How to select an Anti Vibration Mounts - the basics.

How to select Anti Vibration Mounts

Selecting Anti Vibration Mounts starts with knowing whether you are trying to solve a shock or a vibration problem - there is a difference between the two and how they are solved!  If you want to isolate vibration, one of the common tools to help us do this is a vibration isolator, or an Anti Vibration Mount.

It is, naturally, important to do everything that is possible to reduce the vibration generated at its source, but there will normally be some vibration remaining which can transfer into surrounding structures, which you might not want.  Vibration Isolators are there to interrupt, attenuate, absorb, reduce this transfer of structural vibration.  For example, no matter how well balanced a car engine is, it will still need to be fixed to the car structure via anti vibration mounts, otherwise the passengers will experience a lot of vibration. 

Warning: This guide is simplified.  We would always suggest asking for advice on mount selections for all but the most simple vibration attenuation requirements.

For this type of vibration challenge, which is the most common, there are two questions we need to answer when selecting anti vibation mounts:

1. How much static deflection do I need
2. Which mounts will give the required static deflection

Step 1: How much Static Deflection do you need for vibration isolation?

To work this out, our best friend is called a Vibration Isolation Nomogram.

Vibration Isolation Nomogram

There are three related pieces of information on this graph:

• Disturbing frequencies.  
• Level of vibration energy transferred to the surrounding structure
• Deflection, by which we mean "Static Deflection"

Let's look at these one at a time.

What is a Disturbing Frequency?

Disturbing frequency, sometimes called 'forcing frequency', is the frequency of the vibration being created by the source (e.g. motor, engine, generator).  This is typically in either Hz (cycles per second), RPM or CPM (cycles per minute).  RPM and CPM are essentially the same thing.

Some applications have a range of disturbing frequencies.  For example, a car engine may idle at around 800 rpm, but can rev much higher.  Usually, we would take the lowest frequency commonly experienced, there are exceptions to this, but let's stick with the basics. 

It is worth noting that when starting a car engine it quickly accelerates from 0 rpm to, typically, around 800 rpm.  As it accelerates to 800 rpm it transitions through lower frequencies, but as it spends little-to-no time at these frequencies, we can ignore them, and take 800 rpm as our datapoint.

What is the Level of vibration energy transferred to the surrounding structure?

This is about energy transferred from the source of the vibration to the surrounding structure. If the motor or engine is bolted directly to the structure, then 100% of the energy is transferred.  We would not be looking at Vibration Isolators if we were prepared to accept 100% transfer, so the question being asked really, is how much of the vibration energy do you want to be absorbed by the anti vibration mounts you are selecting.

Typically, anti vibration mounts are used to absorb at least 50% of the energy produced.  

What is Deflection, or Static Deflection?

Let's say that an anti vibation mount, when no weight is placed on it, has a height of 20mm.  When the load is placed on the mount, that 20mm will probably reduce.  Deflection is the change in that measurement.

It is important to note that the change in that measurement should be in the same direction as the load.  E.g. vertical for most applications.

Normally, static deflection is the unknown we are trying to establish when using the vibration nomogram.

How much Static Deflection do you need?

This is normally the key question that needs to be answered before mounts can be selected.

We typically start on the left hand side of the graph - the disturbing frequency.  Let's say, we have a pump.  The engine rotates at a steady 2,800 rpm, but the pump only rotates at 1,400 rpm.  We would take 1,400 rpm as our frequency.  The lower number is considered as 'playing safe'.

Note, 1,400 rpm is the same thing as 23.3 Hz, which is the conversion of cycles per minute to cycles per second.

So, we find the 1,400 cpm point on the vertical y-axis of the graph, and track right from that point.  In so doing, we will pass a number of diagonal lines.  These diagonal lines tell us how much of the vibration energy will be absorbed, or attenuated.  It is important that we move far enough across the graph to get into the white "absorption zone".  If we are still in the grey zone we could be in trouble.

As we track right, across the graph, the first vertical line we meet in the white zone is at 1.00mm of deflection (x-axis).  For this example, that means we do not want less than 1.00 mm of static deflection in our final solution, which means that the anti vibration mounts should compress by at least 1.00 mm when we add the load of the pump to the mount.

Continuing with the above example, if we want at least 90% of the vibration to be absorbed, we would continue tracking right, across the graph, from 1,400 cpm until we meet the 10% line (the diagonals on the graph give the amount of vibration energy that is transmitted, which is the opposite of absorbed).  We can see that around 5.00 mm of static deflection would give this result.  To be certain, we could go a little further and aim for 6.00 mm.

So, we now know that we need at least 1mm static deflection to avoid the 'danger zone', which could lead to resonance, and ideally nearer 6mm to achieve 90% absorption.

Step 2: Which mounts will give the required Static Deflection

Now that we know the amount of static deflection we need, we are almost ready to look for anti vibration mounts.  However, before we do, we need to understand the load that each mount will be taking.

Let's say that our pump weighs 200kg and has 4 feet.  Keeping the example simple, we will say that the load is evenly distributed across the 4 feet, so 50kg per foot.

It is not exactly correct, but reasonable to say that 50kg per foot is the same as 50 daN per foot - some mounts have loads specified in daN rather than Kg.  Whilst there is a difference, assuming that the mounts are in use at sea level, the tolerance in the mounts' spring rates is greater than the 2% difference between the two measurements.

Most mounts listed on AVMR.com  have load range in the title.  Look for products with 50kg or 50 daN in the load range. As long as those products have a static deflection of over, say, 5mm when loaded up with 50Kg, it should be a reasonable option.

Not all mounts clearly show the deflection level by load.  Some only give an indication of the loads they should take.  Ideally, mounts will have spring rates as part of their dataset, but not all do.  AVMR have not yet managed to upload all datapoints for each mount - apologies for that.  If you cannot find the right mount, please drop us a line, we will have the information, or a good idea, even if we have not managed to load the data yet.

An example Anti Vibration Mount Selection:

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 be 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.

What do I do with this Information?

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.

This is normally an easy step.  Once we understand the static deflection requirements, we need to look at the load we are trying to isolate on each mounting point (or foot).

Key questions:

To understand the load per mount, we need to understand:

1. How heavy is the equipment we are isolating?
   • Use the max load incl. any fuels, pipework etc that can increase the supported weight.  We have seen overloaded mounts due to the shipped or dry weight being used.
2. How many feet does it have?
3. Is the centre of gravity evenly positioned over the feet (so, is the load evenly distributed)? If not, we need to take some moments to calculate the loads per foot.  This is not tricky, but I will cover it separately.  For now, let’s assume loads are evenly distributed as they are for most of our day-to-day enquiries.

From this, we can work out how much load is on each foot or mount.

Why does load matter?

Well, there are a few reasons:

1. Mounts have a working range.
   • If you overload a rubber mount the worst-case scenario is tearing, failure and/or collapse. At a minimum the mount will be less effective at its job as when an AV mount is over compressed it ‘bottoms out’ and becomes solid, which prevents it from isolating.
   • If your mount is insufficiently loaded it won’t do its job either – it might not deflect enough resulting in reduced isolation and potentially resonance.
2. We want all the mounts to have a similar level of deflection to avoid an imbalance in the natural frequency of the system we are generating. We will talk more about this in another post.

So, we need to know that the load will sit in the AV mount’s “goldilocks zone”:

1. Not too much load to avoid overloading
2. Not too little load to work the mount properly and to avoid the risk of resonance.

For simplicity, assume our pump from the example in “Part 2: Static Deflection” weighs 1,000 kgs, and that weight is supported evenly by 4 feet, so 250kg/foot.

Select the right Mount

Now that we have our desired deflection and our load per foot, it is time to find products which fit the bill.  There are plenty on our shop at AVMR.com

Well tested AV mounts will normally be specified with deflection graphs, like the one below, for our SW-R mounts:

AVMR’s SW-R Sandwich Mount range: compression data and image Note: 1 daN is broadly equivalent to 1kg

This graph has a series of deflection lines which show how much each variant deflects with load.  This will normally be given in partnership with the maximum suggested loads for the product.

We have decided that AVMR’s SW-R range of products is right for our application, so want to find a variant which deflects around 10mm under 250kg (or circa 250daN)

Looking at the variants available, we can see that the following options are closest:

SW-R mount compression data (excuse the wobbly lines drawn)

1. SW-R3 will deflect circa 14mm*
2. SW-R4 will deflect circa 10mm
3. SW-R5 will deflect circa 7mm

When the anticipated static deflections are taken from the product deflection graph, this can be reviewed in the Isolation Graph from Part 2 and isolation levels predicted (see info in brackets, below).

1. SW-R3 will give 90-95% isolation*
2. SW-R4 will give 90% isolation
3. SW-R5 will give 80-90% isolation

SW-R4 gives us exactly what we were looking for, while options 3 and 5 are close.  We could push to increase the isolation with SW-R5, or reduce work that the mount has to conduct and go for SW-R3.  The ultimate decision depends on overall requirements and priorities.