How design came to the rescue for Silverson

17 February 2009

Design innovations are increasingly creeping into the engineering industry and when high shear mixer maker Silverson experienced sourcing difficulties it turned its attentions on completely re-thinking the lift mechanism on its laboratory scale high shear mixer.

Silverson adopted an Origa linear actuator, replacing a complex chain drive built in-house with an out-of-the-box mechatronic unit.

Silverson builds about 95 per cent of its mixers for export, a business level that has built up steadily over the 20 years the product has been in production.

Alan Pepper, chief engineer at Silverson, said: ‘Mixer buyers know what they want, and that is proven technology for what is in fact a heavy-duty application. We do put a lot of design effort into developing high shear mixer heads for particular applications - which can be as diverse as toothpaste, downhole slurry, bitumen and feedstocks for the brewing and distilling industries.

‘But they tend to want the same mixer drive and stand again and again, a design they trust from years of good experience.’

For more than 60 years Silverson has specialised in the manufacture of high shear mixers for the worldwide processing and manufacturing industries. Many of these are enormous and bespoke to particular requirements, but they are built to the same principles as the far smaller laboratory and pilot scale units.

A couple of years ago Silverson began to worry about the continued supply of the square section tube they use for the stands' main column or upright. Coupled with this was the realisation that getting them chromed, a necessity for clean room environments, would also become harder as specialist chroming seemed to be a dying art and from an environmental view is a pretty unfriendly process.

Pepper added: ‘We could have gone for a like-for-like replacement, probably sourcing from India or China, and of course we explored these avenues. The logistics would have been difficult, so we'd have had to hold larger stocks. But we decided to turn a negative into a positive and look at innovative design solutions.’

The upright acts as a housing for the lift mechanism which raises and lowers the shear head into the mixing vessel. Traditionally Silverson has used a chain drive to do this, which they built up in-house. This arrangement had a few issues that although accepted by the industry as a matter of course could be addressed to improve the overall design of the mixer. The component count was high and the build time significant. No matter what, the chain drive always gave an over run of 20-30mm, making fast accurate lowering of the shear head very difficult. And the chain was subject to wear (it works quite hard lifting and lowering a 10kg overhanging load through viscous liquids to a total of 4-5km over a 10 year working life) so had to be re-tensioned or replaced under a regular service pattern.

Pepper said: ‘The industry view was that it was ever thus and our competitor products were exactly the same.

‘But the more we thought about an innovation, the more excited we became.

‘The technical issues we were looking at were solvable; the unknowns were marketing issues - we could offer an improved mixer, but would people buy it?’

Pepper and his technical team began to review options for buying in a linear actuator to both support the mixing head and provide vertical drive. Silverson wanted a fully enclosed unit with smooth lines for hygiene purposes. It had to be capable of driving the load at 100mm/s, yet resolve the overrun issue of the chain drive. The design life of the actuator had to match or better the 10 years of the Silverson mixer.

There were several products that fitted the bill, so stricter criteria were introduced in order to derive a shortlist. One of these was that the unit should have integral limit switches so that there were no trailing leads or cable management requirements. This highlighted Parker-Origa's OSP-E range as the primary candidate and the company was called in to make a presentation.

The OSP-E is available in a range of sizes and with a choice of various screw and toothed belt drive options. A trapezoidal screw drive was identified as best suited to the Silverson application, a size 32 unit could satisfy all of Silverson's technical requirements, including that the drive was not back-drivable. (The sister product OSPP is a rodless pneumatic cylinder, ideal for many jobs where compressed air is available). It is designed to be an out-of-the-box solution that incorporates all necessary parts, and to have considerable static and dynamic load bearing capabilities.

‘There was a slight issue on the overall shape of the actuator, a minor detail that I was quite prepared to design around,’ said Alan. ‘But Parker-Origa instantly said that they would design a bespoke extrusion profile for us and have samples ready within a couple of week. Sure enough the first samples were on time and exactly met our spec.’

Silverson put several units through extensive test programmes, carrying 20kg loads at 200mm/sec (double the specification) for 24 hours a day. The laboratory mixer requires capability for 4km of linear motion if it is to reach its 10 year design life. The Origa actuators reached 18km with no sign of wear, at which point the test was discontinued.

For Silverson the OSPE is coupled with a reversing motor from Oriental, with the limit switches flicking it into reverse at the end of stroke to act as a brake and minimise overrun.

As for the marketing issues, Silverson took a slow but steady approach. First of all it showed prototypes to all its customers, then introduced the new version one country at a time to the metric world. It is currently introducing an imperial version to America (Silverson's largest market) and Pacific Rim countries. The US version has a 1hp motor, three times the size of that for Europe, but this says Pepper is a cultural requirement rather than a technical necessity.

‘We're trialling an RS232 port so that intelligent devices can be connected in, are developing more sophisticated electronics and readouts and have a string of other ideas to show to customers.’

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