How does water in threads affect tightening torque?
How does water in threads affect tightening torque?
(OP)
I know that thread lubricants and contaminants cause the fastener torque requirement to be lower. How does water affect torque though? We have implemented torque equipment on a hydraulic motor assembly where we have 21 3/4-10 bolts to mate the plates together. The drawing is calling for 56 kg-m on the fasteners. We have the equipment set for 56 kg-m and we keep breaking fasteners. The plates are shipped with a light coating of oil on them to avoid rust, and we are pressure-washing them off before assembly. There is definately water and possibly light oil in the threads during assembly. I am assuming this is the problem, so I am about to start testing this theory. Just wondering if anyone has run into this before, and what was done. The bolts have been checked and are okay.





RE: How does water in threads affect tightening torque?
nick
RE: How does water in threads affect tightening torque?
I would question how the installers are measuring that torque...there are not many wrenches calibrating in KgM.
If the failures are a result of excessive lubricity, it is most likely coming from residual oil. At the contact stresses that you achieve on the pressure flanks at yield, water will not provide much lubrication.
Dick
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
Interesting that you can get angle on the drive guns, that may give us another way to get to your target preload. How thick is the clamped component and is there a gasket in the joint? It may be possible to come up with a rough Torque / Angle stratgey via calculation.
I questioned the gun calibration because it is very often the source of the problem and we spend huge amounts of time chasing problems that cease to exist once the units are properly calibrated.
Dick
RE: How does water in threads affect tightening torque?
Plain water, soap, detergant, solvant?
Are you blasting each fastener individually, or tossing a bunch in a bucket and blasting away?
It seems quite possible that you are creating some sort of emulsion of soap and oil.
What is your tightening sequence?
RE: How does water in threads affect tightening torque?
Perhaps it would be a good idea to wash & dry the parts before assembly? then paint or preserve afterwards.
Either that, or RTV seal the thread/blind hole leak paths be for blasting them full of water
RE: How does water in threads affect tightening torque?
The fasteners are not pressure washed, only the threaded motor housing is washed. It is straight water that they are using just to blast some of the oil coating off. The tightening sequence is the 4 corners, and then all the rest in a line from one end to the other. The bolts are arrayed in a c-shape.
The weird thing is that we have two different motors and casings that we use. This is the larger set of the two. The other uses slightly smaller bolts and is torqued to 28kgm and we never have any problems.
RE: How does water in threads affect tightening torque?
If these are blind holes, are you sure that you have enough thread depth? If the tapped threads are running out before the part is fully seated you will be putting all ofthe torque onto the shank of the bolt and you may be getting a torsional failure before the parts see a significant amount of clamp load.
Is there any pattern to what location the broken bolts occur? If you are seeing them in just some of the locations that will tell us a lot about possible causes.
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
Solutions are varied. Increase thickness of 25mm plate to increase stiffness. Change fastener to higher strength such as Bowmalloy bolts. Add more fasteners at the high stress points. Go to a larger fastener. Redesign mounting structure to eliminate localized loading if enough strength is supplied by the fastener.
I would also recommend using the correct units when torquing either Newton meters or foot pounds. Kilograms are a unit of mass not force.
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
Is there any caoting on the bearing surface that the bolts are seating on? We just solved a sporadic breakage issue (<1%) that turned out to be caused by contamination of bearing surface with an aluminum bearing undercoat. It increased the lubricity enough to cause breakage during installation.
RE: How does water in threads affect tightening torque?
RE: How does water in threads affect tightening torque?
I also am a manufacturing engineer but I do not have blind faith in a design until I know it works. Check what grade of bolt you are supposed to be using. Verify the bolts you are using meet that specification. There has been many cases where inadequate bolts (usually foreign) are substituted. Your purchasing department may not even know to buy the correct bolts. Your Quality department should already have that answer but if not have the bolt supplier certify the bolts. Changing a torque spec is a design engineering change as you are changing the clamp load on the motor.
A blind tapped hole has a lot of room for the chamfer for the tap and unless the hole is completely full of water I don't think the water will matter. The thread has clearances between the pitch diameter and also clearance in the crest and root of the threads. You could be bottoming out the fastener in the hole but even then the torque should not be enough to twist the bolt off. I think you have the wrong grade of bolt.