Non-contact torque sensor assesses ferrofluid seals

A non-contact torque monitoring system that could guarantee an infinite lightness of touch has proven to be the only way to test the seals of super-high-performance vacuum systems.

The ultimate fields of precision manufacture, such as electronics, biophysics and thin-film deposition, where tolerances are measured in atoms, are often conducted in hard vacuum to remove airborne contaminants and avoid the performance-reducing effects of tiny air movements.

However, the vast majority of vacuum chamber designs require seals for rotary drive shafts and feedthroughs for the passage of materials, components, tools and finished products. In high-vacuum applications, conventional seals are unlikely to be able to achieve the performance specifications required, so magnetic fluid seals are used.

Ferrofluidic seals

A ferrofluid is a stable colloidal suspension of sub-domain magnetic nano particles in a liquid carrier. The particles, which have an average size of about 100Angstroms (10nm), are coated with a stabilising dispersing agent (surfactant) that prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid.

With over thirty years of experience producing seals for the world's most demanding applications, Ferrotec of Woolwich, London, is able to optimise ferrofluid materials for the most intense performance requirements and incorporate them into bespoke vacuum system designs.

Jeff Lewcock of Ferrotec comments: "About half of our work is to bespoke design, and we have to test every seal to the 'nth degree' to meet our customers' needs. Basically we build the seal, mount it on a bearing of known - and very very low - drag, and use it to seal a test chamber that we fill with helium. Over the course of a prolonged test, we monitor for helium loss through the seal."

Starting and running torques

As part of the test, the starting and running torques of the seal are measured and the power loss through the seal is calculated. These readings allow Ferrotec to troubleshoot the individual seal, analyse the design's performance and add to the ferrofluids knowledge bank.

Lewcock says: "With the sort of tolerances we work to, a torque sensor that touches the seal adds far too much drag to the seal, so we were delighted to discover Sensor Technology's non-contact Torqsense."

With Torqsense, a tiny piezo comb is glued to the seal and supplied with power via a radio signal. The comb 'opens up' as the seal rotates faster, changing its electrical resistance. Thus, the frequency of the returning radio signal is an indication of the torque within the seal.

Because it is non-contact, the Torqsense exerts no drag on the testpiece whatsoever, making it highly suitable for Ferrotec's needs.

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