Manipulator Torque (A Bit Scary for Us)
 

After doing some calculations for my team's manipulator, I realized that a particular shaft needs to be able to take about 190 ft-lbs. of torque. While we have a motor and sprocket ratios that can handle it, we can't quite figure out how to set up the shaft itself and the hubs attached to it. We intend to use the Taigene van door motor mounted to a 22-tooth IFI sprocket linked to a 48-tooth IFI sprocket on the same shaft as a 9-tooth McMaster sprocket going to a 60-tooth IFI sprocket on a third shaft attached to a lever mechanism.

As stated, the shaft attached to the 60-tooth sprocket needs to be capable of exerting a maximum of about 190 ft-lbs. of torque. My problem is that I have absolutely no idea how to calculate the thickness of the shaft required, what material it should be comprised of, or how to couple the 60-tooth sprocket to it if the shaft must be thicker than .5 inches (since IFI sprockets have a 1.875" bolt circle, AndyMark hubs with a 1.875" bolt circle only go up to .5" bore, and I can't find any other hubs for this sort of application on the internet). Further, the only 9-tooth ANSI #35 roller chain sprockets that I can find with keyed bores use a 3/8" bore (McMaster 2500T11). Maybe this middle shaft really can be just a steel 3/8" diameter one like McMaster 1497K952 (since 190*9/60 is only 28.5 ft-lbs.), but I must admit that I really have no idea.

Perhaps this wouldn't be such a big deal if my school had a machine shop (to simply make really big hubs and widen the bore on the 9-tooth sprocket) or engineering classes (so that we could do these calculations) or my team had any mechanical engineers as mentors (so that they could tell us how to do these things), but as the situation is right now, we're pretty much stuck.

Can someone tell me how big the shafts to take 190 ft-lbs. should be and what material? Does anyone know whether a 3/8" keyed steel shaft would be sufficient for 28.5 ft-lbs. of torque? Is it possible to get something other than an AndyMark hub to translate these forces to the axles? Or would it be better to do a dead-axle design and put bearings in the sprockets instead (somehow linking the tiny 9-tooth one to the huge 48-tooth one)? Can regular ball bearings like the KoP ones even take these sort of loads? My team and I are almost completely out of options, and I hope that this design doesn't turn out to be impossible, too (we first tried a scissor lift... that will never happen again).

Re: Manipulator Torque (A Bit Scary for Us)
 

Well, I'm somewhat confused about the set up you are using, but is it something along the lines of this? http://i42.photobucket.com/albums/e3...0112081353.jpg,
at least the front part of that. I think we need a better understanding of what you are doing, but if you see the sprocket attached to the "arm", we ran that six inch sprocket to around a 2 inch one on the keyang motor, and were able to pick up the ball. We just used the plastic caps they give you with those motors, and hard mounted to a keyed half inch axle all the way across...


a good way to mount a lever is maybe attach it straight to the output sprocket, and have an axle to take some load but definitely not all of it, and if you can mounting straight onto the sprocket that helps, and is how we have done arms/levers in the past...


now that I think of it, I don't know how much help I was =x

Re: Manipulator Torque (A Bit Scary for Us)
 

You may want to beef up the steel that you wish to use as an axle if i understand you correctly is what you are asking. Last year our team (2028) had a claw at the end of our design of arm tha only weighed 6 pounds, and we used the denso window motor with a 1 to 4 ratio (i think, at least it was for every 4 turns of the motor there was 1 turn for the pulley at the top) with pulleys and timing belts. Our axels were made out of .5 in solid steel. After about 3 matches, we noticed that we had BENT ALL of our axels! If you can find a better material than steel, i think you would be better off...

just throwing that out there...

Re: Manipulator Torque (A Bit Scary for Us)
 

Actually, we were thinking something like this, but with a claw attached via another shaft to the arm segment so as to be able to "dunk" the ball over the overpass. But that claw's linkage isn't my concern at the moment - I'm more worried about the primary joint (in the upper left corner of the picture).

That is to say, our planned arm is a two-stage articulated system with a total reach of about 8-9 vertical feet (for hurdling purposes), and the linkage in question is the primary joint of the arm (hence the reason for the large torque requirement).

I do believe that mounting the arm directly to the 60-tooth output sprocket is probably the best means of attachment, but I still do not know how big to make the shaft to which said sprocket is attached, or how to attach the sprocket to the shaft (we intend to use a 60-tooth IFI sprocket to attach to this high-torque arm, but the only keyed hubs that I can find are these AndyMark ones, which seem grossly insufficient for this application).

The idea that a 6-pound claw at the end of maybe a 5-foot arm (30 ft-lbs.) bent a .5" solid steel shaft is a bit disturbing - it almost certainly rules out the 3/8" shaft in the middle of our assembly (which must take 28.5 ft-lbs.) unless we can find some funky alloys (or, God forbid, titanium, although if that is the only feasible avenue then we may well consider it).

I guess that I'm just looking for hard numbers here, or some way of calculating them.

Re: Manipulator Torque (A Bit Scary for Us)
 

In all actuality, if you attach the arm to the sprocket then you -don't- have to attach the sprocket to the axle, as it can just be free spinning and still work...


As you might be able to see in that picture, that's how they did it, and their sprocket looks free spinning on that axle although it is hard to tell.


If ultimately you do want to attach the axle to the sprocket, then something like that hub is really all I can think of... Sorry I'm not of more help to what you are asking. (and sometimes testing outweighs hard numbers. there can always be something you are forgetting, overcompensating for, etc...)

Re: Manipulator Torque (A Bit Scary for Us)
 

i would strongly recomend using the pre-drilled bolt hole circle(s) to transmit the torque. these bolts will be able to handle farily high torque, leaving the shaft to deal with the relatively tame shear loading. also, i'd check the loading specs on the bearing, but you're probably not gonna overload them.

aditionally, i'd also double check your torque calculations; if i recall, the tiagene motor has a torque of 34 Nm, or roughly 25 ft*lbs; with a 60:9 reduction, you'll be stalling (or close to stalling, depending on the rotation direction) the motor. a stalled motor is never a happy motor. a good rule of thumb is to not load the motor more than 25% of its stall torque. also, is 190 ft*lbs correct? i have no idea what your mechanism is, but to generate a 190 ft*lb torque at the end of an 80" lever arm, you'd need almost 30 lbs of load, seems pretty hefty.

Re: Manipulator Torque (A Bit Scary for Us)
 

Wow, I can't believe that I never thought of that! Of course I don't have to attach the sprocket to the axle if the mechanism itself is attached to the sprocket!

Now I just need to find a shaft material that can take the weight (or some way of taking the weight mostly off of the shaft), which will probably be quite high (since the chain on the back side of the 3.5825" radius sprocket will have to exert about 600 lbs. of force :ahh: to balance the weight at the end of the lever on the other side - we should probably look into doubling up that 60-tooth sprocket and using double-strand chain, since 600 lbs. is almost twice as high as the working load that I recall seeing for ANSI #35 chain, but it should be doable at least). According to the IFI spec sheet, the sprocket can accept a .5" bearing, and if the shaft is really well-supported that should be possible (since there won't be any torque exerted on the shaft itself).

I guess I still need to work out how whether the middle shaft (with the 48-tooth and 9-tooth sprockets) can be attached to said sprockets or whether it will be necessary to somehow rig them together on a dead axle as well, but that seems to be a somewhat easier problem to overcome.

Re: Manipulator Torque (A Bit Scary for Us)
 

bolting the arm to the sprocket should work ok. The Igus 5/8" (from last year) or the new KOP 1" shaft and bushings is nice for making stuff like this pivot.

A 3/8" bolt is usually torqued to about 35 ft lbs when we use them on cars, so it should handle 28 ft lbs just fine, although the key may or may not be up to the task. you can calculate the shearing stress in it by figuring the torque is all transmitted to the key at the outer diameter of the shaft, and the key has a known cross section that must withstand that force.


and ditto on the motor torque, you want to load the motor much less than the stall torque, so it will last. 20% would be nice.

Re: Manipulator Torque (A Bit Scary for Us)
 

Agreed on the bolt holes - mounting the arm directly to the holes in the sprocket seems to be a much better idea that attempting to mount the arm directly to the shaft itself.

I'm sorry if my explanation of the reduction was poorly done: what I meant was this: Taigene coupled to 22-tooth, linked to 48-tooth, coupled to 9-tooth, linked to 60-tooth - total reduction: ~14.5:1, and according to JVN's design calculator (my gracious thanks go to him), perfectly ok (close to the peak of the power curve). Stalled motor = hot motor = dead motor = bad situation.

Also, yes it is 190 ft-lbs. in a worst-case scenario (ball in claw, arm completely extended) - the arm, when fully extended with the claw, measures 108" long. While we cannot fully extend it and stay within the 80" bounding cylinder (the robot will have to do a "dunking" motion), it is important that the physical mechanisms be able to handle the arm being in this position for testing purposes and in case of programming glitches (we might get a penalty, but at least the arm won't break off).

I will definitely check the loading specs on the bearings as well - another point of good advice for which I am very grateful.

P.S. Are master links available for double-strand chain? Would we need a different kind of chain breaker for it? Or should I just use two loops of single-strand (on two 9-tooth sprockets on a keyed 3/8" shaft) and couple the double 60-tooth sprockets really tightly together to ensure even loading? Also, is 28.5 ft-lbs. an acceptable torque on a short 3/8" steel shaft, or will I need to find some sort of exotic material or larger-bored sprockets? Current opinions on the 3/8" shaft appear somewhat discouraging, and I'm wondering if there is any way to obtain perhaps 3/8" keyed titanium or something uber-strong. That shaft, of course, will also have to be supported against the large load from the 60-tooth sprocket linkage, although it will be pulled downward by the linkage to the van door motor as well (that will have to be supported really well, too).

Re: Manipulator Torque (A Bit Scary for Us)
 

Ok, so assuming that I can find a nice alloy and that the key holds (would the key just shear in half if something went wrong?), a well-supported 3/8" shaft should not cause problems. How could we get the IFI sprocket to fit on a 1" shaft, though? The hole in the middle is 1.125" and meant for a .5" bearing, and we don't have access to any machining facilities. I guess that we could use something like McMaster's 5905K68 on the inside of the sprockets, but we would still have to increase the diameter of the hole in the middle by 1/8" (and I'm not sure that would even be safe). Or we could use McMaster bearing 7929K21 on a 5/8 shaft - actually, that seems pretty feasible. Thanks for the advice!

Re: Manipulator Torque (A Bit Scary for Us)
 

there's a formula for this somewhere....

J = pi d^4 /32

max shear stress = T r /J

i figure about 33,000 psi shear stress on a 3/8" shaft with 30 ft lbs torque

Re: Manipulator Torque (A Bit Scary for Us)
 

what's the OD of the 1" igus flanged bushing?

Re: Manipulator Torque (A Bit Scary for Us)
 

Hmm... actually, that Igus bushing might work - it looks like the OD is 1.125 from the picture in the manual (I can't tell right now since I'm not at the team's work area to measure it), in which case no sprocket modification would be necessary. I'm somewhat concerned that the radial load on it (something over 600 lbs.) might be too high, though.

In the formula, what do J, d, T, and r represent (or is that T subscript r)? This appears to be the same formula that I found earlier and was confused about because I didn't know what the variables stood for.

Re: Manipulator Torque (A Bit Scary for Us)
 

we do that just to confuse you! and making the proper greek symbols is beyond my patience level :)

T = torque on shaft

r = radius of shaft

d = diameter of shaft

J = polar moment of inertia of shaft

And there are some assumptions made when the forumula is derived, so having a mechanics of materials book or a mechanical engineering handbook that explains it would be nice...but that's really 3rd year engineering school stuff....nice to see you're getting a head start

Re: Manipulator Torque (A Bit Scary for Us)
 

look up the igus specs on their online catalog, they list the allowable load, and you might be surprised how strong those little plastic bushings are!

Re: Manipulator Torque (A Bit Scary for Us)
 

So I need to find a shaft made of a material with a shear strength of at least (a safe margin from) 33,000 psi? Based on what I've seen so far, that should be doable.

I'm not entirely sure how to handle the key. 30 ft-lbs. = 1920 lbs. at .1875", which would have to be resisted by a 3/32" x 3/32" key about 1.5" long in total (to go under the two 3/4" wide 9-tooth sprockets), and I'm not sure what to do from there to determine if it can handle it or not. I think that it would go something like: (3/32)^2 = ~0.008789 in.^2, and 1920 lbs. / 0.008789 in. = ~218,453.3 psi. I'm not sure whether I did that correctly or that that is a reasonable number, though....

Re: Manipulator Torque (A Bit Scary for Us)
 

The key would have to support the load along the length which is contacting each sprocket. So, if the key is fully under each sprocket, and each sprocket is (guessing) 1/2" inch thick, then the load at each one is spread over

3/32" x 1/2" = .0047 sq inches

you have 30 ft lbs torque, at 3/16" radius, so the shearing force on the key is

30 x 12 / .188 = 1900 lbs

so the stress on the key at each sprocket is

1900 / .0047 = 400,000 psi which is a bit much!

so it looks like keys won't do the job. Could you drill some holes in the sprockets and bolt them together? might put the load out at a larger radius which could help.

Re: Manipulator Torque (A Bit Scary for Us)
 

Oh - I had the cross section messed up. Actually, the sprockets are 3/4" wide, and there are two of them, so that's a total of 1.5" of key stock. 3/32" * 1.5" = 0.140625" and 1920 / 0.140625 = ~13,653.3 psi. Or should I only take one sprocket into account at .75" for a pressure of ~27,306.7 psi? In any case, 3/32" * 1/2" = ~0.047, not 0.0047, so your calculated pressure for 1/2" long keys would be ~40,000 psi, not ~400,000 psi. Are any of those numbers workable, or are they still excessive?

Edit: and yes, we could definitely bolt the sprockets together - that seems like a good idea given the situation.

Re: Manipulator Torque (A Bit Scary for Us)
 

There are two keys, but remember that one sprocket is turning the shaft, and the shaft is turning the other sprocket...so you need to look at each key by itself.

if I understand your design correctly, that is

Re: Manipulator Torque (A Bit Scary for Us)
 

oops! Good to see you're checking my math. Sorry bout that

Re: Manipulator Torque (A Bit Scary for Us)
 

Ah, I see. Assuming that the 3/8" hub had two 9-tooth sprockets chained to two 60-tooth sprockets on the above shaft attached to the arm, then there would be 1.5" of key to transfer the force (since both sprockets would be driven by the axle). However, I did indeed forget to consider the torque on the 48-tooth sprocket that drives that axle (with the chain running downward to a 22-tooth sprocket on the motor). Since a .375" AndyMark hub (of the type to which this sprocket would be mounted) has a width of 1" and its key would also have to take ~30 ft-lbs. of torque, that key would be the limiting factor.

1 * (3/32) = 3/32 = .09375

1920 / .09375 = 20,480 psi on the key of the sprocket driving the shaft, as compared to 13,653.3 psi on each of the keys of the two 9-tooth sprockets being driven by the shaft (if my math in my last post is correct). So 20,480 psi is the highest required by the design (plus some safety factors to compensate for acceleration and any downward impacts on the arm). Is that reasonable? What sort of material would it have to be made out of?

Just based on what I could find, shear strength is approximately 0.75 * ultimate tensile strength for alloy steel (according to this site), and McMaster-Carr has key stock made of alloy 8630 steel with a tensile strength of 112,000 psi. 0.75 * 112,000 psi is 84,000 psi, which seems to be quite safely in excess of the 20,480 psi required at the key with the smallest cross section (the one on the 48-tooth sprocket that will be driving the shaft).

However, the AndyMark hub that I found for .375" shafts is constructed from 6061 aluminum, which, according to this site, has a tensile strength of somewhere between 18 and 45 ksi (depending on the specific type of 6061, like -O, -T4, or -T6). According to the site from which I obtained tensile strength to shear strength conversion factors for the keys, aluminum alloys generally have a shear strength of about 0.65 times their tensile strength, so the shear strength of the aluminum in the hub should be somewhere roughly between 11.7 ksi and 29.25 ksi. However, I am not sure how to find the cross section in this case (or whether this is even an application that that formula is meant for), and hence cannot determine whether the hub could theoretically take 30 ft-lbs. I have made a request to AndyMark.biz for the maximum allowable torque on the hub, though, so hopefully I will know soon.

Thanks for all of your help!

Re: Manipulator Torque (A Bit Scary for Us)
 

Figuring out the strength of the slot for the key is indeed a bit more complicated, especially when the key is stronger than the shaft or hub.

I think you have a pretty good idea now of where the weak links would be with your design, glad I could help!

Re: Manipulator Torque (A Bit Scary for Us)
 

I imagine that AndyMark should be able to assist with it - they've probably done some testing with the hubs to determine how much torque can be transmitted safely. Thanks again for helping me to work through this design!

Re: Manipulator Torque (A Bit Scary for Us)
 

Well... here is a simple solution; If you have big sprocket on there it is most likely parallel to the arm at most points, so as close to the outer edge of the sprocket put a large (3/8"-1/2") bolt from the sprocket to the arm.

This single bolt at 3-4" from the point of rotation can take more more torque than a 6 bolt pattern of #10's at 15/16" (That's AM's pattern).

Here is a good example; zoom in on the sprocket on beachbot's arm and you'll see.

There is really no reason to depend on the shaft for torque transmission, and most likely no reason why a bolt like that can't be used.

Re: Manipulator Torque (A Bit Scary for Us)
 

Yes, I have definitely decided not to use the shaft for torque transmission to the arm (the 190 ft-lb. joint) - it would have to be really thick and thus heavy and hard to work with. If we put bolts through the arm and the sprocket, then the torque will be transferred at a greater distance, and hence with a smaller force, leaving the shaft to deal with just the downward forces - a much more practical solution.

The AndyMark hub is intended for the middle shaft, where a 9-tooth sprocket has to be coupled to a 48-tooth sprocket, so a large bolt several inches from the axle isn't much of an option there (since the 9-tooth sprocket is so small), so the torque will be at least partly transmitted through the shaft (we do intend to put bolts through the sprockets, but they likely will not be able to be far enough out to take all of the torque safely). That axle only has to take ~30 ft-lbs., though, and based on the calculations that squirrel so generously demonstrated, the shaft and the key should both be alright. Now it's just a matter of determining whether the 3/8" AndyMark hub can take it, and if not, then the bolts through the sprockets should be able to take some of the load off.

Re: Manipulator Torque (A Bit Scary for Us)
 

Okay, let me crack out some math. Our team is doing something very similar with a four bar design. I calculated our torque for the arm at 124.97 ft lbs (aprrox 170 Nm) at it's worst (the perpendicular). Based on that I know that at best I could hope to load the van door motor at 50% of stall and 20 amps (240 watts). The torque at that point is 17.5 Nm and the rpm under that load according to this chart (http://www.chiefdelphi.com/media/papers/2064) is also roughly 17.5 rpm. Sooooo here is the problem. You take 170 Nm and divide by 17.5 Nm provided to get 9.714, essentially 10 to one gearing needed somewhere. Based on that you know that from that original 17.5 rpm under load you can expect 1.75 rpm out of the gearing reduction. In our particular case out arm lifts 115.97 degrees, this is 3.1 "lifts in one revolution (360/115.97). Multiply 1.75 by 3.1 to get 5.425 "lifts" per minute. Then divide 60 seconds by that number. Finally you have that it would take 11.06 seconds to lift the arm.

Now I am sure there is something really wrong with my math, and that's why I posted it, or I hope there is because the van door motors make life nice. But I think both or our teams here might run into the same problem so I wanted to share the calculations I have had trouble with. How do you solve the speed issue? Power is no problem relative. I can pick up a ten to one gearing reduction just about anywhere (joking, we all know nothing is that simple). But if I want to raise the arm in a minimum of three seconds where do I get the 368% increase in rpm? I think this is something that any team using the single Van Door motor for an arm faces so I just wanted to bring it up for discussion.

Re: Manipulator Torque (A Bit Scary for Us)
 

Actually, power is the problem! torque is increased by decreasing speed, but you can only get so much power (work per unit time) out of the motor and that makes the arm really slow.

So....if you use this setup you will have to be lifting as you are lapping, should be exciting to drive!

Re: Manipulator Torque (A Bit Scary for Us)
 

:D :D :D sorry, horrible physics terminology, yes power is the problem, because time is the problem and torque or force is not.... though I ran the same calcs with the two globes and got 7.5 seconds, then with the CIMM and got .644 seconds. Soooo I think I will try to see if using the CIMM is viable. The only problem with the CIMM is it doesn't mechanically lock like the worm gear, so we have to supply massive amperage just to keep the arm from falling down, is there any way around that that's not nasty and complicated? I'm from a rookie team so I'm full of questions and not a lot of technical knowledge. :(

Kind of stinks though because we wanted to use four CIMMs on the drive train. But I looked and I think you can take the Andy Mark super shifter and a planetary gear box for the FP motors and set it up so the super shifter takes one FP and one CIMM instead of two CIMMs

Re: Manipulator Torque (A Bit Scary for Us)
 

You might try to see if you can put a gas spring (available from McMaster-Carr) to support the weight of the arm, that will greatly reduce the load on the motor...we did that on last year's bot and lifted tubes with a long arm and a small banebots motor.

Re: Manipulator Torque (A Bit Scary for Us)
 

That would be nice, someone mentioned gas springs to me, the only reason I don't do it is because I can't figure out how to get them to help the motion through a full 120 degree arc like this.

Re: Manipulator Torque (A Bit Scary for Us)
 

I don't know what your robot looks like, so it's kind of hard to figure out what to suggest.

maybe send me a sketch?

Re: Manipulator Torque (A Bit Scary for Us)
 

I have some simple CAD screen shots with no drive train I could show you. The CAD is still in progress...



do those help at all?

Re: Manipulator Torque (A Bit Scary for Us)
 

If you can help support the arm thru the part of travel where it is sticking straight out, and maybe 30 degrees above and below that point, it would probably help quite a bit. Maybe make another shorter arm that is supported by the gas spring, that the main arm can rest on, and as the arm travels way up it lifts off it

Re: Manipulator Torque (A Bit Scary for Us)
 

I think I see what you're saying, that would make perfect sense too. I guess you really don't need to support it all the way through the arc do you, just the perpendicular where torque is greatest.

Re: Manipulator Torque (A Bit Scary for Us)
 

If you do use a gas spring, keep in mind that the rated force is at one end of it's travel--so a 50 pound spring will have more than 50 pounds force when it is compressed. I don't recall what the force is when fully closed, maybe 20% higher? I think there's one or two in the garage I could play with on the scale...

Re: Manipulator Torque (A Bit Scary for Us)
 

Okay, sure. The information you have given me is extremely helpful, Thanks so much for helping me out. Though I think I'm going to get some sleep, I'm on EST so it's 12:53.

Re: Manipulator Torque (A Bit Scary for Us)
 

We had an interesting situation last year, the first time we just calculated the torque to lift the arm at a constant speed. When the smoke cleared, we decided that torque calculations also need to account for acceleration of the load.

This essentially doubled our numbers, but on the second try it all worked.

Re: Manipulator Torque (A Bit Scary for Us)
 

Useless Math: no real need to read......

Alright, I think I have this figured out. We plan to go with two super shifters with two FP motors on the AM Planetary Boxes (to simulate a CIM) as well as two CIMs for drive. Then we use another CIM for he arm. The CIM according to the guidlines manual section operates best at 4350 rpm and 64 oz-in in torque and 27 amps (pretty good). Then I converted that 123.9 foot lbs I need to 125 ft lbs or an even 24000 oz-in. Soooo this is where it gets interesting, you use the CIM with the planetary (184:1) gear box. That gives 11776 oz in, nooooot quite enough (184*64). So we decided to use a short length of #35 chain rated at 269 lbs with a 23 tooth steel hubbed sprcket on the planetary and a 54 tooth on the axel for the arm (which would probably be a live axel with the sprocket dead mounted). That gives 432:1 gearing ((54/23)*184) or 27648 oz in. That is 15.2% over engineered, so it should be perfect. That would raise the arm in 1.92 seconds, which is also perfect (60 seconds/((4350 rpm/432 gear)*(360 rotation/115.97 degree arm arc))).

My Succinct Question:

Math now works the question is mechanical. How do you mount this? From the bottom up the plan was this....Dead mount the 23 tooth sprocket to the planetary with key and pin. Then a short #35 chain to the 54 tooth sprocket. That sprocket is dead mounted to a one inch axel (keyed) and then the two top bars of the four bar are also dead mounted to the axel with a keyed hole and then bolted to two steel hubs (keyed) on either side. Kind of like each arm in a hub sandwich about 6 inches apart with the sprocket in the center. This entire thing might be assisted by a gas spring for 60-120 degrees in the arc of the arm. But I figured I would design it to work without the spring and then if weight and time allows the spring would only help. Do you see any obvious problems with this? Or improvements that we should look at? I have to thank imensely anyone who read this entire thing, I realize it is a bit of a thome.

Re: Manipulator Torque (A Bit Scary for Us)
 

Yeah good point, this is a ten pound ball moving at speed. We are hoping that with a 2 second lift that is just enoguh to not be painfully slow and just slow enough not to make acceleration a huge problem. Plus we can put some ramping in the code for the arm so it doesn't just jolt right up.

Re: Manipulator Torque (A Bit Scary for Us)
 

Remember that the arm and ball are starting at rest.

Our numbers are in the 2007 notebook over at the school, but roughly I seem to recall that it was almost the same amount of work to accelerate the mass as it took to lift the weight. And they add.

Re: Manipulator Torque (A Bit Scary for Us)
 

After thinking about it I realized my torque calculations for the "normal torque" are for the perpendicular ( the arm is parallel to the ground) or worst case scenario. They are also set to the maximum efficiency rating of the CIM. Because the arm starts at about 30 degrees it needs only a half of that previously calculated normal torque (vectors and such, 30-60-90) which means the way I have it set up you have at the beginning of the arch a 12000 oz in normal force to overcome and the motor producing 18720 into acceleration torque. I don't remember how to calculate acceleration from acceleration torque but I think that will be plenty. I hope.
So my big question now is how to mount the dang thing to the shaft. We figured we would key and pin the 23 tooth sprocket directly to the CIM planetary gear box shaft. Then stretch 35 chain about 12 inches up to the 7.7 inch diameter 60 tooth sprocket. But that's about where I start drawing blanks. That 60 tooth sprocket needs to be hard mounted to some type of axle which is also hard mounted to the two top arms of the four from the CAD above (the bottom two arms spin freely) and then that entire axle assembly with sprocket needs to be free spinning. But the axle must be something that can withhold substantial stress (though the closer I put the arms to the sprocket the easier it is on the axle), the sprocket must be mounted in a way that can take that 125+ foot pounds and the arms must be mounted in a ways that can take 62.5+ foot pounds. I have not even the foggiest of what diameter axle to use, whether to key it alone, to use hubs, for it to be steel or aluminum, to weld the sprocket to the axle, to use a threaded axle. it's kind of mind boggling, but this is the single most important joint I have to deal with. I was told belt could actually better transmit the torque but that in this particular application it's difficulty to replace makes it completely not worth the effort, is that correct also?