Author Topic: worm-drive vs linear-actuator vs belted-pulley  (Read 1776 times)

ben

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worm-drive vs linear-actuator vs belted-pulley
« on: September 07, 2015, 06:30:57 AM »
    Hi All,

    I understand there are three main types of mechanics used to control the angular position of an axis.
    - worm-drive - fixed shaft rotates which turns a toothed disk
    - linear-actuator - fixed shaft rotates which slides along a nut
    - belted pulley - small wheel on stepper drives a large wheel on the axis using a belt.

    Can we please discuss PROs and CONs of each?

    Of interest to me is the following:
    - holding torque of stepper,
    - strength of components
    - weather proof
    - ease of build and ease of maintenance
    - back lash or slack in interconnections.
    - price

    What have you used, and would you recommend it?

    This discussion will hopefully help me choose an approach for my own design.
    Thanks,
    Ben,.


Paul L

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #1 on: September 07, 2015, 09:54:17 AM »
Hey Ben!

    Nice to see you being so active on the forums lately - I like the enthusiasm!  :)  Of those three systems, I've used the worm drive and linear actuator.  Both these systems do what they're supposed to do - track/redirect sun.  But there are strengths and weaknesses to both. 

Linear actuators - The best thing about this setup is the relatively cheap, readily available parts you need to build it.  Once all the bugs are figured out - its a fairly robust system.  But setting it up can be a pain - the measurements have to be dead on and there's a lot of re-adjusting.  It also has limited range, which is my biggest problem with the setup.  Can be a little noisier than worm gears - I apply oil to the lead screws periodically.

Worm gear systems are tougher to find, and way more expensive than linear acutator systems.  On the plus side, setup is way more simplified, movements are quicker and there is no limit to the range that it can track.  I managed to salvage and modify two worm drives from an old electric wheelchair and they work really well.  Worm gears are also self locking, so you don't have to worry as much about misalignment due to high winds.  And mine are in a sealed unit, so it's really quiet and can handle the weather.

Belt n pulley I've never tried, but I'd be a little concerned about the holding power of such a system in high winds since it's not going to self lock, and you cant depend on the holding torque of the stepper, which is powered down between moves with Gabriel's program to save power.

The big companies use slewing drives, which would, in an ideal world,what I would use.  Basically, they're large self contained worm drives that can take alot of weight.   

Winner in my books?  By far a worm gear setup.  Nicer to look at, easier to set up, quiet, efficient movement, gears are self contained and protected from elements, self locking.


Gabriel

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #2 on: September 10, 2015, 02:22:54 PM »
Hi Ben,

Over the last year I have attempted to build probably 5 different heliostats that used
worm gears. I can't say that I have been super happy with any of them. It is hard to completely eliminate
backlash and inprecisions when building from scratch. When building them, they seem quite rigid, but after mounting
the mirror the weight from it seems to cause a fair bit of flex. This happens even when the mirror is balanced. It seems
that the extra inertia is enough to make the mirror move further when pushed.

I also can't seem to find wormgears for as cheap as I would like. I have seen people modify used wormgears from
other sources by typically removing the DC motor and replacing it with a stepper motor.


I'm not especially happy to do so, but I ended up mounting linear actuators on my most recent heliostat. It's not a huge deal
where I live, which is roughly a mid latitude, but their limited range of motion isn't so good for many locations.

I did actually have an idea recently to build a heliostat with linear actuators that is capable of moving a full 360 degrees if need be.
I think that it would be possible, although the program would have to be modified to make such a thing work.

It probably won't happen for awhile sense I have a lot of other things to work on, but it is on the list. I think that it is totally worth doing
though because linear actuators are a lot cheaper than worm gears and it seems like you generally need less in the way of tools to build with them.

Gabriel

Paul L

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #3 on: September 12, 2015, 08:44:48 AM »
Hey Gabriel!

    I've been scratching my head for the last two days - how exactly would you get a linear actuator system to rotate a full 360?  The only mechanism I could imagine for the job would be a Scotch yoke, so the program would have to be modified to change direction at solar noon.  Is that what you were thinking? 


Gabriel

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #4 on: September 12, 2015, 05:00:13 PM »
Hey Paul,

I think is more than one way to do it, but I think the easiest method to understand would be to be to hook the linear actuator to a big gear which meshes with a smaller gear with maybe a 3:1 ratio (or more). So although the linear actuator might not be able to move the big gear 360 degrees, the big gear would be able to move the small gear 360 degrees or even further. There might be other ways too that don't require a relatively complicated gear system, but I would have to actually build them to understand them.

Why I didn't think of this a long time ago I don't know. I guess I figured that cheap wormgears with minimal backlash actually existed. If it works out, it would be a good way to build a cheap heliostat, although at the expense of making it more complicated.


ben

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #5 on: September 13, 2015, 06:02:38 AM »
i have never used a worm-drive, but i do know what they are.
attached is an example for our reference.

i think that if the output of the worm-drive (the worm gear) is used directly as the rotational axis, then backlash (slop in the gears) would cause problems. especially if the worm-gear had a small diameter, like 100mm.

imagine if the worm-gear was 1000mm, using the same worm, the same slack in meshed teeth exists, but as this backlash is now farther away from the center of rotation of output worm-gear, the amount of angular movement/lash would be considerable less using a bigger output gear.

How big are the worm gears that you use, and do you use the output worm-gear to drive directly the axis of rotation?

ben

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #6 on: September 13, 2015, 06:15:42 AM »
i think with whatever of the three types of drives (worm/linear/pulley), to get precision and minimise backlash the intersection of mechanical components needs to be as far away from the axis of rotation as possible.

I say that for angular precision and to reduce the effects of backlash:
- the worm-gear needs to be as large as possible,
- the linear actuator needs to be as far away as possible from the axis or rotation,
- the pulley ratio needs to be maximised, and possibly distance apart (?) needs to be maximised

Can someone please start a thread on linear actuators? How should they be designed>?
- does the nut on the thread move, or does the thread move within the fixed nut?

ben

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #7 on: September 13, 2015, 06:20:48 AM »
ive not built one before, so maybe shouting my mouth-off without experience, but...

my preference is the belted-pulley system,
PROs
- it is linear, ie the ratio of movement from input to output is linear (same with worm-drive)
- it is simple
CONs:
- holding torque relies on the strength of the belt, which is not as strong as the metal teeth of worm and actuator systems

Gabriel

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Re: worm-drive vs linear-actuator vs belted-pulley
« Reply #8 on: September 21, 2015, 06:51:11 PM »
I think Paul mentioned this, but one thing to keep in mind is that the stepper motors are turned off when the machine isn't moving. (Although you could leave them on all the time I suppose.) Stepper motors don't typically have much holding torque when they are off.

I think you could still make a belt system work, but you may have to add a bit of friction to keep the mirror from moving on its own in the wind.

It's definitely worth trying out though as a belt system would be a lot easier to make than a worm gear one. Also, it's possible that such a system might be more robust too. You figure if someone did something dumb like run into the heliostat with a lawn mower, it would just rotate out of the way instead of trying to withstand the full brunt of the blowf.