Crab Drive

For the past two weeks, I’ve been thinking up a crab drive for next year. Its very simple, yet does the job. I hope to have the frame of one of the modules at Nationals, so if you want to stop by, I’ll be glad to talk to anyone whos interested. O yeh, its going to made out of lexan so you can see the insides! So if you see us in Atlanta come on over and I’ll be glad to talk with you. I do have a 2d autocad file of the plates with the bearing pockets and holes ect. if anyone wants it, pm me but I wont have acess to it until next monday. Granted the robot isnt broken, and I’m not at a match driving

sweet i will definatly stop.

Lexan? That’s got to be expensive.

Well we built basically our whole robot out of lexan(polycarbonate), so we have a bunch of scraps. Now, since its basically just so everyone can get the general idea, not function, I’m making out of 1/4" lexan. The hole module is pretty much 3 36 pieces of lexan, 2 33 pieces, 2 1*3 pieces, a few standoff’s, and some angle iron. I have a parts list for one that is strong, made of aluminum, inclueds driveshafts, gear, keys, bearings, and costs less than two andymark 2-speed shifters.

Parts (11.76:1 gear ratio)

Modules
8 Steel Plain Bore 14-1/2 Deg Spur Gear 20 Pitch, 48 Teeth, 2.4" Pitch Diameter, 3/8" Bore
4 Steel Plain Bore 14-1/2 Deg Spur Gear 20 Pitch, 15 Teeth, 0.75" Pitch Dia, 3/8" Bore
12 Steel Ball Bearing Plain Open for 1/2" Shaft Dia, 1-1/2" Od, 7/16" W
8 Steel Ball Bearing Plain Open for 3/8" Shaft Dia, 1-1/8" Od, 3/8" W
1 Fully Keyed Steel Drive Shaft 3/8" Shaft Od, 3/32" Keyway Width, 12" Length
1 Fully Keyed Steel Drive Shaft 1/2" Shaft Od, 1/8" Keyway Width, 24" Length
1 Plain Steel Oversized Key Stock 3/32" X 3/32", 12" Length
1 Plain Steel Oversized Key Stock 1/8" X 1/8", 12" Length
1 Alloy 7075 Aluminum Rectangular Bar 1/4" Thick, 4" Wide, 6’ Length
1 Alloy 6061 Aluminum Rectangular Bar 3/16" Thick X 3" Width X 6’ Length
1 Alloy 6061 Aluminum Rectangular Bar 1/4" Thick, 5" Width, 3’ Length
1 Alloy 6063 Aluminum Round Tube .750" Od, .620" Id, 8’ Length
20 Aluminum Female Threaded Hex Standoff 3/8" Hex, 1" Length, 10-32 Screw Size
10 Aluminum Female Threaded Hex Standoff 1/2" Hex, 3" Length, 1/4"-20 Screw Size
(angle iron was at our school)

Total Cost: $654.93

How complicated is the mechanism? Do just have the legs attached to pneumatic pistons that extend and retract or do you have a motor that rotates like the old style locomotives? I’m very interested on how this works. I will defiantly stop by and see what you’ve got so far. good luck and I hope to see you soon. :smiley:

I had a crazy idea last year to do a crab drive and use 2 AndyMark’s on their side; from there I would run chain to each “pod” and the pod would drive the wheels using bevel gears… Chainzilla’s counterpart anyone? :stuck_out_tongue:

Its actually quite simple. Well if you have a CNC but yeh. Its basically 3 milled plates, standoffs, two plates for holding the sides together and some 3" wheels.

in my initial swerve design, i used a system similar to what you described, with bevel gears and ‘remote’ gearboxes (dewalts, in my case). in the end, i decided against this and mounted the gear box inside the module because of ‘torque-steer’. this thread and this thread offer some pretty good insight into it. obviously, torque steer doesn’t preclude this style of design (since its been done successfully before), but it is an additional factor to consider.

Making some revisons, found away to make it cheaper and easier to make. Now there is only one piece to weld and the rest is held togetherby stand offs.

Parts (11.76:1 gear ratio)

Modules
1 Alloy 6061 Aluminum Rectangular Bar 1/4" Thick, 4" Width, 3’ Length
1 Alloy 6061 Aluminum Rectangular Bar 3/16" Thick X 3" Width X 6’ Length
8 Steel Ball Bearing Plain Open for 3/8" Shaft Dia, 1" Od, 5/16" Width
12 Steel Ball Bearing Plain Open for 1/2" Shaft Dia, 1-1/2" Od, 7/16" W
1 Fully Keyed Steel Drive Shaft 1/2" Shaft Od, 1/8" Keyway Width, 24" Length
1 Fully Keyed Steel Drive Shaft 3/8" Shaft Od, 3/32" Keyway Width, 12" Length
1 Plain Steel Oversized Key Stock 3/32" X 3/32", 12" Length
1 Plain Steel Oversized Key Stock 1/8" X 1/8", 12" Length
8 Steel Plain Bore 14-1/2 Deg Spur Gear 20 Pitch, 48 Teeth, 2.4" Pitch Diameter, 3/8" Bore
4 Steel Plain Bore 14-1/2 Deg Spur Gear 20 Pitch, 15 Teeth, 0.75" Pitch Dia, 3/8" Bore
32 Aluminum Female Threaded Hex Standoff 3/8" Hex, 2" Length, 10-32 Screw Size

Total Cost: $479.14

We did crab drive last year. It worked well after we fine tuned the steering algorithm. The setup we used four cim motors that were vertically mounted on drive modules, which had the custom gearing in them. The robot moved about 9.6 FT/sec. We turned these modules by having two steering modules, one in front and one in back. The modules were powered by window motors, so the front and back drive modules were independently linked, not like traditional crab drive. It was controlled by close loop control using a derivative, which slowed the turning the wheels away from center and speeds up when returning to center, so you maintain accuracy. There where two pots on the steering modules which would turn the wheels in opposite directions so it could turn. If you have any questions about team 176’s crab drive just ask?