> ## Documentation Index
> Fetch the complete documentation index at: https://docs.researchanddesire.com/llms.txt
> Use this file to discover all available pages before exploring further.

# 1. Rear Tensioner

> Install the rear tensioner and thread the belt to prepare the linear rail assembly

The rear tensioner is the first component you'll install on the linear rail. It provides the mechanism for adjusting belt tension and secures one end of the belt to the rail assembly.

## Required Components and Tools

### Components

| Component            | Count | Specification / Notes                                |
| -------------------- | ----- | ---------------------------------------------------- |
| Linear Rail          | 1     | With end stops installed                             |
| Belt                 | 1     | Main belt with grooves                               |
| Rear Tensioner Piece | 1     | Main tensioner component                             |
| M3 Bolt              | 1     | 20 mm (for tensioner bridge)                         |
| M3 Nut               | 1     | For the M3 20 mm bolt                                |
| M5 Nut               | 1     | Slides into tensioner opening                        |
| M5 Bolt              | 1     | Approximately 20 mm (for initial tension adjustment) |

### Tools

| Tool                       | Count | Specification / Notes                         |
| -------------------------- | ----- | --------------------------------------------- |
| 2.5 mm Hex                 | 1     | To fit the M3 bolt                            |
| 4 mm Hex                   | 1     | To fit the M5 bolt                            |
| Small Flathead Screwdriver | 1     | Or equivalent tool to help hold nuts in place |

<Info>
  The rear tensioner installation includes initial belt threading. You'll perform final belt tensioning later in the overall build process after all components are assembled.
</Info>

## Step-by-Step Assembly

<Steps>
  <Step title="Prepare the belt and tensioner">
    Orient the belt so the grooves face the bottom of the linear rail. Curve the belt and thread it through the rear tensioner piece's first opening.

    Loop the belt back under itself so the grooves of the exiting belt touch the grooves of the incoming belt. Pull the belt out just enough so roughly four teeth are showing.

    <Check>
      The belt should form a loop through the tensioner with approximately four teeth visible on the loose end.
    </Check>
  </Step>

  <Step title="Mount the tensioner">
    Remove one of the small rubber end stops from the linear rail.

    Slide the rear tensioner piece onto the linear rail. Wiggle it gently to ensure the rail's internal bearings stay in place.

    Pull the loose end of the belt until the belt tightens up against the assembly.

    <Tip>
      Gentle wiggling helps ensure the tensioner slides smoothly onto the rail without dislodging the internal bearings. If you feel resistance, check that the rail is clean and the tensioner is properly aligned.
    </Tip>
  </Step>

  <Step title="Secure the tensioner bridge">
    Insert the M3 20 mm bolt through the tensioner piece and secure it with an M3 nut.

    Tighten this bolt just enough to reinforce the bridge of the tensioner piece.

    <Warning>
      Do not overtighten the M3 bolt. It should be snug enough to reinforce the bridge, but excessive force could damage the tensioner piece.
    </Warning>
  </Step>

  <Step title="Install the initial tension bolt">
    Slide the M5 nut into its rectangular opening on the tensioner piece.

    Thread the M5 bolt into the M5 nut.

    Tighten this bolt until you feel it lightly touch the linear rail. Do not overtighten; leave a gap to allow for final belt tensioning later in the overall build process.

    <Info>
      The M5 bolt provides initial tension adjustment. You'll perform final belt tensioning in a later step after all actuator components are assembled.
    </Info>

    <Check>
      The M5 bolt should make light contact with the rail, but there should still be a small gap to allow for final tension adjustment.
    </Check>
  </Step>
</Steps>

## Next Steps

With the rear tensioner installed and the belt initially threaded, proceed to install the end effector on the opposite end of the linear rail.