Unusual Thread Profile 2

[PDPumpGuy]

I have recently experienced two different fatigue failure on a large diameter studs used on reciprocating mud pumps. When examining the failure, we found a very unusual thread profile that we first thought was a manufacturing defect, however we have now learned it was by design. The pumps were made in China, and they call this thread profile a Zig-Zag, or Hackle profile over there. It's basically a normal angle on one side of the thread, but the other side is flat. It's apparently used when loads are applied in only one direction to help prevent vibrations from loosening the threads. Both the stud and the nut have a similiar profile.

Has anyone heard of this, and if you have any information about where I can learn more about it I'd very much appreaciate it. I'm trying to understand how this profile might affect the load carrying properties of the joint (as compared with a more standard thread profile), and how to best determine how to preload it.

 

[MikeHalloran]

What you describe sounds like a Buttress thread.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

Look at the "saw thread" at the end of the article

http://www.hexagon.de/dose/dose1e.htm#2.6

 

[yates]

Seems weird that a normal tension bolt would have this kind of asymetrical thread - the saw thread is normally associated with lead screws, as ACME threads on machine tools. It has a distinct disadvantage in that the nut must be put on the right way ! 'It's apparently used when loads are applied in only one direction to help prevent vibrations from loosening the threads.' appears to be incorrect, maybe 80% of all bolt applications use a bolt in tension and this need is fulfilled by standard threads. I fail to see why this type of thread unscrews less easily with vibration. This property is associated with the friction factor of the bearing surface (or washer) and is basically not affected by thread geometry.

 

[Kenneth]

A literature search (and/or a little bit of pondering) will confirm that thread flank angle has a direct influence on resistance to loosening in a vibratory environment.

 

[BillPSU]

Buttress threads are common in hydraulic cylinders and come as either 0 or 10 degree forms. Look in Machinery's Handbook. I assume there are metric equivalents out there.

 

[Kenneth]

Seems weird that a normal tension bolt would have this kind of asymetrical thread...

Couple of reason why not:

"Normal" bolts are thread rolled, for cost and other reasons. The asymmetrical buttress thread does not lend itself to thread rolling with acceptable tool life or without developing unacceptable defects. "Cut" threads both cost more to make in large quantities and have a poorer finish and defect level (and fatigue life) than rolled threads.

Buttress threads (while very efficient for transmitting load in a single direction) are also really crummy in bending. While proper joint design will minimize bolt bending loads, they are a reality that must be considered. I once ran a test program on some titanium aerospace fasteners with a modified buttress thread form that the designer was convinced was going to revolutionize the fastener industry. While they performed well enough when exposed to "pure" (laboratory) axial tension loading, they failed miserably in fatigue when compared to the UNJ thread “control” lot when exposed to the slightest off-axis loading.

As noted in the references, the low radial forces generated by the buttress thread form (due a loaded thread flank that its nearly perpendicular to the thread axis) make it particularly well suited for joining tubular members subjected to “pure” tension loads in a single direction (e.g. oilwell casings, hydraulic system components, propeller hubs, gun breaches, etc.) Their relative efficiency has also found them use in plastic threads.