1B: Power Transmissions
Chain and Sprocket Basics
Roller chain and sprocket drives are very similar to belt and pulley transmissions. They consist of two main components: a chain, which is a series of interconnected links, and sprockets, which are toothed wheels that mesh with the chain. As the sprockets rotate, they engage with the chain, causing it to move and transmit power from one shaft to another. Bikes are an everyday object that use chain to transmit power. Chains excel at transmitting high force over long distances.
In order to change the torque and speed from the input to the output, different sized sprockets must be used. The mechanical advantage for chain transmissions, similar to gears and pulleys, is based on the ratio between the number of teeth of the output sprocket to the number of teeth of the input sprocket. Similar to pulleys, the sprockets will spin in the same direction.
Types of Chain
The two commonly used sizes of roller chain in FRC is #25 and #35 chain, with 0.25" and 0.375" pitch respectively. For chain, the pitch is the length of each link. The pitch length of the chain is then the pitch (0.25" or 0.375") multiplied by the number of links. Try to use an even number of links to avoid the use of a "half-link" which is weaker than a full-link. This means your chain pitch length should be a multiple of 0.5".
Additionally, the chain clearance diameter describes the diameter of the sprocket with the chain wrapped around it. The following equation can be used:
Clearance Diameter = PD + PitchBelow is an illustration of the pitch, pitch diameter, outside diameter, and chain clearance diameter of a sprocket.
In FRC, #25 chain is most commonly used as it is strong yet relatively lightweight. #35 is sometimes used on very high torque transmissions, but it is heavy and bulky.
Modeling Chain Transmissions
The modeling workflow for chain is the exact same as for belts in the previous page. You will need the pitch diameter of the two sprockets and a correct center-to-center distance.
Utilize the Origin Cube Featurescript function to dimension the pitch diameters of two sprockets on points you want connected with a chain.
#SprocketPD_25(n): Calculates the pitch diameter of a #25 pitch sprocket withnteeth.- Ex:
#SprocketPD_25(16)returns the pitch diameter of an 16T #25 pitch sprocket.
- Ex:
Generate a 3D model of the chain with the same Belt & Chain Gen Featurescript that you used for belts, check the chain link count that it generates, then go back to the sketch and dimension your center-to-center distance using that chain link count to get the correct center-to-center distance. Use the following Origin Cube function:
#ChainCTC_25(n1, n2, n3): Calculates the c-c distance of an1link #25 pitch chain connecting sprocket with tooth countn2and sprocket with tooth countn3.- Ex:
#ChainCTC_25(80,16,48)returns the center distance for an 80 link #25 pitch chain connecting a 16T sprocket to a 48T sprocket.
- Ex:
Chain Tensioners
One difficulty when designing with chain is that it will physically stretch as it is used. This means the distance between each link will slightly increase, making the overall chain longer in a non-insignificant way. Loose chain can be difficult to fix if the chain transmission is not designed with chain tensioning in mind. Although you will not be learning about chain tensioning methods quite yet, you should keep this idea in the back of your mind.
In Stage 2, different chain tensioning methods are introduced in the context of different types of robot mechanisms. The Design Handbook page also dives deeper into this topic.
Example of Chain Tensioning
