13) Why does prestress control by torque have low reproducibility?

FAQ: What is the problem when a target pressure of 45 MPa is set and torque is applied to the transducer with a torque device - a situation that 100 % satisfies 45 MPa in PiezoClamping®, but a variable torque of 15-20 Nm range is found? In other words, it is a story that the torque value appears different each time measured. There is nothing wrong when setting other parameters. Grounding is performed at every measurement to eliminate the cause of ceramic charging and discharging. In this case, in terms of equipment reliability, it is a part that I cannot understand. Our torque equipment is digital with an error of 2 %. So, where is the problem originating from and what is the solution?

Correlation between prestress and torque for different bolt conditions.

Correlation between prestress and torque for different bolt conditions.

The ratio between torque and prestress is not stable. It is very sensitive to the surface’s cleanliness, finishing and roughness, as well to the mechanical tolerances. If you assembly the same transducer twice to the same prestress, the torque will be different because you polished the threads during the first assembly. Any lubricant, even a finger print, will increase the ratio between prestress and torque. On the other hand, any dust or surface scratch will decrease it. All these factors combined turn the stress-torque correlation unpredictable and this is one of the main reasons why the PiezoClamping® is necessary. It is not possible to control prestress by torque because the prestress standard deviation is too high. Please, take a look in the image above: for the same torque (60 Nm), the prestress varied from 20 to 64 MPa. It is interesting for you to keep using the torque wrench, however, only to protect the bolt in case of a damaged thread. If the prestress is right, but the torque needed is too high, the bolt may fail prematurely by fatigue.

To test the PiezoClamping® accuracy and precision, we suggest for you to use a hydraulic press and apply the same force over the transducer without bolt. The induced prestress should be always the same.

Learn more about prestress technology.