Belt (mechanical)
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- For other belts, see Belt
Belts are used to mechanically link two or more rotating items. They may be used as a source of motion, to transmit power at up to 98% efficiency between two points, or to track relative movement.
As a source of motion, a conveyor belt is one application where the belt is adapted to continually carry a load between two points. A belt may also be looped (or crossed) between two points so that the direction of rotation is reversed at the other point.
Power transmission is achieved by specially designed belts and pulleys. The demands on a belt drive transmission system are large and this has led to many variations on the theme.
The earliest was the flat belt, used with line shafting. It is a simple system of power transmission that was well suited to its time in history. The Industrial Revolution soon demanded more from the system, as flat belt pulleys need to be carefully aligned to prevent the belt from slipping off. The flat belt also tends to slip on the pulley face when heavy loads are applied. In practice, such belts were often given a half-twist before joining the ends (forming a Möbius strip), so that wear was evenly distributed on both sides of the belt.
Round belts are a circular cross section belt designed to run in a pulley with a circular (or near circular) groove. They are for use in low torque situations and may be purchased in various lengths or cut to length and joined, either by a staple, gluing or welding (in the case of polyurethane). The early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great affect.
Vee belts (also known as v-belt or wedge rope) are an early solution that solved the slippage and alignment problem. The V-belt was developed in 1917 by John Gates of the Gates Rubber Company. 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 vee belt an effective solution. They can be supplied at various fixed lengths or as a segmented section, where the segments are linked (spliced) to form a belt of the required length. For high-power requirements, two or more vee belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves.
Timing belts, (also known as Toothed, Notch or Cog) belts are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tensioned, they have no slippage and are often used to transfer direct motion for indexing or timing purposes (hence their name). Camshafts of automobiles and stepper motors often utilize these belts.
Timing belts with a helical offset tooth design are available. The helical offset tooth design forms a chevron pattern and causes the teeth to engage progressively. The chevron pattern design is self-aligning. The chevron pattern design does not make the noise that some timing belts make at idiosyncratic speeds.
Belts normally transmit power only on the tension side of the loop. However, designs for continuously variable transmissions exist that use belts that are a series of solid metal blocks, linked together as in a chain, transmitting power on the compression side of the loop.
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The flat belt alignment problem for the most part was later solved by machining a radial crown or a straight double taper to the pulley's flat face. This modification effectively acts as if the pulley's angular alignment was shifted when the belt drifts to the edge of the pulley face. The crown gives the pulley center a slightly greater diameter than it's edges, effectively balancing the forces when the belt tracks in the center. This difference in diameter is usually 1/8" per foot from the center to either edge. Greater taper will cause increased slip or break the back of the belting. check out Machinery's Handbook to verify The angle of the input shaft to the output is 90 degrees
