Sensible dome rotation speed?

[Rusted]

Hi,

I was going to rotate my 3m/10' plywood dome with a 4" Urethane industrial wheel. ?‍♂️

This would be pressed up under the base ring and driven using a simple hand crank.

[See mock-up image below]

I'm trying to avoid dragging the dome around by its ribs.

Which is hard work at times and aggravating my "tennis elbow."

 

Then I remembered I had a smallish, 56rpm mains, gearbox motor.

The 4" friction wheel will rub against the 10' ring providing a 1:30 reduction.

So the dome will probably rotate at 56/30 = roughly 2rpm. 

Isn't that a bit too much like a fairground ride? 

Any thoughts on a sensible speed for "slewing" a dome between objects? 

Thanks

 

[sloz1664]

That is a bit too quick in my opinion. When I automated my dome the consensus was for one rotation every 45 - 60 seconds or roughly 1 rpm. 

Steve

[lenscap]

So at 2 rpm the perimeter would be moving at about 1 ft/sec.  Not outrageously fast?

[tony210]

I think unless you are planning to use an encoder that's fine but for slaving your dome with an encoder slow is best and leads to less mistakes and inaccuracy - mine slews about 90 seconds for a whole rotation but slaves well - Tony

[sloz1664]

5 hours ago, lenscap said:

So at 2 rpm the perimeter would be moving at about 1 ft/sec.  Not outrageously fast?

It may not look outrageously fast but on a sudden stop inertia kicks in, which could cause damage to the dome.

Steve

[JamesF]

48 minutes ago, sloz1664 said:

It may not look outrageously fast but on a sudden stop inertia kicks in, which could cause damage to the dome.

Steve

Or allow the dome to slide against the drive wheel and wear flats on it.

My recollection from the OP's build thread is that it's a pretty hefty dome.  If it isn't possible to vary the rotation speed near the start and end of the rotation I  think I'd be inclined to keep it fairly slow to start with.  Perhaps fix a gear wheel to the face of the friction wheel and then experiment a bit with different gear ratios by trying a few different gears on the motor shaft?  Using bevel gears would allow the motor to be mounted with a vertical axis, or with the axis parallel to the wall, which might be neat.

James

[Rusted]

13 hours ago, lenscap said:

So at 2 rpm the perimeter would be moving at about 1 ft/sec.  Not outrageously fast?

It is when you have the reaction times of a septuagenarian.

Thank you for giving me an alternative way of looking at it. This was a valuable insight.

I was imagining whole turns and fractions rather than linear units passing a given point.

 

When slewing, my mounting takes two minutes to turn 90° In RA = 0.75° per second.

This does seem very slow when moving over long arcs but still has to be watched carefully when clearances are so tight.

I should have built a bigger dome! 

 

The dome is indeed heavy and will not like being accelerated and then decelerated with a simple, electrical motor switch. 

I'll postpone any thoughts on tracking and slaving for another day.

 

My immediate decision is to play it safe and have a manual [probably bicycle] crank on the drive wheel's axle.

This allows me to choose my own speed of rotation, acceleration/deceleration and direction.

I can iron out any problems with a friction drive before adding a motor in the future.
 

More importantly, a crank should provide a considerable reduction in effort from physically dragging the dome around.

It also provides the potential for a built-in chainwheel for gear reductions if they should prove necessary.

It might also be handy[?] to have a lowered crank rather than right up at head height.

Thanks again to all who responded. It is always valuable to have different viewpoints and ideas.

 

[sloz1664]

I you want to automate your Dome at some point in the future, have a look at my DIY automation project, for any pointers:-

 

Steve

[Gina]

I've just looked at that thread and replied, Steve.  Great job!!

[Rusted]

3 hours ago, sloz1664 said:

I you want to automate your Dome at some point in the future, have a look at my DIY automation project, for any pointers:-

 

Steve

Hi Steve,

Neat workmanship and creativity!

I seriously doubt my heavy dome would move without a very big motor.

Linear torque required to start it moving is between 8 and 15kg! Drops back to about 5kg once under way.

It was fine when it was a bare skeleton. Now it weighs a ton with the 4mm birch ply cladding and shutters! 

[Rusted]

Progress with the 4" friction roller, bicycle crank and bottom bracket bearing:

I removed the needle roller bearing from the roller to provide more room.

Then I cut the thick end of another single bicycle crank and turned it down in the lathe to fit tightly in the wheel bore.

I'll use some Loctite to secure the turned adapter in the roller. 

Now I just need to make some temporary, sprung supports for the bottom bracket bearing assembly to give it a test.

I think it is too risky to remove the nearest dome support roller and rely completely on the friction roller instead.

No expense so far except for a couple of hours in the workshop on a heavily overcast day. 

[Peter Drew]

I'm by no means a pessimist but I think it's likely that the urethane wheel won't have enough friction to turn the dome reliably. Our drive wheel is metal with a rubber rim and the dome ring is clear all round on its upper surface so we can use a spring loaded pinch wheel to adjust the amount of grip. Our motor ramps up or down for start and stop and the speed (which I've never measured) is programmable. I also think you will get tired of cranking the handle, if you could arrange the input effort from a different position you might be able to employ two pedals and form a fitness bike effect.   ?

[Rusted]

Thank you, Peter.

I get enough exercise as a cyclist already and I was quite looking forward to winding my dome gently instead of dragging the thing around.

Your pinch roller idea won't work for me. But, I could simulate the same effect by having my friction roller at the end of a long, 'see-saw' lever.

Screw adjustment on the far end should provide enough lift to overcome any "leaves on the tracks" syndrome. 

 

[Rusted]

I have produced a rough [virtual] drawing to show the general idea behind my <cough> alternative technology contraption.

In theory I can provide enough upward pressure to unload the nearest dome support/rotation roller.

I shall add a strong spring to the adjusting stud to take up any variation in base ring truth.

The white support rollers can easily turn the dome by friction alone when hand driven. 

The 4" Urethane roller has much better adhesion [than the white rollers] in manual scrub tests against plywood.

I'll lash up some scrap timber into a working model to test whether blind optimism can overcome [resident expert] pessimism. 

 

[Stub Mandrel]

I would 'replace' ther white roller with the cranked roller. That will ensure you get maximum grip.

[Rusted]

Thanks, Neil, but it's not quite that easy. I'm [rather desperately] trying to limit the maximum load which falls on the friction drive roller.

The dome's base ring ended up less than flat. So the individual loads on the [white] rollers varies considerably with dome rotation.

The difference in base ring level is about half an inch due to totally rebuilding the dome "upstairs" directly onto the rollers.

Mass loading the far end of the crank roller lever sets a safe and [hopefully] constant load limit on the friction drive roller.

If the load goes any higher the crank roller dips further and the nearby white roller takes over to limit the degree of roller deflection.

I'm hoping that mass loading the far end of the 12:1 lever will provide enough pressure for a constant friction drive.

The image shows the latest mock-up to judge lever size and pivot height and positioning.

A length of 2"x 4" should do for the lever. It can easily be tapered if it needs more room at the far end.

The far end may rise and fall about 6" but I'm hoping for much less.

At least, that's the theory.  

[Stub Mandrel]

Looks good!

[Rusted]

3 hours ago, Stub Mandrel said:

Looks good!

Thanks, Neil. I have ordered a longer axle bottom bracket @ 131mm for better clearance for the crank.

Once I have the geometry correct and the friction roller functioning well [if ever] I can make a much "prettier" setup. 

[Stub Mandrel]

Thought, would it be easier to spring the white roller, so  the crank could be 'rigid'.?

[Rusted]

If I use a white roller for the drive I'd need to rethink its bearing and crank drive connection.

The variation in height of the 4" roller on its pressure balancing lever should be small enough to ignore.

The friction roller designs I have found so far probably lacked enough pressure.

Or, were too rigid to react to variations in truth of the ring it was driving.

My design is still unproven but I can easily load the roller at a 12:1 ratio to provide considerable upward force. 

I can even add my own strength to the pre-load if I find the roller is  slipping.

The ideal roller pressure, to avoid slip, can easily be measured at the far end of the lever with a luggage or fishing scale.

Then the reading multiplied by 12.

 

[Stub Mandrel]

3 hours ago, Rusted said:

If I use a white roller for the drive I'd need to rethink its bearing and crank drive connection.

I meant make the white roller sprung, but still have the drive through the red roller. That way it won't 'bob' when you crank it.

[Rusted]

Hmm.  I'm going to have to think about this idea before committing myself. 

[Peter Drew]

I would try with what you've already got for starters.   (Dome Pedant)   ?

[hughgilhespie]

Hi Rusted,

As another septagenarian, I would definitely go with a motor drive - I did with my admittedly puny 2.2 metre Pulsar observatory. You say it needs up to 15 kg linear to get it started. With a 10 cm diameter drive wheel that is less than 10 Newton metres torque. Have a look at a good, meaty stepper like this https://www.omc-stepperonline.com/nema-42-stepper-motor/nema-42-cnc-stepper-motor-bipolar-30nm-4248ozin-8a-110x201mm-4-wires-42hs79-8004s.html

Puts out 30 Nm so more than enough to get it started plus, as it's a stepper, you can use a little Arduino to drive the main stepper driver and that gives you loads of possibilities to use a library routine such as Accestepper to provide gentle acceleration and deceleration so that nothing is ever very stressed.  Attached is a picture of the stripboard layout of the Arduino driver that I use for my dome rotation. Yours would actually be simpler and I would be happy to do a layout for you.

Regards, Hugh

WallBox36.pdf

[appistar]

Hello Steve

I would listen to Hugh, much easier to flip a switch than to wind the handle, easier to automate as well.

I have some questions about the Sun - don't want to get you into trouble can you private PM me?

 

Best regards

 

Ed