EQ-6 New steppers and gearboxes

[bojan]

Worm gears are standard solution for a telescope reduction - so do not keep away from them :-)
Steve (Kinetic) even build one himself.. Backlash is not such a big problem, and it can be controlled and compensated for in software
(Bartels again.. I know I am boring, mentioning this guy all the time.. but I cant help it, his software solution is fantastic and low cost approach, perfect for DIY people like you are and it is freeware.. so please consider :-)

Worm gear can be made also by pressing epoxy resin or similar by threaded shaft - Rod did this for his dobson and it works OK (he has this described in this thread here:
http://www.iceinspace.com.au/forum/s...hlight=bartels


My dobson uses 1:12 friction reduction on both axes, driven by 1:240 worm-gear, which is coupled via rubber tube to 200-step motor shaft directly.
This way, all backlash present in worm gears (and this is less than one step) is effectively reduced by 12 (because friction reduction has none), and it is further compensated for in software.. Not an issue at all.

If you are using Bartels software and have no encoders, then clutch is not a good idea because the software doesn't know where the scope is moved if it was done manually.. So in order to have GoTo (or clutch and Push -to) you must have encoders, coupled all the time to both shafts.
Well, with friction reduction (like I have on my dobson) the same applies, because when I am moving the scope tube manually, encoders are still sending information on position to the software, despite the induced slippage in the friction reduction stages.

[alistairsam]

Hi,

The rubber tube coupling from the motor looks interesting. could you tell me what sort of tube this is and how rigid it is.
if solid rubber, it can be used to couple shafts with dissimilar sizes.
is there any flexion due to torsion, it might be an issue if there is no position feedback after the gear stages.

i've uploaded a video showing my controller. link is at this thread
http://www.iceinspace.com.au/forum/s...603#post602603

comments welcome.

[bojan]

This rubber tube is used for fuel, I found it in autoparts store.. it has nylon fabric inside, so it must be usable for high pressure applications as well.
I used it to minimise the audible vibrations from steppers.. It is fairly rigid, and dampens the sound from PWM significantly..
There is no visible issues because of flexion.. the torque is not that high at this point in reduction train, so we can consider it 100% rigid (for this application).

[alistairsam]

hi,
had a question about gear reduction.
if the inertial load is high, does it matter if the reduction at the last stage is not high when compared to another system with similar overall reduction ratio, but with a higher reduction at the final stage?

eg. if its a three stage reduction using pulleys, overall reduction 90:1, will the torque required to overcome the inertial load at the final stage be higher if the final stage is 1:1, or will it be lower if the final stage is 10:1, but overall reduction ratio is still 90:1?

reason i ask is because i tried friction drive using rubber rollers at the final stage but with identical sized rollers on the driving shaft and the driven shaft. I noticed that there is a "lag" between when the motor starts to turn and the assembly starts to turn.
i'm guessing this is because the rubber rollers are quite soft and therefore are stretched due to torsion and once thats overcome, power is transferred.
so if i use a larger wheel on the driven shaft, will the same effect be noticed?

[bojan]

You have to take one step at a time...
The torque on each shaft is inverse proportional to reduction ratio.
That means, higher the reduction ratio on particular stage, lower the torque at the input (the shaft which rotates faster).

The end torque (at the motor shaft) will be the same for the same overall reduction ratio, regardless of the reductions for each individual stage.. and of course, if we do not count the mechanical losses (friction).

That is the reason why people are using worm gears at output shafts, because this way the torque on the worm is much smaller (the reduction of this stage is high).
However, the mechanical losses (friction) of the worm gear stage are also very high (because of the way the surfaces of the worm helicoil and teeth are sliding against each other, they are much higher (compared to conventional spur gears or belt or chain reduction stages). Those losses (essentially friction) are depending on the load, so that is why the telescope must be properly balanced, to reduce the wearing of worm gear and required motor torque (in some extreme cases steppers may even stall because of imbalance).
Adequate lubrication of worm gear stage is also very important.

[alistairsam]

Hi,

Did some reading, and its apparent that reduction ratio at the final stage is important as the torque required on the final driving shaft in a multistage reduction train is the total torque required to overcome the load's moment of inertia (dependent on radius of driven pulley), and is not dependent on preceding gear reduction stages.
so higher the reduction ratio at the final coupling stage, lower the torque required at the final driving shaft.

I was looking at your metal friction drive and my hypothesis on why slippage is not such a problem is because the ratio at that stage is very high and torque required is relatively low.

if the diameter of your driven metal wheel was say 1/4th the size and the driving wheel was twice the size, the torque required would be much higher even if reduction was added at preceding stages and slippage might occur.

sorry to brag on in this thread, but its just clearer to understand now.
I'm quite confident i'll be able to get the ratio needed which is around 700:1 just using timing pulleys and a planetary gearbox.
backlash was quite low, but will need to measure it later.

as you mentioned, balance will be critical. thats another aspect altogether.

Pies australia and powergrip have timing pulleys that use taperlock so can accomodate a range of shaft sizes.