Air Compressor Vibration

Give us some more clues please. Type of compressor, bearing type, which bearing etc.

it is a boge screw compressor (Germany) with SKF bearing NU. The measurements are done at the scution side of the air end.

As stated by Del and others, this is not uncommon. NU bearings are designed for high radial loads, high speed and axial freedom of the shaft.

You will see from the catalogues that there is a minimum radial load below which you should not go because at very low radial loads the cylindrical rollers tend to skid and the cage does not know whether it is driving the rollers or is being driven by the rollers resulting in noise (vibration). As you are measuring using shock pulse it is possible that you are picking up the skidding of the rollers and the rattling of the cage.

You do not give the bearing designation so we do not know the cage material. If you are picking up the cage frequency then I would guess that you have either steel or brass cages. Polyamide does not make much noise. Polyamide is also considered to be better for lightly loaded bearings, but I am not a fan of "plastic cages" (except maybe in a washing machine) and would insist on using a one piece machined brass cage suffix ML.

Running at low load for extended periods with high vibrations of this sort could lead to a reduction in bearing life.

It happens.
A string with no weight on it will waggle about in the breeze, but with a weight attached it will be stable. Maybe the bearing chioce is poor and it should have some pre-load?
...the term 'hunting' springs to mind...
Del

can't understand well.

pls give me a more detailed reply

thanks

If the inner and outer race are not pressing on the balls (pre-loaded) they may be free to vibrate, oscillate or hunt. Once under load they may roll smoothly.

E.G.
If you put a load of golf balls in a tray and shake the tray the balls will rattle around. If you place another tray on top of the balls and stand on it you will probably be able to smoothly slide about without rattling. (Don't try this at home)
I don't say that this is necessarilly your problem, I'm just try to explain what I meant in my previous post.
Del

thanks del

For a cat, that makes pretty good sense. As you suggest, this is rather common in rotating devices.

My mechanic uncle once told me that letting an engine run with no load wears the bearings more quickly than running with a load.

Cheers.

I think it could be true. That should be the reason that guys on the street run their machines in a more "loaded" condition... of course they spend some more tires in white smoke clouds but... all in name of good maintenance practices... I guess... Just to stress that an unloaded mechanical system is subject to movements in the free play of its loose-fitted parts that lead to vibration. But, it does not mean that the system is bad, it just means that a load will make it rest to "one side" of the plays and put the vibration to a rest. If it can make you more comfortable, this effect is also observed in aircraft engines gearboxes (due not only to the bearings unload, but also due to a series of gears being moved in the unloaded condition).

The valves in each compressor stage slapping shut under no load will drown your shock pulse meter with high frequency ringdown events. This is not bearing noise, which at worst will be an order of magnitude less severe. Overall vibration levels must always be regarded simply as an indication that a more detailed frequency investigation is required.

The valves in each compressor stage slapping shut under no load will drown your shock pulse meter with high frequency ringdown events.

BS - These slapping valves are precisely where???????????