You knew it was only a matter of time, here it is....
How to ball bearing modify a 700 model Roomba CHM
Here is a step by step procedure to add three sealed ball bearings to your stock 700 model Cleaning Head Module. Although the 700 model CHM has many design changes to help prevent the entry of debris into the brush gear assembly, it still occurs just at a much slower pace.
The three sealed ball bearings used in this mod are the same three bearings that are used in the 500/600 model CHM. Brush motor gear (6 x 10 x 3 mm flanged), small rubbrt flapper brush gear (10 x 15 x 4 mm), and the large bristle brush gear (15 x 21 x 4 mm) There are several new additional steps required in performing this mod.
The first step is to remove the CHM either from the Roomba or from the shipping box you received it in then remove the wire brush retainer and the two cleaning brushes. Of course you will need a work space and the following tools.
First thing to do is to remove the brush deck assembly from the blue plastic module. To do that the paddle board must be loosened from the blue plastic module by unscrewing the single phillips head screw. Once the screw is loose the paddle board can be pushed down and out of the opening, then the wiring that is looped in a U bracket can be lifted up just a little to clear the U bracket. Now there is enough slack in the two wire bundles to allow the paddle board to be pushed downward towards the left side. The paddle board has a small notch on the right hand top portion that is used to keep it in place when the screw is installed so when loosening the screw only the left side will move when you first push down on it. Remember this little tidbit when installing the paddle board.
Once the brush motor is removed it can be secured to the brush deck using some tape and to keep track of the motor mounting screws, just insert them back into their openings and secure with a couple of turns. Remove the 4 screws indicated in the picture. Remember that the screw on the far right is shorter than the other three. Once those four screws are removed then the gear case assembly can be removed from the side of the brush deck. Now the three gears to be modified for mounting sealed ball bearings onto can be removed from the case and wiped clean of grease.
The first gear to install a ball bearing onto is the small brush motor gear. To accomplish that task requires that a bolt (4-40) be inserted through the gear and be secured using two nuts on each end of the bolt. In the picture instead of nuts, I happened to be using brass inserts to perform the same function. Once the gear is secured on the bolt it is then inserted into a drill. Use a metal file to turn down the diameter of the gear shaft a little at a time. Test fitting of the ball bearing is required several times during the turning down process so that a tight fit can be reached.
To install the brush motor gear into the gear case housing requires the use of a stepper drill bit. I prefer to perform the step by hand instead of using a drill. Use the stepper drill to increase the opening diameter to 7/16 and then use the awl to make the final diameter adjustment. Again several test fittings and very mild diameter changes should be done until the bearing can be pressed through the opening. Once again a tight fitting bearing is the goal.
Now on to the two brush gears. Here is where the sealed ball bearing modifications begin to become different than the Green CHM mod. The rubber brush and bristle brush gears both have an additional plastic ring that will need to be removed before a ball bearing can be installed. The same two gears also will need to have a
7/64 opening drilled through each gear in order to allow a 4-40 bolt and
nuts attachment to enable the gears to be turned with an electric drill
to cut the additional plastic ring off as well as turning the gear to
make the gear diameter smaller for mounting the sealed ball bearing. Either a small hack saw blade or even an Xacto micro saw blade can be used to cut the additional plastic ring off of the gear. Once that is accomplished the gear can be turned using a file and once again obtain a gear shaft diameter that will create a tight fit when inserting the ball bearing.
Also when enlarging the gear case opening for the rubber brush gear, the non-stepped bit is be used and several trial fittings should be done to ensure a tight fit of the gear and bearing. The same method is also performed on the large bristle brush gear except that instead of the non-stepped drill bit, the stepped drill bit is used to enlarge the opening in the gear case.
Once all the gears are modified and installed into the brush gear housing then the other gears can be inserted and lubrication applied. White lithium grease is a good lubricant to use. Install the gear case cover and install the assembly back into the brush deck. Install the brush motor and then connect a power supply using 15 vdc to the red(+) and black(-) leads on the paddle board. Verify that the current required to turn the gears is less than .4 amps. This will ensure that there will not be false failures of the motor current sensor that detects dirty brushes.
When installing the brush motor back into position check that the plastic tab on the motor is next to the brush motor stop. One additional step is to add a short piece of tape over the deck height control cable once it is slid back into its position in the blue module.
Extra picture shows the brush motor front end difference between the 500/600 CHM and the 700 CHM.