Skip to main content

Reply to "671 Repair Questions - 1946 turbine"

Gauging of Wheels

The picture below is a summary of the measurements I have taken using a caliper. What I learned from this exercise is that there is much as .063” variation when taking absolute measurements between measuring points A) distance from flange to flange, caliper contact points, and B) distance between inner edges of the nickel rim. I might add that B is really an estimation since the nickel tires mostly appear to be situated evenly with the intersection of the wheel tread and wheel flange.

671 wheel measurements

Surprisingly (to me), the most consistent measurements seem to be from the inside flange-to-flange measurements, having only a max variation 0.003” This appears to be any easy measurement to obtain when gauging wheels, and in my case, the most consistent.

Next I need to find suitable washers to add to eliminate the side-to-side play so prominent in my rear and front drivers – that situation can’t bode well for keeping the worm centered on the worm wheel! I also need to consider the fact that the left rear wheel flange is rubbing on the frame.

Theoretical Discussion about replacing one or both worm gears (decided not to do this!  

Now, about the seating of a new worm gear, should one actually decide to attempt it... To @David Johnston's point, what is the requirement for matching up the positions of the front and rear worm gears with respect to the front and rear wheel drive rod mounting holes? As @Ted S stated, a situation could arise where the rear driver is really using just the side rods to drive the front wheels, i.e. no direct drive through the front worm gear at times, resulting in what I would think would be some very jerky motion, or some gear grinding (growling?) at times. To solve this, I took the easy way out and counted the worm gear teeth, finding 23 in total. That seemed such a strange (odd) number and having older eyes, I counted gear teeth six times, 3 front, 3 rear, so I’m comfortable with the count. That would indicate there are 360/23 = 15.65 angular degrees between teeth.

Now if I could identify the absolute need to mount each worm gear in exactly one spot, or in this case one of 23 same exact spots, then my maximum variation would be half the angular distance between gear teeth, or + 7.825 degrees - anywhere within that + range should produce acceptable performance. Next I measured the angular variation allowed by the “slop” in drive rods, and their associated mounting hardware. I used the evenly spaced wheel spokes to accomplish this – I counted 16 spokes, or 22.5 degrees between spokes. This is where measuring gets “iffy.” I connected the front and rear drivers on one side only using the hardware that came with the train – no wear marks appear on the screw shoulders that attached the drive rod, no obvious wear on the inside of the drive rod mounting holes, but that can be hard to see. By holding the front driver in a fixed position, I moved the rear driver CW and CCW, and found the variation to be about 2/3 of the distance between adjacent wheel spokes, or 0.667 x 22.5 degrees = 15.008 degrees. That’s roughly double the variation that could be realized by mounting a worm gear in a worst case scenario. My conclusion is that you can’t screw it up!! Is my nerd showing yet?

That said, I am comfortable with all my measurements, but less so with my conclusion. This is where I could use some confirmation or explanation of what I might have thunk wrongly!!

While I'm waiting to hear back some thoughtful response(s), I think I'm ready to quit talking and take some next steps.



Images (1)
  • 671 wheel measurements
OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653