Placement of vibration sensors for strobic fans

Good morning-

I'm wondering if anyone could give me some feedback on how they are doing vibration analysis on their strobic fans.  Of course, you mount the probes as close to the motor bearing as possible. That's easy to do for the motor non drive end bearing, not as easy to do for the motor shaft end bearing (the motor is vertical mount, direct connected to the fan wheel.  

My installations sit high high above the roof, have no access platforms whatsoever and have never been tested (very poor setup for testing.  I imagine some folks out there had to install permanent sensors. If so, what brand worked best?

Would anyone be willing to send me some photographs of their setup? The only way I can test mine with portable equipment is to install temporary scaffolds and platforms.  No wonder the tings have never been tested!

Forget what the manufacturer tells you, nothing last forever. You need to do trend analysis over the long run to predict your old age failures before they fail all at once!






James R. Karl, P.E. C.M.R.P.

Original Post

Go to for General Brochure that shows typical fan installations and cutaway drawing. Also look for 2 Technical Bulletins for bearings and vibrations (no help for vibration measurements).

Can you provide a photo of your installation?

My 1st suggestion is to use a pole with accelerometer to reach target location instead of ladder or scaffold. I hope you do not think you have to reach the top of the stack, because the motor is at the bottom!

I would only measure vibrations on casing ring just above lift lugs in both radial and axial (thrust/vertical) directions. This is direct vibration path to motor DE bearing and indirect path for NDE bearing. Motor NDE bearing is not accessible, and unit would probably require disassembly to install permanent accelerometer.

Lots of options for permanent accelerometers with cables to terminal box for portable analyzer connection (preferred) or multi-channel monitor, or consider wireless monitor system.


Walt -

Please see attached.

I see the casing ring. It's the one just below the large circular motor shroud  that bisects the air flow. I wondered about that location. I knew the motor was at the bottom and the mounting plate appeared to be welded directly to the ring. 

I have a mag pickup with a long cable , so I'm sure I can measure while standing at roof level.

Can you mark on the picture where I should place the probe and send it back to me?  The axial direction is parallel to the airflow, so should the probe sit on the flat face of the flange?

The radial direction is perpendicular to the airflow so should the probe be flush against the face of the casing ring?

Thanks for your help!




The motor in the brochure cut-away view appears to be foot-mounted on a vertical base plate. If that is the case, then find location of base to select Ring location for accelerometer (arrow). An underside mounting may work better for axial (vertical) measurement. It may be possible to see motor through casing crotch. If yes, then may be possible to reach in with hand or accelerometer on short pole to mount accelerometer onto motor case for NDE (not on fan shroud). If multiple fans, be sure to make measurements at identical locations for comparison. Choose radial direction on Ring surface or below it (depends on accelerometer size and magnet base).



Photos (1)

Thanks. That's how I thought it would be set up.

I do have Strobics mounted at roof level as shown in the picture, so reaching in through casing crotch to place accelerometer is feasible.  For the ones elevated above the plenums, not yet. because there's no grating or access ladders for safety.  I will try shroud measurements on those units for starters.


I just saw your installation photo with 5 sets of 4 fans (20 fans total). The fans can be difficult to measure because of cross-talk vibrations between 4 fans, and especially if variable speed. If possible run only one fan in set at a time and at full speed. Get speed from VFD controller, vibration at 1xSS, or vibration at 1xBPF. You need to know number of fan blades; from OEM or open access cover and count them. The blade pass frequency (BPF) amplitude should be highest on lower casing at fan wheel. If reaching with a pole, then accelerometer location may not be optimal, but at least should be consistent and identical on all fans for alarming, trending and diagnostics.

Pick a sunny day for survey!


|James I fitted remote sensors only 6 weeks back on 2x strobics like the ones pictured. 15kw Abb Motors fitted(I think) The fans are about 5-6 feet above the level of the roof.(see ladder)

I fitted 3x studs on each motor with epoxy quick fit studs, then hand tightened the transducers on. There are drain holes on side of the cowling and I ran the cables out and down to a Metal powder coated BNC junction box.

See Image attached the blue line is where I ran the cables.

Pics attached.I have none of the sensor mountings though.

I haven't had a chance to take the first readings on them yet.  The Cowling is a pain in the butt to get at the motor, but it can be done.



Sensors used.


Photos (2)

Walt -


How is your two sensor system performing for the Strobics? Did you use triaxial or single axis accelerometers for the setups?  Can you tell me the manufacturer?

I'm not having much luck getting Strobic to look into this kind of system .Strobic uses a vibration switch mounted on the outside of the fan cowl.  A fan switch is a trip device - it gives you no predictive capability.


Were you able to install the sensors without taking the fan apart?  I have some Strobics whee a noise suppressor section is used. As a result, the motor is very deep in the cowl section and the upper section of the fan has to be removed....

I did not install permanent accelerometers, but merely indicated suitable locations. If you do not mount accelerometers (1 or 2) directly onto motor, then the next best choice is on the exterior structure that connects directly to the motor support. Either location should be able to detect fan unbalance and significant motor bearing fault. The Strobic fan is very similar to tube-axial fans with construction that make direct vibration measurements difficult, but not impossible.


Hi James, I fitted My sensors on that installation in images above, to the sides of the ABB motors inside the fan cowling, was awkward as hell to fit them but it worked. In the image the blue lines I have drawn are showing the route of the cable, there is an access hole approx 2' diameter at the base and I ran the sensors in through that and have them epoxied to the motor at the motor non-drive and drive end locations.

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