Self-Tightening Grips for Elastic Materials

A test run in these grips gives the values rubber and plastic specifications ask for: tensile strength, elongation at break and, for elastomers, the stress at a set elongation, which the rubber industry calls modulus. The grip's job is to carry the specimen to the break without letting it slip or tearing it at the jaw.

Tensile strength is the break force divided by the original cross-section of the narrow part of the specimen, so width and thickness are measured before the specimen goes into the grips. Soft material compresses under a micrometer anvil, which is why rubber thickness is read with a light, controlled foot pressure.

Elongation is best taken from gauge marks or an extensometer on the narrow section rather than from crosshead travel, because the wide ends of a specimen also stretch a little near the jaws. The grips mount on NextGen frames, including our electromechanical testing machines for rubber.

The load and elongation curve shows it first. A slip appears as a sudden drop in force, or as a sawtooth, followed by a climb back as the jaws catch the specimen again, while a clean test on rubber or thin plastic rises smoothly to the break.

The specimen shows it afterwards. Drag marks on the tabs, or tabs that sit further out of the jaws than they went in, mean the material moved under the grip. When elongation is taken from crosshead travel, that movement is counted as stretch, so the reported elongation reads high even when the curve looks normal.

The self-tightening grips are built to cut slippage without the operator adjusting anything mid-test, and without the jaw pressure that damages a soft specimen. For thin film and sheet, our eccentric roller grips work on a related principle, with an eccentric cam roller that grips harder as the load rises.

Seat it straight, centered and to the same depth in both jaws. The upper and lower grips each take one end, and the specimen's axis should run along the line between the two pin connections, so the load passes through the narrow section without bending it.

On a dumbbell, the width limit of each grip applies to the wide tab that goes into the jaw, not to the narrow section in the middle. Clamp the tab up to the shoulder where the specimen starts to narrow, but not onto it, so the break happens in the gauge length and not at the jaw edge.

Rubber dumbbells are normally die-cut, and a clean edge matters as much as the grip: a nick left by a dull die starts a tear long before the material reaches its real strength. For cutting rubber tensile specimens we supply the NG T-Press M Series manual cutting press.

All of them turn the pull on the specimen into clamping force. The self-tightening family comes in scissor grips, cam-action grips and lever-actuated jaws, all built to mount on NextGen universal testing machines, and each design closes harder on the specimen as the tensile load rises.

In a scissor grip, two crossed arms pivot so that the pull on the specimen draws the jaw faces together. A cam-action grip holds the specimen against an eccentric cam that turns with the pull and narrows the gap. A lever-actuated jaw uses a lever to multiply the pull into side pressure on the faces.

The difference between them lies in how the force builds up at the jaw face, and the right one depends on how soft the specimen is and how slippery its surface is. We confirm the design together with the model and your specimen sizes when the grips are configured for your program.

Plan for a body width of 4.1″ (104 mm) on the 2 kN NGA203A and 5.9″ (150 mm) on the 5 kN NG5203A. That is the dimension to check against the clear space between the columns of a two-column frame, with room left on each side for the operator's hands while a specimen is loaded.

The body is wider than the specimen it holds, because it carries the clamping mechanism around the jaw. The NG5203A is built with a heavier clamp and a wider jaw than the compact NGA203A, which is where its extra body width comes from.

Check the space around the grips as well as between them. An extensometer, a specimen guard or a light curtain close to the load line needs clearance from the grip body, and on a frame shared with other work the wider grip leaves less room for accessories that stay mounted.

Choose the load cell by the force at which your specimens break, not by the grip rating. The grips set the upper limit of the setup, 2 kN on the NGA203A and 5 kN on the NG5203A, but many soft rubbers, foams and films break at a small fraction of that.

A load cell measures most accurately inside its verified range, which starts at a set fraction of its capacity. A cell sized for heavy work can read a light elastomer test at the bottom of that range, where the force resolution is too coarse to show the detail of the curve.

On a frame that runs both light and heavy tests, a lower-capacity cell for the soft materials is the usual answer. Its capacity should still sit above the highest break force you expect, with a margin, because a cell rated below that force can be overloaded before the specimen fails.

A specimen needs the larger grip when it is wider than 30 mm, thicker than 10 mm or expected to break above 2 kN. The NGA203A takes specimens up to 10 mm thick and 30 mm wide at 2 kN, while the 5 kN grip takes up to 12 mm thick and 45 mm wide at 5 kN.

Size decides more often than force, so check the widest specimen in your program before you look at capacity. A wide rubber strip that breaks well under 2 kN still needs the larger grip.

The NGA203A is the smaller and lighter grip, 75 mm high and 0.49 kg per half, against 137 mm and 1.5 kg for the 5 kN grip. Both mount through the same Ø10 mm pin and share the same ambient working range, so they are interchangeable on one frame.

Put the material, the specimen width and thickness and the expected break force into a quote request, and our technical sales team will come back with the grip that fits.

Their clamping pressure rises by itself as tensile force is applied. An elastomer or a thin plastic gets narrower and thinner as it elongates, so a fixed jaw set at the start slowly loosens around it. A self-tightening grip follows that change, and the hold grows with the load.

A fixed jaw forces a compromise on soft or low-friction material. Set lightly, it lets the specimen slip out, and set tight enough to hold at peak load, it crushes the specimen before the test has begun.

No one has to stop the test to retighten, and alignment and gripping force stay steady from the first load to the break. Specimens from one batch are therefore loaded the same way, which is what makes their results comparable.

Yes. They are built for deformable and elastic materials: elastomers and rubber, foams, thin and soft plastics, textiles and biomedical specimens. Soft elastomers and foams compress under clamping pressure, while films and many textiles have smooth surfaces with little friction, and the grips are made for both cases.

Both models are specified for tension tests on plastic and rubber. They suit quality control, where the same test has to give the same answer on every lot, and research, where the full load and elongation curve is the result.

They are built to mount on NextGen universal testing machines, including our electromechanical testing machines for plastics, the frames on which rubber and plastic tensile tests usually run.