Caution; trying to claim 100% right on either end of the spectrum can be equally wrong.

Let’s not mix up to two issues of spur gears vs helical; planetary vs non-planetary.  Note that most of those big WWII engines used planetary, but they used spur gears. Helical gear were developed and used primarily for the lack of noise.  We don’t like whining noises in our commercial vehicles.  Helical gearing has the disadvantage of significant axial loads which add both stress and the friction. Contact area, even when computed correctly (looking at how many teeth are in contact, and the area of contact on each) is only one factor in a more complex design process.

Using a planetary for a compact, lightweight design is a good idea. We use helical gears because they are readily available at a very reasonable price.  Using a custom designed spur gear planetary in our case could be a better approach, but at what cost? It could maybe eliminate some of the lost power that goes into heating of the gears and oil – not much, but something.  When I disassembled my drive after about 40 hours, the only evident wear was where the thrust bearing bears onto the mounting plate, and evidence of heat on the nylon snubber washers – the things that take the axial loads from the helical gears. 

And I would think most gear designers would agree that using a lubricant optimized to the gears could be better than using engine oil. But it certainly adds complexity, and is it worth it?

FWIW, just another point of view.

Al

Al,

The Black and White "Everything is bad mine is the only one that you can take your grandchildren flying with" is what angered me the most about this guy's statements. I have flown behind (or in front of) belts, spur gears, and planetary gears, plus I have lots of hours in cars with chains driving the cam or the final drive (front wheel drive car), Haven't experienced a failure yet. Anything can be made to work well and all things are a bunch of compromises.

Some additional things about helical gears, they actually have higher load carrying capacities all things equal than a spur gear. This is because more teeth are in mesh and the tooth loading is more gradual. That "noise" they eliminate is the gear teeth banging together. The downside to helical gears is they produce axial thrust, which is no bid deal if the bearings are designed to handle it. The other downside is they are more difficult to machine. The additional shearing in the lubricant produces more heat as well. Helical planetary gear sets are typically arranged back to back to that the thrust loads cancel most of the time. In large planetary sets with lots of reduction handling all the thrust loads under all conditions in a light compact way becomes pretty difficult. That is why spur gears are preferred for turbines.

A big deal is typically made about the lube oil for the planetary being engine oil. This is another red herring. When you design a gear set, you always design with a particular lube in mind. The viscosity of the lube determines what amount of contact load can be had without squeezing out the lube and producing metal to metal contact. In the automatic car trans the lube is a compromise between what the gear set needs and what the hydraulic control unit  and clutch packs need. The gear set is designed to run in a light weight hydraulic oil with a bunch of additives to tolerate wet clutch packs. Needless to say this is not an ideal gear lube. That is probably one of the reasons for so many small teeth and so much contact area. If you run it in a multi weight synthetic engine lube, say 10W-30W or 20W-50W you are going to have lube chains from 10 to 30 or 20 50 weight viscosity. The gear set will tend to produce more heat because of the longer heavier chain molecules, but the load rating will actually go up. It will also tend to degrade the heavier wt molecules more rapidly. Heavy wt hypoid lube in the automatic trans planetary would be a disaster. 80-90 gear lube is made for much larger mesh areas and much larger slower gears.

Is a Ford automatic trans planetary as good as a purpose designed unit? Probably not in theory. But there is the teething stage of any development program, and the Ford planetary has a proven track record. I get sick of the Johnny come lately with unproven hardware claiming "every drive on the market is a death trap but mine" advertising pitch. This kind of pitch immediately sends up red flags for me.

I hope that nobody took what I posted to mean the only thing that will ever work is a planetary. That is simply not the case, and for a V type engine that is not inverted, a two element spur gear would be my choice.....besides I kind of like the sound of gear noise ;-)

Monty