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Thread: Forces on bolt during vibration

  1. #1
    Associate Engineer
    Join Date
    Jul 2018
    Posts
    3

    Forces on bolt during vibration

    Hi

    I am trying to do an analysis on what is the minimum size of bolts I would need to use on an electrical enclosure that is to be wall mounted. This whole unit would be mounted on the inside of a vehicle subjected to vibrations. I have attached a pdf to explain.

    The enclosure is composed of 2 halves (front panel and chassis). At a state of rest, the axial load on the bolts would be purely due to the preload after tightening. The shear force would be solely due to the weight of the panel.

    During vibration, I assume that these forces would be magnified. How do I calculate the effective forces?
    Attached Files Attached Files

  2. #2
    Lead Engineer
    Join Date
    Aug 2013
    Location
    Houston TX USA
    Posts
    417
    Unfortunately, there is no other way to do an accurate maximum vibration loading determination without actual vehicle field operation vibration monitoring and recording to determine the frequencies of the the vibrations in the vertical and horizontal planes your vehicle body will experience under normal and anticipated extreme operating conditions.
    Once you have that data; then, you must identify the worst case vibration frequency in terms of acceleration based upon the combination of cycle amplitude and time with a = 2s / t^2 where: t = 1/2 cycle time and s = the the peak to peak distance of the 1/2 cycle. From there using f=ma (m being the mass of your box) you should be able to determine the resulting worst case box vibration loading on the bolts.
    In addition, you take into consideration in selecting your mounting bolt sizes and strength the potential G loading on the box(s) in the event of a truck accident or rollover which will, in all probability, be much higher than any loadings experienced form body vibrations.

  3. #3
    Associate Engineer
    Join Date
    Jul 2018
    Posts
    3
    Thank you, your comments have been very instructive. After reading your comments, it became apparent that I needed to define vibration in terms of acceleration and from there, I can use good old F=ma. Fortunately, I do have vibration profiles of the vehicle I am interested in. Thanks again.

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