Theme: [Original] Types of fixtures for tensile testing machines

1. Features of the fixture

a. We know that the sample (or product) is clamped by the fixture to apply force to the sample. The magnitude of the test force that the fixture can withstand is an important indicator of the fixture. It determines the jig junction

The size of the structure and the labor intensity of the clamp operation, the sample material is divided into metal and non-metal, and the shape is divided into sizes. The composition of the material is various, and the test force that the sample can withstand is as small as tens of centimeters (such as spandex yarn for textiles), as large as tens of tons (such as ordinary steel, etc .; the largest electronic universal testing machine in China The test force is 600KN, 0.5 machine), the sample size is as small as a gold wire with a diameter of φ0.006mm, and a PVC pipe with a diameter of 1m. This requires the design of different fixtures according to different test forces and the shape and size of the sample.

b. Requirements for fixture materials:

â‘  For general metal and non-metal samples, the jaws of the fixture are in direct contact with the sample. Generally, high-quality alloy structural steel, alloy high carbon steel (or low carbon alloy steel), cold work die steel, etc. are selected. The heat treatment process (quenching and tempering, carburizing and quenching, etc.) increases its strength and wear resistance. Sometimes special steel is also installed at the jaws, or gold steel sand is sprayed on the jaws.

â‘¡. For some fixtures with small test force, sticky soft rubber is used for the surface in contact with the sample. (For example: plastic film, fiber wire and other sample clamping surface.)

â‘¢The clamp is generally made of high-quality medium-carbon steel and alloy structural steel, and its mechanical properties are increased through an appropriate heat treatment process. Sometimes non-ferrous metals such as aluminum alloys and special metals are used to reduce weight. Sometimes casting structure is also used (cast steel, cast aluminum, etc.)

c. Requirements for fixture structure:

â‘ . The design of the fixture is mainly based on the test standard of the material and the shape and material of the sample (specifically the finished product and semi-finished product). The test standards mentioned above refer to ISO, ASTM, DIN, GB, BS, JIS, etc., as well as enterprise standards and industry standards. These standards generally have strict regulations on sample preparation and test methods. We can design different fixtures according to different samples and test methods. For the fixtures used for special samples (finished and semi-finished products), the fixtures are mainly designed according to the shape and material of the sample.

â‘¡. The fixture itself does not have a fixed structure (such as the wire can be clamped by winding, or two flat plates, and the thin metal sample can be clamped by wedge or by clamping). There is a clear difference from the host. The mainframes are similar in domestic and foreign countries, while the fixtures are very different between foreign countries and domestic ones, and there are also big differences between different companies. This mainly depends on the company's overall level and the accumulation of designer experience. Foreign fixtures, such as those of INSTRON, MTS, ZWICK, etc., generally have meticulous workmanship and high availability, but the price is high and they are in the high-end market; and our fixtures have a large market share in the domestic market due to their wide involvement in the industry To a certain extent, it can replace some foreign fixtures and is in the mid-to-high-end market. However, there are still some gaps between domestic and foreign standards in some new materials and special materials fixtures.

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③. The fixture itself is a locking mechanism, we know that the locking structure on the machine is: thread (screw, screw, nut), inclined plane, eccentric wheel, lever, etc. The fixture is a combination of these structures. There is no fixed pattern on the structure of the test machine fixture.According to the different samples and the test force, the structure is very different. (The samples with large test force generally adopt the inclined surface clamping structure.As the test force increases, the clamp The tightening force increases accordingly, and the shoulder sample adopts a suspension structure, etc.), if the fixture is divided according to the structure, it can be divided into a wedge-shaped fixture (referring to a fixture with a bevel locking principle structure), a clamp type fixture (referring to a single-sided or Fixture with double-sided threading principle structure), winding fixture (means the sample is locked by winding), eccentric fixture means (the fixture with eccentric locking principle structure), lever type fixture (means lever force) Amplification principle structure jig), shoulder type jig (referring to the jig suitable for shoulder samples), bolt type jig (referring to the jig suitable for testing the thread strength of bolts, screws, studs, etc.), 90 ° peeling type jig ( Refers to a jig suitable for two samples for hanging, straight peeling), etc. The structure of these fixtures has its own advantages and disadvantages, such as: wedge-shaped fixture, the initial clamping force is small, with the increase of the test force. The clamping force increases accordingly. For clamping fixtures, the initial clamping force is large and increases with the test force. The clamping force is reduced accordingly.

Tensile testing machines are similar, but the fixtures are diverse!

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