How to Choose the Right Anti-Vibration Mount for Your Machinery

Choosing an anti-vibration mount comes down to five questions: how much load each mounting point carries, what frequency the machine is generating, how much deflection the mount needs to achieve at that load, what direction the loads act in, and what the mount is exposed to in service. Get those five right and the mount type usually selects itself. Get any one of them wrong — most commonly the frequency — and even a well-made mount will underperform or fail early.

This guide walks through each factor in the order an engineer would normally work through it.

Why mount selection is not a like-for-like exercise

Vibration mounts are often bought as replacements: the old one has perished, so a similar-looking part is ordered. That works only if the machine, its duty cycle and its mounting arrangement have not changed. In practice, machines get uprated, foundations change, equipment is relocated to a mezzanine floor, or a diesel set that ran at one speed is now variable-speed.

The consequence of a wrong selection is rarely a dramatic failure. It is usually slower and more expensive: cracked brackets, loosening fasteners, premature bearing wear, structure-borne noise carrying into offices above, and mounts that need replacing every few months.

The terminology you need

A few terms come up repeatedly, and mixing them up is the source of most selection errors.

• Excitation frequency (or disturbing frequency) — the frequency the machine produces. For rotating equipment, the primary excitation is usually the running speed: frequency in hertz equals RPM divided by 60. A generator set at 1500 RPM has a primary excitation of 25 Hz.
• Natural frequency — the frequency at which the mounted machine wants to bounce on its mounts. This is a property of the mass and the mount stiffness, not of the machine’s speed.
• Static deflection — how far the mount compresses under the weight of the machine at rest. Softer mounts deflect more and give a lower natural frequency.
• Resonance — the condition where excitation frequency and natural frequency coincide. Vibration is amplified rather than isolated. This is the one condition to design away from.
• Isolation vs damping — isolation reduces how much vibration is transmitted into the structure. Damping dissipates energy and controls the amplitude at resonance, which matters during run-up and run-down. Rubber provides both; steel springs provide isolation with very little inherent damping.
• Compression, shear and tension — the three ways a mount can be loaded. Rubber is strong in compression, softer and more compliant in shear, and generally should not be relied upon in tension.

Step 1: Establish the load at each mounting point

Start with total equipment weight, then work out how it is distributed. An even split across four points is convenient to assume and frequently wrong — engine-driven sets, machines with an overhanging tool head, and skid-mounted packages often carry substantially more weight at one end.

You will need:

• Total mass of the equipment, including any base frame, fluids and enclosure
• Number of mounting points
• Approximate weight distribution, or the centre of gravity position
• Any dynamic or shock loads over and above the static weight

For mobile plant, transported equipment and anything subject to impact, the dynamic load can be a multiple of the static load. Mounts sized only on standing weight will be under-specified.

Step 2: Identify the excitation frequency

Note the running speed in RPM and convert it to hertz. Where the machine has more than one significant source — an engine and an alternator, or a compressor with a reciprocating action — record each of them. Reciprocating machinery also produces harmonics at multiples of running speed, and low-speed equipment is generally the harder isolation problem.

If the machine is variable-speed, the lowest operating speed usually governs the selection, because that is where the excitation frequency is closest to the mount’s natural frequency.

Step 3: Work out the deflection you need

Isolation only begins once the excitation frequency is meaningfully above the natural frequency of the mounted system. Below that point, and particularly at a ratio close to 1:1, vibration is amplified. As a general engineering guideline, a frequency ratio of around 3 or higher is a common design target for machinery isolation, though the appropriate target depends on the application and on how much movement the installation can tolerate.

For a first approximation, an undamped single-degree-of-freedom system with a linear spring rate gives:

fn ≈ 15.76 / √δ

where fn is natural frequency in Hz and δ is static deflection in millimetres.

Two important caveats apply. First, rubber is not a linear spring — its dynamic stiffness is higher than its static stiffness, so the natural frequency in service is typically higher than this formula suggests. Second, this assumes a rigid supporting structure. A machine mounted on a suspended floor or a light steel frame behaves differently. Use the formula to narrow the field, then confirm against the actual load-deflection data for the mount you are considering.

Where deflection requirements are large and rubber alone cannot achieve them — low-speed equipment, HVAC plant on upper floors — spring mounts or acoustic spring hangers are usually the appropriate route.

Step 4: Determine the direction of loading

This is what decides the mount family more than any other single factor.

• Predominantly downward, static machinery on a solid floor — compression-loaded types such as machinery mounts, cone mounts and levelling feet.
• Loads acting in several directions — rubber bobbin mounts and cone mounts handle compression and shear, and are widely used where the machine can move in more than one plane.
• Risk of the equipment lifting off or being inverted — captive transit mounts and failsafe designs, which retain the equipment if the rubber element is damaged. Anything mobile, airborne or transported should be assessed on this basis.
• Impact and end-of-travel control rather than continuous vibration — rubber buffers and bump stops, which limit travel and absorb shock rather than isolate a steady-state frequency.
• Uneven floors or machines needing alignment — levelling feet, which combine load support with height adjustment.

Step 5: Match the material to the environment

The rubber compound is chosen against the service environment, not against the load.

• Natural rubber — good resilience and fatigue performance for general industrial use, but limited resistance to oils and fuels.
• Neoprene / chloroprene — a common choice where moderate oil, weather and ozone exposure is expected.
• Nitrile — used where contact with oils and fuels is routine.
• EPDM — suited to outdoor exposure, ozone and weathering.
• Silicone — used where wide temperature range is the driver.

Also confirm the metalwork. In marine, coastal and washdown environments, the bonded inserts and housings matter as much as the rubber — corrosion at the bond line is a frequent cause of failure in marine engine mounts and outdoor installations. Specify the finish or material requirement explicitly at enquiry stage.

Operating temperature, sunlight, chemical splash and washdown regime should all be stated. A compound that performs well in a machine shop can degrade quickly in a food-processing washdown area.

Common selection mistakes

• Sizing on static weight alone. Shock and dynamic loads are ignored, and mounts fatigue early.
• Assuming equal load sharing. Four identical mounts under an unevenly loaded skid means two are overloaded and two are barely working.
• Choosing the softest available mount. More deflection is not always better. Excessive movement can cause pipework strain, coupling misalignment and instability during start-up.
• Overlooking the run-up through resonance. Every machine passes through its natural frequency when starting and stopping. Damping matters here, which is one reason undamped springs are sometimes paired with additional restraint.
• Over-tightening bonded mounts. Compressing the rubber element beyond its intended working range changes the stiffness and stresses the bond. Follow the fitting guidance for the specific product.
• Mixing hardness grades across mounting points without calculation. This is sometimes correct for an uneven load distribution — but it should be a deliberate decision, not an accident of stock.
• Ignoring lateral restraint on mobile plant. Cab mounts, compactor mounts and transport applications need to control movement in all directions, not just vertically.

When the standard range is not the answer

If the mounting envelope is fixed, the original part is obsolete, or the load case falls between standard sizes, a bespoke component is often the better route. Custom rubber mouldings and rubber-to-metal bonded parts can be produced to a drawing or from a sample, which is frequently the practical solution when replacing a discontinued OEM part.

A note on safety-critical applications

Where incorrect mount selection could affect operator safety, structural integrity, or compliance with regulatory or classification requirements — lifting equipment, marine classification work, defence and aerospace equipment, or any installation where mount failure allows the equipment to move — the selection should be reviewed and signed off by a qualified engineer against the specific application data. General guidance of the kind in this article is a starting point for a shortlist, not a final specification.