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Thread: Resolution of Forces and Moments on a machine structure

  1. #1
    Associate Engineer
    Join Date
    Nov 2018

    Resolution of Forces and Moments on a machine structure


    I am designing a machine which is able to freely move through a total of 4x 1m long guide rails (each have 2 linear guides).

    I have attached a link to the guide rails:

    My concern is the total moment on each of the guide rails, depending on the direction the guide rails allow around 670nm of moment. However, since I am using several rails I am confused as to how the forces/moments are distributed between the rails and guides. Please see below:

    FBD DIAGRAM_zps876oy5dc.png

    The diagram above contains a side view of the machine. The squares (1,2,3 and 4) are pads which mount the linear guides shown in the link (the guide rail I am using is the WS-20-80-15. The machine is free to move in the direction of the force marked 'F', however where a Driveshaft protrudes from the bottom of the structure, A reaction occurs, 'R', which I am assume is equal and opposite to F. There is also a Weight force, W, which acts through the center of mass. The weight of the machine is 800Kg. For arguments sake, take F and R to be 1000N.

    If I wanted to calculate the moment about (1), the linear guide in the left hand corner of the machine, would the moment be as follows:

    (Assuming ACW is positive)

    M (1) = (F * 0.535m) - (R * 1.127m) + (W * 0.4145)

    Assuming Weight = 800KG and F = R = 1000N, then M (1) = 1275.1 (Nm)

    Here is what I am confused about. The guide rails are mounted in the exact positions on BOTH sides of the machine, so in essence, at point (1), there are two guide rails, one on either side of the machine. Does this mean the moment is shared between them, and hence, halved? Or does this moment act on BOTH of these guides rails at position 1?

    These guide rails only have about 670Nm moment allowance. However, with two of them, should they be able to take 1340Nm at point (1)?

    In addition, will the total weight, 800KG, act on (1)? Or, will the weight be distributed by the 8 linear guides and then act as a moment on (1)?

    Is there a more accurate way of calculating this? I appreciate this is quite a simple problem!

    Please advise me, any help would be very much appreciate.


    Last edited by rhys_faulkner; 11-10-2018 at 05:58 PM.

  2. #2
    Technical Fellow jboggs's Avatar
    Join Date
    Mar 2011
    Myrtle Beach, SC
    Rhys, Its a good thing to seek advice from others on a forum like this. But sometimes you may get responses you don't anticipate. This might be one.

    Before I even looked at the numbers I saw a feature of your design that is almost certain to cause problems - too many rails! Those little short gray hairs on the back of my head stood straight up when I saw that. Please don't do that.

    Getting two rails to remain in perfect alignment from installation through operation is hard enough, but three? Extremely difficult. Four? I doubt it.

    Very short stroke applications (like die sets) might work acceptably with 4 guides, but any issues like alignment, binding, wear, offset loading, are all just multiplied by stroke length.

    I'm sure some amount of the white hair on my head has come from dealing with the issues caused by designers that tried to create similar configurations on other machines. What you're trying to do may be theoretically possible, but the real world will bite you, hard.

    Plus - There is an added benefit to going with just two rails, most of your questions about shared loading just go away! I would strongly recommend you find a way to mount the whole thing on two rails. (If I was a little too harsh in my response please accept my apology.)

  3. #3
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    The easiest way to determine the moments and acting forces is to model a free body diagram in Engineers Edge Statics Modeler
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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