The vacuum pump is the part of the machine most easily overlooked, because it is hidden inside the cabinet and while it runs it produces only one continuous sound, which makes it look as though there is nothing wrong with it. Yet as soon as the pump loses output, what the machine shows is a vacuum level that will not come up, and the search is easily led into the vacuum system and comes back round to the pump itself after a long detour.
Oil level: the sight glass on the pump
The oil level is the most direct of the four. Look at the sight glass on the pump body; the level should stay between the upper and the lower mark. Below the lower mark the lubrication and the cooling of the pump are reduced and the pump oil has to be topped up; above the upper mark the amount of oil returning is too high, which usually points to leakage at the shaft seal or the end cover. The level is itself an indicator of leakage, and both ends of the scale carry information.
A level that falls quickly is often more worth watching than the level itself, because it means oil is going somewhere else. Record it once after topping up and look again a week later; if it is back at the lower mark, there is a continuing leak and the machine should be stopped to check the shaft seal and the end cover, rather than making the topping up a routine action.
Oil temperature: a level the back of the hand can judge
Hold the back of the hand near the pump body and the oil delivery pipe, taking care not to touch the metal directly. Compare the feeling with the way the machine feels when it is running normally; a clear difference is the signal. A temperature that stays high means the load on the pump is too heavy or the cooling is not what it should be, and a long period at high temperature ages the pump oil faster, which brings the oil level down with it, so the two indicators usually move together.
This one corroborates the current reading. When both are abnormal at the same time, the problem is most likely in the mechanical part of the pump itself rather than in the external conditions; if only one of them is abnormal, that direction is checked on its own and there is no need to pull the pump apart straight away.
Abnormal noise: listen for a change, not for a sound
A vacuum pump is supposed to make a sound while it runs; the problem lies in listening for a change. An even running sound is normal; a metallic knocking usually means the coupling or a mounting bolt has loosened; a sharp whistling note often comes from a blocked inlet or from a lack of oil; and a speed that rises and falls irregularly points to the supply or to the load.
When it is not easy to tell, stop the machine and start it again and compare the two, because the difference is easier to pick out than the sound on its own. Write the description into the record, for example a short sharp knock that repeats or a continuous whistle, which is a good deal more use than writing only the word normal.
Current: compared with the rated value
The reading on the ammeter in the control cabinet is the easiest of the four to keep a record of. It should be close to the level seen in normal running and should not sit against the upper limit for long periods. If it stays high together with a rise in the oil temperature, the cause is most likely an increased mechanical drag; if it fluctuates, the quality of the supply and the state of the phase failure protection have to be checked.
| Check | Where to look | Normal state | Signal that needs action |
|---|---|---|---|
| Oil level | The sight glass on the pump body | The level is between the upper and the lower mark | Below the lower mark the oil has to be topped up, above the upper mark the return is too high |
| Oil temperature | The pump body or the oil delivery pipe | Close to the feel it has in normal running, not uncomfortably hot to the touch | A temperature that keeps rising means the load or the cooling has a problem |
| Abnormal noise | The pump body and the coupling | An even running sound with no periodic knock | Metallic knocking, a sharp whistle, a speed that rises and falls |
| Current | The ammeter in the control cabinet | Close to the reading in normal running, not against the limit | A reading that stays high or fluctuates calls for a stop and a check |
Putting the four into one daily record
- Do the check at a fixed point in time after every start, away from the transient right after switching on.
- Record the four separately and describe the noise in words rather than writing only the word normal.
- Compare the current with the reading from the last time under the same conditions; the change says more than the absolute value.
- When the oil level falls quickly, look for leakage at the shaft seal and the end cover.
- Keep the spares ready according to the service interval, and do not wait for a failure to order the pump oil and the sealing parts.
- If there is a continuous abnormal noise or an abnormal current, stop the machine first and do not try to compensate by raising the vacuum setting.
Checking the vacuum pump is not about seeing whether it turns, it is about seeing whether it is still the same as it was last time.
