PulleyBot: a Pulley Driven Robot

The PulleyBot is a simple, single actuator 3D printed toy with a transmission entirely made up of pulley systems. This instructable is a tutorial on how to assemble the PulleyBot, but if you are interested in pulley, cable, and chain mechanisms, then I would suggest checking out this Pulley Mechanisms Instructable. I hope you have fun building this, and even more fun designing your own!

PulleyBot is entirely driven by belt and pulley systems, which are a type of mechanical transmission. A belt is basically a loop of material, be it a bike chain, a rubber band, or a tank tread, that can be used to transmit mechanical power. It is looped around sets of pulleys, which are simply wheels with grooves or teeth in them for the belt to catch on. As a pulley rotates about an axis, it drives the belt via friction between the belt's surface and the groove of the pulley. This in turn drives the other pulleys linked to the same belt. PulleyBot is a simple system composed of multiple belts and pulleys rotating multiple axes to drive the robot forward.

The first thing you'll need to do is gather all the off the shelf parts that you'll need for this assembly. Here's the list:

1. Pololu Micro Metal Gearmotor 1000:1
2. 3 AAA Enclosed Battery Pack with on/off switch
3. 3 AAA batteries
4. Pololu Universal Hub for 3 mm shaft with 4-40 Holes
5. Pololu Micro Metal Gearmotor Bracket
6. 2 4-40 Socket Head Cap Screws
7. Pack of 2 inch ID, 1/8th thick O-Rings
8. 1 foot of .25 diameter D profile rod stock

The tools to assemble this are as follows:

• Small screwdriver
• Wire strippers
• .05" Hex Wrench (comes with motor hub)
• 3/32 Hex wrench
• Solder and soldering iron
• Superglue
• File
• Hack saw or band saw
• 3D Printer
• Grease

Of course, the first thing you're going to need to do is 3D print all of the components. I printed out the parts on a MakerBot Replicator 2, on standard resolution. All the parts have been designed to avoid the need for support material. Printed all in one job, it took about 5 hours and 45 minutes in total. For my final version, I printed out different parts in different colors to show which pulleys were looped together.

The part files are all available here. The quantities you'll need are as follows:

1. Back_Chassis X1: Large, fancy piece with the cutout of the motor in the back, and various holes for shafts
2. Front_Chassis X1: Smaller fancy piece with various holes for shafts
3. Large_Drum X1: Large cylindrical piece with a conical top
4. Small_Drum X1: Small cylindrical piece
5. Hubcap X2: Short shaft caps that have the profile of the D-shaft in them
6. Wheel X3: Textured wheels with D-profile hubs.
7. Drive_Pulley X1: Double-grooved pulley with holes on its face
8. Speed_Shifter_Pulley X1: Double-grooved pulley with a hub sticking out of its top
9. Wheel_Driver X1: Pulley with no external hub and a single groove
10. Drive_Wheel X1: Pulley with a groove on one side and a textured wheel on the other. I printed this in two colors by changing the filament midway through.

You'll need 4 pieces of rod stock for this design. Here are their lengths:

• 3 inches X1
• 2.3 inches X2
• 2.1875 inches X1

After your parts are cut, you'll want to file down any sharp edges or burrs and make sure the rod ends are flat.

I designed the 3D printed pulleys to have a tight fit around the shaft and the frame to have a loose fit. The tolerances on the MakerBot I used may be different than the printer you used, so the fit may not be exactly the same. Try sliding your parts onto the D-shaft and see if they fit well. You may need to use a file to widen the axle holes if the fit is too tight, or you may want to reprint some of the parts and scale them up or down by a small amount.

Note: If you don't want to, or can't reprint the parts, You could use an arbor press or clamp to press the shaft into the part.

The battery box should slide right into the back of the Back_Chassis piece. Slide it in with the power switch facing down and toward the back of the chassis. You'll notice on the bottom of the that there is a nice slot to give the switch clearance. You'll also need to keep track of where the wires end up. As you slide the box in, make sure the wires are within the small channel on the top right side of the Back_Chassis enclosure. This will prevent them from getting squished or tangled in the chassis.

Place the motor bracket on top of the motor's gearbox (with the top of the Pololu logo toward the motor shaft). The bracket should have come with 2-56 mounting bolts and nuts. Put two nuts into their cavities on the bracket, and slide the bracket and motor into the bracket-shaped hole on the back of the chassis. The motor and bracket should be flush with the back face. Next, stick the two bolts up through the holes in the bottom of the chassis and use the screwdriver to screw them into the nuts and tighten everything down.

To complete the circuit, you'll need to solder the battery box wires to the motor leads. It doesn't matter which wire goes to which lead, because we can change the direction of spin mechanically with the pulleys later on. The first thing you'll want to do is cut the wires down to minimize their slack, because this may get in the way of the pulley mechanism. Strip the ends of the wires and feed them through the holes in the motor leads. My battery box had multi-stranded wires, so I needed to twist the ends tight before feeding them through. Secure the wires by looping the ends through the lead and twisting the wire around itself. After they are connected, solder the wire to the leads. Be careful not to melt the plastic casing on the back of the motor!

At this point, it's safe to put in the batteries and test the motor. You can slide the battery box back out (but don't slide it too far) and put the AAA batteries in place. Slide it back in, flip the on switch and make sure your motor is working properly.

Although each pulley has a close fit with the axle, there is still a chance the pulley may fall off the shaft. You'll want to superglue the following pulleys and shafts together to ensure a secure connection:

• Speed_Shifter_Pulley to 3" shaft
• Wheel to 2.3" shaft (X2)
• Wheel_Driver to 2.1875" shaft

To glue each pulley to each shaft, add a drop of superglue inside the axle inset, and slide the D-shaft inside, lined up with the axle profile on the part. Make sure the axle is perpendicular to the face of the pulley.

Note: Don't apply too much glue. If you do, it will flow up and onto the shaft above the wheel, and this will make the fit between the shaft and the axle holes tighter and harder to turn, which is not a good thing! Just add a small dab of glue to the bottom of the hole in the pulleys.

Before you put anything together, you'll want to take some grease and either spray or rub it onto the axles and holes for the axles in the chassis pieces. I also sprayed the axles and fed them through their respective holes and slid and rotated them a bit to spread the grease around. The Speed_Shifter_Pulley axle goes through the top set of holes, the Tread_Drive pulley shaft goes through the holes in the middle of the chassis, and the two Wheel pulley shafts go through the bottom sets of holes. Greased joints will ensure a smooth rotation once everything is assembled, and prevent anything from catching.

Slide the Wheel_Driver (a flat pulley with no extended hub) pulley and shaft through ONLY the right (looking at the chassis from the back) of the two holes on either side of the top of the Back_Chassis piece, as shown above. Before sliding the axle all the way through to the second hole, put the Small_Drum piece with a single O-ring around it between the two holes, and slide the axle through it until it comes out the other side of the chassis. Once the axle is all the way through, place the Hubcap piece on the exposed end of the shaft. Add a dab of superglue to a hubcap piece and secure it to the other end of the shaft.

Slide the two wheel and axle parts you glued together into the holes along the bottom of the front and back chassis pieces. Put them in on the left side (looking from the back of the chassis). Add a dab of superglue to the inside of the Drive_Wheel piece, and attach it to the back axle shaft on the left side of the machine. Do the same with the last Wheel piece on the front shaft.

Place the Drive_Pulley piece on top of the motor hub, with the large side flush with the hub. Place the 4-40 screws into opposite holes in the pulley and screw them into the hub with the 3/32 hex wrench. Once this is complete, take one of the set screws that came with the hub and thread it into the hole on the side of the hub with the .05" hex wrench. Before tightening it all the way, slide the hub and pulley onto the motor shaft as far as they can go (hub first), and align the set screw hole with the flat face of the shaft. Now tighten the set screw with the small hex wrench.

Take the Speed_Shifter_Pulley and axle and only slide it into the back hole of the chassis. Place the Large_Drum piece between the two chassis pieces, and loop the O-ring attached to the Small_Drum around it while you do it.

This is when it is important to note which direction you want your bot to move. Turn on the motor and see which way it turns. The axle with the Large_Drum will spin the same way. You want the Small_Drum axle to spin toward the front of the bot (clockwise when looking at the Tread_Driver piece), because this will spin the wheel forward. As the Large_Drum rotates, you'll want the side of the O-ring that it is "pulling" to come up the back of the Small_Drum. Twist the O-ring as necessary to ensure this (either you'll be twisting it 90 degrees to the left or right).

Slide the Front_Chassis piece onto the battery box, largest face first. Make sure that the axle on the top of the bot slides through the hole on the face of the Front_Chassis piece. There are snap-fit joints on either side of the Back_Chassis piece, and you may need to spread them a little with your fingers as you slide the Front_Chassis part on. They will bend around the slight insets in the sides of the part and snap into the holes above the insets. Once each hook clicks into each hole, the entire chassis should be secure. Note that the movement of the battery is stopped by small chamfers at the front of the piece.

Put a second hubcap on the tip of the Large_Drum axle to secure it. You may not want to superglue this part down just because you may still want to access the battery box.

Loop a second O-ring around the smallest radius groove in the Drive_Pulley and the largest radius groove in the Speed_Shifter_Pulley. I'll explain how to use these two pulleys to change speeds a bit later.

Slide another O-ring around the back right Drive_Wheel piece and loop it to the Wheel_Driver. You''ll want to loop it over the wheel before looping it around the Wheel_Driver, so that you don't have to stretch the O-ring so much.

You're finished with the PulleyBot assembly! Turn it on and see if it moves! If not, you may want to check to ensure the O-rings aren't slipping. If they are, grease the axles around their pivots in the chassis.

You'll notice the pulleys in the back have two different grooves with different radii. You can use these to change the speed and torque output of the bot. These systems are common on variable speed machines like lathes, drill presses, and milling machines. The grooves on each "set" of aligned pulleys are dimensioned for the specific band length, so don't try putting the O-ring over two grooves that are not aligned. The outermost set will make the bot move the slowest, as the top pulley has a larger radius than the bottom, with a 2:1 ratio (so it takes 2 rotations of the bottom pulley to turn the top pulley once). The innermost grooves have a 1:1 ratio, so they will spin at the same speed.

To change the speed, pull up on the O-ring near the pulley with the larger groove radius, and move it down to the adjacent smaller radius. Next, pull the O-ring near the other pulley from its current groove onto the next larger one, spinning the pulley as you do so.

If you want the bot to drive backward, twist one side of the O-ring 180 degrees before placing it in the groove. This will make the top pulley spin in the opposite direction of the bottom one.

There are lots of fun ways to make pulley-driven bots, and they can have any range of complexity. Make your own PulleyBot using some of the tools and ideas I've provided. If you want to learn more about other pulley and cable mechanisms, check out this Instructable on pulley mechanisms. Have fun!

There are definitely a couple ways I could have improved this design. I probably could have made the axles spin smoother by using bearings instead of just by greasing them, and with more time I probably would have incorporated more pulley systems. There's always room for improvement in design!

If you have any questions or want to share your own versions of PulleyBot, please leave them in the comments!