Selection method of test machine fixture

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Fixture of electronic tensile testing machine:
The fixture is an indispensable part in the electronic tensile testing machine. Many people ask what is a fixture, and today we will talk about the type and function of the fixture.
According to different test methods, clamps can be roughly divided into: tensile clamps, compression clamps, bending clamps, peeling clamps, shearing clamps, etc., among which the tensile clamps account for about 80% of the total clamps.
1. Features of the fixture
a. We know that the sample (or product) is clamped by the clamp to add force to the sample. The magnitude of the test force that the clamp can withstand is an important indicator of the clamp. It determines the size of the clamp 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 of cattle (such as spandex for textile), and as large as tens of tons (such as ordinary steel
Etc .; the largest electronic universal testing machine in China has a test force of 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 directly contact 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
Sex. Sometimes special steel is also installed at the jaws, or gold steel grit 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 specimen 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 cast structures are 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, industry standards, etc. These standards are generally tested
Sample preparation and test methods have strict regulations, 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), which There is a clear difference from the host. Hosts at home and abroad
The difference is that the fixtures are very different from abroad and domestic, 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 INSTRON, MTS, ZWICK, etc., generally have meticulous workmanship and high availability
, But the price is higher and it is in the high-end market; and our fixtures, because of their wide involvement in the industry, have a large market share in the domestic market, and to a certain extent, they can replace some of the fixtures in foreign countries and are in the high-end market. However, in some new materials and special materials fixtures, domestic and foreign water
There is still a certain gap.
American TestResources mechanical testing system, all-electric servo static tensile, compression, torsion testing machine, dynamic & fatigue series testing machine, equipped with various professional fixtures, can also be used for other brand testing machines â‘¢. The fixture itself is a locking mechanism, we know the machinery The locking structures on the top are: thread patterns (that is, threads, screws, nuts), inclined planes, eccentric wheels, levers, etc. The fixture is a combination of these structures. There is no fixed pattern in the structure of the test machine fixture, according to different
The size of the sample and the test force vary greatly in structure. (The sample with a large test force generally adopts a sloped clamping structure. With the increase of the test force, the clamping force increases, and the shoulder sample adopts a suspension structure, etc. ), If the fixture is divided according to the structure, it can be divided into wedge-shaped fixtures (refers to the use of inclined planes)
Locking principle structure clamp), wafer clamp (referring to the clamp with single-sided or double-sided threading principle structure), winding clamp (referring to the clamp where the sample is locked by winding), eccentric clamp refers to the use of (Clamp with eccentric locking principle structure), lever clamp (
Refers to the fixture using the principle of lever force amplification), shoulder type fixture (refers to the fixture for the shoulder sample), bolt type fixture (refers to the fixture for testing the thread strength of bolts, screws, studs, etc.), 90 ° Stripping fixture (refers to two samples for hanging, straight stripping
Away from the fixture) 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.
2. Current status and development trend of fixtures
a. The development direction of the testing machine is from sample preparation testing to product (i.e., finished and semi-finished products) testing, which requires that the corresponding jig be changed from the jig originally used for standard sample testing to the jig used for product testing development of.
b. The use of fixtures is developing in the direction of high efficiency and low labor intensity. In the past, fixtures were generally mechanically locked, which was time-consuming and labor-intensive, labor-intensive, and inefficient. With the improvement of the working environment and the need for large-scale testing (random random inspection of the production line), the clamping method of the fixture
From the original mechanical clamping to pneumatic clamping, hydraulic clamping and other directions.
c. Fully automatic fixture: from the measurement of the sample size to the clamping, to the start of the test, and finally the test report is completed at one time. Such fixtures are very costly and are only suitable for testing and inspection of large quantities of the same samples or finished products.
d. The increase of environmental tests (high and low temperature tests), the increase in the use of high and low temperature fixtures. The increase in environmental tests (high and low temperature boxes) makes it more difficult to design fixtures. We know that the national standards for high-temperature tensile testing have provisions: threads for round specimens and holes in plate specimens. by
Because the connection method is fixed, the design of the fixture is relatively simple. However, the high and low temperature test is different. It is generally tested in a high and low temperature box, and its sample generally has a short gauge length (generally a normal temperature sample). In this way, the fixture must be installed in a high and low temperature box.
Due to the limited travel of the testing machine (the testing machine travels when the standard fixture is installed), this requires the fixture to be small in size, meet the test force, and be resistant to high and low temperatures. It is generally difficult to design.
e. Increased number of continuous test fixtures. In the past, samples were generally tested for testing, and the stretching and compression of the samples were carried out separately (that is, the stretching and compression were performed with different fixtures), but now more and more finished products are tested, and the samples are in the same test To be stretched and compressed,
To have high efficiency, only the same kind of jig can be used for both stretching and compression.
f. Increased test fixtures for special industries. With the development of science and technology, some emerging industries have put forward new requirements for test fixtures, such as the requirement that the fixture structure is small, non-magnetic, and corrosion-resistant (tested in solution) and so on.
Suzhou high-quality testing instrument mechanical test system all-electric servo static tensile, compression and torsion testing machine, dynamic & fatigue series testing machine, equipped with various professional fixtures, can also be used for other brand testing machines. Due to the particularity of the fixture used in the testing machine and the continuous emergence of new materials, the fixture design has been in a passive situation. We encounter new materials every day and need to design new fixtures. We constantly sum up the past successful experience to adapt to the new development trend.

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