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.
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 15mm Build System is designed around #25 Roller Chain (REV-41-1365) using compatible #25 Sprockets.
The most common and important features of a sprocket are called out in the figure below.
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.
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 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.
3mm (15mm with Locking Motion Hub)
REV Robotics sprockets are a #25 pitch. Plastic Sprockets are designed to fit a 5mm hex shaft which eliminates the need for special hubs and setscrews. Most Metal Sprockets use a Locking Motion Hub (REV-41-1719) in order to connect to a Hex Shaft. The REV Metal Sprockets are at less risk for wear than the Plastic Sprockets.
All REV Robotics Plastic Sprockets have a M3 bolt hole mounting pattern that is on an 8mm pitch. This makes it easy to directly mount REV Robotics Brackets and Extrusion to sprockets. The 8mm pitch is also compatible with many other building systems.
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 15T:20T. 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).
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.
Like with other motion components, REV Sprockets drive motion with the 5mm Hex Shaft. However, in order to use a Hex Shaft with the Metal Sprockets, a Locking Motion Hub will also need to be used. To learn more about using Hex Shafts and proper motion support and constraint visit the pages linked below:
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.
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.
When using the slots on REV structural elements its is very easy to adjust and tension the chain if the sizing is off. When using the Extended Motion Pattern in conjunction with a chain drive, use Tensioning Bushings (REV-41-1702) and Standoffs (REV-41-1492).
Sometimes in a design it may be desirable to stack together multiple of the same sprocket on a shaft. In the cases where the number of teeth on the sprocket is not divisible by six, because of how they are oriented when put onto the hex shaft, the teeth may not be aligned between the two sprockets. To ensure all of the sprockets are clocked the same way, use the alignment shaft notch to put all the gears on the shaft with the same orientation.