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jaydigs (Mechanical) (OP)
25 Sep 09 14:15
I've been tasked to design an oscillator for a fan unit and I must confess this is really not my forte.  I have the bar linkage worked out but I need help selecting a linear actuator.  Basic requirements are to rotate a head that weights approximately 200lbs.  I'm guessing the speed of the actuator to be around .5-1.0 in/sec.  In a perfect world the duty cycle would be 100% but I know I'm going to have to let it rest some.  I've found some 1500lb capacity, .5in/sec, 36VDC actuators for around $200.  Does this seem reasonable?  Are there other parameters I need to be considering in my selection?  Thanks!
Helpful Member!  itsmoked (Electrical)
25 Sep 09 18:39
Very hard for us to say with the little info presented.

Most would NOT consider even doing it this way.  The standard for oscillating something like this continuously is just a gear motor with an eccentric wheel on it's output shaft.

This gives a steam engine like rotary to linear result.  This is far more robust than what you're contemplating.  And by far more I mean like 30 or 40 times more longevity. And, at probably 1/10th the initial cost.  Meaning over a few years your method is going to cost probably 50X more.

Use an off--the-shelf gear motor.
Put a disk on the shaft with a pin somewhere near the edge.
Put a bearing on the pin.
Get two linear slide blocks.
Run a shaft to your fan thru the two slide blocks.
Put a rod between the bearing on the disk and the driven end of the shaft.

Speed control the gear motor if you want.

Here's what I'm talking about:
http://tinyurl.com/yb2x6pj

Keith Cress
kcress - http://www.flaminsystems.com

jaydigs (Mechanical) (OP)
26 Sep 09 15:38
I don't know why this didn't occur to me earlier, it's so simple.  Thanks for the tip and I appreciate the .pdf.
itsmoked (Electrical)
26 Sep 09 16:05
You bet!

You can find gear motors of all sizes.  I'd probably set up the linear aspect and then use a spring fish scale to find the force required to pull the linear as it moves the fan.  Then using my hand I would estimate the 'push' force. If it was about the same or less I would stick with the scale value. Otherwise alter the force estimate as you feel you should.

Once you have that number triple it.

Use that number for the force needed out at the pin in the disk.  Calculate the torque needed from the motor.

Also run, by hand, the linear shaft to determine how fast you want the fan to oscillate.  Use this result to pick the gear ratio of your gear motor.  Once you have the gear ratio and the required torque you have all you need for picking a gear motor.  Note, if at all possible you want the motor to be just a fixed speed gear motor at the correct speed.

Hook it all up and see..

One warning..  These type systems have VERY dangerous pinch points.  If this thing is anywhere a human can put their hand somewhere, you MUST include guards.

Keith Cress
kcress - http://www.flaminsystems.com

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