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Thread: Identifying a force

  1. #1
    Associate Engineer
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    Confused Identifying a force

    Hello,

    I am not an engineer and I am trying to identify the scientific name of a force/tension that occurs in a particular situation.

    We have two pins on one side and two boreholes on the other side.
    The distance between center of the pins is shorter than the distance between the center boreholes.
    When the two sides are put together, the pins go into the boreholes but a force/tension is created due to the uneven distances.
    When they go into the boreholes, the pins are deformed or put under tension.

    I am desperately searching if there is a scientific name or term for this.

    Many thanks !

  2. #2
    Administrator Kelly Bramble's Avatar
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    It's not clear what the loading is from your description but there are six types of stress induced by loading:

    Compression
    Tension
    Shear
    Bending
    Torsion
    Fatigue

    There can also be combinations of loading, like Bending-Torsion.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Principle Engineer
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    The deliberate misalignment of locating pins creates an "interference fit".

  4. #4
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    Quote Originally Posted by Hudson
    The deliberate misalignment of locating pins creates an "interference fit".
    "Deliberate misalignment of locating pins" is a definition for interference fit that I have not come across before.

    As someone who had distant proximity to machining in general and tool & die work in particular, my recollection is that while misalignment of locating pins occured on jobs, it was NOT deliberate. I will readily concede that one machinist of my acquaintance whose style of workmanship led to misalignment of locating pins would have attempted to dismiss the error by claiming that he INTENDED for this to happen.

    Quote Originally Posted by laster View Post
    We have two pins on one side and two boreholes on the other side.

    ... When the two sides are put together, the pins go into the boreholes ...
    To visualize the description of your object, I think of a Master Link: one plate has two pins, the other plate has two holes.

    Under circumstances where the holes are sized for a CLEARANCE FIT (their ID is larger than the OD of the pins) and the c/c distances of the pins on one side and the holes on the other are the same and the pins are parallel, then the plate with holes will install onto the pins of the opposing plate and slide along the length of the pins until it rests flat against the plate the pins are installed in without creating any of the individual forces posted by KB or combinations thereof.

    Under circumstances where the holes are sized for a TRANSITION or INTERFERENCE FIT (their ID is a little or a lot smaller than the OD of the pins) and the c/l distances of the pins on one side and the holes on the other are the same and the pins are parallel, , then the plate with holes will install onto the pins of the opposing plate WITH EFFORT and REQUIRE FORCE to slide along the length of the pins, but it will still be possible for the plate with holes to rest flat against the plate the pins are installed in.

    While the plate is being moved, there will be several of KB's forces at play, ie: axial compression force on the pins, radial compression/tension force on the holes/pins localized at contact points along the pin length. When movement of the plate with holes stops, axial compression force ends, leaving just the forces of the tight holes acting on the pins, assuming common pin/hole c/l's and parallel pins.

    IMO, shear, bending, torsion and fatigue forces would not be factors in the above scenario. Alternatively, where the c/l's of a master link's pins on one plate and holes on the opposing plate were NOT the same and/or if the pins were not parallel, then bending and shear forces would likely occur, possibly accompanied by fatigue forces in a dynamic application. Torsion seems unlikely in my master link analogy, buttt ...

  5. #5
    Associate Engineer
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    Thank you so much for your input.

    I am aware of the interference fit that occurs when the diameter of the pins is larger than the one of the holes.

    Combined with this interference fit, is the other force for which I am searching a name.

    The object is indeed composed of two plates (in fact two sides of a plastic casing), one side having the holes and the other side having the pins.

    The combination of the interference fit and the other force ensures that the two parts of the casing can be fitted.

    So there is no movement involved. Only the two forces : intereference fit and the other one which I would like to identify.

    Thank you so much already. Your help is appreciated !

  6. #6
    Associate Engineer
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    In the absence of more informed contributions to your question, I offer the following.

    Since the c/l spacing of the pins is less than that of the bores, my response is that, at the interface where the pins pass through the bores, there will be a lateral compression force (perpendicular to the axis of the pins) between each pin/bore pair. If the pins project up vertically and we are looking down at the circumferences of the pin/bore pairs, this pin/bore compression force will be localized where the bores are closest to each other.

    If you are seeking a different term for this particlar force, I am unable to be of further assistance.

    Be aware that the pin/bore misalignment will create other forces: bending along the length of the pins, shear where the pins enter the plate they are mounted in, compression in the plate with holes at the point of the web between the bores. I presume these are of secondary interest.

  7. #7
    Principle Engineer
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    force on pins

    The projecting pins will be pushed outward or inward by the mating holes.

    This force exerts a bending moment on the pins attempting to rotate them.

    The force also tries to move the pins away from each other parallel to the applied force. This produces a shear force at the cross section of the pin perpendicular to the bending force.

    The problem in calculating these forces is knowing where to apply them along the pin.
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