Due to the elastic components in the forging, machining, heat treatment, surface grinding, resistance strain gauge bonding and pressure curing process, various residual stresses are generated, and the load cell is continuously released and released as time and usage conditions change. The performance fluctuations are mainly manifested in zero point and sensitivity instability.

In order to make the load cell through the initial unstable period in the production process, the process of simulating various conditions of use to test and make it as stable as possible is called stability treatment, also called artificial aging test. The stability treatment method for the residual stress released by the load cell, in addition to the temperature aging and electric aging treatment commonly used in the manufacturing process, there are mainly two methods, namely the heat treatment method and the mechanical method.

1, heat treatment

It is mostly used in aluminum alloy load cell, after the blank is processed into elastic components, mainly including anti-quenching method, cold-heat cycle method and constant temperature aging method.

(1) Anti-quenching method

Domestically, it is also called deep cold and hot method. The aluminum alloy elastic member was placed in liquid nitrogen at -196 ° C, and after being kept for 12 hours, it was rapidly sprayed with fresh high-speed steam or placed in boiling water. Due to the opposite direction of stress caused by deep cooling and rapid heat, they cancel each other out to achieve the purpose of releasing residual stress. Tests have shown that the use of liquid nitrogen - high-speed steam method can reduce residual stress by 84%, using liquid nitrogen - boiling water method can reduce residual stress by 50%.

(2) Cold and hot cycle method

The thermal and thermal cycle stability treatment process is - 196 ° C × 4 hours / 190 ° C × 4 hours, cycle 3 times, the residual stress can be reduced by about 90%, and the structure is stable, the microplastic deformation resistance is high, and the dimensional stability is good. The effect of releasing residual stress is so obvious. First, the energy of atomic thermal motion increases while heating, the lattice distortion decreases or disappears, and the internal stress decreases. The higher the upper limit temperature, the larger the atomic thermal motion, the better the plasticity, and the more favorable the residual stress. freed. Second, the thermal stress generated by the hot and cold temperature gradient interacts with the residual stress to redistribute it to obtain the effect of residual stress reduction.

(3) Constant temperature aging

Constant temperature aging can eliminate the residual stress generated by machining and eliminate the residual stress introduced by heat treatment. When LY12 hard aluminum alloy is tempered at 200 °C for a constant temperature, the relationship between residual stress release and aging time shows that the residual stress can be reduced by about 50% after 24 hours of heat preservation.

2, mechanical method

Mechanical method stability treatment, mostly after the load cell circuit compensation and adjustment and protective sealing, is basically formed when the product is formed. The main processes are pulsating fatigue method, overload static pressure method and vibration aging method.

(1) Pulsating fatigue method

The load cell is mounted on a low-frequency fatigue tester, and the upper limit is the rated load or 120% of the rated load, and the cycle is performed 5000 to 10,000 times at a frequency of 3 to 5 times per second. It can effectively release the residual stress of the elastic element, the resistance strain gauge and the strain adhesive layer, and the effect of improving the zero point and the sensitivity stability is extremely obvious.

(2) Overload static pressure method

It is theoretically applicable to various ranges, but in actual production, there are many applications of aluminum alloy small-range load cells.

The process is: applying a 125% rated load to the load cell in a dedicated standard weight loading device or a simple mechanical screw loading device for 4 to 8 hours, or applying a 110% rated load for 24 hours, two The process can achieve the purpose of releasing residual stress, improving zero point and sensitivity stability. Because the equipment used in the overload static pressure process is simple, low in cost and good in effect, it is widely used by aluminum alloy load cell manufacturers.

(3) Vibratory Sterss Reliering method

The load cell is mounted on a vibration table whose rated sinusoidal thrust meets the vibration aging requirements, and the applied vibration load, operating frequency and vibration time are determined according to the rated range of the load cell. Resonance aging is better than vibration aging to release residual stress, but the natural frequency of the load cell must be measured. The characteristics of vibration aging and resonance aging process are: low energy consumption, short cycle, good effect, no damage to the surface of the elastic component, and simple operation. The mechanism of vibration aging is still inconclusive. The theories and viewpoints put forward by foreign experts include: plastic deformation theory, fatigue theory, lattice dislocation slip theory, energy viewpoint and material mechanics viewpoint. Only a different degree of explanation has been made, but there is no sufficient, persuasive and authoritative testimony.

These theories and viewpoints often cross each other, so the mechanism of vibration aging can be considered a complicated process. After the experimental study of vibration aging, some experts tend to explain the mechanism of vibration aging by the viewpoint of repeated stress overload of material mechanics. That is, the vibration stress acting on the elastic member interacts with the residual stress inside it to relax and release the residual stress.

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