Belt drive

(mechanical engineering) The transmission of power between shafts by means of a belt connecting pulleys on the shafts.

The lowest-cost means for transmitting power between shafts that are not necessarily parallel. Belts run smoothly and quietly, and they cushion motor and bearings against load fluctuations. Belts typically are not as strong or durable as gears or chains. However, improvements in belt construction and materials are making it possible to use belts where formerly only chains or gears would do.

Advantages of belt drive are: They are simple. They are economical. Parallel shafts are not required. Overload and jam protection are provided. Noise and vibration are damped out. Machinery life is prolonged because load fluctuations are cushioned (shock-absorbed). They are lubrication-free. They require only low maintenance. They are highly efficient (90–98%, usually 95%). Some misalignment is tolerable. They are very economical when shafts are separated by large distances. Clutch action may be obtained by relieving belt tension. Variable speeds may be economically obtained by step or tapered pulleys.

Disadvantages include: The angular-velocity ratio is not necessarily constant or equal to the ratio of pulley diameters, because of belt slip and stretch. Heat buildup occurs. Speed is limited to usually 7000 feet per minute (35 meters per second). Power transmission is limited to 370 kilowatts (500 horsepower). Operating temperatures are usually restricted to –31 to 185°F (–35 to 85°C). Some adjustment of center distance or use of an idler pulley is necessary for wear and stretch compensation. A means of disassembly must be provided to install endless belts.

There are four general types of belts: flat belts, V-belts, film belts, and timing belts. Each has its own special characteristics, limitations, advantages, and special-purpose variations for different applications.

Flat belts, in the form of leather belting, served as the basic belt drive from the beginning of the Industrial Revolution. They can transmit large amounts of power at high speeds. Flat belts find their widest application where high-speed motion, rather than power, is the main concern. Flat belts are very useful where large center distances and small pulleys are involved. They can engage pulleys on both inside and outside surfaces, and both endless and jointed construction are available.

V-belts are the basic power-transmission belt, providing the best combination of traction, operating speed, bearing load, and service life. The belts are typically endless, with a trapezoidal cross section which runs in a pulley with a V-shaped groove. The wedging action of the belt in the pulley groove allows V-belts to transmit higher torque at less width and tension than flat belts. V-belts are far superior to flat belts at small center distances and high reduction ratios. V-belts require larger pulleys than flat belts because of their greater thickness. Several individual belts running on the same pulley in separate grooves are often used when the power to be transmitted exceeds that of a single belt. These are called multiple-belt drives.

Film belts are often classified as a variety of flat belt, but actually they are a separate type. Consisting of a very thin strip of material, usually plastic but sometimes rubber, their widest application is in business machines, tape recorders, and other light-duty service.

Timing belts have evenly spaced teeth on their bottom side which mesh with grooves cut on the periphery of the pulleys to produce a positive, no-slip, constant-speed drive. They are often used to replace chains or gears, reducing noise and avoiding the lubrication bath or oiling system requirement. They have also found widespread application in miniature timing applications. Timing belts, known also as synchronous or cogged belts, require the least tension of all belt drives and are among the most efficient.

The most common belt-pulley arrangement, by far, is the open belt drive (Fig. 1). Here both shafts are parallel and rotate in the same direction. The cross-belt drive of Fig. 2 shows parallel shafts rotating in opposite directions. Timing and standard V-belts are not suitable for cross-belt drives because the pulleys contact both the inside and outside belt surfaces.

Open belt drive.

Cross belt drive.

Industrial belts are usually reinforced rubber or leather, the rubber type being predominant. Nonreinforced types, other than leather, are limited to light-duty applications.

Belts probably fail by fatigue more often than by abrasion. The fatigue is caused by the cyclic stress applied to the belt as it bends around the pulleys. Belt failure is accelerated when the following conditions are present: high belt tension; excessive slippage; adverse environmental conditions; and momentary overloads caused by shock, vibration, or belt slapping. See also Pulley.