Factors affecting the friction coefficient of bolts

I. Introduction:

The influence of different process conditions on the bolt equivalent friction coefficient provides a basis for accurately controlling the bolt pre-tightening force and adopting a suitable coupling structure.

The pre-tightening force of the bolt connection has a great influence on the reliability and fatigue life of the joint. The greater the pre-tightening force, the better the joint reliability and the longer the joint life. However, a large preload may damage the joint sandwich, so it is necessary to control the preload of the bolt. However, in actual installation, it is very difficult to directly control the preload, but indirectly control the torque by controlling the tightening torque.

Therefore, it is very important to understand the relationship between tightening torque and preload, which is also the key to controlling the preload.

Second, the theoretical basis:

The relationship between the bolt tightening torque and the preload force:

In the formula:

M: tightening torque

P: preload

R: outer radius of the nut bearing surface

r: inner radius of the nut bearing surface

γ: Thread angle

ρ: spiral equivalent friction angle

Fc: coefficient of friction between the nut and the friction surface

t: pitch

β: half angle of the thread

f': thread pair friction coefficient

f: equivalent friction coefficient

The factors affecting the friction coefficient are:

1. Lubrication conditions;

2. Support surface material;

3. Surface treatment;

4. Bolt specifications;

5. Bolt material;

Test data: (Measured using the HY-1000N.m multi-purpose bolt fastening analysis system.)

Table I

Table II

Table 3

Table 4

Table 5

Third, the impact of the test results on the friction coefficient:

1. It can be seen from the table that the high-strength bolt has less influence on the friction coefficient of the material; because the strength is close, the surface hardness is relatively close, so for high-strength bolts, the influence of the material on the equivalent friction coefficient smaller;

2. The bolt specifications are different, and the friction coefficient is completely different. Therefore, the friction coefficient is completely different for different specifications;

3. The surface treatment is different, the friction coefficient is completely different, so the same bolt specifications, different surface treatments, the friction coefficient is completely different;

4. The material of the support surface is different, the friction coefficient is different, and the higher the surface hardness, the smaller the friction coefficient.

5. The lubricant has different effects on the friction coefficient and is completely different;

Fourth, the conclusion:

1. Different bolts and different combinations of support materials are combined and their friction coefficients are different;

2. The support surface material, surface treatment, surface hardness, surface roughness, surface lubrication conditions have a great influence on the friction coefficient;

3. The preload force is indirectly controlled by the tightening torque, and the corresponding friction coefficient must be selected according to the specific conditions.

Fifth, the scope of use:

The machine is used for torque, torsion performance testing and analysis of various materials. It is widely used in aerospace, petrochemical, machinery manufacturing, wind power bolts, bolts and bolts, vehicle manufacturing, wire and cable, plastic rubber, ceramic building materials. , metal materials and products, etc., is the only domestic company based on the British ARM 32Bit-ARM micro-processing structure , through the Japanese AC digital controller control servo motor and high-precision reducer with Japanese photoelectric encoder to make the test station static rotation, test bench It can be operated at a speed of 0~1000°/min (or according to the actual situation). The high-precision torque sensor is used on the force measurement source, and its accuracy is 0.02%, the sensitivity is high, and the whole system is above 0.5 accuracy. The effective force measurement range is 1/500 to 100% of the maximum force value; the torque accuracy is the indication value. Within ±1%; the torsion angle accuracy is within ±1% of the indicated value; the angular velocity accuracy is within ±1 of the indicated value; the rotation angle measurement accuracy is within ±1 of the indicated value. According to GB, ISO, ASTM, JIS, DIN and other standards, mechanical tests such as torque, torsion angle, corner, torsion force, constant rotation torque, and cycle low cycle fatigue can be performed to detect the torque-torsion and torque of the material. - The corresponding parameters such as time, twist angle - time.

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