Posted by Charlie on March 09, 19100 at 23:44:22:
In Reply to: Another NP 540 question??? posted by Aaron A. Arnold on March 09, 19100 at 17:14:50:
Gordon's comments are all correct and to the point, but one more thing: the weak link of any OD 5spd including the NP540 OD is the input shaft and its' bearing. That's because any transmission gets the most torque put thru the input in its' highest gear. In direct high gear transmissions the power flow is straight thru with no side forces on the bearing or any part of the shaft, just centered twisting force. But in 5th the pedal is frquently to the metal and the torque goes from the gear to the countershaft. In lower gears, the power flow is the same but the engine and input shaft have the mechanical advantage of gear reduction which decreases the stress on them. Anyways, input gears for OD version NP540s are incredibly scarce. So don't buy one with a bad input shaft unless you have a replacement.
From an engineering standpoint, the better design is a 5 spd with 5th direct, all the gears below it lower (numerically higher) by the inverse of the desired OD ratio, and the diff ratio numerically lower (therefore a stronger ring and pinion set) by the desired OD ratio. The only reasons for a mfg to design a vehicle with an overdrive ratio in the transmission is to save money by not needing a different R&P ratio, or to save money by not having to slightly upgrade driveshaft and transfer case strength.
Charlie