Hi all,

I've got a small motor/gearbox unit(5.5kW)with a single reduction, driving a feed screw via belts. A new unit was fitted approx 3 months ago. Since then there has been a noticeable whirring noise coming from the box. The peak at 1x gearmesh frequency has sidebands of 2x motor (input)rpm and the demodulated spectrum also shows harmonics of 2x motor rpm. The spike energy level is very erratic and high and when I freeze the back of the motor with a strobe, I can see the fan blades "rocking" back and forth.

I'm thinking that I could have a loose/misaligned gear on the input. Has anyone else got any thoughts? Are there any electrical problems that could cause the motor to behave in this manner?

Thanks in advance.

Al,

What type of drive do you have? In the states a "screw conveyor drive" is usually a motor mounted on an adjustable bracket that is attached to the gear reducer. The motor is connected by a v-belt drive to the reducer which is shaft mounted on the screw conveyor drive shaft and bolted to the conveyor endplate using a heavy cast iron flange that also serves as a packing seal. The European vesrions that I have seen are made by Sala and sold by David Brown. They are ball bearing throughout the gear reducer (as opposed to tapered roller in the US) and require a thrust bearing mounted to the conveyor endplate.

I have seen this type of drive be badly overtightened causing motor shaft bending, distortion of the mount, premature motor bearing wear, etc. You may have a bent motor shaft that causes continuous loading and reloading of the high speed gears.

Good luck,

How does the motor spectrum look like? Please post data.

Danny,

The motor is attached directly to the reducer and comes as a single unit, with the pinion mounted on the motor shaft. Effectively the motor shaft is the input shaft of the gearbox. This then drives the output shaft via a gearset. The output shaft drives the feed screw via V belts. It was made by Danfoss Bauer and is now an obsolete drive unit.

David,

All the motor spectra look normal, including high resolution data. I've attched spectra of the gearbox I/P.

Mtr-gbx.doc (86 Kb, 44 downloads) Mtr-gbx

Al,

A v-belt drive on the output? That is not very common unless the speed is fairly high like 300 rpm ar higher.

Refering to the spectrum you posted, are you sure of the geartooth count? It appears that the tall peak that is at about 14000 cpm has sidebands around it spaced at what I would suppose to be output shaft speed. Are you sure that this isn't gearmesh frequency?

This type of drive uses what is called a shell pinion which can have as few as 8 teeth. The distance from the corner of the keyway to the root of the gear teeth can be very small and makes for a very weak point. You could have a pinion that is bored offcenter of still a bent motor shaft.

Does anything show up in the acceleration twf?

Good Luck,

Danny,

The screw turns at 240 RPM. It's the feed screw on the disperger, hence it's a relatively small piece of equipment, as opposed to the larger heat screw or break screw, which turn much slower.

I had the same thoughts as you about the largest peak being the GMF. The large peak actually occurs at 10x I/P speed and I've confirmed with the manufacturer that the pinion has 12 teeth. So I'm pretty sure that I've plotted the GMF in the correct place. If the other peaks are sidebands then this places them at 2x I/P speed. There is also a small peak peak to the left of GMF (sideband?) which corresponds to the O/P speed of 265rpm.

I don't have any waveforms, I'll try and grab one when I'm over there next.

The engineer in charge is quite happy to change the drive unit, as it's a relatively quick job and we have a spare on stock. So there's no great threat to the plant, but I don't suppose I'll ever get to find out what's going on inside it.

Here is the acceleration waveform from the gearbox I/P. It shows events occurring twice per rev of the I/P shaft.

waveform.doc (76 Kb, 19 downloads) Waveform

Al,

That looks like cracked gearteeth of possible a cracked shaft. I suspect that if you look at a longer twf, you will see a beat frequency in time with the output shaft.

If the engineer is happy changing it out, it may be a good idea to go ahead with the change and disassemble the drive to find out exactly what the problem is. That way it can possibly be repaired and used as a spare and you can know what to look for next time.

Good Luck

Here's the latest update.

A new motor/gearbox unit was fitted today. I split the old motor from the gearbox and checked for obvious faults, excessive lift, loose gears, chipped teeth, unusual wear pattern on teeth, rough bearings, etc. I found nothing unusual on either the motor or gearbox, except that the motor was a bit stiffer to turn than I was expecting. I also confirmed the number of gear teeth was 12 on the pinion as per OEM's information. So the large peak at 10x rpm definitely wasn't GMF.

Feeling slightly despondent (as I expected to find something) I strolled over to the new unit now in service only to hear it making the same whirring noise that the previous unit was making. The belts were removed and the unit was run unloaded and it still made the same noise. It doesn't sound good.

The electrical people checked out the supply back at the control cabinet and are happy that the phases are balanced.

I've taken a load of data including high res spectra and TWF and basically these readings are a hell of a lot lower than the previous unit, but there is still impacting at 1x I/P in all the TWF readings and there are running speed harmonics in the demod spectra. The peak at 10x I/P rpm has now virtually disappeared.

We will be running through the weekend with this unit and taking further readings next week. I'm wondering what the chances are that I've got two gearboxes with the same fault built in.

Al,

Have you considered temporarily removing the cooling fan?

It's probably pretty easy and could eliminate at least one more possibility.

I have experienced this problem twice once on a dryer section on a paper machine (motor and gearbox oscillating back and forth. And the other time it was on a 1st press gearbox and motor. Same type of problem motor and gearbox oscillating back and forth as you describe it in your post. Both times it was found that we had electrical problem on our drives, can't remember exactly what it was it was awhile back and i'm getting too old to remember. But nevertheless it was deffinitly electrical.