Film-Tech Forum ARCHIVE: Century Main Drive Shatf Speed

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Posted by Cary Loeser (Member # 2923) on 03-31-2008, 11:37 PM:

Does any body know the correct rotational speed (i.e., RPM) of the main drive shaft for a Century Model C, H, or SA?

Posted by Monte L Fullmer (Member # 2797) on 04-01-2008, 12:15 AM:

Kinda easy for you to figure that one out: Intermittent assembly flywheels attached to the cam, spins at 1440 rpm. The drive gear on the vertical shaft that drives the assembly is a larger gear than the gear on the assembly. Thus, do the reduction math: Put a mark at the 12:00 position on the flywheel and at the same position of the main drive shaft sprocket and turn the main shaft where the mark on the assembly flywheel comes around in one revolution. Then, see how much rotation percentage it takes that main drive gear to turn that flywheel one rotation and times the percentage into 1440 to get your shaft speed RPM.. (for example: if the shaft rotates .75 to the one rotation of the flywheel, then the main shaft RPM would be 1080 rpm)

We used to do this when figuring out rear end final gear ratios on cars-by counting how many turns of the crankshaft to turn the back wheels one revolution by jacking up one of the rear wheels, take the sparkplugs out of the engine so the engine would crank a lot easier with the wrench on the crank pulley, put the transmission in the final gear (manual) and do the count.

good luck-Monte

Posted by Sam D. Chavez (Member # 1841) on 04-01-2008, 11:12 AM:

If you only jack up one wheel you would need to divide by half as one wheel spins twice as fast if the other is not turning, right?

Unless you have limited slip, in which case the car will try to take off in the middle of the experiment.

Posted by Cary Loeser (Member # 2923) on 04-01-2008, 01:42 PM:

The reason that I posted the original question was that I tried measuring the ratio and came up with some strange numbers (22 revolutions of the main drive shaft per 68 revolutions of the of the intermittent flywheel - giving a rather odd ratio of .3235 - giving a main drive shaft speed of 465.88 RPM, which I don't think is correct). That is why I was wondering if anyone knew what the rotational speed of the main drive shaft is really suppose to be.

Posted by Monte L Fullmer (Member # 2797) on 04-02-2008, 12:37 AM:

or, count the teeth on the INT assembly drive gear and its drive gear on the vertical shaft and do your ratio thingey...

Posted by Steve Guttag (Member # 268) on 04-02-2008, 06:47 AM:

Or just take a strobe and stop the gear. Normally when I need such trivia....I just ask Hugh...but that is an inside joke sort of thing.

Posted by Jim Cassedy (Member # 4115) on 04-02-2008, 09:07 AM:

Several years ago I got myself an laser-optical tachometer.
It comes in handy for checking projector & platter speeds.
Here it is in use checking shutter rotational speed.
(oops! This one appears to be off a bit!)

You can see the laser dot & reflective tape used by the tach to read the speed.

Posted by Louis Bornwasser (Member # 3063) on 04-02-2008, 09:46 AM:

The main drive shaft of an R3 soundhead is 360 rpm. a 22 tooth gear turns a 17 tooth gear. RPM should be 465.88 on the projector main drive shaft.

This assumes standard drive. Louis