Gear pitch preference

[Andy Baker]

For you drivetrain gearbox designers, hear is a question for you: what is your preferred gear pitch?


Andy B.

[Max Lobovsky]

I have been meaning to ask about pitch, also. I'd be interested in a bit more information, also. Specifically, face width (and wether the full face is engaged or not, i.e. shifting gears by sliding them in and out of each other) and estimated maximum torque the gear withstands

[Tristan Lall]

Originally Posted by Andy Baker

For you drivetrain gearbox designers, hear is a question for you: what is your preferred gear pitch?

Just one preferred pitch? With these 3-motor monstrosities I design, and the fact that we (at 188) never figured out exactly how to replace the gears (nor really tried), gears of all types find their way into my designs.
Older Woburn gearboxes used mostly 20 pitch, 14.5° pressure angle gears from Boston, because they were easily available, machinable, reasonably durable, pretty cheap, and featured in the Small Parts catalogue (back in the day...).
And the current Woburn design still uses 4 of them. But I was introduced to SDP/SI's line of hardened steel gears last year--and they're pretty neat too (hard to machine without an EDM, though). If cost were no object, I'd like to make greater use of them. I used the 24 pitch, 20° pressure angle ones, but they also come in 32 pitch, if my memory serves me correctly.
I don't like metric gears, because PIC Design sells them....
Edit: To clarify, last year's Woburn gearboxes contained 0.7 M/20° PA, 32 P/20° PA, 24 P/20° PA and 20 P/14.5° PA gears.

[Rickertsen2]

This is an interesting thread. One thing that i have noticed is that ALMOST EVERY gearbox i have sene in first is grossly overly beefy. I have seen some beefier than some stages of my car's tranny. Last year we used metric module 1 (i think) gears with face widths varying btw 6mm and 8mm. They stood up just fine with no tooth chipping breaking etc. There was a very small about of wear and we may go to module 1.5 next year but they held up very well and we didn't have a single problem. Except for the final output shaft, all of our shafts were 10) REALLY hard steel (soo hard that is was difficult to machine. We actually shattered one of them during machining). We also used needle roller bearings.

[patrickrd]

I haven't designed a gearbox for FIRST (at least in a few years), but if I were to do so: I'd first need to know the the transmitted force is on the gear teeth. I'd then decide on a material (or a couple candidate materials) that are available, reasonably priced, and appropriate for the torque and speeds expected.

Once I have that, I'd look at several different pitches. For each pitch, I'd calculate the minimum required gear thickness to handle the load without tooth deformation. This can be found using the formula:

b = F/(mYs) (mm)
where:
b = tooth width (mm)
m = 25.4/P (P=pitch diameter in mm)
Y = lewis form factor (which is a function of the pitch, which can be looked up in an emperical chart in a machine design book)
s = yield strength of the material i've chosen (Pa)

Note that the formula does not take into account fatigue or misalignment, or other factors that might damage gears.

Finally you add in a safety factor (usually > 2), so you multiply your thickness by at least two. Then you can look at all the gears you want (and know that they are strong enough) and find the one that is cheapest, easiest to integrate into the gearbox, or whatever other criteria you desire.

This is how I chose the gears used on Cornell's RoboCup team. We ended up using plastic gears with 32 english pitch, if my memory is correct.

[Andy Baker]

Originally Posted by Tristan Lall

Just one preferred pitch? With these 3-motor monstrosities I design, and the fact that we (at 188) never figured out exactly how to replace the gears (nor really tried), gears of all types find their way into my designs.

This is why I made the poll so you can pick more than one choice. I agree that using a combination of gear pitches could be the best solution.

As for over-designing the gears with regard to pitch and face width, I think that most FIRSTers do this for multiple reasons:
1. Larger pitch gears seem to be easier to get, with a shorter lead time (especially from Martin and Boston).
2. For most FIRST design applications, it is better to design things more durable than they need to be. Of course, we have the weight limit, but we all know that this is an impact game with some highly dynamic loads and jerks on your mechanisms.


Andy B.

[Andrew]

We have used 20 dp on our final stage of the gearbox for 2003, 2004.



In 2003, we used 5/8" shaft with a 3/16" key way and a .5" face width, and the key ways started to run. I.e. this was a marginal condition. We performed a lot of pushing with 2003's robot, so this may be about as far out as we would ever go with a FIRST design. The gear teeth showed no appreciable wear.

We had a six wheel design, so the robot's weight was distributed over six wheels and two gear boxes. The middle wheels were chain driven to the front wheels.

The worst wear in the key ways occured in the gear box which had the BDMs powering and the middle+front wheels. I.e. we had more torque available and we had more normal force on the ground contact side.

In 2004, we used 1/2" hex stock and a .4" face width. We had six driven wheels, each with its own motor, so that, nominally, each wheel had the same normal ground contact force. We saw no appreciable wear in either the hex shaft or in the hex 'hole' in the gear. We saw no appreciable wear in the gear teeth. This robot also did a lot of pushing (we tried to move the center platform in autonomy a couple of times); however, since we had added another set of motors into the drive system, we backed the gear ratio down a bit.

Nonetheless, since the force on the output gear is more strongly affected by the available force at the ground than by the gear ratio, the loads on the output gear and shaft would be similar. I still think 2003's robot saw more load on the BDM gears than in 2004. However, the difference in performance with using hex stock instead of round shaft+key ways was remarkable.

In the intermediate stage in the gear box, we use 24 dp in 2003 and 20 dp in 2004. The reason for the change to 20 dp in 2003 was that we decreased the number of teeth in the pinion gear. We used 3/8" hex stock for this stage and the ID for a 24 dp gear was so close to the hex size that we did not have enough wall thickness. In order to keep the design gear ratio, we bumped up the pitch from 24 to 20 and moved the center distances.

We used a face width of 0.2" for these gears and saw no appreciable wear in teeth, shaft (aluminum yet!), or gear 'hole.'

In our mini-gearboxes for the FP motors, we use 32 dp to match the motor's pinion gear. In our mini-gearboxes for the BDMs, we use 0.7 module gears to match the motor's pinion gear. Although we used metal gears in 2004, we would probably use plastic gears in this stage in the future, both to save on weight and cost.

However, we'll have to see what the design rules for 2005 are (i.e. can we buy from anywhere? do we have BDMs and FPs? Etc.)

[JVN]

20DP Baby!

We standardized to this last fall, and it hasn't given us ANY problems.

Probably a little bit overkill for the early stages of our gearbox, but if anyone has seen our current layout, they'd understand why we go 20DP from the motor to the output.

It just makes things nicer.

JV

[Tristan Lall]

Originally Posted by Andy Baker

This is why I made the poll so you can pick more than one choice. I agree that using a combination of gear pitches could be the best solution.

Andy, you'd better ask Brandon to fix that...as it stands, you've got "radio buttons" in the poll, rather than "checkboxes". That means, I can only actually vote for one item. (Look at that last election poll to see what I mean.)

[Denman]

Go metric....... i'm afraid i dont know what the others are tbh.....being in england, most things are metric.

[Tristan Lall]

Originally Posted by Denman

Go metric....... i'm afraid i dont know what the others are tbh.....being in england, most things are metric.

Do you have easy access to 0.7 module gears? Because they're virtually nonexistent in North America. (We can get a limited selection of plastic ones, and expensive metal ones with long lead times.)

If so, your services might be invaluable...far too many teams have spent far too long trying to figure out an elegant and inexpensive way to adapt Bosch motors to their custom gearboxes--if we get another Bosch with a 0.7 module gear, the same problem might present itself. So any ideas on where to order them from would be helpful.

[Ian Mackenzie]

I also like 20 pitch, although with SDP/SI selling a good selection of metric gears, I'm tempted to switch at some point. I like 20 pitch for the reasons everybody else has mentioned - they're common, quite strong, and not too heavy. However, another advantage of 20 pitch over something smaller is that they're less sensitive to small displacements - if for whatever reason the two shafts are not exactly the right distance from each other, larger-pitch gears are much less likely to get damaged or become inefficient.

[team222badbrad]

Last year we used 20 DP,20 degree PA. All gears also were 1/2'' in width.

Martin gears...

What are the Tigertrons cooking up for this years transmissions? Who knows.

[Ken Patton]

we have used 32 pitch (or 0.7 mod for the Bosch) for a lot of our gears in the past. However, for some of the high power systems (three motor drives) we used 20 pitch. We have tended to vary face width as a way to (0.25", 0.375", 0.5") as a way to optimize beyond the pitch.

Ken

[maclaren]

"The" gear supplier is Linn Gear. They are one of the only full service gear Manufacturing facilities in the US. We got all of our gears and sprockets there. The have good prices and fast turn around.

Web site is: www.linngear.com

If they don't have it made and on the shelf they will just make one. They do english and metric gears and sprockets along with some timing belt pulleys.