In the world of bolt assembly, we often focus on the torque value – that clearly visible number on the wrench. Engineers strictly set the torque according to standards, and operators accurately perform the tightening. However, a hidden "controller" plays a decisive role behind the scenes: the friction coefficient.

Many mistakenly believe that "tightening" is everything, but they don’t realize that up to 90 % of the tightening torque is used to overcome friction. Only a mere 10 % of the torque is actually converted into the preload (clamping force) we need. Understanding the influence of the friction coefficient is the first step toward high-quality bolt assembly.

The “Triple Consumption” of Tightening Torque

To understand the friction coefficient, we must first know where the tightening torque goes. When we apply torque, it is mainly consumed in three places:

• Thread friction (≈ 40 %): friction between the bolt threads and the nut or tapped-hole threads.

• Bearing-surface friction (≈ 50 %): friction under the bolt head or nut flange against the surface of the clamped parts.

• Generating preload (≈ 10 %): the portion that finally stretches the bolt and produces the clamping force to hold the parts together.

As can be seen, friction consumes the vast majority of the input torque. The friction coefficient is the core parameter that measures the magnitude of these frictional forces.

How the Friction Coefficient Directly Affects Assembly Quality

Fluctuations in the friction coefficient will directly cause huge scatter in preload, leading to two extreme risks:

Scenario 1 – Friction coefficient too low (too smooth)

• Result: When the target torque is reached, because friction is too small, the proportion of torque used to create preload becomes relatively large.

• Risk: The bolt is overstretched, producing excessive preload. Mild cases cause the clamped parts to be crushed or the gasket to fail; severe cases make the bolt yield, elongate, or even break, leading to catastrophic failure.

Scenario 2 – Friction coefficient too high (too rough)

• Result: When the target torque is reached, most of the torque is consumed by friction.

• Risk: The preload is insufficient. This causes the joint to loosen, parts to slide relative to each other, and the interface to open; under vibrational loads the problem worsens quickly, again resulting in joint failure.

Key Factors Influencing the Friction Coefficient

To control the friction coefficient, we must first understand its influencing factors:

• Surface treatment and coating: Different treatments such as zinc plating, Dacromet, phosphating, and oxidation significantly alter the friction coefficient. Lubricant coatings (e.g., MoS₂, PTFE) can stabilize and lower the friction coefficient.

• Lubrication condition: Is oil or grease used? Which type? The uniformity and stability of the lubricant are crucial. Dry friction, oil lubrication, and grease lubrication show huge differences in friction coefficient.

• Material pairing: the interaction between bolt and clamped-part material hardness and surface roughness.

• Tightening speed: In some cases, the tightening speed affects lubricant distribution and the friction regime.

• Repeated tightening: When the same bolt-nut pair is disassembled and re-used, the surface condition and friction coefficient change.

How to Manage the Friction Coefficient Effectively and Achieve Reliable Assembly?

Facing the challenges posed by the friction coefficient, modern industry offers mature solutions:

1. Torque–angle method
   Instead of relying on torque alone, this method first applies a starting torque to eliminate clearance and then turns through a specified angle. Because bolt elongation is proportional to rotation angle, the method controls preload more directly and reduces the influence of friction-coefficient scatter.

2. Use friction-stabilizing agents
   Apply specified lubricating oil or paste during assembly, and ensure the brand, model, and quantity are consistent. This guarantees batch-to-batch stability of the friction coefficient.

3. Direct preload measurement
   For critical joints, adopt more direct methods such as bolt-elongation gauges or instrumented bolts with sensors to monitor preload directly, completely bypassing the friction-coefficient issue.

4. Standardize the assembly process
   Establish strict work instructions that clearly define bolt/nut coatings, lubricants, tightening tools, and tightening procedures, and provide professional training to operators.

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