Posted by Matt Wilson [172.70.131.74] on Thursday, December 01, 2022 at 12:21:23 :
In Reply to: Re: WC54 Steering posted by Clint Dixon [172.70.126.143] on Wednesday, November 30, 2022 at 19:38:02 :
Well, technically, the worm just has one continuous tooth, spiraling down its length. I'm definitely not a worm gear expert, but I think the gear "ratio" is achieved by the spiral angle (maybe I should call it the helix angle). The steeper the angle, the more motion is seen at the tires for a given amount of steering wheel motion.
It's just like with a course vs fine thread bolt. With each full turn of the bolt, a coarse thread will move it into or out of the mating hole (or nut) a father distance than would a single turn of a fine-thread bolt. In other words, you can thread or unthread a coarse-thread bolt significantly faster than a fine thread bolt. The idea is the same for the worm gear setup. Of course, the finer the thread (shallower helix angle), the slower the tires respond when you put in a steering input at the steering wheel, but you also have more leverage, which is important for our trucks, since they usually did not have power steering.
So there are two requirements that dictate what the helix angle of a steering box should be. One is the need for steering backdrive, in order to return the tires to their center (straight ahead) position after making a turn. This influences the helix angle to be somewhat toward the steep side. The other need is for the operator of the truck to be able to actually steer the truck without needing bionic arms. This influences the helix angle toward the shallow side. So you can see that these two needs are competing against each other in terms of the helix angle. In my estimation, the need for backdrive capability wins, and to make up for the increased operator effort at the steering wheel, the sector gear is made larger, so that the distance from where it contacts the worm gear is large enough to provide more leverage, and the number of teeth on the sector is increased (effectively increasing the gear ratio), thus reducing the steering effort. The steering wheel is also made larger to provide more leverage. As with many things in life, there is a balance, and there are probably more factors involved that I'm not thinking of. In fact, I feel like I'm not envisioning a complete picture.
Again, I'm no worm gear expert, so I could have some of this wrong, but I think this makes sense from where I sit.
One thing I should note is that, although I suspect the winch drive is a self-locking worm type, I would not rely on that alone to keep the winch from backdriving the winch input shaft. I believe the winch has a brake on it, for added safety.
Also, even though the steering gearbox may be a backdriving unit, I've noticed that the original Power Wagons don't seem to do a good job of returning to the straight-ahead position following a turn, but what I've read is that this is caused by the lack of caster angle. With the appropriate caster, I think the steering would return nicely on its own.