Eugene Vogel
EASA Pump & Vibration Specialist

On occasion, service centers are asked to balance fan blades that are designed for an overhung mounting. The fan blade may be received mounted on the shaft, or without any shaft. The decision has to be made about how to mount the rotor in the balancing machine. One solution is to fabricate a mandrel to balance the fan blade between the machine pedestals. The other alternative is to mount the fan blade on the end of the shaft in an overhung configuration, with the fan blade outboard of both balancing machine pedestals. This would be the more expedient method if the fan blade is already mounted on the shaft in the overhung configuration.

As long as the fit of the fan blade to the shaft doesn’t change (when using a mandrel), it can be mounted in either configuration for balancing without affecting the results. If the fan blade is balanced in one configuration, it is balanced for the other. 

How the fan blade is mounted doesn’t change the balance, as long as the fit to the shaft doesn’t change. So the question is, “Which is easiest?” Often it is easiest to mount the rotor in the overhung configuration, but balancing in that configuration presents some challenges. Those challenges are addressed here.

Soft bearing balancing machines
Most of the challenges presented by overhung rotors apply only to soft bearing balancing machines (as opposed to hard bearing balancing machines), so this discussion is focused on that type of machine. The first challenge is handling the potential upward load on the pedestal opposite the fan blade. It is unsafe to rely on the balancing machine drive belt to hold the shaft down. If the belt broke, the rotor would careen out of the machine. Many balancing machine manufacturers have optional “negative load hold down” assemblies available. While it is possible to fabricate your own hold down, it is important to engineer it so that it does not restrict either the lateral or pivotal motion of the bearing support on the pedestal. Specific concerns for building negative load “hold downs” vary with the type of balancing machine.

In regards to actually balancing the rotor, the static-couple method is the best approach. Normally the static component is balanced first, so that the only remaining unbalance is a couple. (For a more in-depth explanation of the static-couple method, contact me at gvogel@easa.com.) While there are vector calculations to separate the static and couple unbalance components, the balancing machine will have an easy way to isolate the static component. Simply lock one of the two pedestals; with only one degree of freedom, the unlocked pedestal will respond to only the static unbalance component. Normally the pedestal opposite the fan blade is locked.

Correction weight placement
When balancing the static component, try to keep the correction weights in a plane as close as possible to the center of gravity (CG) of the rotor. The CG is located at the point where the rotor would balance if lifted with a choker strap; this can be easily checked when you load the rotor into the balancing machine. For most fan blades, that means the best correction plane for the static component is furthest inboard edge of the blade (Figure 1).

With the static component balanced, the only remaining unbalance will be a couple. A couple unbalance is corrected by adding two equal correction weights (at the same radius), in different planes, 180⁰ apart. Choosing two correction planes that are the maximum distance apart will minimize the amount of couple weight required. Typically the outer edges of the fan blade are used (see Figure 1). When both pedestals are unlocked, two-plane unbalance will be indicated. However, since the static component has been balanced, the balancing is approached as being a pure couple. Although two correction planes are used for a couple weight, the couple is a single force. So it is only necessary to use data from one of the pedestals. Usually the pedestal opposite the fan blade is used for balancing the couple. When balancing the couple, the amplitude readings on both pedestals should be proportional; that is, increasing or decreasing together. So when the couple is balanced, readings on both pedestals should be reduced.

May need to repeat procedure
As with any two-plane rotor balancing, there is cross effect – correcting on one plane affects the other. So when the couple correction is made, it is likely the static unbalance may be increased slightly. If correcting the couple does not reduce the unbalance indicated at both pedestals to within tolerance, the procedure should be repeated by locking a pedestal to isolate the static component and then unlocking to check the couple unbalance.

This interaction between the static and couple components is identical to the interaction between left and right planes when balancing between pedestals. Often a two-plane balancing calculation program is used to solve the between pedestal balancing problem. (For a more in-depth explanation of two-plane balancing programs, contact me at gvogel@easa.com.)

The same program can be used for overhung fan blades with the static couple method. The trial weight for the first correction plane is the static weight and the trial weight for the second correction plane is the pair of couple weights. When applying this approach both pedestals are unlocked; the program separates the effect of the static and couple trial weights. The correction results from the program are applied as a static weight for the first correction plane and a pair of couple weights for the second correction plane.

When balancing overhung fan blades, it is common for the inboard shroud to be the plane location for the static correction weights and to be one of the two-plane locations for the pair of couple weights. So when acceptable balance is achieved with trial weights, there will be multiple weights on the inboard shroud, some static weights, and some couple weights. It is good practice to combine all temporary weights in the same correction plane into a single correction weight. Simple vector addition can be used to calculate the equivalent combined weight.

When faced with balancing an overhung fan blade, there are a variety of approaches available. The ones presented here are used commonly. There are many different types and configurations of fan blades, and other techniques may be more appropriate for certain ones. 

For example, axial flow blades (propellers) present some unique challenges since there are no shrouds to mount correction weights. But an understanding of the approaches here will be helpful in any overhung fan blade balancing situation.

Categories: Rotors, Balancing, Fans, Vibration

Tags: Balancing Rotors

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