Looking After Your Grips

Any successful gripping solution can be adversely affected by poor maintenance. Many common gripping techniques rely on friction or local surface deformation of the specimen to function. If the gripping surfaces become worn or contaminated, a loss in gripping efficiency occurs. Ultimately, this causes the specimen to slip, leading to an invalid test.

The first response to slippage is often to increase the gripping force by over-tightening the mechanical grips or to increase the pressure of pneumatic or hydraulic grips. Although this may temporarily solve the slippage, it can also bring new problems:

• Simple screw action grips are often tightened with a spanner or wrench. It's very easy to exert high torque loadings to the load cell unless care is taken. Excessive tightening can easily damage low force load cells. Taken to extremes, it's possible to damage the grips themselves. Using a small torque wrench will allow you to achieve consistent gripping force.
• Increasing the pressure applied to the specimen by the gripping system can also increase the chances of influencing the mechanical properties. This is especially true of materials that are weaker in compression than in tension. We find that increases in jaw breaks often accompany increases in the gripping force.

Poor maintenance can result in uneven or inconsistent gripping. High-frictional losses in screw grips reduce the clamping force on the specimen for a given tightening force. Friction effects in wedge grips can induce bending if the faces move unevenly.

Here are some golden rules for reliable gripping:

• Regularly clean and lubricate moving parts with the correct grades of lubricant as advised by the manufacturer. This is especially important for wedge action grips, which rely on the smooth sliding of the jaw faces along an inclined plane for their correct function.
• Periodically inspect the grips for defects, such as cracks or leaks in hoses.
• Periodically verify that the pressure gauges are accurately registering air or oil pressure to the gripping system.
• Replace jaw faces when the surfaces become worn, damaged, or contaminated. Some jaw faces, such as rubber-coated types, can degrade over time simply due to exposure to air and light. This degradation can accelerate if the jaw faces are used under non-ambient conditions in an environmental chamber.
• Do not use more gripping force than necessary to provide reliable, slip-free gripping.
• Old grips don't necessarily work with new materials or specimens. You may find that special grips or different jaw face surfaces are needed. You can try a variety of things to modify existing gripping methods including emery cloths, sticky tape, etc.