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

# Belt tensioner arm

> Install the tensioner piece that removes belt slack for smooth, accurate print-head motion.

## Tensioning the belt

The belt you just routed has slack in it. The **tensioner arm** - a black piece with a metal wheel and two T-nuts - installs at the end of the bar, wraps the belt around its wheel, and is pushed outward to pull the belt tight. A tight belt means crisp, accurate prints.

<iframe src="https://iframe.mediadelivery.net/embed/697805/5d73e81e-d0db-46d3-b492-7524116760d6?autoplay=false&preload=false" style={{ width: "100%", aspectRatio: "16/9", border: "none" }} allow="encrypted-media; picture-in-picture" allowFullScreen />

<Info>
  This part uses **T-nuts** again and the **number 4 Allen wrench**.
</Info>

## Step-by-step

<Steps>
  <Step title="Loosen the T-nuts">
    Crack each T-nut screw just enough to rotate the nuts to **vertical** so they can enter the rail slot. Do not loosen more than needed.
  </Step>

  <Step title="Wrap the belt on the wheel">
    Lift the loose belt and slip the tensioner's metal wheel underneath it, so the belt wraps around the wheel before the piece goes into the track.
  </Step>

  <Step title="Slide it into the track">
    Insert the vertical T-nuts into the end of the bar's slot so the piece sits flush.
  </Step>

  <Step title="Push out and tighten">
    Push the tensioner **outward** to pull the belt tight, and while holding it there, tighten both T-nut screws with the number 4 Allen wrench to lock it.
  </Step>
</Steps>

<Check>
  The tensioner is locked into the end of the bar, the belt has no more slack, and the print head moves left and right **smoothly** with no looseness.
</Check>

<Tip>
  A properly tensioned belt should feel firm, not floppy. Too loose causes "ghosting" and layer shifts; too tight strains the motor. Firm with a low twang is right.
</Tip>

## Up next

The gantry is done. Now you cap it with the **top rail**.