Chains - the workhorses of industry

Chain is possibly the most hard working and ubiquitous mechanical engineering component that is employed right across industry for driving, lifting and conveying.

Chain is possibly the most hard working and ubiquitous mechanical engineering component that is employed right across industry for driving, lifting and conveying.

It comes in many forms and variations from the standard configurations through to derivative specials that perform more than their "basic" duties.

In essence, there are two types of chain, roller and leaf.

Roller chains are employed primarily for driving functions as a component of a power transmission system for delivering power and torque and are normally driven by toothed sprockets.

Leaf chains are used for lifting and pulling applications with power being applied at the opposite end to the load, usually via an hydraulic cylinder.

The benefits of using roller chains as a means of delivering a motive force are numerous.

They include high efficiency (up to 98%), are suitable for both long and short centre distances, ideal for linked processes such as engine timing, permit use of smaller bearings for driving and driven shafts (unlike belting no initial tensioning required), withstand shock loadings, easy to install and maintain, and longevity.

Both types of chain are manufactured in various forms either to suit the application or the industry and, in some cases, a combination of the two.

As well as single strand designs, roller chains are supplied frequently in multi-strand configurations, usually for high speed applications (up to 23m/s).

Also, by modifying a chain's design it can perform a secondary duty.

For example, by extending the pin or adding attachment links with integral mounting brackets, the chain is able to become a transporting device as well as being part of the transmission system, such as in conveyors on packaging machinery.

Similarly, for leaf chains, side flanges can be added to act as travel guides for the chain.

Chains are available in a range of materials and finishes, including stainless steel, nickel-plating and zinc-aluminium based coatings, making it possible to optimise material choice according to application.

Self lubricating and low maintenance chains are also available for applications where lubrication is problematic or undesirable.

Today's manufacturing techniques and materials have improved significantly the performance and durability of chains as well as making it possible to produce products ideal for specific markets with particular demands.

Modern manufacturing processes include sophisticated heat treatments that achieve a combination of both high surface hardness for (pins and bushes), and through hardening that imparts shock resistance to the plates and rollers.

To further enhance fatigue resistance, plates, bushes and rollers are shot peened.

This cold working process, which involves bombarding the components with tiny balls, improves the load bearing properties of the plates, bushes and rollers by pre-stressing the surfaces through work hardening, thereby improving fatigue resistance.

However, hardness levels need to be controlled carefully because if they exceed certain levels components become brittle and fatigue strength diminishes.

The components also become more sensitive to aggressive media (free hydrogen, acids etc) and more vulnerable to corrosion.

To further improve chain performance, a technique called ball drifting is employed.

This process involves forcing precision balls through plate holes to remove all punching flutes and sharp edges left after stamping whilst simultaneously introducing strain hardening around the aperture.

Preloading is also a significant element in the manufacturing process.

After assembly, chain is subject to a high tensile load which minimises the initial stretch that all chains experience during the first hours of use.

Some manufacturers have also developed new link plates.

These are the product of lengthy R and D work that looked at every aspect, including plate height, thickness and eccentric profile of link heads, all in conjunction with the adoption of enhanced materials and manufacturing techniques.

The result is a 30% improvement in fatigue resistance.

Initial lubrication is a key factor for chain manufacturers.

Factory prelubricants are chosen both to lubricate and provide some protection against corrosion.

However, increasingly specialist pre-lubricants are being developed as coatings.

These reduce the amount of maintenance lubrication required due to the superior protection they provide.

In fact, salt spray tests (DIN50021SS/ISO 9227SS) have demonstrated such coatings to be up to 10 times better than conventional prelubricants, resulting in maintained link articulation and longer service life.

In addition to practical performance, some of these coatings also have environmental benefits, in that they do not contain heavy metals, no Teflon and no silicon.

Further, they comply with NSF-H2 food industry requirements and the electric/electronic industry's RoHS:2005 standard.

When specifying roller chains the breaking strength has little significance.

Instead, it is the bearing pressure and fatigue resistance that determine the life and performance of the chain.

In fact, chains with comparable breaking loads can have fatigue strengths that can differ by as much as 100%.

Chain selection is dependent on a number of factors governing the complete drive system.

Considerations include the number of teeth for both the drive and driven sprockets in conjunction with the drive ratio required.

It is then necessary to consider the impact load classification following which it is possible to select the correct chain pitch.

The economic and technically superior solution is the single strand chain of the smallest pitch based on the output values to be transmitted and the revolution of the small sprocket.

If the design envelope is too small to accommodate the sprocket size for a chain with a single strand, a smaller sise chain with multiple strands can be employed.

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