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a few design questions...
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Nexus
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quote:
Originally posted by Karmond:
Does something like this sound like a good idea? Is going for a low RPM high torque gearbox setup a good way to go? Biohazard's actuators are the other way around, high RPM low torque. The 12v motors for it's actuators are running at 24v, with a ratio of about 9:1.


Good Question, IN my own experience have had more luck with High RPMS.
The setup in Poochy was running a pair of jaycar 540 size motors (25 watts each) on the first stage, these motors where placed mirrored to deal with back lash then they went through an angle grinder head finally turning a ball screw. End result was a gear ratio of 18:1 with pissy motors that could run in series or parralel but they could lift a lot very quickly with low amps.

Compared with Spyder which had a ratio of about 120:1 with a higher powered motor than poochys combined. The strain placed on the chain and mech by the fulcrum effect made it suck so I personally favour an efficent lifting mech rather than a powerful motor.
Try to reduce energy losses in your system rather than increase power in your motors, this also has the benefit of letting your drive motors get the juice thay need. One thing I liked about Poochy was that when the batteries where going flat and it struggled to move, the lifter would still work fine
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Post Thu Apr 12, 2007 5:24 pm 
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Karmond



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Thanks for that Nexus.

According to that page the fewer threads per inch the more efficient the acme rod. So you get more force out of the same motor, while maintaining the same speed of travel.

This is what the numbers would look like the other way around...

RS550 16:1 (2 stage)
1488 RPM @ 14.8v
= 24.8 RPS
1530.32 oz-in stall torque
= 95.65 lb-in stall torque
= 110200 g-cm stall torque
= 110.2 kg-cm stall torque


Acme Thread
6.4mm (1/4") = 20 per 25.4mm (20/in)

4782.5 lb force
2169 kg force
31.496 mm/s

Acme Thread
6.4mm (5/16") = 16 per 25.4mm (16/in)

3678.8 lb force
1669 kg force
39.37 mm/s


RS550 20:1 (2 stage)
1190 RPM @ 14.8v
= 19.8 RPS
1740.23 oz-in stall torque
= 108.8 lb-in stall torque
= 125300 g-cm stall torque
= 125.3 kg-cm stall torque


Acme Thread
6.4mm (1/4") = 20 per 25.4mm (20/in)

5440 lb force
2468 kg force
25.146 mm/s

Acme Thread
6.4mm (5/16") = 16 per 25.4mm (16/in)

4184.6 lb force
1898 kg force
31.4325 mm/s

Post Thu Apr 12, 2007 7:28 pm 
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Nexus
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quote:
Originally posted by Karmond:

According to that page the fewer threads per inch the more efficient the acme rod. So you get more force out of the same motor, while maintaining the same speed of travel.


The speed comment doesnt make sence for some reason.
I remember when I was toying with actuators and preffered to get fewer threads per inch to increase the speed. Every revolution of your gearbox would move one thread so increaseing the TPI would slow the travel speed. I saw it simlar to a gear reduction in that the motor/gearbox would have to turn more to move just a little on the actuator if you have high TPI which would increase the force whilst slowing down the speed.
A low TPI would incease speed but reduce force.
Increasing the threads also increase the surface area contact which I dont know if it is good or bad for efficiency. If the comment above is true then a smaller contact area is more efficient tha a larger contact area but there would be more force applied to a smaller area.
But I could be wrong, thats how it seems to me anyway
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Post Fri Apr 13, 2007 1:30 pm 
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Karmond



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quote:
Originally posted by Nexus:
The speed comment doesnt make sence for some reason.
I remember when I was toying with actuators and preffered to get fewer threads per inch to increase the speed. Every revolution of your gearbox would move one thread so increaseing the TPI would slow the travel speed. I saw it simlar to a gear reduction in that the motor/gearbox would have to turn more to move just a little on the actuator if you have high TPI which would increase the force whilst slowing down the speed.
A low TPI would incease speed but reduce force.
Increasing the threads also increase the surface area contact which I dont know if it is good or bad for efficiency. If the comment above is true then a smaller contact area is more efficient tha a larger contact area but there would be more force applied to a smaller area.
But I could be wrong, thats how it seems to me anyway
The speed comment is basically that with same motor, a low TPI and high gear reduction could travel at the same speed as a high TPI and low gear reduction.

RS550 16:1
Acme Thread
6.4mm (5/16") = 16 per 25.4mm (16/in)
1669 kg force
travel = 39.37 mm per second

vs.

RS550 125:1
Acme Thread
9.5mm (3/8") = 2x4 per 25.4mm (2/in)
2882 kg force
travel = 40.64 mm per second

Very similar speeds, vastly different forces.




I took a little look at http://www.smallparts.com.au and I have a few questions.
What kind of pulleys/belts do I want for drive?
Is cutting gears from lengths of spur gear stock a good idea?
Would Single Row Radial Economy Ball Bearings (6mm ID, 10mm OD) be adequate to support the lifting mechanism shafts?

Post Fri Apr 13, 2007 2:55 pm 
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Knightrous
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If I was in your situation, I would be using 12mm 4140 shafts for the lifting mechanism and Bronze Bushes instead of bearings. Your doing very little rotational work, so bushes will be stronger and cheaper.

Belt wise, I'm not much of an expert and I'm currently looking at this issue myself, but looking at toothed belts, I reckon a 5mm pitch, 10mm wide toothed belt should be plenty good. Just make sure you use a pully with a good 30+mm diameter so you don't strain them as much under load.
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Post Fri Apr 13, 2007 4:04 pm 
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Karmond



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quote:
Originally posted by TDT:
If I was in your situation, I would be using 12mm 4140 shafts for the lifting mechanism and Bronze Bushes instead of bearings. Your doing very little rotational work, so bushes will be stronger and cheaper.
Reason I was hoping to go smaller shafts was to make the lifting arm pivot point lower and the lever arm points higher.

Lets assume that I was to use a 12mm shaft and that I used a 12mm ID, 16mm OD Bronze Bushes.

The center of the shaft attaching the lever arms to the lifter arms would be 8mm above the base. The lever arms would be a max length of 40mm vertically from the base. The center of the shaft attaching the lever arms to the actuator would be 6mm from the top of the lever arm. So the piviot points between both shafts would only be a distance of 26mm.

Post Fri Apr 13, 2007 4:48 pm 
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Knightrous
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Well I just thin 6mm is a bit weak. If your lift is up and it copes a kiss from a spinner, it might not work again Smile
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Post Fri Apr 13, 2007 4:52 pm 
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Karmond



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quote:
Originally posted by TDT:
Well I just thin 6mm is a bit weak. If your lift is up and it copes a kiss from a spinner, it might not work again Smile
I don't expect it to survive a strong hit when it's up. Biohazard's never does, even breaks when it's down.

6mm might be too weak, but I think 12mm is too bulky. I might look into 8 or maybe 10mm but I doubt I'll go any higher. If the arms, shafts, or lifter bar breaks or bends then the cost of the metal to replace them should be pretty cheap, assuming I source them from local suppliers. They're not very big.

Post Fri Apr 13, 2007 5:01 pm 
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Philip
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This is another option to consider. http://www.teamstorm.com/heavies/storm2/series7/weaparmour/index.htm The author questions the need to "use linear actuators and convert rotary motion, into linear motion and back into rotary motion."

I don't know if it is a good idea or not. It is just more information.
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Post Fri Apr 13, 2007 5:53 pm 
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Karmond



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Thanks phillip, although I took one look at the setup and figured it may as well have been made from gold as I couldn't afford something like that.


I was however considering having the actuator in the middle but decided against it as it made the lifter itself wider and weaker and didn't really save space.

Post Fri Apr 13, 2007 7:06 pm 
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Nick
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ACME thread is very deep, so a 6mm thread has a much smaller central load bearing core than,say, an M6 screw. I would definitely go for a minium 10mm thread. The lower threads = higher efficiency claim seems suspicious to me; if it were so, everyone would have big gear reductions on their actuators to make the specs look better. As a total system the difference may be quite small.

Carlo went for ball screws in his bot - he specifically mentions converting the original actuators from acme threads for higher efficiency.
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Post Fri Apr 13, 2007 9:03 pm 
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Karmond



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Ok, I'll go for the 3/8 - 16 thread then. That's a diameter of about 9.5mm. The bronze nut has a diameter of 17.272 mm so that should fit ok I think. Not sure how much speed it'll lose, but using the no load speeds of a RS550 will give a travel of 39.37 mm/s @ 16:1, 31.4325 mm/s @ 20:1, 25.1917 mm/s @ 25:1.

Post Fri Apr 13, 2007 9:30 pm 
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Karmond



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quote:
Originally posted by Nick:
Carlo went for ball screws in his bot - he specifically mentions converting the original actuators from acme threads for higher efficiency.
You know how he mentions that they're so efficient that he can push the lifter down by hand? I can see that happening with acme thread with low TPI but not with a high TPI. So maybe the efficiency thing is correct.

Post Fri Apr 13, 2007 9:49 pm 
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Nick
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Or it might be viewing only part of the system... There is going to be a sweetspot between TPI and efficiency when viewed as a complete system. i don't pretend to know the answer but ACME thread is fairly cheap so if you go with the low TPI and it doesn't work as well as you want, then swap to ball screws or a different TPI configuration.

Just had a thought - actuator makers would surely spend R&D money on this question to make the most competitive product. If you used an average of the typical TPI screws in commercial units, then you would likely be close to the most efficient combination.
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Post Fri Apr 13, 2007 10:09 pm 
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Valen
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i think that your probably best off trying to aim for a particular motor RPM and letting that set your TPI, it is after all the source of your power and if its near stalled it isn't going to be efficient no matter whats after it.

TPI is going to be a trade off between lifting force and speed. Aim to get your motor at its peak power lifting 13.6kg.
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Post Fri Apr 13, 2007 11:12 pm 
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