BIZOMAT MAKEOS · Mechanics
Vibration Isolator Selection
What mount stiffness and static deflection isolate this machine at its running speed, and how well do my chosen mounts work?
Selection of resilient mounts for a machine: the frequency ratio needed for a target isolation efficiency with damping, the natural frequency, static deflection and stiffness per mount that achieve it, and, for a chosen mount stiffness, the actual natural frequency, transmissibility and isolation, with a warning near resonance.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Machine mass on the mounts | kg | Mass carried by the mounts (machine plus base frame). |
| Number of mounts | Number of mounts sharing the weight equally. | |
| Lowest running speed (forcing) | rpm | Lowest steady speed producing vibration (use the lowest forcing frequency). |
| Target isolation | % | Share of the vibrating force you want kept out of the structure. |
| Damping ratio of the mounts | Damping ratio (about 0.05 for rubber, 0.01 for steel springs). | |
| Chosen stiffness per mount (0 = none) | N/mm | Vertical stiffness of the mount you are considering. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Machine mass on the mounts
- What it is
- Total mass resting on the isolation mounts: the machine plus any base frame or inertia block.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Stiffness needed per mount rises in proportion.
- Get it right
- Include the base frame; if the weight is not evenly spread, mounts carry different loads.
What this means: Number of mounts
- What it is
- Number of mounts sharing the weight. The tool assumes they share it equally.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More mounts lower the stiffness needed per mount in proportion.
- Get it right
- If the centre of gravity is off-centre, mounts carry unequal loads; check each one separately.
What this means: Lowest running speed (forcing)
- What it is
- Lowest steady running speed of the machine, which sets the lowest forcing frequency to be isolated.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher speed allows a higher natural frequency and less static deflection.
- Get it right
- Use the lowest speed the machine runs at continuously, as lower frequencies are hardest to isolate.
What this means: Target isolation
- What it is
- Percentage of the vibrating force you want kept from reaching the floor or structure (1 − transmissibility).
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 99.5%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher target needs a larger frequency ratio, softer mounts and more static deflection.
- Get it right
- Type a percentage; higher targets demand much softer mounts.
What this means: Damping ratio of the mounts
- What it is
- Damping ratio of the mounts: their damping as a fraction of critical damping. It limits motion at resonance but reduces isolation above it.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More damping needs a softer mount for the same isolation.
- Get it right
- Take it from the mount maker for the mount type; enter a ratio, not a percentage.
What this means: Chosen stiffness per mount (0 = none)
- What it is
- Vertical stiffness of one mount you are considering, at the load it will carry, to check its isolation.
- Units and format
- Enter a number in N/mm: newtons per millimetre. The form accepts 0 N/mm to 10000000 N/mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 N/mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to skip the check; use the maker’s loaded stiffness, not a nominal value.
Method and formulas
- Transmissibility T = √{[1 + (2ζr)²] ÷ [(1 − r²)² + (2ζr)²]}, with r = forcing ÷ natural frequency; isolation = 1 − T.
- Required r solved for the target isolation; natural frequency f_n = forcing frequency ÷ r.
- Static deflection δ = g ÷ (2π f_n)²; stiffness per mount = (m ÷ n)(2π f_n)².
- Example: 90% isolation at 1500 rpm needs 4.5931 mm of static deflection, about 266.88 N/mm per mount.
Limitations
- Single-degree-of-freedom vertical model; rocking modes and a flexible floor change the result.
- Mount stiffness varies with load and temperature; use the maker’s loaded stiffness.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose anti-vibration mounts for a compressor, fan or pump
- Check whether existing mounts isolate or amplify
- Explain why soft mounts are needed for slow machines
Who it is for
- Maintenance engineers
- Plant engineers
- Design engineers
What you get
- Required frequency ratio and natural frequency
- Static deflection and stiffness per mount
- Performance of the chosen mount
Interpreting the result
- Isolation only starts above √2 times the natural frequency; stiff mounts on a slow machine amplify vibration.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 197–198, 304–316. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Vibration isolation and structure-borne noise
In Machine Noise Control by the Numbers
What it covers: Stop machine vibration reaching the building: natural frequency from static deflection, transmissibility, mounts, flexible connections, and catch hard pads tuned to the motor's speed.
Method explained here · BIZOMAT original book
Isolating the enclosure
In Random Vibration by the Numbers
What it covers: Choosing isolators against both a machine tone and broadband random vibration: frequency ratio and transmissibility, the isolator tool, Miles' response on soft mounts, sway space, damping and run-up.
Method explained here · BIZOMAT original book
Isolate the fan on springs
In Vibrating Machine Elements by the Numbers
What it covers: Choose isolator deflection from the lowest running speed, check transmissibility across the drive range, and catch the mount swap, uneven loads and soft roofs that undo it.
Method explained here · BIZOMAT original book
Misalignment, looseness and resonance
In Vibration Monitoring by the Numbers
What it covers: The signatures of misalignment, soft foot and looseness, how resonance magnifies an ordinary force, bump tests, the isolator selection tool, and a pump skid that could not be balanced.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.