Can Foam be used for reducing vibration 1

[busguy]

I got an old bus frame that gets excited by the powerpack and creates an unacceptable ringing noise when it is in 5th gear and at 1500 rpm engine speed. I am looking at changing the axle ratio to reduce the eng speed at cruising speeds. But I was wondering if filling 2" X 3" frame members with the foam seal product (that some have had success with stiffening their chassis) could dampened out this noise?
Thanks,

 

[GregLocock]

Might work, other people have used sand or lead shot to damp out noise in tubular members.

BUT it'll only help if the frame members are actually part of the problem.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[rmw]

I take it that 5th is your top gear???? And 1500 rpm is right at your running speed.

I have found that sometimes foam gets hard once it sets up which doesn't then give any dampening. So see if you can find a non hardening foam.

Changing the axle ratio for the reason that you state will also change your low gear 'take off from a stop' ratio/speed. Make sure your bus still has enough grade-ability to get off in low gear on a stiff grade. (Yes, I have had to back one down a long grade and start off at the bottom again. Hell to do with passengers on board.) I re-ratioed a bus for that reason once. But that turned the engine up at cruising speed.

I think the change I made was from 3.70 to 4.11 if memory serves.

rmw

 

[msquared48]

Usually a change of mass is used to alter the response frequency, but the use of foam is an interesting idea in light of the mattress ads with the wine glass and person jumping on the bed. Seems to dampen the response too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Tmoose]

Is this made of all OEM components? Anything interesting happen if you rev it up to 1500 in 3rd, 4th, or in neutral? How about 1500 in 5th up on frame jacks? How about in neutral and idling at the 1500/5th road speed?

Do you have some odd exhaust or linkage mount that bypasses or short circuits the rubber engine, transmission, and spring mounts? Studded snow tires? Vibration analysis might reveal the frequency of the ringing is related to some damaged bearing or gear, or a heavily lugged tread pattern, that I'd expect typical rubber mounts might normally be able to isolate.

 

[busguy]

Thanks for all the replies. Sorry for my slow response. Per the transmission supplier, the gear mesh is 1000 Hz (frame's natural frequency) only when it is in fifth gear and the engine is spinning at 1500 rpm engine. The vibration requires high throttle, but went away when transmission was put in neutral after obtaining 70 mph (1500 rpm Neng).

We did change the axle from 4.27 to 3.54 (and fortunately there is enough tractive effort with the 3.54). Running at 1300 rpm instead of 1500 rpm did help reduce the noise/vibration. There seems to be another source of excitation that we have to minimize.

Since I wanted to make sure that I can reverse the frame filling, I will probably try filling with sand (cheaper than lead shot). I will close this thread.

 

[FeX32]

You can do that? Close the thread?
Why not add passive damping (or reduce it) by changing the engine mounts.


Fe

 

[GregLocock]

That's not how engine mounts work, for high frequency problems, in fact damping in engine mounts is counterproductive for HF attenuation.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[unclesyd]

Cans of high expansion foam, from the big boxes store, isn't very expensive so you could seal it up and go for it. You want to keep the foam confined to increase the density.
If you go this way it will be hard to add anything else because of the lost space.
GM, Fisher Body, use to glue wood blocks to frame and body to do the same thing, I've never seen any on the frame but was told that this happens. The one I saw were on the body panels.

The foam approach does work. We installed a 30" hollow tapered Al mast, a light pole, on a 46" boat that had the same problem, it vibrated, bad, around the optimum cruising speed. We changed all the stays, no help. Went to pipe stays again no help. I was checking out an insulatin job at the fish house on day while they were foaming in a new freezer. A quick discussion and assurances that the density of the foam could be raised we fill the mast with the foam through 4 holes. plugged the holes with taper pins and let the foam expand. After the expansion the mast would hardly ring sitting still and not a vib while running.

Brain thing:
Just a though find you an old piece of frame ring it and fill it with the highest density foam as possible. After the foam expands ring it again. A little work, but it might tell you something. Maybe a yea or nay.

 

[Rob45]

You could also investigate Lodengraf damping; a paper on this by J.R. Fricke suggests that filling the closed frame with other light materials could also help, including even vermiculite.

 

[autogyro46]

In general, increasing stiffness/rigidity doesn't get rid of resonances, it only raises the frequency, and the higher mechanical "Q" makes them even more pronounced. What is required is something lossy to damp the resonances: there are foams out there that have butadiene in the formulation that might work.
The ideal damper would be a highly viscous, gloppy, substance (tar would be a good example)

Resonances like to talk to each other. Stiffening can sometimes work by pushing a resonance of of the range that it can be "heard" by other resonances. But that requires a lot of cut and try experimentation, unless you have access to good acoustical analysis tools.

 

[JayMaechtlen]

I heard that gravel can also be used for vibration damping. Of course, after long enough it might become sand anyway?

Jay Maechtlen

http://www.laserpubs.com/techcomm