Need help from a top notch gearhead

[Jags]

I am trying to build a barge with a boom on it. I want a motor to be in charge of pulling up about 6-7lbs. of weight and hold for about 5-10 sec. and then release back into the water automatically and continuing the process.w/o stopping the process.  If someone can help me what size and type dc motor and other accessories will I need. To make this automatically pull and release after a short amount of seconds. Pretty simple process just dont know much about robotics. Is there any timers needed to allow motor to run for a particular amount of time and then the release. Can or is there a motor out there that will run and then stop and spin freely letting the object spin back into the water. There maybe one pulley involved. But letting me know the type motor and sz. will help tremendously.  I know I am asking alot but 2nd question.  Where can I find a site that will allow me to purchase a geared track that a motor can travel on. Basically a site with different accessories for gears and tracks. Any help will be much appreciated. Thanks

[ArcMan]

Are you talking about a boom crane?  If so, you will need a gearmotor to drive a spindle that winds up the crane cable (or string).  You will need a clutch between the gearmotor and spindle shaft if you want it to freewheel back down.
I buy alot of mechanical components at Stock Drive (http://www.sdp-si.com).
There's a million places that sell gearmotors, like http://www.robotmarketplace.com/products/motors_main.html, for example.

What's your mechanical fab experience?  This could be quite a challenge for you.

[ArcMan]

So the torque you will need could be determined by the following:

T = (7 lbs.)*(Rspool)*(16 oz./1 lb.)
   where Rspool is the maximum wire spool radius (when all the wire is wound up on it)

For example, let's say your max. spool radius is 0.5"
T = 7*0.5*16 = 56 oz.in.

You'll also need to select the right motor speed.  Let's say you want to raise the weight 12" in 3 seconds (Vl = 4" / sec.).  The motor speed you'll need can be calculated by:

RPM = (Vl / (Pi * Dspool)) * 60
   where Dspool is the maximum spool diameter

So RPM = (4 / (3.1415 * 1)) * 60 = 76 RPM

[Jags]

ArcMan Thanks, A few more questions. Your info. will help considerable.   Next. What is used to make the motor stop pulling on the weight.  Or will it keep pulling until clutch kicks in. Concerning clutch is there a sz. on it or does it come complete with motor/cultch assy. Not sure of clutch assy. How it connects  Main idea is to pull weight and release when gets to top of boom. Is there something needed to make this continous without sending signal to motor. I would like this to be continous until motor is turned off. Again thank for your help.

[ArcMan]

My first thought was to use a microcontroller to control the motor and an electrically actuated clutch based on sensor input.  That's my background and what I'm most comfortable with.  But a clever mechanical design using a mechanically actuated clutch could accomplish your goal, too.