Date Published:Sep. 27. 2016
Vol.1 Thinking about "Galling and Seizing"
Hello everyone. My name is Nedzigon.
I am a monster made up of screws.
Today, I will start a lecture called "What is a Screw? ".
I would be more than happy to assist you with any issues you may have.
Today, I would like to talk you about Thread Galling and Seizing.
Do you have any experience with screw threads getting stuck and seized up ?
If you try to drag a seized screw out by force, you will end up wasting tremendous time and effort on a broken screw.
So, how can we solve the problem? I will teach you the solution today.
What exactly is Galling and Seizing?
Galling is the phenomenon in which a screw cannot rotate for some reason. This is also commonly referred to as seizing.
Why does a screw gall?There are two reasons for this occurrence:
1) friction and adhesion (cold welding)
2) outside factors, such as foreign matter/burr
Friction and adhesion (cold welding)The screw expands due to friction heat(cold welding) that occurs during screw tightening, causing adhesion in the threads which leads to galling and seizing. Stainless steel screws are more likely than other screws to seize for the following reasons:
・they have a higher coefficient of friction (twice that of general steel), and generate heat more easily
・they have a lower rate of thermal conductivity (1/3 that of general steel), meaning heat is not easily released
・they have a larger expansion coefficient (twice that of general steel), meaning that the male and female, i.e., the screw and hole, adhere more easily
Therefore, threads are more prone to adhesion. Since galling and seizing are even more likely to occur in high-temperature environments, steel screws are not entirely safe either.
Outside factors such as foreign (external) matter/burrOther causes of galling and seizing in steel screws include the following:
・Screws are tightened with burr, etc. still attached
・Excessive tightening torque
・Surface roughness or dents are present in the thread
How can we prevent Galling and Seizing ?
First, regarding outside factors, galling and seizing can be prevented by checking the condition of the thread before tightening and using an appropriate torque.
So how do we prevent galling and seizing that is due to friction heat and adhesion(cold welding) of threads? While changing the thermal conductivity or thermal expansion coefficient is difficult, the friction coefficient can be decreased.
For example, by:
・ lubricant application
・ surface treatments like Anti-Galling and Anti-Seize
or other countermeasures.
Lowering the friction coefficient makes the screw easier to tighten, however tightening at the same torque could still damage the thread; there's no point in it all if galling and seizing then occur due to outside factors.
Therefore, when changing the friction coefficient, be absolutely certain to adjust the tightening torque as well.
Refer to the discussion of axial force change due to surface treatment.
＞＞Comparison of axial force with regard to different surface-treatments
For situations such as, "Can't use lubricant" or "Plating keeps peeling off" consider using the surface-hardened screw SNSL-PN to prevent seizing even in clean environments.
That's it for today.
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