SIMPLE TENSILE WEDGE
GRIPS
As also noted in Section E-1, there
are two basic types of mechanical wedge grips. Either the wedges move
as they tighten on the specimen and the bodies of the grips remain
stationary, or the grip bodies move and the wedges remain stationary
while being tightened on the specimen. In Wyoming Test Fixtures, Inc.
terminology, the former are termed “simple grips” and the latter are
termed “standard grips“.
The “simple”
wedge grips are easier to fabricate and thus are less expensive. The
“standard” wedge grips, because the wedges do not move axially relative
to the specimen during clamping, do not have the potential of putting
the tensile specimen in compression, possibly damaging it as they are
tightened. But of course, with most specimen materials this may not be
a damaging effect. And when using simple wedge grips it is possible to
disconnect the one grip from the testing machine while engaging the
specimen, thus avoiding the potential problem.
An example of a simple wedge grip is
shown in the following photograph. Note that a screw on each side of
each grip, pushing on a plate against the tops of the wedge grip
inserts, is used to tighten the grip onto the specimen prior to the
test.
10,000 lb. Capacity Simple Wedge Grip
1” Wide Flat Specimen, 1.5” Grip Length
Of course, once a loading is applied to
the test specimen, the wedging action takes over, to grip the specimen
in proportion to the magnitude of the loading being applied, just as for
a standard grip. The springs retract the grip inserts when opening the
grips.
The photograph below shows a similar
simple grip, but with the tightening screws in the top of the grip,
another option. Although the screws cannot be seen in this photograph,
the push plate can. Note that this grip has vee-grip inserts mounted in
it, for gripping round specimens. All types of grip inserts can be
provided (see Section F-3).
15,000 lb. Capacity Simple Wedge Grips
Thermal Sprayed Vee-Grips for Round Specimens
The next photograph is
another example of simple grips with top screws and pusher plates.
These grips are for wide specimens such as geotextiles tested per ASTM D
4595.
Four-Inch Wide Tensile Wedge Grips, 1000 lb Capacity
Sometimes space is very
limited, such as when performing tensile testing at cryogenic
temperatures in a dewar. The next photograph shows 20,000 lb capacity
compact wedge grips fabricated of 310 stainless steel.
Compact Tensile Wedge Grips, 310 Stainless Steel, 20,000 lb. Capacity at Cryogenic Temperatures
A very attractive version
of simple wedge grips is that shown in the following photograph. Rather
than using screws, at either the top or at the sides of the grip body,
to push the wedges against the test specimen, these grips have a hand
wheel to tighten the grips.
Simple Wedge Grips with Hand Wheels for Tightening Wedges
Grips Shown are 5,000 lb Capacity, 1” Wide
