In article , N_Cook
wrote:
On 08/09/2013 01:33, anorton wrote:
"Ned Simmons" wrote in message
...
On Sat, 07 Sep 2013 12:41:00 -0500, dpb wrote:
On 9/7/2013 10:39 AM, N_Cook wrote:
I need to do a hub-pulling type operation on a small piece of equipment
that is not made to be taken apart, so no purpose-made anchor points.
There are 8 off , 1mm tapped holes with 5 threads per hole , in the
aluminium available (used for undemanding normal fixing to the chassis)
, if I drill a plate to take 8 screws . The only screws of that
thread I
have are brass, probably preferable to steel anyway. For a similar
hub-pulling where there was purchase , after heating (limited as to
amount that can be used ), required about 4Kg of force to extract.
Would
4Kg , assuming even distributed loading over the 8 screws be above or
below the stripping force ?
I found these sites but not applicable for my purpose it would seem
www.tribology-abc.com/calculators/e3_6e.htm
http://www.roymech.co.uk/Useful_Tabl...ead_Calcs.html
The _rough_ back-of-envelope estimate here comes out at only about 20
lbf loading each assuming a soft Al. T6061 w/ a hard fastener might be
closer to 100 lbf.
I get 30 pounds, but maybe your aluminum is softer. And I didn't have
the benefit of a napkin. g
In any case, if the screws are not silicon bronze or some other high
strength alloy, it's probably a toss-up between stripping the threads
and breaking the screws.
--
Ned Simmons
In practice I think both your estimates are overly optimistic. In this
tiny, fine thread, the clearance is usually a large fraction of the
total thread depth, so the threads just engage near the tips. Usually
the failure mode is that the tips round over instead of shearing off.
Optical equipment I work with sometimes has these tiny screws.
Yes I would not say they were a tight fit. Decided to suck it and see
and potentially sacrifice half a tapped hole. So 3 turns only engaged in
one tapped hole and a spring balance, I took up to 2Kg with no problem.
So I'm now confident that 8x five thread screws and a compressible
washer/pad of silicone rubber under each screw head , will take the 5Kg
via an added hole-matching load-sharing plate.
I'd use belleville spring washers under each screw head, not rubber.
Alternately, a big washer to spread the load onto the rubber.
http://www.mcmaster.com/#belleville-disc-springs/=ofahsv
http://en.wikipedia.org/wiki/Belleville_washer
Joe Gwinn