Information on Pneumatic Isolators? 2

[jheidt2543]

I recently read an article on the use of pneumatic isolators to reduce machine vibrations. Can anyone out there in cyberland provide information on references and websites that can help me find more info. I would assume that there are size ranges that make pneumatic isolation economical; like size of machine, magnitude of vibration, etc.

I am researching a machine vibration problem where spring isolators and a tuned damper don't seem to be enough (it may also be that the tuned damper is out of tune <g>). The machine still sends objectionalbe vibrations into the building structure.

Thanks in advance for any help.

 

[pjhype]

jheidt,

Firestone makes a pneumatic air-bladder style mount -

http://www.firestoneindustrial.com/productinfo.htm

TPC Dayton has a spring/neoprene mount -

http://www.tpcdayton.com/vibration/productinfo/lowfreqmnts/index.asp

pj

 

[okdb]

Just some tips,

air springs have almost none sideway strength. So, if your vibration are in 1 direction, 1 axe, it's ok, or else you have to install air springs in every axes.

Second, air springs have a large displacement. Check wether this wouldn't interfere with the good function of the machine. That could cause some problems.

But first, verify that the your tuned damper is accurately tuned since it's normally as effective as air spring.

okdB

 

[MHONE]

May I ask what the offending frequency is? If this frequency is not truly resonant the dynamic absorber will not suppress the vibration whether tuned to it, or not tuned to it. Is this equipment isolated from the building that it is tranfering the vibration to? Have you attempted an impact test to find out if turning speed is exciting a natural frequency of the frame or the building?

 

[jheidt2543]

I started this post in another forum before I found this one. Here is the basic information on the problem, with a few changes made for updated information. I have received a number of helpful posts both here and in the other forum to set me on the right track.

I am looking at an existing three story, steel frame building in which a vibratory machine has been installed on the third floor. The machine is transmitting vibrations through the steel structure. The Owner has installed the manufacturer's recommended vibration dampers on the machine, but there is still enough vibration to be uncomfortable to the workers.

The steel frame is concrete encased for fire protection. My gut feeling is that this should help dampen the vibrations. The beam cross-section is a standard wide flanged steel beam, within a rectangular concrete fireproofing covering. I am assuming that the concrete cover ADDS to the dampening of the building.

The machine vibrates at about 650 rpm with a product weight of 350 lbs. and a machine weight of 7,500 lbs., for a total moving weight of 7,850 lbs. The machine is supported on four legs bolted to a 6&quot; thick concrete floor slab over steel beams on a bay spacing of 20' x 30'.

We finally had a firm run the vibration tests with a nifty looking, hand-held accelerometer. We took about 15 readings on the floor slab at various locations around the machine and the plant. The information was downloaded to a PC and the data printed out in a chart for each reading location. The primary motion is vertical.

Here is one set of the readings. I estimated the major peak values from the graph I was given, frequency on the x-axis and acceleration on the y-axis:

FREQUENCY (Hz) ACCELERATION (g's)

7.5 1.30E-04
11 1.30E-02
19 1.85E-04
38 1.75E-04
42 1.35E-03
49 1.60E-03
67.5 1.00E-03
77 1.85E-04
83 1.20E-03
94 1.00E-03
97 1.10E-03

There are locations with higher acceleration values than I've shown above. All of the references I have graph amplitude rather than acceleration against frequency, where do I go from here? I will be getting recommendations from the damper manufacturer; I would just like to learn a little something in the process.

 

[tomaspin]

Hi Gents,

What you need here is really good isolation of the machine from the building floor.
Your disturbing frequency is about 11Hz, so a vertical natural frequency of about 3Hz should attenuate the vibration by abour 90%.To get a frequency this low, you will need a spring stiffness to give you a static deflection of about 25mm on the mountings.
I would recommend that you look at bonded rubber springs, as these can be engineered to give the low vertical stiffness for good isolation, but with high lateral stiffness to maintain the stability of the machine. Air springs will probably be more trouble than they are worth, as you will need extra lateral restraint to stop the machine 'floating'.

Any competent manufacturer of rubber/metal bonded antivibration mountings should be able to supply something suitable. Try Lord, Barry Controls, Freudenberg, Trelleborg(Metalastik) for example.

I hope this helps.

Tom Aspin

 

[jheidt2543]

Thanks to all for some helpful information!

 

[Strong]

The vibration frequency near 11 Hertz is very close to the expected natural frequency for this type of construction. A simple impact-response test can be conducted to confirm this. Most catalog values for vibration isolators, including pneumatic, are based on an infititely rigid support that you don't have. It will be difficult, but not impossible to get any isolator to be effective at 11 Hz on a &quot;soft&quot; floor. Consider these recommendations first:
1) Relocate machine to exterior corner of floor to minimize floor excitation. It would be far better to put machine on 1st floor.
2) Be sure there are no vibration short circuits such as power conduit, pipes, ducts, control conduit, and material handling attachments.
3) Be sure to track down other paths that vibrations may be taking to other floors such as air ducts, pipes, and partition walls.
4) Be wary of bad advise and simple (yet expensive) solutions.

 

[okdb]

Hi,

Maybe it's a stupid proposal, but if the machin has its own supporting structure, would it be possible to separate that structure from the building? no contacts, no vibration transmission, right?

OkdB

 

[jheidt2543]

okdb,

Your suggestion is the first one I made, that is to isolate the machine. Although, I suggested moving it to the ground floor and setting it on an isolated pad. No dice, the IE's want it on the third floor. So, how about a separate frame at that level? Well, then the framing would have to go all the way down through the building to the ground floor. No dice again! They want it where they want it, now find a way.

It looks like the solution will be along the lines of adjusting the mounting springs and tuned damper, along with &quot;flexible&quot; connections at all pipes, hoppers and conveyors feeding into the vibratory dryer.

Thanks again for all the suggestions. The great thing about Eng-Tips.com is the variety of reponses to a question all help in thinking through the problem. Keep those cards and letters coming - metaphorically speaking!