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বাড়ি / মামলা / Rubber Vibration Isolator Engineering Case Study II

Rubber Vibration Isolator Engineering Case Study II

2026-10-15
সম্পর্কে সর্বশেষ কোম্পানী কেস Rubber Vibration Isolator Engineering Case Study II
Case Detail

  1. Executive Summary

Core judgment: For a 55 kW screw air compressor running at 2,950 r/min, shear-type rubber isolators with a natural frequency of about 8 Hz yield a frequency ratio r ≈ 6.2, a theoretical transmissibility of about 2.7%, and an isolation efficiency of about 97.3%. Because the compressor operates in an oil-mist environment, an oil-resistant rubber compound (NBR) is mandatory; a flexible discharge connection and rigid, separate mounting of the air receiver prevent secondary sound paths.
Conclusion: Mount the compressor skid on an inertia base with 6 isolators rated at ≥ 400 kg per point (design load ≈ 347 kg/point). The calculated performance exceeds that of the pump-unit case (r ≈ 5.4, efficiency ≈ 96.4%) because of the higher running speed, and the broadband rotor-meshing harmonics are also well attenuated.
Basis:
  • GB 50019-2015 requires the ratio of equipment running frequency to isolator natural frequency to be ≥ 2.5, and recommends rubber or other elastic-material isolators when equipment speed exceeds 1,500 r/min.
  • Disturbing frequency f = 2,950/60 ≈ 49.2 Hz; with f0 = 8 Hz, r ≈ 6.15, T = 1/(r²−1) ≈ 0.027, isolation efficiency ≈ 97.3%, force transmission reduction ≈ 31.3 dB (undamped formula).
  • The 2nd harmonic (rotor-meshing) frequency is 98.3 Hz, giving r ≈ 12.3 and T ≈ 0.7% — broadband content is isolated even more effectively.
  • A vendor compressor case reports about 90% isolation efficiency with foundation vibration below 2.5 mm/s RMS (third-party claim).
Constraints: Calculations use an undamped single-DOF model; in practice rubber damping and piping-borne secondary transmission affect results, so field re-measurement governs. Oil mist and discharge heat degrade ordinary rubber, so an oil-resistant compound and periodic aging inspection are required.
Action: Execute the workflow “measure speed → calculate frequency ratio → select model by load → install on inertia base with flexible connections → re-measure and accept”; additionally verify oil-resistance of the rubber compound and the separate rigid mounting of the air receiver.

  1. Case Overview and Isolation Objectives

  1. Project Background

A plant compressor room houses one 55 kW oil-injected screw air compressor on a factory skid (compressor, oil–gas separator, and control panel as one assembly), adjacent to a product inspection area and offices. Rotor-meshing excitation produces broadband vibration and noise (fundamental plus harmonics) that propagates through the floor slab and the discharge piping; field measurements showed excessive micro-vibration in the inspection area and excessive noise in the adjacent office, calling for a vibration-isolation retrofit. This document is a typical engineering case (equipment parameters are replaceable); all calculations and parameters are annotated with their source or basis.

  1. Isolation Objectives and Acceptance Criteria

Objective
Indicator
Target/Limit
Basis
Frequency ratio
r = f/f0
≥ 2.5, preferably 4~5
GB 50019-2015, Cl. 12.3.4
Isolation efficiency
I = 1 − T
≥ 93% (at r ≥ 4)
GB 50019-2015 commentary
Vibration transmissibility
T
≤ 0.1
Engineering practice / vendor case figures
Noise in adjacent office
Daytime equivalent level
≤ 45 dB(A)
Project target (office area)
Sources: GB 50019-2015 “Code for Design of Heating Ventilation and Air Conditioning for Industrial Buildings”; GB 22337-2008 “Emission Standard of Environment Noise for Social Life” | Targets are common engineering values; follow project-specific requirements

  1. Vibration Source Analysis and Isolation Calculation

  1. Equipment Vibration Parameters and Isolation Form Selection

Item
Speed (r/min)
Disturbing frequency f = n/60 (Hz)
Recommended treatment
Screw air compressor (skid)
2 950
49.2
Shear-type rubber isolators (speed > 1,500 r/min)
Rotor-meshing 2nd harmonic
—
98.3
Even higher frequency ratio, well isolated
Air receiver (static)
0
—
Rigid mounting on floor, connected via flexible hose
Source: GB 50019-2015, Cl. 12.3.4; disturbing frequency computed as f = n/60

  1. Frequency Ratio and Transmissibility Calculation: r ≈ 6.2, Isolation Efficiency ≈ 97%

Disturbing frequency: $$f = n/60 = 2950/60 approx 49.2 mathrm{Hz$$
Shear-type rubber isolators with an oil-resistant compound provide a natural frequency of about 8 Hz; this case assumes $$f_0 = 8 mathrm{Hz$$.
Frequency ratio: $$r = f/f_0 = 49.2/8 approx 6.1$$
Undamped transmissibility: $$T = 1/(r^{2}-1) = 1/(6.15^{2}-1) approx 0.02$$
Isolation efficiency: $$I = (1-T) times 100% approx 97.3$$; force transmission reduction: $$R = 20 lg(1/T) approx 31.3 mathrm{dB$$
2nd harmonic (rotor-meshing): $$f_2 = 2f = 98.3 mathrm{Hz$$, $$r_2 = 98.3/8 approx 12.$$, $$T_2 approx 0.7$$ — the broadband content is attenuated even more strongly.

Fig. Transmissibility and isolation efficiency vs frequency ratio (calculated curve) | Basis: undamped single-DOF model T = 1/(r²−1), sampled at r = 1.5~10; GB 50019-2015 requires r ≥ 2.5, and its commentary notes efficiency >93% at r = 4~5, with diminishing returns beyond r = 5
Conclusion: r ≈ 6.2 falls in the “efficiency >97%” region; because the frequency ratio is higher than in the pump-unit case, the screw compressor achieves better isolation with the same isolator family, and its harmonic content is isolated even more effectively.

  1. Isolator Selection

  1. Selection Principle for the Compressor Environment

Per the selection ranges in the “Code for Design of Noise Control in Industrial Enterprises” and the GB 50019-2015 commentary, shear-type rubber isolators (natural frequency 5~12 Hz) suit medium/high-speed equipment; metal springs (2~5 Hz) are intended for equipment at or below 1,500 r/min, and air springs (1~8 Hz) for precision low-frequency isolation. For this compressor, rubber is the economical choice and its internal damping additionally controls the startup passage through resonance.
Isolation form
Natural-frequency range
Suitability here
Shear-type rubber isolator
5~12 Hz
Chosen: f0 ≈ 8 Hz, high damping, economical
Metal spring isolator
2~5 Hz
Not required: speed > 1,500 r/min; low damping
Rubber isolation pad
10~20 Hz
Only as auxiliary layer, not primary isolation
Air spring isolator
1~8 Hz
Over-specified and costly for this application
Source: isolation ranges from the “Code for Design of Noise Control in Industrial Enterprises” and GB 50019-2015 commentary

  1. Load Calculation and Model Selection

The compressor skid (compressor + separator + control panel + steel base) weighs about 1,600 kg; with 6 mounting points, the static load is about 267 kg per point. Applying a dynamic factor of 1.3 for startup torque and load redistribution gives a design load of about 347 kg per point.
Selection follows “rated load ≥ design load, and deflection not exceeding 1/3–1/2 of the ultimate deflection”. Selection result:
Item
Value
Note
Mounting points
6 (symmetrical)
Skid corners plus mid-span points
Static load per point
≈ 267 kg
1,600 kg / 6
Design load per point
≈ 347 kg
Dynamic factor 1.3
Selected rated load
≥ 400 kg
Shear-type, NBR compound
Target natural frequency
≈ 8 Hz
gives r ≈ 6.2
Basis: rated-load matching with manufacturer parameter tables (e.g., E-type series); oil-resistant NBR compound required by the oil-mist environment | Values are case assumptions and calculations, not measured data

  1. Why Rubber Rather Than Springs for This Compressor

Dimension
Rubber isolator (chosen)
Metal spring isolator
Natural frequency
≈ 8 Hz (shear type)
2~5 Hz
Damping
High internal damping (ζ ≈ 0.05~0.15), controls startup resonance
Low damping; large resonance amplitude at startup
Cost and simplicity
Low, direct mounting
Higher, often needs added dampers
Oil/heat resistance
NBR compound; needs protection and inspection
Durable; corrosion protection needed
Source: combined from code selection ranges and engineering experience

  1. Installation Implementation

  1. Base and Foundation

  • Weld a steel inertia base beneath the skid to stiffen it and distribute load; level the base so that the center of gravity is as close as possible to the geometric center of the 6-point isolator array and the overall CG is low (GB 50019-2015, Cl. 12.3.5).
  • Where the inspection area is extremely sensitive, add a “floating double-isolation base”: a precast concrete slab between the equipment base and the floor, with rubber isolation pads beneath the slab (Cl. 12.3.8).
  • Mount the air receiver rigidly on the floor as a separate item — do not place it on the isolators, so it does not rock and re-radiate vibration.

  1. Isolator Installation Requirements

  • Design compression deflection should be taken as 1/3–1/2 of the manufacturer's ultimate deflection; no isolator may exceed its allowable working load (Cl. 12.3.4).
  • Check that deflection is equal across all 6 points after installation; a flexible skid redistributes load if leveling is poor, causing uneven deflection and rocking modes.
  • Use an oil-resistant (NBR) compound; shield isolators from oil mist, discharge heat, and direct sunlight, and include deflection/aging checks in the annual maintenance plan.

  1. Piping, Discharge Connection, and Structure-Borne Sound Control

  • Connect the compressor discharge to the piping via a rubber flexible connector; install elastic pads between pipe supports/hangers and the piping; seal gaps where pipes pass through the compressor-room enclosure with sound-insulating elastic material (Cl. 12.3.6, Cl. 12.3.7).
  • Connect the air receiver to the discharge piping through a flexible hose so that receiver pulsations are not transmitted back to the isolated skid or to the building.

  1. Performance Verification and Case Data

  1. Before/After Comparison

Indicator
Before retrofit
After retrofit
Data nature & source
Vibration transmissibility
Rigid mounting, near 1
< 5% (calculated T ≈ 2.7%)
Undamped formula, this case
Isolation efficiency
—
≈ 97.3% (calculated)
Undamped formula, this case
Foundation vibration
Excessive
< 2.5 mm/s RMS
Vendor compressor case (third-party claim)
Noise in adjacent office
Exceeded 45 dB(A)
Reduced; meets project target
Project measurement (planned)
Sources: see references at the end of this document | Calculated values are undamped-model results; vendor figure is a third-party claim for reference only; acceptance is based on field measurement
Conclusion: at r ≈ 6.2 the screw compressor case reaches a higher isolation efficiency (≈97.3%) than the pump case (≈96.4%) thanks to the higher running speed, and the 2nd-harmonic content is attenuated to below 1% transmissibility.

  1. Acceptance Conclusion

Recheck the calculation against GB 50019-2015 (frequency ratio ≥ 2.5), then re-measure vibration velocity, transmissibility, and adjacent-office noise after the retrofit: acceptance is granted when the targets are met. Record isolator deflection, base levelness, the condition of flexible connections, and the oil-resistance/aging status of the rubber.

  1. Lessons Learned and Notes

  • Higher running speed makes isolation easier: r ≈ 6.2 here versus r ≈ 5.4 for the pump, with efficiency rising from about 96% to about 97%.
  • Screw compressors emit broadband rotor-meshing vibration; rubber damping not only isolates but also controls the startup passage through resonance, which low-damping springs would amplify.
  • Oil mist and discharge heat are the main rubber killers: use an NBR compound, shield the isolators, and inspect for aging and deflection annually.
  • On a flexible skid with 6 mounting points, leveling and equal deflection are critical; otherwise load redistribution creates rocking modes and uneven compression.
  • Isolation addresses only the “equipment–foundation” path: the discharge pipe, supports, wall penetrations, and a rigidly mounted receiver must be treated, or secondary sound paths undermine the retrofit.
  • Calculations use an undamped model; actual rubber damping ratios of about 0.05–0.15 suppress resonance amplitude, and the above formulas are adequate for engineering judgment.

  1. References