Fastener Torque and Lubrication
When and where lubrication on a fastener is used may seem to be obvious or of little importance to some of us, not least in relationship to the assembly and the torque requirements for installation. The facts of the matter could not be more important if the integrity of the bolted joint is to be maintained.
Too often the need to understand the effects of torque and the relationship to the coefficient of friction between the bearing area under the head of the fastener and mating threads, seems to be a minor issue to be considered, until something goes wrong!
The need to understand and appreciate the fundamental importance is vital both for the specifier as well as the installer of the fastener. To often technical evaluations are calculated, only to be demolish by some seemingly minor flaw, which may have been actioned with the best of all intention, leading to the failure of the fastened joint.
In this Case Study the failure of the joint was due to lubrication and was actioned in the best of intent by technicians working on the assembly track.
An automated systems manufacturing company.
The company had developed a new design of clamping mechanisms in a robotic hoisting arm. The design was lighter and had a greater capacity for lifting large loads.
Prototype designs had been built and tested within the research and development department and the approval and certification of the authorities ensured that the robot and the sundry equipment met the requirements of the health and safety legislation.
Having launched the product, with a substantial invested in marketing and promotion, the revolutionary development of the product became a leading milestone in the industry until failures stated to occur to the dismay of the company and their customers.
It meant that recalls were required and a full investigation into the reasons for the failures had to be found quickly. It was not just an issue of damaged equipment, but also of pride in the market place.
The fasteners were M8 x 50 hexagon head socket cap screws manufactured to strength grade 12.9. These connected to the hinge-points at the elbow of the robotic arm. It was observed that the fasteners were elongated in the thread area.
There were washers under the head of the head of the screws and all-metal locknuts were being used. No apparent damage was exhibited to either the heads of the screws, or the nuts being used. That is to say, there were no marks on the mating surfaces, which could suggest over tightening.
The manufacturers head marking was present and was considered to be a very reputable manufacturer.
It was imperative for everyone concerned to have the problem fixed in order to insure that the clamping mechanism was capable of working in a reliable and consistent manner.
Laboratory tests confirmed that the screws were to the correct hardness. The flat washers were through hardened and correct to the international standards.
Closer examination using a scanning electron microscope showed that on the fracture surfaces, the break exhibited a ductile condition. This is a normal appearance for hexagon socket cap screws when the product has been pulled in a tensile test.
Attention was next focused on the installation process in the plant. The production supervisor said that he had used the torque value given by the supplier, exactly as printed in the technical data catalogue.
He went on to say that he had his people put "a little grease on the under head of the screws to make a smoother installation". They had also put on some liquid anaerobic adhesive on the threads so that the fasteners would not loosen.
The supervisor unwittingly caused the failure problem by using a torque value for as received fasteners from the supplier and then adding two sources of lubrication.
This excessively slippery combination was sufficient to yield the fasteners whilst the lifting arm action on the clamps, pulled the screws to failure.
Any substance or material that is in contact with the underhead bearing surface of the fastener or the mating threads, must be taken into account when selecting the torque value.
Torque tensioning charts can easily be misunderstood. When calculating the torque valuation, then the tables for the coefficient of friction must be reviewed and the appropriate friction factor put into the equation.
If in doubt, then consult with a fastener application engineer who will have the knowledge and expertise in this area.
Submitted by Pat Monaghan - Technical Support Manager SPS Unbrako.