Looking for a way to make an inexpensive bot, I decided to modify several 41G T-Pro SG5010 Ballraced Servos I got for $4.90 each (below). The first thing to know is that servos that have the pot/motor directly soldered to the servo board CAN NOT be easily removed from the servo housing. The pots are held in place internally with tabs and a screw. If you pry hard enough the pot will eventually pull apart (or possibly the board will break). Been there/done that (slow learner!). The good news is that these servos can be quickly modified with just a small screwdriver and a small wire cutter. I made a number of pix of my mod process, but looking back the below pix is about all that is needed.
hobbypartz.com/kahaoubrmo14.html
Modification steps:
- Remove the four screws holding the servo case together.
- Carefully gently wiggle the top servo cover until it comes lose. Small parts alert: The some of the gears and the two very small gear shafts may come off with the servo top. Be situated such that these small parts don’t get lost.
- Carefully remove the big servo output gear. Due to the top bearing being pressed on the big gear, the center top gear will also need to be removed/replaced together due to interferance.
- Remove the stopping tab on the servo output shaft. I used a small wire cutter like Radio Shack (and others) sell. In two cases where the tab was cut flush with the big gear, the tab totally popped itself off the shaft and flew across the room. If the first cut doesn’t get the same results, a little wiggling with needle nose pliers should make it cleanly break off.
- On the top of the pot shaft is a small black collar that keys the pot shaft to the servo output gear. Remove this collar so the pot and output shaft are no longer connected.
- Before reassembling the servo, connect the servo to a servo controller set for position 1500. Very carefully adjust the pot until the servo motor stops rotating. This is a little more taxing than it sounds, but can be done. This should make programming for servo speed control a little easier. Note that you may want to remove the gears while doing this to reduce the possibility of them comming off and getting lost.
- Carefully reassemble the the servo without disturbing the pot shaft. The easiest way is to have the gear shafts mounted on the servo body and stack the gears on in correct order, then carefully put the servo top back on (I took advantage of having the servo top off to put some aditional silicone grease on the gears). The top should easily fit back on if the the small gear shafts fit back into their holes in the top. Reinstall the four servo screws that hold the servo together.
http://web.comporium.net/~shb/pix/cr9.jpg
Advanced tip:
Per mods on other servos, the servo pot can be made adjustable from the outside by doing the following:
- While you have the the servo output shaft removed, get a small drill that just fits in the servo horn screw hole in the output shaft (5/64" in this case). Drill thru the bottom of the hole. If needed enlarge the bottom of the hole a little from the bottom side of the hole with a little bit larger drill (3/32", turned by hand). This makes a tiny shaft down thru the output gear to the top of the pot shaft. Take care to not enlarge the servo horn screw hole itself.
- Carefully make a tiny slot in the top of the servo pot shaft for a tiny screwdriver blade to fit. I used one of the small thin dremel tool cutoff wheels to make the slot. The pot shaft is soft metal so be careful.
- Now when the servo pot drifts from the desired preset position, one can remove the servo horn screw and insert a tiny screwdriver (a toothpick with a screwdriver type tip also works) to tweek the pot shaft to regain the desired pot position. This can be done with the servo rotating.
http://web.comporium.net/~shb/pix/cr10.jpg
http://web.comporium.net/~shb/pix/cr13.jpg
Below is a very good tool for adjusting the internal pot. Made from a plastic jar lid and a round toothpick. The sharp ends of the toothpick are snipped off, with one end cut to a chizel shape and the other low temp hot glued to the top of the jar lid. The wide jar cap makes the delicate pot adjustments much easier than using a skinny screwdriver.
http://web.comporium.net/~shb/pix/cr15.jpg
http://web.comporium.net/~shb/pix/cr14.jpg
Below is the servo that the pot was damaged prying the motor/pot out of the servo. The pot has been ~repaired, but this is not the way to go with this type servo. Once opened, the pot was bent over 90 deg and the slot in the servo case for the servo wires was enlarged to also accomidate the pot.