American V8's - Curious 5

[ryandias]

The mechanical drive fans on american v8's are never symetrical. They have equally spaced blades except for one or two locations that are oddly spaced. Whats up with that?

Just a Friday afternoon off in the clouds kinda question.

Any suggestions?

I have come up with;
balancing for other items (viscous clutch)
maybe the designer had really thick glasses?
fluid flow dynamics?? (not sure what might require this)

 

[Cheddarcaveman]

I remember seeing this on my old Chevy convertable back in 1968 but didn't think twice about it.

However, is it possible that they make a standard fan and, depending on the application or power available to drive it, they remove some of the blades to reduce load?

Or perhaps they got it all wrong in the first place and "overloaded" the fluid drives. To save throwing the 20,000,000 fans they had already made they just reduced the blade numbers??????

 

[evelrod]

I have been told by some who supposedly know, "noise attenuation". I have accepted that for want of a better idea.

Rod

 

[JStephen]

Noise sounds good. Could also be to allow better access to get something on or off, either during assembly or when working on the vehicle.

 

[dvd]

It might just be to allow the fan belt to be installed.

 

[MikeHalloran]

It's for noise reasons. Not attenuation, exactly, but modified generation.

Typical steel fans have an odd number of blades, and they're all unevenly spaced, some a little, some a lot, so they won't form constructive interference with each other or with nearby stationary objects, e.g. radiator fins.

Contrast them with a mechanical siren, comprising evenly spaced vanes of a frustrated centrifugal air pump, interacting closely with evenly spaced stationary bars and slots.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Yes, that's it. If you space the fan blades equally you get a strong tone at N*rpm/60 and its harmonics.

If you stagger the blades then the result is the same overall sound level, but the tonal component is not as high.

They do the same with tread patterns on tires, and possibly on alternator cooling fans.

Bear in mind they still need to balance, so you only have a limited choice.



Cheers

Greg Locock

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

 

[GregLocock]

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

might work, otherwise look in the Gallery at

http://www.geocities.com/greglocock

it's the top link. This is the (predicted) noise spectrum of a five bladed fan.

red is the baseline, blue is with 4 of the blades staggered by 10 degrees, although I didn't check the balance.

The strength of the fifth order tonal drops a bit , and oddly the tenth order drops a lot. The penalty is that you get all the other orders springing up. The improvement is about 4 dB at the peak tone, but I didn't optimise this. Incidentally, the balancing vector is fun - it's a pentagon with equal length sides, so it is symmetrical. A six bladed fan gives you many more possibilities.



Cheers

Greg Locock

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

 

[CoryPad]

Great information Greg. Thanks.

Regards,

Cory

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[dimjim]

I remember designing cages for bearings with
non standard spacings for helicopters to reduce
the noise level. Do not know if they still do
this or not.

 

[bklauba]

There is another reason, which is a bit more low-down dirty practical (besides those listed, which certainly make sense): With the additional spacing between some blades, it permits both visibility and (the mechanic's friend!!) access to items behind the fan that can then be reached with tools (sockets, extensions, etc.)

Putting it another way, it reduces the sickness of "knuckle-busted-itis". Whether this was a planned advantage for the mechanic or an inantvertent one, I am not sure, but I have been thankful for it a few times.

BK

 

[GregLocock]

BTW I think you can do a non symmetrical, balanced, pattern for a pentagon- imagine two vectors at 90 degrees, there are many games you can play with the other three. For a 4 bladed fan I think you always end up with a rhombus.

Cheers

Greg Locock

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

 

[MikeHalloran]

I think that typical 'merican V8s had seven or nine bladed fans starting in the mid- sixties, with the introduction of flex fans. They had to be irregularly spaced because the blades were very large, so you needed at least one big gap to get at the attaching bolts. And still you got cut.





Mike Halloran
Pembroke Pines, FL, USA

 

[ryandias]

wouldn't the odd spacing create balance issues?

 

[sms]

The fan doesn't care about the spacing as long as the mass is evenly distributed. So if one blade is heavier and another lighter, or if you compensate by adding balance weights to the hub, the fan can still be in balance even with the odd spacing.

-The future's so bright I gotta wear shades!
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[GregLocock]

The balance contribution from each blade is just a vector along its axis. So long as the loop of vectors ends back at the origin then that arrangement will balance, assuming a balanced hub and identical blades.

Or you could add a counterweight, but that's ugly.

Incidentally I think modern automotive cooling fans are balanced individually, but that may just be prototype ones.



Cheers

Greg Locock

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

 

[chapp333]

You know, I seem to recall replacing the non-symmetric fan on my '70 Nova with a symmetric flex fan that had more blades (and yes, I cut my knuckles many times afterwards). And I recall that the new fan was very loud, though at the time I figured it was because of the greater number of blades and the more aggressive nature of each. A quick trip to the exhaust shop took care of the loud fan problem though.

 

[handleman]

They do the same with tread patterns on tires, and possibly on alternator cooling fans.

I know for a fact that this is done on alternator cooling fans - I work in a facility that makes alternators. Not only is the spacing uneven, the blade sizes and angles are different as well.

 

[UMDpru]

the AH64 Apache Helicopter has a tail rotor with an X type pattern. I think thats what you are refering to? They did it to attenuate the rotor noise.

 

[RickFoley]

Safety is the reason for the seemingly odd spacing on radiator fans and alternator vanes. It was designed to reduce the stop motion effect while using stroboscopic timing lights. The illusion of a stationary fan blade only centimeters away from the pulley/timing mark can lead to obvious hand injury.