Crossed Thread -v- Binding Thread

[1MarcoD]

Is there a good or even foolproof way to determine whether a threaded joint has been damaged by cross-threading (bad assembly) -v- binding (bad manufacturing tolerancing or gauges). We believe the attached are caused by binding - if it were cross threaded the damage should be at the start of the thread. However our supplier differs.

 

[zdas04]

I don't see how a bad assembly could have skipped the first half dozen threads before damaging the next thread.

David

 

[CoryPad]

I know of no method other than visual inspection to differentiate between cross-threading and binding after assembly and disassembly of the parts.

I don't see how this example could be cross-threading. You are correct, that would be at the beginning of the threads, not at the eighth thread.

Can you measure the threads (external and internal) with functional gauges?

 

[mcgyvr]

Is this a tapered thread?

 

[Timelord]

It looks like the thread galled. What materials are running against each other?

Timelord

 

[SincoTC]

I agree with Timelord; it does look like galling/cold-welding.

Some materials, particularly those that form a self protective oxide surface can experience rupture of this layer during assembly and some combinations have a definate affinity for each other and can soon become immovably fixed. A smear of MolySlip and lower assembly speed can help and a waft of WD-40 has saved my bacon when I felt a stainless fastener grab in an auluminium tapped hole.

I have also seen this type of failure in non suspect materials, it was initiated by stray beads of glass after the parts (not the threads which were masked-off) were bead-blssted, the fine glass dust is very invasive.

 

[1MarcoD]

Thanks for all the comments.

For Info: the part in the photograph is a SAE4340 (BS970 817M40) tube which was screwed into a cap of identical material. No glass beads or other blasting was used. The interior of the tube (i.e. not the threads) was treated with a phosphating-type treatment. The threads were treated with Lubriplate before assembly. The threads are not tapered, i.e. they're not supposed to interfere.

It's clear to us it's not cross-threading and our contention with the supplier is that the threads were made without gauges and therefore were not interchangeable - this is the result of "make-to-fit" gone wrong (make-not-to-fit, in fact). We don't have actual measurements, however.

 

[SincoTC]

I was worried about trapped glass (or other debris) because of the numerous black spots visible on the threads and the sudden start of the damaged area, which is what you get when something gets crushed and rolled along a thread groove, it disrupts any surface treatment and exposes virgin metal which then cold-welds, tears a bit out which adds to the jam and so on. The fact that thread and cap are both made of the same material may not help it resist this galling.

Are those black spots corrosion, if so that could be a failure starter, especially if there are high local pressures due to profile mismatch, poor geometry, fit or surface finish. Noting that the you say the parts were "made to fit" without gauges, this seem quite likely!

 

[Timelord]

Large diameter fine threads are especially prone to galling. Some materials will jamb even when well lubricated. I don't know about 4340, perhaps your suppler can help. If the material is prone to this behavior, the threads can be dimensionally correct and within tolerances and it will still happen. I had a 2"dia 32 TPI thread jamb while hand rotating. It was a 300 series stainless running against itself and there was no load on the well lubricated threads.

Timelord

 

[tbuelna]

1MarcoD,

It looks like those threads were single pointed, as opposed to being ground. Plus it appears that there is debris in the thread roots, which would cause galling.

It's not clear what diameter those threads are, but large diameter threads always have tolerance issues. If you want a threaded joint more than about 2 inches, you should let the machinist produce both halves of the joint. And you should pay the extra money for a "go/no-go" thread gauge to make sure the threads meet spec.

Finally, keep the number of engaged threads to a minimum. This will minimize the problems with pitch tolerance accumulation.

Good luck,
Terry