Balancing

CF34-3 engine rotors


The CF34-3A compressor rotor incorporates loose blades in Stages 1 and 2. Stage 1 and Stage 2 compressor blades are mass weighed and distributed about the compressor 1-2 spool to optimize mass distribution. The CF34-3B compressor rotor incorporates an integrally bladed Stage 1 rotor which is commonly referred to as a ‘Blisk.’

The compressor 1-2 spool assembly is dynamically balanced in two planes using a stub shaft tool that serves as a bearing journal. For the CF34-3A, unbalance correction can be accomplished by blade swapping Stage 1 and Stage 2 compressor blades. Only the loose Stage 2 compressor blades can be mass weighed and distributed on a CF34-3B compressor 1-2 spool.

The compressor Stage 10-14 spool assembly is dynamically balanced in two planes using a stub shaft tool that serves as a bearing journal. This spool features annular dovetail grooves to retain Stage 10 through Stage 14 compressor blades. Unbalance correction can be accomplished by shifting blades at each stage of the spool. Locking tabs secure the blades from shifting about in their dovetail groove.

After completing the 1-2 spool and 10-14 spool balancing tasks, the major compressor components including the 1-2 spool, 3-9 spool, and 10-14 spool are assembled together to complete the compressor rotor. The compressor rotor is then dynamically balanced in two planes to complete the CF34-3 compressor assembly and balancing tasks. Final unbalance correction in forward and aft correction planes is accomplished by addition of mass.

High-pressure rotor
The CF34-3 high-pressure turbine rotor assembly consists of two stages. Schenck recommends Stage 1 and Stage 2 turbine blades be mass weighed and distributed about each disk to reduce the initial unbalance of the high-pressure turbine rotor assembly. Unbalance is corrected at Stage 1 by the addition of cup washers secured by bolts. Unbalance is corrected at Stage 2 by the addition of small unbalance correction masses under the Stage 2 turbine blade roots. These correction masses are axially retained by a rotating seal plate on the aft face of the Stage 2 disk.

Low-pressure rotor
The CF34-3 low-pressure turbine rotor incorporates four stages. The GE Transportation Aircraft Engines overhaul manual requires each stage be statically balanced. An alternative procedure permits each stage of the low-pressure turbine rotor to be optimized for unbalance using a computer-based solution. Schenck recommends Stage 1 through Stage 4 low-pressure turbine blades be mass weighed and distributed using a mass weighing and blade distribution system such as Schenck’s BLADIS online system. Once the blade sets are optimized, the low-pressure turbine rotor is assembled into a bladed rotor and dynamically balanced in two planes. Unbalance correction to reach the unbalance tolerance in each correction plane of the rotor assembly is accomplished by the addition of mass.

This article has been an introduction to balancing requirements for the CF34-3 jet engine. One should always refer to the engine manufacturer’s maintenance manual for specific instruction prior to attempting any engine overhaul task. Mechanics should be specifically trained and certified for completing the balancing tasks referenced in this article.

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