On Fri, 02 Nov 2007 21:04:26 -0700, Eric Anderson
wrote:

I have an application where I have to move a 3 lb object in a 3 ft
cubic space. I am not looking at high performance or high accuracy.
It can take as much as 30 seconds to traverse the 3 ft distance and
the accuracy could be no better than 0.020 or 0.050 inch (possibly the
spec could be even looser).

Linear actuators would appear to be the best solution, but I am
finding that they are hundreds or even thousands of dollars. The
product I want to build will have to cost no more than $2,000 for a 3
axis device with controller, overhead and profit.

What is it in a linear actuator that costs so much that you could not
build a low cost version using a threaded rod, servo or stepper motor
and an extruded frame for a lot less?

Precision, speed and power. Given your light load, low speed, and
low accuracy requirement, I see no reason why threaded rod and a small
motor wouldn't work acceptably.

With a 20-pitch thread, torque would be minimal with 3 lb thrust but
speed would be about 1500 RPM to make your speed requirement. That's
a bit fast for a steppermotor, though not out of the question. You
could also use a DCPM motor with simple rotary encoder feedback. A
suitable optical encoder is about $18 at Digi-Key. You'd also need
limit switches at ends of travel.

Does anyone know of a linear actuator that can provide the kind of
performance I am looking for for a couple hundred dollars?