Lubrication - Smooth operator06 June 2005
In recent years, there has been a significant increase in the number of lubrication-free chain products on the market - and manufacturers are continuing to develop more. But why do we need so many different types of lubrication-free chain and what precisely do manufacturers mean by the term 'lubrication-free'?
Dr Chris Lodge, engineering manager at Renold Chain's factory in Bredbury, Stockport, explains. To understand the history of lubrication-free chain, he says, we have to go back at least as far as the 1950s to find the first commercially available products. "They weren't very satisfactory, though," he says, "because we didn't have the type of production processes, high-tech materials and lubricants that we have today. Even where we did have suitable materials, the processing of them was cost prohibitive and so we have had to wait for innovations in production techniques before they could be used."
Chain needs to be lubricated to prevent rapid, uncontrolled wear of the bearing surfaces between the pin and the bush, and between the bush and the roller. When these surfaces wear, the chain elongates. Excessive wear will result in the chain jumping the sprocket teeth, fluctuating drive loads and possible chain failure.
A well-lubricated chain is still one of the most efficient and reliable methods of power transmission, but lubrication incurs additional costs and there are many applications where it is either impractical or undesirable. For example, chain lubrication is undesirable in the food industry, because lubricant could contaminate the product, and it is impractical on applications where the chain is inaccessible or difficult to reach.
Until quite recently, there were many applications where companies had to run with the chain unlubricated and bear the resultant high cost of frequent chain replacement and expensive downtime, simply because a lubrication-free alternative was not available or was not cost effective. In some arduous applications, unlubricated chain might only last a period of days before needing to be replaced, so it is easy to see why the development of lubrication-free solutions has been given such a high priority.
Most lubrication-free chain is probably more accurately described as chain which does not need to be re-lubricated. The chain effectively comes with its own supply of lubricant which, in most cases, is so cleverly engineered within the chain that, to all intents and purposes, it is not there at all - certainly not as far as maintenance engineers are concerned. For the lifetime of the product, the chain will not need to be lubricated again and there is no visible lubricant on the chain to contaminate the product being produced. The lubricant is there, it is just trapped inside the chain.
Essentially, there are two categories that all lubrication-free chain falls into: those that contain a solid lubricant and those containing a liquid lubricant. Within each category, there are numerous technologies and mechanical techniques that are used.
Solid lubricants have been around for the longest period of time and include surface coatings which are applied to the chain's components during the manufacturing process. Certain coatings are particularly well suited to very high-temperature applications where an oil-based lubricant would be burned off. The problem with coatings is that, once they have worn away, the chain is effectively no different to a standard unlubricated chain.
Other types of solid lubricant include wax-like coatings that are applied by immersing the chain in vats of hot, melted lubricant and leaving it there until the chain has attained the same temperature as the melt. By this time, it will have permeated the chain and coated the inner components.
Clearly, these chains are not really lubrication free; they either contain a supply of lubricant or are treated with a coating and will provide maintenance-free operation until the lubricant is exhausted or the coating has worn away. If a way can be found to trap lubricant inside a chain for longer, then the lifetime of that chain will be increased. It's not surprising, therefore, that much investment of both time and R&D budgets is directed at finding hightech ways of doing just that.
One product which has made a significant step in this direction is Renold's Syno brand, which the company described at the time of its launch as an 'intelligent chain that lubricates itself.' In a sense, that is exactly what it does. The chain is manufactured with an oil-impregnated bush and, due to capillary action, the oil remains within the bush when the chain is not running. During operation, the oil is released on to the bearing surfaces to provide the necessary lubrication. The faster the chain runs, the more lubricant is released - almost as if the chain really is thinking for itself and providing just the right amount of lubrication, exactly where it is needed. What's more, when the chain stops running, the oil returns to the bush, rather like water being soaked up by a sponge.
This type of 'lubrication-free' chain is suited to a variety of applications. It is high-tech chain that can be used virtually anywhere and lasts hundreds of times longer than running standard chain without lubrication.
The oil-impregnated bush at the heart of the chain is a relatively recent innovation. It is manufactured by a sintering process where metal particles are suspended in lubricant and then fused together by exposing them to very high temperatures and pressures. The result is a porous steel bush that is honeycombed with lots of tiny pockets and capillaries containing the vital lubricant.
Although the sintered bush was a breakthrough in lubrication-free technology when it was launched, there are still applications where an alternative solution is required. Such applications are found in industries where frequent machine wash-downs are common, where the chain is being operated outside or on very heavy-duty applications. Exposure to extremes of temperature and wash-down solutions would eventually remove the oil from the bush, while very heavy-duty applications may cause it to fail prematurely.
To provide a solution for such applications, Renold has now launched a polymer bush chain where there is no metal-to-metal contact at the bearing surfaces and the chain operates without any lubrication at all. This recent innovation truly is lubrication-free technology. Lodge showed Plant Engineer a sample of the chain that could be dismantled to demonstrate the way in which it is constructed. A polymer sleeve fits between the pin and the bush and the chain's rollers are made of a specially designed polymer material, instead of the traditional steel. As it contains no lubricant, it is completely unaffected by machine wash-downs or when operated outdoors, and is ideally suited to heavy-duty applications.
The breakthrough here came with the development of a special polymer for the chain's rollers that would deliver the necessary performance at a fraction of the cost of a high-performance engineering polymer. As Lodge explains: "We wanted something that could deliver 90% of the performance of a high-grade engineering polymer, but at 10% of the price. This was important so that we could keep the cost of the polymer rollers in line with those of steel, otherwise the solution would not have been cost effective."
Renold, understandably, will not divulge precise details of the polymer, but Lodge says it performed exceptionally well in tests and exceeded the design team's initial expectations.
With the development of this polymer bush chain, there are few applications for which there is not an off-the-shelf, lubrication-free, solution. Of course, some situations do still defy lubrication-free technology, but they are becoming increasingly rare. The sintered bush solution is certainly a move closer to the Holy Grail of true lubrication-free chain.
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