jvian
Aerospace
- Aug 13, 2009
- 119
Hello,
I am performing a random vibration analysis and test on a part which pilots into a test fixture and mounts via a bolted mounting flange. The part has various masses attached to it which created an undesired overhung mass system but similar designs have worked for us in the past and this is required for envelope. I have attached an image (very crude but approximate) to help explain. Normally when setting up this analysis I would fix a volume of 1.5 times the bolt diameter around the fasteners and perform the modal and vibration analyses, and as needed modify the housing to lower the rms stresses in order to pass our fatigue failure criteria. Normally this can be achieved without a significant impact on product weight and the lower risk is desirable. This program however is very stringent on weight requirements and now I am trying to minimize additions as possible. With the 1.5D constraints the mounting flange is free to interpenetrate into and/or lift off from the mounting plane of the test fixture. This is only partially true as in application the model cannot penetrate into the plane of the fixture. Again normally this is not a concern as the stresses are always conservative with the freedom but now I have to question how real is the situation. When I fix the mounting face to a planar condition (and therefore neither lift off nor penetrate the fixture plane) the stresses are significantly lower and I show positive margin for fatigue but worry that is over constraining the model. The results of the preliminary analyses report max deflections of .0015" at the back end of the part with a 'interpenetration' of .0005" into the test fixture.
Does anyone here have a feel for what is the more appropriate method to constrain the model or any better alternatives? I am using Creo Simulate (pro/mechanica) for the analysis and I am limited to linear bonded or free interfaces between the parts so utilizing contacts or non-linear springs is not a valid option. Again any help would be appreciated and thanks in advance.
- J -
Hope that helps and good luck,
- J -
I am performing a random vibration analysis and test on a part which pilots into a test fixture and mounts via a bolted mounting flange. The part has various masses attached to it which created an undesired overhung mass system but similar designs have worked for us in the past and this is required for envelope. I have attached an image (very crude but approximate) to help explain. Normally when setting up this analysis I would fix a volume of 1.5 times the bolt diameter around the fasteners and perform the modal and vibration analyses, and as needed modify the housing to lower the rms stresses in order to pass our fatigue failure criteria. Normally this can be achieved without a significant impact on product weight and the lower risk is desirable. This program however is very stringent on weight requirements and now I am trying to minimize additions as possible. With the 1.5D constraints the mounting flange is free to interpenetrate into and/or lift off from the mounting plane of the test fixture. This is only partially true as in application the model cannot penetrate into the plane of the fixture. Again normally this is not a concern as the stresses are always conservative with the freedom but now I have to question how real is the situation. When I fix the mounting face to a planar condition (and therefore neither lift off nor penetrate the fixture plane) the stresses are significantly lower and I show positive margin for fatigue but worry that is over constraining the model. The results of the preliminary analyses report max deflections of .0015" at the back end of the part with a 'interpenetration' of .0005" into the test fixture.
Does anyone here have a feel for what is the more appropriate method to constrain the model or any better alternatives? I am using Creo Simulate (pro/mechanica) for the analysis and I am limited to linear bonded or free interfaces between the parts so utilizing contacts or non-linear springs is not a valid option. Again any help would be appreciated and thanks in advance.
- J -
Hope that helps and good luck,
- J -
