Oil filled vibration damper
Oil filled vibration damper
(OP)
What is the best way of designing an oil filled vibration damper ? The frequency range of concern is perhaps 30-80Hz, at low amplitude (0-.001"). Should such a damper utilize a relatively long viscous restriction or is a short orifice good enough ? According to my edition of the Vibration Handbook (Harris & Crede), there is a rapid increase in damping as a function of frequency in this type of damper. Also, does anyone know of any textbooks/papers covering this subject? I haven't found much on the web.





RE: Oil filled vibration damper
I don't thonk you'll find much on the web, we use hydraulic engine mounts which have tuned channels for the damping, and designing them seems to be a black art.
Cheers
Greg Locock
RE: Oil filled vibration damper
I am obviously going to have to make one and test it, but I was just wondering what direction to go in.
RE: Oil filled vibration damper
I anxiously await further discussion in this thread.
My feeling (and it's just that, nothing more) is seal friction effects may be detrimental to what you are trying to accomplish and you'll have better luck with the gap viscous loss approach.
RE: Oil filled vibration damper
Our hydromounts use rubber as the spring and seal (ie no stiction problem), and then an inertia tuned spiral channel to get the damping. This is coupled to the main fluid chamber via another spring, in effect (actually a diaphragm).
The system model has upwards of 50 elements... and correlation is good enough across the frequency range 5-300 Hz.
I don't like the sound of your friction pad, because of stiction and wear.
The most foolproof solution would be a dashpot, maybe with some ancillary ports if you want to get fancy. How critical is this thing? How will you cope with variations in viscousity with temperature?
Cheers
Greg Locock
RE: Oil filled vibration damper
RE: Oil filled vibration damper
OK, first problem is that your vibrations are small, and frequencies are low, so the velocities are small. Stiction will be your biggest problem in this scenario, I suspect. I think you'll be in the 'roll back' portion of any sliding seal - ie you might as well go with a bonded elastomer joint. With such small deflections I think you'll struggle with any fancy valving unless you can get the fluid velocity up by using a piston and a small orifice.
I've never designed a dashpot - where does this frequency dependent behaviour come from? just omega*x?
Cheers
Greg Locock
RE: Oil filled vibration damper
RE: Oil filled vibration damper
Having read this thread, I am wondering whether a viscous damper is required at all.
To summarise, you have a large machine weighing several tonnes, producing low amplitude vibration in the range 30-80Hz, and you wish to damp or isolate this vibration. Freedom to move in a lateral plane is also required.
Hydraulically damped mountings tend to be tuned to produce maximum damping in the 10-30 hz range, but at higher frequencies the damping is usually little different from an undamped elastomer mounting.
If you want hydraulically damped mountings, then ContiTech, Lord, and Trelleborg have a range of machine mountings, but I'm not sure that they will allow much lateral movement. A damped elastomer bush will damp in the radial (loaded) direction, with low axial stiffness, but the only 'industrial' bush design I am aware of is made by Freudenberg (simrit). These are limted to about 400kg static load, and again the peak damping is in the 18-20 Hz region, so I'm not sure how effective these would be in your application.
Depending upon the movement required, it sounds like like a good application for a laminated rubber bearing / compression pad. If you aim for a vertical natural frequency of about 10Hz (not difficult), then the transmissibility should be no more than 15%, and the natural hysteresis of the material will provide some damping. Laminated rubber bearings are relatively soft in shear, and can easily accomodate shear strains of 50%.
Does this sound like an option?
Regards
Tom
RE: Oil filled vibration damper
Greg Locock: I read about the frequency dependent behavior in Harris & Crede - it is apparently a very marked effect and they give a graph (experimentally derived). It seems to be related to the inertia of the oil in the viscous gap. Presumably it can be compensated for (up to a point) by using a bigger gap.