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

 

[TheTick]

Floral arrangers like odd numbers.

 

[ornerynorsk]

Don't ball and roller bearings generally have an odd number of elements? Will be curious to see others input on this one.

 

[TVP]

Is this why a baker's dozen is 13?

 

[Twoballcane]

halfway from being too much or too little?

Tobalcane
"If you avoid failure, you also avoid success."

 

[williedawg]

It may be a slight modification of the "Goldilocks Principle" LOL
In the case of car wheel lugs or nuts, 4 seems too little, 6 seems too much, 5 seems just right.

When increasing the load capacity of the wheel, the number of lugs or nuts goes to 6, or on up to 8, & sometimes 10 but I've never seen a 7-hole pattern used for this; odd, eh? (no pun intended)

Also, it could be a harmonics thing. Odd numbers would be less likely to set up harmonics, such as number of balls in bearings.

If at all possible, gear trains and PD belt assemblies are usually made up of odd numbers of teeth or a combination of odd/even that allows "hunting", so that wear is maximally spread.

 

[RebelBrill]

I believe 3,5,7, etc ribs on a casting or weldment, for example, will allow it to stress relieve (move) better than 2,4,6 etc ribs will because they are not diametrically opposed. It is also sometimes recommended to have unequal spacing between the ribs.

 

[SomptingGuy]

Odd is better than even for (4-stroke) radial engines for reasons of timing.

- Steve

 

[tygerdawg]

Good discussion, always enjoy hearing what jewels of wisdom "old timers" have to offer.

I would go along with RebelBrill. It would seem logical one would want to avoid any sort of "diametrically opposed" construction, whether it's mechanical or some analogous situation. It would indicate the reason would be due to induced harmonic tendencies manifesting in a variety of ways, structural weakness, perhaps easier manufacturing,...whatever. But always it seems the end result is designed-in robustness.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[ginsoakedboy]

In radial geometries, 7-pattern is not practical as that introduces "directionality" in the product. If you have to go odd, go with 3, 5 or 9 as it is easier to divide 360° that way.

I once stirred up a small storm when I tried to put 7 holes in a part to avoid lining up with 4 pins on a mating part (4 holes would have been too few, 6 or 8 holes would be liable to line up with 2 diamterically opposed pins simultaneously). I did end up putting 7 holes, but I know better now. I should have gone with either 5 or 9 holes.

That being said, I agree with tygerdog and RebelBrill. Cheers ...

 

[gwolf2]

On jet engines it is desirable to have a prime number of blades on disks, and also a prime number of vanes on the adjacent stators. By doing this one can minimize aerodynamically induced resonances. It's a target rather than a hard and fast rule. If there are ball-races nearby then it is also worth having carefully chosen numbers of bearing elements.

It's to do with 'aliasing'

 

[arunmrao]

Wheel spokes ,trolley wheels, sheave pulleys,centre ribs are always odd in number and I have preferred them too. There is a break in symmetry during stress relief,thus minimising distortion or cracks.

If they are cast parts and during cooling of the castings in the molds those with even numbers are found to crack near the hub. Thus in the case of odd members too,we do not provide straight elements but have curved ones.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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[IceStationZebra]

"Odd is better than even for (4-stroke) radial engines for reasons of timing."

It's not just better, it's required! (And realize that rotary engines with an even number of cylinders have two rows of an odd number of cylinders - essentially two stacked engines.)
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"Is this why a baker's dozen is 13?"

Per Wikipedia (and other various sources): The oldest known source and most probable origin for the expression "baker's dozen" dates to the 13th century in one of the earliest English statutes, instituted during the reign of Henry III (r. 1216-1272), called the Assize of Bread and Ale. Bakers who were found to have shortchanged customers could be liable to severe punishment. To guard against the punishment of losing a hand to an axe, a baker would give 13 for the price of 12, to be certain of not being known as a cheat.

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When designing gears or sprockets you should have at least one with an odd number of teeth the maximize wear. When both gears are even not every tooth with mesh with every other tooth of the second gear. It is similar for sprockets, where a roller element will not engage every tooth.

ISZ

 

[ScottyUK]

ISZ,

Radial engines? Rotary engines passed into history a long time ago.

Interesting re. the Baker's Dozen.


----------------------------------

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

 

[SomptingGuy]

Maybe defunct, but still loads more interesting than a vanilla I4. I shudder when I see pictures and videos of the JRL Radial Flyer (7-cyl radial-engined motorcycle).

http://www.youtube.com/watch?v=GUx35zuI5GI

I guess it's no more idiotic than a flat or boxer 6 and probably handles similarly.

- Steve

 

[Ron]

Rock. Paper. Scissors.

Always a tie breaker.

 

[GregLocock]

Bicycle wheels are carefully optimised, and they have even number of spokes. Possibly they have so many spokes that odd vs even is subordinate to the difficulty of indexing odd numbers.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PhilBW]

I appreciate all the interesting responses. It sounds like there is some engineering logic behind the statement, at least in some applications.

Skywalker09, I'm not sure I understand the concept of seven creating directionality. Can you explain more?

Phil

 

[zdas04]

Most of the real reasons that us "old timers" have these rules of thumb are lost in the mists of time. Some of them are actually about fear and superstition instead of engineering, but some of them developbed because some smart guy observed that one configuration worked and another didn't. Occasionally, the profession just uses the rules that work without ever working out the arithmetic of why "a" works better than "b". This is ok except when the rule masks a "c" that is actually better than "a" or "b".

Maybe somebody noticed the "prime number" case above and since all primes after 2 are odd, it evolved into "use odd numbers" even though many of the odd numbers are not prime?

David

 

[benta]

SomptingGuy, if you shudder at that one, you'll probably faint over this one:

I wonder if it flips over on the side when you "blip" the gas handle?

Benta.