Century Sound Impedance Drum and Flywheel Issue

[Ryan Gallagher]

Hoping this is an easy adjustment/fix. Would appreciate anyone with experience installing/adjusting this assembly confirm my logic:

Over time I have become aware that our No. 1 Century has a tendency to impart some optical sound drum/flywheel rotation even when no film is threaded. It's drive side is the most difficult to access and service being pressed against our Christie Digital system. But I was in there yesterday for grease/inspection/dampening fluid, and I was able to spot the culprit.

Our main drive belt just barely edge contacts the rear thrust collar (CL-0082) enough to rotate it and cause a fine dusting of rubber around that collar. To avoid the contact the collar would have to move towards the bearing (and there is room there to move it on the flat ground in the shaft for the set screw).

Since moving collar inward would clear the contact, I also tried pushing on the flywheel itself, and sure enough, there is movement in the whole assembly when doing so. It seems to settle back to a seemingly correct drum position as soon as you let go, perhaps the result of the loading spring?

Here is a not so illustrative video of that end play when pressing on the flywheel assembly from the drive side, can't see my hands, just watch for the drum to jump in and out. I didn't think to take a photo of the belt contact issue when the covers were off unfortunately. I'll try to snag a photo of the thrust collar/belt/bearing current situation today.

So my logic is I can just loosen the rear thrust collar set screw and move it towards the bearing until that end play is gone (while also moving it out of the path of the drive belt)? I don't know how the drum position is set relative to the film path, my hope would be there is no adjustment necessary to that aspect?

My worry would be the loading spring actually has a bunch of force behind it, and i'll need extra hands or tools to compress it more than it currently is?

I would just use the other projector with zero end play to confirm the correct drum position, but our two Centuries are such unique snowflakes (different brand optical readers for example), that I don't trust the comparison.

Sound Impedance Drum Flywheel Assembly_Parts.png

Sound Impedance Drum Flywheel Assembly.png

[Steve Guttag]

There should be loading on the spring. There is also nothing wrong with pulling it down and rebending the sprint a little bit. When setting the collar, you should be pressing in on the bearing enough that it moves in a little (engages the loading spring some). It should, absolutely return on its own.

Truth be told, as with most Century things, their machining sucks. The bearing receiver is a bit overly large and conical (particularly over time). A better method of loading is to remove the original spring, push the bearing all of the way in until it bottoms out. Then use a wave-washer/spring that engages the inner race. the collar would press in on that loading spring directly which, in turn, applies the loading to the bearing. This will keep that bearing from moving within the receiver and just the impedance drum itself will be able to move back/forth...which it won't as it will be under loading from the new spring.

Component Engineering came up with the newer loading method when they did basement readers because Century drums orbited enough to cause tracking issues. The improvement was pretty significant...which means it was affecting optical performance too but jitter on analog optical sound isn't as profound (flutter) as for digital, where it will mistrack.

[Ryan Gallagher]

There should be loading on the spring. There is also nothing wrong with pulling it down and rebending the sprint a little bit. When setting the collar, you should be pressing in on the bearing enough that it moves in a little (engages the loading spring some). It should, absolutely return on its own.

Truth be told, as with most Century things, their machining sucks. The bearing receiver is a bit overly large and conical (particularly over time). A better method of loading is to remove the original spring, push the bearing all of the way in until it bottoms out. Then use a wave-washer/spring that engages the inner race. the collar would press in on that loading spring directly which, in turn, applies the loading to the bearing. This will keep that bearing from moving within the receiver and just the impedance drum itself will be able to move back/forth...which it won't as it will be under loading from the new spring.

Component Engineering came up with the newer loading method when they did basement readers because Century drums orbited enough to cause tracking issues. The improvement was pretty significant...which means it was affecting optical performance too but jitter on analog optical sound isn't as profound (flutter) as for digital, where it will mistrack.

Yeah I can't determine the spring design from their diagram.

So I just need someone to hold the drum towards the casting, and then compress the bearing slightly (engaging the spring) using the thrust collar before tightening. (Presumably with the flywheel off for access)?

I would hesitate to access the spring to implement an upgraded loading method at this time, cause I feel like getting that rear bearing out might be challenging without removing the shaft, and I don't want to have to do optical alignment right before our season starts. My impression is the reader would have to come off to pull the shaft out of the operator side (or to access whatever holds the drum on the shaft). Not trying to go that deep just yet.

[Steve Guttag]

You don't need anyone to help you. Just pull out on the shaft while you set the loading.

[Ryan Gallagher]

You don't need anyone to help you. Just pull out on the shaft while you set the loading.

Copy. Body position will be the challenge. It's quite the booth yoga to get to that area. I may have an opportunity tonight to attempt by myself, if not I'll have a helper on Tuesday.

[Pete Naples]

I generally used to remove the spring (28), and set the pre-load by moving (27) in, enough to remove all on-axis movement in the shaft, but not tight enough as to drag the bearings. It's a bit of a fiddle to find the sweet spot.