How to determine if motor bearings are going bad?

[WxMan]

I have a Delta Rockwell Super 990 turret-style RAS, circa 1962.  I bought the saw six years ago.  I cleaned it up, and after having a new king bolt machined for it, I put the saw back into service a year ago.

The saw has 1HP induction motor which spins up quite nicely.  The arbor turns freely, and I don't detect any abnormal vibration or rubbing when spinning the arbor by hand.  The arbor spins silently.  There's no end play that I can detect by hand.

When I turn the switch off after making a cut, the blade will spin for a very long time.  It spins silently, and the only way you know it's spinning is to see it.  The motor is equipped with a manual brake for stopping the blade.  I normally use the brake to stop the spinning blade.

I've read in a couple places that motors that spin for a long time after shutdown are signaling emerging bearing issues.  My search around the net does not result in any meaningful discussion about "freewheeling" bearings.  The articles I've seen all describe various symptoms like noisy bearings, vibration, or rough spinning.

Do I need to be concerned about these bearings?  I really like this machine, and a sudden motor seizure from a failed bearing would be most unfortunate.

Thanks for any advice.

[oakey]

My radial saw experience
I bought a dewalt radial arm saw 6 or 7 years back
Motor had bad bearings it growled and rumbled and had shaft play
I installed new bearings now it sounds great and when it is turned off it coasts for ever
Probably twice as long as it coasted with bad bearings
As long as the saw sounds good and there is no play I would not worry about the bearings what so ever

[fredhargis]

That would be the way to tell...the blade keeps spinning. On one of my Dewalts, the blade would take well over a minute to spin down (I never timed it). After I replaced the motor bearings, the blade would spin down in less than 30 seconds (I did time that). I had picked that info up over at the Dewalt RAS forum. Changing the bearings on a Dewalt isn't a bad job, and it's pretty cheap.

[TDKPE]

Bearings that are dry, and bearings with shields instead of [contact] seals, will present lower friction.  New bearings, which are usually sealed and full of grease, tend to have higher rotating resistance.  

Having said that, if you can't feel any feedback through the shaft when spinning it by hand with no blade, and you can't hear any sound from them, and you can't feel any play, I'd judge them to be still serviceable.  If it bothers you, take your time and do the research on replacement bearings (high quality, name-brand, usually Japanese) and procedures.  


But I would still use the saw and not worry about it until it's time to actually do something about it.

Oh, and there's a lot of inertia in that saw blade, so absent an automatic brake, it'll spin for a long time.  When I was a kid, I used to get a kick out of the big saws in lumber yards that would run forever coasting down, to the point where the guy would come back and make another cut or two without even restarting the saw.  

[Stwood_]

Your bearings wouldn't concern me. I have a grinder (bought new 1980?) that takes 10 minutes to stop after I've shut if off.

[Lumber Yard®]

Like others have said, if it takes forever to spin down the bearing grease has most likely solidified/ dried out and is no longer doing much of anything. With that being the case, it is probably a matter of time before the bearings go out. It might take years in a hobby shop but I think it is worth replacing the bearings prior to doing any other motor damage when the bearing(s) do decide to go. On those older saws it is easy to crack the motor open and replace the bearings. I have done it on probably a dozen saws and it usually takes about 30 minutes and the bearing might cost you $20.

[daddo]

Bad bearings make noise or have end play/runout.

If they spin more freely, they have less grease for friction or you have a weight.

Might be the bearings need a little grease, but opening it up and greasing them might not be a good idea at this point.

I wouldn't worry about it.

[WxMan]

Thanks, all.

I don't think I want to crack the case until I really need to.

I'll keep my good eye on it.

[Wild Turkey]

I have a "mechanic's stethoscope" that lets me listen to the noises from certain areas via a metal probe so I can easily listen to the suspect area.  

really helps me locate a noisy bearing, but wouldn't help if quiet is the problem

[lift mechanic]

The main concern with waiting until you hear a "bearing " noise is damage to the armature shaft or the outer bearing housing. It will be quite difficult to repair once the damage is done.
As others have said the original grease is most likely dried up and not doing it's job. Bearing replacement is quite easy.