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1B Power Transmissions

Belts and Pulley Basics

Timing belt and pulley drives are mechanical systems used to transmit motion and power between rotating shafts using flexible belts and pulleys. The system consists of two main components: the belt, which is a flexible loop made of a material like rubber, and the pulleys, which are wheels with grooves that the belt wraps around. As one pulley rotates, it drives the belt, which in turn drives the other pulley, transferring motion and power from one shaft to another.

A belt and pulley transmission. (Image Source: ReCalc)

In order to change the torque and speed from the input to the output, different sized pulleys must be used. The mechanical advantage for belt transmissions, similar to gears, is based on the ratio between the number of teeth of the output pulley to the number of teeth of the input pulley. Note that unlike gears, pulleys will spin in the same direction.

Types of Belt

Like gears, belts also have a pitch. The pitch is defined as the distance between each tooth on the belt. In FRC, this is typically 5 mm. The pitch length of the belt is then the pitch (5 mm) multiplied by the number of teeth. Belts will always have an integer number of teeth so the pitch length will be a multiple of 5mm.

For example, an 80T belt will have a length of 400 mm.

Belts also come in various widths. In FRC, you will typically use either 9 mm or 15 mm wide belts.

Modeling Belt Transmissions

To model belts, you will need two featurescripts, the Origin Cube Featurescript and the Belt & Chain Gen Featurescript. In order for the 'Belt & Chain Gen' Featurescript to create the belt, it needs the pitch diameter of at least two pulleys or sprockets and a correct center to center distance, both of which will use the functions from the Origin Cube featurescript.

Note

The Origin Cube also has additional functionality for robot and mechanism assemblies that will be discussed in later stages. You can also check out these practices on the assembly best practices page. The Origin Cube feature must be the first feature in all part studios from here on out in order for the functions to be available.

  1. Insert the Origin Cube feature using the Origin Cube Featurescript. The exercises in 1B do not require the cube generated by the feature so we will uncheck it.

    Origin Cube Feature Options

  2. In your layout sketch, draw two circles to represent pulley pitch diameters. Dimension the circles using the #PulleyPD_5mm(# of teeth) function.

    • #PulleyPD_5mm(n): Calculates the pitch diameter of a 5 mm pitch pulley with n teeth.
    • Ex: #PulleyPD_5mm(18) returns the pitch diameter of an 18T 5mm pitch pulley.

    Tip

    This is typically done in your layout sketch because distances between shafts connected by belts will be driven by the center-to-center distance for the closest belt size.

  3. Draw the center line to connect the pulleys and set your target c-c distance. In this example, our target c-c distance is 5". Make sure all sketch entities are construction. Optionally connect the circles with tangent lines. Confrm the sketch.

  4. Use the Belt & Chain Gen Featurescript to generate a 3D model of the belt. The reference plane mate connector sets the location of the centerline of the belt. You can modify the selected mate connector to offset the location of the belt. You can also choose to generate belt teeth, but this will significantly increase rebuild time and is not recommended.

  5. The Belt & Chain Gen Featurescript will return the closest whole number belt size and the pitch length. Assuming that you only stock belts in 5T increments, the next closest belt size is 80T.

  6. Modify the c-c dimension to use the #BeltCTC_5mm() function to get an exact c-c distance for the 80T belt. Checking the Show Expressions box enables you to see the belt pitch, pulley tooth counts, and belt tooth count.

    • #BeltCTC_5mm(n1, n2, n3): Calculates the c-c distance of a n1 tooth 5 mm pitch belt connecting pulleys with tooth count n2 and pulley with tooth count n3.
    • Ex: #BeltCTC_5mm(80,18,36) returns the center distance for an 80T 5 mm pitch belt connecting an 18T pulley to a 36T pulley.

    Capturing Design Intent

    Before the advent of these two Featurescripts, designers would need to use online calculators, such as ReCalc, to calculate c-c distances. However, this method does not capture design intent as it relies on copy-pasting a calculated value into the layout sketch.

    Design intent is captured in the top sketch but not the bottom.

  7. The Belt & Chain Gen feature automatically updates and we can see that the belt tooth count is correct (80T) and the pitch length is a multiple of 5 mm, meaning that the belt tooth count is exact and not being rounded.