Friction in a bolted joint: friend or enemy?
There are over a hundred parameters that can cause a badly tightened assembly. The final goal of a tightened assembly is to generate the correct tension in the bolt resulting in the correct clamping force in the assembly. Through this concept and functionality the bolt is meant to give us the warranty of the expected tension. To get initial pre-tension (just after the tightening has been executed) we must apply a torque. In fact, a bolted joint acts like a combination of two springs:
- A spring (the bolt) which is put under axial load (stress) and elongates upto its “elongation” point
- A spring (the parts) which are compressed due to the executed forces.
Friction in a joint
A tightened assembly absorbs torque in three different ways:
- The torque required overcoming the friction under the head of the screw; these are forces that appear when the head of the screw reaches the surface of the parts to be assembled.
- The torque required to overcome the friction in respect of the threads of the bolt and the threads of the nut.
- The torque required to create tension in the bolt by forcing the nut thread into the bolt thread.
In other words, 90% of the torque applied on a tightened assembly is used to overcome friction in threads and under the head of the bolt and only 10% of it is converted in potential energy (pre-tension) in the bolt or clamping force in the assembly. As this force only represents 10% of all forces present in the assembly, it must be kept under perfect control. But friction forces are needed because they keep the assembly together!
Coefficient of friction
The degree of friction is quantified by a number commonly called « coefficient of friction ». It is influenced by two main factors:
- The surface under the bolt’s head
- The lubrication of the surfaces to be assembled
Lubricants present on threads and other surfaces have great impact on the tightening itself, on the « working » assembly and on possible disassembly. They make the assembly process easier by ensuring a controlled tightening and restrict the spreading of torque/tension characteristics. Lubricants look like microscopic ball bearings that keep threads apart and consequently guarantee a quick and easy disassembly. Lubricants can be divided into 3 groups:
- Liquid lubricants (based on oil)
- Lubricants looking like silicone (mix of oils and/or grease)
- Dry lubricants
Liquid lubricants are made of petrol based oils and include a.o. olefin glycol, etc … Under high pressure they can be repulsed resulting in a metal/metal contact which looks like a “cold welding”. Silicones are solid lubricants dissolved in liquid and/or grease. They are most of the time used in applications such as maintenance or repair.
Dry lubricants are made of dry components dissolved in « binding material », the best known being Molybdenum Sulphide (MoS2).
Friction coefficients also depend on the tightening method used. There are only « recommendations » for the friction coefficients based on following parameters:
- Roughness of the surface
- Type of components to be assembled
- Purification (oil, MoS2, dry, …)
- Surface treatment (phosphate, not finished,…)
The choice of a good lubricant or a correct friction coefficient must be based on a proper analysis of the assembly process and its associated torques in order to make sure that the tension efforts finally generated are within the agreed limits (calculated or preliminary tested).
Lubrication reduces ultimately the friction in the threads and under the bolt’s head; in other words, the ratio torque/tension decreases. If we apply a same torque after having lubricated the assembly, a larger part of that torque will be converted in tension, which in the end will result in a too high elongation and the bolt could break! If the assembly is entirely dry, the tension will be too low and the bolt will tend to unscrew under dynamic efforts.
A simple worked sample teaches us that an M8 tightened at 25Nm with a friction coefficient of 0.11 will induce a tension of 23.4 kN. If we tighten the same M8 at the same torque but in its non lubricated version (with a friction coefficient of 0.16) the tension will then be 14.6 kN. In other words, a difference of 40% in tension that we do not « see » at all in the torque value!
In order to better explain this phenomenon, let us take a look at the following screws of which only the head is different. Which one will give the best tension for the same torque applied?
In the left screw, friction under the head is less important than on the right screw; hence, a larger part of the torque applied will be converted in pre-tension and afterwards in clamping force. Friction has thus a large influence on the torque/tension relation, which under certain circumstances can lead to an important scatter of this ratio. Therefore, in the last years, assembly methods less sensitive to the « friction » factor have been used, like e.g. the tightening to a pre-torque followed by a (controlled) angle tightening. This is only feasible with electric nutrunners or digital dynamic wrenches with torque/angle functions.
It is hence particularly important to insist on the fact that friction is one of the most meaningful parameters that must be controlled in order to assure a correct tightened assembly. If in a life application, friction deviates even only slightly from calculated or tested values, this could have disastrous consequences in terms of pre-tension or clamping force when we will apply a tightening to torque.