Clutches provide "clean" tension control

Edited by the Engineeringtalk editorial team Apr 17, 2002

The combination of high levels of tension control and noncontact zero-contamination operation provided by ZF's range of hysteresis brakes and clutches has proved to be just right for Maersk Medical.

The combination of high levels of tension control and noncontact zero-contamination operation provided by ZF's range of hysteresis brakes and clutches has proved to be just right for Maersk Medical at Stroud, on a new special purpose machine for producing multilayer intravenous dressings.

Maersk Medical is a global company specialising in the development, manufacture, and marketing of sterile single-use medical devices for hospitals and the health sector.

The new special purpose machine is assisting this development, providing high quality, reliable products, which, by being faster, easier, safer and more effective to use, help to get people out of hospital faster.

Designed and built by Danish company TriTech, the multilayer dressing machine is used in a clean room under sterile conditions.

Its operation consists basically of taking three rolls of material and combining them under accurate web tension control to produce final laminated NikoGuard intravenous dressings.

If at any stage during the laminating process the tension in any of the three rolls varies outside set parameters - especially during the cutting operation, which includes three cutters operating in a timed sequence, then the dressings will be unusable.

The machine employs a total of seven of ZF's hysteresis clutches and brakes on winding and unwinding operations during the manufacturing process.

The ZF clutches were preferred for the application because, unlike powder clutches, their unique operation leaves no possibility for contamination with wear residue.

"The ZF clutch is one of the few clutches that doesn't work on the powder clutch principle", said David Dowling, Maersk's Engineering Supervisor.

"This is important for us because working in a clean room environment we cannot take the risk of a seal failing on a powder clutch unit, resulting in contamination of otherwise highly sterile medical products".

The possibility of product spoilage through contamination does not occur with the ZF hysteresis units because, uniquely, they transmit torque via a magnetic flux.

Consequently, there is no physical contact and none of the usual disadvantages resulting from friction.

There is also no fluctuation in friction levels due to external influences, no stick-slip effect and no wear during slip operation.

This reduction in friction and wear right across the board with the hysteresis products means that in many instances users can expect an operating life much in excess of alternative systems.

"This application is the latest of an increasing number to realise the non contact wear free and precision benefits of using hysteresis clutches and brakes", commented ZF's Account Manager, Rob Pearson.

"The products are the ideal OEM components for modern machine builders.

In this application they were used in preference to powder clutches, but in many applications they are fast becoming the favoured alternative to servos.

What both OEMs and users are realising in increasing numbers is that whilst a servo system is almost always the first choice for precision tension control applications, it may not always be the best choice in terms of cost and complexity and especially not if some alternative proprietary method of achieving the desired mode of control is available".