Vibration reduction..

[JerryFlyGuy]

In regards to taking-out/changing the vibration freq/ resonance [sp] of a structure. Has anyone ever tried putting something inside the structure to stiffen or improve/change this? I'm thinking of my situation in particular with hollow tube structures. Would adding some type of pourable, expanding foam increase or change the resonance?


I don't have my machine up and running yet but, in my current construction phaze, it would be easy to either add it, or make the nesc allowances to add it later. If I do it later, however, it would have to be some type of pourable product to get it in there..

Of course, adding weight is not hugely desirable..

Jerry [just curious more than anything at this point..]

[mxtras]

Low shrinkage grout, packed sand, lead shot, concrete, low shrink epoxy - all have been used for vibration dampening in industry. Using foam would not have much impact on dampening vibration.

Scott

[JerryFlyGuy]

I'd thought of all of those, however.. the weight factor comes into effect w/ each of them. I was thinking foam simply because it will add some stiffness yet its light. [I assume it would help w/ the stiffness simply from seeing other products use it.. in baking furnace wall panels for example..]. I know that typically to change the vibration or resonance of a structure they usually add weight, but.. adding a couple thousand pounds to my gantry isn't something I'm too excited about

Jerry

[Geof]

As Scott says various things have been used. You are never going to get rid of resonant frequencies in your structure but you want to avoid exciting them under normal conditions. One approach is to make things really heavy so they are below normal operating frequencies; this to a large extent is why cast iron works well but it means lots of weight. This is primarily the effect of grout, concrete or very tightly packed sand; the weight is just being increased but a secondary effect is the packed in material dampens the drumming of flat surfaces. Lead shot also increases the weight but when it is loosely packed it can also dampen vibration because the shot rattles around. Maybe rattle is not exactly the correct word because it doesn't bounce off the walls but it moves and absorbs and dampens vibrations over a range of frequencies.

With all these you cannot avoid significantly increasing the weight but there is one vibration damping method that does not increase weight as much and that is to attach weights using rubber mounts with the stiffness of the mount and the mass of the weight chosen for the location it is mounted. Crankshaft dampers on engines which have a metal ring mounted on an elastomeric ring are an example of this. These work because the resonant frequency of the attached weight in conjunction with the stiffness of the rubber is differs by a fractional multiple from the resonant frequency of the structure at that point. I don't know if this method is used for the structural as opposed to rotating parts of machine but cannot think of any reason it should not be. It is used on a large scale for damping the vibration of buildings in earthquakes and the smallest application of the principle I know about is on a pedestrian bridge over the Thames in England.

[JerryFlyGuy]

Interesting.. well.. I think for now I'll just have to continue the route I'm on. I'll see when its running what is what.. If it becomes a problem in the future.. I'll address it then..

Jerry [btw Geof, I've got all my stuff in order to get the rails potted on now. I've got all the jig's D&T'd, the substructure is all drilled.. I've just got to hatch up the mounting surface so the epoxy has something to stick to on the sub structure..& finally.. wax the bottom of the rails and potting fences.. then its setting the rails in place.. adding the potting fences and jig's and start putting the epoxy in.. might even have it ready for epoxy this weekend..

Making progess slowly.. ]

[mxtras]

Jerry -

Please photograph this procedure (potting) for others to see!

Scott

[Geof]

Originally Posted by JerryFlyGuy .... If it becomes a problem in the future.. I'll address it then..

That is the best approach.

Regarding your potting with tons of gooey stuff. I agree with Scott - pictures would be nice. Mount your camera on a tripod and buy a wireless remote for it and every 30 seconds or so push the button. I have a vision of a camera thoroughly potted in epoxy; not a nice vision.

[mxtras]

I have potted many things over the years - prior to my involvement with this forum. I have potted mounts for large diesel engines, prop shaft bearings, machine bases and structures and even made prototype molding tools from potting compound but I have not ever been in a position to take photographs nor have I had the need to until my involvement here. I have never seen a good pictorial on here about how it is done, the steps taken, it's purpose, etc, etc and I just thought maybe this would be a good time to make something happen - thanks to Jerry, that is!!!

Scott

[JerryFlyGuy]

I'll most definatly be taking some pictures of the proceedure. I don't kow if the 30sec interval thing will happen but some will be taken anyway.. The results will also be documented.

I've attached a PDF that is a simple "How To" of what I'm planning on doing.

Jerry [How did we get from vibration to potting rails!?? ]

[mxtras]

You started it, Jerry!



Scott

[JerryFlyGuy]

BUT MOMMMMMYYY.........

J

[ZipSnipe]

Hey Jerry , I,m in the same boat as you except my milling machine is already built. My column is a 2x3x 1/8 hollow tube. This I plan to fill with a grout/cement. I used to work construction along time ago and we did this job of removing the grout off theses steel column feet(HUGE) anyway I remember swinging a sledge hammer(big one) to break up the grout and it just bounced off the grout, you could not chisel this stuff, it was as if it melded with the steel column and turned to steel. We ended up grinding it off. I never knew what its actual name was but thats the stuff I,m hunting down to fill my column. What do I think its impact on performance will be by filling it? I figured a 3-5% increase in performance of the mill as far as rigidness goes and every % counts in my book. Heres a link to my never ending project

My Manual small mill project
CNCzone.com-The Ultimate Machinist Community - My Manual small mill project