Untangling rubber reasoning

Specifying the right compound for rubber components is a deceptively tricky task that leads many engineers out of their comfort zone. Justin Cunningham reports.

Unlike their polymer cousins, elastomers such as rubber are almost exclusively used for their mechanical properties rather than for aesthetic reasons. But rubber is a complex material that is often misunderstood, both in terms of the material science behind its unique properties and in applying it as an engineering material.

As a result, rubber selection is outside the comfort zone of many design engineers, who are often more au fait with rigid materials such as metal and plastic.

Even the experts can have problems getting it right. For example, Pirelli has recently been criticised for engineering too much sudden degradation into its Formula One tyres. Although Pirelli says it has specifically engineered its tyres act in the way they do, the tyres look much more degraded, and seem to 'go off' much more suddenly than in previous years. The point is, if the experts can find it difficult to it exactly right, what chance is there for the rest of us?

Elastomers frequently find applications as seals, dampeners and belts on all sorts equipment in almost every industry. There are potentially hundreds to choose from that can be formulated and compounded for very specific properties.
Specifying the best compound for applications can be deceptively tricky and is often akin to cooking a complicated recipe: best left to the experts.

As with any material, there is a trade off and limitations in terms of what is possible. Ultimately, though, it is about accurately predicting the in-service behaviour, and ultimately failure, of a component so it can be managed. This comes down to specifying the right material at the early stages of the design process.

Principal consultant at rubber and plastic consultancy Smithers Rapra Gary Crutchley says: "You need to understand your application in detail, the operating environment and the temperatures that you are likely to encounter. Consider also periods, no matter how short, where the material might be exposed to extremes outside the normal temperature range. Also the stresses and strains that the application will apply to the material, both initially and long term."

The physical interactions on the material from the initial stresses and strains to the longer term ones are vital. Rubber can be in use for decades. Bridge bearings, for example, can have a service lifetime of up to 80 or 100 years.

"Very often engineers rely on the recommendation by a manufacturer of a particular material to decide on the most suitable material," says Crutchley. "That is not necessarily a bad thing, as the manufacturer would have an excellent understanding of the properties of the materials they are producing. But it can restrict you sometimes in terms of the number of materials that might be available."

There are two main reasons why rubber components fail prematurely: human causes and the operating environment. Premature failures are the ultimate nightmare for any firm, being damaging to reputation and potentially resulting in extremely costly recalls.

Material mis-selection is the biggest cause of failure that Crutchley and his team see, accounting for almost half of all failures. Product design is another area where, although an adequate material has been selected, the design of the component is causing it to fail. For example there could be sharp edges, or stress may be concentrated in one area. Rapra recommends carrying out lifetime analysis to understand how the material and component will perform over its service lifetime.

"The interesting thing about the human causes of failure is that material mis-selection, poor product design and poor material processing account for up to 85% of the total human causes of failure of products," says Crutchley. "So those are causes of failure that you introduce before the product has even gone in to service."

Degradation in rubber properties is usually the result of its environment and this can have an adverse effect on strength, colour, or shape, which can eventually lead to a component failing. Heat, light and chemicals can cause the rubber to perish and cause it to degrade and possibly crack.

However, physical interactions that cause abrasion are also important considerations. "Tyres on a car will abrade as they run in contact with the road," says Crutchley. "But we can use that abrasion as it helps with frictional properties of the material for better grip. So there is a trade off to be made there."

For sealing applications, stress relaxation and set are important parameters to consider. If an elastomer is deformed and held under compression for a period of time, the material will not recover to its original dimensions when it is released. That permanent deformation is known as 'set'.

If the level of set is too high, this can result in a reduction of sealing force. While under stress, an elastomeric material will tend to relax in order to relieve the applied stress, this stress relaxation can reduce sealing force and is as important as set in sealing applications.

"We use elastomer materials for their elastic properties, but have to balance this with their viscous flow properties, which can lead to stress relaxation and set," says Crutchley. "But you will see recommendations in certain sealing applications where the seal itself can only be used once and this is why. Hence, selection of the right elastomeric material for an engineering application can be hugely advantageous, but to get this process wrong is to engineer failure into your product at the outset."

Justin Cunningham

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