Universal Parallel Wedge Grips -TJ15

Wedge grips are mainly designed for testing of metals, plastic dog-bone shape samples as well as wooden specimens with soft to mid-hard surfaces. Wedge grips possess a solid body with interchangeable jaw inserts with a mechanism for opening and closing of the grip.
The wedge grips come with a pre-tensioned spring, which generates the pre-gripping force. The main gripping force will be applied through the wedge action. The gripping pressure adjusts itself using a moving wedge in a constant ratio to the tensile strength.

Description:

The manual wedge action grip is designed for easy specimen loading, alignment and positioning. After initial face to specimen contact, gripping force will increase as the testing load increases. Virtually no pre-load is seen during specimen clamping due to Dak’ s design of moving grip bodies. This makes them particularly suitable for testing high strength materials, such as metals and composites, ensuring that the specimen slippage is eliminated.
There are two basic types of mechanical wedge grips. Either the grip bodies move and the wedges remain stationary while being tightened on the specimen, or the wedges move as they tighten on the specimen and the bodies of the grips remain stationary. In Dak’s phraseology, the former are known as the “standard grips” and the latter the “simple grips”
In “standard” wedge grips, the wedges do not move axially relative to the specimen during clamping, and they do not have the potential of putting the tensile specimen in compression, possibly damaging it as they are tightened. This is obviously a more desirable condition in general. However, the “simple” wedge grips are easier to fabricate and thus are less expensive.

Principle of Operation

The wedge action principle of these grips allows them to be tightened onto a specimen without altering the vertical position of the faces in relation to the specimen. This is accomplished by a design, which moves the grip body to close the faces. This feature makes it possible to pre-select the exact point at which the specimen will be held with consistent gage length and no compressive force applied which may cause specimen buckling.

A strong clamping force can be applied to materials that are difficult to hold through a high mechanical advantage achieved by the tightening mechanism. The grip faces are spring loaded against a shoe which is in a fixed position in respect to the testing frame holding mechanism, only a light initial gripping force needs to be applied to the specimen. As the grip is tightened, it’s frame is drawn up and the inclined sides push against the matching side of the faces which move laterally against the specimen. A shoe incorporated to the driving shaft prevents the faces from moving.

The open front design of the grip frames allows faces to be easily interchanged as well as easy specimen loading.
Because the faces are held in fixed position, there is no recoil, or loosening, when the specimen ruptures and if an extensometer is attached, it will remain in place.