Vibration Phase Reference for Rotating Shaft

[Jieve]

Hello,

I was curious what provisions (if any) are usually provided in rotating industrial machines to use as a phase reference for condition monitoring. One example would be a keyway or notch in the shaft for a proximity sensor. Is it common for manufacturers of rotating equipment to provide provisions for this in the design, or is it usually just up to the technicians who are balancing to figure out how to get a phase reference? (Using an optical sensor and reflective tape, for example) What way do you guys find to be the easiest, and what methods are most commonly used?

Thanks!

 

[JJPellin]

Large, special purpose machines generally come equipped for continuous vibration monitoring which would ordinarily include a keyphasor® probe for each shaft speed. This is usually a standard proximity probe on an area of the shaft with a keyway. If no keyway is accessible, then a target dimple will be drilled into the shaft.

If we want to do phase analysis on a machine that is not equipped with a keyphasor®, then we have a few options. Many of our machines run constantly and cannot be shut down to allow for the addition of reflective tape. In those cases, we can use a two-channel analyzer. One probe is fixed and set to lock in on the run speed peak as the reference. The other probe is used to rove around to all the other points providing a phase reference back to the run-speed peak on the fixed probe. We have also used a laser tach looking at a keyway, installed with the machine running as the phase reference.

If we have the luxury of shutting the machine down, we can install reflective tape and use a tachometer as the phase trigger.

If you are building a new machine, I would suggest drilling appropriate target dimples into each shaft to allow for phase angle reference probes. If you are planning to use the phase reference for field balancing, then you can shut the machine down to add the target and use a proximity probe on a mag base mount.

Johnny Pellin

 

[GregLocock]

I imagine a spraycan and a paintball gun might come in handy if you really can't stop a machine. But if the machine is that expensive, yes it should be a standard fixture.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Jieve]

Thanks so much for the responses. So what should be considered when sizing the slot to be used for the proximity Probe? I have an application where a small shaft 18mm diameter is used On the machine to be balanced. If we cut a key slot 4 mm wide and 2mm deep, since the machine runs as 2250 rpm I'm looking at a Time duration of the sensor over the slot something like .002 secs each rotation. As far as ive understood, the optimal shape of the resulting Signal from the Probe is like a downward spike, which makes measuring the phase On the scope easy, it sorta points down if that makes sense. If the slot is too wide, you get a flat or step from the signal, from which im assuming you could use the initial point of signal change as the reference. Do you guys have any tips On what the signal would ideally look like and how the key slot should be sized?

Also, is it better to just drill a hole or divet in the shaft as opposed to a keyway, assuming its at an unloaded end of the shaft? Machining is certainly easier, but is there an advantage to One or the other?

 

[JJPellin]

A keyway is fine. The requirements for depth and width should be specified by the probe manufacturer. You velocity doesn't seem too high. We have compressors running up to 50,000 rpm with keyphasor® probes. If you are very particular about maintaining balance, you can drill a hole and then drill another identical hole on the opposite side a short axial distance from the first one. Make sure that the probe cannot see both holes or you will read double the actual speed.

Johnny Pellin

 

[Jieve]

Thanks again for the response. As this is in an academic setting, we likely won't be purchasing a keyphasor sensor but will be using an inductive sensor that we already have. In that Sense we don't have any documentation about what size the slot needs to be or what the signal should look like specifically. We will likely machine a keyway slot into the shaft, can you (or anyone with experience using balancing equipment) give me any tips On what to aim for with the signal and approx what size the keyway should be (or some guidelines)? Would be very appreciative. Thanks!