Why won't cheap servos work?

[Ubarch]

I've been browsing around, and have found some of the rectangular-type control servos that are pretty beefy.

Here's one: http://www.hobbyengineering.com/SectionM.html#CatMS

Scroll down to the "Extreme Duty Servo". It's rated at 375 oz/in. It costs $80.

Lets assume it comes with a crummy potentiometer position sensor which you have to have to hack anyway to get continuous movement, as well as file down a stop in the gear assembly. No big deal. You'd have to find a place to bolt your own optical encoder, but I'd be really suprised if that was hard. Ignore the control servo line, and use the encoder line and the DC wires as you would an expensive servo.

Why wouldn't this servo be acceptable for a desktop system if you restricted it to, say, half its rated torque? Is there some fundamental difference in these servos that makes them unacceptable?

Unless someone knows of a source of decent gearhead motors for $80. Please share.

[WoodSnarfer]

It's rated at 375 oz-in, but it's already geared down to 125RPM

Let's say you direct-connect this to a 4 TPI lead screw, you have a jog speed of 31 ipm.

There's an outside chance that if you gear it back up (say 1:3), you can get a jog of 93 ipm and your linear thrust will end up being 157 pounds with that 4 TPI screw. Maybe for a very small setup (like a 18" x 18" table), this will pass.

But, at this point $80 and another $40 for an encoder makes it totally unreasonable. I'd try to find steppers to direct-connect to the axis, instead. Everything will be cheaper: the motors, the hardware and the driver electronics.

eBay has been good to me so far, hunting for this stuff.

-Chris

[harolda]

Well bud you could use big servos with encoders and drives with the proper power supply .Yes ebay is a good place to find stuff