General Category => General Discussion => Topic started by: Deerslayer on December 06, 2017, 06:34:27 AM

Title: Servos - how they work
Post by: Deerslayer on December 06, 2017, 06:34:27 AM
Here is an excellent and easy to understand video that shows what goes on inside that tiny electronics package in every servo. There are other videos that depict the mechanical actions. Sometime, grab an old servo and pull it apart to see for yourself. I have repaired many servos, as most likely cause of problems is gear damage due to binding or otherwise overstressing the system; for most servos, from the cheap Hextronic ones through to the most expensive, you can buy the gear train and spend a few minutes to restore one to new (sometimes better than new!) condition. (
Title: Re: Servos - how they work
Post by: ganguy on December 06, 2017, 09:27:39 AM
Yes, the HXT kits are peanuts, and after a try or two, easy to fix the 900's. I still haven't figured out how to open the 500's.
It's always the second gear from the top so I have quite a collection of the others. Sometimes I change the whole set, others, I just change the #2.
But as yet, I haven't found any kits for the other brands, especially Turnigy metal gear. Any ideas?
Title: Re: Servos - how they work
Post by: Deerslayer on December 06, 2017, 12:14:07 PM
In my experience, it is almost always the second gear from the top. They tend to split when over-torqued. There may also be other gears with damage. Sometimes, this is from the initial breakage up top, with teeth getting jammed into other gears. I, too, have an extensive collection of unuused gears! As a mechanical engineer, I cannot bear to throw out some of Man's greatest products; they are bound to come in useful someday, right?
This is another good insight into how servos work. In particular, starting at around  the 4:20 mark, you will see the typical damage that I usually encounter. On my Heron, very poor quality connectors from Multiplex were eventually found to have caused thrashing and subsequent damage to several servos. The servos would be driven rapidly onto their internal mechanical stops, splitting the second gear and grinding other gears with the broken teeth.

I found HiTec gears at Leading Edge. A set usually costs a significant fraction of a new servo but it is still worth fixing. HK gearsets for the XT900 cost less than a buck - of course, the whole servo costs only about $3.

Sometimes you run into an unusual servo, such as the red "Tiny" ones on my Multiplex Heron. I discovered that they are HiTec, so LE has replacement gearsets for about $6. Until you get into really large and expensive servos, most likely they all come out of the same one or two factories.

 Nylon gears really require little or no lubrication. I give the assembly a quick shot of my DuPont lube with Teflon (available at Lowes), but just about anything will do the job.