# Making the Chain Links
### Basics of Chain
This drivetrain uses sprockets and chain to transmit motion from the UltraPlanetary Gearbox and Drive Shaft to the rest of the drivetrain. The figure below shows the major components of chain.
***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***. The REV 15mm Build System uses #25 (0.25”) chain.
To find more information about chain check out the [Sprocket and Chain section](https://docs.revrobotics.com/duo-build/motion/sprockets-and-chain#anatomy-of-chain).
### Introduction to the Chain Tool
In the FTC Starter Kit is a #25 Chain Tool. This custom-designed tool 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.
For more information on the #25 Chain Tool, see the [Chain Tool page](https://docs.revrobotics.com/duo-build/motion/sprockets-and-chain/chain-tool) and [How To video](https://www.youtube.com/watch?v=Y4Ur0f5kra8\&t=1s).
### Creating the Chain Loop – Resetting Chain Method
For this drivetrain, four chain loops that are 56 links long (14 inches or 35.56 cm in length) are needed. When counting links for chain, both the inner and outer links need to be counted. Counting the number of bushings is another way to get the correct chain length. To successfully reform chain, the total count of links must be an even number with one inner link and one outer link exposed. This allows the chain tool to press the pin back into the bushing, through an inner and outer link, reconnecting the chain. Below are the steps to use the #25 Chain Tool to reset the pin into the chain. If you would like to use master links see the [#25 Chain Tool page](https://docs.revrobotics.com/duo-build/motion/sprockets-and-chain/chain-tool) for more information.
{% hint style="info" %}
The steps below show how to make a chain loop and call out the specific link counts for the correct chain loop needed to complete this guide. The images use a smaller chain link sample. Make sure you make chain loops that are 56 links long (14 inches or 35.56 cm in length).
{% endhint %}
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| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
|  | Unscrew the Pin Screw and Compression Screw such that they are flush with the chain channel.
Ensure the Cup Point Set Screw is fully engaged in the Chain Tool.
|
|  | Insert #25 chain into the chain channel and align the desired link between the two vertical pins in the channel.
Note: Count 56 bushings from an exposed inner link.
|
|  | Next secure the chain in place with the compression screw. Tighten until the chain cannot shift within the channel. |
| **Note:** Be careful to not overly push the pin out in the next step it will be impossible to put it back in, and a master link will be needed. As result of manufacturing tolerances, the cup point set crew may be close enough to use as a hard stop for the pin. | |
|  | Screw the Pin Screw down until the pin almost touches the Cup Point Set Screw. The user should stop pushing the pin out before it leaves the back plate the outer link. Considerable pressure will be felt before the pin comes all the way out. Removing the chain from the tool to check if the pin is fully unseated from the bushing is recommended.
The final result will be the pin still partially connected to the chain (see photo as example).
|
|  | Put the Inner Link bushing into the Outer Link. Align the Pin in the Compression Screw hole. |
|  | Turn the Compression Screw until the Pin is fully seated back into the chain channel. |
|  | 56 Link Chain Assembly Complete
Repeat these steps step until you have four 56 Link Chain Assemblies.
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