High pinion rear axle with reverse cut gears- design limitations

[2009JKlover]

Good morning, I’m new to the forum. I’ve been learning about axles lately, and have found myself at a sticking point. I’ll try to be concise here, sorry it’s so specific and long. This is in the context of 4wd vehicles like the Jeep wrangler.

I understand that overall a low pinion rear axle with a standard cut ring and pinion is the strongest set up, as normally the pinion pulls inward on the ring, while pushing against the Drive Side of the tooth (rather than the Coast side) and flings oil upward for lubrication.

For off-road vehicles- I know it’s common to have a high pinion front axle, with reverse cut ring and pinion; and a low pinion rear axle with standard cut gears. As I understand this setup is used because on the front axle, it also allows the pinion to pull inward on the axle, and the pinion drives the Drive Side of the ring. As opposed to a high pinion front axle with standard cut gears, which drive the Coast Side of the gears; or a low pinion front axle with reverse cut gears, which does the same.

I see that some custom 4x4s are using a high pinion solid rear axle with reverse cut ring and pinion, where the pinion normally drives the Drive Side of the ring gear. I keep reading that this setup is still weaker than the more common low pinion rear axle with standard cut ring and pinion. In both of these setups the pinion drives the Drive Side of the ring gear.

Am I then inferring correctly, that the high pinion rear axle with reverse cut gears is still weaker- because when this set up is used as the rear axle, the pinion is trying to climb/ push away from the ring? Is this just the inherent weakness because of the axle orientation?

If I missed anything or misunderstood I’ll try to clarify. Thank you for your help!

 

[BrianPetersen]

First I've heard of any such thing.

Low-pinion hypoid rear axle gears came about because of a desire to have a smaller clearance hump in the floor underneath a traditional rear axle drive sedan. Trucks kept that arrangement so as to use the same internal parts.

I don't know what you mean by "pulls inward" and so forth. Any gears that have a pressure angle (which is all of them) want to push each other apart (in whichever direction the pressure angle applies) when torque is applied in either direction. This will lead to a side thrust on both the pinion and ring gear of a 90-degree set. Any 90-degree rear-axle gearset that is carrying drive torque will have forces leading to the pinion trying to "climb" and the diff housing trying to be driven downward (opposite for a front axle). Due to the helical/hypoid design, there's going to be an end thrust on the pinion because the pressure angle is being applied at an angle. I can't see any of this being any more relevant or not whether it's a high-pinion or low-pinion design. Oil is going to be flinging everywhere because the ring gear is dipping into it.

A low-pinion gearset inside a rear axle with a clockwise-spinning forward-entry pinion becomes a high-pinion gearset when turned around and flipped over so as to use it in a rear-entry diff in a front axle with the driveshaft spinning in the same global-viewpoint direction, doesn't it? (edit: No. too hard to visualise. On second thought ... The front axle layout is turned around, but not flipped over.) With a high-pinion front axle, because generally the front axle is going to be sitting lower than the transfer case front output shaft no matter what you try, this should give better (smaller) U-joint angles.