How to tighten a bolt?

This problem looks like “how to put an elephant in the refrigerator”, open the refrigerator, and put the elephant in the fridge.

So is it really that simple to tighten a bolt? Let TorcStark explain in detail below.

The way we guarantee the torque is usually to use a torque wrench (commonly known as a click wrench) to tighten the torque, and the torque multiplier has a similar function. But the torque is not the element we need to tighten the bolts. The real tightening is to clamp the connecting parts by the clamping force generated by the thread.

Therefore, the standard for us to judge whether the bolt is tightened is the clamping force.

The bolt tightening process is as follows: After pre-tightening the screw with the air gun, pick up the torque wrench and insert it into the screw head, apply torque vertically, turn the torque wrench at a constant speed, and hear a click, and you’re done.

What happened to the bolt at this point?

After the bolt is rotated, the screw rod is stretched under force, and the elongation of the screw rod generates a clamping force to clamp the connecting piece.

Applied torque is not as simple as clamping force.

In the general formula: Force (F) * Moment (L) = Torque M That is to say, the more the bolt is rotated, the greater the torque obtained.

But 90% of the torque is consumed by friction, and only 10% is converted into clamping force.

For example, when you tighten a bolt with a process requirement of 10Nm torque, we really need the 1Nm axial torque, which is consumed by friction.

What is the relationship between friction and clamping force?

Usually, follows the 50-40-10 principle, that is, 50% of the friction under the bolt head, 40% of the friction in the thread pair, and 10% of the clamping force. But the ratio of clamping force can be changed under some conditions.

For example, when a bolt is found to have bruises or impurities on its thread, once you put it into the screw hole, what kind of clamping force will such a bolt produce? It is generally believed that there are defects (impurities, bumps, etc.) in the thread pair. After assembly according to the assembly torque, there will be 50% of the friction force under the bolt head, 45% of the friction force in the thread pair, and only 5% of the clamping force we want. At this time, the assembly torque of this bolt has been reached, but it is far from the clamping force we need. If the bolts are on the moving parts such as the flywheel and the crankshaft, it is very easy to fall off, which causes what we often call “false tightness”.

The softening of the elastic material will also attenuate the clamping force, which is also the torque attenuation of what we usually call a soft connection. For example, if the material of the cylinder head gasket is soft, we use the method of secondary tightening to reduce the attenuation of the clamping force, and the attenuation of the clamping force of the oil pan bolts is often caused by the oil pan gasket (soft material) under the bolts.

Imagine that we need the screw to stretch to generate clamping force. The greater the torque, the longer the screw can be stretched. Is the greater the torque the better? The more torque we apply, the more elongated the bolt will be, and the bolt will stress fracture beyond its yield strength limit.

Thereby losing the link effect of the bolt.

Several methods of tightening bolts

## 1. Torque control method (T)

The torque control method is the simplest method. It is based on the threaded connection, when the axial clamping force F is tightened, it is proportional to the tightening torque T, which can be expressed by a formula T=K·F, and this K is the torque coefficient. When a bolt is designed, its axial clamping force F is known, and the tightening torque T is set through the process. Our tightening torque is also regulated by the process department. However, in the assembly workshop, the tightening torque is often reached but the assembled bolts are still unqualified. Why is this?

The key lies in the torque coefficient. The main fluctuation factor for the change of the torque coefficient K is the comprehensive friction coefficient u, that is to say, the accuracy of bolts and screw holes, impurities, and bumps will affect the comprehensive friction coefficient u. Moreover, this K value is also related to temperature. The experiment by Sumitomo Corporation in Japan proves that the torque coefficient K will decrease by 0.31% for every 1°C increase in the ambient temperature.

Can the torque control method be accurate? According to the tightening test report of the German Association of Engineers, when the error of the tightening torque T is ±0 (that is, the applied torque without error), the error of the axial clamping force of the bolt can reach ±27.2%.

Therefore, the advantages of the torque control method are: low cost and a simple tightening tool torque wrench can be used to check the tightening quality.

Its disadvantages are: insufficient tightening accuracy, unable to give full play to the potential of the material, and great environmental impact (temperature, bolt threads, impurities, bumps, etc.)

## 2. The torque-angle control method (TA) is also called the hyperelastic control method.

The torque-rotation angle control method is to tighten the bolt to a small torque, generally 40%-60% of the tightening torque (developed after process verification), and then start from this point to tighten a specified rotation angle control method.

This method is based on a certain rotation angle, a certain axial elongation of the bolt, and a compression of the joint. The purpose of doing this is to screw the bolts to the tight contact surface and overcome some surface unevenness and unevenness, and the axial clamping force required later is generated by the rotation angle. After calculating the rotation angle, the influence of frictional resistance on the axial clamping force no longer exists, so its accuracy is higher than that of the simple torque control method. The point of the torque control method is to measure the starting point of the rotation angle. Once the rotation angle is determined, we can obtain a fairly high tightening accuracy.

Due to the more advanced tightening method, a tool that adapts to productivity has been produced, which is the electric tightening tool (tools such as an electric torque wrench). It is composed of a motor-drive gear-elbow gear-sensor, etc. It is relatively easy to set the early warning torque and starting angle.

Torque-Angle Control Method (TA) Advantages: High tightening precision and large axial clamping force can be obtained.

Disadvantages: The control system is relatively complicated, and it is necessary to measure two data of pre-tightening torque and rotation angle. It is difficult for the quality department to find an appropriate method to check and follow up on the tightening results.

## 3. Yield point method

At present, this method is only used by car engine manufacturers that produce high-end brands, because the cost of the required equipment is too high, and we can only make some guesses about the equipment used based on its principle.

This method is developed from the torque-rotation angle control method, and the yield point is calculated by continuously calculating and judging the slope of the torque-rotation angle curve.

During the tightening process, it is necessary to measure the natural length of the bolt. It is necessary to monitor the bolt at all times to know the stretched length of the bolt when it is tightened. This requires X-ray or infrared devices that can see through and measure. For each bolt and judging the maximum gradient and yield point when the maximum gradient drops, a computer and corresponding calculation software are required to control the automatic tightening equipment to apply yield torque, and automatic tightening equipment is required here.

Pros: Very precise, bolts can be reused, utilizing maximum compression force potential.

Cons: expensive equipment

The above is TorcStark’s description of how to tighten a bolt. So what questions or good practices do you have about how to tighten a bolt? Please leave a message to discuss this!