diff and axle shear loads and gear selection

[gulfcountry]

Rear axle torque shear stress query.

Id like to know what is happening in the way of torque applying shear stress to the driven axles .
If we pick a speed , say 100kph .
Is the load on the final drive in any way different depending on the gear the vehicle is in . If in 5th is there more load placed on the axles than if doing the same speed in 4th gear .
Ive heard labouring your vehicle in higher gears puts more strain on the vehicle components and selecting a lower gear reduces those loads and stresses on moving parts .
Is moving the vehicle at 100kph no matter the gear selection using the same amount of power and torque no matter the gear selected . Like physics requires a certain amount of force to move something so the stresses on the rear drive are constant no matter the gear . Would this be correct thinking ?

[Rob_Wood]

"X" amount of force in = "X" amount of force out.
Exactly which component is letting which little bit of outgoing force free will vary dependant on things like which gear might be selected at any given time, whether in 2wd or 4wd and probably a million other things.
It might help to know your goal /endgame?

[Gipsy]

Driving at the higher revs is easier on most of the driveline because the momentum of the components flattens the curve when extra loads are expected. The engine in particular is damaged by lugging at low revs with a high load. The engine power/torque curve is suited to a specific rev range (around 2000 to 3500rpm) so it's always best to use this range. Particularly diesels hate grunting at high loads/low revs, despite the torque levels at anything over 1000rpm. Just don't lug your engine, it hurts everything especially fuel consumption/wastage.

Edit: if I read your question correctly though, I guess you are specifically asking about the final gearing; rear axle/diff etc. As noted previously, the physics of torque in torque out, the load on those components are basically not affected by the gear selection but they are affected by shock loads at low speed because as you throttle up to overcome a load, slow moving components can suffer shock loads as the power increases to overcome the load no matter which gear you are in. There is a problem with shear loads at very slow speeds as the torque from the engine can provide a shock load to those components as the gearing is magnifying (multiplying) the torque from the engine. Thus as your question asks specifically, no the shear load is not really variable at 100kph no matter which gear you select because the power required to maintain that speed is relatively constant irrespective of which gear selection. You will however use more/less fuel and affect engine wear depending on which gear you are using.

[gulfcountry]

I can see from the descriptions above how it works with the force in and force out .
If I add another factor in to the 100kph and that factor is a big hill . Say the vehicle has ample power and torque available to continue up that hill without labouring or loosing speed is there going to be more of a chance of that vehicle breaking its diff or axle if in 5th gear instead of gearing down to fourth .
Im not trying to achieve anything with this post but simply get an understanding of how forces are acting on thr rear drive train . I have once a long time ago broke a rear axle but that was just turning left from an intersection in 1st gear and broke the rear left axle in an old car .

[Rob_Wood]

5th Gear (in "MOST" vehicles) is actually an overdrive and not an extra gear. It is designed for cruising at higher speeds with lower revs in an effort to help a bit with fuel economy (and other stresses). Most of these vehicles will need to down gear when going up steeper hills.
Not knowing what the vehicle was that was turning left, it's hard to comment, but I would have to guess that previous damage was the main cause of the event and that your "turning left" was probably just the straw that broke the proverbial camels back.

[Gipsy]

Not much chance of breaking a rear axle on a Hilux just by going up a steep incline in 5th gear, but as Rob said, 5th is an overdrive gear and this is where the rev range window comes in... Keep the revs in that 2-3k rpm and it should be easier on all components especially the engine... If you try this, going up a steep incline, shift to 4th and you will notice that you can back off the throttle and maintain speed whereas 5th you need to put some throttle in to maintain speed. You will also notice less fuel being used... That's why you have 5 gears... Select the appropriate one for the condition.