Assembly Fit-up Issue Investigation of a First Stage High Pressure 
Gas Turbine Rotor 
Antonio Miranda Rivera
Advisor: Dr. Rafael Nieves Castro
Graduate School, Polytechnic University of Puerto Rico
ABSTRACT INSPECTION APPROACH INSPECTION RESULTS EVALUATION
A Gas Turbine engine generates mechanical energy from a Preliminary engineering visual review of parts performed at the Chart 3 shows the resultant circumferential clearance at the MIN neck The circumferential clearance increment at the MIN Neck region will
combustible fuel that could then be converted into electrical power by customer assembly facility indicated high risk of interference fit region between the disk and blades (92pcs) based on the individual be achieved by reducing the MIN Neck radius on the blades. This
means of a rotating shaft as opposed to the pressurized thrust between blades and disk located along the MIN Neck region interface data collected for each component. It is evident that on many slot change will increase the minimum clearance from line on line (0.000
generated by a gas turbine engine. Within the different variations of (see Figure 2). locations the clearance is almost zero or close to. clearance) to 0.0020in.
gas turbine engines, the Turbofan is the configuration that is most Analysis were completed following a comparative approach. The
commonly used in the commercial and military aviation field because it current, proposed and successful designs were compared to validate
provides high thrust and good fuel efficiency. DELTA - MIN NECK CLEARANCE the change.
A gas turbine engine can be break down onto five main modules: a) 0.004 Results shows that the proposed design will exhibit P/A stresses 0.2%
inlet/fan, b) compressor (high pressure and/or low pressure), c) 0.003 higher than the current design but 4.4% below legacy experience.
combustor chamber, d) turbine (high pressure and/or low pressure), 0.002
CALCULATIONS
and e) outlet/nozzle. This investigation will focus on the high-pressure 0.001 Analyze DELTA
turbine module of a Turbofan engine, more specifically, the high- 0
pressure Disk and Blades components. 0 20 40 60 80 100
The blades incorporate a dovetail design with surfaces that will load Slot Location
against parallel surfaces on the disk during engine operation to
distribute loads due to the centrifugal force. These surfaces are called Chart 3. MIN Neck Resultant Clearance
pressure faces. The intent of this Disk and Blade sub-assembly is to
Figure 2. Disk Dovetail Sketch – MIN Neck Table 1. Centrifugal Force Variables
extract energy coming from the burner (compressed air mixed with Centrifugal Force: Fc
fuel) and rotate the shaft that is linked to the compressor. Historical inspection techniques were reviewed to determine the best ANALYSIS OF RESULTS Fc= wt*W2*RCG
approach to inspect the affected hardware. G
Inspection results indicated that both components met the drawing
INTRODUCTION Disk: Calibrated pins were selected to measure the maximum width of Where:requirements and should assembled together. Looking back at thethe MIN neck opening on the disk at all 92plc. The calibrated pins are RCG-Radius @ Center of Gravity
inspection results it can be seen that the fielded disk and new make
precise to the third significant figure and provide an accurate measure W-Rotational Speed
This article will summarize the investigation of fit-up issues blades ran at the low and high side of the tolerance band respectively.for the disk Wt-weight
encountered during the sub-assembly of high pressure turbine blades As a result, some of the disk/blade slot combinations would provide aBlades: Coordinate measurement machine (CMM) inspection was G=Gravity
and disk. The dovetail design of the blades incorporates 4 total line on line fit during the disk/blades sub - assembly and any otherconcluded to be the best method to inspect blade thickness at the MIN 
pressure faces. These blades will slide in into the disk which will factors that may occur during the assembly process might contributeneck region.
incorporate a similar dovetail design. This is shown on figure 1. The to the fit-up issues that were localized on this MIN Neck interface.
fitup issues occur during the installation process of blades and disk. INSPECTION RESULTS Customer highlighted that during the inspection of the fielded disk the
Each component (turbine blades and disk) is manufacture by different pin checks exhibited a tight fit meaning that inspector had to exert
manufacturing sources and assembly is performed by the customer some force and rotation at some slot locations to fit the pins in.
Inspection results for the high-pressure turbine disk are summarized on
which adds up to the variables and potential factors influencing the fit- Afterwards, pin checks were performed over again and measurements
Chart 1. These shows that the MIN neck met the drawing requirements were identical but no force or rotation on the pins was needed. Same Table 2. P/A Comparisonup issues.
but process was targeted towards the lower end of the drawing
A set of engine blades (92pcs) and Disk exhibiting this issue was blades exhibiting the fitup issues were tried again after the pin checks
tolerance band. However, measurements run consistently on the lower
made available by the customer to support this investigation. The and were successfully assembled.
CONCLUSION
side which closes to a minimum the amount of room to install blades.
affected hardware was segregated for detailed inspection and Review of the cleaning process at engine overhaul showed that no It was concluded that both blades and disk met the drawing
analyses. detailed cleaning was performed on the MIN Neck. Lack of detailed requirements and it was the lack of cleaning on the fielded disk that
cleaning at this location in addition to the line to line fit at the MIN neck
drove the interference fit. In addition, it was also determined that the
would influence fit.
tolerance circumferential stackup also contributed to the fitup issue with
Furthermore, the engine manual instructions prevent the use of any
a worst-case scenario that creates a line to line contact.
force to install blades that are slide in manually into the disk. This
Short term corrective actions were implemented consisting of detailed
means that if there is any sign friction during the installation process
cleaning of the MIN neck area of the disk.
these are not acceptable for use and would need to be called
The long term and final fix for the fitup issues consisted of redefining
unacceptable.
the blades MIN neck area to provide additional circumferential room on
ROOT CAUSE AND CORRECTIVE ACTIONS a worst-case tolerance stackup.
Changes were validated by comparative analyses. The smaller radius
Chart 1. Disk Dovetail Sketch – MIN Neck CLEANING: Short-term solution for potential future fitup events will be translates into a smaller MIN neck cross sectional area that will carry
to incorporate a detailed cleaning of the MIN neck area of the disk.
For the blades, results summarized on Chart 2 shows that MIN neck is the same centrifugal load. This constitute a stress increment. Three
However, performing this detailed cleaning process require additional
conforming but running on the higher end of the drawing tolerance band. different blades geometries were compared: a) Current Design, b)
time that will translate into cost-in for the customer. This was
However, measurements run consistently on the higher side which Proposed Design and c) Legacy Design. In summary, stress on the
determined to be acceptable on the short term until the long term fix
closes to a minimum the amount of room for installation. Opposite to the proposed blade configuration is 0.2% higher than the current but still
Figure 1. Turbofan HPT Rotor Sub-Assembly was implemented.disk, the blades were new make, meaning that they came right from the 4.40% below legacy. Successful experience of the legacy blade
manufacturing source. operating on the same disk under a harsher environment validated the
DRAWING TOLERANCE STACKUP: The final fix to eliminate any proposed change.
OBJECTIVES future fitup events is to reduce the MIN neck thickness of the blades Final change to the blade MIN neck design will provide adequate
by 0.002in. This change will increase the current worst-case minimum circumferential clearance at the MIN neck region and will mitigate future
The intent of this investigation is to identify the root causes and clearance from line on line (0.000 clearance) to 0.002in. This minimum fitup issues.
implement the corrective actions necessary to mitigate the current and limit will provide adequate clearance to prevent future fitup events.
any potential future fit-up issues between high pressure turbine blades During the investigation process it was determined that the current line REFERENCES
and disk reported by a customer. on line minimum allowable fit between disk and blades at the MIN
The corrective actions implemented here after will improve the quality Neck area is not practical and represents a risk regardless of cleaning
process on the disk given that inspection techniques utilized on both [1] “Gas Turbines. Learn about the history and development of the gasof the affected products and reduce waste in the form of added cycle 
turbine”, Edison Tech Center.
time during the assembly process which represents cost savings for the blades and disk manufacturing sources combined with the level of
[2] “Types of Gas Turbines”, Glenn Research Center. National
the customer. Results will also be leveraged across multiple engine stackup tolerances provide room for deviations that increases risk of
Aeronautics and Space Administration.
lines to prevent similar issues. an interference fit.
Chart 2. Blades MIN Neck Measurements. Data collected using CMM
MIN Neck (IN)