cost of linear actuators

Look at:

For your application, I'd consider air cylinders. Exhaust restrictors to control speed, stops on the rods for location, solenoid valves for the electric interface. Very low cost,especially if you look at surplus items. Good for many 1000s of cysles without maintenance

Karl

Eric, The cost can be attributed to feedback. Not using feedback can be dangerous to people or machinery. You can use digital or analogue feedback. Neither is cheap. At the very least, you must use limit switches. Please also note that steppers are very wasteful of energy and can slip when transporting loads during acceleration and deceleration. Hydraulic, pnuematic and pulse driven PM Motors are good choices, some require brakes, all require feedback. Steve

Try Surplus Center.

Pete Stanaitis

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Eric Anders> I have an application where I have to move a 3 lb object in a 3 ft

Given you want to position your load, not just push it into a stop, I'd avoid air. Air is compressible, so you're essentially pushing your load with a spring. When your feedback system says "done", you still have a cylinder full of pressure air, which will keep it moving. As others have said, some kind of electric motor system (screws, wires & pulleys, etc.) looks good.

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.