Thursday, November 11, 2010

MECHANICAL BELT

A belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used to move objects, to efficiently transmit mechanical power, or to track relative movement. Belts are looped over pulleys. In a two-pulley system, the belt may either drive the pulleys in the same direction, or the belt may be crossed so that the shafts move in opposite directions. A conveyor belt is built to continually carry a load between two points.

Power Transmission

Belts are the cheapest utility for power transmission between shafts that may not be parallel. 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. Belts run smoothly and with little noise, and cushion motor and bearings against load changes, albeit with less strength than gears or chains. However, improvements in belt engineering allow use of belts in systems that formerly allowed only chains or gears.

Pros and cons

A belt drive is simple, inexpensive, and does not require parallel shafts. It helps protect a car from overload and jam, damping it from noise and vibration. Load fluctuations are shock-absorbed (cushioned). They need no lubrication and only little maintenance. They have high efficiency (90-98 percent), higher tolerance of misalignment, and are relatively inexpensive. Clutch action is activated by releasing belt tension. Different speeds can be obtained by step or tapered pulleys.
However, the angular-velocity ratio is not constant or equal to that of the pulley diameters, due to slip and stretch. Heat accumulation is present, and speed is limited to approximately 7000 feet per minute (ft/min), and a power of only 500 horsepower (hp). Temperatures ranges from -31 to 185°F. Adjustment of center distance or addition of an idler pulley is crucial for balancing the wear and stretch. To install endless belts, the relevant assembly must be dismantled first.

Flat belts

Belts on a Yanmar 2GM20 marine diesel engine.
Belts on a Yanmar 2GM20 marine diesel engine.


Flat belts were used early in line shafting to transmit power in factories. It is a simple system of power transmission that was well suited to its time in history. It delivered high power for high speeds (500 hp for 10,000 ft/min), in cases of wide belts and large pulleys. However, these drives are bulky, requiring high tension leading to high loads, so vee belts have mainly replaced the flat-belts (except when high speed is needed over power). 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. Because flat belts tend to slip towards the higher side of the pulley, pulleys were made with a slightly convex face (rather than flat) to keep the belts centered. 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 (DB). A good modern use for a flat belt is with smaller pulleys and large central distances. They can connect inside and outside pulleys, and can come in both endless and jointed construction.

Round belts

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). Early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great effect.

Vee belts

The Vee belt (also known as V-belt or wedge rope) provided an early solution to the slippage and alignment problem. It is now the basic belt for power for the transmission. It provides the best array of traction, speed of movement, load of the bearings, and longer service life. It was developed in 1917 by John Gates of the Gates Rubber Company. They are generally endless, and their general cross-section shape is trapezoidal. 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, needing less width and tension than flat belts.
V-belts trump flat belts with their small center distances and high reduction ratios. The preferred center distance is larger than the largest pulley diameter but less than three times the sum of both pulleys. Optimal speed range is 1000-7000 ft/min. V-belts need larger pulleys for their larger thickness than flat belts. 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. The strength of these belts is obtained by reinforcements with fibers like steel, polyester or aramid (e.g. Twaron). This is known as a multiple-belt drive.
When endless belts do not fit the need, jointed and link vee-belts may be used. They are, however, weaker and speed up to only 4000 ft/min. A link v-belt is a number of rubberized fabric links held together by metal fasteners. They are length adjustable by dissasembling and removing links when needed.

Film belts

Though often grouped with flat belts, they are actually a different kind. They consist of a very thin belt (0.5-15 millimeters or 100-4000 microns) strip of plastic and occasionally rubber. They are generally intended for low-power (ten hp or seven kW), high-speed uses, allowing high efficiency (up to 98 percent) and long life. These are seen in business machines, tape recorders, and other light-duty operations.

Timing Belts

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, run at constant speed, and are often used to transfer direct motion for indexing or timing purposes (hence their name). They are often used in lieu 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 (150 kW) at speeds of 16,000 ft/min, and there is no limit on speed.
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, and is more efficient at transferring power (up to 98 percent).
Disadvantages include high starting price, grooving the pulleys, less protection from overload and jam, no clutch action, and backlash.

Specialty Belts

Belts normally transmit power 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.
"T belts" that simulate rolling roads for wind tunnels can be made to reach speeds of up to 250 km/h.


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