The inspection method for static torque

In many cases, the torque value during production is different from the torque value measured during end-of-line testing. For example, the torque displayed on the controller is 100 Nm, but after tightening is completed, the measured torque is only 80 Nm. So, which torque value should we focus on?

Let's first clarify the torque values under different conditions.

The manufacturing process department sets the parameters of the electric tightening tool based on the torque value specified on the drawing. The peak value displayed on the electric tightening tool at the moment tightening ends is the dynamic torque. Dynamic torque is measured during the tightening process. For assembly purposes, dynamic torque is an input from engineering/R&D to the manufacturing process, not a requirement for the final tightening outcome.

Dynamic torque is controlled by a torque-limiting mechanism or sensor within the tool. It offers high accuracy but requires high equipment costs. Because it is the peak value during rotation, it does not directly reflect the final tightening result.

So, the question arises: when the electric tool reaches the set torque, how do we know that the bolt or nut is truly tightened to the required specification? This is where the concept of static torque comes in, and periodic spot checks using static torque are performed during production.

Static torque generally refers to a re-inspection of a bolted joint after tightening to confirm the amount of torque decay. Static torque is measured after fastening. In some cases, it is also called residual torque.

Static torque is mostly measured manually. The inspection tools are simple, easy to operate, and require low equipment costs. Static torque provides a more direct indication of the final tightening result.

In short, dynamic torque is used for production, while static torque is used for inspection.

Static torque inspection tools and methods

Static torque wrenches are generally divided into two types: dial-type torque wrenches and digital torque wrenches.

Dial-type torque wrenches are inexpensive but have relatively low accuracy.

Digital inspection torque wrenches come with sensors and can set angles. Their accuracy is far higher than that of dial-type torque wrenches, though of course the price is also much higher.

Static Torque Measurement Method

For the installation integrity of bolted connections, some uncertainty is quite normal. A commonly used inspection method is to measure static torque to assess whether a nut has been tightened to the correct torque or has loosened. The common methods are as follows:

1. Loosening Method
   Use an indicating torque wrench to slowly apply reverse torque to the assembled bolt/nut until it loosens, and read the instantaneous torque value at the moment of loosening. This method has a high margin of error and, except for special circumstances, is rarely used in production.

When loosening a bolt/nut, the thread extension torque component helps with loosening, so a lower torque is required than for tightening. You can think of it this way: to some extent, pushing an object downhill is easier than pushing it uphill. In any case, the loosening torque will be lower than the tightening torque. This method disrupts the original tightened state of the threaded joint and is not suitable for batch production inspection in the workshop.

2. Tightening (Re‑tightening) Method
   Use a dial‑type torque wrench or a digital torque wrench to gradually and steadily increase the torque on the assembled bolt. When the bolt begins to make a slight rotation, continue applying force. The torque will increase and then gradually decrease. Record the torque value indicated on the dial‑type or digital torque wrench.

This method is also called the Breakaway method. Different companies have certain differences in the time allowed after tightening is completed. For example, Volkswagen stipulates that the measurement should be taken within 30 minutes after tightening, while General Motors requires the measurement to be taken within 5 minutes after tightening.

This method is simple to operate, requiring only a dial‑type or digital torque wrench, and there is no need to re‑tighten the fastener after inspection. It is currently the most commonly used static torque measurement method among major OEMs.

3. Reset Method
   Before inspection, first mark a line between the bolt or nut being inspected and the workpiece. Then loosen the bolt or nut by 10–30°, and use a dial‑type torque wrench to re‑tighten it back to the original marked position. The torque value read at that point is the measured value.

This method is suitable for situations where high static friction makes test data unreliable. When the static friction in a bolted joint is significantly higher than the kinematic friction, using the re‑tightening method may fail to determine the true condition of the joint.

This method is particularly suitable for measuring static torque of bolts after bench and road tests. As can be seen from the screenshot below of the Volkswagen standard, Volkswagen recommends this method for static torque testing after bench and road tests, especially after engine tests: loosen and then re‑tighten to the marked position. For the Chinese national standard GB/T 19055, "Automotive engine reliability test method", the method for measuring the loosening amount of bolt tightening torque during engine reliability testing is similar to the Volkswagen standard, using a marking method.

First loosen the fastener, then accurately tighten it back to its original position (mark it beforehand). The difference between this re‑tightening torque and the torque measured before testing is the loosening amount. The reason this method is better than the continue‑tightening method is that after bench and road tests, the product may have been in operation for a long time, and the bolted joint may have rusted, seized, or encountered other issues. If the continue‑tightening method is used directly in such cases, the test results may be abnormally high and inconsistent with actual conditions.

However, by first loosening the joint, the effects of rust and seizure can be eliminated. Even if rust or seizure has occurred, loosening can bring the joint back to normal rotation. Then, tightening again to the marked position allows an accurate measurement of the torque at that point.

4. Snug Torque Method
   This method is commonly used for inspecting products with threadlocking coatings. The common procedure is: continue tightening in the tightening direction until reaching a specified check torque (Mp). If the bolt head does not rotate, the joint is deemed acceptable; otherwise, it is deemed unacceptable. The snug torque method is suitable for inspecting bolted joints with pre‑applied chemical threadlocking adhesives, but it cannot avoid damaging the already cured threadlocking adhesive.

   Taking TSLG's Precote 30, Precote 80, and Precote 85 as examples: these three adhesives are all chemical anti‑loosening sealants, and this method can be used for residual torque inspection.

Value requirement: There is currently no unified standard. Based on experience, the value is generally set at 80–90% of the preload torque.

The above methods are commonly used for static torque inspection in production. The torque measured is a comprehensive value that combines the fastener preload, thread geometry, and current friction conditions.

The most direct approach is to measure the preload itself. This can be achieved by machining the bolt head and the bolt end. The specific method is: using an ultrasonic stress meter, the longitudinal wave transit time and temperature are measured. Based on the relationship between longitudinal wave transit time, temperature, and stress, the instrument calculates the actual clamping force.

This method can directly determine whether a bolted joint meets the process design requirements. However, the test equipment is expensive and less convenient to operate than a digital torque wrench. It is not suitable for production floor testing, but is applicable for development testing in laboratory departments.

Summary of experience from the screw expert

For all methods, the inspection torque should be applied slowly to minimize dynamic effects on the gauge reading. After the tightening operation, the torque value should be checked as soon as possible before subsequent processes such as painting or heating. Torque readings depend on the coefficient of friction under the nut face and within the threads.

If the fastener is left for too long or is exposed to different environmental conditions before inspection, friction and torque values may change. If the bolt sits for two days, the torque value can vary by as much as 20%.