iRobot Roomba 5xx 6xx 7xx Gray CHM mod

by Updated: Dec 14, 2020diy-irobot-roomba0 comments

Roomba Gray CHM mod

How did I modify 5xx CHM with bearings

This mod assumes that you already cleaned the gearbox (see this page) and/or found that the plastic housing had started to deform due to massive friction. With this modification dust and hair entering to the gearbox will be blocked. Worn to the gearbox wheels, wheel holes and to brush hubs will be reduced dramatically as ball bearings will replace the plastic to plastic friction. Doing this modification on time (when no serious damage was done to the plastic holes) will ‘save’ and extend Roomba CHM life and will add many hours of smooth and quieter work.
Note Note: This page describes Roomba 560 but it is the same for all 5xx 6xx 7xx series models that uses Green / Red / Gray CHM though procedure might be slightly different. For the Red CHM mod details instructions go to this page.

DISCLAIMER - Important!

A. Preparing for the fix:

Suggested tools and materials: 

  • Phillips head screwdriver
  • Old toothbrush or some other small cleaning brush
  • Lithium grease
  • Vacuum cleaner (to be use it in each step where you find dust and hair buildup. Be careful not to vacuum small parts as well.)
  • Electric screwdriver or drill
    (This will be used as a kind of lathe that holds the gear wheel in order to contract its diameter.)
  • 3.0mm drill
  • Fine tooth blade arc hand saw
  • Set of files, straight and round.
  • Screw and nuts 4-40 or M3 to retain the small gear wheel to the drill.
  • Step drill bits
    or any other tool that can help expand the housing holes diameter such as grinding stone, awl or a file.
  • Swivel burrs removal tool.
    (This is a great tool to help in expanding housing holes.)
  • Vise grips or a bit handle
    (This is to hold the drill bit etc. It is advised to expand the holes by hand and not with an electric drill to gain precise and more controlled process.)
  • 3 sealed ball bearings with the following dimensions:
    (in x out x thick, Metric system)
    – 6x10x3 mm (for motor gear)
    – 10x15x4 mm (for small beater brush gear)
    – 15x21x4 mm (for large bristle brush gear)

Note To save you time, I have prepared for you a set of the 3 needed bearings that you can purchase right now. Click the following Buy Now banner to review the set and purchase.

B. Disassembly of the robot and modding the gearbox.
Note Note: since most of the screws are screwed to plastic bosses, it is advised to use hand screwdriver for fine torque control.

1. Place the Roomba top down on a solid work surface. Remove the screw from the rotating side brush and gently pull it out.
2. Remove the four screws holding the bottom plate and remove the plate
3. Take out the battery, lift it using the 2 plastic tabs. Take out the dust bin.
4. Unscrew the CHM 4x screws and take it out by pulling it gently.

5. This is the complete CHM – “Cleaning Head Module”.
Arrow points to the gearbox side

6. Open the module top door and take out the bristle and the beater brush

7. This is the mating area where bristle brush and beater brush meet the gearbox.
The gap between the gear wheel cylinder to the plastic housing is the area which this mod is dealing with.

8. Take apart the CHM module by first unscrewing 1x PCB screw and releasing the motor tension cable. See next images.

9. Take off the small round sticker that keeps the motor tension cable head.
Grab the head of the tension cable with a small pliers/tweezers, pull it up and
slide it through the cable slot to release it.

10. Disassemble the PCB by first unscrewing its single screw.
Next, release the cables from its hook
Now push the PCB down to release it from its slot.

11. Separate the Gray CHM module from the Blue external frame by first spreading its ‘wings’ (one after the other) and by rotating the CHM inner Gray case.
Be careful not to use excessive force.
Note Though the image shows the module prior of PCB/tension cable disassembly, you should perform this step AFTER you disassemble the above PCB/tension cable.

12. You now have access to the motor.
Unscrew the 2x screws of the motor bracket that holds the motor.

13. Pull the motor aside and temporary secure it with a tape.

14. Remove the gearbox sub-assembly by unscrewing its 4x screws.
Pay attention that one screw is shorter! This is important when you’ll assemble it later on.

15. Pull the gearbox and separate it from the module.
Unscrew the 2x screws holding the gearbox cover plate and open it.

16. This shows the open gearbox with all its wheels assembled.

17. 3x wheels needs to be modified by adding bearings to the wheels.
The gearbox wheels openings of those 3x wheels needs to be expanded to fit the bearings .

18. Now contract the gear wheels cylinder diameter.
Modified diameter should meet the inner diameter of the relative bearing. Mating should be tight.

Note Motor wheel needs to be contracted and on both beater and bristle wheels we need to first remove its external ring wall and then contract its diameter to meet the bearings

19. Diameter contraction is done here with the help of an electric screwdriver and a file.
First image shows the motor gear wheel which is held with a 4-40 screw
Second image shows the ring wall removal with an arc saw
Third image show drilling the bristle wheel in order to allow restrain it to the drill.

If possible, prefer using table drill or a lathe for perfect centering and alignment
Pay close attention not to file too much !
Check fitting to the bearing often till you reach needed diameter.
Note Don’t file the gear, just its mating cylinder diameter !
Make sure that file’s flat surface is pointing towards the gear.


  • Motor wheel cylinder size fits bearing inner diameter of 6.0mm. Stop modifying at 6.3mm
  • Beater wheel cylinder size fits bearing inner diameter of 10.0mm. Stop modifying at 10.3mm
  • Bristle wheel cylinder size fits bearing inner diameter of 15.0mm. Stop modifying at 15.3mm
20. Now expand the case holes to meet the outer diameter of the bearing. Do it by hand for better control and use any of the above mentioned tools. If possible, prefer using table drill or a lathe for perfect centering and alignment Mating should be tight. All mating holes diameters are expanded. Check fitting to the bearing often till you reach needed diameter.
  • Hole size for motor wheel fits bearing outer diameter of 10.0mm. Stop modifying at 9.7mm
  • Hole size for beater wheel fits bearing outer diameter of 15.0mm. Stop modifying at 14.7mm
  • Hole size for bristle wheel fits bearing outer diameter of 21.0mm. Stop modifying at 20.7mm

21. When reaching needed diameter, insert the bearing with a little force.
Lubricate the gear wheels using Lithium grease, assemble the gear wheels and gearbox cover.

Here you can see the gearbox with the bearings installed.
If using a flanged bearing, the flange should be on the gearbox side.

22. Assemble the gearbox to the housing.
Slide the motor to its place and pay attention that its rod hex shape will mate with the gearbox wheel. Also pay attention to the motor position in the gray housing.

23. This is the assembled module from inside.

24. Test CHM. You can use a 9V battery or a power supply if you have one.
Connect + to the Red motor wire and – to the Black motor wire. At 15-16VDC, current should be less than 0.5A
If you test it without the outer frame, hold the tension wire to allow it to work.

25. Assemble the CHM outer Blue frame.
Start with the PCB and pay attention to its direction in the slot of the outer Blue frame.

26. Return the tension wire round head to its place.
Attach the old / new small sticker to cover the tension wire head recess / slot.

C. Assembly – assemble all parts in reverse order to what is listed above. If you have completed the assembly and left without any screw, hit the Clean button and check if your Roomba is working…

D. Voila – you now have a working iRobot Roomba with modded Gray CHM gearbox.

Have fun and enjoy your Roomba, Avi Clap

Want to improve your Roomba with this mod as well? Buy Now the bearing set and please consider to donate.

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electric hazard sign


1. iRobot Roomba contains electronics, some works on high voltage.

Any modification made to your Roomba robot may cause damage to your equipment and harm to yourself or to others. 
Any modification made to your Roomba robot will void manufacturer warranty.
None of the following steps is recommended by me nor by the manufacturer, iRobot. 
This post show only my personal experience and know-how.
If you follow any of the steps, you do it on your own risk and responsibility.

2. Copyright © 2011 Avi. All rights reserved. 

Content including all text and images in part or in whole may not be reproduced, retransmitted, stored, distributed, disseminated, sold, published, broadcast or circulated in any medium
without written consent of the author

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