MRO Magazine

Tough chain

The performance of a drive chain is governed by a number of key factors. The tensile strength is the most obvious, since this is the means by which a chain installation is roughly sized. However, the ...


September 1, 2008
By MRO Magazine

The performance of a drive chain is governed by a number of key factors. The tensile strength is the most obvious, since this is the means by which a chain installation is roughly sized. However, the environment in which the chain is operating also has a significant effect on its performance.

If a chain is correctly selected, it should be capable of running for 10,000,000 loading cycles or 15,000 hours of continuous operation. Again, there are many external factors which will affect the chain performance.

Lubrication is one of the most important factors affecting chain performance. Insufficient or inadequate lubrication causes increased wear rate, chain seizures, and galling of pin surfaces. Replacement of the chain is the only real answer to an incorrectly lubricated chain.

The other main factors affecting chain performance are speed, power and environment.

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Many chains will fail due to overload. In these cases, failures will be seen by elongated plate holes prior to plate failure and pin bending prior to breakage. These failures are solely due to an incorrectly selected chain and should never occur. Fatigue failures can be seen on chains that are running above their maximum working load and again can be avoided if correctly selected.

There are a number of occasions — such as operation in harsh or severe-service environments — where standard roller chain is not suitable and must be replaced by ‘problem solving’ chains. Examples include applications in high temperatures, low temperatures, wet conditions, abrasive conditions and corrosive environments.

Temperature

High-temperature lubricants can be used to eliminate the lubrication breakdown effect and improve wear resistance; however, depending on actual temperatures, the annealing (softening) of components will always be the main problem. For temperature ranges above 250C (480F), it is preferable to use stainless steel chains. The type and grade of stainless steel is dependent on the actual temperature to which the chain is subjected.

When heat-treated carbon steel chains are run in temperatures above their components’ tempering temperatures, a number of things should be considered:

• Chain lubricants can carbonize or scorch and break down.

• Hardness of components will draw back (soften), and increased wear will be observed.

• Chain strength will reduce and stiff joints will result.

• Accept that strength will be reduced and up-size accordingly.

When using chains in low-temperature applications, problems will occur with brittleness of chain components, leading to a reduction in shock resistance, stiffness of links caused by the freezing of condensation, and the reduction in performance of the chain lubricants.

The lower temperature limit for using a carbon steel chain is -40C (-40F); however, the tensile strength should be reduced to allow for the reduction in transmission capabilities of the chain. Stainless steel chain is recommended for temperatures below -40C. The type and grade of stainless steel, as with high-temperature applications, is, again, dependent on actual temperatures.

Abrasive conditions

When using chains in abrasive conditions, the primary mode of failure is an increased wear rate due to abrasive particles collecting inside the journal bearings of the chain. Various methods can be adopted to improve chain performance in these applications:

• Increase the surface hardness of the pin and bushing so that the surface is harder than the abrasive particles.

• Use a high wear resistance chain. • Use a chain with seals or O-rings to keep particles out of the bearing area.

• Use a chain with increased clearances so particles can escape.

Wet conditions

In wet conditions, standard carbon steel chains will suffer from insufficient lubrication and corrosive attack from the environment. There are various ways of reducing the effect of a wet environment and increasing chain performance:

• Use a lubricant with good hydro-capillary properties, which will help remove moisture from the bearing areas of the chain.

• Take into account the corrosive environment in the selection calculations. Increasing the chain size will reduce the bearing pressure and increase wear life.

• Use corrosion-resistant plated chains.

Corrosive environments

In corrosive environments, there is the tendency for a reduction in fatigue performance due to corrosion pitting and oxidation. There will also be a reduction in wear performance from abrasive corrosion debris in the bearing area and a reduction in strength.

In mildly corrosive environments, plated chains can offer some protection without a reduction in tensile strength; however, in more severe environments, many different stainless and plastic materials could be used.

One of the most significant variations of the carbon steel chain is a maintenance- free or self-lubricating chain. Typically, these chains are used in food or hygiene-sensitive environments where it is impossible or impractical to lubricate the chain.

Generally, the main feature of these chains is a sintered steel bushing, which is oil impregnated. Plates, pins and rollers can have additional surface treatments or plating processes added, which further prevent corrosion. Note that some of these types of chain cannot accommodate the same power and speed as conventional carbon steel chains.

Some chain manufacturers will incorporate polymer sleeves between the pin and bushing as the self-lubricating feature, but these tend to be typically on larger pitch chains.

This article is adapted from a report by Mike Barlow, Applications Engineer — Roller Chain, Renold Jeffrey, Morristown, TN. For more information on drive chain for specific harsh environments, visit www.renoldcanada.com.

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