Influence of bolt fasteners friction coefficient on torque

Influence of bolt friction coefficient on torque

The coefficient of friction of a bolt is the ratio of the force required to the load in the case of forceful friction between two surfaces. It is the ratio of the bolt loosening torque to the tightening torque.

The friction coefficient is an important parameter affecting the assembly's axial force and torque, and its value's stability directly affects the assembly's quality. Bolts with large fluctuations in friction coefficient are prone to torque and angle monitoring window alarms during assembly. The friction coefficient is one of the most critical parameters of the fastener assembly, and its value and stability directly affect the quality of the connecting sub. In the automotive assembly plant, the coefficient of friction is the most influential factor in monitoring window alarms. In general, the advantages of a low coefficient of friction are a higher conversion of torque to axial force, the possibility of using smaller fastener sizes and lower grades, and lower ranges of tightening equipment. The advantage of a high coefficient of friction is that after tightening when the axial forces are the same, the connection will resist loosening better. So, how does a change in the coefficient of friction affect assembly torque?

(1) Effect of friction coefficient on two assembly methods

The most common assembly methods are the torque and over-yielding torque corner methods. The torque method has a final tightening target torque value, so even though the coefficient of friction fluctuates, the bolts are tightened directly to the set torque. Although the tightening torque values are the same, differences in friction coefficients can lead to differences in final assembly shaft forces.

The primary consideration of the effect of the coefficient of friction on the assembly torque is only for the over-yield torque corner method of tightening. The final target value of the torque corner method is the corner, so the final tightening torque value is not fixed and changes with the fastener friction coefficient, strength, and the state of the connected parts. So, how does a change in the coefficient of friction of just the bolt affect the final assembly torque?

(2)Influence of the friction coefficient on the final tightening torque (example calculation)

Case 1: A bolt, size M16, pitch 1.5mm, grade 10.9, hexagonal flange bolt (diameter of the hole of the connected part: 17.5mm, size of the outer flange 33mm), yield strength of the median 1000Mpa.

The following figure shows the relationship between the final assembly torque and the friction coefficient for this bolt. Different friction coefficients and other conditions remain unchanged after tightening by the over-yield torque corner method.

Friction coefficient of the calculation range of 0.08-0.29, with the increase of the friction coefficient, the final tightening torque rises. However, the relationship is not entirely linear; the slope of the curve tends to decrease, and the two are positively correlated.

Case 2: a bolt, size M8, pitch 1.25mm, grade 8.8, hexagon socket flange bolts (hole diameter of the connected parts: 9mm, the outer flange size 12.3mm), take the yield strength of the median value of 700Mpa.

Bolts with different friction coefficients, other conditions remain unchanged, after the yield torque corner method of tightening, the final assembly torque with the friction coefficient of the relationship between the change.

Friction coefficient of the calculation range of 0.08-0.29, with the increase of the friction coefficient, the final tightening torque rises. However, the relationship is not entirely linearly increasing, and the slope of the curve tends to decrease. The two are positively correlated.

Calculating the 2 cases reveals that for fasteners of different sizes, grades, and shapes, the relationship between the change of torque and coefficient of friction is basically the same.

1. A more significant coefficient of friction increases the torque: the larger the coefficient of friction, the greater the corresponding friction and the more excellent the resistance. Under the same external force, the bolt must withstand a greater clamping force, thus increasing the torque value.

2. Smaller coefficients of friction reduce the torque: Unlike more significant coefficients of friction, smaller coefficients produce very little friction and require very little force. Therefore, when the external force is fixed, the clamping force on the bolt is small, which reduces the torque value.

(3)Calculation principle

According to the definition of the minimum value of the yield strength of the bolt according to ISO 898-1, as shown in the table below, in the calculation of the above case, since only the friction coefficient is considered to affect the torque, the value of the yield strength is fixed, and the intermediate value is taken.

The relationship between axial force and yield strength, coefficient of friction, assembly torque and assembly axial force in the case of bolts tightened to yield is as follows: the relationship between final tightening torque and coefficient of friction can be obtained by bringing Equation 1 into Equation 2 as follows:

Where:

MT: final tightening torque, the

FAssy: assembly shaft force, and

RP0.2: yield strength.

μG: thread friction coefficient, μK: end face friction coefficient

μK: coefficient of face friction

A0: Minimum cross-sectional area of the thread, d2: Medium diameter of the thread

d2: thread center diameter, d0: minimum thread cross-section

d0: Minimum cross-sectional diameter of thread, d0: Minimum cross-sectional diameter of thread, d0: Minimum cross-sectional diameter of thread

p: pitch

DKm: end face friction diameter.

According to Equation 3, the final tightening torque MT and μG: thread friction coefficient, μK: end face friction coefficient is a nonlinear and relatively complex relationship, but the two are positively correlated. The first half of Eq. 3, where one of the μG: thread friction coefficients is in the denominator position, is on a decreasing trend as the friction coefficient increases. Therefore, the slopes of the final tightening torque and coefficient of friction change curves slightly decrease in Cases 1 and 2.

（4）on the angle of fracture of the bolt

The angle of fracture of a tightened bolt can be calculated from the yielding of the bolt to the final fracture of the bolt. By testing two bolts with different friction coefficients and the same other parameters and overloading them to failure, the high friction coefficient is significantly higher than the low friction coefficient.

The torque with a high coefficient of friction is significantly higher than the torque with a low coefficient of friction. In contrast, the angle of rupture with a high coefficient of friction is relatively small. A high coefficient of friction of the bolt leads to a high tightening torque and a high shear stress.

The higher the shear stress,

the minor the neck shrinkage, the more negligible the elongation of the bolt when it breaks, and the smaller the fracture angle. When designing a threaded connection, we prefer a higher fracture angle of the bolt. For the over yielding torque corner method of tightening, the angle of the safety margin will be higher, the bolt is less likely to fracture.

（5）Impact on fasteners pre-coated with anti-loosening adhesive

Pre-coating fasteners with a microencapsulated adhesive layer of anti-loosening adhesive are effective in preventing the loosening of threaded joints from occurring. As the coefficient of friction decreases, the ratio of loosening torque/tightening torque decreases. This is because the adhesion of the microencapsulated adhesive layer decreases when the surface of the bolt is lubricated, which reduces the adhesive strength of the cured adhesive and, therefore, the loosening torque value. Therefore， the higher the coefficient of friction, the better the fastener loosening performance of the pre-coated anti-loosening adhesive.

(6) Factors affecting the coefficient of friction

The production and surface treatment of the parts (including fasteners and connected parts), the accuracy of the cold piercing of fasteners, the accuracy of thread processing, the roughness of the tooth side, and the roughness of the supporting surface (including fasteners and connected parts) affects the coefficient of friction of the threads, the higher the processing accuracy, the more polished the surface, the lower the coefficient of friction, and vice versa, the greater the coefficient of friction.

The surface treatment of bolts and nuts can change the coefficient of friction of the threads and supporting surfaces. Nowadays, the common types of surface treatment are electro-galvanized, electro-galvanized zinc-nickel alloy, zinc-aluminum coating, copper plating, phosphating, etc. The coefficient of friction of different surface treatments is different.

1. Material selection: The choice of material is the key to controlling the bolt's coefficient of friction. Different materials have different hardness; the friction coefficient of a high-hardness material is also more significant, and the opposite is smaller. Therefore, choosing a suitable material can control the bolt's friction coefficient.

2. Thread precision; composed of thread tolerance band and screwing length, thread precision is the comprehensive embodiment of thread processing quality; the higher the thread precision under the same conditions, the smaller the coefficient of friction.

3. Fastener surface roughness: The thread manufacturing process, including thread surface roughness and supporting surface roughness, has a direct relationship with fastener production in the cold heading operation process parameters.

4. Fastener surface treatment process: surface treatment is also an important method to control the coefficient of friction of the bolt. There are many kinds of treatment, such as sandblasting, coating, plating, and so on.

Different surface roughness after treatment will impact the coefficient of friction, including phosphating, plating, and zinc-aluminum coating.

Affected by the type of surface treatment layer material, local thickness, transformation process, and other factors, different surface treatment processes to get the fastener friction coefficient vary greatly, but the lubrication process or adding lubrication in the surface coating agent can effectively reduce the coefficient of friction. Therefore, in the actual production and assembly, according to the needs of the connected structure, the process parameters of fasteners should be reasonably selected to ensure that the friction coefficient of dispersion is controlled within a small range. When designing the threaded connection, the friction coefficient should be determined according to the test results. Then, this friction coefficient should calculate the torsion torque to ensure the consistency between the theoretical value of the axial preload and the exact value and to improve the threaded connection's reliability and the design's accuracy. The bolt friction coefficient is an essential factor affecting torque, and its size is directly related to the bolt's loosening torque and tightening torque. Therefore, when designing and using bolts, choosing the appropriate material and treatment is necessary according to the specific situation to control the friction coefficient and ensure the regular work of bolts.

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