Modifying servos for continuous rotation is a common and well-documented hack to create an easy-to-interface and inexpensive gearmotor.  The steps required to perform the modification can differ between manufacturers. 

The following instructions tell how to modify the Hitech HS-322HD for continuous rotation.  This servo has very smooth operation and long life so it makes a very nice robot motor.  While these instructions are specifically for the Hitech HS-322HD, it may be possible to modify other Hitec servo models using this same procedure.  It takes a bit more work to modify these servos when compared to other servo brands but when compared to older Hitec servo models, modifying the HS-322HD is much easier. 

Tools you will need:

• small phillips screwdriver
• small flathead screwdriver
• small diagonal cutters
• tiny hand saw or other plastic cutting device
• drill and 11/64 inch bit

Below are the steps we take to modify the Hitech HS-322HD servos. 

Remove the single black screw holding the round white servo horn.  Make sure you get a good grip on both the horn and servo case when removing the screw.  The screw can be pretty tight in a new servo.  Once the screw is removed, pull the servo horn off.  It may also have a snug fit so it may take some force to remove it after the screw has been taken out.  Sometimes rocking the servo horn a bit can help. 

Remove the four screws from the bottom of the servo.  These screws are quite long reaching the entire height of the servo.  Take care as these screws can be stripped easily. 

Now, carefully remove the cover of the servo.  The geartrain will be exposed and don't be surprised if some of the gears fall out.  There may be a lot of grease on the gears so avoid getting dust on them.  Find a clean safe place in your work area (perhaps a small bin) and set aside the servo lid, servo horn, black screw, four long screws, and the gears.  The last gear will be in the middle and you can rock the post it sits on back and forth slightly while pulling to remove both the post and the gear. 

Locate the tiny black knob attached to the potentiometer.  It's made of plastic and rotates easily through 180 degrees.  With the tiny hand saw (the smaller the better) or other plastic cutting device, saw off this knob.  It is very important to make a clean cut as close as possible to the servo case. 

You'll need a controller (we use a BrainStem GP 1.0 module) that outputs a 1.5ms centering pulse and supplies power to the servo.  Plug in the servo.  The motor will probably be spinning.  Adjust the potentiometer (whose knob you just removed) with a small screwdriver until it stops rotating.  As an optional next step, you can put a tiny bit of glue on the pot to hold it in that centered position. 

The biggest gear, the one with the output shaft and splines, has a small plastic stop on it.  Using diagonal cutters, a tiny saw, or some other tool, remove this plastic stop to allow this gear to rotate continously. 

Look on the bottom of the big gear.  You'll see the rectangular hole that fit over the potentiometer knob.  Use the drill and 11/64" bit to widen that hole and make it a little bit deeper.  It needs to be rounded out so no part of it will touch the sawed-down pot.  Be careful not to remove too much material since that will weaken the output shaft.  Test the fit of the big gear and make sure it spins freely when seated over the potentiometer.  It should not move the potentiometer.  If it does, drill the hole a little deeper. 

If you have a servo ball-bearing, you can replace the plastic donut bearing on the output shaft with it.  The ball bearings don't have a top or a bottom and can be installed either way.  The fit is tight so it may take a bit of pressure to get the bearing to seat down on the gear.  Once you get it aligned, it will typically snap down in place. 

Make sure the gears are aligned properly and re-install the top of the servo and the 4 long screws to hold the servo together. 

The servo is done! Test it out to make sure it spins freely and can be run in both directions.  If there are problems, try verifying the input to the servo with another servo before taking it apart again.  You may want to re-install the white round servo horn depending on your intended use. 

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