[xj-s] Torquing Bolts

[pjsmith <pjsmith@gil.com.au>]

I was recently reading something about torquing bolts and thought you
might be interested if I reproduced it here:


"When bolts are tightened they stretch.  Like a piece of elastic rope,
it is the bolt's stretch that supplies the pressure to keep the engine
components together.  However only about 10 percent of the torque
converted into bolt stretch; about 90 percent of the applied torque is
lost to friction.

With that in mind two points emerge:
1.  "Giving it a nudge" is not worthwhile as 90 percent of the "nudge"
is also lost in friction.
2.  With 90 percent of torque lost in friction, reducing this friction
by only a small amount will significantly increase bolt stretch.

To demonstrate this effect ACL manufactured a device to measure the
stretch load in cylinder head bolts.  The device is portable and can be
placed directly onto a cylinder block to give a realistic result.  As an
example, let's look at a bolt from a popular six cylinder engine:
	size:	M12 (12mm)
	grade:	12.9 (high tensile)
	washer: intergral with bolt head
Stretch load at a torque of 115Nm:
	clean & dry		2.1 tonnes
	oil on the threads	2.6 tonnes

Lubricating the threads gave us 23 % more bolt load but what would
happen if we added oil under the bolt head?
	Oil under the bolt head 6.4 tonnes

Because this bolt has no washer, the relatively hard bolt head quickly
welds itself to the soft aluminium of the cylinder head.  A drop of oil
is enough to significantly reduce the friction under the bolt head and
allow more torque to reach the thread.

What about a bolt with a washer?
	Size:	M10(10mm)
	Grade:	10.9(medium tensile)
	Washer:22mm diameter

Stretch load at at torque of 84Nm:
	Clean & dry			3.9 tonnes
	Oil on thread & bolt washer  	5.5 tonnes

With a washer, the washer fretted onto the aluminium leaving the hard
bolt head to rotate against the hard steel washer (hence reduced
friction).

A summary of our testing is as follows:
1.  Threads and bolt heads, or nuts and washers should all be
lubricated.
2.  A bolt designed to have a washer should not be used without one. "

Now I am not an engineer so all that raises an immediate question -- if
lubricating the thread and washer can increase the torque by up to 41 %
doesn't that mean that the bolt is stretching a great deal more and
doesn't that risk damaging aluminium threads?  Is this advice wise only
on iron blocks?

Regards Peter Smith

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