Power Transmission
7-98
 Technical Feature  


Working on the chain gang

Lubrication is a necessity for chain systems – but what happens in sensitive applications, such as food production, where lubricant may be undesirable?

Engineers trying to determine the life expectancy of a chain in service have used a calculation to estimate the life of power transmission chains with some degree of accuracy.

The life of mechanical handling conveyor chains though, has always been more difficult to predict, as they are more dependant on the quality of maintenance they receive – particularly lubrication.

Recent cost-motivated trends in the manufacturing industry have been to reduce the level of maintenance staff with the inevitable detrimental effect on the quality and maintenance of the chains and therefore chain life.

As a rule of thumb, to achieve an acceptable life from mechanical handling chain operating under poor lubrication conditions, the load-carrying capacity is reduced by a factor of five over a well-lubricated chain. The present situation has given impetus to the development of lubrication-free chains.

Other motivating factors were: the environmental and safety pressures to remove hazardous substances – such as excess lubricant – from escalators following the King’s Cross fire; lower maintenance costs; and longer life.

Step chain wear is a major cause of escalators being taken out of service for overhaul and is probably the most costly single item to replace on a heavy duty public service escalator, in terms of both material and labour. One innovation was to adapt and built on the polymer bushed chain concept, originally developed by the London Underground Escalator Engineering Department and one of the UK’s leading tribology consultants.

This followed extensive trials and testing, in an effort to obtain a cleaner operating environment below the escalator, not only to reduce fire risk but to ensure that every link was effectively lubricated.

Polymer materials have been available for many years, but the criteria was to achieve endurance and reliability when operating at typical bearing pressures of around 20N/mm˛, without the benefit of a rotating bearing effect. A typical escalator step chain is designed to last for a minimum of 20 years. The production techniques developed by the chain manufacturer have been a key factor in the successful use of polymer bush chains in the past few years. Performance monitoring results, by independent inspectors, are predicting chain life well in excess of 20 years, with the additional commercial benefit of considerably reduced maintenance costs.

One major problem was that the chain had to fit into the existing space envelope on the escalator. The addition of any non-lube component between the pin and bush had the inevitable effect of increasing the bush outside diameter and reducing the strength of the chain.

After some research a material was found that was thought to be suitable and the size problem overcome. The chain shown in the diagram is the result of this development, with the design being used on many escalator installations.

The key to the design is the polymer bush fitted between the steel pin and bush and the material used in the manufacture of the pin. The wear life of the lube-free chain compared with the standard greased chain is considerable.

The success of the polymer bush fitted to the escalator chain has led to the same principle being applied to other industries with very different conditions.

An escalator can be said to be an ideal environment for a minimal lubrication chain. Non-lube transmission chains have been available for some years, fitted with plastic bushes between the pins and the bushes, but have always been restricted in their performance. One restriction is the extreme difficulty of fitting a polymer bush into a transmission chain and still offering a chain that complies with the BS, ISO or ANSI standards. Another was the very limited load capacity of the bush insert.

The answer proved to be the Renold Syno chain. It has a special oil-impregnated bush which serves to lubricate both the pin and roller with a USDA-approved food quality lubricant. The chains are now in service in many applications where lubrication is difficult, impossible or undesirable, notably the food industry (where lubricant contamination is a major concern with conventional chain types), electronics, paper, packaging, clothes and textiles.

There are, of course, applications where non-lube chains cannot, at present, be used; for the time being, the operator will still be required to lubricate chains as before in these places. But even in these difficult applications, chains can be treated and maintained with special lubricants tailored for specific industries. In most cases there is no excuse for allowing chains to fail prematurely due to lack of a suitable lubricant.

Renold, the inventor of the bush roller chain, has led the world in chain technology for more than a century. Renold Chain is part of a diverse, worldwide engineering group that specialises in power transmission products. For further information, contact: Andrew Freeman, marketing manager, Renold

Chain, Renold House, Styal House, Wythenshawe, Manchester M22 5WL. Telephone: 0161 437 5221. Facsimile: 0161 437 7782. E-mail: enquiry@renold.com

The group’s website is at: http://www.renold.com

Taking the biscuit
A major biscuit manufacturer was having serious problems with short chain life on the infeed conveyor of a flow wrap machine. The installed chains needed replacement after as little as three months, due to difficulties experienced in lubricating the chain.

Debris from the product was building up on and around the chain parts and effectively blocking the penetration of oil into the chain. The oil-wet product debris then attracted more debris by adhesion, which compounded the problem.

The machine design provided only a small amount of tension adjustment for the chain and so the rapid wear caused by lubrication starvation severely restricted the chain life. The adapted transmission chain used on the conveyor also suffered from misalignment, which had led to abrasion against the conveyor tracking.

Renold suggested some small modifications to the track layout and the fitting of an attachment variation of Renold Syno chain to replace the original chain. This new Syno chain remains fitted and continues operating without problems after nine months – a threefold increase in life.

The chain has required no tension adjustments to allow for wear. The biscuit manufacturer is delighted with the new chain development and the savings enjoyed.

Benefits include: reduction of downtime; saving of maintenance time for tension adjustements; saving of lubrications costs; extension of chain life; and saving of chain replacement costs.

The plant, including the chain, is washed down on a regular basis with hot water. This has not affected the self-lubricating feature of the chain. The Renold Syno chain now features in the customer’s plans for future replacement and is already used in several new applications.

 

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