Genset vibration limits turn a complaint about shaking into a figure that can be measured and compared. Check the specification against the mount type and the floor, then compare supplier offers before the baseframe is built. Tesla Power puts the limit and the measuring point on the drawing; send the slab detail to the technical desk and we confirm it.
- Write the limit as a velocity in millimetres per second and name the measuring point.
- Measure on the frame and on the floor, because the two readings answer different questions.
- Check the mount natural frequency against the running speed before the mounts are ordered.
- Look for a soft foot before blaming the mounts, since one loose shim shakes a whole frame.
What does a genset vibration limit measure?
It measures how fast the frame moves rather than how far, expressed as a velocity in millimetres per second and read as a root mean square figure. Velocity is used because it relates to the energy in the movement and it holds across a range of speeds. A displacement figure at one speed says much less about the damage the movement causes. A frame that moves fast enough to crack a weld reads well above the limit long before the crack appears.
The limit only means something with a measuring point and a load attached. A reading taken on the baseframe at full load is a different number from a reading taken on the canopy roof at idle. Tesla Power names both the point and the load on the drawing, because two suppliers quoting the same limit against different points are not comparable.
Which figure belongs in the specification?
A rigidly mounted set and an isolated set carry different limits, because the isolation changes what the frame is allowed to do. A common figure for a rigid frame is about 7 mm/s, with an alarm at 80% of it and a trip above the limit. An isolated set moves more by design, so its frame limit is looser and its floor limit is tighter. The instrument class matters as much as the figure, because two classes read the same set differently.
The specification should state the limit, the point, the load and the instrument class, because a reading taken with a phone application is not the same measurement as one taken with a calibrated probe. A 500kVA set at 0.8 pf delivers 400kW, and the frame limit on that size sits near 7 mm/s with the alarm at 5.6: 7 x 80% = 5.6.
| Mount type | Natural frequency | Note |
|---|---|---|
| Rubber pad under the frame | 8 Hz to 12 Hz | Cheap and ages with oil contact |
| Moulded rubber mount | 6 Hz to 10 Hz | Common choice for a 1500 rpm set |
| Spring isolator | 2 Hz to 4 Hz | Best isolation, needs a restraint |
| Rigid, bolted to the slab | Above 25 Hz | Passes the movement into the floor |
Why does a soft foot cause vibration?
A soft foot is a mounting point that does not sit flat, so tightening the bolt bends the frame and preloads the rubber. The frame then vibrates at a level the mounts cannot absorb, and the reading climbs without any fault in the engine. It is the most common cause of a vibration complaint on a new set and the cheapest to fix. A soft foot also loads the coupling, so the symptom often appears at the driven end first.
Check it by loosening one bolt at a time and watching a dial gauge on the foot, or by sliding a feeler gauge under each foot before the bolts are tight. A gap of 0.2 mm is enough to double a frame reading. Tesla Power Equipment checks all four feet on the test bed and records the shim thickness, because a soft foot that leaves the bay becomes a site visit.
How are the mounts chosen?
The mount is chosen so that its natural frequency sits well below the running speed, which for a 1500 rpm set means a natural frequency near 8 Hz or lower. A mount that is too stiff passes the vibration straight into the floor, and a mount that is too soft lets the frame move further than the pipework allows. Both failures look like a mount problem and are a selection problem. A mount chart is read against the weight carried by each corner, so the four mounts are rarely the same part.
The set of mounts also has to be matched as a set, because one mount of a different stiffness tilts the frame and loads the others. Replace mounts as a set rather than one at a time. Yangzhou Tesla Power lists the mount part number and the natural frequency on the baseframe drawing, so a replacement is ordered against the build rather than against a catalogue page.
What does torsional vibration add?
Torsional vibration twists the shaft rather than shaking the frame, and it is invisible to a probe on the baseframe. It appears as a cracked coupling, a worn driven end or a gear rattle at a particular speed. A set that reads well on the frame and fails a coupling has a torsional problem rather than a mounting problem. A coupling that fails at a particular load rather than at a particular speed points to the torsional case.
The cure is a different coupling or a damper rather than a softer mount, and the calculation belongs with the engine and alternator combination. A coupling that is too stiff for the combination passes the torque ripple into the alternator shaft. Tesla Power checks the torsional case for every engine and alternator pairing rather than for the frame alone.
How is vibration measured on site?
Take a reading on the frame near each mount and a second reading on the floor beside the frame, with the set at the load the complaint describes. Compare the two, because a high frame reading with a low floor reading points to the mounts and the reverse points to the floor. Then sweep the speed if the controller allows it, since a resonance shows as a peak at one speed. A sweep over a 10% band is enough to find it, and it takes ten minutes on a set that is already running.
Log the readings with the load, the speed and the mount age, and repeat them after any change. A reading that has doubled in a year is a warning even when it sits under the limit. Record the instrument and its calibration date as well, because a comparison between two instruments is worth very little without it. Tesla Power keeps both readings on the service sheet.
What should a buyer put on the enquiry?
Send the floor or slab detail, the mounting arrangement, the pipework connections and the speed of the set. Add the limit the site has to meet and the point it is measured at, because a specification written without a point cannot be tested. Those six items decide the mount set, and they show whether the offer has quoted a limit at all.
The wording on the offer matters too. Genset vibration manufacturers often quote a limit with no measuring point, and Chinese genset vibration suppliers sometimes ship a mixed set of mounts from two batches. Ask for genset vibration limits for a roof slab, genset vibration for a container build and genset for a hospital as three separate cases. Tesla Power writes the point on the drawing, and the 1000kW container build shows the mount arrangement.
What does the handover file record?
The file carries the limit, the measuring point, the frame reading, the floor reading and the shim thickness at each foot. One page covers the mounting when those five items sit under the set serial number. Add the conformity assessment for the mount set when the installation is part of a permit package, because a surveyor will ask for the readings rather than for the mounts.
Hand the page to the technician who will replace the mounts rather than to the buyer who placed the order. the Yangzhou factory describes the bed where the readings are taken. the commissioning notes lists the frames that ship with spring isolators, the build range explains how a soft foot is found with a feeler gauge, the ISO 8528-1 duty definitions sets the rating basis the load case comes from, and the NFPA 110 maintenance routine covers the load steps the set was accepted on.
Frequently asked questions about genset vibration limits
A phone can show that a set is shaking and cannot produce a figure that stands against a limit, because the sensor and the mounting are not controlled. Use it to find a resonance and a calibrated probe to prove a limit. A phone reading is a screening tool rather than a measurement.
Rubber mounts stiffen and take a set, and a mount that has been oil soaked loses its properties faster. A soft foot can also develop as the slab settles. Read the frame and the floor together, replace the mounts as a set and check the feet before the new mounts go in.
It matters more on an upper floor, because a flexible slab amplifies the movement the mounts were meant to stop. A set on a mezzanine needs the slab checked before the mount is chosen. A mount chosen for a ground slab on a mezzanine is a common and expensive mistake.
It sounds like a beat or a growl that comes and goes as the speed drifts, and it appears at one narrow band of speed. A set that is smooth at full load and rough at 80% has a resonance rather than a fault. Sweep the speed to confirm it before changing any hardware.
The canopy sits on the baseframe and moves with it, so isolating the frame isolates the canopy. A canopy bolted to a separate floor slab picks up the movement and rattles, which is why the canopy should follow the frame. Check the canopy fixings after the mounts are replaced.
Replace them when the frame reading has risen by half against the commissioning figure, or at the major overhaul, whichever comes first. A set in daily service reaches that point sooner than a standby set. Tesla Power writes the commissioning reading on the page so the comparison is possible.