The number of mounts is more or less irrelevant, since it is control of the resonances that you are interested in. 3 mounts has fewer vibration paths, but 4 mounts probably suits your floor structure better. 3 mounts are in determinate equilibrium, 4 are indeterminate, but given that they are soft this is not usually a problem.

I do wonder why the mounts are not as widely spaced as possible. There can be advantages in keeping them close together in some axes, but it is not, done like that as a general principle.

I realise it is probably just a sketch, but the baseplate design you show is going to cause more problems than it solves. You also haven't included the pipework, that can be another source of angst and may govern your choice of mount location.

Cheers

Greg Locock


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Quote:

When both pumps are running where it'll oscillate where they may cancel each other out, combine to have a higher vibration, and most likely somewhere in between.

Just a comment - if there is a time when one runs and the other doesn't, that could be the more concerning scenario due to the potential for vibration to cause "false brinneling" of the rolling bearings of the non-running pump.


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(2B)+(2B)' ?

The mount locations is my guess on transferring as much vibration as possible to the soft mounts; 2 aligned mounts to lock in 2 axis and the single third mount to lock 3 axis. Figured with this 3 mounts setup, the baseplate with pumps movement would be absorbed better on the soft mounts than 4 point mount which is more restrictive. This is going on a cart so I need remove as much vibration as possible with piping to be plastic or rubber hose/tubing.

I was thinking false brinneling was not going to be a big issue as there is no long periods of time (<3 min) when only 1 pump is running, the rest of the time both pumps are running.

There are three design keys that are perhaps more important than choice of 3 or 4 mounts:

1) Common support plate must be stiff to transfer dynamic forces from each machine base to the vibration mounts.
2) Vibration mount stiffness must provide adequate isolation based on machine speed and the fact that foundation (mobile cart) is probably not infinitely rigid. The mount stiffness will probably be much softer than from a simple calculation or chart on last page of catalog.
3) The foundation (cart frame) should not have natural frequencies near 1xSS and pulsation frequencies.

Walt

ESTinker,

Is it possible that your mounts are too stiff for proper vibration isolation?

In that case, three is better than four. Otherwise, I agree with the comments above.

--
JHG

The pumps are 20lbs each (40lbs total) and the isolation mounts are rated for 35lbs each (too large but already have these on hand). I'm thinking of going with 3x 20lbs rated Sorbothane mounts instead. I plan on adding 1/2" flanges on the 4 edges to stiffen the baseplate. The goal is to try to reduce as much vibration as possible, in anyway possible since this is going to a mobile cart.

It's the stiffness (rate) of the mounts in shear and in compression that is of interest.

Cheers

Greg Locock


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Yes 3 point mounts with 2 on the so called torque axis were a thing for car engines once, but it is just a way of thinking about the problem, it doesn't actually change much. In many cases wide separation is far more important than elegant theories about axes.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

How much margin should I add to the isolation mounts? Say 4x of 10lbs for a 40lbs total or use 4x 20lbs mounts?

ESTinker,

Are you concerned about resonant frequency, or the pumps tearing loose under some sort of impact?

You should be using anti-vibration mounts rated for your suspended mass, regardless.

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JHG

Resonant frequency and the overall vibration from the pumps, I want to reduce as much of the vibration transferred to the cart. I'm adding limit stoppers stop any impact issues.

ESTinker,

You want anti-vibration mounts with a low spring rate. Mounts rated for higher masses will have higher spring rates, and will resonate at higher frequencies.

Anti-vibration mounts isolate your pump from the floor. How are your pumps attached to your fluid system? If they are attached through rigid pipes or tubes, your mounts will not work.

--
JHG

piston reciprocating pumps I', gUessInG. If so, they likely vibrate and buzz a plenty. Probably highly directional vibration, too/
Are the pumps to be mounted on the bottom shelf of the cart, or 32" off the floor?

I picture the sheet metal cart shelves and the casters uniting to create a wimpy, light weight 2 or 3 mass system under the compressors.
I'd Pay special attention to "static deflection" and ignore the weight rating of the mounts you are contemplating.

https://mason-ind.com/ashrae-lecture/

The piping are either silicone/rubber or hydraulic hose so nothing rigid.

Pumps are rocking piston twin, mounted on the bottom shelf of cart about 8" off the floor. The plan is to make cart is made of 3/16" thick aluminum so shouldn't have issues with static deflection.

Could use 4 Sorbothane stud mounts rated at 11-16lbs each 0510330-50-10 to mount the baseplate: https://www.sorbothane.com/Data/Sites/31/pdfs/prod...

I may be able to separating the pumps and mount them individually if this is the best way to reduce the vibration, but have limited space and may become a cost issue.

I fear your perception of a 3/16" thick aluminum shelf's stiffness may prove flawed.

Over 2 score years ago it was not uncommon to hang the turn table ( record pLayer ) in the party house from the ceiling with bungie cords etc to keep the floor vibration from causing skipping.

I think you might have better results trying this with very flexible springs ( > 2" static deflection with your pump assembly on board) with their upper ends attached to the 2nd shelf's corners.

A simple spring's natural frequency dictates how much attenuation you get at any higher frequency; the softer the spring, the lower its natural frequency, and the more it attenuates higher frequencies.

The old Cadillacs had extremely soft suspensions (springs) such that you felt you were riding on a comfy cloud; high frequency bumps, etc., was only road noise. Of course, the fact that it was a tank also helped.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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ESTinker,

There are two ways to reduce your resonant frequency. You can reduce the spring rate, and you can increase the mass. There is more than one reason that Tmoose's suggesting above is correct.

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JHG