Machining rubber anti-vibration mounts

[KeithB]

As we prepare for our upcoming move one of the things I wanted to make was some anti-vibration mounts for our air compressors and rotophase unit. When I recently swapped out our old compressor for a new one I mounted it directly to the concrete floor. The old one had been mounted on 2" thick oak blocks, it helped quiet things down and it raised the drain valve underneath enough to make it easier to get to. The new one is a lot noisier.

I ordered a piece of 70 durometer Buna N rubber from Grainger. It was 1" thick, 4" wide, and 36" long. A thumb indent "test" gives me the feeling that it flexes about like a truck tire. (Maybe some of the mechanics here can tell me what hardness a truck tire measures.) It turned out there was no problem cutting it into blocks using a cutoff bandsaw and lots of soluble oil. The problem was trying to drill a 3/4" hole in the middle. Twist drills just tore things up, so after a little thought I bought a hole saw and tried it. Worked fine, except I had to take the drill off the arbor each time to remove the rubber plug inside. Took a couple holes to figure out exactly what to do. I took the twist drill out of the hole saw arbor, that left a solid plug, which I pushed out using an arbor press. I used air tool oil as a lubricant, it really helped push the plug out of the drill. Took a few minutes each time but the holes came out fairly clean.

I cut twelve pieces of 1/4" aluminum plate measuring 4" x 4" and eight pieces measuring 3" x 3". The larger pieces got a 3/4" hole drilled in the center, the smaller ones got a 1/2" hole. One side of the rubber sheet has an adhesive film covered by a tear-off strip of paper. I will put some double sided tape on the other side and stick an aluminum plate to each side to make a sandwich.

 

[uncle jimbo]

I find this very interesting. They look good. Keep us posted as to how well they work. I guess time will tell as to how long they last.

 

[RicinYakima]

Looks Good! Lots better than the ones I got with my Grizzly Mill/Drill.

 

[fiver]

basically an old school motor mount.

the hardness of a truck tire is too relative to try and pin down.
most of them are now like 60% synthetic material, and have wear ratings [hardness] that span half the alphabet.
[you basically have from pure lead to lino-type in the mix]

 

[Ian]

For drilling rubber you got it right with the hole saw. I was freaking out about soaking all that expensive material down with barious oils until I saw the Buna-N part, those are resistant (not proof) to most oils except esters. For future reference, tire mounting compound is our lubricant of choice for drilling, sawing, and punching rubber (I pre-punch a ton of mud flaps for the guys since it is thin enough).

 

[KeithB]

Ian, appreciate the info, but since I don't work in a mechanic's shop I don't have any tire mounting compound in stock. I know that there are Buna N seals and O-rings in various air tools we use so I figure that it would be OK. Mostly I just needed something slippery to help get the plug out of the hole saw. And the soluble oil we use in the cutoff saw is also used in equipment with Buna N seals so I figure it's OK also.

In any case, as soon as all the machine work was done I wiped everything down with 409 spray cleaner to get all the oils off. I'll wipe them down one more time before we glue them together. And they won't be soaked in oil during use.

 

[Bret4207]

Dish soap Keith, dish soap. Tire mounting compound (lube) is a type of soap. Can't give you the chemical breakdown, but GoJo hand cleaner or even cheapy dollar store soap works about the same in a pinch.

As far as truck tire hardness, a lot of that depends on age, at least to judge by how hard the 1950's tires I often get to work with are! Leave a tire on a manure spreader for 50-60 years, they get pretty doggone stiff!

 

[Intheshop]

Ru-Glyde is what we use,not only for mounting M/C tyres though. It is,without a doubt the best gosh darned,leather preservative I've ever used. Even nicer than Lexol.

We recover rollers from big sanders with rubber and cut it with razor sharp lathe tooling,"dry". The "chip" management,learned from wood moulding machines,applies to an even higher importance,machining rubber in my experience.

 

[KeithB]

Dish soap, I'll be darned, never thought of that. I always have a good supply of Dawn around, it's what I use to wash the mineral oil off the gel sleeves that I wear underneath my prosthetic. It can be slippery, probably would work fine now that I think about it.

 

[Rick]

Back in the days of playin grease monkey and twistin wrenches dish soap was about all I ever even heard of being used in tire mounting, diluted and applied with a brush. No doubt somebody markets some high dollar goop for the purpose, it's probably just dish soap though.

 

[RBHarter]

Bead grease or something like that ......

Just as a geewiz why not something preformed like a truck body mount ? Many off roaders also use hockey pucks for body lift without noise transfer , but they typically used between body the mount and body .
Nylon or hard neoprene Ford radius arm bushings are fairly cheap also and would serve nicely for this I would think . As cheap as $19/pair .

 

[KeithB]

I paid about $45 for the rubber, from which I got 10 mounts. I had some 1/4" x 6" and 1/4" x 8" 6061 aluminum stock on the rack leftover from other jobs. Somebody else had already paid for that so it was essentially $0 out of pocket expense. Spent about $25 for the hole saw and arbor but I'll reuse that, it's tax deductible anyway. I think I got off OK. I thought about trying to find something already made up, maybe from the automotive or industrial area, but I wasn't really sure what to look for and what I needed was simple and easy to make.

 

[fiver]

and just as good as what you were gonna buy..

i'll tell you what I would have done.
took the sawzall out back and cut one of the tires out there into some strips.
drilled a hole just big enough to push the floor bolt through, and pounded them in place with a 2x4 with a hole drilled in it before setting the machine down.

 

[Ian]

I'm not sure how your pads are engineered, but NVH reduction involves isolation of the parts (decoupling) and that is where heavy truck engine mounts would come into play. The systems have sleeves and donuts or stud plates vulvanized into the butyl block itself. The butyl elastomer completely isolates the two objects from each ither and converts oscillation into heat. I imagine yyou have one plate against the floor and bottom of the machine foot and the other plate on top of the machine foot with a stud passing through the whole sandwich with a washer and self-locking nut on top. If so, that will be great unless the the machine migrates and grounds-out its feet against the stud. An appropriately sized hole in the mounts and machine feet and piece of automotive heater or multi-purpose rubber hose slipped over the stud will ensure total isolation/decoupling.

Another thing you can do to mitigate noise somewhat is apply constrained-layer damping tiles to the large areas of the tank and any sheet metal or plastic shields or housings. Hide the tiles on the back side of parts except for the tank. The tiles will trap vibrations between the moving surface and the aluminum outer layer of the tile, forcing the mechanical energy into the butyl mastic layer where again, the sound energy is converted into heat. Adding better reed valve mufflers and even building easily removable baffle panels from 2x4 frame and asphalt sheathing to box in the compressor will make a world of difference to the output noise level of an air compressor.

 

[KeithB]

I like the idea of a rubber hose sleeve over the bolt and a rubber washer under the nut, think I'll try to do that. We planned to put a free standing folding screen lined with sound dampening material around each unit.

One of our units is a twin cylinder single stage unit, the other is a twin cylinder single stage unit. The former is rated at 11 scfm at 175 psi, the latter is rated at 13 psi at 125 psi. They sound distinctly different. The new compressor for the old unit came in, I just need the time to mount it and do whatever is necessary to redo a little piping.

 

[Ian]

Free-hanging mass-loaded vinyl sheets will soak up line-of-sight sound through a pretty broad spectrum but is spendy. Asphalt sheathing is about 7-8 dollars per 4x8 and is cheap even if you double it and attach to itself with some butyl caulk (edges and a few big S-shaped beads in the center) to leave an isolated air gap. Just throwing ideas out there to consider. Anofher soendy but effective option that I've seen used in various factories is acoustic curtains made for machinery. The curtains make easy access and easy to seal sound iut at the floor level where oanels can leak if you don't put a foam gasket against the floor.

 

[Bret4207]

Ian, what is this asphalt sheeting you mention? Gotta link to an example?

 

[KeithB]

I appreciate your ideas very much Ian. I've read your posts about controlling noise in your vehicles with interest, it is obvious you have a good understanding of things.

I'm not going to spend a lot of time and energy beyond what I've talked about on sound related issues until we get in and see how noisy the place actually is. The metal ceiling has small holes punched in it that are supposed to help absorb sound. At this point there is no echo or resonence in the building. For a metal building it is suprisingly quiet.

We usually wear earbuds and listen to podcasts or Pandora when we are working on our individual projects. We take them off when we work as a team so if its quiet enough to communicate at reasonable range that would be a good minimum noise level, although of course the lower the better. As time passes we will try to continue to improve our working environment.

 

[fiver]

compressors can be loud.
they are basically an engine just without the gas explosion noise we enjoy so much out the muffler.

a simple double walled drywall and rolled insulation box can be pretty effective at noise mitigation too.

one thing I would do for a new compressor is put a street ell in the bottom and a ball valve extended out the front for a water drain.
it just makes the things so simple to drain and you don't have to get down on your hands and knees to do it.

 

[KeithB]

Another great idea, I can get down on my hands and knees easily, getting up is another story!