Martin Schatz
SCHATZ USA Inc.
martin@schatz-usa.comwww.schatz-usa.comDetermining the clamp force remains a complex procedure when it comes to assessing the conditions during bolt fastening. Despite this, efforts are made, using simple methods, in production to estimate frictional conditions within the bolted joint. All assembly procedures during production are indirect methods for achieving the desired preload. The torque-controlled tightening method is the most frequently used procedure as it can be carried out using fairly basic assembly tools. The torque applied and the clamp force thereby produced are to a large extent proportional which means that where torque is applied within the predefined tolerances, it may be assumed that this produces a clamp force that lies within the design limits. But to what extent can it be assumed by those carrying out the assembly work that the two variables torque and clamp force are proportional to one another and what influences change this relationship?
For one thing, the geometry of the fastener determines which torque will achieve a specific clamp force. The thread pitch is a decisive factor in this regard. A bolt is a geometric entity that represents a “rolled up skewed plane” thus influencing the way in which forces are distributed throughout the threaded connection. The second factor that influences the interdependency of forces within the bolted joint is its inherent frictional relationships. This is the friction in the bearing surface that is turned above the material or a washer and also the friction in the thread that turns in the material or in a nut thread.
Depending on the frictional relationships, up to 50% of the torque applied is required just to overcome the frictional resistance below the bolt head. This means that during fastening the conditions of the bearing parts under the bolt head must be particularly carefully observed in order to ensure that the fastening operation is of a high quality. Factors that contribute towards changes in the friction characteristics are:
- Roughness of the bearing parts contact surface
- Hardness of the bolted joint material
- Surface treatment of the fastener and assembled components
- Lubrication condition of the bolt
- Speed of the power tool
- Temperature of the bolted joint
- Surface contact pressure under the bolt head
- Material pairing
While this list is by no means exhaustive, the most important parameters influencing the frictional characteristics of the bolted joint are listed. This means that a bolt connection can fail simply as a result of the way in which it has been handled. A drop of oil on the worker’s hand or a drop of oil falling from the tool may considerably change the frictional characteristics of the fastener. For example, a bolt that is to be tightened dry (not lubricated) with a friction coefficient of µ=0.14 exhibits the following fastening torque distribution:
- Clamp force 13%
- Thread friction torque 40%
- Bolt head friction torque 47%.
If the friction under the head of the bolt changes by only 10%, i.e. from 0.14 to 0.126, this would produce the following change in force distribution:
- Clamp force 18%
- Thread friction torque 40%
- Bolt head friction torque 42%.
This means a clamp force that is roughly 40% higher than was originally intended. Where the design of the bolted joint is critical this may result in over elongation or even breaking of the bolt.
So how is it possible to identify or determine the friction coefficient inside a bolted joint during production without having to carry out time-consuming measurements in the laboratory? A simple analysis that can be carried out under production conditions to determine the friction coefficient is the tightening/loosening test whereby the torque used to fasten the nut or bolt is determined and the loosening torque is subsequently measured.
The clamp force, and therefore also the friction coefficient, can be calculated based on the difference between the two torques. It is recommended that a graphical analysis is carried out during this trial. In the event of breakaway of the bolted joint, the measured loosening torque may be too high. Breakaway occurs if the loosening torque is initially much higher than is required and then falls again—frequently following a noticeable audible “click“. This occurs when the static friction torque is considerably higher than the dynamic friction torque. In this case the torque that occurs immediately following breakaway must be evaluated. With metric bolts the friction coefficient can be calculated according to the formula below.
This method only produces good results if the same friction characteristics exist when tightening and when loosening the bolt and the bolted joint does not settle between tightening and loosening them.
Literature: Volker Schatz, 2008, 10 Steps for reliable Bolted Assembly, Tectum Verlag, Marburg, Germany
