Is odd better than even? 4

[PhilBW]

Another engineer and I were discussing a design for a manifold and how many parallel channels we should use in this manifold. The other engineer said an old timer once told him to use odd numbers whenever possible when designing things. He said that is why most wheels have an odd number of spokes. The old timer didn't explain why odd numbers are better. I've seen many designs using even numbers and they seem to work fine.

Has anyone else heard this? Does anyone have an idea why odd numbers of things might be better than even?

Phil

 

[GregLocock]

Just thinking about it a lot of splines are odd numbers of teeth as well (sometimes we use different odd numbers at each end of a shaft if we want really subtle adjustability).

The wear/phasing argument can't apply to a spline, is it just that gear makers habitually use odd numbers?



Cheers

Greg Locock

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[williedawg]

my head hurts

 

[ivymike]

well, you've only got one.. works well enough for everyone but Zaphod Beeblebrox. Another odd winner.

 

[Denial]

Go for prime, every time.
But remember that the cicadas got there first, with their life cycles.

 

[SomptingGuy]

Horizontal axis wind turbine blades. Never an even number, usually three. There is a very simple technical reason for this. You don't want a design were there are opposed blades - one lightly loaded in the tower shadow and one heavily loaded high up in the faster air. Cyclic stress loading and all that.

- Steve

 

[KENAT]

Sompting, funny, I see lots of smallish 2 blade wind turbines on my drive to & from work. I believe they're mostly for water pumps.

Never say never?

KENAT,

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[tbuelna]

Excellent and entertaining discussion (that comment about the Simpsons was hilarious).

As for using prime numbers for gear teeth, this is always done unless there is a need for a specific ratio like that of a timing gear. The reason is two-fold. First, prime numbers of gear teeth are less likely to produce vibration problems due to coupled meshing frequencies. And second, mating gears with different prime numbers of teeth give the best results for even wear.

Splines don't really care about the number of teeth, since they are rotationally coupled. If your quill shaft needs a vernier feature for indexing, that is best accomplished simply by using a one tooth difference from one shaft's end spline to the other.

And with regards to the topic of rotor blade symmetry, how about a single rotor blade:

Regards,
Terry

 

[ivymike]

prime numbers for gear teeth, this is always done

Always done... where? I can't recall a single example, and I've been around for a number of engine geartrain designs at several OEMs. I can't recall prime numbers ever coming up, although hunting teeth are good for a fight - they'll even out wear, but prevent thorough validation of the geartrain in a reasonable period of time. As I recall, the number of teeth on each gear was usually defined by the rough geartrain centers layout, the desired pitch and contact ratio, and the desired (approximate) drive ratios.

 

[rmw]

I suspect a lot of the 'old timer's' rules came from the horse and buggy days when the wheels were wooden and so were the spokes. With even numbered spokes, placing a couple of spokes at the high end of their length tolerance opposite each other would guarantee that the 'lower tolerance' spokes would be too loose to fit. Odd numbered spokes would prevent that. Wheel out of roundness probably wasn't a show stopper in those days.

Same principle applies to rollers turning inside a circular path, such as heat exchanger or boiler tube rollers, not that there aren't 4 roll tube rollers out there but the ability to center up the mandrel and evenly load all the rollers is much better in a odd numbered tube roller than an even numbered roller. I would think, but can't state it that the same would be true for similar devises like some bearings.

rmw

 

[Artisi]

A simple example of odd /even is 3 leg stools/chairs are better than 4 leg.

 

[SomptingGuy]

That's because we live in three dimensions though. A 5 leg stool would be over-constrained (wobbly) too.

- Steve

 

[ScottyUK]

Artisi,

Three legs are better than one or two legs too...


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If we learn from our mistakes I'm getting a great education!

 

[ivymike]

so how about the animal kingdom? seems like pairs of limbs are pretty common. allows some redundancy, but with minimal waste, perhaps?

 

[rhodie]

So. Is zero odd or even?

 

[vpl]

SomptingGuy

Have you noticed how many rollable office chairs have 5 legs? Much more stable than 3 or 4; doesn't tip over anywhere near as often.

Patricia Lougheed

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[SomptingGuy]

Poo!! I wrote that last message sitting on a five-legger.

I was also told today that 3-bladed wind turbines were more prevalent because of their aesthetic not engineering attributes.

Prime numbers for gear teeth though still seems in the running.

- Steve

 

[ScottyUK]

vpl,

That is true but only because most office floors are, to a reasonable approximation, two dimensional planes. A five legged chair on an uneven surface will almost always rock, while a three-legged one will almost always find a point where the three legs touch the ground.


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If we learn from our mistakes I'm getting a great education!

 

[KENAT]

Scotty, unless you've got extreme irregularities, like the hole on a golf course, or an extreme angle such that C of G is an issue, a three legged stool or chair should never rock.

How level the seat is is another matter.

I double checked on the way home Friday, lots of 2 blade wind turbines in the Santa Paula/Fillmore area of CA.

KENAT,

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[fcsuper]

That's because we live in three dimensions though. A 5 leg stool would be over-constrained (wobbly) too.

that all depends on which way the 5th leg is pointing.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ScottyUK]

Kenat,

That's the reason for the 'almost' qualifier in my comment - if the surface is at a sufficiently steep angle then the stool will tip beyond its balance point and none of the legs will touch the floor.


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If we learn from our mistakes I'm getting a great education!