This is the sixth post in our ongoing series on Project Super C. We are chronicling the rehabilitation of our 1954 Farmall Super C tractor, which has taken up residence outside of the Farmers Business Network office. If you are new to this series, please start with the introduction to the project in our first post.
When we left off, we had rehabilitated the components of the gear shifter mechanism and we're getting ready to reassemble it so that we can find out if it still functions after all of the rust damage it sustained. We’ll get to that shortly, but first let’s spend some time looking at how the mechanism actually works. If you don’t have the patience for mechanical explanations and want to cut to the chase, skip ahead to “The Test” below.
You can see the shift pattern for the tractor in the picture above. There are four forward gears and one reverse gear. Let’s look at what actually happens as a gear is selected. For this example, we’ll use fourth gear which, as can be seen in the picture, is selected by moving the gear shift lever left and then backward.
Because the shift lever pivots on a shaft, moving the top of the lever to the left actually causes the bottom end of the lever to move in the opposite direction (right in this case). Moving the lever backward to engage fourth gear also causes the bottom end of the lever to move in the opposite direction, in this case forwards. So moving the top of the lever left and backward actually causes the bottom of the lever to move right and forwards.
Does that sound a little bit confusing? Here’s a video that shows it in action:
When the mechanism is assembled, the bottom end of the lever is meshed with the fork and rails apparatus that we saw in the prior post. These pictures show what that looks like:
Looking at the picture on the right, the leftmost rail is responsible for reverse, the rail in the middle is responsible for 1st and 2nd gears, and the rail on the right is responsible for 3rd and 4th gears. Moving the lever side to side selects which rail to move, and moving the lever forward and backward moves the rail back and forth to select and deselect gears. How does moving a rail actually change what gear is selected though?
Each rail has a fork connected to the bottom of it, and the fork slides gears back and forth on the upper transmission shaft, causing them to mesh with gears on the lower transmission shaft (or not to mesh, when in neutral). When the gears are meshed, a mechanical connection is made that allows power to flow through from the engine to the rear wheels. Let’s look at an animation that shows a side view of how moving from neutral to fourth gear actually works:
You may have noticed a spring and ball in the animation above. This is a “detent mechanism,” which serves to keep the rails from moving except when pushed by the shift lever. When shifting gears, the spring is compressed and the rail travels under the rolling ball until the ball lands in the new slot. Each rail has its own spring and ball, and there are slots in the rail for the ball to fit into (which can be seen in the pictures above).
You can see in picture below that the springs fit into tubes on the underside of the transmission case lid:
OK, enough technical stuff. We need to find out if our damaged but repaired shifter mechanism works, or if all of the hard work rehabilitating it up was in vain. We reassembled the mechanism, and reattached it to the transmission case lid:
Finally, we were ready to try out the shifter mechanism:
In the video you can see that we were able to engage every gear successfully, and despite the loss of metal the mechanism worked correctly and didn’t allow any unnatural acts (e.g. engaging multiple gears at once). Moving the shifter required more effort than we expected, but that will probably change once we oil the mechanism.
In the next post, we take a break from the transmission and work on the front wheels. Stay tuned for more updates on Project Super C.
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