Servo motor question

Can you provide more details on exactly what you want to do?  Pictures of the Cat would be nice to get an idea of how much weight is involved.  Are there treads or just free-rolling wheels? 

Are you trying to just go fwd and backward, or some idea more interesting pattern?  I recall one animation with a farm tractor going back and forth in rows and then returning to the original point after after the last row was harvested.

Are you sure you want a servo motor?  When I think of servo motor (like used in R/C planes), these only rotate maybe 1/2 to 3/4 of a turn but have lots of torque from gearing.  I'd be thinking more of a standard DC gearmotor which has full circle turning following with some rack-and-pinion or other rotary-to-linear transmission.

I think we need to know exactly what you want it to do before recommending a solution.  If you're somewhat up to speed with software programming, you could do something tricky with an Arduino and one of the stepper motors that you can get on eBay for $5-6.  The Arduino and motor would be less than $20, and you can do whatever crazy pattern of movement you like.

First of all, thanks for your response and questions.  My idea was to have the tractor move back and forth.  As you can see, it has treads, which turn easily.  Since this is my first attempt with animation, I wanted to keep it relatively simple.  The tractor weighs 6 oz.  I'm not locked into a servo, but like I indicated, I am totally new with this.  My idea is to attach whatever device (rod?) to the bottom of the tractor.  But, I am open to any and all suggestions.  As you requested, there are photos(3) attached. I hope that I have answered all your questions.  Thanks again.  

In that case, I think Stan's idea of a simple gear motor would probably do the trick. 

Thanks for the reply.  First time out should be simple.  As I get comfortable with this, I can think of several applications for the future.

For a basic back-and-forth reversing animation, I'd probably go with a right-angle DC gear-motor driving a rack.  The rack would drag the Cat with a pin(s).  If the tractor needs to be held downward to engage the treads (rather than slip on the surface), you could use a magnet in the tractor being pulled by a magnet (instead of a pin) below the surface.  The magnet method would pull the treads against the surface and eliminate the need for a slot on the surface.

Here's a short video with one way to do this. The DC gear-motor reverses direction by applying the oppositive polarity voltage.  So a limit switches or other end-of-travel detector activated by the moving rack could reverse the motor so the Cat goes fwd and back as long as power is applied.

I got the right-angle gear-motor assemblies surplus for a few dollars each but years ago so I realize it doesn't help.  The rack and pinion (gears) parts are readily available on eBay (or various DIY robotic stores).  A 5" rack might be $1-2 and the bag of mating gears (worm, crown, etc.) in the photo I think was less than $5.

Here's a 6V one for less than $5: DC 6V Mini Gear Motor

I like the idea of the magnet.  Many thanks to you and all who responded to this rookie's question.

I searched and put it up again.

I dug up this animation, from 2003, also using a DC gear-motor that only rotates in one direction but performs back-and-forth motion.  I think it speaks for itself and should be fairly easy to replicate.  What's interesting about this is the motion slows down when reversing direction which is required to animate a swing motion.  This was a proof-of-concept video before I built the actual animation for a friend of mine which had children on a swing set.  It uses the magnet technique which was handy as a moving-pin protruding from the ground into the child's posterior would have been unseemly.

That looks great Stan, very realistic motion!

I like your idea of using a servo to control movement. Gearboxes are OK and I've used them a lot, but they are so last year's technology. This year, I authored an extensive article on the use of servos in layouts. I can provide details and would be happy to offer my assistance in this area.

Bob, I am definitely interested in reading your article, and appreciate the offer of assistance.  Right now I'm on a hiatus to get my knee back in shape so we can do some traveling.  I will pick back up later this summer.

Let me know when you are ready to go and I'll provide the info you will need.

Stan- the animation in the video is exactly what we use. As a matter of fact, the same design has been used for years & years in model railroading. We could go to rack & pinions, or servos and use complex circuitry, reversing switches etc. BUT I have found that simpler the design, the more reliable. Always been a KISS kind of person... Less to go wrong.

In my humble opinion.