The call came on a Tuesday. It always comes on a Tuesday, because Tuesday is exercise day.
“Your generator is getting louder,” the building manager told me. “People on the second floor can feel it in their teeth.”
I drove out. The set was a 400 kVA unit in the basement, maybe eight years old, exercised thirty minutes every Tuesday morning. It started. It ran. It sounded—normal. Exactly like a diesel generator sounds.
But the manager wasn’t wrong. The complaint was real. It just wasn’t a noise problem. It was a rubber problem.
Every diesel generator sits on anti-vibration mounts—rubber blocks between the engine feet and the skid. Their job is simple: keep the engine’s shaking in the engine, not in the building. A diesel at 1500 rpm fires its cylinders 750 times a minute. That energy has to go somewhere. Good mounts soak up most of it. Bad mounts hand it straight to the concrete.
Here’s the part nobody budgets for: rubber mounts are consumables. Heat, oil mist, ozone, and eight years of a two-ton engine sitting on them—rubber hardens. It sags. It cracks. And it does all of this without making a sound, which is why the mounts are the one part of the generator that dies quietly, on schedule, with no warning light. They deserve a place in the foundation and mounting plan from day one, not as an afterthought.
Put a new rubber mount next to one that’s been under an engine for a decade and they barely look like the same part. The new one gives when you press it. The old one feels like a hockey puck. That’s not an exaggeration—rubber compounds keep cross-linking as they age, and the hardness can roughly double. The mount was designed to flex. Now it barely moves.
There’s a field test I do with nothing but my thumb. Press your thumbnail into a new mount and it sinks in a couple of millimeters. Do the same on a ten-year-old one and it doesn’t leave a mark. That’s the whole comparison in one thumb—and it tells you more than the spec sheet the mounts came with.
A hardened mount does the opposite of its job. Instead of absorbing vibration, it conducts it—metal bolted to hardened rubber bolted to steel is just a slightly softer version of bolting the engine straight to the floor. The hum the office workers felt in their teeth? That was the firing frequency traveling up through rock-hard rubber, into the skid, into the slab, into the building frame.
Then there’s the sag. Old mounts don’t just harden; they compress and stay compressed. A few millimeters of sag at each mount drops the engine’s shaft centerline, and on a two-bearing set that eats straight into your coupling alignment. I’ve seen mounts sag enough to push a laser alignment out of tolerance—and the alignment tech gets blamed for a rubber problem.
This is the comparison customers actually ask me about, usually when they’re already replacing dead rubber ones. So here’s my honest take.
For a standby set on a slab at grade—a basement, a ground-floor plant room—good rubber mounts are enough. They handle the vibration, they’re cheap, and there’s nothing to adjust. I’ve got standby sets on rubber mounts that have run twenty years with one mount change. Don’t overthink it.
Spring isolators earn their money in exactly three situations: the set sits on an upper floor or a roof, the building has a quiet zone it has to protect (hospital wards, offices directly above), or the local code demands a specific isolation efficiency. Springs isolate lower frequencies better than rubber, and that matters when the structure above you is light.
But—and this is the part the spring-isolator brochures skip—springs without lateral restraint are worse than rubber. A generator on free springs will walk during a load step. Every spring installation needs snubbers or seismic restraints, properly gapped, or you’ve traded vibration for a machine that wanders. An old-timer I work with swears springs are always better. On a ground-floor standby set, I disagree with him. Rubber, changed on time, wins on simplicity every time.
You don’t need a vibration analyzer to find dead mounts. You need a pry bar and five minutes.
The Tuesday-morning building? Before I got there, they’d already paid for a vibration survey. The report said vibration at running speed was up and recommended checking the coupling alignment. Fair enough—so they paid for a laser alignment. The hum stayed.
Then someone suggested the alternator fan might be out of balance. They were about to pull the end bracket when the manager called me for a second opinion. I put a pry bar under the number-three engine foot. It didn’t move. None of them did. Eight mounts, all hockey pucks. (If you want the full tour of the vibration you can’t see, start there—but check the mounts first.)
New mounts cost a fraction of what they’d already spent, and the hum was gone the next Tuesday. The alignment tech hadn’t done anything wrong. The fan was never the problem. The rubber had just finished dying, quietly, over eight years, and nobody had a line item for it.
So if your generator has started bothering people it never bothered before—and the engine itself sounds the same—don’t start with the expensive theories. Add mounts to the maintenance checklist, get under the engine feet with a pry bar, and see what the rubber has to say. Or ask the Pavo engineering team—we’ve pried on enough dead mounts to know the feeling by heart.
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