Helical gears are quiet because they "keep" more tooth contact (during rotation). Helical gears can be tough on shaft end loading (the old "no such thing as a free lunch" rule.)
The finer tooth pitch transmission, while still controlling end loads, gives the gear sets a chance to absorb those hammer blows from the diesel engine. Surprisingly, the input gear-to-countershaft gear is the fellow that can get hammered to death by a diesel...while idling in neutral! Things start rattling like a jack hammer, in there, and that must be minimized.
Picture an air hammer in each hand, you put the hammer heads together...and pull the triggers. The bone breaks occur where your lower arm bones connect at the elbows....not the little bones in the hands. My friends wife was a nurse and she was PI$$ED at him....considering she would now be sole income for several months. (back in my line mechanic days)
While we are ruminating on silly mechanical stuff that thumps and chatters....how about "dual mass flywheels"?
My first experience was the Supra Twin-Turbo with Getrag 6-speed trans. Many of those started rattling bad while idling in neutral. A new trans didn't change a thing....hmmm. What's up??
Well, it all starts with a turbo engine at idle....in the early days of OBD II and "misfire detection" rules. You see, lower compression gasoline turbo engines don't "fire" very hard at idle. Misfire detection is accomplished by closely monitoring the acceleration and deceleration of the crankshaft DURING each rotation (three times per rotation on a 6-cylinder 4-stroke).
In order to lighten the "accel-decel" parts, we had to split the flywheel and add damper springs between the two parts (sort of a clock spring arrangement). Unfortunately, when you put the damping in the flywheel you CANNOT used damping in the clutch disc. When the springs in the flywheel start getting weak, the trans gears would rattle like crazy at idle. Man-oh-man, there were a lot of perfectly good 6-speeds dumped in the trash! That whole product disappeared in the late 90's.
So I got old, retired, and got out of that strange world.
And then in 2018 I go and buy me a new Jeep to tow behind the motorhome, and I wanted the new 6-speed Aisin manual trans. Guess what?! It's got the durned dual-mass flywheel, which makes clutch feel funky....and now there's a recall because if you get into clutch slip situation, there is not enough flywheel mass material clamped against the disc to absorb a lot of heat.
In worst cases (only a few....so far) the pressure plate overheats, cracks, flies apart, explodes the bell housing, cuts the EFI fuel lines, and your new Jeep burns to the ground. There are pics all over the internet. It blows the engine up so hard that cylinder bores are pointed every which direction in what started as a V6.
I really suspect it's the old "low power misfire detection" animal. This engine is a gutless wonder at very low RPM. But the trans is talking at idle...already.
You just can't rattle metal parts against each other and get away with it.
Don't even get me started on overdriven gear ratios and input bearing life. Another "no free lunch" deal if you plan to drive the vehicle using a (wait for it).......driveshaft with u-joints!
