Harmonic Vibration At 1000rpm

[wa5cab]

Once you get the other motor installed and running, you can quickly determine whether the vibration was from the motor or something else on the machine. If the Round-head is like most of the era, with a countershaft of some sort and with back gears, you can probably disconnect just the direct drive without engaging back gear and run the countershaft alone.

 

[Str8jacket]

Once you get the other motor installed and running, you can quickly determine whether the vibration was from the motor or something else on the machine. If the Round-head is like most of the era, with a countershaft of some sort and with back gears, you can probably disconnect just the direct drive without engaging back gear and run the countershaft alone.

I have been testing the old motor in this way, no vibration? It is strange because without the vfd the speed of the motor never changed, and the vibration only ever reared its head when the gear box was in high gear, (spindle at 1000) chuch, no chuck, faceplate you name it I have tested it!

I should clarify the vibration was more of a unbalance / wobble rather than a rumble or grinding. it always sounded and felt smooth, just the whole machine would develop a wobble. I suppose that is what has sent me round in circles. Under any load there is a slight knocking noise from the motor when listened to with a screw driver up to my ear, disconnect the belts and it is quiet.

In retrospect I have had engines at work 45litre cummins that have had a build up of dirt in the harmonic balancer and a vibration will occur at different rpms, change the throttle 100rpms and it will disappear. Doesn't take much either.

 

[wa5cab]

Could be one of the bearings.

 

[Str8jacket]

In the motor? Or the head stock gear train?

 

[wa5cab]

In the motor. You said that you could hear something in it using a screwdriver as a stethoscope. That's just a guess, of course.

 

[Str8jacket]

Yep was using it as stethoscope. Noise was there before and after i replaced the motor bearings. Only under load. Belts off it's perfect. Old and new bearings the same

 

[Str8jacket]

Well new motor fitted and done. The unbalance is still there at around 950rpm, 50 rpm either side ot is gone? Must be something in the gear box that gets into sync at that rpm and creates the harmonics. It happens when using the high speed selector and it will happen if i put the range at 700rpm and over speed the motor to get to 950. Or if i have it at 1000 range and slow the motor a little. It just so happens that it was at 50hz when i noticed it. Which is what my mains supply is.

I finished the Vfd install and made a frame for the box. Used all the machine start forward reverse levers to control still so its easy to use. Picked up a tacho that seems to work well. Overall happy with it and can easily work around the wobble.