Stainless steel fasteners

[womble]

Hi!

I am having a few problems being reported to me about the stainless steel fasteners used on a machine currently being built. The fasteners are of A4-70 grade and use nyloc nuts. When they are being tightened up by the fitters some of the fasteners are suffering from thread stripping (on the screw),some have sheared off completely. The ones that have tightened up ok generally shear off when trying to undo them! I specified a torque value given by the fastener supplier and the fitters are saying that the nylon patch in the nut is "jamming" up the threads on the nuts. I'm not sure as to wether the torque value is wrong or the fitters are right. Has anyone experienced a similer problem with locking nuts? Which are the best to use on stainless steel fasteners? Ive contacted the supplier about this but they are busy and can't get an engineer over until next week and I need an answer sooner (management!)
The screws are M8x1.25-6g A4-70, Nyloc style full nuts, 25Nm torque value (for reusable fastener),washer under screw head and nut. Hope someone can help!

Thanks.

 

[motorman]

I had this happen once and discovered that a cleaning solution was being used in the area (to save time degreasing) and the fumes were degrading the nylon. Solved that problem then it came back to haunt me and that is when I discovered that the nuts were being stored in the store room on the shelf above the cleaners! Anyway, check for possible solvent vapors, paint thinners, along with (heaven forbid) thread lubricants, etc. next check for heat sources. Nylon does not like any of that stuff. If you draw more blanks, check nylon manufactures website for a rather large list of all the other things that are in this world that will degrade (soften) nylon.

 

[wh]

We experience a similar problem with our 1/4-20 316SS threaded rod and lock nut with nylon insert. It has happened here at the plant, but it happens more frequently in the field when our customers install our product. The problem is assembling 2 like materials either too fast or under a heavy load which causes galling. Find out if your fitters are using power tools or examine the load during assembly. We ship a packet of Never-Seez Pure Nickel along with each product. It does not appear to affect the nylon locking feature.

 

[Kenneth]

Also see responses at thread725-18693...