Offset rear axle, final drives

[piller]

Looked at utility tractors this weekend. Noticed most tractor brochures list planetary final drives. One manufacturer's web site compared it's tractors to Kubota M tractors, it said the kubota M machines had bevel gear final drives, while it had planetary final drives. Kubota advertises that it has planetary final drives. So what does this really mean?

Next, I was looking at some new Case/IH and New Holland tractors and saw that the rear axle is offset from the rear wheel center, there is some kind of gearbox attachment. Just did not look "right" to me. I'm used to seeing a big rear axle lined up with the center of the rear wheel. Any thoughts on these offset rear axle drives?

 

[Wombat125]

One manufacturer's web site compared it's tractors to Kubota M tractors, it said the kubota M machines had bevel gear final drives, while it had planetary final drives. Kubota advertises that it has planetary final drives. So what does this really mean??

I'm guessing they were talking about the front axle where Kubota does use beveled gear sets. The beveled gears add a little complexity, but are beneficial for ground clearance and turning radius.

Next, I was looking at some new Case/IH and New Holland tractors and saw that the rear axle is offset from the rear wheel center, there is some kind of gearbox attachment. Just did not look "right" to me. I'm used to seeing a big rear axle lined up with the center of the rear wheel. Any thoughts on these offset rear axle drives?

They look a little odd to me too, but have several advantages. First, you can get better ground clearance by clocking the gear reduction hub so the output shaft is lower. And second, you can reduce the torque load on the intermediate driveline components by moving the gear reduction as far downstream as possible. This should allow for a lighter, more reliable tranny and differential.

This is not a new design. Evidence the old Allis Chalmers and Unimog tractors.

 

[3RRL]

Don't know much about the offset differential, but there is a simple explanation for planetary and bevel gears.
Here is the short version...
Generally, planetary gears refer to a gear within a gear.
There is a center gear on a shaft referred to the "sun" gear and a gear or cluster of gears that surround it referred to as "planets or planetary" gears. Then there is usually a ring gear around those "planet" gears. That is what most rear end drives have in them. It is a very strong, robust manner of gearing the rear axle.

The bevel gears allow for 90 degree power transition and change of speed or gear ratio. Miter gears are just like like bevel gears except they usually don't change the gear ratio. They can be straight cut or curved (hypoid). Those are generally used in the front drives. The advantage is like Wombat said, you can gain ground clearance. You use a shaft mounted in one position (with a smaller diameter bevel gear on it) to drive the wheel (with a much larger diameter bevel gear on it), but the wheel gear is not on the same centerline as the drive shaft. So for added ground clearance, the drive axle is mounted above the wheel gear, making for more ground clearance. Drawback is this is a much weaker gearing condition and therefore front drive "assist" is what you get out of it. Yes, it is a 4wd but much weaker in the gearing set up than planetary.

Hypoid bevel gears are more expensive than straight cut bevel gears. They are stronger, quieter and last longer than straight cut. I believe Kubota and other high end tractors use hypoid bevel gears whereas my Chinese Kama uses straight cut bevel gears. I sheared mine from overload and had to replace them.



I used both kinds of gearing systems for some of the fully automatic unwinding plastic injection molds I used to make.
Maybe someone else will jump in with more technical detail?

 

[flusher]

Looked at utility tractors this weekend. Noticed most tractor brochures list planetary final drives. One manufacturer's web site compared it's tractors to Kubota M tractors, it said the kubota M machines had bevel gear final drives, while it had planetary final drives. Kubota advertises that it has planetary final drives. So what does this really mean?

Next, I was looking at some new Case/IH and New Holland tractors and saw that the rear axle is offset from the rear wheel center, there is some kind of gearbox attachment. Just did not look "right" to me. I'm used to seeing a big rear axle lined up with the center of the rear wheel. Any thoughts on these offset rear axle drives?

Tractors like the Ford 960 and 961, the Farmall Cub, A and Super A and many of the old Allis Chalmers tractors have this type of rear axle arrangement, mostly to raise the tractor into some form of "high crop" configuration.
For example, here's a rear-end view of my Farmall Super A

Other vehicles have this kind of axle. Here's an extreme off-road version of a Ford Bronco with axles from a Mercedes Benz Unimog 404, called "portal axles"

 

[farmerjim]

basically plantery gear final drive is a series of gears that rotate around a "sun" main gear. they are very strong and maximize effiency, hence their use on tractors. most all older tractors came w/ this set up as a final drive, for that reason. many newer tractors come with this final drive set up as well, some makers also use a spur gear final drive which is also a good strong platform. here's a link to a planetary gear how it works.
http://science.howstuffworks.com/gear7.htm

 

[piller]

So basically it sounds like I should not be overly concerned about whether the rear axle is offset from the wheel center or not, that the offset could be an advantage with regard to the upstream drivetrain loads. Ground clearance is not really an issue, it seems that the clearance will be just fine with the centered axle or with the offset variety.

Regarding final drive, to me it seems that the offset axle will have some gearing at the wheel hub and that this would be the "final" drive, and I don't think that this is a planetary setup, although I could be wrong on this.