A machine tool that got noisier after installing soft vibration isolators. The customer bought the softest rubber mounts they could find, because “softer must isolate better.” The machine shook more. The isolators were wrong for the machine — they put the machine’s resonance right into the running speed range.
Vibration isolators work only if the isolator’s natural frequency is well below the forcing frequency. Rule of thumb: isolator frequency should be under half the lowest forcing frequency. A machine running at 1450 rpm (24 Hz) needs isolators with a natural frequency below 12 Hz. That means soft mounts — rubber in shear, or air springs. The customer’s soft rubber mounts were rated at about 8 Hz. That should have worked. But the machine had an internal resonance of its own around 10-12 Hz (the frame and the spindle head), and the soft mounts dropped the whole machine’s bounce frequency to about 9 Hz. The machine now bounced at 9 Hz, close to its internal resonance, and the isolators amplified instead of isolating.
The transmissibility curve everyone forgets
An isolator amplifies vibration below √2 x its natural frequency. Above that, it starts attenuating. The customer’s mounts at 8 Hz natural frequency amplified everything below 11.3 Hz — including the machine’s own 10 Hz resonance. The result: the machine shook more at startup and during load changes. The isolator was doing exactly what the physics says it would.
Choose the isolator by measuring the machine’s lowest forcing frequency, then picking mounts with a natural frequency at most half of that. For a 24 Hz motor, 12 Hz or lower. For a machine that ramps through 0-50 Hz (VFD), you can’t isolate the whole range with fixed mounts — you need air springs with leveling, or accept that the machine passes through resonance during ramp-up.
How to measure what you actually need
Don’t guess the forcing frequency from the nameplate. Measure it: run the machine, put a phone with a vibration app (or an accelerometer) on the base, and log the dominant frequency. Then check the isolator datasheet for its natural frequency at the actual load (not the unloaded rating — rubber mounts soften under load). If the mounted natural frequency is above half the forcing frequency, you need softer mounts — or heavier machine mass.
Adding mass to the machine base lowers the bounce frequency. A concrete inertia block under a machine is the classic fix: 3x the machine mass in concrete drops the bounce frequency by a factor of about 2. The block + soft mounts = proper isolation. The customer’s machine got a 200 mm concrete pad and the original soft mounts. The bounce frequency dropped below 5 Hz, far from the 10 Hz internal resonance. The noise went away.
The stiffness that matters
Isolation is about stiffness, not about softness per se. An air spring at 4 Hz natural frequency isolates far better than a rubber mount at 8 Hz, but it also lets the machine rock under load. For a machining center that takes heavy cuts, too-soft isolation means the machine rocks during the cut. There’s a tradeoff between isolation and stability. Machine tools often use mounts at 15-20 Hz natural frequency — not great isolation, but the machine holds its position. The customer’s error was going to the extreme soft end without checking what the machine needed.
Soft mounts aren’t automatically better. If the mounted natural frequency lands near the machine’s own resonance or the forcing frequency, the isolator amplifies. Measure the forcing frequency, pick isolators at half of it, and add mass to lower the bounce. The machine got noisier because it was bouncing on its own resonance.
