11/22/2022

Choosing a simple Anti-Vibration Mount? Part 2: Static Deflection

 Written by Daniel Lawes

We now know that flexibility is key when holding a vibrating machine the next question is “How flexible should it be?”

Before we get into this, there’s a worked example at the bottom of this page, in case you get lost, that should help.

The answer to this “how flexible?” question has to be derived from two pieces of information and a graph.  The two pieces of information are:

1. What is the forcing frequency we are trying to isolate? For things like generators or pumps, this might be the RPM of the motor, engine or pump.  For most applications, there is an obvious answer.  However, if you are looking at transport applications, or variable speed applications, things can get complicated.  If in doubt, drop us a line to see if we can help you.
2. How much do you want to isolate the vibration by?  50 – 90% reduction is a usual target, depending on how problematic the initial vibration is.  It goes without saying that, if you want to isolate 90% of the vibration, 10% will be remaining – that’s important later.

Now that you have this information, use the graph, below, to work out the required deflection.  Don’t let this graph intimidate you, it is quite simple once explained:

1. Find the “forcing frequency we are trying to isolate” (in cycles/minute) on the left-hand side (Y-axis) of the graph. If in doubt use a lower frequency on the graph.
2. Trace across to the right until you meet the relevant diagonal %age line (this is How much vibration energy can remain which you found from answering question 2, above).
3. From that point draw a line down to the X-axis, and note the deflection (if in doubt, round up).

This deflection is referring to the difference in height of the mount when the load is applied compared to when it is not applied.  This difference is measured when the equipment is switched off and everything is stationary, hence why it is called the ‘static deflection’.

If the mounts were 30mm tall originally, then the load was applied (e.g. the mounts are now supporting the pump) and now they are only 27mm tall, the static deflection is 3mm – the difference between before and after the load is applied.

An example:

Question: I have a pump which spins at 1,000 rpm and is vibrating through the factory floor.  I want to isolate 90% of the vibration, how much deflection do I need in the mounts/feet that I put on it?
Note: The motor might spinning at 3,000 rpm, but through a gearbox, the pump is spinning at 1,000 rpm.  It has been determined that the pump is causing the vibration issue.

You can scroll to the answer below, but you should be able to work out the answer using the information in the question and the isolation graph.

Nearly there…

Answer: After finding the 1,000 cycles per minute line on the left-hand side, I tracked across to the 10% (remaining) line (see yellow highlighted line on graph), which represents 90% isolation.  From there I went down to the X-axis to find that I require 10mm of static deflection on each foot of my pump.

So, the answer is 10mm of static deflection to isolate 90% of the vibration caused at 1,000rpm.

Ok, so hopefully you are now comfortable with the basics of how to establish the amount of deflection you require.

What do we now do with this information?  The answer is nothing – yet.  We need one other piece of information first – the load each AV mount will carry.

Relevant Links:

For other parts of of this lesson, click here:

1. Introduction to this lesson
2. Basic Principles of Vibration Management
3. Load
4. Mount Selection