# Help - several things

1. May 30, 2007

### apatton

hi, my name's drew, i'm 19 and this is my first post on this forum. I plan to start college for mechanical engineering in fall but i have a project i'm working on that i want to be able to finish, well, in less than 4 years time

see, i also study martial arts, and for awhile now i've had this idea for a training tool that actually hit back, but just recently found myself sitting down & actually figuring out the mechanics of such a machine, partly due to a newly acquired & extremely boring job

now i have all the gears & all that figured out, but for this project i decided to use an electric motor rather than gas powered like all my other projects, mainly because gas prices are ridiculous now & i wanna be able to just turn it on & go for as long as i please, not as long as i can afford

So far i've run into a couple problems. First of all, electric motors are more expensive so my usual "get as much hp as i can for under $150" is out the window, i can't just pull a motor off an old motorcycle & have more hp than i'll ever need for this project, so i have to be pretty precise. I'll have one main horizontal gear running 2 side vertical gears, and the side gears need to be able to turn about 160 rpm max. For cost & conveniance all the gears will be mostly made of wood, but the side gears are going to be about 2 feet in diameter, and for overall size reasons, i don't want the horizontal gear to be much bigger than that, if at all bigger than that. So, while i'll be using bearings to cut down on resistance, i still have no idea how much HP i'll need to run it at that rpm. I know to calculate that i'll have to find out how much torque it takes to turn the gears, but i don't know how to calculate that either. The other problem is controlling speed. I'd like to use something like a potentiometer to control the rpm instead of just a switch between lets say 80-120-160 rpm, that way i can gradually increase the rpm, but i don't have expirience in controlling electric motor rpm, so i'd like some advice on that too thanks in advance, and sorry for the long post. Also, If any of you are feeling generous and would like to volunteer to be my indefinate consultant on this project via e-mail or something like that i would greatly appreciate it. 2. May 31, 2007 ### ank_gl cud u get a diagram or somethin like that showing the gear arrangement, torque calculations r pretty much easy(only 1 equation torque*rpm=constant), so u only need output torque n output rpm to calculate wut u need. for controlling the motor, tell me which type r u using, DC, induction or wut. u can use an induction motor frm any old fan like an exhaust fan or something(i got from my old cooler), its pretty much easy. n still i didnt get wut ll the hit back mechanism do, u can get it to reciprocate, but if u want a hit back, go for a spring best of luck anyways 3. May 31, 2007 ### Danger There really isn't enough information there to determine what sort of hp you'll need. That's primarily determined by the final load, which you didn't specify, not just the size of the gears and the speed required. It might be simplest to obtain a good strong motor that already has electronic speed control built into it, such as from an electric saw. And ank_gl, please try to use proper language on PF. I could just barely read your post. 4. May 31, 2007 ### berkeman ### Staff: Mentor Welcome to the PF, ank_gl. Please spell out your posts without using texting shorthand. You are using a full keyboard for your posts, most likely. It's okay to use standard forum abbreviations like IMO and BTW and OP, but please don't use texting shorthand. We need to keep the posts as easy to read as possible, and texting shorthand is not standard for Internet forums, especially technical ones like the PF. Thank you. 5. May 31, 2007 ### berkeman ### Staff: Mentor Welcome to the PF, apatton. Have you considered using pneumatics as the main power transfer means for your hit-back punching dummy? It seems like pneumatic actuators would be better suited to the explosive motions of a punching robot. You store the energy as air pressure, and can release it quickly with a lot of power. Are you picturing this punching robot moving around the mat? I'd think it would be easier (and still useful) if it was stationary, with the ability to rotate 360 degrees as you move around it, and the ability to block and punch and counterpunch. You might even be able to make it into a real product.... Product liability issues might be a problem, however. You'd need to ensure somehow that the robot wouldn't punch somebody who just walked by unknowingly.... 6. May 31, 2007 ### Danger Now that you mention it, Mythbusters devised a 'punching arm' for Buster the Dummy to check out the idea of shooing a shark away by poking it in the snout. It was actually a bit more complicated than one would expect to get it right. They did indeed use pneumatics for that, but I don't know if there was a design reason for it other than it being used underwater. Perhaps, apatton, you can get a chance to view the episode to see how they did it. 7. May 31, 2007 ### ank_gl sorry for that, will keep it in mind the next time. sorry.. and yes how come i didn't think of pneumatics. it will surely pack a punch. and i saw that rotating head machine used as a practice machine in movie "master of disguise". also i never got to practice with a machine, my teacher always acted like a punching bag and yes he sure did use to pack a punch on my face and once again sorry for using shorthand 8. May 31, 2007 ### ank_gl which season was it, i have first three 9. May 31, 2007 ### berkeman ### Staff: Mentor Here's a link that shows the robot underwater. I didn't poke around much (no pun intended) to see if it shows it operating. http://www.discoverychannel.com.au/mythbusters/shark_special/index.shtml [Broken] Last edited by a moderator: May 2, 2017 10. May 31, 2007 ### Danger It doesn't show any more than just appears on that page, as far as I can tell, and doesn't mention what season it was in. Nice link, though; I bookmarked it. I only started watching the show less than 2 years ago when I moved in with W and got cable, so they're all re-runs that are probably not in their original sequence. 11. May 31, 2007 ### apatton wow, lots of replies, this is awesome anyhoo, no i had not considered pneumatics, primarily because i'm not that familiar with them. It seems like a good approach, but a little pricey. I'm a poor teenager so i'd like to keep this whole project under$500

i tried to find the mythbusters episode on youtube but no such luck, must not be out on dvd yet.

i've revised my original plans a bit, instead of using big gears i think I'm gonna turn the motor sideways & just use a belt to run, well, you know those big spools that single people use for coffee tables? something like that, suspended from the top, with the arms on the side, aaand yeah have the arms & guides similar to a piston rod in a cylinder, except instead of a crankshaft there'll be a wheel, aaand insead of a piston head there'll just be more flexible rod

i think why the mythbusters used pneumatics was mainly to conserve space & reduce under-water complications. Luckily i have a place to store a huge box. Pneumatics really wouldn't help conserve space either as each of the arms is 32" long, so either way it'd have to be fairly deep. I could see it as the best option if you wanted it to act like an actual arm, but for my purposes i'd have to end up making a shoulder joint as well, which i won't lie i'm scared of even attempting something that represents the functions of a human shoulder. I'd much rather just wrap long springs in neoprene & duct tape

With the belt/spool design i'm pretty confident I'll stay under budget, but if someone would like to shed a little more light on a pneumatic design/cost i'd love to see it.

But, if the belt/spool design DOES end up being the most cost effective & realistic, do you think a motor from one of those big shop fans would work? Assuming the spool was on bearings & stuff like that. That motor has a few speeds so that'll come in handy, but i'll still definately need some way to control speed within those settings, like a pot or something that on any setting can gradually decrease the speed

any ideas for such a circuit?

thanks

12. May 31, 2007

### Danger

I'm afraid that I have no knowledge of AC motor speed control; that's why I crapped out earlier and suggested a pre-wired unit.
The shop fan idea might or might not work, depending upon the rest of the system. For one thing, it'll take a long time to get up to speed. Those motors don't typically have much torque, and cable spools are heavy. On the bright side of that, there might be enough 'flywheel' effect from the angular momentum that it won't stall out when you do apply an intermittent heavy load (which is what I assume the punching action will result in).
From the way that you worded it, I get the impression that it's going to be primarily just a big wheel with spokes made of strong padded springs that whap you upside the head when they come around, rather than some straight-line thrust. Is that correct?

13. May 31, 2007

### apatton

no, they'll be straight line. There'll be a guide, similar to a cylinder wall in an engine, that'll straighten it out

the spool was just for visual purposes, the actual thing will be much lighter

are those AC or DC motors? AC right?

14. May 31, 2007

### Danger

Okay, so the 'punches' are run through some sort of cam plate? You do realize that the required torque from the motor will be drastically increased by that, I hope. Translating rotational to linear motion and then back again is not really easy. (Physically, yes; efficiency-wise, no.)
The motors that I refer to are indeed AC. Personally, I use DC whenever possible. That's not a matter of efficiency or practicality; I just happen to know how to work with them and feel more comfortable with them because of it.
I'm glad to hear that you don't really mean to use a cable spool. In that case, there's no need to use a spool-shape at all. A simple disk will serve the same purpose, with a negligible percentage of the spool's mass.

15. Jun 1, 2007

### ank_gl

but i still didnt get how you are going to get a hit back mechanism. what you are suggesting is merely a reciprocating motion, so they would be like dumb guys punching at your face in a predefined order. that aint a hit back mechanism. you ll be bored of practicing in seconds. what i thought was like when you hit it, it would hit back with either a punch or a rotating arm or something.
and talking of the piston like punch(as u say it is going to be based on the crank slider mechanism or ic engine mechanism), it aint a punch, it is going to travel the same path, not like when you hit it, it would go back. it ll keep coming at you, if you are in its path. you are gonna hurt your self.
and yes if you put some spring and then a boxing gear over it and the spring compresses by ,say 10 inches, then you are on right track.
and thanks for the link berkeman. i just love mythbusters.
and apatton, best of luck

16. Jun 1, 2007

### Danger

It would be very helpful, apatton, if you could post some sort of sketch outlining your idea. I'm having a bit of trouble envisioning just what you're up to. There are a lot of different ways to accomplish it, and I'm not sure which way you're going.

17. Jun 2, 2007

### apatton

yes it'll just be a repetetive motion for practicing blocks & parying & all that, but that's the point. It'll have adjustable height & speed

I'll sketch something out this week & post it, but i warn you my art skills are terrible :)

18. Jun 2, 2007

### Danger

That is irrelevant. Any visual aid will be beneficial. Even if you just do a 'stick-man' type drawing showing the orientation of the various parts, it will help greatly.

19. Jun 2, 2007

### ank_gl

ok just show it in a sketch, it ll atleast let us know what design are you going for