Wedge Grips
Wedge grips consist of a solid body with interchangeable jaw inserts or fixed jaws, as well as an operating unit for opening and closing the grip.
Two different operating principles are available for wedge grips:
Lever principle
Wedge grips operating on this principle are self-clamping grips. On clamping a pre-loading force is applied to the specimen via operation of the lever combined with a spring. However, it is only during the tensile loading that the main gripping force is attained via the wedge effect. The gripping pressure maintains a constant relationship to the tensile force through displacement of the wedges. Grips of this type have the advantage that testing can be performed very quickly and requires little physical effort from the operator.
’Body over wedge’ principle
Wedge grips operating on this principle apply a greater pre-loading force on clamping. The pre-load on the specimen is transmitted by a screw drive via the body to the jaws. In this case also the main gripping force is not attained until during the tensile loading (via the wedge effect). Grips of this type have two main advantages: specimens inclined to slippage are gripped more firmly right from the beginning; the clamping length remains constant, even if thickness varies from specimen to specimen. In addition, strain-rate control to ISO 6892-1 (2009) Method A (closed loop) and ASTM E 8 (2009) Method B can be applied with greater precision.
General advantages of both operating principles:
- wedge grips are ideally suited to specimens prone to shrinkage, as they automatically re-tighten, compensating for decreasing specimen thickness
- wedge grips close symmetrically, automatically positioning the specimen in the tensile axis and eliminating the need for thickness adjustment
- large clamping length and good jaw guiding properties allow contact pressure on the specimen to be kept low
- high temperature resistance and low overall height make them ideal for use in temperature chambers.
