I figure this is the best forum to ask this question in, and get the kind of answer I am looking for. Why do high power cars, such as racing vehicles, opt for straight cut gears over their quieter helical brethren? I got into an arguement today with someone about it, and he insisted helical gears are stronger in an automotive application. I know I have read to the contrary of this multiple times, but I really didn't have a solid answer as to why they are, just several of my own assumptions. Feel free to get into as much detail as need be about this; I am looking for an in depth answer. Any links to any websites or anything would also be great, as I would have something to print out and show him. Much appretiation to anyone with any feedback on this.

 

I was under the impression Helical were much stronger as well.

Check the pinion and ring gears, or internal tranny constant mesh gears,
they are most likely helical.

This allows more than one tooth to be engaged at all times to handle more load.

I think the only down side is the drag used to turn a helical gear. Maybe this
is why a straight cut (or spur gear) is more efficient in high end race cars?

I'll try to snap picks of my tech book later this weekend. It doesn't mention
anything about race applications, so my other thoughts are just assumptions.

 

I would imagine that helical cut gears are stronger because they have more surface area to distribute the power.

But I think straight cut would be good for less friction/drag... so if they're strong enough I guess there's no need to make them helical?

 

I don't think there is a clear cut answer on which is stronger - it will depend on a number of details. What is clear is that helical gears are quieter and spur (straight cut) gears are more efficient.

Rich

 

For a given size, helical tooth gears are stronger than straight tooth gears for the reasons mentioned above...more surface area in contact. With spur gears, or gears on parallel shafts like in transmissions, the tooth friction is nearly the same in both types. Hypoid (rear end) gears are another story altogether.

The down side to helical gears is THRUST. Transmitting loads causes axial thrust which must be resisted. If you are transmitting lots of power for a period of time, you need to be looking at thrust bearings at the ends of the gears, and if they aren't rollers, lots of heat.

An old example: Turbo Hydramatic 425, which was used in the Olds Toronado FWD from 66 on was basically the guts of a THM 400 with helical planet pinion gears in the gearsets, as are virtually all automatics. Worked fine in the cars, but it was then used in the (neat) FWD GMC motorhome. On long hills at higher altitudes, even the 403/455 Olds engines needed to use 2nd or even first gear under high power for many miles, something the cars never did. The helical planet gears wore themselves into the carrier due to heavy, continuous thrust. The fix was to go to straight-cut pinions, in spite of some extra noise. However, when made from the same material and heat treatment, the straight tooth gears were not strong enough. A material change plus a heat treating change was needed to get the durability back. Remember they couldn't go to larger, stronger gears because of space limitations in the planetary gearsets.

This is all from an old memory, but I think the material changed from a low carbon steel like 1117 or even 1018 to a alloy like 8620. Material price of 8620 could be 2X that of the carbon steel or more. OEMs only do expensive stuff like that when they need to. LS7 engined Z06 has bigger, stronger driveline gears, so they had to save weight elsewhere.

Think of puttng 1300+ hp thru a ProStock trans. I believe they use straight cut gears to keep the friction from the thrust loads low, and for shifting purposes because they need every hp they can get to the driveshaft. One team (at least) is fanatical about driveline losses. The gears are as short as they can make them, but horsepower the last few years has overloaded them so much that bearings are a problem. Gear materials and treatments are cutting edge exotic, and not cheap.

The obvious solution is more spacing between the shafts, but that means bigger cases, shafts, bearings and gears with more rotating inertia. That's less power getting to the output shaft especially in the lower gears where engines may be accelerating above 2000 rpm/sec. I don't know if shaft spacing has increased this year, but I haven't noticed many failed transmissions. I guess if everyone has to go to a larger trans the playing field stays level. Even ceramic bearings can only take so much before they, too go south.

 

Helical gears also create thrust. In a differential, the pinion gear is trying to climb the ring gear. Since it's held in by a bearing, it forces the ring gear to turn. Pushing down on the helical gears is forcing the pinion gear forward out the front of the diff and it's also trying to force the ring gear out the back of the diff.

With straight cut gears there is no thrust like that. All the energy is being used to rotate the gears.

Best example is in a powerglide transmission. All automatic transmissions have helical gears. They have to to keep gear noise down. Aftermarket gears sets are available to change the powerglides 1.76 first gear to a 1.80 straight cut gear set. These gears are considered unbreakable and because the don't create thrust, the transmission doesn't eat up thrust washers when behind a high HP engine.

 

I thought maybe we were posting simultaneously, but that's not the case. We do agree on a few things.

I do love differences of opinion...straight cut planet gears are weaker than their helical brothers, as mentioned above, so they have to be made from better material. Yep, the thrust IS the driving force (pun intended) for them being straight.

FWIW, rear end ring and pinions are really not considered helical, but rather a special case of spiral bevel gears called hypoid gears. The axes of the ring and pinion do not intersect like they would in a normal spiral bevel so lots of friction is created. The up side is that the driveshaft can be offset for a lower floor pan, and the gearset, even with 6:1 + ratio and very few pinion teeth (43:7) is very strong because there is so much tooth contact with the spiral teeth. I believe you'll find the major force on the ring is trying to push it sideways along its axis. It works the same way in the straight cut bevel gears in a Model T rear end. They are very weak, by the way, because they are not spiral bevels. Noisy, too.

FWIW-2, all gears try to "climb" each other when under load.

 

Thanks guys. A lot of great information here. It appears I may have been wrong, but I am glad to have cleared up many of my misconceptions.

 

And a Liberty 5 speed prostock transmission incorporates 2 cluster shafts to deal with the load, unlike the G-Force 5 speed, in case anybody cares.

 

Thanks for the info. The Liberty website has a good explanation of that trans.