Balance is Key in Shaft Couplings

When it comes to connecting components together within an aircraft, sturdiness is key. The two components should be solid and continuous, without any wiggle room. Under no circumstances should the coupling experience any sort of vibration. In general, vibrations jeopardize the strength and tension of a fitting or joint. While it may seem like a comfort issue, vibration can actually cause significant damage to the hardware of an aircraft. If a loose fitting is not adjusted the vibration can even lead to failure of the component.

One cause for vibration between hardware pieces is an incorrect balancing of the shaft coupling. Like all other aspects of aircraft systems and components, balance is a heavily regulated. A component is either balanced by a manufacturer, or it is balanced by the addition or subtraction of material during construction or maintenance. International Standards Order 1940-1:2003 outlines the balance quality requirements for rotors in a rigid state. The balance quality grade is used to define the limits of residual unbalance.

There are multiple options when it comes to correctly balancing a shaft coupling. An additional weight can be added to one side or plane only. This is known as static balancing and is typically used when the length/diameter ratio is less than one. If you were to imagine a cylinder, static balancing is the equivalent of drilling a hole in just one side. Dynamic balance is used if the length/diameter ratio is greater than one. For this balancing technique, adjustments are made to more than one area on our cylinder example.

The actual balancing of a shaft coupling can be split up in light of the placement of the shaft coupling on the aircraft. A manufacturer will cover the specified balance quality for a component. To ease assembly, components can be balanced in part, fitted, and then further balanced up to the industry standard. An alternative way of balancing a shaft coupling is to fit and balance one half of the shaft first. The other half of coupling can be assembled individually, with each of the components singularly balanced. The benefit of this way of balancing is the ability to interchange the various components of a system.

For example, individual brake discs can be removed and replaced, without having to rebalance the entire brake system. Ultimately this increases the rate of efficiency of any repair and maintenance, which in turn, mitigates the cost of aircraft maintenance. Due to their location within an aircraft or their load bearing properties, certain mechanisms dictate balancing procedures. For example, gear couplings must be balanced using a combination or sub-assembly. The relationship between the hub and the sleeve are not lined up until they are under a load, therefore any prior balancing would be incorrect.

Balance is a key characteristic of a sturdy, well-functioning system. If balance is not given the correct amount of care and attention, vibration will occur which leads to further issues down the line. 


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