Isolation has a threshold, and stiff mounts sit on the wrong side of it
A machine on mounts is a mass on a spring. That system has a natural frequency, f-n, set by how far the mounts deflect under the machine weight. The machine itself produces a disturbing frequency, f-d, from its rotating speed. Everything about isolation follows from the ratio between those two numbers.
Transmissibility — the fraction of vibration that gets through to the floor — is T = 1 / |r squared minus 1|, where r is f-d divided by f-n. Work through that expression and something uncomfortable falls out. When r equals 1, the denominator is zero and transmissibility is theoretically infinite: that is resonance. Isolation only begins once r exceeds the square root of 2, roughly 1.41. Anywhere between r = 0 and r = 1.41, the mount transmits more vibration than a rigid bolt would.
The practical rule
Your mounts must be soft enough that the system natural frequency is below about 70% of the running frequency. A mount that is too hard does not isolate a little less well — it actively amplifies.
Working it through with real numbers
Take a 600 kg compressor on four mounts running at 960 RPM. The disturbing frequency is 960 divided by 60, which is 16 Hz. Each mount carries 150 kg.
Natural frequency relates to static deflection by f-n = 15.76 divided by the square root of the deflection in millimetres. So a hard pad that squashes 1 mm under load gives f-n = 15.76 Hz. The ratio r is 16 divided by 15.76, which is 1.02 — almost exactly resonance. That machine will shake itself apart, and the mounts are the reason.
The same compressor on mounts that deflect 10 mm sees 91% of the vibration stopped. The only thing that changed is stiffness.
Why people get this wrong
- They choose the mount by load rating alone. A mount rated for 200 kg is not the right mount for a 150 kg load if it barely deflects at that load.
- They use the maximum running speed. Isolation is hardest at the lowest speed the machine runs at — that is the case to design for.
- They add more mounts to be safe. Six mounts instead of four means less load on each, less deflection, higher natural frequency and worse isolation.
- They fit a hard pad because a soft one feels unstable. Stability is a real concern, but it is solved with mount geometry and placement, not by making the spring stiff.
More mounts is usually worse
This one surprises people. Isolation depends on deflection under the actual load. Spreading the same weight over more mounts reduces the load each one carries, so each deflects less, and the system gets stiffer. If you need more mounts for stability, you need softer mounts to go with them.
Where rubber runs out
Rubber mounts practically deliver up to about 25 to 30 mm of static deflection before creep and stability become problems. Below roughly 500 RPM, the disturbing frequency drops so low that the deflection needed for good isolation exceeds what rubber can give. That is the point at which a steel spring isolator, often with a rubber pad in series for high-frequency noise, becomes the honest answer.
We would rather tell you that than sell you a mount that cannot work. If the numbers say springs, we will say springs.
What to send us
- 1Total machine weight, including anything bolted to it
- 2The lowest speed it runs at in normal service, in RPM
- 3How many mounting points it has, and roughly how the weight is distributed
- 4Whether the machine is bolted down or free-standing, and the available height for a mount
With those four numbers we will tell you the deflection you need and whether a rubber mount can deliver it. The mount selector on this site runs the same calculation if you would rather do it yourself first.
Have a part that fits this description?
Send the dimensions and the operating conditions. We will tell you what compound it should be in and quote it — including saying so if it is not a job for us.