9 Gram Servo Rebuild

I can't stand to toss something just because it's "broken".  So after breaking a few 9 gram servos (these are TG9e's but the replacement gears are for HXT900's so they appear to be interchangeable) I noticed that Hobby King had replacement gearsets for sale: http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=4950

The gears are listed as being for the HXT900 servo's and I have all TG9e's so far - but they both have the same tower sticker inside of them so I wasn't very concerned about the hxt900 gears fitting in my tg9e servo's.  I was concerned enough that I only ordered one set of gears so far - that was in retrospect the major mistake in this little endeavour.  

Alright, got your replacement gears?  Or have gears from other broken servos that failed differently, looking at the parts I suspect the failure I had is the most common so just swapping bits from other broken servos doesn't seem likely to work very often...but if I had broken servos that broke differently I'd reuse the parts that way.

To open the servos there are just four screws you remove from the bottom with a small phillips screwdriver.

Once you have the screws open the case can be opened at two places, one just under the mounting tabs - which gives access to the gears.  And one just above the servo wires that gives access to the circuit, motor and for our needs gives a way to brace the bottom of the potentiometer shaft.  Just be gentle opening them up so you don't send parts flying everywhere, you'll want to see how it all goes together so you can put it back that way when you're done!

Inside you'll find two shafts with four gears.  You can see how mine broke on the top pinion of the shorter shaft. On both the original and replacement gears this sure seems like the weakest point so I suspect it's a very common failure.  Be sure to note the position of the gears.  The finest toothed gear is on the lowest position and they get coarser as you go stacking them up alternating shafts as you go.  They all just drop in and spin freely on the shafts except for the last gear (the one your servo arm attachs to) which is pressed on. I was able to pull that gear off fairly easily after which the rest basically fell off.  The second shorter shaft is free floating so it will probably just fall off with it's two gears still on it.  

Before pulling the old gears off I gave them a bit of an eyeball against the replacement gears to make sure they looked the same (other than color): (<a href="http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=4950">New Gears</a>)

They looked good so I pulled off the old gears.  I was able to easily do this just with my fingers.  As I mentioned only the top gear that your servo arm attaches to is actually attached to either shaft, but even it came off fairly easily.  The one thing I noted was the position of the two teeth on the bottom of the top gear which was pressed on in relation to the shaft it was pressed on.  I figured getting this right on reassembly would assure the servo doesn't have more travel in one direction than another.

I laid the gears out in the positions they were on the shafts, and then layed out the new gears to match so I could check them once again.  

Everything looked good so I startd restacking the gears.  It went easy until that first (now last) gear which is pressed on.  It took more effort to press on than I expected and I was worried about damaging the potentiometer or the servo case.  

Looking inside the case I could see the end of the shaft - so I figured bracing that against something would be the best bet.  All I had handy was a small hex bit which was far less than ideal...but it got the job done.

I couldn't photograph it as I did it, since it took both hands.  But basically I turned the servo upside down and pressed the new gear against the table.  I then used the driver to press down on the end of the shaft until it was fully seated.  The secondary shaft fell out while I was doing this - but I was easily able to slide it back into place.  With the new gears installed the top of the servo case went right back on.

I reinstalled the case screws and plugged the servo in to test it.  All looks good!  Less than $1 for the replacement gears is cheaper than a new servo, and it just took a few minutes.  With practice it should be really easy.  I'm not sure what the lube is on these - I need to open this one back up and put some on before it goes into a plane..I wanted to see it worked or not before I made things messy.  It sounds like lithium white grease is a good bet and thre's a good chance I've got some laying around.  At these prices and as often as I'd probably break these I don't mind doing some experimenting.  If it's true that the hxt900's have better gears than the tg9e's then this sould be a nice upgrade for my servos as I strip them out.  If not it's still cheaper than buying a new servo and I feel good about not wasting as many "broken" parts.

P.S. - Sorry about the quality of the photos.  I didn't think of doing a write up before I started this so these were just quick and dirty reference photos with my point and shoot so I wouldn't forget how things went together.  But I figured it was knowledge worth sharing so I made an article around them anyway.