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.
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.
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.
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.
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!
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.
Sprockets are one common way to transmit power and change the output torque or speed of a mechanical system. Understanding these basic concepts is required to make optimized design decisions. This section will briefly cover the definition of these concepts and then explain them in relationship to basic sprocket and chain designs.
Speed is the measure of how fast an object is moving. The speed of an object is how far it will travel in a given amount of time. The SI unit for speed is meters per second but speed is also commonly expressed in feet per second.
Torque is roughly the measure of the turning force on an object like a sprocket or a wheel. Mathematically, torque is defined as the rate of change of the angular momentum of an object. A common example of torque is a wrench attached to a bolt produces a torque to tighten or loosen it. Torque is commonly expressed in N⋅m or in⋅lbs.
When torque is turning an object, like a sprocket, the sprocket will create a straight line (linear) force at the point where the teeth contact the chain. The magnitude of the torque created is the product of the rotational force applied and the length of the lever arm, which in the case of a sprocket, is half of the pitch diameter (the radius).
Power (P) is the rate of work over time. The concept of power includes both a physical change and a time period which the change occurs. This is distinct from the concept of work which only measures a physical change. It takes the same amount of work to carry a brick up a mountain whether you walk or run, but running takes more power because the work is done in a shorter amount of time. The SI unit for power is the watt(W) which is the same as one joule per second (J/s).
Often in competition robotics the total power is fixed by the motors and the batteries available. The maximum speed at which an arm can lift a certain load is dictated by the maximum system power.
Selecting sprockets with different sizes relative to the input sprocket varies the output speed and the output torque. However, total power is not effected through these changes.
Sprocket and chain is a very efficient way to transmit torque over long distances. Modest reductions can be accomplished using sprockets and chain, but gears typically provide a more space-efficient solution for higher ratio reductions.
When a larger sprocket drives a smaller one, for every rotation of the larger sprocket, the smaller sprocket must complete more revolutions, so the output will be faster than the input. If the situation is reversed, and a smaller sprocket drives a larger output sprocket, then for one rotation of the input, the output will complete less than one revolution- resulting in a speed decrease from the input. The ratio of the sizes of the two sprockets is proportional to the speed and torque changes between them.
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 calculate exactly how the sprocket size ratio effects the relationship from input to output, use the ratio of the number of teeth between the two sprockets.
Some designs may require more reduction than is practical in a single stage. The ratio from the smallest sprocket available to the largest is 64:16, so if a greater reduction then 4x is required, multiple reduction stages can be used in the same mechanism which is called a compound gear reduction. There are multiple gear or sprocket pairs in a compound reduction with each pair linked by a shared axle. When using sprockets and chain in a multi stage reduction, it’s very common to use gears for the first stage and then use sprockets and chain for the last stage. The figure below is an example of a two-stage reduction using all gears, but one of the pairs could be replaced with sprockets and chain. The driving gear (input) of each pair is highlighted in orange.
Reduction is calculated the same for gears and sprockets based on the ratio of the number of teeth. To calculate the total reduction of a compound reduction, identify the reduction of each stage and then multiply each reduction together.
Where:
CR is the total Compound Reduction
Rn is the total reduction of each stage
Using the image above as an example, the compound reduction is 12:1.
For any gear system, there are a limited number of gear and sprocket sizes available, so in addition to being able to create greater reductions using compound reductions, it is also possible to create a wider range of reduction values or the same reduction of a single stage, but with smaller diameter motion components.
Each additional compound stage will result in a decrease in efficiency of the system.
Chain Loops can be used with ION Sprockets and structure featuring the MAX Pattern. Any 1:1 ratio will have the correct center-to-center distance for a properly tensioned chain, without the need for tensioning bushings. To calculate how many links you will need, multiply the center-to-center distance by eight, and add the number of teeth on one sprocket.
Links of #25 chain = (Center-to-center Distance x 8) + Teeth in one sprocket
If a ratio other than 1:1 is needed when using the REV ION Build System, use our Ratio Plates to accommodate for the change in center-to-center distance. An ION Ratio Plate provides an offset from the standard MAX Pattern pitch that creates the center-to-center distance.
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 the 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 and reliable operation.
In the image below, the ratio of the number of teeth from the input sprocket to the output sprocket is 20T:15T, which means the input needs to turn 1.3 rotations for the output to complete one rotation
Creating a loop of chain requires breaking off the correct number of links by removing a specific chain pin and joining the ends together. Chain can be broken using many methods, including a Chain Tool or various steel cutting blades, like a dremel. Once you have counted the number of links necessary for your application, the chain can be joined using a master link or by replacing the chain pin.
This custom-designed #25 Chain Tool () also commonly referred to as a "chain break" or "chain breaker", allows teams to easily break and re-assemble #25 Chain (). The mandrel is used to push out the chain pin. If using Master Links (), the pin can be completely removed, but the depth guide screw allows the option of partially pressing out the pin and then re-assembling without master links.
1 Chain Tool Block
2 Set Screw Mandrels
1 Depth Guide Screw
1 Cup Point Set Screw
1 4mm Allen Wrench
Before using the #25 Chain Tool for the first time, remove the thread pin screw and use WD-40 or compressed air to remove any shavings left in the tool from the manufacturing process. This will ensure the chain break works smoothly and efficiently breaks your chain. Reinstall the thread pin screw. Once this is complete, the chain break is ready for use.
In almost all applications, chain links are connected to form a loop. While chain can sometimes be purchased in specific length loops, it is more common and economical to buy chain by the foot and make custom loop lengths to fit the application. It’s recommended to use a specialized tool, a chain breaker, to cut chain into desired lengths to prevent accidental damage.
Chain breakers do not actually cut the chain; instead, they are used to press out the pins from an outer link. After the pins have been removed, the chain can be separated, leaving inner links on both ends of the break.
Chain Tools have two methods for resetting chain. Using Master Links and resetting the chain pin. Resetting the pin is results in a stronger chain than using a master link.
The REV Robotics #25 Chain Tool () comes with the following:
Roller chain is typically connected into a continuous loop. This can be done using a special tool to press the pins in and out of the desired outer link as described in the section, or, if the chain is already the correct length, a common roller chain accessory called a master link, or quick-release link, can be used to connect two ends of the chain.
1) Unscrew the Pin Screw and Compression Screw such that the chain channel is free of obstructions. |
2) Insert #25 chain (REV-41-1365) into the chain channel and align the desired link between the two pins above the Cup Point Set Screw. |
3) Secure the chain in place with the Compression Screw using the Allen Wrench. Tighten until the chain cannot shift within the channel. |
4) Put the Allen Wrench into the Pin Screw and tighten until the pin is entirely removed from the chain. Make sure to have a Master Link (REV-41-1366) on hand. |
1) Unscrew the Pin Screw and Compression Screw such that the chain channel is free of obstructions. |
2) Insert #25 chain (REV-41-1365) into the chain channel and align the desired link between the two pins above the Cup Point Set Screw. |
3) Secure the chain in place with the Compression Screw using the Allen Wrench. Tighten until the chain cannot shift within the channel. |
4) Put the Allen Wrench into the Pin Screw and tighten until the pin almost touches the Cup Point Set Screw. You should stop pushing the pin out before it leaves the back plate of the outer links. Considerable pressure will be felt before the pin comes out, but removing the chain from the tool occasionally during the process to check if the pin is unseated from the bushing is recommended. The final result should be the pin still partially connected to the chain, as seen in the second photo. |
5) Unscrew the Compression Screw until the channel is empty, and place the unseated pin and outer plates into the open channel. Place the desired empty inside link in between the outer plates and unseated pin. |
6) Tighten the Compression Screw using the Allen Wrench until the pin is reseated. |
7) Once the pin is fully reseated, release the chain from the tool using the Allen Wrench- your chain should be connected! |
