Semi helical vs S/C final drives.
Discussion
I was reading some old stuff about gearboxes and BDA Conversions. JK used to make SC final drives down to 2.76:1 which I thought was nice. I always wondered why the only SC ones made are 3.7 and the rest are semi helical. I was considering the forces involved, looked at the diff bearings and the helices of the gears (input, main, pinion) and noticed the directions. It can't be good. Despite having SC drop gears and main gears, the old pinion is still chucking a huge thrust at all the bearings. I reckon this is a waste of power and also not good for wear.....
So, what to do, I want to get a 3.2 CWP cut for an LSD (and you can) and it would be cheaper to get it done SC than semi helical. I figure that people like MED, KAD, etc would use a SC item if there wasn't a hidden catch. Do they make SC drops sound quiet? Does the diff assembly slop around too much? I'm curious.
Noise is not a snag as the car is an earplugs job anyway
So, what to do, I want to get a 3.2 CWP cut for an LSD (and you can) and it would be cheaper to get it done SC than semi helical. I figure that people like MED, KAD, etc would use a SC item if there wasn't a hidden catch. Do they make SC drops sound quiet? Does the diff assembly slop around too much? I'm curious.
Noise is not a snag as the car is an earplugs job anyway
love
you have to much spare time on your hands for thinking.
the reason cwp's arnt often made sc is simple, 1) it makes them very noisey (even the semi helical ones are pretty loud and 2) they wear very fast due to the small area of contact, especially on the lower ratios due to the size of the pinion. the variences between the centres of the gear case cannot be altered either, so the backlash beteen the crownwheel and the pinion cannot be adjusted.
on high powered minis the semi helical pinions need changing every few thousand miles, the s/c ones last even less - the only s/c one now is 3.7, the root patttern isnt big enough on the others to really make it viable.
you have to much spare time on your hands for thinking.
the reason cwp's arnt often made sc is simple, 1) it makes them very noisey (even the semi helical ones are pretty loud and 2) they wear very fast due to the small area of contact, especially on the lower ratios due to the size of the pinion. the variences between the centres of the gear case cannot be altered either, so the backlash beteen the crownwheel and the pinion cannot be adjusted.
on high powered minis the semi helical pinions need changing every few thousand miles, the s/c ones last even less - the only s/c one now is 3.7, the root patttern isnt big enough on the others to really make it viable.
Intersting points. I would have thought the same was due for drop gears being even smaller and having less contact. Hmmmm.
I'll compare prices. I image my cog cutter man is a lot cheaper than mini spares, etc. I'll have a chat with my engineering chum.
Makes you think about double helical gears. Hmmmm.
I'll compare prices. I image my cog cutter man is a lot cheaper than mini spares, etc. I'll have a chat with my engineering chum.
Makes you think about double helical gears. Hmmmm.
its all to do with the movement of the centres during machining and use of the gear cases. the nominal gap is 4" but if its less than that the teeth on the gears are forced together, if its more they move apart. its measured in 'thou' but the point at where the tip of one tooth touches the side of another will start to pit. this is why the cwp are made with a set amount of backlash.
it also depends on useage, most cars using semi helical cwp are high reving motorsport things
on big power stuff - and particually with the thinner later a plus cases the case flexes like crazy round the back, this moves the gap in and out in useage with these cases - they will also start to break up on really big power machines
its lack of backlash and case flex that sets the whole broken pinion in motion, though there often isnt much to study when the pinion lets go, as it usually exits through the rear (not so bad) or the front (far worse, particually of you have a dog leg first kad dog box.......)
my race car gets the pinion 'turned' every time the box is stripped, normally after. say, half a season the teeth will be pitting - dont forget that on the lower ratio cwp the pinion is working very hard because its turning so many more times than say on a 3.4:1 - but because its so low power i just dont have (touch wood) the problems of the pinions breaking
the cwps could be made of harder stuff, but then you run the risk of the teeth pulling out by the roots, so the trade is made for root strength.
the s/c drops are slightly different, the only time that they are ever a problem is if people dont use gaskets between the block and the box when they reasemble the engine, this allows the primary to start to 'gain' on the idler and every couple of rotations it will be wanting to jump a tooth, the gear set then goes really tight, goes over center and sets off free again.
this is why when the box is built its important to turn the diff over and over as you torque the diff carrier up, any backlash problems become apparent as the diff will tighten right up if there are any issues - its even worse if someone has swopped the diff case from another gear box!
it also depends on useage, most cars using semi helical cwp are high reving motorsport things
on big power stuff - and particually with the thinner later a plus cases the case flexes like crazy round the back, this moves the gap in and out in useage with these cases - they will also start to break up on really big power machines
its lack of backlash and case flex that sets the whole broken pinion in motion, though there often isnt much to study when the pinion lets go, as it usually exits through the rear (not so bad) or the front (far worse, particually of you have a dog leg first kad dog box.......)
my race car gets the pinion 'turned' every time the box is stripped, normally after. say, half a season the teeth will be pitting - dont forget that on the lower ratio cwp the pinion is working very hard because its turning so many more times than say on a 3.4:1 - but because its so low power i just dont have (touch wood) the problems of the pinions breaking
the cwps could be made of harder stuff, but then you run the risk of the teeth pulling out by the roots, so the trade is made for root strength.
the s/c drops are slightly different, the only time that they are ever a problem is if people dont use gaskets between the block and the box when they reasemble the engine, this allows the primary to start to 'gain' on the idler and every couple of rotations it will be wanting to jump a tooth, the gear set then goes really tight, goes over center and sets off free again.
this is why when the box is built its important to turn the diff over and over as you torque the diff carrier up, any backlash problems become apparent as the diff will tighten right up if there are any issues - its even worse if someone has swopped the diff case from another gear box!
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