What is thread galling?
Thread galling (also known as locking up, freezing up or seizing) can occur when:
- Excessive pressure is used to close a joint
- A “nut runner” is used at high speed
- Pressure is applied by an operator who thinks that the nut will tighten the joint quicker
The above actions and techniques increase the friction between the mating threads and during fastener tightening the pressure builds between the contacting and sliding thread surfaces. This in turn causes the surface protective oxides to temporarily break down and the underlying metal-to-metal surfaces to “jam”.
This clogging-shearing-locking action causes increasing adhesion and in extreme cases, galling can cause the threads to seize. If tightening is continued, the fastener can be twisted off and sheared.
Notes: Thread galling commonly occurs with fasteners made of Stainless Steel, Aluminium. Titanium, and other Alloys which self-generate an oxide surface film for corrosion protection.
Nut runners are tools used for tightening nuts. Nut runners are pneumatic, electric or hydraulic power tools.
If a “nut runner” is being used then simply slow down the installation RPM speed and ask the operator to not apply any excessive pressure.
This should reduce or sometimes will completely solve the problem of Galling.
How does this work?
Due to the installation RPM speed being slowed, there is less heat generated with friction during tightening which decreases your chances of Thread Galling.
Reduce the friction by applying a suitable lubricant. This will eliminate Thread Galling.
Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating.
Whilst applying a lubricant will help to reduce friction on the mating threads it will have an adverse effect on the tightening torque of the fastener.
To check this tests will need to be conducted to ensure that the joint will perform as it should.
Always check the end use of the fastener before selecting / applying a lubricant. Stainless Steel is frequently used in food related applications which may make some lubricants unacceptable.
Using different stainless alloy grades for the bolt and nut will reduce Galling.
If one of the items is A4 (316) and the other is A2 (304) they are less likely to gall than if they were both the same alloy grade.
How does this work?
The different alloys work-harden at different rates.
We would recommend using the following product ranges to help to prevent Thread Galling:
Action Can AG-90 Lubricant – AG-90 is an extreme pressure rated anti-galling and anti-seizing lubricant
Loctite Anti-Seize Solutions – http://bit.ly/1Exu8GA
If you would like more information on these ranges or to purchase any of the products from this range please contact us.