The machine shakes the floor. The operator next door complains the desk rattles. You bolt the machine down harder and it gets worse. Bolting a vibrating machine to the building is the wrong fix.
Resonance is the enemy
Every mounting system has a natural frequency. If the machine’s operating speed matches that frequency, the mount amplifies the vibration instead of isolating it. This is resonance. You don’t need softer mounts; you need mounts whose natural frequency is well below the disturbing frequency.
The rule of thumb: the mount’s natural frequency should be one-third or less of the disturbing frequency. A machine running at 1800 RPM (30 Hz) needs mounts with a natural frequency below 10 Hz. A rubber pad might be at 20 Hz, which is in the danger zone. A spring mount gets to 3-5 Hz, which isolates well.
Rubber vs spring
Rubber mounts are cheap and good for high-frequency vibration. They work for motors, pumps, and small gearboxes running above 1500 RPM. They don’t isolate low-frequency vibration well, because their natural frequency is too high. If the machine runs at 600 RPM, rubber transmits the vibration straight to the floor.
Spring mounts handle low-frequency isolation. They’re the choice for presses, punches, and heavy recip equipment. But springs can bounce. Add a damper or an air spring if the machine moves around during startup and shutdown. A bare spring mount on a press will walk across the floor on its own.
Center of gravity matters
Four mounts under a machine don’t share the load equally if the center of gravity is off. One mount takes more weight, compresses more, and sits lower. The machine isn’t level, the shaft alignment drifts, and the coupling wears. Calculate the static load at each mounting point, not just the total weight divided by four.
For uneven loads, use mounts with adjustable leveling. Don’t shim with washers; that adds a hard contact point that defeats the isolation.
Isolation doesn’t mean floating
An isolated machine still needs to stay where you bolted it. If a side load from a belt or a hydraulic hose pushes it, it should move a millimeter, not slide across the floor. Some mounts have a captive bolt that allows vertical isolation but prevents horizontal walking. Use those where a lateral force exists.
Anchor bolts should not be torqued down tight on the isolation mount. The bolt goes through, but the mount itself sits between machine and floor. Tightening the bolt clamps the rubber and kills the isolation.
Measure before and after
Don’t guess whether the mounts are working. Put a vibration pen on the machine base and on the floor next to the mount. The floor reading should be a fraction of the machine reading. If they’re the same, the isolation is shorted out, usually by a bracket or a pipe connected rigidly to both sides.
Bottom line
Isolate by choosing a mount whose natural frequency is well below the machine’s operating speed. Rubber for high frequency, springs for low. Calculate loads at each mount, and measure the floor to confirm it’s actually working.
