I admit I'm a TT guy who's never had an ordinary T and I've never had an opportunity to learn much about differentials, but I have a question. I've read numerous threads on this forum you guys have posted about all the stuff that goes into a regular T rearend and making it be right. It all sounds pretty complicated and more than a little intimidating. I've had my TT worm drive apart and it coudn't be any simpler. I would think that from a production point of view the worm unit would be much easier (read that cheaper) to produce due to the fact that you just bolt it together with no "adjustments". You pretty much can't screw it up. They are known to be "rugged" and virtually trouble free (if not abused).
So, the question: why didn't they just use a worm drive on all the cars, even if the one for a car might be made a little smaller and lighter? It just seams that it would have been easier all the way around.
I imagine there's a really good answer to this question, but it escapes me. I'd like to start a T project (hopefully this spring, probably a '17-'23 touring) and frankly, checking out and repairing a rear end is the only thing that worries me. Maybe a little better understanding will help.
As always, THANKS!!
Ford made all their parts within tolerance close enough that all new parts could be assembled into a new unit that was satisfactory. With today's parts, you pretty much have to verify fits on the installation of each item, each step of the way. Buy the Model T Ford Club of America book on Restoring the Rear Axle from the club or any dealer. Read it completely through, and follow the directions presented there. You are much more likely to have a satisfactory rebuit rear end if you follow the advise in this book than if you go into it blind. The info in the Model T Service Manual is also helpful.
Being "on the other side of the fence", I rebuilt several T car rear axles before I built my first TT rear axle. I have to admit the TT rear axle is much simpler, but there are a couple traps that are not warned about in any books I've seen. The worm roller bearings have an uneven number of rollers, and because of such, have a tendency to creep in one direction depending upon which way they are installed. The rear one appears to make no difference, but the front one has to be installed in such a way that it has a tendency to "creep" to the rear. Also, the TT rear axle has to be filled with lubricant even with the middle filler plug. Any less oil and it does not properly lubricate itself (found that in a service bulletin). Many T car owners don't fill TT rear ends up far enough for fear of leakage and the worm gear roller bearings don't get proper lubrication. Modern seals work great on the TT's, but you have to make your own. The available seals are modern seals captured between two discs that are spot welded and they tend to leak.
Henry, I am pretty good at guessing answers and I believe that the TT rear end weighs more and the worm shapes are more difficult to machine, so more metal and more machining make it cost more, that's it in a nut shell, and not a clam shell.
Regardless of what Henry did the simple answer is that worm drives are better for lower gear ratios and and ring/pinion gears are better for higher gear rations. Simple geometry, each has its own place and they are not interchangeable.
The worm drive is stronger for pulling heavy loads. However the worm also has more friction drag than the ring and pinion. For lighter loads such as carried by a car, the ring and pinion is better. It is also better for higher speeds.
This helps a lot! A little understanding goes a long way. Thanks for the responses!!
(Jim, I take it you mean the other Henry. Yuk, Yuk.
To add a little - to get a 3 to 1 ratio, not at all uncommon with a ring and pinion, a worm drive set up with say 30 teeth on the worm wheel would need ten "starts" or "threads" on the worm itself. Talk about friction city......whole lot of sliding going on there on the tooth flanks.
Peugeot cars had a worm drive diff, made from brass, upto the 1960's maybe even later, the oil for them, I don't remember the grading, was just short of liquid grease, never any trouble for what I can remember and Peugeot's were raced from day dot...Kerry
Is it just the ratio, or do worm drives resist feeding back in the other direction?
Along that line of thought, why did Henry stick with the planetary steering gear all those years? Why did it take from 1928 to 1937 to settle on one they liked?
It's both the ratio and all the friction.
Bevel, spur, and helical gear drives have no sliding friction between gear teeth.
Hypoid and especially worm gear drives do.
Some worm gear drives do resist feedback, but not at the low ratios found in a TT rear end. In industrial gearboxes ratios are commonly 30:1 on up. Those do not feed back. But at only 7.25:1 they do feedback. If they didn't, your rear wheels whould come to a screaching halt every time you let off the gas.