Fastener preload.
Fastener preload.
(OP)
Can any one tell me an easy way to measure the preload on a small screw. I have a #10 screw used to assemble 2 housing halves. I am looking to switch to a thread forming screw to elliminate the secondary opperation of tapping. The dillima comes from the driving torque of the thread rolling screw. It takes 16in/lbs to drive the thread rolling screw and my #10 has an assembly torque called out at 20in/lbs. The thinking around our office is that this takes away from the end clamping force by using most of the torque while driving. The screw manufacturer however disagrees with this and says that the torque will not effect the clamp force.
So I need a quick, cheap way to measure this to decide which school of thought to go with.
Any suggestions??
So I need a quick, cheap way to measure this to decide which school of thought to go with.
Any suggestions??





RE: Fastener preload.
RE: Fastener preload.
RE: Fastener preload.
it's going to 16 ftlbs of torque to form the thread; maybe it'll take more to form the thread, maybe less. once the thread is formed, who cares about this. as israelkk says, back off one turn then tighten to 20 ft.lbs and you'll have the required preload.
RE: Fastener preload.
These two oppinions are what I am currently trying to decide between (my boss & screw manf.)
Now the question becomes: How do I measure preload with out a load cell, (no budget)??
RE: Fastener preload.
Cheers.
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RE: Fastener preload.
Preload = Torque/(Screw Diameter x 0.15)
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RE: Fastener preload.
RE: Fastener preload.
I was assuming that the 16lb-in to form the thread is a fact. Therefore, to get the clamping force equivalent to a non forming screw you have to use 36lb-in (16lb-in goes to form the thread and the 20lb-in to create a clamp).
However, if the 16lb-in is varrying too much then you have to form the thread first and then back off one turn and re-tighten the 20lb-in.
RE: Fastener preload.
RE: Fastener preload.
------------
See FAQ569-1083 for details on how to make best use of Eng-Tips.com
RE: Fastener preload.
RE: Fastener preload.
RE: Fastener preload.
Measrue the torque needed to drive a screw the first time while forming threads.
Remove the screw and measure the torque needed to run it down into the now threaded hole.
Are they the same? On the first shot the screw is cutting threads - always. The torque needed to cut the threads needs to be subtracted.
This is the same as with a prevailing torque fastener, the prevaining torque needs to be accounted for.
RE: Fastener preload.
------------
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RE: Fastener preload.
It seems to me the only way the prevailing torque of thread cutting would NOT be additive is IF the thread cutting stopped when the screw first seats.
After the instant the screw seats there is still some thread cutting to be done if the screw is to advance the 1/8 turn (or whatever it takes to tension the screw). I'd note that whether the screw is mostly stretching or a gasketed joint is mostly compressing the screw must still "turn more."
One test beats an hour of my intense speculation.
RE: Fastener preload.
Ok, it doesn't really speak to the "how to measure preload" question, but the thread has diverged to discuss the "does the effort of cutting the threads need to be accounted for?" question.
Torque-o-meters are available, however several simple (albeit rather crude) methods are:
Use a beam type torque wrench.
Use a micrometer-adjust type torque wrench, start with it set as low as it will go and increase incrementally in small amounts. Note the lowest setting that results in rotation before the click.
Use a force-o-meter at the end of a common wrench.
RE: Fastener preload.
The difference between the two sides of debate is great enough you won't need anything fancy. The shape of your parts will dictate the type of linkage or lever you will need to hook the scale to your part.
Hook up the scale and measure the amount of torque required to move the scale.
If the original clamp spec was to ensure the screws would not come loose you made need to change the spec for the new style of screw.
Barry1961
RE: Fastener preload.
I have machined a jig and by using 2 die springs and a scale have found that the tapped hole compresses the springs more than the thread forming screw at the same torque. So I now believe that the drive torque should be additive.
RE: Fastener preload.
Since you have the test jig now you might as well go the extra step and test to determine the torque you need for this application to get the preload that you want.
More better and no harder than figuring out what torque you need to add to the "standard" 20 lb-in.
RE: Fastener preload.
Hydrae
RE: Fastener preload.
In this instance, a blind hole with no relief at the bottom, the torques must add.
If you're working fast, add a little vigorish for whatever air is being compressed in the hole, and be stingy with the Loctite.
Similar to the danger of hydraulic locking, some of the point forms on thread rolling screws will force a few chips ahead of the screw, so be sure the cored/drilled hole has a little extra depth, or better, make it a through hole.
Mike Halloran
Ft. Lauderdale, FL, USA
RE: Fastener preload.
RE: Fastener preload.
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