The stress handled by ANY gearbox is the function of two forces: the torque that is inputted, and the inertia beyond the output that must be overcome for the box to do its job. A number of variables will affect how these forces interact in a powertain , from the cubic inch displacement of the engine, to the resistance supplied by vehicle weight, gear ratio and tire.
For example, in my mind an engine would rather tear itself free of its mounts and flip over when it meets the dead weight of a 6000lb truck. It is only when it is unsuccessful at flipping itself over that it applies, as a secondary option, motive power. Likewise, a rear differential would rather turn itself 90 degrees from the driveline and break the driveshaft than move the weight of a vehicle. But because the axles are harnessed by leaf springs and/or link bars, the differential reluctantly turns the tires. Point: motive power is not the first option.
How efficiently a gearbox transfers power not only depends on how the gears interface, but how the gears are supported. Regarding the interface, a straight-cut gear will always transfer power directly, while a helical cut gear will ALWAYS have some tendency to want to push itself away laterally from what it is required to move.
Take for example a transmission like the Muncie SM420, designed specifically to handle a lot of torque and a MASSIVE amount of resistance (think dump trucks). It uses a straight-cut, non synchromesh 1st gear, thus removing the dynamic potential of the gear and its counterpart to move away from one another laterally. Even the lowly 1st/reverse gear of the LC 3speed transmission was designed with the same thought, with the additional advantage of having massive, direct spline contact with the output shaft.
The transfer case is a whole different ballgame. Originally designed for a 4200lb vehicle inputting 100 ft/lbs of torque on the one side, and turning a 4:11 gear set and 28” tall tires on the other end. Toyota’s engineers deemed helical cut gears AND bronze bushings a satisfactory combination. The sheer volume of wiped out bushings and output shafts that I have seen on even bone stock rigs over the last 34 years is testament that this wasn’t an optimal decision. The helical cut gears of an H41 or H42 (or even the H55 for that matter) rest on either a precisely matched gear/shaft interface, or better yet, a set of caged roller bearings.
Those gears and shafts rarely suffer any damage. Coincidence?
So here comes AA, trying to fill a much-needed void in the Landcruiser off road world for a ‘bolt-in’ transfer case option. My hat is off to them. They accomplished something that has made a lot of people happy rock crawlers. The problem is that they inherited all the inherent weaknesses of the original design, along with the force multipliers of end users whose rigs have A. much more torque than what Toyota designed the gearbox to handle, B. much more weight than it left the factory with,( and more likely more than the GVWR) C. much larger tires. Or a combination of those things.
And since the Orion mimics the factory design well enough to reuse the factory output shaft and hi-lo shift collar, they inherit the limitations of those components as well.
Reconfiguring the splines on the output shaft is not the answer. The force that drives the collar off the gear needs to minimized at the source, not ‘contained’ by all the stop-gap measures that have been dreamed up over the last 13 years.
The I.D. of the output gears (and possibly the O.D. of the output shaft) need to be revisited by engineers with the mindset of replacing the bronze bushings with caged roller bearings of tighter tolerances. That, along with the stepped thrust washers Poser first envisioned, will minimize the pressure at its source, the gear itself.
The smooth sides of the splined center section of the output shaft will still be galled over time even with this improvement. But it will be at a much lower rate once the gear is both adequately supported by roller bearings and caged by the stepped thrust washers.
