Stepper motor torque

I want to use the following stepper motor to build a device which can rotate a tray of lab vials. How do I know if the stepper motor I plan on using is powerful enough for this task

stepper motor:Stepper Motor - 68 oz.in (400 steps/rev) - ROB-10846 - SparkFun Electronics

Hi jkarimi

at first, you must realize, how large will be youre tray. What about weight
(roughly)? On the sparkfun site you posted there are features about the stepper motor. You can read the necessary attribute Holding Torque: 48N.cm. It means if you fix a small rod by the right angle to the turning rod of the stepper motor, it can hold 48N (4,8 Kg) weight on the end of this fix rod. The motor must be conected to the source of course. So you can imagine, how strong is youre mentioned motor. Also you will need special electronic circuit for driveing the motor.There are several ways, how to connect youre motor to the driveing electronic circuit. Each of them will give you another holding Torque and features. Learn something about it on this site : Stepper motor - Wikipedia

I hope, my post will help you ;).

Martin Juráš, Czech Republic

I wanted the tray and overall design to look like this
http://www.phenixresearch.com/products/s-5500-mini-labroller.asp

So if the motor can do 48 N*cm wouldn't I have to calculate the displacement each step makes and divide it by the torque to get my force for each step and then verify that my force for each step is equal to or greater than the weight of the tray (and its components)? I guess I just don't understand how to interpret torque or its units when it comes to real world applications

I draw simple image. Im confident, that youre device will be running properly. If you don't sure, just do one simple test. Take a rod and place there a second rod 1cm long by right angle. Hang up here some weight, which has 4,8 kg and compare the intensity with youre tray. :wink:

If I were building that I’d use an RC servo. A typical servo will be able to swing your tray 90 degrees; if you need a wider range of motion you can look into “sail winch” servos which are capable of 360 or 720 degrees of motion.

See http://www.youtube.com/watch?v=GeUKNmzcZV4 for an example.

I don't know anything about rc servos; what advantages do they present that a stepper doesn't? the rotation can be programmed into the stepper so I achieve any angle I wish and by incorporating delays I can adjust rotation speed

jkarimi:
I don't know anything about rc servos; what advantages do they present that a stepper doesn't? the rotation can be programmed into the stepper so I achieve any angle I wish and by incorporating delays I can adjust rotation speed

The primary advantages of a servo is that it has its driver built in (a stepper will require an external driver, such as the EasyStepper or Pololu A4988) and contains an internal mechanism so it can keep track of its current position. The primary limitation of a servo is that it cannot perform continuous rotation and still keep track of its position.

Either the stepper or servo can have its rate of rotation adjusted. With a stepper its a matter of what rate you tell it to make steps, and for a servo you just pause at intermediate angles (i.e. when commanding it to turn to 90 degrees you just tell it to stop at 1 degree, 2 degrees, etc.).

You cannot tell a stepper to rotate 90 degrees per se; you can only tell it to "take 50 steps clockwise". If your tray gets knocked or its unbalanced it's quite possible that it will eventually flip itself over as it keeps swaying those 50 steps back and forth; you need to add some type of extra physical stop to prevent that.