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

 

[SomptingGuy]

I'm just coming round now...

- Steve

 

[btrueblood]

Symmetry and harmonics have similar roots. In summing sine waves, odd numbered waves tend to cancel, even numbers to add. Three-cylinder piston/plunger pumps have a theoretical output "ripple" of about 13%, 4-cylinder pumps have nearer 50%.

 

[dinjin]

When the numbers gets large, it probably makes little difference. For bearing designs under slow revolutions,
we maximize the number of rolling elements either odd or even. I know that in some early designs of tapered roller
bearings for helicopters, the cage used differential spacing to help reduce the amplitude but I do not remember any even or add requirement.

 

[SVanden]

Odd is better in wheel design/manufacturing. By not having an even number of spokes, any distortion from sub-par heat treat and quench, and stress relief after machining is reduced. When i was in the industry, I justified it as "off-axis stress in cornering fatigue" to my counterparts, by stating that adding one spoke to an even spoke design increased the likelyhood of having at least one spoke close to being in pure tension or compression at any time (just a reason to add a spoke, thin down all of the spokes, and reduce the weight of the wheel, thus reducing unsprung weight). Some battles were won, some lost due to cosmetic aesthitics, but FEA usually proved odd was better "lighter, stronger, truer".

 

[tbuelna]

PhilBW,

Using prime numbers (for things like numbers of teeth on gears, etc.) is more beneficial than simply using odd numbers. Prime numbers of gear teeth are less likely to produce coupled frequencies within various shafts in a driveline.

Other things like aircraft propellors, helicopter rotors, or turbine engine compressor/turbine discs might benefit from odd number symmetries if they are subject to vibration/turbulence due to passing frequencies. And for noise sensitive applications, radial assymmetry is also sometimes used.

As for using odd numbers of elements for things like rolling element bearings, the exact opposite is usually true, but for very practical reasons. Cylindrical roller bearings typically have even numbers of rollers. The reason for this is so that the square (or rectangular) retainer cage pockets can be through-broached. With ball bearings, the number of balls in a deep groove or Conrad bearing is dictated by the max number of balls that can be accommodated by the assembly procedure used. Angular contact ball bearing numbers are limited by how many ball can be fit around the race pitch circle.

Regards,
Terry

 

[SomptingGuy]

This is a fascinating thread! I've been counting things for the last couple of days, looking for odd/even numbers, primes and (here's where things get really sad) Fibonacci numbers.

- Steve

 

[GregLocock]

I posted this a while back, but here's an example of irregular spacing of fan blades to reduce tonal noise

http://www.geocities.com/greglocock/fanblade.png

red is the original (calculated) spectrum, blue is with the blades still correctly positioned for balance, but staggered to reduce the tonal content at 5th order.





Cheers

Greg Locock

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

 

[tbuelna]

GregLocock,

Here's an irregular spacing of blades on a tail rotor for noise reduction:

 

[mauricestoker]

Odd numbers are better for items that are paired, like your fingers or toes. I'd much rather have an odd number of fingers on each hand.

 

[GregLocock]

Well, now you've revealed your bias against the Simpsons -why?

Fewer than two fingers and a thumb seems cumbersome, but 4,5 or 6 dactyls seem more like an aesthetic choice than a rational one (perhaps I am biased, my little fingers aka pinkies are effectively non functional these days)

Cheers

Greg Locock

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

 

[mauricestoker]

I always liked Ned Flanders-until he got gang probed by the aliens.

 

[AlyB]

Fun thread.

Bicycle spokes were brought up earlier as an exception to the odd better than even thing. These are paired evenly because each spoke on the left side of the wheel is paired with one on the right side of the wheel, so that you can adjust the tension on each side in the vicinity of each spoke, thus truing a wheel. That maintenance seems a lot more important than minimizing noise or vibration under the circumstances.

Well, I'm off to go count fan blades now. Cheers.

 

[SnTMan]

AlyB, well, true, but that does not imply an even number of PAIRS.

Regards,

Mike

 

[KENAT]

AlyB - it's easier if you turn the fan off first.

I've got to say this thread, and some of the hypothesis/rules of fun are really rather interesting.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[SnTMan]

KENAT, I second that, this is a most interesting thread.

 

[ivymike]

oh man, the shark has been jumped.

take the "4-wheeled" vehicles which are so popular these days... in reality, they've got an odd number of wheels (once you count the steering wheel), or you could simply say that they've got one set of wheels, one being an odd number.

 

[KENAT]

ivy, if you're counting the steering wheel, don't you have to count the spare, which brings you even again unless your car only has a can of fix-a-flat.

I've been thinking about helicopters & even props and there's quite a mixture of odd and even even though you'd expect some of the dynamics issues to come into play.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[zdas04]

Since the car is not designed to rotate, the number of wheels is irrelevant to this discussion. Just as the number of spokes is less relevant than the number of pairs of spokes.

Rules like this should be rigerously developed and published. It seems that it is pretty common to have an odd (or prime) number of structural elements on a rotating shaft, but from the above none of us has described the reason why.

David

 

[ivymike]

oh, my bad, I didn't realize we were talking about rotating manifolds.

 

[zdas04]

I seemed to be developing that most of the interesting examples were rotating elements. The number of wheels on a vehicle didn't seem germane. Maybe it was me that was bad.

David