Replies

1. 
   mrbird90 | Jan 23, 2009 04:49am | #1

   A lot of machines when operating are running above the first critical speed of the machine and when you shut them down they have to coast through the first critical and this is usually a  shudder or other such noise. This is particularly noticeable with a bench grinder.  Above the first critical speed the rotating parts are turning around their mass center, prior to going through the critical speed they are turning around their geometric center and the two centers may only be a few thousands apart. Probably nothing to be concerned about. The greater the unbalance of the rotating parts the more noise and shudder when going through the critical speeds. ( A critical speed being the natural vibration of the machine)

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   Edited 1/22/2009 8:51 pm ET by mrbird90

   
   

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   Edited 1/23/2009 9:04 am ET by mrbird90

1. 
   MikeHennessy | Jan 23, 2009 02:53pm | #3

   Interesting post. Never heard of "first critical speed" (or second or third, which I assume exist, for that matter). Also interesting concept of rotating around geometric vs. mass centers. Thanks for that info!

   Mike HennessyPittsburgh, PA

1. 
   pezzi | Jan 23, 2009 05:04pm | #4

   When the nuclear reactor in Tchernobyl blew up, this critical resonance when slowing down the turbines was allegedly a major factor. Just a thought. Chances are slim that your saw would experience a nuclear core meltdown. Still, with these newfangled table saws, you don't really know.

2. 
   mrbird90 | Jan 23, 2009 05:10pm | #6

   Mike, you are right, there are many harmonics, but each is smaller than the previous. Almost always the first is the one you hear. My planner has a horrible shudder when coasting down and it has done it since day one, that was 15 years ago. When operating at speed is is as smoth a silk.

2. 
   ulstadts | Jan 23, 2009 05:08pm | #5

   I understand that there may be some vibration.  My jointer and band saw have a similar belt drive and they are much smoother.  The motor actually bounces up and down when powering down.  This doesn't happen with the belt off or the blade off.  the table is shimmed solid but the motor almost makes it bounce a little. 

   Sorry,  it does happen with the blade off.

    

   Edited 1/23/2009 9:09 am ET by ulstadts

1. 
   mrbird90 | Jan 23, 2009 05:17pm | #7

   The amount of shudder is a function of unbalance, putting the machine on vibration dampeners (special pads) helps but does not solve the problem. A perfectly balanced roating part would be  undetectible as there would be no force to create a vibration. Check for a loose pulley, missing set screw or anything that can cause an unbalance.

1. 
   ulstadts | Jan 23, 2009 05:50pm | #10

   I use to be a math and physics teacher and I understand and agree with what you are saying, I'm just trying to limit it a reasonable amount.  I'll take a closer look at the pulleys and then try a new belt.  As it is now I'm not comfortable leaving things, like my cross cut sled, on the table when powering down.

    

2. 
   WillGeorge | Jan 23, 2009 05:19am | #2

   The only problem I have is when it powers down, about a second or two before it stops it starts to shake.

   I do not have a Steel City but my old faithful Ridgid (Yes a Big Box saw) did that. I found a loose pulley setscrew on the motor shaft.

   I would call their technical support. Nothing to loose but some time on the phone.

   EDIT:

   I forgot to mention.... With a thin kerf blade I will get this on occasion. I have no idea why. It comes and goes. I would suspect heat related on the blade causing it to be out of balance? Just a guess on my part...

   
   

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   Edited 1/22/2009 9:23 pm by WillGeorge

3. 
   BruceS | Jan 23, 2009 05:20pm | #8

   As mentioned before,  critical speeds are a very real thing.  I'am a retired power plant engineer.  Our turbogenerators all had their own particular critical speeds and it was quite easy to feel the "beasts" pass through the critical speeds.  Always gave them an extra shot of steam at those points to get them through those speeds quickly.

   Work Safe,  Count to 10 when your done for the day !!

   Bruce S. 

    

4. 
   Frozen | Jan 23, 2009 05:40pm | #9

   It's possible that replacing the belt with a link belt would solve the problem.

   A link belt would make the saw run smoother when operating as well.

   “Expectation strolls through the spacious fields of Time towards Opportunity.”

   Umberto Eco, The Island of the Day Before

5. 
   knuts | Jan 23, 2009 06:56pm | #11

   I've seen this on machines with loose (slightly even) motor pulleys and also with pulleys not co-planner. However, with the mis-aligned pulleys there was also minor vibration at speed.