Like Duncan Carter, I also use dental cement since 1980's and can attest to the fact that about 90% of the discs are still intact.
The worse case are those attached to our cooling tower gearboxes and motors and even then I usually expect them to last 3 to 5 years.
Even those on my turbine bearings have remained for over 10 years.
It is particularly useful when mounting on vertical surfaces since the cure time is very short.
As all have noted, prep is essential. I take down to bare metal and "score" both surfaces.
Jim P
As Duncan said

quote:

Originally posted by Sam Pickens:
Why does the base of the transducer have as much mass as it has? How does the little thin disk compare?

So with such mass and obviously plenty of room with plenty of exposed surfaces well worthy of data collection; why would you use a washer as a medium? Am I missing something here? But I also use acceleration.

In a nut shell... An accel with a magnetic base attached to a machine is after all a K, M, C object with a single degree freedom. Vibration from the machine under investigation is being transmitted into this mechanical system happened to be also a measurement system which has its own resonance frequency and magnification factor/damping.

We would prefer for it to have the resonance frequency as high as possible (much higher then frequency of interest) and high transmissability/low isolation. Therefore, for these purposes we need to have highest K, lowest M, and lowest C. Good K and C require strong attachment force and good surface condition. M should be at minimum. Therefore, old big mag bases now days are being replaced with small ones but as forceful as big.

Assuming that a washer is part of machine (perfect bond) it will provide all these conditions.

A flat high finish machine surface does not require a glued washer.

This message has been edited. Last edited by: David_G,

Sam,

As David_G states, the mounting surface of a pad will usually be superior a mounting surface directly on the machine.

The other important fact is that the measurement will be taken at the exact same location each time. You may remember from machine to machine where you've taken measurements each time, but if more than 1 person is taking readings, using mounting pads helps ensure consistent readings. Afterall, we want changes in vibration level to be caused by changes in machine consition, not because the measurement location keeps changing.

Jon
http://www.spintelligentlabs.com

I'm not so sure we live in a perfect world. I will also assume that the second law of thermodynamics applies here as well. I like direct contact whenever I can have it. The disk may be pristine on day one but? If I have a large machine with a flat surface I can easily 'X' marks the spot. Can you improve on that reading by adding material?

quote:

would prefer for it to have the resonance frequency as high as possible (much higher then frequency of interest) and high transmissability/low isolation. Therefore, for these purposes we need to have highest K . . .

Interesting, the K is coming from the magnet. Whtat type of K is this? Negative K. So we really want it as low, i.e. most negative, as possible. A negative spring that we like.

Is this a negative spring with a pre-load, i.e. does it take a minimum amount of force to move the spring?

K is positive and is coming not from the magnet itself. It is a spring property characteristic of the mechanical interface between the magnet and machine.

One can picture it as an isolator between a machine and accel.

With a magnet the more one pulls it away the lower the attractive force. With a mechanical spring the more one pulls it away (from center) the greater the attractive force; this is a positive spring.

I agree with you rusty I have just recently thought of just using the spot facing tool on some machine with lots of depth and room.I think this should be the best reading one could get.after all no middle man usually improves things. lol.Anyone else tried to just spot face then take your reading I would be interested in your findings.time saved installing washers could be better spent.

Lee,

A spot face only works well the day it was made. They can get painted over, corroded, crudded-up and may need to be scraped out before attaching a magnet, assuming you or someone else can find "the spots" again. A lot of folks on this board have their own "tried and true" way of temporarily mounting an accelerometer. I have my own way that differs from those discussed.

Walt

Bill,

One can think of a virtual spring between a magnet and a surface as follows. Neutral (static) status of the spring is preloaded. In this case it becomes an equivalent of a real positive spring.

I think a spot-face is a good alternative to glue-down pads, not-withstanding the comments about paiting and etc.

I don't like to recommend this approach for a couple reasons. If done properly, the surface is flat and an ideal mounting surface. If done poorly, the results are worse than no effort at all. My second concern is that the machine can be damaged by careless use of the spot-facing tool. A grease cavity or oil channel drilled into or another "opps" could make this an expensive exercise. I know none of us....er...make that you....would make a mistake like that, but mistakes do happen.

A flat, well identified location for making measurements is the goal. A glue-down pad is one way to get there that can work well.

Jon
http://www.spintelligentlabs.com

I don't disagree with the spot facing method at all. Taking readings from the same spot every time, on a flat area, with no paint would be perfect, assuming that the machine is magnetic. I only fit discs to non-magnetic machines, which then gives the benefit of using a magnet mounted transducer. I agree with Sam that collecting data throgh an extra medium is not ideal, but in my experience, it's much better than physically holding the accelerometer directly against a non-magnetic machine. The results are more consistent and it's the acceleration readings that have improved more than anything else.