Understanding the fundamentals of sprockets

There really is more to sprockets than meets the eye.

The power transmission industry is dominated by new developments in chain products, solutions to problems on chain wear etc.

However, there is another side to power transmission that is commonly overlooked - the sprocket.

The sprocket seems to be the last item ordered, or even replaced, when the chain wears out or when the system needs to be serviced.

One likely reason may be that it's considered a commodity item.

All it does is connect the chain to the shaft.

However, there really is more to sprockets than meets the eye.

In the power transmission industry, the sprocket transmits power from one source to another, the first source being the motor drive, the second, the receiving source - the chain.

There are three basic types of sprockets used in PT drive applications.

The drive sprocket is considered the 'power source' sprocket.

It transmits the power toward the particular application.

The driven sprocket is driven by the drive sprocket and turns the shaft for the application (such as a conveyor).

The idler sprocket is used to take up tension on the slack side of the chain.

It is not considered part of the powering system.

It typically rides on a shaft with bearings.

Most standard sprockets are manufactured from steel.

They can be made from many types of material, but the offerings from most manufacturers are based on their own equipment limitations and the tooling available to cut the teeth.

Below is a listing of typical sprocket materials and their most frequent application environments.

Steel is considered the most typical construction material.

It is available in different hardness levels (covered later) and is used in all types of applications.

Bronze is a metal used in nonmagnetic applications where 'no sparking' is required.

It can also withstand the abuse of some corrosive environments.

Brass is also a nonmagnetic application material with the ability to stand up in a number of corrosive environments.

Stainless steel is the most common material used for corrosive environments.

It is widely applied throughout in the food processing industry and most manufacturers have types approved for direct food contact.

Titanium is lightweight and very strong.

This metal is a silvery, dark grey colour and is designed for highly corrosive applications or direct chemical exposure such as in the electrical industry where printed circuit boards are cleaned.

Aluminium is silvery lightweight metal that can resist corrosion but is restricted to light duty, light load applications.

Typically used in belt and pulley applications (timing belts).

As with roller chain, nylon is also used for anticorrosive environments, as well as for quietness.

Nylon materials are also generally less expensive than metal.

Nylon sprockets can be used in the food industry, as they hold up well in washdown situations.

These plastic sprockets can be constructed from electroconductive through heat resistant styles - similarly found in plastic chain.

Two of the most difficult areas to understand for individuals learning about sprockets are how to make a selection from the wide variety available and which size is required.

There is no single answer.

It is important to analyse the whole power transmission application when selecting the sprocket.

Typical factors that influence selection include: the size of chain; the number of teeth required for the drive and driven sprockets; the bore size for the sprocket required; the keyway size; the setscrew size; whether the sprocket requires hardened teeth for improved wear; and the type of finish required by the customer.

The following are some typical sprocket styles.

The A-style (A-plate) has no hubs on either side.

It is a basic sprocket plate only.

It can be mounted by bolting through the plate or welding it to a roller or shaft with a drive unit.

The B-style is the 'typical' sprocket.

Manufactured from an A-plate and one hub, welded or bolted together, or is made as a single-piece construction (also referred to as solid construction).

Solid design is usually used up to 4.75in outside diameter.

Larger sizes are made from two pieces of material.

The reason for the change from one piece to two-piece construction is due to the limitations of the manufacturer's equipment, as well as the cost of manufacturing the sprocket efficiently.

Material is the most expensive element in this process and on larger sprockets; one-piece construction is prohibitive as large volumes of metal are wasted.

The C-style is made from one A-plate and two hubs, one located on either side of the plate.

This is a more stable design than the B-style, as it offers a longer length through bore (LTB) measurement - reducing runout.

The design of the taper lock-style, also known as a 'quick detachable' sprocket, physically locks together rather than using a keyway and setscrew design.

The advantages over a standard sprocket are that whereas standard sprockets can be hard to remove, the taper lock offers a bushing style locking device that can be easily removed and eliminates the need for a setscrew.

Multiple sprockets are designed for multiple strand chain.

Extended or double-pitch sprockets are designed for use with conveyor or 'double-pitch' chain.

There are two types of double pitch sprockets, standard roller diameter and oversized roller diameter.

Double single sprockets are designed for two single strands of chain (as opposed to multiple strands) for use on the same drive shaft.

In essence, it is two A-plates connected by a single hub, where the LTB can be any size, as long as the chains do not touch.

Normally, made in single-piece construction, the sprockets must be of the same size, have the same number of teeth, and use the same size chain.

Coupling halves link two shafts in a straight line for drive purposes.

A chain locks the two shafts together.

They are offered with hardened teeth in plain, finished bore, and taper-lock.

The final area to cover is the hardening or heat treatment of sprockets.

This process is basically increasing the surface hardness of the tooth in order to increase the wear life.

In many cases, hardening can significantly increase not only the wear life of the sprocket but also the chain itself, a point some maintenance engineers do not consider.

The surface of the tooth is the only area that should be affected by hardening.

Typical penetration depth of hardening is about 0.8mm.

The balance of the inner core of the sprocket remains the same to absorb shock loads.

Without that flexibility, the tooth would break off or the sprocket would shatter.

Some customers can become confused when offered heat-treated sprockets.

They may understand that the sprocket life will increase, but will it reduce the wear life of their chains? The answer here is that by controlling the heat treatment range it is possible to ensure that the hardness level of the sprocket is not in the same range as the chain itself.

However, there are cases where the customer requests sprockets that are harder than the chain, because the end-user may find it easier to replace the chain than the sprocket when servicing their units.

Most popular sizes and materials are kept in stock by manufacturers or good engineering distributors, and so are easy to obtain, but there are so many variations of material, finish and style that inevitably the ideal solution may not always be available from stock.

In order to address this issue, Tsubaki has installed an Engineering Service and Machining centre at its UK headquarters in Annesley, Nottingham.

A wide range of sprockets is held there in various materials that can be finished and engineered to suit any particular application.

Expert advice on what is the best solution for any application is also always on hand.

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