Cheap Continual Motion Linear Actuator?

[vbsaltydog]

I am not a mechanical engineer but rather a computer programmer so pardon me if my question is simple in your field. I have a need for a linear actuator that has a stroke of about eight inches but it needs to be constant motion so as soon as it reaches the end of its stroke, it needs to reverse itself. It is for moving a light (20 lbs) back and forth over an aquarium.

I have searched for linear actuators on google but they are really expensive and none of them are continual motion. They are all controlled by a manual switch.

It doesn't seem like that hard of a concept, if you fix a nut in position and have an eight inch bolt threaded in it that is driven by a motor that can handle the weight load of the device being pushed/pulled then the bolt shaft would be the "piston" of the actuator and then just put contact points on the nut (both sides) that reverse the motor when they are touched as the bolt hits the start and end positions then it should work great but I don't want to build it if it is already built and can be bought for less than $25.00

Anybody know where I can get a cheap continual motion linear actuator with an 8" stroke (give or take). The device to be moved weighs 20lbs but it is suspended by cables so it takes very little force to move it left to right so the actuator does not need to be able to move 20lbs, more like ~4lbs

 

[FredGarvin]

I have used these with good results. Granted, they are stepper motors.

http://servosystems.com/ead_actuators.htm

 

[vbsaltydog]

Thanks for the link. They were helpful but they were too expensive for my project. I am going to have to fabricate something myself with a bolt/nut/motor/switches. Ehhh.

Any more ideas are appreciated.

 

[Emreth]

Why don't you use something like this?
http://www.youtube.com/watch?v=poDwOv3unD8&feature=related

 

[vbsaltydog]

Perfect. I like the fact that I can use it to rock the lights like a pendulum but without the vertical movement. Thank you.

 

[Q_Goest]

Might want to also consider the simple http://www.youtube.com/watch?v=YV9WkQkUHZ4".

 

[vbsaltydog]

I think the scotch yoke is the winner so far but all are great ideas. I was thinking of doing a spline shaft and drive gear to "climb" up and down the shaft like the steering rack of a car but this too, would require "end of travel" ? switches to reverse the motor but your ideas for converting rotary motion into linear/reciprocating motion are easier.

More ideas are welcome although I can't see it getting much easier than the scotch yoke. Pretty simple design.

Thanks again to everyone.

 

[vbsaltydog]

I assume that in order to increase the stroke length of a scotch yoke driven device you have to increase the diameter of the flywheel. Is there any other way?

Are there any charts that show the ratio of scotch yoke flywheel size to linear stroke length?