I had some nema 17 style stepper motors from an old prusia i3 and tried to use them to automate my blinds but noticed that they weren't powerful enough for my blinds. I used a basic newton meter to check the amount of weight i need to apply to move the blinds and it's roughly 25-30newtons. My stepper motor looks like it can only pull around 15-20 newtons. Is there anything i can do to increase the torque of the stepper motor(i'm using a L293D motor shield on an uno)? And i was thinking about adding gears but wasn't sure about the best type or the needed ratio so it doesn't reliably skip any steps, speed isn't really important? Any advice you have would be really helpful, thanks!
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\$\begingroup\$ Are you at the maximum voltage rating of the motor? Depending on how much you care about reliability/lifetime you may be able to overvolt a little bit in exchange for potentially reduced lifetime. After that, you either need reduction gears or a new stepper motor. \$\endgroup\$– uint128_tCommented Jul 8, 2017 at 15:55
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\$\begingroup\$ Rule for life- don't use stepper motor. Use brushless. \$\endgroup\$– user76844Commented Jul 8, 2017 at 16:00
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1\$\begingroup\$ You need > 2:1 pulley or a counterweight to reduce load torque and if you use BLDC then you need end stop detection. \$\endgroup\$– D.A.S.Commented Jul 8, 2017 at 16:33
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Note that the amount of force (Newtons) available from a motor depends on how far away from the center of rotation you are, because torque is the product of force and distance. (We measure torque in Newton-meters or pound-feet or somesuch unit.)
A gearbox or pulley will give you close to the exact ratio of torque increase; a pinion with 12 teeth driving a sprocket with 60 teeth will give you about 5:1 torque increase, with the corresponding 1:5 decrease in rotational speed. Same thing for a belt or any other kind of rigid ratio.
Regarding "not skipping steps," you will want to add limit switches no matter what. Even with brushless motors, as suggested in the comments, you need to know how far to move, and unless the BLDC has an absolute encoder attached, you can't guarantee the exact stop position without limit switches.
Stepper motors are reasonably tolerant to light over-volting, or even over-current-ing, because they have very few moving parts, and no brushes to wear out. Also, if you only run them for a short amount of time, there won't be as much heat build-up, and thus you can run them at higher current for a shorter duty cycle. If you can crank up the voltage, and turn the motor driver up to maximum current, that might be enough. Or not -- the L293D is not a high-performance driver.
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1\$\begingroup\$ One point ...a stepper motor is a form of BLDC. The stepper motor is very very voltage tolerant, the OP could simply raise the drive voltage for a few ms on the lead edge of each step to improve the pull out torque. Overall though it would be much more reliable to put in a gearbox to get the torque needed. \$\endgroup\$ Commented Jul 8, 2017 at 19:25
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\$\begingroup\$ Rotational motors produce torque not force \$\endgroup\$– user16222Commented Jul 8, 2017 at 20:32
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1\$\begingroup\$ @JonRB: Yes, that's exactly what I said; force has to be measured at a distance from the center of rotation, and is derived from torque and distance. \$\endgroup\$ Commented Jul 10, 2017 at 3:03
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1\$\begingroup\$ @Jack Creasey: The problem is that the L293D limits the current at 1200 mA even for the first millisecond, so there is a practical upper limit on the voltage. You also have to be careful to not exceed the winding insulation voltage! \$\endgroup\$ Commented Jul 10, 2017 at 3:05
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1\$\begingroup\$ @JonWatte True, using the L293D limits current, but the current (LR charging rate) is altered by changing the aiming voltage. This means you get to the current limit quicker, which increases the pullout torque. \$\endgroup\$ Commented Jul 10, 2017 at 4:15