The high cost of commercially available, motorized 2 inch ball valves (>200 USD) prompted me to build my own actuator (and in the process add some IoT functionality).
My 2 inch PVC ball valves can be turned with a max torque of 7 lb-ft (measured). Ball valves open/close with a quarter turn, so I don’t need micro stepping, high turn speed or more than 90 degrees movement. I can live with 5 seconds or more to turn 90 degrees. For design purposes, I’m considering double the measured torque (14 lb-ft).
A $35 USD Stepper (Pololu Item 2256 : 0.8A, 4.3V, weighing 80g) will provide 0.3kg-cm (20 lb-ft) of holding torque.
A $10 USD Servo (Pololu Item 1046 : 0.8A, 4.8V, weighing 17g) will provide 2kg-cm (134 lb-ft) of stall torque.
I understand the difference between holding and stall torque, but in this use case (ball valve actuator), there’s no need for holding torque (the valve doesn’t move unless actuated upon) nor for surges in required torque (no changes in load or terrain).
So if I understand it correctly, a Servo would be the better choice?
There are pros and cons of using either one, but a servo would probably be easier since the built-in control electronics simplify the position control without having to use additional limit switches or keep track of the position in your software.
For servos (which generally use a brushed DC motor inside), I recommend looking for one with a stall torque 4 times your required torque.
For stepper motors, I recommend looking for a pull-out torque curve to get a better idea of the torque capabilities while moving at various step rates since it will be very different than the holding torque. You can typically find a pull-out torque curve in the manufacturer datasheet, which can be found under the “Resources” tab of our stepper motor product pages.
Aside from the Torxis servos, we do not carry any servos or stepper motors that powerful, and it might be difficult to find some that have enough power to comfortably handle 7 lb-ft directly (especially while trying to stay within a tight budget). So you might consider using some additional gearing, but please note that gearing the output down would require proportionally more movement to achieve the same quarter-turn. For example, if you geared down the output 2:1, not only would it double the torque, but the output would turn half as far (and at half the speed).
Thanks! Your information helps me in further understanding servo vs stepper. For this project, I think I’ll have to go with a commercial brass valve (PVC’s will deteriorate over time and require even more torque to actuate) to ensure the gearing, limit switches, etc. last as long as the brass valve. I’ll add the controller and DC power myself.