belt
V-belts are mostly used in factories and workshops, where a large amount of power has to be transmitted, from one pulley to another, when two pulleys are very close to each other.
The V belt solved the slippage and alignment problem. It is now the original belt for power transmission. They provide the best combination of traction, movement speed, a load of bearings, and longer service life. They are usually endless, and the size of their normal cross-section is broadly trapezoidal.
3. Circular belt
The round belt is 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.
V-grooves transmit torque through palpation, which increases friction. Nevertheless, round belts are only for use in relatively low torque situations and can be purchased at various lengths or cut to length and can be connected either by a staple, a metal connector, gluing, or welding. Early sewing machines used a leather belt, either attached to a metal staple or glued, to great effect.
4. Timing belt
Timing belts are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matched toothed pulley. When properly stressed, they have no slippage, move at a constant speed, and are often used to transfer direct motion for the purposes of sequencing or timing.
They are often used instead of chains or gears, so noise is low and a lubrication bath is not necessary. Automobiles’ camshafts, short timing systems, and stepper motors often use these belts. Timing belts need the least tension of all belts and are among the most efficient.
Selection of a belt drive:
The following are the various important factors on which the selection of belt drive depends:
- Speed of driving and driven shafts,
- Speed reduction ratio,
- The power to be transmitted,
- Center distance between shafts,
- positive drive requirements,
- Shaft layout,
- Location available, and
- Service Terms.
Applications of Belt-drive:
Belt-drives are used in various places like:
- A belt drive is used to power transfer.
- Drive to the rotor on the rotor spinning machine.
- The belt drive is used in Conveyor.
- Drive to drafting rollers and other rolling elements on a single delivery drawing machine.
- Drives to opening rollers, friction drums, and take-off rollers on friction spinning machine.
- Main drive on the draw-texturing machine.
- The belt drive is used in the Mill industry.
- Drive to creel-rollers of a high-speed drawing machine.
Advantages of a belt drive
The advantages of a belt drive include:
- It is cost-effective and simple to use.
- New belt-drive efficiency can be up to 95-98 percent.
- Belt drive requires a low maintenance cost.
- Belt drives do not require a parallel shaft.
- They come with overload and jam protection
- Speeds Difference can be obtained by using a step or tapered pulleys.
- Belt drives are the most economical option when a large distance of power transmit between shaft.
- Damp out noise and vibration, Load fluctuations are shock-absorbed, increasing the machinery life
- Clutch action can be activated by releasing belt tension
Disadvantages of Belt drive
However, belt technology also comes with certain disadvantages of belt drive. These are:
- Belt drive not suitable for small distances.
- There is a loss of performance due to high levels of slip and creep.
- Constant velocity ratio can not achieve between drive and driven pulley.
- It produces high noice.
- it has low mechanical efficiency.
The material used for Belts
There are five main materials that conveyor belts are made out of:
- Leather belts
- Cotton belts or Fabric belts
- Rubber belts
- Balata belts
- Plastics belts
There are five main materials that conveyor belts are made out of: thermoplastics, metal, rubber, fabric, and leather. Plastics include polyester, polyvinyl chloride, silicone, and polyethylene.
The metals are stainless and carbon steel and the fabric materials are canvas or cotton. The most popular material is a rubber composite because it is flexible, resistant, smooth, and seamless.
1. Leather Belt
The most important material for a flat belt is leather. The best leather belts are made from strips 1.2 m to 1.5 m long, cut from either side of the backbone of the top-grade step hides.
The side of the leather hair is smooth and hard on the flesh side, but the flesh side is strong. The fibers on the hair side are perpendicular to the surface, while the flesh side parts are interconnected and parallel to the surface.
2. Cotton or Fabric Belt
Most fabric belts are made by folding three or more layers of canvas or cotton duck and stitching them together. These belts are also woven into a strip of desired width and thickness.
They are applied like linseed oil with some filler to make the belt waterproof and prevent injury to fibers. Cotton bands are inexpensive and suitable in moist climates, moist environments. Since cotton belts require little attention, these belts are mostly used in farm machinery, belt conveyors, etc.
3. Rubber belt
Rubber belts are made of layers of fabric that are fastened with a rubber structure and have a thin layer of rubber on the face. These belts are very flexible but are quickly destroyed when allowed to come in contact with heat, oil, or grease.
One principal advantage of these belts is that they can be made endlessly easily. These belts are found suitable for sawmills, paper mills where they are exposed to moisture.
4. Balata belt
These belts are similar to rubber belts except that balata glue is used in place of rubber. These belts are acid-proof and waterproof and are not affected by animal oils or alkalis.
The belt should not be at a temperature above 40 ° C because at this temperature the sanding starts to soften and become sticky. Balata belts have a strength of 25 percent higher than rubber belts.
5. Plastic belt
Plastics include polyester, polyvinyl chloride, silicon, and polyethylene.