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





RE: Is odd better than even?
RE: Is odd better than even?
RE: Is odd better than even?
RE: Is odd better than even?
Tobalcane
"If you avoid failure, you also avoid success."
RE: Is odd better than even?
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.
RE: Is odd better than even?
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
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
www.bluetechnik.com
RE: Is odd better than even?
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 ...
RE: Is odd better than even?
It's to do with 'aliasing'
RE: Is odd better than even?
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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RE: Is odd better than even?
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
RE: Is odd better than even?
Radial engines? Rotary engines passed into history a long time ago.
Interesting re. the Baker's Dozen.
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If we learn from our mistakes I'm getting a great education!
RE: Is odd better than even?
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
RE: Is odd better than even?
Always a tie breaker.
RE: Is odd better than even?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Is odd better than even?
Skywalker09, I'm not sure I understand the concept of seven creating directionality. Can you explain more?
Phil
RE: Is odd better than even?
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
RE: Is odd better than even?
[IMG]http
I wonder if it flips over on the side when you "blip" the gas handle?
Benta.
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
RE: Is odd better than even?
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.
RE: Is odd better than even?
RE: Is odd better than even?
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
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Is odd better than even?
Here's an irregular spacing of blades on a tail rotor for noise reduction:
RE: Is odd better than even?
RE: Is odd better than even?
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Is odd better than even?
RE: Is odd better than even?
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.
RE: Is odd better than even?
Regards,
Mike
RE: Is odd better than even?
I've got to say this thread, and some of the hypothesis/rules of fun are really rather interesting.
KENAT,
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RE: Is odd better than even?
RE: Is odd better than even?
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.
RE: Is odd better than even?
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,
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RE: Is odd better than even?
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
RE: Is odd better than even?
RE: Is odd better than even?
David
RE: Is odd better than even?
The wear/phasing argument can't apply to a spline, is it just that gear makers habitually use odd numbers?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Is odd better than even?
RE: Is odd better than even?
RE: Is odd better than even?
But remember that the cicadas got there first, with their life cycles.
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
Never say never?
KENAT,
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RE: Is odd better than even?
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
RE: Is odd better than even?
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.
RE: Is odd better than even?
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
RE: Is odd better than even?
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
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!
RE: Is odd better than even?
RE: Is odd better than even?
RE: Is odd better than even?
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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RE: Is odd better than even?
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
RE: Is odd better than even?
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!
RE: Is odd better than even?
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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RE: Is odd better than even?
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
RE: Is odd better than even?
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!
RE: Is odd better than even?
for auto engines they are mainly 4,6,8,10,12,16, etc
for steam turbines are 2,4,6, and 8
RE: Is odd better than even?
The simplest division is two outlets (one above and one below) but that tends to create "bubbles" or surges in the delivered gasses. 4 connections is better since that gives nice even fixed nozzles every 90 degrees: 45 up and down from the split line. The more the connections to smoother the pressure.
RE: Is odd better than even?
RE: Is odd better than even?
- Steve
RE: Is odd better than even?
Are you suggesting that a tripod works because it has an odd number of elements?
Perhaps you are thinking of a hunting tooth? Instead of meshing a 40 tooth gear with a 20 tooth gear, you would change the big gear to 39 or 41 teeth.
RE: Is odd better than even?
It's 1.6180339887......
Fe
RE: Is odd better than even?
As you would if you were a gear manufacturer, and observe in passing that it is the lack of a common factor that is important, rather than primeness as such.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Is odd better than even?
RE: Is odd better than even?
RE: Is odd better than even?
-2sin(666°)
THere is a hidden message here I'm sure.
- Steve
RE: Is odd better than even?
Nor does the oddness or evenness matter.
RE: Is odd better than even?
Fe
RE: Is odd better than even?
Impressive and original! That's the second most insightful post I've ever read on this website.
As for the original question regarding whether, in general, is odd better than even when designing something? The answer is: It all depends upon the application.
Most religious types would agree that God is the ultimate designer. And most men would agree that his best piece of work is the female body. She has one mouth, two eyes, one belly button, two big toes, and one heart. Odd, even, odd, even, odd, etc. Can any of you think of a more perfect design?