The other day I was asked to check out a drive that was "shaking a little".

This is a 4-pole ac induction motor connected through a fluid coupling to a jackshaft with 2 pillow plock bearings and an eight grove v-belt sheave mounted outboard on the jackshaft. The sheave drives two spearate sets of 4 belts to 2 different shafts on some type of crushed. The entire motor and jackshaft assembly is mounted on a slidebase to allow for tensioning the belts.

"Shaking a little" is about 2.5 ips at running speed and harmonics with a 2 1/2" steel pipe driven under the fabricated steel base in an attempt to control the vibration. The fluid coupling had been replaced, but neither of the flexible geartooth couplings were even inspected and one was obviously locked up when inspected with a tunable strobe. A visible wobble was present when the atrobe was tuned to allow slow rotation and wear was obvious in the area where the hub and sleeve of the gear coupling meet.

My question is with the fluid coupling and the slip from 1793 rpm to 1769 rpm what would expect to see as far as phase shifts across the fluid coupling?

What I actually saw was a shift of -94 degrees with the accels mounted axially opposed and 0 degrees with the accels mounted horizontal across the coupling. Phase was steady, but coherence fell off steadily.

Thanks,

Danny

Since the shafts are rotating at different speeds, the phase shift should vary continuously. If you think about the phase between keyways on the input and output shafts, the phase must continuously change, as 1 revolution of the input shaft results in less than a complete turn of the output shaft.

Jon
Spintellligent Labs

This is not an answer to your question, but by way of some extra info that may help some one somewhere.

I had a bad experience with a fluid coupling many years ago. The coupling had a delay tube on the drive end and the point where the drive shaft bolted on was not running true. It took two visits to the vendor to get them to admit the problem, and it had killed a Duetz V10 diesal by ripping the crank clean out of it prior to my involvement.

The result was a one times rpm vibration that would rise and fall at a frequency equal to the slip of the coupling. less slip, long period, more load, more slip, short period. 35mm/sec at the peak, and 3 minutes between peaks at no load, from memory.

It turned out the fluid coupling was distorted, and as it rotated it had one position that caused zero runout at the delay tube, and with the halfs 180 degrees apart, max runout at the delay tube.

Jon,

That is exactly what I would expect to see also. A beat frequency just like Simon describes.

The only possible explanation that I can think of is that maybe both coupling halves are locked, although I'm not really sure about that.

I'm going to contact them today and see what happened. I'll post again if I find out anything new.

Thanks,

Danny

Danny,

I wonder if the method you're using to measure phase really works in this case.....

If you had a piece of reflective tape on each shaft and you measured the phase between two phototachs, the phase would have to change unless the shafts were indeed locked together.

I wonder if the analyzer locks on 1x motor on the motor side, and is comparing phase to 1x motor on the other side of the coupling. There will be 1x motor vibration, carried through the structure, and phase at a given frequency is what makes sense, so perhaps this is what's happening.

Have you tried looking at it with a strobe? If the halves are locked, stopping one shaft will stop them both, but I expect you'll see one of them slowly turning with the other stationary.

Jon
Spintelligent Labs

Jon,

I see what you mean and that makes sense. The analyser takes the signal from one accel and compares it to the other. I measured the slip (only 23 rpm) and the difference was obvious with the strobe.

I really didn't spend too much time doing free vibration analysis on a piece of equipment that had about 3/8" runout visible with the strobe in the coupling sleeve. I didn't really start to think about what it really meant until I was 2 hours down the road.

The life of a data dog...

Danny,

That's certainly above my limit where I'll be happy to explain my thoughts...as I back not-so-slowly away from the machine!

Jon

We have a I.D. fan with a fluid coupling that
has been running up to 13 mils of vib. at the coupling output horz. pos.
Instead of following manufactuers recomended installation proceedure they tried lazer dials
between fan and motor.The motor required 120 thou. shims and when run damn near shook the building.The extra shims were removed and the level was brought down to 8 mils p-p .
The same fan was deemed to need cleaning at 3 mils in the past.
I have been after them to re-align properly but they will not put the fan down long enough to do that.The proper method is to line up fan to coupling then with one bolt finger tight in the spider attach a dial from the bolt to the
shaft and turn adjusting the motor til all zero.
I have argued with others that it didn't seem right to set the motor first then alighn the coupling to the fan.It seems to me that that would put undue pressure on the spigot fit at the motor.And that fit is critical.