News articles

STRAIN GAUGES GO DOWN THE TUBES!
How do you measure the circumferential stress at three points inside a tube while it’s being crushed?

That was the problem posed to PCM by an internatational oil company. The tube bore was 2.3 inches and the length 12 inches. To add to the problem, the test temperature had to be 600°C over a long period.

As we had not carried out such an installation before we built a prototype installation and produced special tooling to weld the three strain gauges in place. Once we had proved it would work and the customer had approved the prototype, we were able to go into production confident that the procedure would be successful.

Mineral insulated cable and weldable strain gauges were preformed and assembled before installation in the tube. Then, when the fit was correct, they were permanently positioned.

The system went through several cyclic temperature runs from ambient to 625°C to maximise repeatability and minimise hysteresis, providing valuable data and giving our customer confidence about setting up the test.

GOING ROUND IN CIRCLES
Measuring stress levels in circular components calls for careful strain gauge selection. But when a Wolverhampton based areospace company approached us with their problem there were a couple of added difficulties.

First, the component, a hinge joint ring, would be subject to high levels of stress, which made the choice of strain gauge even more critical. And second, the design of the ring meant that the overall height of the gauge, its interbridge wiring and the necessary coatings could be no more than 2mm.

To make matters worse, the entire assembly would be submerged in thick hydraulic oil for the whole test term of three years. Several rings at a time were to be gauged for a variety of tests including fatigue life and failure.

Finally, we had a very tight deadline to work to. But we turned the project round within seven working days, to the delight of customer, and it has proved completely reliable in use, meeting all their expectations.

HIGH TEMPERATURE, SHARP SHOCK
Imagine the effects on an aluminium block heated to 475°C then plunged into cold water.

That’s exactly what the University of Limerick wanted to measure for a heat treatment quenching process, and they approached us to see if we could help. Because of the combination of the material and the high temperature, we had to use a Wheatstone bridge installation made from Hoskins material and bonded in place using a ceramic cement.

Our concern was that the sudden thermal shock to the hot aluminium being dipped into cold water might crack the cement and make the test void. So we carried out a feasibility study and a series of inhouse tests to make sure that the service we offered would satisfy the university’s needs.