Engaging Gears Perpendicular to Shaft Direction?
 

Many teams, most with reasonable success, have made shifting transmissions by engaging gearsets by sliding gears along an axle with the proper distance between the gears for the correct mesh. This is shown as a top view on my fast sketch below.

What I'm wondering is if there have been any teams that have attempted, with or without success, to engage gear sets in a direction perpendicular to that, as shown as a side view in the sketch below.




1. I am aware this is more difficult
2. I am aware that you need to be careful about backlash and proper spacing
3. I am aware that you run the risk of stripping teeth due to the possibillity of teeth prematuring engaging on the outter-most edges instead of at the pitch diameter.

All that aside, has it been done?

Thanks in advance.

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Team 469, last years 2nd national champion had a shifter that worked like what you are describing. I got to drive it around at IRI and from what I experianced it worked out very well for them. You could ask them about it because it probably required a lot of work and math.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I dont see why this would be any more beneficial than the "traditional" method. You would have to have your shaft and bearing in some sort of slot so that the gear could be engaged. It seems quite a bit more complex for no obvious benefit. Maybe there is one that I just dont see, but as cool as this seems, It doesnt really look too practical to me.

Cory

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Well, it all depends on application.

I agree a sloted shaft and gear wouldn't be pretty difficult and wouldn't save any complexity. That's why I've got a gear moving on a pivot. Just looking for feedback on this. Thanks!

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

i'm sure that some where in the CD gallery (2001, i thought), there is a picture of a robot that uses a 'swing' shifter that works like the one you described. I would that this sort of shifter might be better than a 'sliding' shifter, because the distance that the shifting mechanism would have to move a very short distance to engage the gear's teeth. of course, this could also work against you, for example if you are a little off on your calculations, you might end up engaging the gears for both high and low speed at the same time.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

one drawback for this idea: when you have gears engaging you want the distance between the centers to be fairly precise and consistant

the gears tend to want to push each other apart

when you have a traditional gearbox, the shafts are held in a plate on bearings - they aint going anywhere

but with your idea, the mechanism that moves into place will have to be precise, repeatable, and in effect it will need to lock-in, so the gears dont push apart under heavy load.

There is a common debug problem with FIRST robots - we often get our bots working great on the table, with the wheels spinning in the air

then when we put in on the floor, and try to push something heavy, gears strip, shafts snap, motors cook.... Its hard to anticipate the secondary forces and effects that take place under a heavy load

other than that, I think if you only shift at a standstill, or when the motors are not loaded up, it should work.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

This is where I'm going to need a mechanical engineer with a little more experience than me...

1. With how much force will they want to push each other apart? I assume that this actually varies... so I guess I'm looking for the maximum force. I assume this is some ratio of the torque.

and the other obvious question is:

2. In which direction is this force applied? As for direction, I assume a significant amount of the force is pushing them directly tangent to where the teeth mesh, but I think this is this too much of a simplification. Is the direction actually related to the gears' pressure angle? What about with mutiple teeth engaging? This seems like it could develop into a rather complex problem...

I'll guess it's something like (T / (D/2))*SIN(PA) is the force pushing them apart in the plane of the shaft centers... but I'd really appreciate it if someone had something better than a guess. :)

Thanks in advance,

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

To my knowledge, many teams (including #469, #60, and #67) have used this type shifting mechanism with great success.

If my memory is correct, teams #469 and #60 both used very wide gears that they slid an idler gear and its shaft axially in order to engage high and low gears.

In 2001, Team #67 used a hex drive shaft and slid the gears on this hex with a motorcycle gearbox style shifting fork.

Anyway, I don't see why folks would argue that this method would be more complex than using so "shifting dogs" to engage gears.

It can be made to work either way.

For those who don't remember, this is the machine that I first recall seeing this type shifter used on (Team 60's 2001 robot):



Joe J.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I have often wondered this, although never mentioned it for the reasons you listed above. I have a toy, which uses some demented form of miter gears in a shifting transmission( i think they call them crown gears. Legos have them too.) Anyway it looks like this:


Please excuse the paint art. This design is probaby impractical for a number of reasons, but none the less i think its interesting. It inspired this:

Its probably still impractical but whatever.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Errr, I feel really stupid now. I looked at the drawings and for some reson assumed that you would be pushing the teeth of the gears together, face to face, rather than the diamter, if that makes any sense. This is basically the same system as Team 116 used too.

Cory

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Actually.. I think this is what I want to do?

A repost of that picture with some text.

I know you can slide the gear along a shaft into a position.




I want to move the gear and its shaft in the plane of gear rotation, as shown in the bottom sketch.

Feel free to make comments, apparently I wasn't too clear the first time around..

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Matt,

The force between the gears acts along the line of pressure. This line is determined by the pressure angle.

Therefore, you are correct with your formula.

-Chris

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Matt,

Depending upon the specific circumstances when shifting, and if you are planning to be able to shift "on the fly" then I believe you will have a couple of significant concerns with this type of design. <disclaimer>I am NOT a gearbox expert - engineers with more gearbox/transmission experience are welcomed/encouraged to coment/correct my statements</disclaimer>

1. Stripping the gear teeth could be a huge problem. When the gears first begin to engage (while moving) there will be a significant load concentrated across a very small shear area as just the edges of the teeth of both gears are in contact. At this instant there is also a much larger than usual (about 2X) moment arm which will cause increased bending stresses at the root of the tooth.

2. It will be very difficult to maximize your efficiency while allowing gear shifting in the plane you suggest. A few thousandths of an inch change in center distance can make the difference between gears binding if too tight, or having too much slop/backlash if too far apart.

OK experts ... fire away and educate me ...

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Yes, you could do this, it would take time and in my opinion pretty spiffy enginering. But there would be some issues like grinding. My team had the CIM motors geared down on a the bot we used 2 years agot. Even though the gears were in constant contact, there was a very small bit of play in the distance between two of the gears, this caused the gear to become flat over time. So you would have a big issue with grinding gears in this idea. There is the possibility of binding, too much pressure pulling that gear into the powered gear could cause problems. You also must think about the movement you are creating. You could close out that entire gearbox, so that nothing could get into it, but you probably won't. Having this movement in competition could be a serious problem. If your opponent has an arm or some object protruding out of the bot, that extremety could get between the gears, and you could not shift. You are also creating a nightmare in the repair department. This design, in my opinion, could cause you to spend more money than needed, and cause you to have to repair parts more often.

A tip I use when working on practicle gear box designs is "when designing, draw something would find easy to build and repair, draw something that will accomplish your goal quickly, draw something that will that doesn't have more parts than needed, and draw something that will acomplish your goal in a simple method."
ivey

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I summerized some of you concerns in bullets. I am aware of all of those, I think I made a pretty clear in my first post:

My response is going to sound grouchy, and I'm definitely not:
Before you start stating problems with a "design", make sure you're actually looking at a design! :)

(All of those bulleted concerns, especially the last three extrapolations, were based on a 5 second MS Paint sketch with 4 circles and an arrow. It might have been jumping the gun a bit...)

However, I do appreciate the concern, Mr. Ivey. I know your intentions were in the right place. I've just seen this happen in other posts too, by many other people. It's just something I wanted to make everyone aware of to make the CD forums an even better place!

To stick on topic...

Has sliding gears in their plane of rotation in a gear box been successfully done?

Matt

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.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

eeek... I gotta say I didn't do that! :ahh: (Though admittedly, small things like how much those shafts would deflect when they had a pneumatic on them were definitely calculated (.00045 inches))

To do so real quick:

Maybe 250 in-lbs on that center gear, 1 in diameter, moment arm of a half inch off center... the pneumatic is about 3.4 inches away from the pivot point, at around a 15 degree angle, providing a max moment arm in the opposite direction of around 95 lbs * 3.4 inches with a 1.5 inch bore... There's also the force on the gears through the shaft centers, sin(20)*250 is around 85, times it's moment arm of 1.5.... So we've got around 70 in lbs to spare with the half inch. whew! :cool: Thanks for the reminder...

I appreciate you noticing, and while this is true... sometimes outsiders see something obvious that the "careful thought" folks have missed which will make a device infinitely better... which is why it's out here. :)

I've reread this about 10 times now.. I just can't visualize what you're saying. Are you suggesting having some loose chaing that is engaged and disengaged with sprockets? If you keep the gears engaged constantly.. how would you shift?

I've already thought of some nice ideas about changing the sizes of the blue connecting sprockets to let them affect the gear ratio, and keep the gears a (larger to reduce load) constant size... but I'm certain this is not what you're proposing. If you could clarify, I'd truly appreciate it!

Thanks,

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I think he means to move gear to chain, rather then gear to gear.
Have both final gears spinning, but rotate them to move the proper one into contact with the output chain. Puts all the meshing forces onto the chain.

At least, that is what I understood him to be saying.


Wetzel
~~~~~~~~~~~~~~~
Making and breaking chain with needlenose pliers is fun.
I swear.

How's this kluggie cut up?
 

The idea is to move the gears closer together so that they are always turning with the input gear.

Then put an idler sprocket on the same shaft as the input shaft (now the shaft marked with the green cross).

Then pivot the both output shafts on a lever that also pivots about the input shaft (it is outlined roughly in a T shape in the image attached).

Then when you drive this link, you engage one sprocket or the other with the chain. (The sprockets are those round black circle things)

You can have the air cylinder either be strong enough to take the load you are pulling on the chain -- in which case you sort of get a chain tensioner built in for free. Or you can add a drive link between the aircylinder and the sprocket pivoting gig - in this case, you can make the whole thing non-backdriveable and then tension in the chain cannot push the mechanism out of engagement (I like this solution better... ...but time is limited so I will have to discuss it another time if there is interest).

The chain routing is shown for the mechanism in high gear (the thickish blue line).

I hope this helps.

Joe J.

Re: How's this kluggie cut up?
 

That's BRILLIANT Dr. J !! :D

Re: How's this kluggie cut up?
 

Can the chain &/or sprocket(s) take this type of abuse (shift on-the-fly ...) ?

Re: How's this kluggie cut up?
 

If I had my choice of chain or gear teeth that had to take the abuse I know which one I would pick.

Shift on the fly may be a bit of a stretch, but I suppose that it wouldn't be too bad if you could figure a way to use #35 chain.

Joe J.

Careful is an adjective not to be glossed over...
 

You forgot the input torque of 250 in-lbs (or else you have to include the torque from the internal short chain -- which magically will work out to be the same 250 in-lbs).

I see 4 torques to deal with
In high gear with driving the input CCW,
Tinput = 250 in-lbs
Taircylinder = 95 lbs*3.4 in = 320 in-lbs
TtoothNormal= -(250 in-lbs/.5 in)*.5 in = -250 in-lbs
TtoothSep= ((250 in-lbs /.5) * sin(20deg) ) *1.5 in = 250 in-lbs
This is okay because they sum to 470 in-lbs (and CCW is toward engagement).

If you do the same thing only have the input torque switch to CW you will get
Tinput = -250
Taircyliner= 320
TtoothNormal = +250 (the normal tooth load is trying to ENGAGE the mechanism in this case)
TtoothSep = -250
Sum = 70 in-lbs

Having done the summing carefully, I get the same answer as your "quick calculation", so what do I know?

I suppose I will shut up now ;-)

Joe J.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Jeez!
Doing that would put waaaay more torque through this box than I would think is healthy. (I haven't run numbers on this). In order to function correctly, this thing would have to be as "early" in the gearbox as possible, to minimize torque on the shifter.

Matt--
I'm very impressed with this design.
This may be a step closer to the "rookie solution" to shifting the FIRST community could use so badly. (Sorry Andy ;))

I'm very curious to see how it will work out for you guys. :D

John

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Not necessarily, if you’re doing a multiple motor gearbox you could just combine in the gearbox, go straight to the shifter, then adjust in the chain. Doing it this way could actually be really helpful as only the smallest sprockets come in nice sized bores and you need a very large sprocket if you only want to drive the wheel(instead of the whole shaft) in your chain.

National champion team 469 used the exact same(or at least very similar) shifting design in 2003 with two motors per side without any trouble, even though the shifter was very late in the transmission.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

One change that can be easily made to the shifter to increase the speed change without using different gears could be making the output shaft always be the shaft that currently has the 1:1 ratio, and changing the blue sprocket to a 20 tooth if the other one is a 10 tooth, or something similar. This would give you a 3:1 reduction and then a 2:1 reduction giving you 6:1 and a 1:1. you could do less or more teeth to make the increase more or less dramatic. That way newer can be very competitive with just an atwood drive having maybe 6ft/s and then 1ft/s or whatever they would like.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

The torque applied depends greatly on your motor combining, and what speed they are matched at. Especially if this combiner gearbox is designed to go straight to the wheel after (as Rob mentioned). The final sprocket reduction is limited by several factors. (Such as Chain size... note: I believe "national champion team 469" blew 1/4" chain like nobodies business towards the end)

Also... I'm almost 100% positive, "national champion team 469" used a shifter NOT like Matt's, but similar to 60-2002, or 116-2002 -- where the gears slide laterally. This system is able to handle much more torque than Matt's (in which applied torque would actually try to push the shifter out of gear).


John

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Yes it does, however that torque is the same if you use a dog shifter or a shifter such as this. Plus he said he was possibly using 16 pitch gears, and steel 16 pitch gears are a lot stronger than your average shifter.

Keep in mind, his design doesn't even use #25 chain. While the reductions possible using sprockets is somewhat limited, its still pretty wide. Using #35 chain, you can get as much as a 3:20 reduction, which is pretty good. If you needed any more than that you could just change the ratios inside the shifter or add another box with another reduction after.

Some Info
 

Actually, this really depends on what you mean by "average shifter" I would wholeheartedly rather bet me life savings (which is admittedly negative since I'm a college student) on a dog style shifter than gears engaging face to face, if you're speaking about going on the fly. WHICH THIS DESIGN IS NOT INTENDED TO HANDLE. :)

I've plugged some numbers into a Lewis bending equation spreadsheet I've made, and let me tell you, with the 1" gears, failure is VERY possible, depending on where you place it in the drive train. Using the initial configuration I had, the factor of safety was around 0.7... :ahh:

I most definitely do use #25 chain in this design, since it's loads are planned to be somewhat light. If I were placing this on a drive train, I'd probably slow the Chips down by maybe by 3 or 4, throw that shaft into this magic box, and probably use sprockets on the wheels to slim it down by another 4. By this, I don't expect the gear box to see torques exceeding around 50 or 75 in-lbs. I think that #25 is safe in these situations, even with the small diameter of the sprockets being used. #35 chain might add another half pound per box, for no real good reason.

Just some thoughts,

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I think you fail to understand what I'm saying, and the main principle behind my comment.

Any torque applied through THIS design, will cause the gears to try to disengage from each other. The only thing preventing this is the force of the pneumatic, holding the shafts together.

In a Dog-style, Pin-Style, Mesh-Style... Shifter, the kind of shifters we see every day in this competition: The shafts are held rigidly. Therefore the applied torque CANNOT seperate the gears from meshing (unless it deflects the shaft, which is a discussion for another time).

See the major differences?
See why Matt's design would have MAJOR torque limitations that wouldn't apply to a "normal" shifter?

I can guaruntee you haven't seen any design like this one in the FRC, so it is unfair of you to draw parallels between this box and another and make broad statements concerning it's capabilities.

I love 469, they are a great team, their last 2 bots have been works of art, and their 2002 bot kicked my butt pretty hard. Great designs, in every part of their bots, including the shifter.
But... that doesn't really apply here, now does it?

Unless you understand all the principles involved, and the design itself... which you've shown some confusion about... perhaps you should limit the broad statements you make about it?


John

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Joe Johnson had said earlier that the design should be able to hold in gear just with the piston. If that was ever a problem you could always increase the bore size. With over 120 pounds pushing against it you can put some considerable torque into the shifter and under normal circumstances not have to worry about it disengaging. Matt had even said that he has a spring system in the workings to hold it in tighter, so if you were afraid of it comming out in low gear you could always reinforce it with springs.


I believe the statement about the chain sizings was reffering to the final chain coming from the output shaft and powering the wheels.


John, lets try to keep negative comments out of general discussion. The point of the thread is to express ideas and possible improvements to Matt's design and to TEACH people if they don't understand a concept. You could possibly explain something you feel is being misunderstood instead of suggesting the person not post ideas they may have. Undeserved comments should be kept in private, not on the forum discussions.

With the possibility of using multiple motors, you can still put this shifter as early in the transmission as you would with any other shifter. From my understandings Matt was trying to make a shifter for newer teams who do not have the machining capability that some of us have. No where has anyone said that the design was perfect, but you can always work in some additional support to the shifter to accomodate a more torquey drive to an extent.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I don't see any "undeserved comments" in anything John posted. John has been making some very good points, and he was just responding to some attempts to knock down those points. He was trying to stick to the point of the thread, and has done a good job at following that. He explained his reasons. Don't be one of the people who try to drag good threads like this through the mud over some sentences and wording you didn't like.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I talked with Matt and some other people about a question I've had, but I thought I'd toss it up on CD for some more opinions and discussion. I am not anywhere near as skilled in engineering math as I'd like to be, so keep that in mind, please.

What I was wondering, essentially, is if there are clear benefits to one method of "swinging" a gear into place over another.

Matt's posted design uses this method:


The gear pivots about a point and swings into place such that the line between the center of the output gear and swing gear is perpendicular to the line between the center of the swing gear and the pivot.

After seeing Matt's design, and for no other reason that my irrational fear of chain, I began working on alternative methods of achieving the same "swing" transmission design. My design, in order to eliminate the chain, relies on this method:


The gear pivots about a point that is colinear with the center line between the swing and output gears. It maintains the same center-center distance between those gears with rigidity.

Matt showed concern that the later method of gear engagement was more prone to breaking teeth and other such bad things, but I can't understand why. I don't see how that's the case, but I trust that Matt probably has a better understanding of what's happening than I do, and was hoping someone could explain it in detail.

Re: How's this kluggie cut up?
 

This is probably an elementary concern, but the rule of thumb I remember being told about chains and sprockets is that it's best for the chain to engage at least 90* of the sprockets circumference to ensure it doesn't slip.

Is that a concern here?

Re: Engaging Gears Perpendicular to Shaft Direction?
 

First off, I'd like to say that my primary concerns with engaging gears along the direction of rotation as shown in method number 2 are that of dynamic forces. Assume that one (or both!) gears are rotating at a reasonable rate counterclockwise when they're trying to be engaged. Eek! You have a sizeable angle of motion where the gears are not at the pitch circle diameter, as well as dynamic forces opposing the motion. Outer edge and angle shown in red (circles and angle shown slightly bigger for clarity)




While there maybe be some benefit of one gears in the above case rotating clockwise, essentially "pulling" the gear... you still have "tip of tooth" engagement over it's travel. This scares me.

In the case where the arc path swings towards the center, the distance travelled in the "danger zone" of outer tooth edges is as short as possible. I think this is better.

I think it's important to realize too that this is a very non-traditional method in the first place.. anything I can to make this safer I'm going to do.

In addition, the last time that Miss Krass and I spoke, which may have changed, she was proposing that style number two not engage a single gear as shown in the drawing, but swing in-between two gears and function as an idler gear. I think this would have a dramatic increase in potential binding and would have immense tolerances requirements.

Anyway, that's my two cents. Anyone else care to share?

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Just so there's no confusion about what we're talking about, I'll toss these screenshots of my own design into the mix so that people can see it in practice.

In high gear:


In low gear:


The idler gears swing between the input (blue shaft) and output (gold shaft) gears. They are fixed, however, relative to the output gears and remain engaged at all times to the output gears.

I agree, of course, that the "danger zone" may be greater for the method I used here, but I wonder how significant of a difference it really is. It may be fractions of a second, which may or may not be enough time to noticeably increase the potential for damage to occur.

Re: How's this kluggie cut up?
 

It is a pretty good rule of thumb to say that 90deg of wrap, but it is not written in stone.

With a tight chain, you can do less -- especially if you have more than 3 or 4 teeth engaged (for example you may be able to have fewer than 90 deg wrap with a sprocket with more than 20 teeth on it).

But, that said, I don't think this design necessarily breaks the 90 deg. rule. I believe with some clever routine of chain (and perhaps moving the drive/idler axle higher) you can get the 90 deg. wrap angle that will let you sleep at nights.

Another exercise I will leave for the student...

Joe J.

I STILL don't like it...
 

I know that a lot of time and energy has gone into this type of radially engagement of gears as a means of shifting gears, but I have to tell you all that it is creeping me out pretty bad.

This is not a really good solution. Engaging drive dogs is one thing, involute gears are another still.

For all this, I keep going back to the shifter on the drill itself.

If you really want to design a good rookie solution, crack that nut. Beleive me, this is a much more doable solution for 99% of the rookies you are trying to help.

Joe J.