The 2009 version of my walking robot was in part a test of the feasibility of using the cheap MG995 servo in a walking robot. I expected that even if these cheap servos didn't last long their cheapness meant replacing them wouldn't be unreasonably expensive. These servos lasted long enough in practice (although when they failed the failure mode what not what I expected) but their terrible position-holding ability made them unacceptable for what I'm trying to do. So the MG995 servos have to go.
I'm currently rebuilding the robot with some HS-5645 servos I have on hand, but I don't expect those to last long. One already has a stripped gear set, and the others don't look they have much life left in them.
I need new servos. Preferably metal-geared servos with motors rated for 7.4V lithium battery use. Unfortunately I'm also on a very tight budget for robots at the moment. My wife will only approve buying expensive servos if I can guarentee they will never fail, and who can guarentee that?
There's the HSR-8498HB. The low-end 7.4V robot servo from HiTec. Not cheap at $60, and a little less torque than the HS-5645MG. It has Karbonite gears, whose durability I would be very concerned about. The HSR-5498SG has steel gears, better torque, and is only slightly more expensive at $70. Both these servos are available with a rear axle point built into the case, which would remove the need for me to modify the case that way. On the downside these do not use the standard servo mounting points, which means they would not be useable with the current waterjet frame pieces. Although since all the strucural failures I had were at the standard mounting points, redesigning the servo-to-frame attachment method might be a good idea.
There's the $60 HDS-2288. Metal gears, rated for 7.4V, nice torque, uses standard servo mounting points. I'm not sure if they're actually available with any reliability, they seem to be sold out or on backorder everywhere.
The $20 Associated SHV1504MG has metal gears and is rated for 7.4V, and uses a standard servo case, but is terribly underpowered for this requirement.
Then Dynamixel servos. These look really interesting, having some amazing features way beyond normal servos in how they can be configured and what kind of feedback data you can get back from them. These don't use the standard servo control pulses at all, but instead have an addressed serial bus scheme, which I should be able to connect straight to the serial ports on the Xbee radio. I could use this to feed back voltage and temperature indications to the controller, maybe even translate joint torque into force-feedback somehow. Very interesting possibilities. And all these servos will run at 7.4V. They're actually designed for 9.6V, so 7.4V is a little on the low side but still within the rated voltage range.
On the down side most of the Dynamixel servos are completely outside my price range. The $45 AX-12 servo I could probably afford, but those have plastic gears which I'd prefer to avoid. The Dynamixel servos also would require a complete strucural redesign to use. That might be a plus after all, the AX-12 servos are far better suited to robotics from a strucural point of view than are standard hobby servos.
The last option would be to just make some frankenstein hack from my current servos, homemade die-cast cases, openservo boards, and rewound motors. Considering that as a backup plan.