Compressor and Generator Rubber Mounts: Complete Industrial Isolation Guide

A facilities engineer at a hospital in Riyadh discovered a problem with two standby diesel generators that had been installed two years earlier. Staff in adjacent patient rooms complained of intermittent low-frequency hum — not during emergencies (when the generators ran at full load) but during weekly test runs at partial load. Vibration measurement found that at partial load, the generator RPM passed through a frequency where chassis vibration was amplified by the building structure — a resonance phenomenon. The generator mounts had been selected by load rating alone (2,500 kg static capacity per mount), without checking natural frequency against the building’s structural resonant frequency. Re-specifying the mounts with a different rubber compound (Shore A 45 instead of 60) shifted the isolation natural frequency below the structural resonant range. The hum disappeared completely. Mount replacement cost: $2,400. The alternative — continuous patient complaints and possible regulatory review of the installation — would have cost considerably more.

Industrial compressors and generators represent some of the most demanding applications for rubber vibration isolation mounts. Unlike construction equipment (where rubber mounts attenuate impacts and rough terrain), industrial machinery operates in continuous-duty cycles that create sustained, periodic vibration that must be precisely isolated from buildings, pipework, and sensitive equipment.

This guide covers rubber mount specification and selection for air compressors (rotary screw, reciprocating), gas compressors (pipeline, process), diesel and gas generators (gensets), and industrial rotating equipment.

Need compressor or generator rubber mounts? Request a quote from Babacan Group — engineering support for frequency-matched isolation.

Vibration Isolation Principles for Industrial Equipment

Why Load Rating is Not Enough

The Riyadh hospital case illustrates the most common error in industrial rubber mount selection: choosing by load rating alone. A rubber mount’s static load capacity tells you nothing about its isolation effectiveness.

The critical parameter is the mount’s natural frequency (fn): the frequency at which the rubber-machine system will resonate. For effective isolation, fn must be significantly lower than the disturbing frequency (machine operating frequency):

• Good isolation: fn < 0.4 × machine operating frequency
• Amplification (dangerous): fn = machine operating frequency (resonance)
• Partial isolation: fn = 0.5-0.7 × machine operating frequency

For a 4-pole electric motor running at 1,500 RPM (25 Hz), the mount natural frequency should be below 10 Hz for good isolation.

Static Deflection Method

A simple way to estimate natural frequency: fn (Hz) ≈ 15.8 / √(δ), where δ is the static deflection in millimetres under the machine’s weight.

• δ = 25 mm → fn ≈ 3.2 Hz (suitable for reciprocating compressors)
• δ = 15 mm → fn ≈ 4.1 Hz (suitable for rotary screw compressors)
• δ = 8 mm → fn ≈ 5.6 Hz (suitable for diesel generators)

For detailed isolation theory, see our anti-vibration mount selection guide.

Reciprocating Compressor Rubber Mounts

Reciprocating (piston) compressors generate complex vibration — both the fundamental frequency from piston reciprocation and multiple harmonics. This makes isolation more challenging than for rotating equipment.

Single-Stage and Two-Stage Reciprocating Compressors (Small/Medium)

For workshop air compressors (5-30 kW), typically 2-stage, two-cylinder:
– Mount type: Cylindrical rubber mounts or sandwich-type rubber-metal mounts
– Shore A: 40-55 (medium compliance)
– Static deflection target: 8-15 mm
– Natural frequency target: 4-6 Hz
– Load range: 50-300 kg per mount (4-6 mounts per compressor)

The high amplitude of reciprocating compressor vibration at low RPM (typically 700-1,000 RPM for these units) means mount selection must account for the full RPM range, not just nominal speed.

Large Process and Industrial Reciprocating Compressors

Industrial process compressors (50-500+ kW), often pipeline or gas compression:
– Mount type: Large sandwich mounts or steel spring mounts with rubber pads
– Shore A: 45-60 (stiffer mounts needed for large machines)
– Static deflection: 12-25 mm
– Mounting weight considerations: Large reciprocating compressors may weigh 5,000-30,000 kg — individual mount loads can exceed 5,000 kg

Large reciprocating compressors often use concrete inertia blocks (equipment mounted on a heavy concrete pad) to reduce vibration amplitude before it reaches the isolating mounts. When inertia blocks are used, the rubber mounts sit between the inertia block and the floor structure.

Rotary Screw Compressor Rubber Mounts

Rotary screw compressors generate much lower vibration amplitude than reciprocating types — the smooth meshing action of the screw rotors is nearly vibration-free at nominal speed. The dominant vibration sources are the drive motor and the cooling fan.

Packaged Rotary Screw Compressors (7-75 kW)

For standard packaged rotary screw compressors (Atlas Copco GA, Kaeser SK/BSD, Ingersoll Rand R-series):
– Mount type: Cylindrical or sandwich rubber mounts under the compressor package base
– Shore A: 45-58
– Static deflection: 5-12 mm
– Number of mounts: Typically 4 under the package frame

Factory-installed rubber feet on packaged compressors are often inadequate for sensitive installations. For hospital, laboratory, or office building installations, add isolation pads or upgrade mounts to achieve better isolation.

Oil-Free Rotary Screw Compressors

Oil-free compressors (Atlas Copco ZR, ZT series) run at higher speeds and generate slightly more vibration than flooded-screw equivalents. Mount specification:
– Shore A: 48-60 (slightly stiffer to handle higher-frequency vibration)
– These compressors are often used in pharmaceutical and food processing — building structure isolation is critical

Diesel Generator Set (Genset) Rubber Mounts

Diesel generators combine a diesel engine and an AC alternator on a common base frame. The engine is the dominant vibration source; the alternator adds static load without contributing significant vibration at nominal speed.

Residential and Commercial Standby Generators (20-500 kVA)

For typical standby generators using Perkins, Cummins, Volvo Penta, or John Deere engines:
– Mount type: Cylindrical or double-conical rubber mounts
– Shore A: 42-55
– Mounting points: 4-6 per genset
– Special consideration: Standby generators start infrequently and run short test cycles — mounts must not bottom-out at start transient or resonate at partial load (as in the Riyadh case)

The Riyadh hospital case demonstrated that partial-load resonance is a real failure mode for standby generators. Specifying mounts for full-load isolation without checking partial-load RPM crossing behavior is insufficient for critical installations.

Industrial Generator Sets (500 kVA–10 MVA+)

Large industrial gensets (power stations, hospitals, data centers):
– Mount type: Large sandwich mounts, housed rubber mounts, or steel coil springs with rubber pads
– Shore A: 50-65 for rubber mounts; coil spring mounts preferred at very high loads
– Static deflection: 10-20 mm for rubber; 20-50 mm for spring mounts
– Acoustic isolation: For critical applications, add steel spring mounts with high static deflection to achieve acoustic isolation frequencies below 5 Hz

For guidance on diesel generator rubber isolation including marine genset applications, see our diesel genset rubber isolation mounts guide.

Gas Turbine and Gas Engine Isolation

Gas turbines (industrial power generation, gas compression) run at very high speed:
– Gas turbine speeds: 3,000-50,000+ RPM depending on type
– Gas engines: 600-1,800 RPM for reciprocating gas engines

High-speed turbines require very soft, high-deflection mounts to isolate the high-frequency excitation:
– Shore A: 25-40 (very soft for gas turbines)
– Static deflection: 20-40 mm
– Note: Very soft mounts require care with lateral restraint to prevent excessive lateral movement under unbalanced loads

Gas turbine mounts are highly application-specific and should be specified with engineering analysis of the machine, frame, and structural resonant frequencies.

Compound Selection for Industrial Environments

Industrial compressors and generators operate in a range of environment types that affect rubber compound selection:

The hospital Riyadh case: indoor standby generator, no oil exposure, 40°C+ ambient — standard NR would work for temperature, but EPDM would be preferable for the hot outdoor ambient that the generator room experiences during daytime.

Shore Hardness and Dynamic Stiffness

For industrial isolation applications, Shore A alone does not fully characterize the mount’s isolation performance. Dynamic stiffness (which is typically 1.2-2x the static stiffness for rubber) determines the actual isolation frequency under operating conditions.

Request dynamic stiffness data (K_dyn) from mount suppliers for critical industrial applications — not just static load capacity and Shore A.

Replacement Indicators for Industrial Mounts

• Visual inspection: Cracking, swelling (oil contamination), compression set (permanent height reduction >15% of original)
• Static height measurement: Compare to new-condition height — more than 10-15% height loss indicates compression set
• Shore A hardness test: Hardness increase >15 Shore A points from nominal indicates thermal hardening; decrease indicates oil contamination or ozone degradation

ISO 9001:2015 manufactured mounts provide consistent compound quality for predictable service life and isolation performance.

Conclusion

Compressor and generator rubber mount selection requires going beyond load rating to specify natural frequency — the parameter that determines isolation effectiveness. The Riyadh hospital case demonstrates that incorrect natural frequency specification causes real problems in sensitive buildings, even when the mount is adequately rated for the static load.

Key principles for industrial compressor and generator rubber mount specification:
– Target fn below 40% of machine operating frequency for good isolation
– Check partial-load RPM range for standby generators — resonance crossing during load pickup is a real failure mode
– Match compound to operating environment — outdoor generators in hot climates need heat and weather resistance
– Request dynamic stiffness data (K_dyn), not just static load rating and Shore A

Babacan Group engineers and manufactures anti-vibration mounts for industrial compressors and generators under ISO 9001:2015 quality management. We provide application engineering support to specify correct Shore A and static deflection for your installation. Browse our rubber mount catalog or request a technical quote with your machine type, weight, and operating speed.