Picking the Perfect Pulleys


A quick, but detailed tutorial on how to choose timing belts and pulleys for your bot. Written by Jacob Greenwood, the other technical lead of ASME at UCLA's BattleBots program.

  1. Step 1: Intro

    For BattleBots, we use timing belts and pulleys. There are a few factors to consider when choosing pulleys. Pulleys are defined by material, tooth style, number of teeth, and belt width. Choosing these parameters may seem complicated, but the most important thing is that the tooth style and belt width match for both pulleys. Also, metal pulleys are less likely to strip or break than plastic ones, but you may want to consider plastic due to their lower weight.

  2. Step 2: Where can I find pulleys?

    Check out SDP-SI's website for pulleys.

    There are many different types of pulleys on this website, but there are some that are better than others. For our project, pulleys that have one or two flanges are desirable. These flanges are rims of metal preventing the belts from coming off. Note that using pulleys with flanges does increase the overall radius.

    There are two methods for securing pulleys to a shaft: set screws and Fairloc hubs. Fairloc hubs are much better and more secure for round shafts, while set screws are better for D shafts. If you are planning on attaching your pulley to your bearings directly (i.e. in a shell spinner), set screws will be better than pulley-driving your shell, while your motor pulley should be a Fairloc.       

    Only the HTD and MXL pulleys are made of metal, have double flanges, and Fairloc Hubs.

    Quick side note: this vendor provides SolidWorks models for most of their parts. Please download the CAD files for your pulleys and add them to your final assembly. You may need to force your computer to open these files with SolidWorks by right clicking, selecting “open with”, and then finding SolidWorks.

  3. Step 3: Choosing Pulleys

    You will be choosing two pulleys, one for you weapons motor and one for your weapon shaft.

    Generally, it is advisable to gear down, or use a smaller pulley on your motor than your weapon. For your smaller pulley, it may not be a bad idea to choose the smallest available. However if you elect for a larger one, choose one smaller than the can of the weapons motor. For your larger pulley, make sure that your pulley’s outermost radius is not greater than your weapon’s innermost radius. In other words, your pulley should be completely obscured by your weapon when looking from the side (along the shaft axis). This protects your belt and pulley from the weapon.

    Remember that there are a finite number of pulley sizes, and only certain sizes exist. Generally, it is a good idea to choose the pulley that is actually slightly smaller than your desired radius.

  4. Step 4: Choosing a Belt

    Once you have chosen the sizes for your pulleys, and made sure that all of the other specifications are similar, it is time to choose your belt. Since your pulleys will (most likely) not be the same size, figuring out the length of your belt can be difficult. Thankfully, there is an online calculator for this.

    With this calculator, you input the specifications of your pulleys and a desired center distance. The calculator will come up with a real belt with a center distance greater than or equal to the one you inputted. It is worth trying to find belt that has the next smallest center distance as well, because it may be closer to your desired distance than the next largest. You should consider both belts and decide which makes the most sense for your design.

    This website is convenient because they link the exact part for you to buy, and has SolidWorks files for the belt. Do not include these in your assembly. Due to their construction, they will not stretch around your virtual pulleys, making your assembly a mess. However, you should include them in your bill of materials for their weight and cost.

  5. Step 5: Common Problems

    A common issue with belts and pulleys is how to tension them. If you build a system without some kind of tensioning mechanism, your system will rely on high tolerances, and you will not it will be difficult to construct. In a properly constructed system, you can attach the belt when at least one, but ideally both, pulleys are attached. This means slipping the belt over the pulley flanges. After the belt is attached, it is somehow pulled taut or tensioned. There are two principal ways to do this: applying pressure to the outside of the belt, and applying pressure to the outside of the belt and applying pressure to the inside of the belt. The latter method is more suitable to our application. This involves creating a mechanism for moving the entire motor back and forth, while still assuring the motor will not move when running.

    More details on this issue to be added later... 

  6. Step 6: Quick Recap

    TLDR; the steps above outline how to pick out two pulleys and a belt as a means of driving your bot's weapon motor system. Here are some of the important links included in this tutorial:

    Pulley Vendor: https://www.sdp-si.com/products/Timing-Belt-Pulleys/index.php

    Online Belt Calculator: https://www.bbman.com/belt-length-calculator/