Torque Values.

Probably a dumb question, but that's never stopped me before. Why do
bigger bolts have higher torque settings than smaller bolts doing the
same job? Only reason I can think of is that bolts have to stretch a
bit to stop them coming loose, & the bigger the bolt, the more torque
it needs to stretch. But that makes no sense at all for steel bolts in
alloy holes, where presumably the bolt would strip the thread before
stretching.

 

Err, dunno. Are they definitely doing the same job?

Is this a specific application? If so, are the smaller bolts
higher-tensile bolts?

 

Jobs with the same stresses, yes. E.g. they changed the size of the
sump plug on Triumph 885 triples from M15 to M22 at some point, & the
M22 version has a higher torque. Likewise engine casing bolts - some
are 8mm, some 10mm, again with different torque values.

Not really specific, & no, not as far as I know. Lots of manufacturers
quote general torque settings for bolt sizes not specifically mentioned
too, which seem to be based purely on bolt size rather than application.

 

erm, off the top of my head, coarser thread = steeper thread angle =
more torque for the same pull?

Maybe.

 

That's a real can-of-worms question. Loading a bolt to a given tensile
stress is easier with large bolts because thread form, thread
engagement, heat treatment, tightening friction etc., are easier to
control. Often the limiting factor is the strength of the female thread.
As you say, Ali and cast iron/steel are bad. Rolled threads and nuts
are good.
Unless there are critical design requirements, it's cheaper and more
reliable to use sub-optimum loadings on standard bolts, and use locking
devices to avoid vibration effects.

Having seen your reply to Pip, Triumph have never in their entire
history been able to make a sump plug that doesn't strip, leak or fall
out. However, it's a bung, not a bolt, so not a good example.

 

As an approximation:

T = C x D x S

T = Torque
C = Coefficient of friction
D = Bolts nominal diameter
S = Bolt's desired tensile load (usually calculated to achieve a certain
elastic strain)

I have used random letters because I can't be arsed to work out the ascii
codes for the conventional symbols.

 

OK, that kinda makes sense. And having thought about it some more,
wouldn't a smaller bolt have a higher surface area relative to its
size, & therefore greater friction? Not that I'm sure that's got
anything to do with the price of fish.

<looks out window at two Triumphs with sump plugs that have never
stripped, leaked or fallen out>

Liar!

Still just a short stumpy bolt though init.

 

Well, a larger sump plug would have a larger circumference crush
washer, so more torque needed there.

In the case of other bolts, umm. Some guesswork here:

IWHT that if the bolt is bigger then it's for a reason and that is
most likely to be to withstand greater stress. After all, bigger bolts
usually mean more weight and more crankcase metal around the bolt, so
you'd want to avoid that and use smaller bolts if at all possible.
More. smaller bolts will spread the load, which is especially useful
where you're joining oil-filled spaces and dont want oil weeps.

Another possible reason: if it isn't practical to have a bolt in a
particular location, the bolts on either side might have to be beefier
to compensate. Also, if bolts are longer then they stretch more, so
you might need larger diameter bolts tightened at a higher torque so
the stretch is similar to surrounding bolts.

Another reason might be that stresses will vary around the crankcase,
for example, you might need stronger bolts around the crankshaft
holding a horizontally-split case together, because the forces trying
to split the cases are greater here. The same might apply to the
vicinity of, say, high-pressure oil galleries that cross the crankcase
split.

Another reason I've just thought of: threads of a given series (such
as metric coarse) have pitches that increase with diameter but only
slightly less than in proportion to bolt circumference (so the 'ramp'
angle is roughly similar); the friction from the larger diameter
threads would mean a slightly greater torque required.

 

Load per unit area is what counts. A small standard production type bolt
with a nominal engagement of 75 percent might have anywhere from 40 to
85 percent thread engagement, with a rough finish and poorly formed
thread roots, whereas a precision ground aircrafty type quality bolt
will have engagement controlled to much finer tolerances (possibly +/-5
percent), thread root properly formed to minimise cracking and often
surface treatment to enhance locking.

Well, (looks at own sig) only 50 percent lying.