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This guide describes how to prepare the center pulley assembly and timing belt tensioners for the brushless actuator modules.

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Overview

The center pulley preparation involves:
  • Press-fitting the 10-tooth aluminum pulley to the 30-tooth 3D printed pulley
  • Assembling timing belt tensioner rollers with bearings
  • Modifying washers for proper bearing support

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Required Tools and Materials

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Tools

  • Arbor press or vise
  • Digital calipers
  • Round file
  • Cotton swabs
  • Toothbrush

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Materials

  • Aluminum pulley AT3 T10 (10 teeth, center)
  • 3D printed pulley AT3 T30 (30 teeth, center)
  • Loctite 648 retaining compound
  • 3D printed protection tool for center pulley
  • 3D printed assembly tool for center pulley
  • Ethanol for cleaning

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For Timing Belt Tensioners (2 required per module)

  • 3D printed timing belt tensioner rollers (10mm diameter standard)
  • MR63ZZ bearings (3mm ID x 6mm OD x 2.5mm, 2 per tensioner)
  • M3x10 flat head screws (2 pieces)
  • M2.5 washers (2 pieces) - to be modified
Safety Requirements
  • Wear work gloves when using press or vise
  • Wear nitril gloves when handling Loctite 648
  • Use controlled force when press-fitting - excessive force can crack parts
  • Keep work area clean to prevent contamination of Loctite bond

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Press-Fitting the Pulleys

1

Apply Loctite to 3D printed pulley

Apply a small amount of Loctite 648 onto the internal tooth profile of the 3D printed pulley (30-tooth AT3). Use a toothpick or cotton swab to spread it evenly.
2

Position aluminum pulley

Place the aluminum pulley (10-tooth AT3) on top of the 3D printed pulley. Ensure:
  • The pulley is vertical
  • The tooth patterns are aligned
  • The centering hole on the aluminum pulley faces upward
Push the aluminum pulley in slightly by hand to engage the teeth.
3

Verify centering hole orientation

Double-check that the centering hole is facing upward before proceeding.
Critical: If the centering hole faces the wrong direction, the center pulley cannot be properly installed in the actuator. The parts would need to be separated and re-bonded.
4

Install protection tool

Place the 3D printed protection tool on top of the aluminum pulley. This prevents damage to the pulley during pressing.
5

Install assembly tool

Turn your stack of parts upside down. Place and center the 3D printed assembly tool on the bottom side (3D printed pulley side).
6

Press-fit the parts

Place the assembly in an arbor press (or vise). Slowly and evenly apply pressure to push the aluminum pulley all the way down into the 3D printed pulley.
Alternative: If you don’t have an arbor press, you can use a vise. Apply pressure gradually and evenly.
7

Remove tools and clean

Remove both the protection tool and assembly tool. Using a cotton swab, remove any excess Loctite from the top and bottom of the assembly.
8

Verify alignment

Check the bottom side of the assembly - the aluminum pulley and 3D printed pulley should be flush. If they don’t line up:
  • Remove any scraped-off material with a toothbrush
  • Place the assembly back in the press and apply additional pressure
9

Cure the bond

Let the assembly cure at room temperature for several hours. For full strength, cure at 70°C for 2-3 hours.

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Assembling the Timing Belt Tensioners

Each actuator module requires two timing belt tensioner assemblies.
1

Prepare components

For each tensioner, gather:
  • 1x 3D printed tensioner roller (10mm diameter standard)
  • 2x MR63ZZ bearings
  • 1x M3x10 flat head screw
  • 1x M2.5 washer (to be modified)
Roller diameter options: The 10mm diameter roller is the standard size. Larger diameter rollers can be used to adjust timing belt tension if needed.
2

Install first bearing

Place one bearing on one side of the 3D printed roller.
3

Install roller on bearing

Turn the parts over and push the roller onto the bearing until it’s fully seated.
4

Install second bearing

Place the second bearing on the other side of the roller and push until fully seated.
5

Verify bearing installation

Check that both bearings are inserted all the way and the outer rings rotate freely.

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Modifying Washers for Tensioner Screws

Standard M3 washers are too large and would prevent the bearing outer ring from rotating freely. Therefore, M2.5 washers must be modified to fit M3 screws.
1

Understand washer dimensions

The M2.5 washers have:
  • Inner diameter: 2.7mm
  • Outer diameter: 5.8mm
These dimensions are ideal for the bearing support, but the inner diameter must be increased to fit the M3 screw.
2

Enlarge inner diameter

Using a round file, carefully and evenly enlarge the inner diameter of each M2.5 washer to approximately 3.0mm.
Rotate the washer while filing to maintain a circular shape. Test fit frequently with an M3 screw.
3

Test fit

Verify that the M3 screw passes through the modified washer smoothly without binding.

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Final Tensioner Assembly

1

Insert screw into roller assembly

Insert the M3x10 flat head screw into the side of the tensioner roller that has a ledge (countersunk side), as shown in the reference images.
2

Install modified washer

Turn the assembly over and install the modified M2.5 washer onto the screw. The washer provides a rigid surface for supporting the inner ring of the bearing.
3

Verify rotation

Rotate the bearing to ensure the outer ring spins freely without the washer interfering.

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Quality Check

Before proceeding to actuator assembly, verify:

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Center Pulley

  • Aluminum and 3D printed pulleys are flush on bottom side
  • Centering hole on aluminum pulley faces upward
  • No wobble when pulley is rotated
  • Tooth profiles are cleanly aligned
  • Excess Loctite has been removed

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Timing Belt Tensioners (2 per module)

  • Both bearings are fully seated in roller
  • Bearing outer rings rotate freely
  • M3 screw passes through modified M2.5 washer
  • Washer does not interfere with bearing rotation
  • Screw is inserted from the countersunk side

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