Posted by Clint Dixon [172.68.38.61] on Friday, December 13, 2019 at 21:55:01 :
Didn't want to hijack Tim's thread any further.
Was wondering if you have seen any build cards, or other line sheets etc. that would have specified the speedometer drive gear and pinion for special order trucks that used tire and gearing combinations other than the standard 9.00x16 - 5.83:1 or 7.50x16 -4.89:1?
Attached is a scan from my 1951 Dodge Power-Wagon Parts List, #D-13864.
The chart shows that the standard equipment 9.00x16 tires and matching differential ratio of 5.83:1 required a speedometer drive set with a gear having 4 teeth and a pinion having 14 teeth. A lot of these trucks were sold outfitted with these parts so I have to assume the engineers came up with tooth profiles, pitch diameters, and gear mating distances that provided proper backlash between the gears. A lot of them have survived so no surprises there.
The chart also shows that the optional combination of 7.50x16 tires and matching differential ratio of 4.89:1 required a speedometer drive set with a gear having 5 teeth and a pinion having 16 teeth, resulting in a completely different set with unique part numbers. Again I would assume the engineers did their homework and worked out the tooth profiles, pitch diameters, and gear mating distances to provide optimum backlash unique to this set as well.
Then there are two oddball combinations shown - probably special order.
One shows 7.50x16 tires used with a 5.83:1 differential ratio. I can't imagine there were a great number of trucks sold with this very low speed combination. This special gear set is shown as using the standard 4 tooth gear borrowed from the 9.00x16/5.83:1 set and a new pinion has been added that was not used in either of the other two sets, one with 15 teeth. I do not know how well the 15 tooth pinion mated to a gear that was designed for a 14 tooth pinion, but I would expect there to be some amount of binding or maybe some slop.
The last one shows the possibility of using 9.00x16 tires in combination with a 4.89:1 differential ratio. This is shown as requiring a completely miss-matched set with the the 14 tooth pinion borrowed from the common 9.00x16 set and the 5 tooth gear borrowed from the less than common 7.50x16 set. This is the one that I am having trouble understanding how they made two miss-matched gears work together without quickly wearing off teeth.
Not to doubt Vaughn. I believe it was done and there may have been no problems, but how? Maybe it is just the inspector in me and I am getting too wrapped up in tooth profiles, root diameters, addendum circles, etc. To me it kind of sounds like if one were to take a 5.83:1 ring gear and trying to mate it to a 4.89:1 pinion gear. Something would grenade quickly.
Then, I see further down in the parts list two different gear spacers and two different pinion spacers under the Speedometer Gear and Pinion section. There is no further explanation for those particular parts. Were they shimming the relationship between the gears to try and dial in backlash?
So the list shows 3 different pinions, 2 different gears, and 4 different spacers. Not much chance of getting something wrong there.
;^)
I will stay content knowing my speedometer is exactly 20% off.
Junior
Thoughts?
Junior
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