Ever wonder how the manual transmission in your car works? So did I. I always thought that gears were moved around on shafts to mesh different sets depending on the gear selected. Some early transmissions did work that way, but it wasn't long before someone came up with a better idea.
The problem with moving and meshing gears is that unless both gears are moving at the same speed already, you're going to have an awful clashing and grinding to get them to mesh. There had to be a better way.
There is. This is a simplified model of a 2-speed manual constant-mesh transmission. Constant-mesh means just what it says: all the gears are always meshed with each other. So how does this magic happen? the gears on the driving shaft (the shaft coming from the engine in a car; the input shaft) are fixed to the shaft and rotate with it at all times. The gears on the driven shaft (the shaft that goes to the wheels, the output shaft), on the other hand, are free to spin on their shaft. However, there is a collar that is fixed to the output shaft through splines and rotates with it, but can also slide back and forth on the shaft. This collar has "dog teeth" (yes, they're really called that!) that can engage matching holes or slots in one or the other of the gears. If the teeth are not engaged in either gear, the transmission is in neutral. All the gears turn , but no rotation is transmitted to the output shaft. Once a dog is engaged, the engaged gear is locked to the output shaft, and rotation is transmitted, with the speed depending on which gear is locked. A 4-or 5- (or more) speed transmission simply has more gear sets and more collars and dogs.
Aside: A modern manual transmission has additional collars called "synchros" that spin up the freewheeling gears to match speeds before the dogs engage. This is what prevents the clashing and grinding that used to be such a problem. I will not be duplicating these in this model.
I wanted to build a wood model of this function, but couldn't figure out how to make a splined shaft, until it dawned on me that any shape other than round would work. I first considered using a square piece of wood, but it still would have taken a lot of work to make the ends round to spin in bearings. Then it hit me: why not just glue two short dowels on either side of a longer dowel shaft? It worked great. Now the design could commence in earnest.
Note: Yes, I know my small gear wobbles. Something was slightly out of adjustment.
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Step 1: Materials and Tools
This entire project can be made using one sheet of 1/4" plywood (or MDF, or acrylic) about 15 by 16 inches. I cut it on a laser although it can also be made with a scroll saw and some patience. A laser is certainly preferable for precision and repeatability.
The only other thing needed will be a couple of feet of 1/4" dowel (and some means to cut it), some wood glue, and medium and/or fine sandpaper. Optional: Eight 4-40 X 5/8" machine screws and nuts to assemble the case. You can just glue it together, but if some fine-tuning is necessary, you want to be able to disassemble it.
Patterns are attached in .svg and .pdf format. The svg files can be opened in Inkscape, my favorite (free!) vector design program.
Step 2: Assemble the Shifting Collar and Dogs
The output shaft is going to require the most time and precision. Begin by cutting four pieces of 1/4" dowel 1.25" long. Sand the ends smooth and round them slightly. Get the three parts with the oval holes in them. Stack and glue them with the smaller diameter piece in the center, all the oval holes lined up, and all four dowels centered in the stack - in other words, you want 1/4" of the dowels protruding on each side. These are the shifting dogs. Use a minimum amount of glue and try to keep glue out of the oval hole. Insert and glue the dowels while the glue on the stack is wet so you get everything lined up properly. Clamp the stack together and let dry.
Step 3: Assemble Output Shaft Gears
Get the two gears that have four extra holes in them, and the two small round pieces with five round holes. These are the holes for the dogs, and are larger than the dowels to make engagement easier. Stack and glue one of the round pieces on the large gear, aligning the center holes and the dog holes. A scrap of the 1/4" dowel will align the center hole, and a 19/64" drill bit can be used to align the dog holes. Don't run out and buy that drill bit if you don't already have one; anything of the approximate diameter will work, or just line it up by eye. This is one place where a little imprecision is acceptable.
Do the same thing with the small 4-hole gear, clamp and let dry.
Step 4: Assemble Output Shaft
Cut two pieces of your 1/4" dowel 4 1/2" long, and three pieces 1 1/2" long. Sand the ends smooth and slightly round them. Save one of the 4 1/2" pieces and one of the 1 1/2' pieces for the next steps.
Mark or eyeball the center of one 4 1/2" piece (this is your output shaft) and glue one of the 1 1/2" pieces to the side of the output shaft, centered on it's length. It might help fit if you sand one side of the short piece slightly flat and glue that side to the shaft. Let dry lying flat on a table so the pieces line up. Now glue the other short piece on the other side of the shaft. Let dry thoroughly, then sand the shaft smooth.
Test fit the shaft in the collar you assembled previously. The tripled shaft should slide smoothly in the oval holes of the collar. Adjust fit by sanding if necessary, then slide the collar to the center of the shaft. Slide the large gear onto the shaft, small disc first. Do the same with the small gear. The gears have holes that should allow them to turn freely on the shaft. If they are tight, sand the shaft slightly until they rotate freely.
Do not glue any of these parts to the shaft!
Okay, I'm done shouting at you. Now, when you slide the center collar, it can engage either the large or small gear and lock it to the shaft so it turns with the shaft instead of freewheeling.
Slide (don't glue) a spacer onto each end of the shaft.
Set this assembly aside, and build the input shaft.
Step 5: Assemble Input Shaft
The input shaft is comparatively easy. Glue one of the small spacers to one side of each remaining gear, aligning the holes with a piece of dowel or a 1/4" drill bit. Don't accidentally glue your alignment dowel into the hole! After these two assemblies are dry, test fit them on the other shaft you saved. They should be a snug fit, because these gears will rotate with the shaft at all times. Slide the gears onto the shaft (spacer toward the center) so that the outsides of the gears are about 1" from the ends of the shaft. Slip a spacer onto each end of the shaft. Do not glue anything at this time.
Step 6: Test Assemble and Adjust
Temporarily assemble the two short sides and one long side of the case. Install the output shaft, large gear first, into one of the holes, and the input shaft into the other hole, meshing the gears. Set this assembly, open side up, on a couple of jars or spacers so the shaft ends can protrude. Test-fit the second long side on the assembly so the shafts are through those holes too, adjust the shafts so about 1/2" of shaft is coming out of each side. Adjust the positions of the input shaft gears so that both sets of gears mesh evenly. When you're happy with everything, you can glue the input shaft gears into position on their shaft. Don't glue the outer spacers to the shafts. Remember, do not glue anything to the output shaft. Make sure both shafts can rotate freely in their case holes.
Now, you can finish assembling the case with either screws or glue.
Step 7: Final Assembly
On one side of the case, glue a spacer on each shaft end. On the other side, glue the disc with the cross pattern on the end of the output shaft. Take your last 1 1/2" dowel and glue it into the 2-hole rectangular piece. This will form a crank. Slip a spacer onto the input shaft, then glue the crank onto the end of the input shaft. Check for proper operation before the glue dries.
If the operation isn't smooth enough, you can lubricate the shafts and gears with a little talcum powder. You could also use graphite, but that will make a black mess! Oil will probably not work, since it could make the wood swell.
Now, when you slide the shifting collar on the output shaft, the dogs (dowels) can engage either the large gear (high gear) or the small gear (low gear), or if they engage neither, the gearbox is in neutral. In low gear, the output is half the speed of the input, and in high gear, the output speed is double the input speed.
Congratulations! you are now well on your way to a promising, exciting career as an auto mechanic!
Other ideas: Motorize it, or make a larger version with more speeds and a reverse gear.
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