Radial Vibration

Alarm levels are set on a machine by people that know the machine's operational limits. You haven't even told us what is this machine. The manufacturer and model identification are useful to know. Volts Direct Current (VDC) are not a unit associated with mechanical vibration. In fact by definition a DC voltage should not vary at all. Gaps are typically measured in units of linear dimension; millimetres, inches, microns and sometimes when measuring the gap of a chasm in miles.

Now the voltage across the magnetic gap between stator and rotor cores in an electric motor will be the voltage arcing across the bearings. This arcing accelerates the bearings wearing out due to pitting and should be kept as low as possible. That is if this is what you are measuring to cause an alarm.

If the manual doesn't say you can change this limit or you do not have a manual to look this up don't touch the limit setting.

It is impossible to answer this question without the operation and maintenance manual for the equipment in question, the details of which have been withheld from the forum.

You need to adjust the alarm limits when you change the excitation voltage. In the range you are looking at, it will be close to a linear relationship. Your old scale was 50% displacement at 5 volts, now 50% will be 4.5 volts or 5.5 volts, so you need to adjust the alarm according to the same proportion.

If your are referring to Bently Nevada proximeters then the answer is no difference at all. The probes are usually set at -10V, but will read between -2 and -18V.

They register relative movement of the shaft to the bearing housing and as long as you are within the linear range of the probe it will make no difference if the voltage is set anywhere in the range that you mention.

Axial probes are a completely different matter.