The Outer Foot Drive Assembly

Previous Version 01/02/14
There are several phases to the Outer Foot Drive Assembly. We'll start with the structural components, then move to the mechanical parts. Then those parts get fitted together to make a complete Outer Foot Drive. 
Parts are described on the mc.outer.foot drawing available here as a .pdf file. Unlike most of my styrene designs, the Outer Foot Drive has both mechanical  & electrical components to it that require some tinkering to adapt them to our purposes. See the Mechanical & Electrical Parts page for a descriptions and supplier information.

Structural Assembly - putting the styrene frame together
Mounting Sprockets on the Wheels -  we attach sprockets to the wheel hubs
Cutting the Chain - We need 2 pieces of drive train and some other mechanical parts.
Assembling the Drive - Putting it all together
Electronics -  Making it Go!

Structural Assembly -  The first step is to separate all of the parts from the sheets they were machined from, remove the tabs and flash, if any.

this drawing is out of date, but basically correct. the parts in the upper right corner go with the outer foot shell.

you may need to drill out the holes for the axles and the bearing mounts

square off the corners of the mounting hole for the square nut

make sure the nut fits now, once assembled it's a lot harder to get to. It's OK if the hole is oversize, you can use silicone cement to hold the nut in place.

To mount the motor you need to remove the sprocket,

The nut, at least on my motor, is left hand threaded! Hold the sprocket while loosening the nut. Don't loose the lock washer!

check the fit, then remove the motor till the end of the assembly process.

verify the fit of the axles, bearings, and the shoulder bolt.

begin assembly with the inner and outer side plates
Left: insert the left and right top cross pieces and the xxx cross pieces. note the orientation of the side plates make sure the motor cutouts are oriented properly!
Above: insert the 4 spacer ribs

finally, insert the inner rib.

These are pieces of 1/2 in MDF that I'm using as spacers
Left: note the piece of .125 styrene being used as a temporary filler. There is a properly shaped cut part on the sheet of shell parts!
Clamp the two side plates, then turn over and drizzle Weldon #3 into the joints from the underside.

glue the two short spacers onto the ends of the drive assembly.
not shown. Turn the assembly right side up and drizzle Weldon #3 into the rib joints on the topside.
Left: Finally, glue the guide ribs onto the outside of the drive assembly.
This Completes the Outer Foot Drive Structural Assembly.

Mounting Sprockets on the Wheels
- There are two separate drilling operations to be done. We have to drill a hole pattern in both the sprockets and the wheel hubs. Then we have to tap the holes in the wheel hubs. Finally we have to enlarge the center hole in the sprocket.

The two styrene disks on the left are hub spacers, one for each wheel. The recess goes over the delrin bearing. The other two disks are drilling guides.

use the jackshaft (.250 shaft) as an alignment tool for the sprocket hole tool and a piece of axle shaft ( .500 shaft) for the wheel.

The holes in the drilling guides are .125. I'm using a .125 transfer punch to mark hole locations. Any punch with a .125 diameter head will work. Mark just a single location on  each sprocket & wheel.

I've marked the punch points with a marker to make them easier to see. Drill and tap the holes for a 4/40 screw.

The sprockets are steel. Remember to hold them with pliers, not your hands when drilling. If they 'catch' on the drill the spinning sprocket will cut your hand!

Anchor the drilling guides with the 4/40 screws and then mark the locations of the other 5 holes. Don't skip this step. If the drill guide slips even slightly your bolt holes in the wheel won't match the holes in the sprocket.

Drill all of the holes. Use a clearance drill for the sprocket and a tap drill for the wheel. Make sure you make the holes in the wheel deep enough for a 1/2 inch screw.

Left: tap all of the holes.
Above: check that the holes are tapped deep enough for the screws

you can test the assembly at this stage, or wait until you drill out the sprocket hole.

put tape on your step drill so you know at what level to stop enlarging the hole. That should be 1 step larger than .5 inch since you don't want the sprocket to rub on the shaft.

the hub spacer goes recess side down over the protruding delrin wheel bearing. mark one of the hole location on the wheel to make it easier to line things up.

Finished wheel/spacer/sprocket assembly.

Cutting the Chain - Cutting, also called 'breaking', the chain into 2 pieces of the length we need for the drive and other mechanical work that needs to be done before we can finish the drive assembly. I don't know how the chain you will buy will be delivered to you. Mine was packed with a thick black grease. Every time I touch the chain my fingers come away black, and that's after I've wiped off most of the grease. I'm going to suggest that you actually wash the grease off of your chain (kerosene, or a similar solvent should work) and then coat your chain with white lithium grease (or at the very least a good heavy duty oil - not WD40!) That way, if you are out and about with R2, and you have to touch the chain your hands won't get really dirty!

Chain has inner links and outer links. We need 2 pieces, one nominally 8 inches, the other 20 inches. Here is how I measure the chain.

The 8 inch piece.

the 20 inch piece.

"breaking" the chain consists of removing the outer link at the correct point.  Once you've measured, put a piece of tape over the chain as a marker. Then use a dremel grinder to remove the top of the rivet.
Left:  you can use a pin punch (nail) to drive the rivet out
Above: once the river is driven part way out you can pull it the rest of the way with a pair of pliers.

The replacement outer link is a 3 piece assembly. The only tricky part is getting the retaining ring to snap in place. Buy extras in case one launches itself into a dark recess and you can't find it.

I use a pair of pliers spanning the pin and the closed end of the retainer while I hold the open end in place.
Left:  Drill holes in the ends of the wheel axles to hold them in place. Cotter pins would work fine. I didn't have any so I  drilled & tapped holes for retaining screws.
This is the Jackshaft assembly. After doing a trial fit, so you know where the sprockets are located, mark them on the shaft, disassemble and file flats on the shaft. Don't skip this step. If the sprockets slip the set screws gouge the shaft and disassembly becomes very difficult.  Oh, and driving your Droid with one drive 'slipping' is impossible.
Cut two wheel spacers .3 inch long. I used copper pipe.
Cut a piece of 3/8 inch Delrin/Acetal rod for the  chain tensioner.  I found  that  a  tubing cutter works and gives nice clean cuts.
Left: You'll need 4 washers as spacers on the wheel axles. I had some plastic ones in my junk box, but you can use your step drill to enlarge the center hole in some standard washers to fit the 1/2 inch axle. The center hole should be oversize.

Assembling the Drive - Mounting the motor, installing the jackshaft, the main drive chain tensioner, and the wheels.
I'll apologize for the directions that follow. The chain tensioner was re-designed after these assembly pictures were taken. while there is a complete set of pictures of the new tensioner, the assembly pictures show the old design. For example, in the picture below (left) the old tensioner is shown between the jackshaft assembly and the shoulder bolt & square nut.

Above: The parts needed for these steps.
Right: you can use the shoulder bolt as a handle to insert the square nut.

Check that the shoulder bolt fits the mounting holes in the structural frame. Enlarge with a 3/8 drill if necessary.

Remove the sprocket from the motor shaft.

Mount the motor. [Not shown, re-install the sprocket!]

Install the jackshaft. Location of the 9 tooth sprocket is approximate for the moment.

Above: Remember to install the 20 inch chain too!
Right, and following, is the (new) chain tensioner installation. Note the centerlines and punch marks for the drill holes.

Either enlarge the slots, or lightly sand the Delrin rod so that it easily slides up and down in the slots.

Drill threw the edge of the frame to tap a hole for a 4/40 bolt.

While holding the tensioner rod, use the drill to 'dimple' the surface of the rod. It's not necessary to drill threw the rod.

Tap the hole

Install a bolt and use the end of the bolt, in the 'dimple' made previously to hold the tensioner rod in place. Repeat the drill/dimple process on the other side.
Remove the tensioner rod and drill one dimple all the way threw using the tap drill. Switch to the oversize drill and use the tap drill as a handle to position the rod.

To align the rod while drilling the second hole use a screw. The rod needs to slide up/down smoothly on the screws.

re-install the rod to verify the smooth fit.

Install one spring on each side.

Verify the fit of the two wheel axles. Note that we have reverted back to the old tensioner. Note the chain routing. It's Above the 9 tooth sprocket and below the tensioner to give more "wrap" to the chain around the small sprocket.

Above: Drape the chain so it's out of the way. Insert axle shaft, add washer and spacer.
Right: slide wheel & sprocket assembly onto shaft.

Add washer on the other side of the wheel.

slide axle threw second washer and use (bolt) hole to secure axle in place or install cotter pin

Repeat the installation process for the other wheel

Now that you know where the jackshaft sprockets are located, mark them on the shaft, disassemble the jackshaft and file flats on the shaft. Don't skip this step. If the sprockets slip the set screws gouge the shaft and disassembly becomes very difficult.  Oh, and driving your Droid with one drive 'slipping' is impossible.

Loosen the motor mounting screws. Slip the chain over the sprockets

Rotate the motor so the chain has no slack but don't make it overly tight. Then tighten all 3 screws.

Finished outer drive wheel assembly. This picture shows the chain path over the sprocket under the (old style) tensioner. The spring loaded tensioner should provide enough tension on the chain so that it does not sag.

Electronics - there are lots of vendors for ESC's (Electronic Speed Controllers) and an even larger number of folks who sell connectors capable of carrying the high currents involved. I chose the Polulu ESC's, LIPO batteries and XT60 Connectors for my Droid since I wanted to be able to put the ESC's and the batteries for the motors into the Battery Boxes.

Strip the ends of the wire

I use soldering paste for tinning the wires

cut two pieces of heat shrink. about 1/2 inch.

XT60 connectors are polarized male/female connectors capable of carrying 60 amps.

use the connector to measure and trim the tinned wire to length

then solder the wire in place.

Solder both sides of the connector.

apply hot air gun to shrink the insulation so there are no exposed points.
Finished XT60 connection. I used pigtail wires off of the Polulu ESC's and an adapter to convert the LIPO connections to XT60 since I didn't want to try soldering the live end of the battery connections.
Left: Here's a Outer Wheel Drive assembly wired up to an ESC and LIPO battery. The ESC has been set up to use a potentiometer to control speed so I can test out the drives on the bench.

Congratulations! Do it all a second time and you've got a completed set of Outer Foot Drives for your R2!

Please send me comments, questions, or suggestions on making the design better to me at: fpirz (at) media (dash) conversions (dot) net