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Sprockets and Chain

Sprockets and Chain are ideal for transmitting motion over long distances. A chain consists of a continuous set of links that ride on the sprockets to transmit motion. The two most commonly used sizes of chain in FIRST Robotics Competition are #25 and #35. When choosing between chain sizes, it is important to consider the pitch of the chain and the weight and forces that your mechanism will be experiencing. The REV ION Build System is designed around #25 chain using compatible #25 sprockets.

Sprocket Basics

Sprockets are rotating parts that have teeth and can be used with a chain and another sprocket to transmit torque. Sprockets and chain can be used to change the speed, torque or direction of a motor. For sprockets and chain to be compatible with each other, they must have the same thickness and pitch.
Sprocket and chain is a very efficient way to transmit torque over long distances.
Sprockets consist of a disk with straight teeth projecting radially. Sprockets will only work correctly with chain and other sprockets if they are on parallel shafts and the teeth are in the same plane. A chain consists of a continuous set of links that ride on the sprockets to transmit motion. The REV ION Build System is designed around #25 Roller Chain (REV-41-1365) using compatible #25 Sprockets.

ION Sprockets

Our #25 ION Sprockets are compatible with the REV ION system and designed for use with #25 Roller Chain. These sprockets are flat and feature a MAXSpline and a 2in bolt circle that patterns outward radially, allow for bolting to structure easily. #25 Hub Sprockets feature a 1/2in hex bore or MAXSpline and transfer torque through a shaft. 1/2in Hex - 16 Tooth - Double sprockets available for chain in tube applications.
A photograph of two sprockets, both with 1/2 inch MAX Spline bores.

Anatomy of a Sprocket

The most common and important features of a sprocket are called out in the figure below.
Number of Teeth
Pitch Diameter
Pitch
Outside Diameter
Chain Clearance Diameter
Number of Teeth is the total count of the number of teeth (projections) around the whole circumference of a sprocket. For sprockets with very few teeth this number is easily physically counted, but for high tooth counts this may not be isn’t very practical.
Pitch Diameter (PD) is an imaginary circle which is traced by the center of the chain pins when the sprocket rotates while meshed with a chain. The ratio of the pitch diameter between sprockets can be used to calculate the gear ratio, but more commonly and much more simply the ratio of the number of teeth is used for this calculation.
Pitch represents the amount of pitch diameter in inches per tooth. Sprockets with a larger pitch will have bigger teeth. Common pitches are 0.25”, known as #25, and 0.375” (#35). The REV Robotics building system uses #25 chain.
Outside Diameter (OD) will always be larger than the pitch diameter but smaller than the chain clearance diameter. The outside diameter does not account for the additional diameter added by the chain, so it should not be used to check for assembly interference.
Chain Clearance Diameter is the outside diameter of a sprocket with chain wrapped around it. The chain clearance diameter will always be larger than the pitch diameter and the outside diameter. The chain clearance diameter should be used when checking for interference when placing sprockets very close to other structures.

Anatomy of Chain

Roller chain is used to connect two sprockets together and transfer torque. Roller chain is made up of a series of inner and outer links connected together which forms a flexible strand.
Outside Links
Inside Links
Pitch
Outside Links consist of two outside plates which are connected by two pins that are pressed into each plate. The pins in the outside link go through the inside of the hollow bushings when the inner and outer links are assembled. The pins can freely rotate on the inside of the bushings.
Inside Link consist of two inside plates that are connected by two hollow bushings which are pressed into each plate. The teeth of the sprocket contact the surface of the bushings when the chain is wrapped around a sprocket.
Pitch is the distance between the centers of two adjacent pins. Common pitches are 0.25”, known as #25, and 0.375” (#35). The REV 15mm Building System uses #25 chain.

Using Sprockets and Chain as a Powertrain

Transforming the torque and speed of the motion is accomplished by changing the size of the sprockets.
A sprocket size ratio is the relationship between the number of teeth of two sprockets (input and output). In the image below, the input sprocket is a 15 tooth sprocket and the output is a 20 tooth. The sprocket size ratio for the example is 20T:15T. The ratio in size from the input (driving) sprocket to the output (driven) sprocket determines if the output is faster (less torque) or has more torque (slower).
To learn more about ratio calculations for sprockets, check out the Sprocket Ratio section on our Advanced Page!
The 15 tooth sprocket outside of the chain loop is known as an idler. Idlers do not affect the sprocket size ratio and thus are not part of the calculation. To learn more about idlers check out the Idler section on the Advanced Sprockets and Chain page.
Within a chain loop, motion follows the direction set by the input sprocket. In the example, both sprockets inside the chain loop move counter clockwise. Idlers, which sit outside of the chain loop, are pushed in the opposing direction. So, the 15 tooth idler sprocket is moving clockwise.

Chain Tension

In order for sprockets to work effectively, it’s important that the center-to-center distance is correctly adjusted. The sprocket and chain example with the red 'X', in the image below, may work under very light loads, but they will certainly not work and will skip under any significant loading. The sprockets in this example are too close together so chain is loose enough that it can skip on the sprocket teeth. The sprockets, with the green check mark, are correctly spaced which will provide smooth reliable operation.
To learn more about calculating center-to-center distance for sprockets visit the Spacing and Center-to-Center Distance section on the Advanced Sprockets and Chain Page.
The first step to getting ideal chain tension is to manipulate, or cut the chain to the correct size. Using the center-to-center distance calculation is one of the most accurate ways to find the chain size needed. Once sizing is approximated, use the Chain Tool (REV-41-1442) or Master Link (REV-41-1366) to break and reform the chain.
To learn more about using the Chain Tool and Master Link, check out the Manipulating Chain section.