It's due to the opposing forces on the shafts - If leverage is the same, then load can't be added from elsewhere. In that manner helical transfers some of the load through another axis. Force = Blue + Red
250lbft is a lot to put through a mini gearbox.... You mentioned a reason for the centre web cracking. Is it true that it is due to using straight cut gears? I cannot see why the evolution heavy duty gearset would spare the centre web, somebody who can explain? I cannot really see why. Is it still true using a pinion support?
Not sure for others, but I rarely use full throttle in second. Mainly 3rd and 4th. I love the maths to this, a few people at work have crunched the numbers and it isn't as bad as i first thought. My other consideration of straight cut was the numerous gearboxes that have had the centre web split due to the gears trying to climb over each other at high torque loads. The evo helical sets are in a lot of high powered turbo A series engine set ups now. I know of at least 2 now running over 250lbft for a few years now. Time will tell on the cases, and bearings, as to the longevity!
I have also considered using the mini spares heavy duty helical gearset. However, my main concern is about the gearbox housing when putting a lot of torque through the gearbox. Straight cut gears do not produce force transfer losses ending up as axial loads and in forth gear it should be fairly quiet as the load goes straight through the mainshaft. Worst case, using helical gears, should be in second gear during "drive" (opposite to "coast") where the layshaft is pushed towards the centre web of the gearbox housing. (At this very moment I do not have a gearbox at hand to measure the different gear radius to calculate the actual axial force level based on typical torque figures) I guess second gear is a gear that is most likely to be used when using full engine torque...
Just me thinking....
Imagine this as straight cut, all the forces will all be in the blue direction, therefore Force = Blue
Ive not seen a web split on a helical set yet, however that's not to say it isn't possible.
I do understand what you mean with the forces pushing the shafts apart. Still, I cannot see the purpose with using such a gear tooth profile (perhaps there could be a cost benefit...). A straight cut gear should have a gear tooth contact surface normal that is tangential to the gear. If so, I would say the forces separating the shafts would be very small and solely due to existing shaft bending. Is there someone who can share a close up photo of the crack surface? (I am interested to see, suspect the root cause for cracking could be that SC gears excitate a resonance mode of the centre web)
Helix cut gears are mainly stronger due to that they have a tooth engagement number larger than 1. (SC gears typically has 1)