×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

American V8's - Curious
5

American V8's - Curious

American V8's - Curious

(OP)
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)

RE: American V8's - Curious

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??????

RE: American V8's - Curious

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

Rod

RE: American V8's - Curious

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.

RE: American V8's - Curious

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

RE: American V8's - Curious

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

RE: American V8's - Curious

2
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: American V8's - Curious

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: American V8's - Curious

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.

RE: American V8's - Curious

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

RE: American V8's - Curious

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: American V8's - Curious

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

RE: American V8's - Curious

(OP)
wouldn't the odd spacing create balance issues?

RE: American V8's - Curious

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!
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
 

 
 

RE: American V8's - Curious

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: American V8's - Curious

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.  auto

RE: American V8's - Curious

Quote (GregLocock):


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.

RE: American V8's - Curious

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.

RE: American V8's - Curious

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.

RE: American V8's - Curious

Re: Fan Balance.  Since the fan is in a single plane, if the fan is in static balance it will be in dynamic balance.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources