Re: Engaging Gears Perpendicular to Shaft Direction?
 

Yes, I think that it has been done. Team 393 (yet another Indiana team... wooo!), did it in 2002. They did it with sprockets. I don't know why, but maybe it was for easier engagement. I do recall that this system was noisy and the sprockets wore down. Paul Osborne or Caleb Osborne (father and son) are the guys to ask on that team. Libby Ritchie is the team leader.

Here is a link to their site:
Full Metal Jackets

I did not find any pictures of that 'bot, but I am sure that they are "out there" (no time now to look).

Good luck,
Andy

Re: Engaging Gears Perpendicular to Shaft Direction?
 

you mean meshing two sprockets together like gears, or moving a chain?
Also has anybody ever considered/tried something like a bicycle shifter?

Re: Engaging Gears Perpendicular to Shaft Direction?
 

being a mountain bike enthusiast, i've put a little bit of thought into the subject, and have come away with a the main impression that while it can be done, it will have a few limitations, most of them do to the fact that a bike's shifting system is made to run in only one direction: forward. because of this, if you pedal backwards the chain won't shift, and if the derailer is in bad shape (or poorly made) the chain will come off the sprockets. in addition, the shifter would be a strictly on-the-fly shifter. it would be impossible for it to shift if the robot is stationary, or moving too slowly. another thing that may give a FIRST team trouble is the fact that most high-end mountain bikes use specially made sprockets that help the chain climb from one sprocket to the next. these sprockets would probably have to be made from scratch.

http://travel.howstuffworks.com/mountain-bike.htm <--some pretty good stuff on mountain bike design

Re: Engaging Gears Perpendicular to Shaft Direction?
 

As a matter of fact, we on 188 tried this very thing last year. It certainly looked good on paper, but we ran into serious issues with the gears grinding, and the centre distance of the gears being extremely susceptible to changing. One moment the gears would be in mesh, the next, the forces acting on the gear (due to rotation) would push the gears apart, causing a loss of contact. To combat this, we tried pushing harder on the gears, so that the gears would be "happier" while running. The problem with that was that the gearbox was now too tight to accelerate properly--the gears would just jam. (Then all of a sudden, as we tried to carefully adjust the spacing, they'd move out of alignment, putting us into neutral once more. At least the motors weren't getting damaged in neutral :rolleyes:.)

With regard to the jamming issues--don't even think of overpowering it, and then settling down to a happy equilibrium once the robot gets up to speed. First of all, it's a waste of power when you need it the most (i.e. acceleration from a standing start, or in a pushing match), and second of all, our robot last year had 6-motor drive (Bosch, F-P and CIM), and still couldn't overcome the jamming issues with much finesse. It comes down to the fact that your centre distance has to be within a few thousandths of an inch of the optimum spacing (quoted centre distance plus 0.003" to 0.008" is a good bet for the optimum value).

If you do want to try it, keep your tolerances very, very tight (there's no room for error here). Also, we used 20-pitch steel and cast-iron gears--you might want to try these, since despite the unusual forces exerted on the gears, they held up admirably.

Lastly, design the gearbox so that it can be locked into one gear (high or low) if all hell breaks loose. We made some significant last-minute changes to remove the shifter stage and set it into high gear, when we decided that it was too much trouble. It was good enough to come second at the Cdn. Regional and win W. Mich., even without a low gear...

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I agree with that one. While we did not use a shifting mechanism last year (we had some cool ideas, but just ran out of time), we have successfully stripped a set of bevel gears during the Canadian Regional so badly, you could barely tell these were gears once :D There was too much torque on the shafts, and we did not support them well enough. This could definitely be a problem with the mechanism you are describing. Remember, no amount of testing you do will prepare you for the actual competition, since no one can force themselves to put a robot they built with their own hands through some of the stuff that happens to it during the actual competition :D

Re: Engaging Gears Perpendicular to Shaft Direction?
 

There is no reason why this type of shifter wouldn't work(referring to the original drawing). There are a few problems to overcome so it comes down to a matter of pros and cons. The cons being the majority here, lets just list those. The transmission is not very compact, requires precise engagement of the teeth at speed or load, requires firm! locking of the driven shaft in place to constantly mesh with the driving shaft, requires a more complex and weighty shifting mechanism and if any of the above fails or is intermittant, the result is catastrophic. The result is many teams use an alternative that minimizes the cons and maximizes the pros. The last robot standing is not necessarily the better robot, just the last one standing.

From the if-you-are-going-to-hit-yourself-with-a-hammer dept...
 

...use a small hammer.

By this I mean, that I think this is a pretty bad idea for lots of reasons... ...but if you are going to do it, do it as close to the motor as you can.

I have actually seen something like this used on some small toy transmissions. They did not have a linear engagement, but an arc one.

Essentially, an idler gear was mounted on an arm that pivoted about the motor's output gear. This arm's movement would engage gear A or gear B depending on the rotational direction of the motor. It worked for the toy.

I would not like to trust my robot's ability to move on such an idea, but to each his/her own.

Joe J.

Re: From the if-you-are-going-to-hit-yourself-with-a-hammer dept...
 

When asked why the man was hitting himself with a hammer, he replied, "Because it feels so good when I stop!"

Re: Engaging Gears Perpendicular to Shaft Direction?
 

If I were going to make a transmission that used the swing in type system to engage the gears, I would make the swing-arm movement such that it bottoms out or stalls out against a fixed flat on the gearbox to set the mesh at the perfect distance. This would be a perfect location to also simply screw the movement to the gearbox to lock it into gear or 'latch' it in like a swining gate. I can imagine some sort of ball screw driving the assembly back and forth or maybe two opposing swing arms for the two gears such that FWD on the motor brings one in and the other out and vice versa. Not sure if I would trust an Air cylinder for this unless it was fairly substantial or it pushed on a lever fairly far from the pivot point.

See Super rough paint sketch (No AutoCAD on home computer and too lazy to boot up laptop)

Why the question was asked in the first place...
 

Since so many of you have contributed to this post, it's not really fair that I don't put the design out there for some people to look at.



I've been sitting on this for a couple of weeks now, slowly tweaking some aspects of it. For better or worse, I've lost my fear about gears engaging in the direction as show. We'll most likely be using a modified design of this if a transmission is required for this year's game.

Essentially your green shaft is the input, which in turns spins the purple sprocket pair to spin the attached spur gear. This spur gear is located on the shaft and can be shifted to engage either the large or small gear, which outputs to either of the gold shafts (pick your favorite, they're connected by a pair of sprockets)

What's not shown is the spring setup which holds the toggle against the high torque gear, in case of pneumatic failure.

Right now there's some 20 pitch gears with a half inch face width, they may go down to some big 16 pitch if i can find some with a half inch face width.

I've come to discover that there aren't many transmission designs available that are:

1. Low Cost - Below $300 for 2 of these

2. Requires no CNC work - This does require accurate work, but it can be done on a drill press with a quality setup. Tweaking can be done with the stopping rods to ensure proper meshing.

3. Modular - Since this piece isn't completely integrated with a motor setup, it could be built in the 3 day period after a competition. The two gear settings of 1:1 or 1:3, means that if you throw it on board to an existing system, your max speed is the same as without it, and the other setting gives 3 times the max torque.

Since there's been some major conclussions that you can't be competitive with just a 2 motor drive train, it's important that rookie teams have some sort of access to a shifting transmission that is easy to build, tweak, and can be forgiving.

One advantage of the design is that since the shafts to stop the toggle are round, they can be made slightly larger or smaller to accodmadate other innaccuracies in the manufacturing process, providing more or less backlash as needed.

Since the pneumatic is below the engaging gear location, there's some mecanical advantage. Since you're going to want to have this setup near the start of your gearing setup, the kicking out forces that the gears will want to do could theoretically be held still with a 3/4" cylinder at 60 PSI with some reasonable factor of safety, but a 1.5 diameter bore is shown above.

Care in the geometry has been taken to ensure that the location of the red toggle is always perpedicular to the engaging point, which means that the toggle member itself will take the engaging forces down its center, in compression.

The housing is what is undergoing the revising right now, as well as performing some calculations on the gear strength. The pitch diameters are at 1 inch on those smaller gears, and they'll probably be increased to 1.25 or 1.5 in the final version.

There will be a full white paper of this design cranked out by the end of the build season. If you're interested in testing this design along with us this year, please let me know.

Comments and thoughts are always welcome.

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I like the simplicity, and the method on keeping both the different output shafts powered at all times. Only thing I would worry about with those two sprocket sets is keeping the chain tensioned properly. I would also be sure to put a nice little disclaimer for teams in the white paper about some things to watch for and possible breaking points so they could buy spair parts to ensure they will always move.

[edit] Thinking about this shifter a little more, I see some big advantages to possibly 2nd or 3rd year teams out there. Since all this idea is, is a simple 3:1 and 1:1 shifter, teams who were planning on doing a 2 motor, or even possibly a 3 motor gearbox per side can still use that gearbox, run chain from it into the shifter, and then from the shifter to the wheels. The way the design is given leaves all the math entirely open so teams still have to do a majority of the work on thier own. If anything teams can use this as a learning tool in the off-season to make a combiner gearbox and run it into the shifter, and then in the real season they could incorporate the two together and add to the design. Very flexible in the area of speeds too as long as you the rough calculations. [/edit]

Sprocket Center Distances
 

This was a concern for me as well. However, I used the formulae to find the proper distance for the sprockets, which comes out nicely if the two sprockets have the same number of teeth. Since the chain lengths are so small, (less than a foot)... I'm hoping that stretching with not be an issue.

I've attached my sprocket distance calculator for those interested.

Thanks for the encouragement!

Matt

Re: Sprocket Center Distances
 

I must say. This looks like a well though out design, and i am impressed.
It has been mentioned that gear wear may be of some problem. This is just an idea that is probably not workable, but hopefully it may inspire something that is. Jamming asynchronously spinning gears together seems a bit harsh. Would it be possible to put a disk of frictiony compressible material on each of the three shafts, which would engage before the gears mesh bring them up to speed sort of like synchros in a car xmission. This material would prolly wear out pretty quickly, but it if it prolongs the life of the gears at all and if its cheap and easy to replace, then it might be worth it. This may be uselessw/o a clutch, or just useless period, but i figured it was worth posting. It might also be worth experimening w/different gear profiles. This may be useless w/o a clutch, or just useless period, but i figured it was worth posting.

On the fly? Not quite...
 

Thank you, there were a number of people who were influential on 461 and from Purdue who helped bring about its design.

I definitely agree!

This design is either "super-slow on the fly engagement" or from a brief standstill.

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Have to done a careful sum of the moments calculation about the axis of rotation of the input shaft?

I am suspicious that the drive link can stay engaged with just the aircylinder.

I am probaby missing something because my mind is telling me that it must be near the wheels (or else why would I see so much chain? -- I typically only use chain toward the end of the drive train). But, I think you made some sort of statement that had this up high in the geartrain. So... ...I am confused.

But... ...either way, here my thoughts.

#1 I alot of careful thought has gone into this. Careful thought is never wasted.

#2 My brain keeps coming back to, why have the engage/disengage gears at all? You have the chain sprockets. Why not just mount the gears and their shafts on the pivoting link, keep the gears engaged 100% of the time and just pivot the whole mechanism to allow the "high gear" sprocket or the "low gear" sprocket to engage the chain? I think your mechanism gets simpler, your gear loads and forces are more easily managed, and the chain takes all the abuse (which chain is quite happy to do -- chains love abuse).

Just my 2 cents worth.

Joe J.