My current project requires high torque and positional control to move a lever across a 110 degree range. RPMs aren't high, 55RPM(+/-10) at the shaft. I started using a NEMA 34 12Nm closed loop stepper on a 5:1 chain drive (so 60Nm at the driven gear). There is one point where the drive just barely doesn't have the torque and needs a little boost to get over the hump. Looking at alternatives, it seems the predominant options are move up a stepper size to a NEMA 42 with 20Nm, or switch to AC servos that can maintain torque at high RPMs and put a gear reducer on.
Trying to keep costs to the $200-$300 range, so I've been looking at Clearpath servos as well as options on aliexpress. Any advice or suggestions are welcome!
Not enough information to be able to offer advice, except to suggest a larger gear ratio. If you disclosed the stepper data and the controller data and the power supply data, some other suggestions may be useful.
Paul
Which shaft, motor or lever? 55RPM @ 60Nm is ~346 Watts. The pullout torque of that motor @ 55RPM is about 6.5Nm.
How much force is required at the end of the unseen lever? What is the length of the lever and what is attached to it?
Unsure how much force is required. The curent motor puts out 12Nm of holding torque, so probably only half that while running, and it gets held up at one point in the cycle. Here's a picture of how the driven gear is attached to the lever and shaft. The shaft operates a press.
To select a motor, you have to know the maximum force required during the lever swing, what the cycle time is (one end of swing to the other and back to start position) and duty cycle (working time vs standstill time). You can estimate the force by pulling the lever through a cycle with a spring scale, like a fish weighing scale, attached to the working end of the lever..
I've ordered a spring scale to measure the force, but I can assume the torque required is more than ~50Nm or so. The swing is 110 degrees and I need a 100% duty cycle under ambient conditions (could use active cooling).
Don't buy an expensive motor based on assumptions, find out the force needed to move the lever (of unknown length) through the full swing, multiplying that by the lever length from pivot point to the point of force application will give you torque in pound feet or kgf meters. Then, knowing the speed will give you mechanical power (Watts) required, you can spec a motor and gear train.
EDIT: When pulling the lever with the scale, be sure to keep the pull direction at right angle (90 degrees) to the lever.
The dynamic torque of a stepper at speed can be a lot below the holding torque, often a lot less than 50%
of the holding torque. Your 12Nm motor could be less than 5Nm at speed, so <25Nm after the
chain reduction.
A servomotor or DC motor + encoder is probably the way to go, torque doesn't drop with speed, and
you can add reduction gearing before the chain. 5500rpm to 55rpm is 100:1 reduction in speed, so
1:100 increase in torque.
If you want 55rpm at 80Nm, thats at least 440W mechanical power(*), suggesting something like 1kW
electrical power allowing for motor and gearing losses. This is serious amounts of power.
Your stepper isn't even remotely this power level anyway...
(*) angular velocity is approx rpm/10 radians/second, power = torque x angular velocity (SI units
throughout). Thus 55/10 x 80 = 440 watts
Thanks for the tips. I'll update once I have the torque requirements. Printing up a jig for it now.
As for motor selection, do you have any suggestions on models? I've been looking at DMM and clearpath servos, and ways to implement a motor + encoder setup.
I am an applications engineer at Teknic and wanted to offer my assistance.
We would be happy to perform a motor sizing simulation to determine the torque needed for your application. In order to do so, we will need some additional information (such as mechanical information, move time goals, etc.).
Based off the findings of the simulation, we can determine whether there is a ClearPath solution that will fit your needs (there likely is).
One feature of ClearPath that you may find specifically helpful is the built-in diagnostic oscilloscope. The oscilloscope allows you to measure and monitor a number of motor parameters including the real-time torque usage throughout your moves. With this torque information you can determine whether a smaller ClearPath motor can work for your application.
ClearPath also has a 90 day return period. So if you do find that a smaller (less expensive) motor would work in your application, you can return your ClearPath for a full refund.
Please give us a call (585-784-7454) or use the "Contact Us" form on Teknic's website: https://www.teknic.com/contact/. We'll gladly assist.
It took 80N with a .45 meter lever arm, so loking at 36Nm requirement acting on the driven shaft. With the 5:1 reduction, I need at least 7.2Nm on the lead shaft. Probably could put a 30:1 gearbox on the motor since the lead shaft speed is only ~60 RPM
Does anyone have experience with "DIY" servos running on something like the ODrive https://odriverobotics.com/ or I think trinamic makes a smiliar product that allows you to hook a BLDC to an encoder an run it off of step/dir?
So the 55rpm is before the 5:1 chain drive, the 36Nm is after it?
Yes. The lead sprocket (which is attached to the motor or gearbox shaft, depending on the configuration) is what spins at 55 RPM. The follower sprocket is attached to the press shaft after the 5:1 reduction. The .45 meter lever is also attached to the press shaft. Sorry for the confusion.
so 100W motor is probably enough.
Wow that's way smaller than I thought. Definitely opens the door some different drive setups.
Clearly a reloading bench!
I have no idea what you're talking about. My reloading bench sank in a boating accident in international waters.
I thought about it, but I am not as familiar with pneumatics. I also wanted something that provides more control and can easily be adapted to other presses, while keeping noise down. With this current setup, I can design a new follower sprocket to fit onto different presses and it will just work.
I've never worked with pneumatic or hydraulic systems though, so it's entirely possible they would also meet those specs.
