CEM120 gear mesh adjustment calculations

[turboscrew]

Since the RA axis of my CEM120 jammed in last tests, I tried another approach.
I wonder if there's any sense to it.
I was thinking to set it just tight enough to start moving the full load.
I think the acceleration to slew is the highest load to the worm gear.
The gear should barely take that.

The mount is rated 50 kg excluding counterweights.
I estimated that 100 kg 50 cm away from the center of rotation is about
the moment of inertia. 100 kg * ((0.5 m)^2) = 25 kg/m^2

The full slewing speed is 1000 x tracking speed = 4.2°/s
I estimated that the slew speed is achieved in half a second.

The impulse moment is then moment of inertia * angular velocity and its time derivative (assuming linear acceleration) is (25 kg/m^2 * 4.2°/s) / 0.5 s
and that equals the torque needed.

With about 50 cm long counterweight pole, that would equal to about 11 kg weight at the end of the pole.

So I put the pole on (it should balance the mount itself) and added a 10 kg counterweight at the end. Then I adjusted the mesh such that it could barely hold the system in place with the counterweight pole in vertical position.
(The mount doesn't have to be able to move it, but just hold it without giving in.)

Am I totally wrong here, or could that be a start for finer adjustments?

[notFritzArgelander]

I'm not sure. I'm more confident about optics than mechanics.... A lot depends on how the mount is designed in detail. Could you run this by the product support folks?

[SkyHiker]

If the slew speed is reached within 0.5 seconds that indicates to me that that should not be an issue. OnStep takes about 5 seconds to gradually reach slew speed. IOptron could have done the same if it were a problem.

What is a much bigger problem is when the gear binds up due to the main cog wheel being not round. This is what happens with th 6" cog wheel of the G11 a lot. This is why the RA gear always has some play, in th classic configuration, to leave some space for non roundness. When I tried to get it nice and tight it was binding up all the time. After implementing my spring loaded worms the problem was gone.

All iOptron mounts come with spring loaded worms. Have you checked their setting, and played with the spring stiffness? Is there a limit setting that is too tight?

I would test these settings thoroughly in the daytime to get a feeling for what matters. It really should not be a big issue and easy to solve knce you know your mount.

[turboscrew]

It's that spring-loaded pin that presses the worm against the wheel that I've tried to adjust.
Too tight is when the teeth go too far and jams the worm. Too loose is when the spring allows the teeth to jump over.
Just loose is too much free play.

I sent a question about any procedure to "reset" the pin tension adjustment to iOptron, but I doubt...

[SkyHiker]

It's that spring-loaded pin that presses the worm against the wheel that I've tried to adjust.
Too tight is when the teeth go too far and jams the worm. Too loose is when the spring allows the teeth to jump over.
Just loose is too much free play.

I sent a question about any procedure to "reset" the pin tension adjustment to iOptron, but I doubt...

I heard similar comments about how a high spring setting can cause too much friction. My DIY SLW has a rather weak setting and it has to slide into the groove as the worm starts turning. When it is not turning and I move the OTA a bit I can see the worm back off of the cog wheel. At first I did not like that at all - who wants a mount with backlash like symptoms - until I realised that it works fine when in operational mode. I don't know what you consider "just loose" but if it is like what I described it might just work fine when the mount is slewing or tracking. Clearly, it should work somewhere in between too loose and too tight or else all customers would have problems so it's a matter of finding the sweet spot. Of course I don't know exactly what problems you experienced so I could be way off the mark.

[turboscrew]

It's that spring-loaded pin that presses the worm against the wheel that I've tried to adjust.
Too tight is when the teeth go too far and jams the worm. Too loose is when the spring allows the teeth to jump over.
Just loose is too much free play.

I sent a question about any procedure to "reset" the pin tension adjustment to iOptron, but I doubt...

I heard similar comments about how a high spring setting can cause too much friction. My DIY SLW has a rather weak setting and it has to slide into the groove as the worm starts turning. When it is not turning and I move the OTA a bit I can see the worm back off of the cog wheel. At first I did not like that at all - who wants a mount with backlash like symptoms - until I realised that it works fine when in operational mode. I don't know what you consider "just loose" but if it is like what I described it might just work fine when the mount is slewing or tracking. Clearly, it should work somewhere in between too loose and too tight or else all customers would have problems so it's a matter of finding the sweet spot. Of course I don't know exactly what problems you experienced so I could be way off the mark.

I put that in an other posting but https://www.dropbox.com/s/wnkw06p4esm46 ... 7.mp4?dl=0
It happened slewing from Deneb to Hamal.

In the test day (23.10.2021) at about 9:00 PM (-2°C)

Code: Select all

star            AZ              ALT
Deneb       +218°18'16"     +71°16'04"
Hamal       +100°06'0"      +32°24'38"

[turboscrew]

I wonder if my thinking in the original posting is valid, and I wonder if there are numbers that seem to be decades off or something.

The idea was basically, that the gear mesh should be tight enough for the acceleration from zero to full slewing speed, and the load is the moment of inertia. And I tried to produce the same "peak stress" statically with equal torque with a counterweight. I considered the maximum load capacity (50 kg + counterweights).

[SkyHiker]

About the math, your moment of inertia is right, 25 kg m^2 (not kg / m^2). For the acceleration of 4.2 degrees per second per 0.5 seconds, in radians that is (4.2*pi/180)/0.5 so the moment becomes 25 times that or 3.67 kg m^2/s^2 or 3.67 Nm. So putting a mass at 0.5 m in a vertical position to achieve that torque the mass would be two times that, or 7.33 kg. Not quite the 10 kg that you have but close enough for a test. Of course when you keep moving the moment changes significantly after not too long because gravity kicks in and provides an increasing torque. HTH

[turboscrew]

About the math, your moment of inertia is right, 25 kg m^2 (not kg / m^2). For the acceleration of 4.2 degrees per second per 0.5 seconds, in radians that is (4.2*pi/180)/0.5 so the moment becomes 25 times that or 3.67 kg m^2/s^2 or 3.67 Nm. So putting a mass at 0.5 m in a vertical position to achieve that torque the mass would be two times that, or 7.33 kg. Not quite the 10 kg that you have but close enough for a test. Of course when you keep moving the moment changes significantly after not too long because gravity kicks in and provides an increasing torque. HTH

Thanks. Somehow I missed converting to radians.
I wouldn't test running the worm gear with 10 kg unbalanced load. It was just to figure out if it could take the moment of inertia at the speeding. I think I'll correct my calculations and try again. There is a good chance that the gear it too tight now. I did the calculations assuming 50 kg payload.
Thanks a lot again.

[turboscrew]

Argh. My mistake is bigger: 3.67 Nm is 3.67 N with 1 meter pole, that's 7.33 newtons with 0.5 meter pole.
1 kg creates about 10N force on earth, so that would be 0.733 kg with 0.5 meter pole?
That would equal to 1 kg with about 37 cm pole?
Could it be that small?

[SkyHiker]

Argh. My mistake is bigger: 3.67 Nm is 3.67 N with 1 meter pole, that's 7.33 newtons with 0.5 meter pole.
1 kg creates about 10N force on earth, so that would be 0.733 kg with 0.5 meter pole?
That would equal to 1 kg with about 37 cm pole?
Could it be that small?

If you have the pole vertically then for an infinitesimally small displacement where the counterweight moves horizontally, gravity plays no role so it simply applies to a 7.33 kg weight, no factor 1/9.81 needed. I suppose you would have to figure out how gravity ties into this for non-infinitesimal movements.

[turboscrew]

Argh. My mistake is bigger: 3.67 Nm is 3.67 N with 1 meter pole, that's 7.33 newtons with 0.5 meter pole.
1 kg creates about 10N force on earth, so that would be 0.733 kg with 0.5 meter pole?
That would equal to 1 kg with about 37 cm pole?
Could it be that small?

If you have the pole vertically then for an infinitesimally small displacement where the counterweight moves horizontally, gravity plays no role so it simply applies to a 7.33 kg weight, no factor 1/9.81 needed. I suppose you would have to figure out how gravity ties into this for non-infinitesimal movements.

The pole is supposed to be horizontally to statically create the torque that "simulates" the moment of inertia to the gear.

[turboscrew]

If someone else wants to try this, DON'T!
It won't work. The difference between dynamic and static friction alone messes things up.