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2.9.5Belt drives

A sheave is very similar in form and function to a pulley, but designed to grip a flexible belt rather than a rope or a cable. Unlike a pulley which is designed to turn freely to re-direct the tension of a rope, a sheave works more like a gear to couple the belt’s motion to a rotating shaft. The mechanical advantage of a pair of sheaves coupled by a common belt is simply the ratio of sheave radii, just like gears:

Sheave #1

Belt

Sheave #2

	Slow-turning,	Fast-turning,
		low-torque
	high-torque

The following photograph shows a triple-belt drive from an electric motor to an agitator on the bottom of a sawdust storage bin:

As indicated by the respective sheave diameters, the electric motor turns much faster than the agitator, while the agitator spins with much greater torque than the motor.

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Most modern belt drives are either V-belt or toothed belt, referring to the shapes of the belt and how they engage with the sheave. The triple-belt drive system for the sawdust agitator shown in the previous photograph used V-belts. A V-belt has a V-shaped cross-section, and sits in a V- shaped groove around the circumference of the sheave. Toothed belts almost resemble chains, in that their inner surface is characterized by regularly-spaced perpendicular ribs designed to engage with matching cavities machined into the circumference of the sheave. The advantage of a toothed belt is that it cannot slip on the sheave if overloaded, unlike a V-belt. A toothed belt is firmly “locked” into place on the sheave’s circumference so long as proper belt tension is maintained.

An older belt-and-sheave technology is the flat belt. Here, the sheave’s circumference is flat, and the belt itself is nothing more than a strip of flexible material with no special shape. The following photograph shows an antique flat belt drive in a workshop, where a central shaft ran along the ridge of the ceiling to power several machines in the shop, the shaft itself turned continuously by either a water turbine or a steam engine:

Flat belts are still used in modern times, but they tend to be much wider than V-belts or toothed belts of comparable rating in order to deliver adequate “grip” on the sheave. Also, sheave-to-sheave alignment is much more critical for a flat belt, which has no guides on the sheave to keep it centered20.

20An interesting feature of many flat-belt sheaves is a slight “crown” shape to the sheave, such that the diameter is slightly larger at the sheave’s center than it is at either side edge. The purpose of this crown is to help the belt center itself while in operation. As it turns out, a flat belt naturally tends to find the point at which it operates under maximum tension. If the belt happens to wander o -center, it will naturally find its way back to the center of the sheave as it rotates because that is where the tension reaches a maximum.

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Like gear sets, industrial belt drive systems are typically shrouded for cleanliness and for personnel safety. Sheet-metal enclosures such as the one covering the top of this V-belt drive system on a “walking-beam” style of oil field pump. The sheet-metal enclosure protects the belts and sheaves from rain and snow. You will also note a large gearbox following the belt drive, further reducing rotational speed from the electric motor to the pump’s counter-weighted crank:

Belts of all styles are subject to wear and fatigue, and as such must be periodically replaced. Some belt drive systems employ tensioner mechanisms which maintain consistent belt tension by applying a constant force to the belt. Small tensioners are usually spring-loaded, while large belt tensioners (particularly conveyor belts) are loaded by the weight of a large mass. Minimum belt tension is extremely important for belt drives, as loose belts will begin to “slip” under load and quickly fail if the problem is not remedied.

When multiple belts are used to distribute loading between belts in high-power drive systems, it is important that all belts be replaced simultaneously, never partially. If a new belt is installed next to an old belt on the same sheave, the old belt will run “loose” and not bear its full share of the load, thus overloading the other (new) belt(s) in the drive system.

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2.9.6Chain drives

Sprockets are identical in function to sheaves, using link chain rather than belt to couple the two rotating pieces together. Bicycles are perhaps the best-known example of sprockets and chains from everyday life, being the most e cient simple machine for the purpose of coupling a person’s leg power to a rotating wheel for propulsion. Like gear sets, the mechanical advantage ratio of a sprocket set may be determined by counting teeth on each sprocket and then dividing one tooth count by the other, or empirically by rotating one sprocket by hand and counting the number of turns (revolutions) each sprocket makes.

The following photograph shows a pair of sprockets linked together with a roller chain. The sprocket ratio here is 1:1, as both sprockets share the same number of teeth:

Bicycles use sprockets and a chain to transfer power from the crank to the rear wheel. Here, a multi-speed sprocket assembly allows the rider to select the best ratio (i.e. mechanical advantage) for riding at di erent speeds and in di erent conditions. Three sprockets on the crank and eight sprockets on the wheel give a theoretical21 maximum of 24 di erent “speeds” or “gears” from which to select:

Chain drive systems require thorough lubrication and freedom from dirt and other abrasive particles in order to deliver full service life. Open-chain systems such as the two shown in the above photographs are challenging to maintain in good working order for these reasons.

21In practice, not all of these 24 “speeds” are recommended, because some of the front/rear sprocket selections would place the chain at an extreme angle as it engaged with both sprockets. In the interest of extending chain life, it should run as “straight” on each sprocket as possible.