Film-Tech Forum ARCHIVE: Century SA-W Speed Problem

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Posted by Frank Angel (Member # 248) on 06-12-2009, 12:30 PM:

One of the Century SAW is giving speed problems -- a rhythmical variation that is constant -- speeding up and down about 1 cycle per second. The motor is to be getting hotter than normal, at least hotter than the other machine running for the same amount of time. There is no resistance when turning the motor over by hand -- it turns free -- same as the second machine. This happened about four years ago. We called a tech in who looked at everything, couldn't find a thing, and then the problem disappeared on its own. He charged us as if he had fixed it; why not...I was happy. Maybe I should call him back and see if it goes away again. Last time I assumed something in the mechanism but nothing was wrong there. This time I suspect it's the motor. If I press on the flywheel and ADD resistance, the oscillating stops. I may swap motors with the other projector see if it follows the motor.

I can record the sound and upload it as a wave file if u think that will help.

Any suggestions where to look...other than replacing it with an XL? [thumbsup]

Posted by Robert Minichino (Member # 3495) on 06-12-2009, 01:24 PM:

It could be the centrifugal switch failing to stay open and leaving the start winding in the circuit. That would explain the temperature rise, and if the switch was only intermittently contacting that may explain the speed variation.

Edit: That would be the centrifugal switch in the motor, which can be found in many single-phase motors.

Posted by Dick Prather (Member # 1136) on 06-12-2009, 03:42 PM:

Usually if the starter winding switch does not open the starter windings will burn up and motor will not start unless you give it a good turn. Starter windings are not designed to run all the time. This is assuming you have a mechanical and not an electronic starter switch.

I would think some windings may be shorting in the running windings. Swapping the motor for testing would be your best and easiest way to find out were the problem is.
Dick

Posted by Louis Bornwasser (Member # 3063) on 06-13-2009, 08:39 AM:

I think Dick has it nailed! LOuis

Posted by Gordon McLeod (Member # 33) on 06-13-2009, 04:50 PM:

If the machine has a poly v drive belt it could be slipping intermitently causeing eratic loading on the motor

Posted by Frank Angel (Member # 248) on 06-19-2009, 04:49 AM:

Close, Gordon -- after playing around with this thing for hours today, listening intently to the motor with my ear up against it, running it with film, without film....film threaded thru the intermittent, not thru it, film reel on the takeup, nothing on the takeup, and then the revelation: let's see what happens with the yellow rollthane belt tension removed -- I slipped it off the takeup arm pulley. I didn't take the belt completely off, I just slipped it off the pulley and let it ride on the shaft, and it was actually still turning the takeup reel, With that, the motor goes from oscillating to perfectly running smoothly. Rollathan back on the pulley and the motor does its rhythmical speed oscillation, which, btw, is so bad that the sound drum won't even smooth it out....you can see the film in the sound head dancing with the rhythm of the motor sound.

Conclusion: the belt tightness (not the resistance of the takeup reel) was what was somehow loading the soundhead drive train and producing the drag on the motor.

As strange at is seemed to me, it WAS the yellow rollothan belt that's causing the motor to load, although I must say, feeling the taughtness of the belt, it seemed loose enough to the touch. Mind you, it's no tighter than it's ever been -- I mean, this same belt has been on this same machine for years. But there it is, cause and effect: with the projector running film with the rollthane belt engaged, the motor severely oscillates. With the belt taken off the takeup arm pulley, the motor immediately runs smoothly again. Stretching the belt in a kind of mad fit after wasting my time for hours -- I just pulled on the plastic belt with all my might trying to stretch it -- fixes the problem -- the motor's running normally and smooth as a baby's butt.

So WTF? This stuff SHRINKS? Changes with the humidity? -- we are outdoors and it has been raining nonstop since 1910 here in Brooklyn, but that seems unlikely, but there it is, cause and effect. Besides, I never argue with success.

Now I should have as much luck with getting rid of the IREM HUMMMMMMM.

Posted by Tim Reed (Member # 137) on 06-19-2009, 08:53 AM:

There's a bad bearing somewhere, possibly on the soundhead sprocket shaft, which drives that takeup pulley. Belt tension alone shouldn't have that affect, unless it's pulling down on a worn bearing.

Posted by Mark Gulbrandsen (Member # 72) on 06-19-2009, 03:40 PM:

I agree with Tim (holy cats) that there may be something else amuck in there. Having the take up belt on shouldn't cause that even with a full 26" reel. Check the take up tension on the lower arm and everything else in the sound head with a bearing on it. If your soundhead main shaft has bushings rather then ball bearings I'd suspect a partial seizure. The ball bearings also fail quite regularly... they are way undersize for the job. Disconnect the projector drive belt(if not direct drive)and take up belt and go from there.

Mark

Posted by Gordon McLeod (Member # 33) on 06-19-2009, 04:37 PM:

I have had issues with the guidance roller on the takeup arm getting wobbly and causing erratic takeup and wow and flutter

Posted by Frank Angel (Member # 248) on 06-19-2009, 07:10 PM:

Yah, it's going to have to be completely torn apart and checked because if it a bad bearing, it's just going to get worse and fail completely at some point (I expect that if I leave it like it is, sooner rather than later I am going to hear that grinding, crunching sound coming from the soundhead as it grinds to a halt. Funny thing is, at the moment it seems like it's only the downward pressure of a tight belt that causes the motor to drag. The thing will turn a full 6000ft reel seemingly effortlessly, no problem, as long as the belt has been stretched -- i.e., not putting excessive downward pressure on the drive pulley. Weird.

I'll call the tech who couldn't figure it out the last time and give him a second chance. [evil]