torque of motor on coarse gear

aand74

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Dec 2005
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Deinze
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I have an application where a servomotor is driving a a big cylinder.
After the servomotor there is a planetary gearbox with reduction ratio of 20.
And after the gearbox there is a gear with 20 teeth on a diameter of +- 8 cm that is driving a larger gear on the cylinder with 60 teeth on a diameter of +-25cm.
The inertia of the load compared to the inertia of the motor is about 300:1.
This seems huge but once started, the velocity setpoint always stays the same.
Because we hear some noise and vibration in the gearbox, I have made traces of the motor torque. I see peaks in the motor torque.
When I calculate back the motor speed to the speed a the final gear (60 teeth on cylinder), I get the result that a that motor speed (120rpm) we should have a new tooth of the final gear switch in every 500ms ((120 rpm * 60 teeth)/(60 sec * 60 total reduction ratio)).
I would expect the peaks to happen at intervals that are a multiple of 500ms.
But this isn't exactly the case : for peak to peak times I have f.i. :
1639ms,939ms,768ms,892ms,1191ms,844ms,929ms,725ms.
These are times of consequetive peaks.
Maybe I am fixing too much on the fixed time of 500ms while in reality it is more a continuously process of the teeth getting into each other?
I am also doubting that maybe the distance between the big coarse gear (60 teeth) and the small coarse gear (20 teeth) is not correct enough. What one could expect if the distance between these two gears is too small or too big?
I hope some guys with experience in motion and gearboxes could help me in this difficult problem.
 
I guess I am trying to figure out if there is an issue, or you just think there is an issue. Torque peaks will depend on many factors, including what the load is doing and on tuning. Torque graphs are not normally smooth, even though you might think they should be. As long as your desired speed regulation is good, don't worry about it. Also, I really don't think you have a 300:1 inertia ratio. The gearing will greatly reduced the reflected ratio.
 
I have attached a picture of a trace with the cylinder that has high torque peaks (the purple curve) and a cylinder that has 'normal' torque behavior.
The peaks of the high torque reach 2.5 x the nominal torque, the other curve only goes to 0.5 x the nominal torque.
Both are just driving the cylinder without any other load or friction.

torque.jpg
 
Lets look at some basics. You have an overall ratio of 60:1. 20:1 in the planetary gearbox and 3:1 in your final gearing (60/20=3). 3 x 20 = 60.

Inertia seen at the motor for acceleration varies with the square of the speed ratio. So, your motor will see 1/3600 times the accelerating/decelerating torque at the cylinder.

https://www.engineersedge.com/motors/gear_drive_system.htm

If your gear center distance is wrong you should see abnormal wear patterns on the final gears. Whether or not that will produce speed fluctuations depends on if they are spur gears or helical and how bad the error is. It is fairly simple to calculate the correct center distance and check it.

My guess is that you have an issue in the planetary gear box.
 
The inertia ratio will indeed be less, I did not implement the square of the reduction ratio, only the single reduction ratio.
The torque peaks can be seen on twelve similar print cilinders, but on some it is more extreme like in the picture I posted. On the 'bad' cylinder we already exchanged the complete gearbox and final gears and the torque peaks stayed the same.
That is what drives me to the distance between the two final gears (20 teeth to 60 teeth).
These are spur gears, and are rather coarse (diameter of the 60 teeth wheel is +- 25 cm).
The machine builder told me when the spacing is too small, you get continuous higher torque demand, but he had no experience when the spacing is too big.
In the trace it looks like after a torque peak the gears are not touching each other, or at least the motor has to deliver no or few torque for a couple of teeth, after that there is again a torque peak, and so on. This construction has been running for years, but now we upgraded the drives to new ones, and because the initial settings for the speed controller were rather dynamic, this 'problem' with torque peaks came to the front.
 

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