Specimen Grips and Test Tools for Materials Testing

With the pneumatic specimen grip, emphasis is on fast, reproducible and, most importantly, parallel gripping. The full clamping force is available right from the start, so that the stationary jaws transfer the test speed directly to the specimen. With pneumatic grips from the body over wedge portfolio, the position of the jaws remains constant in the test direction as the specimen is gripped. This means that the grip-to-grip separation does not depend on the specimen thickness.

Simple extensions are used in combination with temperature chambers The pneumatic actuator is positioned outside the chamber.

Advantages of body over wedge pneumatic grips:

• Whether you are testing sensitive specimens or specimens with short clamping lengths, these symmetrically designed grips guarantee optimum clamping.
• When determining the exact nominal strain the grip-to-grip separation does not depend on specimen diameter. Setup time is minimized.
• Reduced weight plus a compact design simplify handling, ensure short heating and cooling times in the temperature chamber. and enable long travel.
• Absolutely secure gripping right from the start of the test for specimens prone to light slippage.

Hydraulic grips are primarily used when test forces of 50 kN or more and high gripping forces are required (Fmax of 10 kN to 2,000 kN possible). The contact pressure is accurately reproducible.

Operating principle of hydraulic grips

The oil pressure is set at the hydraulic power pack according to the required closing pressure for the specimen grips. The control unit controls the closing and opening procedure separately for the top and bottom specimen grips.

Furthermore, the closing procedure is divided into a closing phase and gripping phase when a a universal hydraulic power pack is used. In the closing phase, the specimen is initially held on the upper end for safety reasons. During gripping, the full gripping force is applied. The force value is based on the expected tensile force.

Double-sided actuators
All hydraulic grips are equipped with double-sided actuators. This means the specimen grip opens hydraulically and not by means of spring return mechanism. The closing and opening pressure can be adjusted.

In contrast to single-acting actuators that open by means of a spring return mechanism, double-sided actuators ensure that the specimen grips always open safely . This also applies if the specimen is crooked or jammed. Additionally, the full opening stroke is not necessary because the opening procedure can be interrupted at any point using the Stop button. The opening stroke can be adjusted according to thickness. Especially with thin specimens, the test time is reduced and the risk of pinching parts of the body is minimized.

Single actuator hydraulic grips can be used for symmetrical and asymmetrical gripping of specimens (for example, for shear tests). The thickness of the specimen is adjusted using a spindle unit.

The single-actuator hydraulic grips are also available with a tandem cylinder to enable uniform force application to the specimen (especially suitable for tests on fabrics, textiles and geotextiles).

Advantages and features of single actuator hydraulic grips

• Hydraulic grips have an eccentric piston shape with a long piston stroke, achieving a deep clamping surface position and a large opening width.
• The alignment of the jaws, specimen grips, and piston facilitates gripping of short specimens as well.
• Defined force application ensures optimum gripping of sensitive materials.
• Exactly reproducible contact force.
• Specimen grips can be installed in testing systems at three different angle settings (0°, 45°, and 90°).
• T-slots in the lower part of the basic body serve as a mounting for the T-slot system, enabling the accommodation of smaller load cells, specimen grips, test tools, and fixtures (except hydraulic grips, type 8392, Fmax 10 kN).
• Prismatic jaws (jaws with V-slots) can be rotated and have two-fold use: For round and flat specimens or for round specimens with various diameters (jaws with crosswise arranged V-slots)
• The upper and lower grips can be opened and closed either individually or together via remote control or PC.
• The ratio between gripping and tensile forces can be adjusted individually, ensuring that the specimen is held securely (at maximum tensile force approximately double gripping force is required).
• Hydraulic grips are designed to offer optimal specimen feeding.
• An adjustable, scaled stop ensures precise vertical specimen gripping for manual special feeding.
• For easier specimen centering, a centering stop can be used.
• For safety reasons, specimen tongs can be used to insert the specimen.
  

Double actuator hydraulic grips always close symmetrically with respect to the tensile axis. This means the specimen is gripped in a precise axial position, thereby eliminating the need for thickness adjustment.

• The constant gripping force allows for repeatable test results.
• Reliable test results are guaranteed with the optimal interaction between the hydraulic power pack, the electronics, and the testing software. The force-zero control prevents unwanted forces on the specimen during the gripping process.
• Time is saved due to quick and easy adaptation of small specimen grips and test fixtures to large specimen grips via the T-slotted system. Precise alignment ensures reliable test results.
• The symmetrically closing jaws save time required for adjusting to varying specimen thicknesses and ensure that the specimen is held exactly in the test axis.
• Short specimen can also be gripped due to the special design of the specimen grip.
• Fast and easy insertion of the specimen is achieved through the ergonomic and open design
• An adjustable centering stop ensures precise test results, even with high specimen throughput rates.

The body over wedge hydraulic grip is double acting and used for symmetrical gripping. Positively driven jaws enable reproducible gripping of the specimen. The symmetrical design and high degree of stiffness of the main body of the grips make them ideal for tests in which strict requirements are placed on the alignment of the specimen to the test axis.

The specimen grips are symmetrically closing wedge grips, which are opened and closed hydraulically. Only the outer casing is moved vertically via the hydraulics. The movement of the housing causes the wedge jaws to be pushed parallel to one another in a horizontal plane.

Advantages of body over wedge hydraulic grips

• The constant gripping force allows for repeatable test results.
• Reliable test results are guaranteed with the optimal interaction between the hydraulic power pack, the electronics, and the testing software. The force-zero control prevents unwanted forces on the specimen during the gripping process.
• The specimen grip is suitable for alignment-critical applications, such as specimens sensitive to transverse tensile forces.
• With the parallel gripping operation (no differential movement of the jaws) the test speed is transfered directly to the specimen.
• Fast and easy insertion of the specimen is achieved through the ergonomic and open design
• An adjustable centering stop ensures precise test results, even with high specimen throughput rates.
• When testing composite specimens for example, the specimen grip offers advantages due to easy cleaning.
• The body over wedge hydraulic grips are also suitable for servohydraulic testing machines and are characterized by their ease of use. When used with servohydraulic testing machines they are connected to the existing pressurized oil supply. A small power pack is required for use in static materials testing machines.

Wedge screw grips combine the mechanical properties of screw grips and wedge grips. Thickness is adjusted and a preload force is generated via a screw action, which helps prevent specimen slipping at the beginning of the test.

Features of wedge screw grips:

• The main gripping force is generated by the wedge action.
• The gripping force is proportional to the actual tensile force (self-clamping), ensuring secure gripping of a wide range of materials.
• Large grip-to-grip separation allows the surface pressure to be kept low, avoiding jaw breaks.
• Without removing the specimen grips, small load cells, specimen grips, tools and test fixtures can be adapted to the large specimen grips (from Fmax 50 kN) in a time-saving manner using the optional mounting unit
• Easy replacement of jaw inserts without the need to use tools
• Wedge screw grips with Fmax 50 kN and above can be operated via a pneumatic motor, These specimen grips are opened and closed by a pneumatic motor; the actuating unit is located directly on the specimen grip
• Low overall height.
• pneumatic motor; the pneumatic actuator is located directly on the specimen grip
• Strain rate control in accordance with standards DIN EN ISO 6892-1:2009 and ASTM E8 – 09.

Highlight:

Switchable synchronization allows wedge screw grips (Fmax 10 kN and above) to grip both symmetrically and asymmetrically. The asymmetry level is easily adjustable and is maintained reliably, even with regripping. One-handed operation is possible with either hand, including with asymmetrical specimens, ensuring that the specimen attachment point is always in the test axis.

The pincer grip is a double actuator specimen grip. The grip is opened and closed via a clamping lever. The prestressed spring holds the specimen grip open for specimen insertion. If the specimen grip is closed, the prestressed spring generates the required gripping force. The gripping force is increased proportionally by the pincer principle as the tensile force increases. This ensures automatic retightening for specimens prone to shrinkage.

Pincer grips are primarily used to test highly ductile plastics and elastomers. The pincer principle produces a strong increase in gripping force during the test.

Advantages and features of pincer grips:

• The gripping force is proportional to the tensile force (self-tensioning)
• Due to the lateral movement of the pincer grips, the specimens are always loaded centrally to the tensile axis
• Fast and easy insertion of the specimen is achieved through the ergonomic and open design
• Due to their high temperature resistance and low height, the pincer grips are very well suited for use in temperature chambers. They are easy to operate and quickly reach the test temperature due to their low mass

Toggle grips are self-clamping specimen grips. Gripping occurs through action of the toggle lever. The specimen is inserted by lifting the toggle lever. The sample can either be placed around the toggle lever or clamped without deflection. Toggle grips are particularly suited for plastics in almost any temperature environment.

Features of toggle grips:

• Due to the low height and a temperature range of -15 ... +80 °C, the specimen grip is also suitable for use in temperature chambers
• The design ensures quick and easy gripping of the specimen

A force reduction curve is used for specimen grips with deflection. The force in the specimen is reduced over the course of the load reduction curve. The required gripping forces can be reduced in the pneumatic control unit. The maximum test load ranges from 500 N to 20 kN

Advantages of capstan grips:

• With pneumatic end clamping, the clamping force remains constant and supports high specimen throughput
• Automatic centering of the specimen via guide grooves in the load reduction curve
• Due to the symmetrical weight distribution, the center of gravity of the specimen grips is close to the tensile axis
• No transverse forces are transmitted to the load cell via the specimen grips.

The double capstan grip can be used to grip filamentary, high-extension tensile specimens. With this deflection principle a force differential occurs over two stages, allowing specimen failure within the gauge length with no jaw breaks. After the deflection of the double thread, a prestress weight (not included in scope of delivery) is attached. During this phase all three rollers are moving in order to balance the extensions which are created by the attachment of the pretension weight. The three lengths are adjusted to the material being tested. Finally, the double thread is clamped in the height-adjustable specimen grip.

After the specimen grip closes the deflection rollers are locked, This results in frictional forces between the rollers and the specimen, which gently reduce the tensile force prior to end clamping (specimen grip), ensuring that the specimen is gripped securely and prevents the jaws from breaking. A specimen grip with constant gripping force is used to clamp the double thread because the specimen to be tested can frequently slip out from between the gripping areas.

The double capstan grip is mainly used for materials such as rubber threads and elastofibers

Advantages of the double capstan grip:

• Wrapping the specimen around the load reduction rollers ensures gentle gripping without jaw breaks.
• The specimen grip ensures precise strain measurements on highly extensible threads
• Avoiding clamping breaks additionally saves time spent on testing

The roller grip is used for testing strip-shaped tensile specimens. The gripping force is generated through self-gripping by looping the specimen multiple times and applying tensile force. For precise strain measurement, optical extensometers are used.

Advantages of the roller grips

• The specimen grips are available with a maximum force measurement range of 2.5 kN to 250 kN
• Thanks to the ergonomic design, the specimen can be quickly and easily inserted into the clamping area (max. clamping width: 220 mm), which is freely accessible from the front
• The specimen grip is ideal for specimens that are sensitive to clamping
• and for tests in temperature chambers (temperature range: -70 ... +250 °C
• Due to the low height of the specimen grips, specimens with a large elongation can be tested

Form-fit force transmission via shouldered, screw or threaded head. The specimen, along with its corresponding holder, is inserted into the grips, which are freely accessible from the front, and is automatically centered in the tensile axis. The specimen grips are available with Fmax 50 kN, 250 kN, 600 kN and 1200 kN. Tests can be carried out on steel, non-ferrous metals and plastics in accordance with the standards DIN 50125, DIN EN ISO 6892-1, DIN EN ISO 4762, ISO 4017, DIN 6912, DIN EN ISO 898, ASTM E8 and others.

Various design details ensure an absolutely symmetrical flow of force:

• Specimen holder anti-backlash option ensures that the specimen is held securely, minimizing holder recoil at specimen break.
• all specimen holders completely surround the specimen. A support sleeve is placed around the split shoulder head specimen holder to prevent the the holders from spreading apart.
• Symmetrical bearing surfaces in the specimen grip: the specimen holders rest on two symmetrical contact surfaces in the specimen grip.
• The low height saves space in the test room
• Pure mechanical solution: they require neither a pneumatic control unit nor a hydraulic unit.

The specimen grip is suitable for testing elastomer ring specimens to DIN 53 504, standard ring 1 & 2. The specimen grip has horizontally mounted rollers. A spring hinge generates an autonomous rotary motion. This rotary motion facilitates form fitting force transmission. The spring hinge snaps out of position if the maximum permissible travel range is exceeded.

Advantages of the specimen grips for ring specimens

• The rollers rotate independently during the test by means of toothed belts or a roller spring belt and spiral drive spring
• Easy insertion of the specimen on the freely accessible rollers (up to max. 80 mm sample width)
• Protective device to protect against flying or bouncing specimen remains is available as an option
• Fmax: 2.5 kN
• Temperature range: -40 ... +150 °C
• The freely accessible rollers ensure easy specimen insertion

The specimen grip for tension springs is available in two versions, type I with Fmax 50 N for fine spring wires and type II with Fmax 10 kN. The force is transmitted via horizontal suspension bolts, which are available in various diameters (for a maximum spring wire diameter of up to 6 mm).

Advantages of the specimen grips for tension springs

• Easy mounting of the specimen on the mounting bolts, which are freely accessible from the front
• Suitable for temperature environments of -70 to +250 C°