Torque Values.

Discussion in 'UK Motorcycles' started by Krusty, Jul 15, 2009.

  1. Krusty

    Krusty Guest

    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
    Krusty, Jul 15, 2009
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  2. Krusty

    Pip Luscher Guest

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

    Is this a specific application? If so, are the smaller bolts
    higher-tensile bolts?
    Pip Luscher, Jul 15, 2009
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  3. Krusty

    Krusty Guest

    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.
    Krusty, Jul 15, 2009
  4. Krusty

    Simon Wilson Guest

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

    Simon Wilson, Jul 15, 2009
  5. Krusty

    Dentist Guest

    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.
    Dentist, Jul 15, 2009
  6. Krusty

    Chris H Guest

    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.
    Chris H, Jul 15, 2009
  7. Krusty

    Krusty Guest

    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>

    Still just a short stumpy bolt though init.
    Krusty, Jul 15, 2009
  8. Krusty

    Pip Luscher Guest

    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

    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.
    Pip Luscher, Jul 15, 2009
  9. Krusty

    Dentist Guest

    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.
    Dentist, Jul 15, 2009
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