Vane-axial blade pitch-vibration?

[crug]

Does anyone know what affect, if any, having blades in a fan at different pitch/angles affects vibration in a fan? Any literature? Have a new customer with some 300hp 1200rpm vane-axial fans with 8 fixed-pitch blades each. Apparently some of the blades have 'un-pitched' themselves and there is not one blade with the same pitch as the one next to it, in the same fan. The customer's maint. staff will be checking into why, getting them pitched correctly, and hopefully taking care to prevent this from happening again. Decreased air flow aside, what vibratory affect would this have? Common sense dictates there would be unequal aerodynamic loading of the entire fan as it rotates. Any application notes or white papers on this subject? Thanks in advance.

 

[electricpete]

I would think you may see:
1 - blade pass frequency due to the aerodynamics
2 - 1x from unbalance if the blade movement occurred after balancing.
3 - Harmonics from 1x if there was looseness associated with the blade movement.

 

[fredt]

You don't mention how the blades are clamped in the hub. From your comments I would deduce that it is an individually adjustable pitch and not an externally variable pitch machine.

The problem you mention is unusual on adjustable pitch fans. There is a considerable moment trying to s turn the blades - tending to in-line in the direction of rotation. However the high centrifugal loads usually ensures that friction prevents movement when running. Larger hub ratio fans sometimes have "pitch plates" to positively locate the blade pitch so that there is no chance of this happening.

Axial flow fans do have an issue with blade resonance and should be designed so that the main "flapping" resonant frequency does not coincide with any of the known exciting frequencies. The result of blade vibration is more commonly fatigue failure of the blade rather than pitch variation but I wouldn't rule that out if there is no positive locking of blade position.

 

[crug]

Thanks for timely replies. Yes, in these Novenco fans all the blades are fixed pitch. They can be adjusted with the fan at rest. Each blade has six bolts through the blade base into a locking collar on the inside of the hub fairing. We're thinking these bolts have possibly loosened. And yes, have harmonics of 1X (go figure.) I wonder if there would be some slippage of air back through a blade with less pitch as it follows one with more pitch, if the static pressure downstream of the fan is high enough? Any way you look at it, this is a less than desirable situation, and after a good tongue lashing from the facilities mgr, I'm sure they'll be looking at these closer from now on.

 

[GregLocock]

This is analagous to having one cylinder misfire in a multi cylinder engine, which creates a vibration at all integer multiples of the 'weak' cylinder.


So in your case I'd expect to see 1st, 2nd 3rd etc harmonic pop up. The relative strength of each harmonic will depend on the number of blades with a reduced pitch, and also what resonances are being excited.




Cheers

Greg Locock

 

[fredt]

The blade retaining collar may not be clamped up. There may be some clearance on the collar when bolts are fully tightened. (By design in some cases or by faulty tolerancing). I believe that fans like these should have a locking device that positively holds the blade at the required pitch setting.

There are plenty potential undesirable effects of incorrect pitch setting (such as overloading the motor) and from asymetrical pitch setting. Any vibration issue is much more likely to be an effect and not the cause. The solution is to positively lock all the blades at their correct setting. This should be done by the manufacturer.