As soon as the pump starts, the site becomes tense. But a vacuum machine has a normal airflow noise of its own: treating a normal sound as a fault means dismantling the machine for nothing, and treating a real mechanical noise as normal can let a small problem turn into a major overhaul. Split the sounds into three classes and the judgement becomes simple.
Class one: an even hissing sound, which is usually normal
Both the vacuum pump drawing air and the oil atomising and flowing inside the pump body produce a continuous airflow noise. The characteristics of this sound are that it is even, continuous and free of rhythm changes: it appears once the machine has reached the vacuum state and weakens or disappears when pumping stops. Its intensity varies with the vacuum level, the oil temperature and the feed rate, which is a normal reflection of the operating condition.
The key is regularity: if the hissing runs steadily and is not accompanied by an abnormal temperature or current fluctuation, it generally needs no action; if it rises and falls erratically, as if something were passing through intermittently, it is not a class one sound and the investigation has to move on to class two.
Class two: an intermittent clicking sound, which points to the coupling or the bearings
A clicking sound is rhythmic and becomes more frequent as the speed rises. There are three common sources: wear of the elastic block in the coupling or a misalignment, striking periodically during operation; pitting on the outer race or on the rolling elements of the bearing, which sounds once per revolution as the defect passes; and periodic interference between the impeller or the rotor and a fixed part.
This class is not urgent, but it has to be dealt with. If pitting in a bearing continues to develop, it can turn into a seizure within a very short time and directly damage the pump shaft and the seal.
Class three: a continuous metallic friction sound, which requires an immediate stop
This is a harsh, high-frequency friction noise accompanied by whistling, and it often gets louder. Its source is direct contact between the shaft and the seal, between the shaft and the bearing, or between the impeller and the casing, with the moving parts rubbing. There are only two correct actions at this point: stop the machine immediately, do not stand and watch, and do not wait to see whether it improves.
Watch the accompanying signals as well: a jump in the current, a clearly elevated casing temperature, a burnt smell, and oil starting to drip below the pump body. If any one of them appears, the case for stopping becomes stronger.
How to judge without dismantling the machine
- Listen while it runs: stand in a safe position, note the rhythm and the direction of the sound and compare it with the normal sound of the machine. Listen in particular for whether the abnormal sound changes in step with the speed.
- Listen after the power is off: with the machine stopped, let the set run down on inertia; if metal impacts or a residual friction sound can be heard during the decay, the problem is in the mechanical parts, not in the airflow.
- Turn the pump over after the power is off: rotate the pump shaft by hand as described in the machine manual and check whether it is seized, whether the resistance rises noticeably over one angular range, and whether there is any abnormal noise.
- Compare the temperatures: measure the coupling, the bearing housings and the casing at several points with a temperature gun and compare them with equivalent positions; a local rise in temperature points directly at the rubbing point.
- Watch the current: observe the running current on a clamp meter; a periodic jump in it usually corresponds to rotor eccentricity or misalignment.
- Check the guard and the displacement: check the coupling guard for looseness, displacement and rubbing marks, and check whether the temperature of the guard itself is clearly above that of the motor.
| Sound characteristic | Common source | How to confirm | Action |
|---|---|---|---|
| Even, continuous hissing, varying with the operating condition | Vacuum extraction and oil mist flow | Compare the feed and the stop of extraction, and check that no temperature rise or current fluctuation accompanies it | Normal, keep on recording |
| Rhythmic clicking, becoming more frequent with the speed | Coupling wear, bearing pitting, rotor interference | Turn the pump over with the power off and measure the temperature at each point | Arrange a shutdown for inspection, with the coupling and the bearings as the first items to check |
| A harsh, continuous friction sound with whistling | Direct contact between the shaft and the seal, the bearing or the impeller | Listen to the sound and check the current, the casing temperature and whether oil is leaking | Stop the machine immediately, do not run it again |
| A regular metallic impact sound | Loose bolts or a foreign object that has fallen in | Turn the pump over with the machine stopped and find where it is seized | Stop the machine, open it up for inspection and tighten |
When the machine has to be stopped
If any one of the following appears, stop the machine immediately: a continuous metallic friction sound, a clear jump in the current, an abnormal rise in the casing temperature, a burnt smell, or continuous dripping below the pump body. Stop in this order: reduce the feed rate first, then stop the heating, then stop the main unit. Do not cut the power directly; let the oil circuit and the pump body come to rest smoothly.
Listening has to be done in two periods: listen while running for anything abnormal, and listen with the power off for any residual sound in the mechanical parts. Only when both periods are clean can it be called normal.