14
« on: October 05, 2012, 07:56:25 PM »
If you have to deal with multiple RPMs that vary significantly, that can be accomplished with multiple isolation stand levels each tuned to the required disturbing frequency (RPM). However, normally you will get plenty of vibration isolation across a fairly wide RPM range if you correctly selected the isolator mount spring rate and load rating in the first place. You can see that by using the calculator and observing the isolation output value after varying the RPM once you have a good design.
If you spread out the distance between the feet of the isolation stand, there is no need to attach the isolation stand to anything. Again, if you correctly select the appropriate isolator mount parameters, you shouldn't get any detectable vibration transmitted beyond the isolation stand. This “magic” is accomplished by the physics of using a properly designed isolation stand. I suspect that some folks just kludge together a stand without actually first designing it, get less than good results, and then form an erroneous belief/opinion around their experience. And then there are other folks who just take on blind faith what they have been told.
I have been running on an isolation stand since 2004 in the attached garage of my suburban home. There were many “experts” back then who held the erroneous belief that concrete was the only way to mount these engines. Their primary argument was that concrete is what Lister recommended and therefore any other approach was wrong. What they neglected to consider in their critical thinking (or lack thereof) was that the folks who designed our Lister engines were only a few hairs away from being baboons relative to engine design and structural dynamic mounting knowledge as compared to today. The fact of the matter is that all engines mounted to a properly designed isolation stand will see far less fatigue stress during their operational life than if they are just bolted to concrete. This is doubly true if the engine is grossly out of balance and just bolted to concrete “as is”. In my opinion, if you can’t get your engine to operate nicely on an isolation stand, you probably shouldn’t be operating these engines at all...or if you do, perhaps you should mount it on concrete in a building far away from people...
Bob B.