The formal definition of a spur gear is one that meshes in the same plane as the mating gear (as opposed to different planes, as in bevel, hypoid or worm gears).
In this respect, straight cut gears and helical gears are both spur gears. It is the application of the gears that we are comparing, not whether they are straight or helical cut. That is indeed a different discussion.
For the purposes of this discussion, I had assumed that "spur gear" meant the use of a single gear vs multiple gears (as in planetary) for the torque path. That is the slang that has been been generally accepted in the aviation community.
Tracy
On 7/29/07, Al Gietzen <ALVentures@cox.net> wrote:
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
-----Original Message-----
From:
Rotary motors in aircraft [mailto:
flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent:
Saturday, July 28, 2007 5:27 PM
To:
Rotary motors in aircraft
Subject: [FlyRotary] Re: Gear mesh area? Was [FlyRotary] Re: Gear
redrives.com
Suspicions confirmed! It just seem a more sophisticated method of transferring power from shaft to shaft than a Spur gear. But, as you mentioned, as long as it is designed to me the loads, conditions and environment, any number of methods of power transfer work, belts, gears, hydraulic, turbine wheels, etc. However, for simplicity, weight and cost, its hard to beat a planetary gear set. Just my opinion of course.
----- Original Message -----
Sent:
Saturday, July 28, 2007 6:57 PM
Subject:
[FlyRotary] Re: Gear mesh area? Was [FlyRotary] Re: Gear redrives.com
My quick & dirty answer is that the 6 pinion planet set is equivalent to a spur gear drive with a gear width totaling the width of all 6 planets. That would be:
6 * .785" = 4.71" Thats a mighty big & heavy spur gear!
That's bigger than the one in a P-51 R.R. Merlin engine I think. Not an apples to apples comparison due to the difference in bending strength of the tooth root but the biggie factor in power handling capability is indeed the gear contact patch.
On 7/28/07, Ed Anderson <
eanderson@carolina.rr.com> wrote:
Well, Tracy, its sort of like flying 400 hours with a "Plugs Up" installation - can't possibly work - the "experts" have said so {:>).
It continues to amaze me (as it does you) how people can just accept something sprouted out by some little (or unknown) self-styled expert without attempt to ascertain whether there are any independent sources/ facts supporting that position.
As several individuals have pointed out most high powered (1500+) engines of WWII had planetary gear boxes not to mention the dozens of different turboprop engines. But, I guess all of that evidence does not out-weigh the opinions of "experts".
I have not done this but it would be interesting to calculate the teeth mesh area of one of the spur gear PSRU and then compare it to the area of the six pinion sun and planetary gear area engaged to transfer a similar amount of power. I could be wrong, but my gut feel is that the sun and planetary probably have more metal to metal contact area for transferring power than the Sun gear.
But, like I said - just a hunch, would be interesting to know. Anybody have a quick answer??
----- Original Message -----
Sent:
Saturday, July 28, 2007 1:47 PM
Thought I'd see what else I was doing wrong so took a fresh look at the
geareddrives.com website.
The kiss of death for a set of gears is in not using the proper
lubrication. Separate lubrication systems are required for safety and for proper engine and gear function. Sharing engine oil with the PSRU is asking for engine and/or PSRU failure in advance.
There is absolutely nothing I'm doing right!
But the company does not look like it is for sale though. But Gershwinder (Sp?) drive company is for sale now. Their chain drives were among the best I've seen.
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