Should You Balance Transmission Drums

[Dan McEachern]

We just finished a transmission balance job for a customer and I thought I would share what we found when balancing the low and reverse drums. I have to say that while these drums were out of balance more than most, these were original Ford drums in pretty nice condition. Most times they are not this bad, but what the pictures are showing is a low drum that was over 40 grams heavy on one side measured at the outside diameter and the low drum was over 20 grams heavy on one side. The pictures show the before and after out of balance. Just thought I would share how much some drums need correction. The brake drum/driven plate assembly was out of balance about 35 grams when we started but I don't have any pictures of that.
So if you are building a transmission for your car, keep this in mind.

Reverse drum before- this is Balancer reading before correction:

Here is actual balancer screen after correction:

And Low drum before:

Low drum after- notice how much material was removed from web:

[AndyClary]

What method do you use to remove or add weight to balance?

Andy

[Allan]

I use a carbide burr in my die grinder. Sometimes the casting of the drum is a little off centre when machined originally. if this contributes to the out of balance, I chuck the drum in my lathe and re-machine the inside diameter somewhat to help the situation.

Hope this helps.
Allan from down under.

[Wayne Sheldon]

A good reminder for all, and especially with a bunch of newcomers joining in. Thank you Dan McE
One of the model Ts I had years ago, had a tired old throw together engine in it. When I was driving it, sometimes the vibration was considerably worse than I liked it to be. Sometimes it was fine It didn't take very long to figure out it had two or more drums that were out of balance. Every time one presses the clutch pedal, the drums all rotate in relation to each other. If the random stopping placed the heavy sides together? The vibration was bad. If that random stopping point happened to balance out? It could be near perfect. I got in the habit of feeling the car as I drove. If it was smooth and happy. I was happy. If it shook much at all, I would press the clutch pedal for a moment to see if I could randomly get a better balance. Sometimes I would have to do that several times until I got a balance I liked.
Eventually, I replaced that engine and transmission. And ever since then, I have statically balanced every drum I have put into a motor. What you do to balance the drums is much better, and I do recommend doing so to others. But even statically balancing them helps a LOT! I have been amazed by how bad some drums have been! I had one that was so bad balance-wise, that I put it aside through a few restorations. However, the drum was in excellent condition otherwise. So as good drums have gotten more difficult to find, I put it into my current restoration. Took a bit of effort to bring it into a decent balance. So hopefully some day soon I will get that engine running and see how well it does.

As to how to effect a balance? That is a tough one. Original drums are borderline fragile. I do not like grinding or drilling on them to remove metal. Too likely to create a future fracture point. Sometimes I can remove a small amount very carefully to bring one into a good balance. More often I end up adding a bit of material. I do not like welding or brazing onto the drums. Also a bit likely to create a future fracture point due to heating/cooling cycles or shrinkage issues. I sometimes lace a piece of steel wire around one of the spokes in the webbing. Then being very careful with heat control, braze the wire in such a way that it shrinks when it cools to a very tight fit (so that it can't shake around and wear or break the wire!). Sometimes one needs to wrap wire around two spokes, more on one, less on the other to offset the balance to match against the original issue. A little bit tricky, but seems to work well. So far.

The situation Allan B mentions is an exception. If a drum is off in that way, cutting with a lathe may well be the best way to handle it.

[John kuehn]

Good information.
Another instance that indicates Fords tolerances weren’t always really close as we build engines and transmissions these days. I don’t know for sure but an unbalanced engine in a T especially when the transmission tail shaft is directly bolted on the crankshaft and the unbalanced drums would eventually cause something to give.
Being out of balance that much at higher rpms that engines run these days wouldn’t being tolerated. The engine would come apart!

[Joe Bell]

Dave Nolting sells new ones that are almost perfect in balance when mounted, I have ground and widdled at many to get the originals to balance. I found out with the driven plate and brake drum one out of six positions will ballance really well, so less grinding.

[Steve Jelf]

The idea of taking material off drums that were fragile to begin with has never appealed to me, so I've tried various methods of adding weight. There are good arguments against all of them, so I'm replacing my busted low drum with a new one from Dave Nolting. As Joe says, they come already balanced.

[Tlman]

Dan,
Just curious, was core shift or misalignment front to back the major cause of imbalance?
Tim

[Dan McEachern]

All good points, the bottom line is if you are building a driver engine, balance the drums, triple gears and the flywheel. That goes double for a flywheel with the magnets removed.

Dave N. drums are certainly nice, and because they are not machined from a casting, the are, by default better balanced than one machined from as casting.

The key to removing material for balance correction is to not create any stress risers- as many of us know, the cast iron drums are brittle. With this said, we never drill holes in the web area of a low or reverse drum, either to attach weight or remove material. Brake drums are a little more forgiving and there are numerous thick sections where material can be removed. Rotating the driven plate on a brake drum will many times help with balance, but you still need to check tail shaft runout, and Mr. Murphy says the best balance combination always gives you the worst runout. It's hundred year old iron . . . . but we make it the best we can.

The low drum unbalance in this case was in large part due to the casting not being perfectly concentric with the machined band surface. I could have machined the ID of the drum but they are so thin, I chose to take material out of the web, and only a few grams came off the ID surface to tweak the final numbers.

[J and M Machine]

Probably would of been a better idea to replace the low speed drum than to take that much out of it.

This post was mentioned a few years ago and some one mentioned our drums were off because they are cast as Ford's were.
Took about 15 minutes to balance one for arguments sake as the poster was saying ours were hard to balance.

This example the drum total unbalance was 14.7 grams far better than the factory one Dan used. I balanced this one to two hundreds of a gram in no time.

[Dan McEachern]

Nothing wrong with cast drums as John mentions, and I don't want to imply anything different- its just a different process to get to the same end point. When Herman Harder was making new drums, his were cast from ductile iron and you just had to balance them. Its unavoidable, as no casting will have perfect geometry and density. I have never understood why Ford made the drums out of cast iron when he was using malleable iron in other parts on the car- drive shaft balls, differential center sections etc were all malleable iron. And don't get me started on how thin some drums are. There had to be some level of drum failure back in the day- even with cotton lining.