Vibration isolation for machine foundations - Guidelines
1992 Edition

IS 13301 offers general recommendations for vibration isolation in machine foundations but does not prescribe specific isolator types. Suggested isolators include elastomeric pads like rubber or neoprene, steel coil springs, pneumatic (air) springs for adjustable support, viscoelastic mounts combining elasticity and damping, and lead-rubber bearings for high damping in heavy equipment. The choice depends on machine mass, operating frequency, and vibration amplitude. Properly selected isolators help reduce vibration transmission to foundations and surroundings, promoting smooth operation and environmental protection.

Rubber and cork pads differ notably in their vibration isolation behavior as per IS 13301. Cork exhibits a widely varying dynamic modulus typically between 10 to 40 N/mm² and a damping ratio ranging from 2.5% to 7.5%, with 6% recommended for design. Rubber pads have manufacturer-specific properties, with allowable bearing pressures from 0.8 to 1.6 N/mm² (depending on Shore hardness), and show nonlinear dynamic stiffness and damping influenced by static stress and vibration amplitude, requiring testing. Cork pads are nonlinear and affected by creep, usually enclosed in steel frames to prevent lateral expansion and need protection from oil and water. Rubber pads have lower natural frequencies (5–30 Hz) and thickness limited to one-fifth of width for stability, while cork pads have higher frequencies (25–60 Hz). Rubber offers better adaptability under varying loads but needs detailed testing.

According to IS 13301, rubber springs with Shore hardness between 40 and 70 have allowable bearing pressures ranging from 0.8 to 1.6 N/mm² and shear stresses from 0.3 to 0.5 N/mm², with thickness limited to no more than one-fifth of the pad width to maintain stability. Cork pads generally allow bearing pressures between 0.1 and 0.4 N/mm² (1 to 4 kg/cm²), but their edges must be enclosed in steel frames to prevent lateral expansion, and they must be protected against oil and water to maintain efficiency. Final design values should be confirmed with manufacturer data, considering dynamic stiffness and damping variations with load conditions.

In IS 13301, transmissibility (T) is calculated for a single degree of freedom system under steady-state excitation using the formula: [ T = \sqrt{\frac{1 + (2 \zeta n)^2}{(1 - n^2)^2 + (2 \zeta n)^2}} ], where ( n ) is the ratio of excitation frequency to natural frequency and ( \zeta ) is the damping ratio. Effective isolation is achieved when the frequency ratio ( n ) exceeds approximately 1.414 (( \sqrt{2} )), beyond which vibration transmission reduces. For ( n < \sqrt{2} ), vibration amplitude may be amplified. Typical damping ratios for isolators range from 5% to 10%. Designing isolators to operate with ( n > \sqrt{2} ) ensures reduction in transmitted vibrations.

Trench isolation, as described in IS 13301 Clause 8, is an effective active vibration isolation method for industrial machine foundations. The trench depth must be at least 0.6 times the Rayleigh wave length (L), which approximates the soil shear wave length, calculated as ( L = \frac{\sqrt{G/\rho}}{f} ), where G is soil shear modulus, ( \rho ) is soil density, and f is the frequency of the vibration source. The wave length L should be determined from in-situ wave propagation tests per IS 5249:1991. The trench acts as a physical barrier to reduce surface wave transmission. Proper trench depth and accurate wave property assessments are vital to achieve effective vibration attenuation.