My original rudder servo lasted almost 2 years. When it died, I replaced it with a KM 0940md and have had 2 of these fail in the last 6 months. As these units are rated for 4.8-6 volts, thought by using a 5 cell NMH battery which when fully charged, delivers 6.9volts, I was over loading it. Assuming we had just run into a bad lot of the 940s,I installed a KM 0950MD and used 4 AA alkalis delivering 6 volts, but after about 20 minutes of sailing my rudder continue to swings to hard port, far beyond the stop point, and lock there heating up the battery and shutting down both rudder and sail servos. The bell crank is installed correctly (90 degrees to the centerline and pointing to the starboard side of the boat). I tried setting my transmitter to reduce the swing to only 90 percent with the same results. This servos bell crank sits slightly higher than the original one, allowing the steering rod linkage on some boats to come into contact with the lower edge of the servo tray. Using a Dremel, I cut out a slot to avoid this. We have had several boat burn up the 940s because of the extra load on the servo due to rubbing.
By the way the 950 servos, though more expensive than the 940 deliver, less torque ( @ 6 volts 5.0 KG.CM for the 940 verses 4.5KG.CM for the 950.