A Crowned Pulley Keeps Robot’s Treads On Track

[Angus] at Maker’s Muse recently created a new and tiny antweight combat robot (video, embedded below) and it has some wonderfully clever design elements we’d like to highlight. In particular: how to keep a tracked robot’s wheel belt where it belongs, and prevent it from slipping or becoming dislodged. In a way, this problem was elegantly solved during the era of the steam engine and industrial revolution. The solution? A crowned pulley.

Silicone bracelet and crowned pulley result in a self-centering belt with a minimum of parts.

A crowned pulley is a way of automatically keeping a flat belt centered by having a slight hump in the center of the pulley, which tapers off on either side. Back when steam engines ran everything, spinning axles along the ceiling transferred their power to machinery on the shop floor via flat belts on pulleys. Crowned pulleys kept those flat belts centered without any need for rims or similar additions.

The reason this worked so well for [Angus]’s robot is partly its simplicity, and partly the fact that it works fantastically with the silicone wrist bracelets he uses as treads. These bracelets are like thick rubber bands, and make excellent wheel substitutes. They have great grip, are cheap and plentiful, and work beautifully with crowned pulleys as the hubs. It’s a great solution for a tiny robot, and you can how it self-centers in the image here.

Antweight robots are limited to 150 grams which means every bit counts, and that constraint leads to some pretty inventive design choices. For example, [Angus]’s new robot also has a clever lifter mechanism that uses a 4-bar linkage designed to lever opponents up using only a single motor for power. Watch [Angus] explain and demonstrate everything in his usual concise and clear manner in the video, embedded below.

20 thoughts on “A Crowned Pulley Keeps Robot’s Treads On Track

      1. Train wheels are conical to center using two radiused rail heads. Not quite the same as it relies on the left and right side opposing forces. It additionally allows for the use of a solid axle without the need for a differential without requiring slip to navigate a corner.

  1. Crowned pulley thing is a neat thing to remember. I believe it was known about for power transmission with leather belts from the golden age of water and wind mill power… then when steam first came in all the “learn-ed” engineers were making their pulleys flat and the millrights of old were telling them that the belts would keep falling off… yeah so the belts kept falling off.. and either the millrights did “unauthorised” mods to get crap working or finally the “learn-ed” men saw sense.

    This effect can also screw you over if you get V-belts and pulleys wrong… You’d think “If I use a V belt it’s guaran-friggin-teed to stay on” yeah maybe. Mistakes there would be the belt bottoming out, being of too wide a section, or the angle being too shallow in the pulleys, and the belt will act like the belt on the crown pulley, walk up one high side or the other, and fall off.

    1. I don’t think the engineers were that stupid – they knew that a flat pulley would eventually cause the belt to walk off because of inevitable misalignment between the shafts because there’s obviously nothing holding the belt on. However, crowned pulleys will also fight each other and cause the belt to oscillate, so often only one side of a pair is actually crowned while the other remains flat. Then there’s machines with only flat pulleys and they work just fine – because they’ve been aligned just so to achieve the same effect.

      The problem with crowned pulleys is that they have greater friction, stretch and wear the belt in the middle, and they can’t deal with a lot of torque since the belt can’t be tightened too much. It may cause the belt to bunch up in the middle and buckle. When you try to scale down the machinery to smaller sizes, the belts need to get tighter to transmit the same power and the friction causes problems like chattering that wears down the belts and pulleys.

      1. After two months of thinking about forces and their directions … think of each half of the crowned pulley as a truncated cone trying to roll off the side of the belt. Since the pulley is fixed, this will transport the belt towards the middle of the pulley, where the forces of both halves cancel each other due to symmetry. Add a flat middle part and smoothen the surface to get to the rounded surface of the crowned pulley.

  2. I have a 6″ x 48″ belt sander that had a problem tracking. So, I put a couple of layers of masking tape on the center of the two pulleys. 50+ years later, that temporary fix is still there and working.

  3. I wonder if anyone has considered using those clear sticky soft rubber sheets, used for sticking mobile phones to car dashboards, as track or tire material. No doubt they’d dramatically increase traction.

    1. Sticking to the surface is usually not permitted in this kind of competitions. I haven’t checked the rules for this particular one, but the sumo robot competitions have such clause. They test it by putting the robot on a sheet of paper and check if it stays on the ground when lifting the robot.

      1. The material CRJEEA refers to isn’t actually sticky, just a soft, clean rubber (likely silicone like the bracelets in the article) that prevents horizontal movement (sliding), using gravity. It would probably work great until it picked up enough dust/debris to become dirty, allowing sliding.

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