Annoying harmonic(?-or something) at slow speed

reluctant43 · Post by **reluctant43** » Fri Sep 30, 2011 3:12 pm

I spent a lot of last winter refirbishing a 1970 Mark V, upgrading to a 520 and improving my bandsaw and other accessories. Even before this work though the headstock at slow speeds developed(s) a loud harmonic sounding howl after it has run a few seconds (10-15) that continues at an even pitch as you slowly increase the speed (in other words, the sound does not change in pitch or volume as the speed is increasing) until somewhere around saw/joint setting it disappears and you have just the classic Mark V headstock noise. If you return to slower speeds right away it will usually will reappear, but if you run the machine for awhile at the higher speed then return to slow ( in other words let it warm up fully) , it usually does not reappear. Thought maybe some of you engineers out there might know what is going on. I have tried to isolate the source, but it is not possible with the tools I have - just to say when you open the port it is pretty loud but there is not visible vibrations. Pretty sure it is not the quill bearings. Function does not appear to be affected and the tool performs normally as far as I can tell. The fact that the sound is steady and of the same pitch makes me think it is maybe something in the motor itself as it runs at a constant RPM? Ideas for tests to isolate this problem would be very helpful.

dusty · Post by **dusty** » Fri Sep 30, 2011 4:17 pm

reluctant43 wrote:I spent a lot of last winter refirbishing a 1970 Mark V, upgrading to a 520 and improving my bandsaw and other accessories. Even before this work though the headstock at slow speeds developed(s) a loud harmonic sounding howl after it has run a few seconds (10-15) that continues at an even pitch as you slowly increase the speed (in other words, the sound does not change in pitch or volume as the speed is increasing) until somewhere around saw/joint setting it disappears and you have just the classic Mark V headstock noise. If you return to slower speeds right away it will usually will reappear, but if you run the machine for awhile at the higher speed then return to slow ( in other words let it warm up fully) , it usually does not reappear. Thought maybe some of you engineers out there might know what is going on. I have tried to isolate the source, but it is not possible with the tools I have - just to say when you open the port it is pretty loud but there is not visible vibrations. Pretty sure it is not the quill bearings. Function does not appear to be affected and the tool performs normally as far as I can tell. The fact that the sound is steady and of the same pitch makes me think it is maybe something in the motor itself as it runs at a constant RPM? Ideas for tests to isolate this problem would be very helpful.

I am not going to be much help, because you have already shot down my first reaction.

My first thought was a headstock bearing but there are no headstock bearings that operate at a constant speed when the main shaft speed is changed.

I would still utilize the "screwdriver in the ear approach" (stethoscope) to listen to each of the headstock bearings as you change speed while hoping to have ear to the culprit when the noise suddenly stops.

Then I would remove the drive belt. Leave it set overnight (reach ambient temperature) and listen to the motor (with drive belt removed) to see if the motor makes that same noise.

Since I have no reputation it means nothing but I'd stack my reputation on a bearing (on the idler shaft).

You could also remove the quill and go your the same procedure. Obviously, if the noise does not disappear when the quill is removed - it is not the quill.

beeg · Post by **beeg** » Fri Sep 30, 2011 4:44 pm

When was the last time ya lubed it?

SDSSmith · Post by **SDSSmith** » Fri Sep 30, 2011 5:43 pm

Any possibility the motor fan sheave is rubbing against the housing? I would disconnect the motor drive belt and see how the motor runs independently.

nuhobby · Post by **nuhobby** » Fri Sep 30, 2011 5:53 pm

All the original headstocks have some degree of resonant behaviors. My 2007 unit (prior to the PowerPro upgrade) would have cyclical noises especially around the vicinity of speed "H". And, depending on your level of sensitivity, you can even set out a plain induction motor, connected to nothing, and pick out cyclic modes of vibration. Build it up into a headstock and anything remotely resembling a spring (or torsional spring) can have oscillation modes.

Beyond servicing actual looser-than-normal parts such as bearings, as suggested above you can also try lubing the sheaves, with 10W oil or slightly heavier. If you hear a quieting, this tells you that the oil-wedges are adding dampening to the sounds.

Some folks have used a flex-link belt in place of the usual motor belt and enjoyed a quieter unit.

Some people don't mind the noises at all. Myself, I actually enjoyed the PowePro upgrade with regard to noise-reduction more than any other aspect of the upgrade.

JPG · Post by **JPG** » Fri Sep 30, 2011 6:09 pm

Only the motor shaft runs at near constant speed when the speed control is adjusted.

I would concentrate any sleuthing there.

As Beeg said, Have ya lubed it lately? I would add cleaned it up lately. It is possible that the sheave on the motor shaft is not keeping sufficient tension on the belt at slower speeds when 'cold'. Have you observed the belt when making the noise to see if it is vibrating?

Also I would not use any oil above #10. The sheave does not need 'viscous damping' when responding.

The belt may have taken a 'set' that limbers up after 'warm up'.

billmayo · Post by **billmayo** » Fri Sep 30, 2011 6:57 pm

JPG40504 wrote:Only the motor shaft runs at near constant speed when the speed control is adjusted.

I would concentrate any sleuthing there.

As Beeg said, Have ya lubed it lately? I would add cleaned it up lately. It is possible that the sheave on the motor shaft is not keeping sufficient tension on the belt at slower speeds when 'cold'. Have you observed the belt when making the noise to see if it is vibrating?

Also I would not use any oil above #10. The sheave does not need 'viscous damping' when responding.

The belt may have taken a 'set' that limbers up after 'warm up'.

Excellant ideas to check. Replacing the motor belt is an excellant place to start. I find removing the motor belt from the motor sheaves seldom helps to isolate any motor noise problem. Mostly, I find this variable noise will occured when the Fan Sheave set screw is starting to loosen. This will allow the Fan Sheave spacer to imbed/wear into the Fan Sheave hub. This can allow rim of the Fan Sheave at different points to start rubbing on the motor plate. This rubbing can occur at different quill speeds as the belt goes up and down on the fan sheave vanes.

When the bearing behind the Fan Sheave starts to fail, it can cause a very similar noise. I find disassembling of the motor to check the bearings and the conditin of the Fan Sheave for all headstock repairs and rebuilding will help prevent future motor problems and noises.

reluctant43 · Post by **reluctant43** » Tue Oct 04, 2011 5:13 pm

OK - I checked bearings with the screwdriver stethoscope and while I couldn't identify a clearly bad bearing the excercise identified someplace in the motor as the definite source of the harmonic. The belt was good, .52 inches. Many asked whether I had lubed it. I was thinking of course I lubed it, and I had lubed it- but then I got thinking the unit had been sitting several months when I started to refirbish it and I just lubed it with the normal few drops of oil. Maybe it was really dry, so I hit the oil ports again, cycled the speed control and presto, no more harmonic. Sometimes it is the little things....

. Sorry to trouble everybody but thanks for your time...

beeg · Post by **beeg** » Tue Oct 04, 2011 6:21 pm

No TROUBLE there reluctant43. Just remember to lube it about every 10 hours of use.

dusty · Post by **dusty** » Tue Oct 04, 2011 6:23 pm

reluctant43 wrote:OK - I checked bearings with the screwdriver stethoscope and while I couldn't identify a clearly bad bearing the excercise identified someplace in the motor as the definite source of the harmonic. The belt was good, .52 inches. Many asked whether I had lubed it. I was thinking of course I lubed it, and I had lubed it- but then I got thinking the unit had been sitting several months when I started to refirbish it and I just lubed it with the normal few drops of oil. Maybe it was really dry, so I hit the oil ports again, cycled the speed control and presto, no more harmonic. Sometimes it is the little things.... . Sorry to trouble everybody but thanks for your time...

This has been no trouble at all. The members of this forum are here to be of help whenever possible.