Originally Posted by GeeTwo (Post 1578494)

While weight of the bull gear would eventually be an issue, the decision to go from single-stage to double (or triple) stage is more commonly driven by geometric considerations (the SIZE of the gearbox, esp. relative to the wheels). Pinions for most of the FRC motors typically have 10-15 teeth. Smaller numbers of (spur gear) teeth would result in poor pressure angles. Gears with more than about 50 or 60 teeth (again, in FRC-typical sizes) are often larger than the wheels they drive, leading to untenable gearboxes. That is, at somewhere around 3.5:1, a two-stage gearbox is preferable to drive wheels of 4" or larger. At somewhere around 14:1, a three-stage gearbox is preferable to drive 4" or larger wheels.

With planetary gearboxes, these ratios are somewhat different, of course. Planetary gearboxes can reduce up to 10:1 (theoretically a bit higher, but no more than 14.9:1) per stage.

Originally Posted by sanddrag (Post 1578523)

In my 15 years in FIRST, most of which consisted of reading Chief Delphi and designing gearboxes, and my 5 years spent studying mechanical engineering, I have never once heard the term "bull gear." Is this some kind of new slang or do I just live under a rock? Would someone care to fill me in?

Originally Posted by sanddrag (Post 1578523)

In my 15 years in FIRST, most of which consisted of reading Chief Delphi and designing gearboxes, and my 5 years spent studying mechanical engineering, I have never once heard the term "bull gear." Is this some kind of new slang or do I just live under a rock? Would someone care to fill me in?

Originally Posted by asid61 (Post 1578516)

Why do you say a planetary can only go up to 14.9:1? The formula should just be R/S + 1, and as R (teeth on ring) gets larger it better approximates a gear rack, so S (teeth on sun) can get smaller while avoiding interference.

Originally Posted by GeeTwo (Post 1578539)

The bull gear is the gear which handles the greatest torque in a drive train. I am most familiar with the term in relation to submarine drive trains, but I have seen it in other contexts.


See this thread. Essentially, near or beyond this limit, the sun gear is too small to engage the planetary gears. (I assumed that there are at least three; if you only have two, you can go higher).

Originally Posted by chadr03 (Post 1578547)

If weight is your only concern larger gears can be machined thinner below the teeth and spoked to be much lighter. A gear rim will typically have significant stiffness and rating if there is at least two tooth depths of solid material below the root but beyond that most of the material is not very highly stressed. So you can get quite high ratios without significant addition of weight when compared to a solid gear.

That being said the main limitation in very large reduction single stage gearing is deflection across the pinion under load. The larger the ratio the greater the gear forces and the larger the gear forces the more deflection along the shaft. When the pinion deflects all of the loading will move toward one end of the tooth effectively reducing the face width. If there isn't significant loading then large reductions can be made with a single stage. If there is large loading deflection starts to become a real issue when you start getting much higher than the 4:1 ratios that are common in single reduction boxes.

Originally Posted by sanddrag (Post 1578523)

In my 15 years in FIRST, most of which consisted of reading Chief Delphi and designing gearboxes, and my 5 years spent studying mechanical engineering, I have never once heard the term "bull gear." Is this some kind of new slang or do I just live under a rock? Would someone care to fill me in?

Originally Posted by asid61 (Post 1578554)

Correct me if I'm off, but shouldn't the pinion tooth load only be proportional to the mechanical power the motor is outputting and the pitch radius of the pinion, and be independent of the gear ratio?

Originally Posted by sanddrag (Post 1578523)

I have never once heard the term "bull gear." Is this some kind of new slang or do I just live under a rock?

Originally Posted by asid61 (Post 1578474)

disregarding the weight of shafts, bearings, and plate?

Originally Posted by hrench (Post 1578660)

This was me too!:)

After a quick Google search, I feel pretty stupid that I didn't know the word.

This seems mostly like a great question, but the constraint:



seems to ruin this question. When the bull gear is really large, you don't need another gear pass, which if we recall, each gear pass consumes 1-2% (or more) of your power, decreasing your efficiency. So I can make any ratio I want--even 1000:1 with only one gear pass, but the size of the transmission would be silly. So you can't disregard that stuff.

Also, each gear pass consumes dollars. Fewer passes are less money.

I remember the year we used andymark's "cimplebox"http://www.andymark.com/CIMplebox-p/am-0734.htm which only has one gear pass in it. I was hoping that the gain in efficiency would help to compensate for a really bad gear ratio (4.67 to 1 max). We ended up with an amazing top speed but it was a terrible robot to position for frisbees.

Also don't forget that most of us that are designing transmissions are still using off-the-shelf gears from robotics vendors, but if we were custom designing gears, we could make them look like the beauties in the Chairman's trophy,
http://www.chiefdelphi.com/media/img...3509e9d6_l.jpg
with spans and webs and lots of weight reduction.
But for the complexity of obtaining your own gears, the weight reduction just isn't worth it.

Also, a 'real' gear designer will vary the modulus of elasticity (materials) of the gears in the train so point contact doesn't increase wear. Never see robots do this. We use steel gears.

So though I'd like to see some kind of comparison, but with no changing material weight and no lightening holes in the gears, it would be hard to be fair in the comparison. I think multiple passes would win every time because the bull gears would be so heavy.

A funny aside...once a supervisor asked me to put some 'lightening' holes in a part and I drew a blank. I responded "how big of hole does lightning make?"

Originally Posted by FrankJ (Post 1578634)

Right concept, but wrong terms. Actual it is related to torque. A brushed DC motor at stall is transmitting maximum gear load but no power. The actual force transmitted is the same on both the big gear and small gear. The small gear wears faster because there are fewer of them. There is a practical limit on how small of a gear you can make where the geometry falls apart.

Originally Posted by asid61 (Post 1578708)

Wouldn't the rotation speed of the gear also come into effect (hence why I said mech. power)?

Originally Posted by asid61 (Post 1578708)

this is a mathematical question, not a practical one. Not really sure how I can make that more clear

Originally Posted by asid61 (Post 1578708)

I have designed gearboxes before (look through my submitted images) and weight is rarely my constraint. If you don't like the constraints, then I can't help you.

Originally Posted by asid61 (Post 1578708)

Wouldn't the rotation speed of the gear also come into effect (hence why I said mech. power)?

Originally Posted by asid61 (Post 1578554)

Correct me if I'm off, but shouldn't the pinion tooth load only be proportional to the mechanical power the motor is outputting and the pitch radius of the pinion, and be independent of the gear ratio?