hi friends:

according my information :

seismic probe use for measure absolute vibration bearing at bearing point.

proximity probe use for measure relative shaft vibration at shaft .(non contact probe )

am i right ???

if i am right,
how can put seismic probe on the rotor to measure the vibration on the machine haven t got one ??

any articles or links will be appressiate

best regrds

I'm not certain as to your question due to your use of email "shorthand" but you can't put a seismic transducer on a rotating rotor; hence its use on casings. Proximity probes are used as you stated to observe the shaft relative to the mount of the transducer, typically the casing adjacent to a bearing. You can by various means combine these two signals to establish shaft absolute. There are mounting considerations to be followed when this is accomplished to yield quality data. That's another topic in itself.

John

Just about right.

The seismic or velocity probe/transducer will measure total vibration at whatever point it is attached to - usually the bearing housing of a running machine.

The proximity probe measures actual shaft movement, usually within its bearing housing, onto or through which it is usually mounted.

As PA so rightly states, you cannot mount a seismic transducer onto a rotating shaft as it has to have a connection/cable to a monitoring system for it to work.

As a matter of interest, why do you think you need to mount a probe onto your shaft? Have you got a problem with vibration?

Oh, by the way. The one time you can mount a seismic transducer onto a shaft is when it is out of the machine and you are trying to establish its resonant frequency, a bump test.

I'm not certain I like the use of the term "total" in describing the measurement that a casing transducer can quantify. Seismic transducers only quantify the dynamics of the device to which they are attached. For example, the rotor can be moving 5 mils pp relative to the casing (proximity probe) and the casing can be moving 1 mil relative to some reference ("ground"). The shaft absolute is generated based on the phase relationships of these two signals, again subject to mounting considerations. In the example, shaft absolute could be 4 to 6 mils depending on phase relationships.

Obviously, in the previous example, examination of only the shaft relative signal might yield a false sense of security. Again, this opens up another thread of casing vs. proximity transducers and when each (or perhaps both) should be utilized.

John

Whoops, instead of "...examination of only the shaft relative signal might yield a false sense of security" I meant to say that examination of only the casing signal (1 mil) might yield a false sense of security.

I believe wnistra is refering to what ApectraQuest's glossary calls a dual probe:

Dual Probe — A transducer set consisting of displacement and velocity transducers. Combines measurement of shaft motion relative to the displacement transducer with velocity of the displacement transducer to produce absolute motion of the shaft.

I recall seeing these in the past but I don't remember who makes them.

The "seismic" sensor, which is an accelerometer, measures the casing vibration. The displacement sensor, which is a prox probe, meausures the relative vibration between the shaft and the sensor mounting point, which in this case is where the accelerometer is mounted. Sum the outputs and you have the absolute shaft vibration.

What this tells you is another question.

Jon
Spintelligent Labs

In the old'n days, it was common practice to to mount a seismic sensor (velocity) on the end of a flat wood stick that had a V-notch cutout. This "fish tail" would then be held against the spinning shaft to measure absolute shaft vibrations. This can be a dangerous procedure, and it certainly requires a smooth shaft with no key or keyway.
It is certainly possible to mount an accelerometer on a shaft and use slip-rings or telemetry to get the signal off the spinning shaft.

Both of these procedures can be used for diagnostics, but they are generally not used for routine monitoring.

Walt

Dual probes were initially introduced as a way to replace shaft rider vibration probes on turbine generators. A proximity probe (measuring shaft relative vibration) and velocity transducer (measuring bearing housing vibration relative to free space) are mounted in the same axis on the machine. A vector summation of the signals produces a shaft absolute vibration measurement (as did the old style shaft rider probe). All the major vibration monitoring manufacturer's for large journal bearing machines such as Bently, SKF, Entek-Rockwell, etc. have Turbine Supervisory Instrumentation monitors that can provide shaft absolute measurements.

dear friends :

thanks very much for every one answer me

i am sorry i mean if i want put proximity probe to monitor relative shaft vibration not installed from the manufacture , can i do it ,how can installed it ?

according my inforamtion :

seiesmic probe use for equipment have got ball and rolling bearing .
proximity probe use for monitor rotor which have journal bearing .

am i right?

best regrds

also what is equation to get absolute shaft vibration ?

if there is example i will gratefull ?

after i get this no. what is benifit ?
is there chart to evaluate the condition of machine ?

beat regrds

Well, definition that velocity or absolute transducers are limited to ball bearing machines is not really valid.
Velocity and accelerometers measure absoulute vibration on bearing, bearing houses, regardless of the type of machinery. In my view monitoring of the bearing house vibration is of value in most mchines as the force from rotating parts must pass that way. You can easily mount them with pin bolts.

Eddy probes or relative probes measure distance between the place it´s mounted and the target, normally the shaft. In my sort of unorthodox way to see things the best way to have use for them are to place them at some free part of the shaft to monitor shaft bending. If used like that they can be mounted even during operation, possibly not really taking care of the stringent surface demands there are on the target shaft, on the other hand the signal levels may be larger.
Eddy probes can be mounted at a service stop in the conventional way in the bearings, shaft surface preparation may be worse to do afterwards.

I vaguely remember that there is a ISO standard using the absolute measurement, possibly for hydro turbines. Something including straight lines in the orbit. Maybe I remember incorrect.
Calculating it may be reasonably done for 1 frequency (1xRPM) by integrating and adding, for a range of frequencies it´s very tricky to get good linear results due to variations in frequency response.
Olov

Vnistra, probes can be mounted to observe a suitable shaft target area in many different ways depending on the machine configuration. Quite often the machine is designed to accept proximity probes that are utilized by the OEM for test stand purposes. These provisions are then "plugged" at time of shipment if the end user doesn't purchase an instrument package. For example, since about 1980, virtually every gearbox manufacturer does this to insure quality of products (test floor prior to shipment) that have fluid film bearings. In this case the indicator of these provisons is the use of two pipe plugs in the extension caps at 90 deg to each other. Pull the plugs out and install the appropriate instrumentation...relatively simple.

At the other extreme you can make or purchase small mounting blocks for proximity probes that can be mounted by various methods ranging from a threaded fastener to the use of "speedy" glue for temporary test purposes.

As far as the method of calculating absolute from casing and shaft relative just perform vector addition (at the same frequency of interest) of the relative and casing displacements. R+C=A where these are all vector quantities. Integration of the casing signal to displacement is necessary and depending on the frequency of interest some compensation for phase shift in the tranducer may also be required. Of course a monitor is designed to also yield a direct or overall shaft absolute

Here are mounting brackets for proximity probes for internally mounted sensors as well as external through the bearing to observe the shaft. Mounting prox probes takes a good deal of thought - need to view journal quality shaft surface, need proper mounting brackets (may have to fabricate), proper orientation of sensors, proper gap, need to tie down any loose cable and plan for proper exit of cabling if mounted inside the machine. Get a copy of API670, or hire an experienced service group to install.

Mounting brackets:
http://www.reliabilitydirect.com/eddyprobeaccessories/eddyprobeindex.htm

Application note on prox. probe installation:
http://www.reliabilitydirect.com/appnotes/radialvibration.html

quote:

As far as the method of calculating absolute from casing and shaft relative just perform vector addition (at the same frequency of interest) of the relative and casing displacements. R+C=A where these are all vector quantities. Integration of the casing signal to displacement is necessary and depending on the frequency of interest some compensation for phase shift in the tranducer may also be required. Of course a monitor is designed to also yield a direct or overall shaft absolute

... or after integration you could just add the signals electronically or digitally in an analyzer or software product (after acquiring sample).

Also, I went down to see the new ADRE, recently discussed on this Board. The new ADRE has differential inputs with inversion; so, after integration one could use ADRE to add the signals to obtain a shaft absolute signal.