Question 1

How to select springs for vibration isolation?

Accepted Answer

To select springs for vibration isolation, first determine the mass of the machine and the desired natural frequency of the isolation system. The natural frequency should be well below the operating frequency — typically less than 1/3 of the lowest excitation frequency. Use the formula k = m × (2πf)² to calculate the required total spring stiffness, then divide by the number of springs to get the stiffness per spring. Finally, verify that the static deflection under gravity is acceptable and that the springs can support the load without exceeding their rated capacity.

Question 2

What is the ideal natural frequency for vibration isolation?

Accepted Answer

For effective vibration isolation, the natural frequency of the spring-mass system should be at most 1/3 of the lowest operating frequency (excitation frequency). For example, if a machine runs at 1500 RPM (25 Hz), the isolation system should have a natural frequency below about 8 Hz. Lower natural frequencies provide better isolation but require softer springs and larger static deflections. Typical isolation systems target 2–8 Hz depending on the application.

Question 3

Why should the natural frequency be below 1/3 of the operating speed?

Accepted Answer

The transmissibility of a vibration isolation system depends on the ratio of the operating frequency to the natural frequency. Isolation only begins when this ratio exceeds √2 ≈ 1.41. At a ratio of 3 (i.e., natural frequency = 1/3 of operating frequency), transmissibility drops to about 12.5%, meaning 87.5% of the vibration is isolated. Higher ratios give even better isolation. Below a ratio of √2, the system actually amplifies vibrations, with the worst case at resonance (ratio = 1).

Question 4

How many springs should I use for vibration isolation?

Accepted Answer

The number of springs depends on the machine size, weight distribution, and mounting configuration. Common arrangements use 4 springs (one at each corner) for rectangular machines, or 3 springs for triangular layouts. The total required stiffness is divided equally among all springs. Using more springs distributes the load better and provides more stability, but each spring must be softer. Ensure all springs have equal stiffness to prevent tilting and uneven loading.

Question 5

What is static deflection and why does it matter?

Accepted Answer

Static deflection is the amount a spring compresses under the weight of the supported mass due to gravity (δ = mg/k). It matters because: (1) it determines how much the machine sinks when placed on the springs, (2) it is directly related to the natural frequency — a larger deflection means a lower natural frequency and better isolation, (3) the spring must have enough travel to accommodate the static deflection plus any dynamic motion, and (4) excessive deflection can cause stability issues. Typical isolation springs have 5–25 mm of static deflection.

Spring Selection by Target Frequency

Results

Required Spring Stiffness

RPM to Hz Conversion

Static Deflection & Per-Spring Stiffness

Practical Example

Nikolai Shelkovenko