Load cell and torque/pressure-sensor maker Futek recently had problems assembling load cells to spec.
The load cells employ a metal foil strain gauge that’s adhesively bonded to the sensor. The spec calls for 50 to 75 psi of clamping pressure during assembly. Technicians ran nondestructive tests (NDT) on production assemblies using a pressure sensing film that changes color under different loads. The procedure showed that clamping pressures ranged from 50 to upwards of 200 psi during some production runs, says Futek mechanical engineer James Meiselbach.
The pressure-sensing film called Pressurex from Sensor Products Inc. is a versatile NDT tool for quality control and machine component inspection. It is a candidate for assessing surface contact inconsistencies in virtually any industrial or electronic application, including gaskets, clamps, bolted joints, connectors, heat sinks, heat sealing elements, welding heads, and plastic and composite manufacture, among others. The film easily and rapidly shows inconsistencies in surface pressure from 2 to 43.2 kpsi (0.14 to 3,000 kg/cm2) between mating or contacting surfaces
After determining that clamping fixtures were the root cause of Futek’s assembly woes, Meiselbach used the film to monitor clamp pressures during the redesign of several clamps that now employ silicone die springs to regulate pressure more precisely. “A sample pack of Pressurex with films that revealed different surface pressure ranges allowed production to continue,” says Meiselbach.
To measure the surface pressure of a clamp around the outside of a load cell, Meiselbach cut Pressurex to match the clamp surface area, placed it between the clamp and the load cell, applied force, unclamped it, and removed the Pressurex. He then compared the film to an accompanying color calibration chart to gage pressure.
Meiselbach became aware of Pressurex when he previously worked for an aerospace company. “We were having a problem with the main rotor blade of a helicopter. Interference was causing a fatigue crack in one of the inner spar tubes of the rotor blade. We put a large sheet of Pressurex in the bonding tool, bagged it up, and pressurized it in the autoclave. When we removed the film, we were able to identify the exact amount of pressure causing the crack.”