Smooth ride with Polybush
An insight into a leading manufacturer of suspension bushes
One of the keys to providing a smooth ride for passengers is a good set of suspension bushes. The Polybush brand is renowned in the bus sector, coach sector and beyond for providing them. Not only is there a good chance your fleet has their products fitted, your car may have them too.
Chris Peat visited the Wrexham based manufacturer of these polyurethane bushes to get an insight into the company.
Over three decades
Having the status of a well known brand does not come easily, it requires a lot of expertise and experience and this is certainly not something Polybush is short of. The company has been manufacturing polyurethane bushes for 34 years in its purpose built factory, 22 of those under its own brand name. The business is the suspension division of Bonaprene Products Ltd. It started as an OE manufacturer, with no particular brand name before moving into the aftermarket, giving birth to its current moniker. The story goes that the founder of the business, Roy Hitching, decided to use polyurethane (PU) for bushes after his dog chewed a piece of the material. It was so unscathed by this canine’s chomping that he thought it would be an ideal substance from which to create bushes.
It turned out he was right and once the company went into production one of its first customers was Land Rover. Over the years, the range it supplies has expanded dramatically, rolling out into various markets and vehicle types. Among those it supplies bushes for are Bentley, JCB, Mercedes-Benz and various other big brands.
Sales Manager, Hayley Smith, described the company as a ‘problem solver’. She said this was the case when the company now known as ADL approached them. The bus builder sought them out to supply its aftermarket parts, a task it was able to fulfil. The relationship between the manufacturers progressed and Polybush became the OE supplier of the components. Over the years it has become a number of other bus and coach builders’ preferred choice for the provision of aftermarket and OE suspension bushes.
Hayley said, ‘There’s probably not a lot of buses out there that do not have our bushes on them.’
As well as bushes for suspension systems, Polybush provide them for other applications. For instance, in conjunction with Volvo Bus, it has jointly developed an Alternator Table Mount for the B7TL, which significantly reduces vibration for the rear seat passengers.
Here’s the science
The polyurethane used to make the bushes is a strong, durable substance. However, the material is not the only reason behind various companies’ choice of Polybush as their aftermarket and OE equipment supplier. Hayley explained the quality is also down to the method used to mould them.
There are three commonly used methods to manufacture PU bushes: injection moulding, dispensed casting without cure and dispensed casting with cure (the method used by Polybush). Each of these techniques makes for a different class of product.
Injection moulding PU bushes is considered a cheap and low grade method. To do this, the PU must be created in such a way that it will melt when heated. This allows a manufacturer to purchase inexpensive pre-made PU in granular form, melt it down and inject it into moulds under high pressure. An automated process is most often used to do this, meaning a high production rate. Also, as the substance can be melted down, any waste can be put back into the machines and recycled.
However, according to Polybush, bushes made this way have a relatively low lifespan and limited strength. The reason being that to make a PU that can be melted means it has to have comparatively short carbon chains and weak electrostatic forces holding the material together. Although this makes it easier to mould, it also makes it susceptible to compression set, meaning it no longer returns to its original uncompressed shape after bearing a load. This detracts from the product’s capability to dampen impact. The limited forces holding the material together also mean it can be easily damaged under compression compared to PU bushes moulded in other ways. According to Polybush, an injection moulded PU bush will last no longer, and in some cases even less time, than an equivalent rubber bush.
The dispensed cast without a cure method increases the product’s strength and resistance to compression set. To achieve this, the method ensures the PU is manufactured with long chain molecules and ‘clustering’, where the electro static forces holding the chains together are immensely strong. However, this chemical structure means it is not suitable to be injection moulded and thus must be dispense casted. This process starts by a dispenser measuring out the right ratio of ingredients and mixing them. The liquid mixture is then transferred to open topped moulds. The chemicals will then react, forming long chain molecules, transforming from a liquid into a solid. According to Polybush, the technique gives the product ‘excellent’ performance, making it ‘very resistant’ to compression set.
This process can be improved further by curing the casting after it has been de-moulded. This involves heating the PU for a specific time at the right temperature. The benefit of curing it this way is it raises the extent of molecular bonding within the material, allowing it to withstand even larger loads. Polybush claims a bush manufactured this way can last four or five times longer than a rubber or injection moulded equivalent.
The trouble is, according to Polybush, it is hard to tell just by looking and feeling what method is used to cast any one PU bush. According to the company’s marketing material, comparing the products of the three moulding techniques is like comparing graphite, coal and diamond. They are all made from carbon crystals but are very different materials with differing properties. All of the company’s products are made using the dispensed cast with cure method, helping to ensure high quality. Also, where applicable, its bushes are fitted with mild steel zinc plated inserts, as used on OE bushes, which prevent galvanic corrosion and wear to mating parts.
Moulded for quality
Polybush’s moulding method requires a lot of manual work, which Hayley said makes it seem a little old school compared to the automated factory processes which we are used to seeing and hearing about with manufacturers of this size. This is not to say it is behind the times, the technology used is highly specified, from the mixing equipment to the ovens for curing them. Even the benches used in their manufacture are specially heated to ensure the products cure properly. Hayley explained that the process also needs a considerable amount of skill and know how, with years of training involved before their technicians are considered fully fledged moulders. She likened it to baking a cake while pouring a pint, with the correct mix and the right temperatures required to ensure the optimum outcome. It takes a lot of experience, skill and precision to get right.
On a tour of the factory, I was given a run through of the manufacturing process. The first stage of which involves selecting the correct mix for each product, using a computer controlled machine that can be programmed with the correct blend of polyurethane for whichever application it is to be used. This is then dispensed and poured by hand into open cast moulds. According to Hayley, even though it may not seem so to the casual observer, the pouring is a tricky thing to get right, though the experienced technician I saw made it look easy.
Once in the mould, the bushes sit on the heated bench for a while to settle. Once ready (and knowing when they are ready is something else learnt with experience) they are then put into an industrial oven for curing over night.
Once out of the oven, they are then subject to a quality test. Hayley explained that the slightest of imperfections mean the product is destined for the reject bin. The final stage of their manufacture sees the removal of any flash, the flaky bits of polyurethane clinging to the edges of the bush as a result of the moulding process (anyone familiar with Airfix models may well have encountered this). Again, this is done by hand using a lathe-like piece of machinery.
Having such an intense focus on quality, with the production process mentioned above, means Polybush’s bushes come at a premium price. Even though the initial cost is slightly more than other brands, Hayley said the cost savings over the longer run more than make up for it, with the products lasting at least four or five times longer than cheaper equivalents on the market. Being longer lasting also means the vehicle will spend less time off the road due to the bushes needing to be changed, which effectively leads to further savings. Hayley said, ‘I’m confident that when they have tried our products, customers will be back for more.’
To bring to life the difference between its own and other company’s products, Hayley showed a Polybush bush next to a competitor’s. She said they had both been fitted in the same position on CVs under regular use for six months. The difference was obvious, with the other company’s product appearing much more worn and warped than Polybush’s.
Not only do Polybush’s products last longer, they also provide improved control as a result of what the company’s marketing material describes as ‘consistent steering geometry’. It said its products lead to responsive steering, better road handling and due to the correct suspension alignment, will increase tyre life.
Polybush’s products are designed to be installed with normal workshop tools and equipment. They are intended to be fitted the same way as OE bushes and should not require any grease to fit.
All in the UK
Like the manufacturing process, all of Polybush’s research and development takes place at its Wrexham headquarters. One of the latest developments it is working on is a high temperature resistant, climate control system mount. The product is to be fitted as OE by a third party, but produced by Polybush. To make this, it will be working with high temperature materials that Hayley said are unique to them. Although this new product is for OE, the company still does a lot for the aftermarket. Hayley said the split between the amount it does for the two sectors is probably 50/50.
Demand for the products is persistently high, with Hayley saying the company is ‘constantly chased all the time’ on account of its reputation. When an operator comes to the company with a request for a certain part, if they have it available the business will point them in the direction of a distributor. If a customer requests a specially made item, then Polybush usually manufactures them in batches of 100 plus. This tends to give a commercially viable price to the end product. However, on request, the factory can produce a single bush, but Hayley said these do come at a high price. Before releasing any product on the market, Hayley said, ‘Everything is trialled and tested, working closely with garages and manufacturers to ensure that they meet our standards.’
As well as providing a high quality product, the company also aims to provide good technical support. ‘We always deal with customers as best we can. If anything, you could almost say we’re too nice, we spend a lot of time dealing with the little things because it makes all the difference. We’re far more patient than our competitors,’ said Hayley.