Balancing

CF34-3 engine rotors


The GE Transportation Aircraft Engines CF34 Series of jet engines represent the genre for propulsion in the regional jet market, particularly in the 50- to 70-seat passenger class of airframes. Balancing requirements are established by the engine manufacturer to ensure vibration levels aren’t exceeded. Some of the benefits of minimizing vibration include reduction of friction, ensuring bearing life, reduction of the potential for low cycle fatigue, and achieving efficiency in operation. The engine manufacturer establishes all overhaul parameters including balancing speed, unbalance correction locations, unbalance correction methods, and unbalance tolerances for each rotor assembly.

Typically, jet engine rotor balancing incorporates multiple processes including blade sorting and distribution, static balancing, and dynamic balancing. Blade sorting and distribution can be accomplished through mass weighing or moment weighing, using an appropriate scale suitable for the intended task. Static balancing involves unbalance correction in a single plane specified by the engine manufacturer. Dynamic balancing typically applies to an assembly where unbalance is corrected in two planes.

Unbalance correction can be accomplished by redistribution of mass, addition of mass, or removal of mass. Each balancing task uses one of these methods to achieve the manufacturer-specified rotor unbalance tolerance.

Anatomy of the engine
The CF34-3 jet engine is a dual-spool design. The engine includes four major rotor modules: fan with forward shaft, compressor rotor assembly, high-pressure turbine rotor, and low-pressure turbine rotor.

The high-pressure turbine rotor is directly coupled to the compressor rotor assembly and forms the core engine (also referred to as the high-pressure spool). The fan with forward shaft is connected to the low-pressure turbine rotor via the fan drive shaft and comprises the low-pressure spool. In service, each of the two spools spin at a different speed to achieve performance parameters established by the engine manufacturer. Because of the different operating speeds, weights, effective radii, moments of inertia, and resultant centrifugal forces these spools are subjected to, the engine manufacturer specifies a variety of methods to reduce unbalance to a residual level that limits vibration in service.

Mass distribution, moment weighing, static balancing, and dynamic balancing processes are incorporated in the overhaul requirements for CF34-3 rotor assemblies. An overview of each rotor balancing process will provide the reader with an overview of requirements necessary for establishing a CF34-3 jet engine overhaul facility.

Fan with forward shaft
The single-stage fan with forward shaft is statically balanced on a horizontal balancing machine using a build and balance process. The unbladed fan disk with forward fan shaft unbalance is measured and recorded on the balancing machine. Fan blade retainers and spinner are then installed onto the assembly and unbalance is again measured and corrected using mass distribution of the blade retainer pins. Fan blades are moment weighed and distributed about the fan disk to compensate for the residual rotor unbalance of the unbladed fan rotor assembly. Residual rotor unbalance can be incorporated into the fan blade distribution solution. Finally, the fully assembled fan rotor with fan blades is check balanced.

The fan drive shaft is balanced in two planes by material removal using a fixture on the balancing machine. CF34-3 fan drive shaft balancing is not often required unless the fan drive shaft is being replaced or otherwise subjected to an OEM-approved repair.

Compressor rotor assembly
The CF34-3 compressor rotor assembly is comprised of three major elements including the:

  • 1-2 spool assembly
  • 3-9 spool assembly
  • 10-14 spool assembly
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