Self Tightening Bolts, Self Locking Bolts

Discussion in 'General Motoring' started by karl, Nov 7, 2005.

  1. karl

    karl Guest

    "Self Tightening Bolts theory.
    Warning: this page is only a theory, not a fact."

    That's a good description.

    Could someone please explain what self-tightening and
    self-locking bolts are and give examples. The author may
    have the latter in mind.

    "Figure 4.1 This picture explains the great inertia and
    centrifugal force"

    "When ever there is a difference in inertial force (as
    pointed out with the arrows) the pulley will move. Not
    180-ft-lb torque can hold the pulley still."

    I wonder what this is about.
    karl, Nov 7, 2005
  2. karl

    Matt Ion Guest

    As a complete aside, this reminds me of a former high-school physics
    teacher's pet rant: "centrifual" force, or the observed
    outward-from-center force on a spinning object, he would always insist,
    is not a real force. The real force in play is centripetal force, or
    the tendency of the point on the object wanting to continue in a
    straight direction on tangent to the spin, is the ACTUAL force at work.
    "Centrifugal" force is only an imaginary thing.
    Matt Ion, Nov 8, 2005
  3. karl

    Burt S. Guest

    What this mean is that the object in motion will move in the direction
    of movement. But when there is a force that tries to change that
    motion usually from the engine or transmission the pulley will move
    when it's not intended to. Scroll down to Figure 4.2. It may explain
    more about centrifugal force not centripetal force. Centripetal is
    moving or directed toward a center or axis. The theory is that the
    centrifugal force can effect the bolt's movement in some way or
    just simply tighten up bolt.
    Burt S., Nov 8, 2005
  4. karl

    Matt Ion Guest
    Matt Ion, Nov 8, 2005
  5. I am unconvinced by this theory.

    1) If microscopic ratchet teeth are created to cause the bolt to
    self-tighten, wouldn't they be destroyed when the god-awful tight bolt is
    broken loose? The bolt at least should be specified as a "use once" item,
    regardless of how the mating threads in the crank fare.

    2) In order to tighten, the bolt will have to move with respect to the
    pulley. That means the washer must have similar ratcheting action, and on a
    similar microscopic level to allow the ratchet to occur with miniscule
    motion. That means if the washer is less than pristine and is reused the
    bolt won't self-tighten.

    3) The forces are downright outrageous. In round numbers, if the washer
    diameter is 1/2 inch and the bolt thread diameter is 1/4 inch, to tighten
    past the 200 ft-lb mark the bolt head has to experience 5000 pounds force
    from one side to the other, or 10000 pounds force on one side relative to
    the center. The equivalent force on the thread is double that.

    4) If there is significant motion of the pulley relative to the crank, the
    mating surfaces will wallow out. We see it often enough with splined drive
    axles that are insufficiently torqued.

    Altogether, it doesn't add up. Torsional forces between the pulley and crank
    must act unidirectionally on the bolt, with several tons of force being
    transferred through both sides of the washer and without damaging the pulley
    or crank mating surfaces, with enough movement to materially tighten the
    bolt. The theorized ratchet mechanism has to operate on a microscopic basis,
    not be damaged in removal, and to allow effortless unthreading when the bolt
    is broken loose. It must work over a wide range of lubrication, including a
    penetrant oil film or being cleaned with brake cleaner. I'm glad I haven't
    been asked to design something like that, particularly if I could just
    specify tightening to a different torque in the first place.

    Michael Pardee, Nov 8, 2005
  6. karl

    Elle Guest

    It's the stresses in the bolt, not the forces acting on the side of it, that
    matter. Specifically, torquing down on a bolt is the equivalent of
    stretching it until it holds two things together. The torquing causes the
    threads to act against each other so as to place the bolt in tension (as
    opposed to compression).

    For correlating torque to the axial load it produces, one finds somewhat
    crude estimates like that given at the bottom of . But of course, this formula will
    require tweaking depending on conditions. E.g. fine thread vs. coarse

    Anyway, it's really about 200 ft-lbs. divided over the six edges of the
    roughly 1.7/2 cm (= about .33 inch = about 0.028 foot) radius bolt head (for
    a 91 Civic, for one), anyway. (This Civic's pulley bolt has a 17 mm head and
    14 mm nominal diameter.) So something like 200/6/(0.028) = about 1200 pounds
    is applied to each bolt head edge. Key word being "edge." Then one has to
    think about what it means to "apply" this force to the whole edge. It's
    distributed over the surface of the edge, for one thing. If one took 1200
    lbs. and set it on a bar of steel with a cross-sectional area of about 1/8
    inch by 1/8 inch = 1/64 inch (conservative for this back-of-the-envelope
    calculation), the stress would still be only 1200*64 = 77000 psi, far below
    the yield strength of typical steels. And it's not being applied
    perpendicularly to each face, but more in shear, besides.
    Which mating surfaces?
    If the above is supposed to relate to your earlier calculation, then I think
    there's a conceptual error here.
    I have doubts that a cold bolt-pulley-crankshaft assembly would hold up to a
    hand application of 300 ft-lbs. of tightening torque. 'Cause crude
    estimators like the one I cite above indicate this would produce in the
    neighborhood of 300(12)/(.2*.55) = 32700 lbs. of axial load in the bolt, or
    32700 / (Pi r^2) = about 137,000 psi of tensile stress in the bolt, which is
    mighty close to the yield strength (~ 130,000 to 150,000) of many steels.
    This is too close for engineering comfort.

    Which is why I am led to believe galling, aggravated by extreme heat cycling
    and the high loads of that pulley working on an initially pretty tight bolt,
    plays at least some role and possibly all of it.
    Elle, Nov 9, 2005
  7. I think we are talking about two separate things. I'm looking at what is
    required for force from the theorized pulley movement (in the original link)
    to tighten the bolt beyond 200 ft-lbs, rather than the tightening being from
    application of a socket. I don't see how that could be transmitted through
    the washer, even if pulley/crank movement occurred without wallowing out the
    mating surface between the crank and pulley.

    Miscommunication aside, we seem to be on the same page. The bolt isn't
    turning to tighten itself, it's just sticking.

    Michael Pardee, Nov 9, 2005
  8. karl

    notbob Guest

    Maybe so, but the page is too damn long. Trim your posts, ferchrysakes!

    notbob, Nov 9, 2005
  9. karl

    Elle Guest

    Oh. That is different. Some of my comments still apply, but I think it's too
    much of a morass to sort out, under the circumstances.
    I don't claim the bolt sticks when it tightens in operation (in theory). I
    do propose that the crankshaft-pulley assembly moves relative to the bolt at

    No big deal. Some time maybe we'll get some studies of whether the bolt does
    move relative to the shaft under some operating conditions.

    Related aside: Does anyone know whether Honda specifies replacing this bolt
    after so many timing belt changes?

    Someone here noted that dealer service shops apparently mark the bolt each
    time it has been removed. There could be a few reasons for this. I'm
    thinking one of them is to keep a record of how many times the bolt has been
    loaded yada a certain way.
    Elle, Nov 9, 2005
  10. karl

    karl Guest

    | Design Considerations
    | The first requirement in determining the amount of torque
    | to apply is a knowledge of the desired bolt stress. This
    | stress based on the yield strength of the bolt material. It
    | is recommended that the induced stress not be allowed to
    | exceed 80% of the yield strength. In the design of a
    | fastener application which will be subject to external
    | loading, whether static or dynamic, it will be necessary to
    | establish bolt size and allowable stress in accordance with
    | current engineering practice.
    | The mathematical relationship between torque applied and
    | the resulting tension force in the bolt has been determined
    | to be as follows:
    | T = Torque required (inch pounds)
    | F = Bolt tension desired (Axial Load) (pounds).
    | D = Nominal bolt diameter. (major dia.)
    | EQUATION: T = .2 D F
    | This relationship is based on the assumption that regular
    | series nuts and bolts with rolled threads are used, acting
    | on surfaces without lubrication.

    What a rubbish! This formula is simply wrong, dead wrong!
    The bolt diameter is irrelevant, but the pitch, which is inversely
    proportional to the Force, is missing from this formula.

    "The [CORRECT] mathematical relationship between torque applied and the
    resulting tension force in the bolt," ignoring friction, is:

    T = Torque required
    F = Bolt tension or compression desired (Axial Load)
    P = Pitch

    T = P*F/2*Pi, or
    F = T*2*Pi/P

    It is likely that the constant ".2" in the wrong formula

    T = .2 D F

    is chosen such that for common threads reasonable results are obtained,
    but it is irresponsible not to point out the limitation of this
    karl, Nov 21, 2005
  11. karl

    Elle Guest

    Karl, it's a formula for approximating. Too many
    non-engineers operate under the illusion that engineering is
    an exact science. It's usually not. (Just as medicine is an
    inexact science.) Engineering computations are fraught with
    assumptions and of course limitations. Torque-axial load
    relationships for bolts are a great example of why
    engineering can't be an exact science per se and so
    approximating formulae are often appropriate. For one thing,
    as has been pointed out, friction effects vary a good deal
    and fairly unpredictably over the life of a bolt, and can
    drastically affect the torque-axial load relationship. For
    another, material manufacture means the strength of the
    material cannot be known precisely. For a third, geometries
    are inexact from the get-go. For a fourth, as materials are
    loaded and unloaded, their material properties may change,
    so over the life of, say, a bolt, the load at which it may
    fail can go down.

    We can only approximate the torque-axial load relationship
    and build in factors of safety to anticipate worse case

    Diameter is relevant. The derivation of the formula is
    complicated. I could not do it off the top of my head,
    despite having quite a bit of experience teaching strength
    of materials subjects (that is, teaching the design of
    beams, pillars, fasteners, etc.; anything that has shear or
    axial stress in it upon angular or axial loading).

    Marks Standard Handbook for Mechanical Engineers' has
    another formula which you might like more, assuming you
    could accept that figuring out how the geometry of threads
    "causes" torque to become axial load is not an easy task:

    F = 2 Pi T / (L + kL sec b sec d cosec b + k ' D 3 Pi / 2)

    L = the pitch
    b = thread angle
    k = the coefficient of friction
    d = sec (angle between faces of thread/2)
    k ' = coefficient of friction between nut and seat (bolt
    face and washer?)

    (Hopefully I copied this correctly. It's probably on the net

    As you can see, thread diameter still of course plays a

    I'm sure we could find several more formulae, good for
    certain conditions and to a particular degree of certainty.
    Yet another appears below. Took about ten seconds of
    googling effort. I just pulled up the first site that came
    up in a google search for {torque bolt formula load}.
    Don't know where you got this, but its omission of diameter
    says a lot.

    Here's another formula:

    T = Fp * K * d

    d here is diameter.
    Karl, you evidently missed the qualifier above, stating that
    the formula could be used as an approximation for /regular
    series/ nuts and bolts with rolled threads, etc.

    Any competent engineer knows that formulae such as the one
    at the site above is an approximation and of course has
    limitations, at least some of which are stated at the site.

    In sum, as much as I hate to be dismissive, the reality is
    that this is a complicated subject. Grasp of the precise
    nature of torque-load relationships requires study and high
    achievement in several college level engineering courses.

    OTOH, my sense is that a lot of folks here do have a feel
    for how torque does cause axial load; the effects of
    friction, diameter, and pitch; etc. So some simple truths
    (or attempts to get at the truth) can be discussed and
    analyzed and even debated.
    Elle, Nov 21, 2005
  12. karl

    karl Guest

    Elle wrote:


    Elle, I wish you a nice Thanksgiving, and I will respond after that.
    karl, Nov 22, 2005
  13. karl

    karl Guest

    The basic laws used in engineering are exact, just like in physics.

    Yes, I was wrong stating, "The bolt diameter is irrelevant." This is
    only true without friction. I was too fast - didn't think through it.

    When developing relationships one starts from simple systems and
    refines them as needed. In this case one starts assuming no friction:

    F = force (axial load, tension)
    T = torque
    W = work
    s = distance traveled
    P = pitch

    Example: Lifting 1 pound 1 foot:

    W = F*s = 1 lb * 1 ft = 1 ftlb

    Applying this to bolts and nuts: the axial work (with s=P) is equal to
    the rotational work:

    W = F*P = 2*Pi*T*
    F = 2*Pi*T / P

    This is the basic relationship between the tension in the bolt and the
    applied torque, ignoring friction. It shows that the tension is
    proportional to the applied torque and inversely proportional to the
    pitch (as one would guess). The diameter is irrelevant in this ideal
    case assuming no friction.

    In the real world, with friction present, both components of it - the
    friction at the face and the friction in the threads - are depending on
    the diameter.

    Not I "missed the qualifier" - it is unequivocally stated,
    Now, this is very clear. But this formula omits the pitch on which the
    tension is inversely proportional. Elle, you, it seems, missed my
    qualifier, that the formula I showed applies when "ignoring friction."

    That applies to you, but I bet there are many people who, when they see
    the formula "T = .2 D F" trust it to be true and believe pitch is
    irrelevant - there is no place for it.

    I don't think so. Even incorporating friction doesn't require special
    knowledge, just a little math and physical understanding.
    karl, Nov 29, 2005
  14. karl

    Elle Guest


    The laws of physics are exact, and engineering design
    certainly does rely on laws of physics. But engineering
    design also takes into account the inability to ascertain
    quantities accurately. Above, I named several examples of
    quantities that cannot be measured accurately and how they
    figure into fastener design. Judgment based on experience
    and not without some subjectivity is essential to the
    engineering design process. Science by itself does not--and
    cannot--build car engines, etc. Engineering does.
    It's how much the bolt deflects under the axial load F, not
    pitch P, that should be used here.

    For one thing, when tightening a bolt, the threads actually
    move a little farther apart from each other. So under axial
    load, the pitch changes.

    The higher the axial load, the more the pitch will be off
    from its design value.

    Bolt advance under no load (and due to the effects of
    rotation and the amount of pitch) is different from bolt
    deflection due to axially loading the bolt.
    Omitting friction, it's more something like:

    dW = F(x) dx = T(omega) domega

    x = deflection in the axial direction
    omega = angular deflection in radians
    F(x) = axial load, which is a function of deflection x
    T(omega) = torque, which is a function of angular deflection
    This is a nice first attempt and certainly shows some
    understanding of the force and torque relationships in
    bolts, but it does have, for one, the blatant mistake I
    identify above.
    Elle, Nov 29, 2005
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