Effect of Contact on Modal Frequency?
Effect of Contact on Modal Frequency?
(OP)
Hello,
I perform random vibration analysis for a small company utilizing cast aluminum housings which are typically piloted into to a gearbox of some sort and mounted via a bolted mounting flange. A recent housing has the main body outside of the gearbox and is mounted via a bolted flange. There are various masses/components attached to the body which creates an overhung mass cantilever beam type system. Whenever I perform an analysis I constrain a volume 1.5 times the mounting bolt diameter and apply base excitation. When I do this the results are always the housing mounting flange in between the bolt locations either lifts off of the fixture plane, or penetrates into the fixture plane. This has always been conservative when compared to fixing the entire mounting face, and we have been able to make modifications to the housings to facilitate passing the vibe test with success. This is however not what will actually happen in application as the housing cannot penetrate the fixture plane (though theoretically it can lift off slightly). My question now is is there an effect on the natural frequencies as a result of this contact between the housing and fixture and can anyone provide some insight as to ways of handling this effect? I keep thinking of a guitar string which does nothing when something obstructs its motion, yes it is free in one direction but not in the other and therefore the mode does not exist (at least as I understand anyway). I know that this is more along the lines of a wave rather than a modal analysis but hopefully gets my thoughts across. Is it possible for a mode to exist if half of its motion is prevented?
Again any help would be appreciated and thanks in advance,
- J -
I perform random vibration analysis for a small company utilizing cast aluminum housings which are typically piloted into to a gearbox of some sort and mounted via a bolted mounting flange. A recent housing has the main body outside of the gearbox and is mounted via a bolted flange. There are various masses/components attached to the body which creates an overhung mass cantilever beam type system. Whenever I perform an analysis I constrain a volume 1.5 times the mounting bolt diameter and apply base excitation. When I do this the results are always the housing mounting flange in between the bolt locations either lifts off of the fixture plane, or penetrates into the fixture plane. This has always been conservative when compared to fixing the entire mounting face, and we have been able to make modifications to the housings to facilitate passing the vibe test with success. This is however not what will actually happen in application as the housing cannot penetrate the fixture plane (though theoretically it can lift off slightly). My question now is is there an effect on the natural frequencies as a result of this contact between the housing and fixture and can anyone provide some insight as to ways of handling this effect? I keep thinking of a guitar string which does nothing when something obstructs its motion, yes it is free in one direction but not in the other and therefore the mode does not exist (at least as I understand anyway). I know that this is more along the lines of a wave rather than a modal analysis but hopefully gets my thoughts across. Is it possible for a mode to exist if half of its motion is prevented?
Again any help would be appreciated and thanks in advance,
- J -





RE: Effect of Contact on Modal Frequency?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Effect of Contact on Modal Frequency?
RE: Effect of Contact on Modal Frequency?
If the contact is continuous, then you can apply the continuity conditions between the two structures and express the coupling force as
F=(V2-V1)/(Y1+Y2) where V1 and V2 are respectively the velocities of structures 1 and 2 before coupling and Y1 and Y2 are the mobilities.
If the contact is non-continuous, then it can be supposed that shocks appears between the structures. Generaly, these shocks lead to harmonic distorsions or broad band noises.
RE: Effect of Contact on Modal Frequency?
RE: Effect of Contact on Modal Frequency?
(The natural frequency is always a linear sinusoidal movement.) To be precise, any bolted connection is non-linear, but in most well designed bolted connections the non-linearity is small enough to assume linearity.
If you need to do a FE analysis of the opening bolted connection, it is done by non-linear dynamic analysis. This will of course not give you the natural frequency of the system but reponse values will tell you if your design is OK or not. That is if the analysis is "correct" which is not the case
I would go for a bolted connection that is guaranteed not to open. If it opens you will get hammering and subsequent fatigue failure.
RE: Effect of Contact on Modal Frequency?
You either linearise the non linear system, or analyse it non linearly.
Typically in real modal testing we linearise the system by testing at only only one force amplitude, using sine sweep. We also hot-glue all the rattly bits together. In the case of contact that means we see some sort f average stiffness from the joint, and a fair bit of damping, but we don't mess up the analsysis because the frequencies created by the contact are ignored.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Effect of Contact on Modal Frequency?