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16.2.2.9 Deviations in Dimensions and Part Geometry

This chapter deals with causes and effects of dimensional deviations in finishing processes that are attributed to special influences or secondary effects. Because these effects are not always known and/or understood, they are dealt with here in an overview. This section does not treat dimensional anomalies that are due to imperfections in material-removing shaping processes such as machining. The development trends towards increased performance concentrations and better efficiency in engines demands ever greater exactness in the finished parts. Upon closer consideration, seemingly contradictory tendencies actually confirm this trend. The number of parts is necessarily minimized for cost reasons, which leads to tighter dimensional tolerances in order to make possible the desired operating behavior, which is more demanding than before. Typical examples can be found in modern compressors. Blades with wide chords increase the weight of the individual blade. The extremely high circumferential speeds (rotor RPM) in modern compressors lead to root stresses near the technical limit. This makes the reproducible topography of the contact zones especially important. An optimal distribution of the surface contact pressure must be guaranteed by requirements for dimensional tolerances (Ill. 16.2.2.9-4).

Another example is the shift from “built” parts such as rotor disks with inserted blades to integral parts such as blisks (16.2.2.9-3). The missing friction damping in the root area can place extreme demands on the dimensional accuracy of the blades in order to guarantee acceptable dynamic loads during operation.

Today, modern finishing technology has sufficient possibilities to meet extreme demands for dimensional accuracy. The stricter these requirements are, the greater the amount of work required in the finishing process and quality assurance, which ultimately raises the price of the end product. In order to combat this cost trend, the amount of finishing work is usually limited to the absolutely necessary minimum. Therefore, unnecessary strict dimensional accuracy will be avoided if possible. The problem with this is understanding all of the influences and required tolerances necessary for the function of the parts.

The diagram above shows one example. Thin-walled structures, such as this labyrinth cone or gas-carrying parts, are usually subjected to considerable high-frequency vibrations in engines. Local deviations in wall thickness can occur during forming presses such as pressing and deep drawing. They can shift the natural frequency towards resonance and/or dangerously increase the stress amplitude while lowering stiffness (Ill. 16.2.2.9-2).

Increasing gas temperatures combined with the lowest possible cooling air consumption demands ever more complex cooling configurations for the hot parts. This means, for example, that the shape of cooling air bores for the cooling air film becomes much more important. Roughness, geometric deviations, burrs, and rounded edges must be closely examined in order to achieve reproducible results. These requirements also apply to the air supply for combustion chambers, which must increasingly meet demands for lower emissions.

Due to the high strength utilization of the parts (stress levels, life spans), a general problem in machine construction is especially relevant to engine technology. The operating safety of many highly-stressed parts is dependent on experience and verifications during development and serial production. This is unavoidable if the loads are dependent on a large number of influences that act in combination. In this case, the safety of a design cannot be guaranteed with theoretical considerations alone. It is also not permissible to draw conclusions based on other parts. For example, geometrically similar blades in different stages can be subjected to very different vibrational loads during operation. This can be seen in the dynamic loads on compressor and turbine blades, for which it is impossible to prevent all resonances related to excitement through flow disruptions. The fundamental principle is “the engine will tell us”. Of course, this approach does not preclude that some parts may be operating safely very near their load limits, although this may not be recognized. In this case, minor dimensional changes can lead to unstable conditions, such as dangerous resonance (Ill. 16.2.2.9-1). If, after sufficiently long periods of time, such as over a “personnel generation”, replacement parts are manufactured, finishing details, or at least the reasons for them, may not receive the necessary attention. This is especially true for manufacturing under license. Usually, the licensee will not know the backgrounds of some specific finishing characteristics. Under cost and deadline pressures, this situation can encourage seemingly insignificant “optimizations” of the finishing processes, including dimensions and tolerances. This can promote damages such as dynamic fatigue during operation.

The goal of this chapter is to use typical examples to increase the awareness of responsible personnel towards the necessity of ensuring dimensional quality. A prerequisite for safe operating behavior is that important details and their effects are known and understood.
An additional problem is the warping of parts during or after the finishing process. This is decisively dependent on residual stresses that form in the part before (forging, casting) or during the finishing process (machining, etc.). Understandably, warping due to a stress breakdown during operation (e.g. creep at high temperatures) can also unallowably change a part with total dimensional accuracy.

Illustration 16.2.2.9-1 (Example 16.2.2.9-1): It is not always known how close to their strength limits parts are during operation. The greater the utilization of the strength, the more serious the effects of seemingly minor dimensional deviations. In the described case, there was an increase in the number of dynamic fatigue fractures in turbine blades after the profile underwent seemingly unproblematic rework (see Ill. 17.5-1).

Example 16.2.2.9-1: Dynamic fatigue fractures occurred above the root platforms of rotor blades in the last low-pressure turbine stage (left diagram). Investigations revealed that resonance of the fundamental flexural mode occurred in the design-conforming blades just above 100% RPM. The engine was an older type (right diagram) with forged blades. In individual blades, raw part production or reworking caused the profile to deviate slightly in the transition to the blade root platform. In these cases, the chord length was shortened and the trailing edge was thinner. This was sufficient for a minor, yet dangerous, lowering of the resonant frequency. The number of blade failures increased dramatically when the blades underwent rework a generation later, and this risk was no longer known. The length of time explains why the critical blade dimensions were not given the necessary attention.

NOTE: In thin-walled parts that will potentially be exposed to dynamic loads, such as gas ducts, housings, and seal carriers, the design-specific wall thickness specified in the drawings must be strictly adhered to.
Deviations require consultation with specialized departments and, in some cases, testing in the engine.

Illustration 16.2.2.9-2: The vibration behavior of thin-walled parts with large surface areas, as are found especially in older engine types, reacts very sensitively to anomalies in wall thickness. There are several reasons for this, which are primarily related to changes in wall stiffness.

A thinner wall can have a negative influence in two ways (bottom frame):

  • Lowering of the resonant frequency can result in resonances that parts with design-conforming wall thickness would not be affected by. This effect is usually not compensated for by the simultaneous reduction of vibrating mass, which increases frequencies.
  • Increased dynamic stress on the weaker cross-section at the same excitement intensity.The diagram shows two typical cases of damage. In both cases, damage investigations revealed that the wall thickness was at least locally thinner than in the design requirements.

Case 1: In a propeller engine (top diagram), sections broke out of the surface of a thin-walled metal sheet cone that carried exhaust gas. The damage mechanism was HCF dynamic fractures, which are typical for high-frequency plane vibrations. The cause was most likely a sheet metal thickness at the lower tolerance limit, because only those parts were damaged. In critical cases, operating damages can occur even when design drawing requirements are met but the limits are not verified during testing and serial operation.

Case 2: Static labyrinth cones in the bearing chamber of a fighter aircraft engine were made from thin-walled sheet metal. In several cases, sections broke out during operation (middle diagram). These were also HCF dynamic fractures with damage symptoms that were typical for plate vibrations. In this case, as well, the minimum wall thickness required in the design drawings was not met in several areas. This was apparently related to the forming process of the cone.

Illustration 16.2.2.9-3: Performance concentrations, increased efficiency (lowering specific fuel consumption), and cost reduction all increase the demands on components. Typical effects include higher mechanical loads and/or demands for optimal flow properties and minimal clearance losses. In most cases, this tends to lead to tighter dimensional tolerances. Typical examples include:

Blisks: These are integral bladed disks (left diagram). These parts have been used for a long time in compressors and turbines in smaller helicopter engines. Their use in larger engines up to their use in fighter aircraft engine fans (top right diagram) is a more recent phenomenon. These one-piece parts, as well as welded stators (Ill. 16.2.2.6-6) do not have any friction damping like rotors with inserted blades do. This makes high-frequency vibrations a more important factor than before. Effects such as mistuning (Refs. 16.2.2.9-4 and 16.2.2.9-5) become very important. This term refers to a method of changing the resonant frequencies by redistributing the mass of the individual blades. Differences can already be apparent within the usual tolerances. Changes cause the mass distribution and stiffness to shift along with the resonant frequencies that are influenced by these factors. Coincidental unfavorable combinations can cause vibrational loads that are several times higher than usual in individual neighboring blades. In contrast, large dimensional deviations outside of normal tolerances, such as specific dimensional alterations between neighboring blades, can lead to lower dynamic stresses (Volume 3, Ill. 12.6.3.4-6.2).

Variable compressor guide vanes (middle diagram): These systems consist of a large number of components that are moveable and/or connected by positive-fitting. Angle deviations in the blades of less than 1° (also see Ill. 16.2.2.9-7) can already unallowably influence the operating behavior of modern compressors (e.g. surge limit and efficiency). Therefore, bearing tolerances must be strictly maintained over long run times. New part dimensions are a prerequisite for this.

Rotor blade (right diagram): Dimensional deviations in the front edge can cause rotating stalls and dangerous blade vibrations. If a sufficient number of blades are affected, it will have an unacceptable effect on the operating behavior and efficiency of the compressor (Ill. 16.2.2.9-5).
The extreme circumferential speeds of modern compressors stress the contact areas of blade roots at levels near the limit of their dynamic fatigue strength. The wide-chord design with high blade weight contributes to this. In order to prevent local overstressing of the edge zones and transitions of the contact surfaces, these areas are given optimal dimensions. The dimensioning takes into account the elastic deformations and ensures favorably distributed surface contact pressure (Ill. 16.2.2.9-4). This can only be achieved with very tight form tolerances.

Illustration 16.2.2.9-4: The loads on blade roots increase with the performance concentration/circumference speed and weight of the blades (long chords, see Ill. 16.2.2.9-3). In the root contact surfaces of compressor blades made from titanium alloys, this leads to an increased risk of dynamic fatigue fractures in combination with fretting damage. For this reason, special measures are not required in older engine types to even out the root loads. It is more important to minimize the loads at the edges and the transitions to the contact surfaces (middle diagram). For this reason, the contact surfaces of the blades are given a shape that takes into account the elastic deformations that occur under operating loads. A typical shape is a minimal camber (bowing, right diagram). The task of finishing is to ensure this three-dimensional form with the tightest possible tolerances.

Illustration 16.2.2.9-5: Requirements for performance increases have led to ever higher flow speeds in compressors. This means that the efficiency of the compressor, and therefore the fuel consumption of the engine, reacts ever more sensitively to inaccuracies in the flow on the inlet edge (diagram, Ref. 16.2.2.9-2). Even minor deviations (top middle and right details) from the ideal edge shape can have an unallowable effect. In addition to influencing the efficiency, unfavorable inlet edge shapes also increase the risk of rotating stalls and dynamic overstressing of the blade. For these reasons, manual edge rounding is becoming ever more problematic.

Illustration 16.2.2.9-6: The demands for carrying and distributing the cooling air for hot parts such as combustion chambers and turbine blading are continually increasing. The reason for this is the increase in gas temperature, which is a typical trend in the development of turbine engines (Volume 3, Ill. 11.1-6).

The operating behavior of combustion chambers can react very sensitively to changes in the combustion and cooling air flow (Volume 3, Ills. 11.2.2.1-9 and 11.2.2.1-10). It can be assumed that this will further increase along with the rise in demands for low emissions with shorter combustion chambers and higher energy conversion. The air infeed for the cooling air film on the inner combustion chamber wall usually occurs through a large number of bores (middle diagram). Even seemingly insignificant changes in the boring process can unallowably alter the roughness, shape, and edges of the bores. Experience has shown that this will mean that the necessary operating behavior of the combustion chamber will no longer be guaranteed.
In addition to the cooling air flow, bores are also important for the cooling film on the surface of turbine blades. Next to optimal cooling air flow, an important criterion is preventing blockages from the inside and/or outside. Dust that can cause blockages can be carried by the hot gas and/or the cooling air. Optimizing the shape of the cooling air bores can reduce this danger (bottom diagrams). This places additional demands on finishing to meet form and dimensional tolerances.

Conical bores that are created with the aid of lasers are state of the art, although there is evidently room for improvement.

Sharp edges and burrs on the air inlet side can significantly disrupt the flow relative to rounded edges (top diagrams). This is the case if the possible boring direction on the air inlet side creates undesirable, flow-disrupting sharp edges and/or burrs.

Illustration 16.2.2.9-7: Diffusion welding processes are used in special applications such as hollow fan blades made from titanium or dual property turbine disks (diagram, Ref. 16.2.2.9-1). They require especially exact machining of the joining surfaces, especially if the surfaces being welded cannot be sufficiently pressed together by external forces during the welding process. This applies especially to heat-resistant materials such as nickel alloys. This exactness is required because diffusion bonding can only occur if there is metallic contact. Even tiny local gaps will cause bonding flaws (Ills. 16.2.1.3-37 and 16.2.1.3-38). The roughness of the surface and its influence on microscopic contact flaws are important. In this context, it can be assumed that problems will occur when joining materials with different hardening states and different volume proportions of precipitated hardening phases (g'). Even these tiny volumnal changes can cause worrying dimensional changes at the high temperatures of diffusion welding (Ill. 16.2.2.9-9).
These flaws occur as kissing bonds (collections of microscopic flaws). They are not sufficiently reliably detectable with serially-implementable non-destructive testing methods. If the weld is placed under significant shear stress, crack growth can be expected from these flaws under creep and/or cyclical loads. Experience has shown that this means that safe operating life spans can no longer be guaranteed (Ref. 16.2.2.9-1).

Example 16.2.2.9-2: During the development of a small gas turbine, startup and operating tests were conducted in a cool chamber below 20°C. In several cases, this resulted in failure of the fuel control when the mechanical regulator failed.
Inspection of the regulator revealed that a control piston in the regulator had become stuck. At first, it was assumed that this was due to fretting (cold welding), but this was incorrect. The control piston runs through the fuel with extremely minimal play. Neither the sliding surface in the opened housing, nor the sliding surface of the piston showed signs of fretting. Measurement of the diameter revealed that the piston diameter had evidently increased and bridged the play necessary for sliding. The control piston was made from a high-strength heat-treated steel. The structure had not been completely transformed after the heat-treatment, and there was a relatively high proportion of residual austenite. In order to change this (Ill. 16.2.2.9-8) into a hardened structure, deep cooling is required if the strength requirements prevent a sufficient heat treatment temperature. The structural changes are related to a minor increase in volume (Ill. 16.2.2.9-8) and corresponding dimensional changes (larger volume in the hardened structure than in the residual austenite). This led to jamming of the piston and failure of the regulator. These findings were verified on other parts that had not yet been installed. The remedy was deep cooling of the hardened parts before final finishing.

Illustration 16.2.2.9-8: Structural changes such as occur during heat treatments or finishing processes (e.g. welding and diffusion coating) can result in small permanent volume changes. The mechanism is an expansion or relaxation of the metal lattice, during which atoms are moved to other parts of the lattice.

A typical example is the hardening of steels. Martensitic structures (hardened structures) have an expanded lattice relative to the soft annealed state. This is caused by carbon atoms. During case hardening, this creates desirable compressive stresses in the surface (Ill. 16.2.1.8-5). In steel parts, it affects the final dimensions.

Even in hardenable light metal alloys (aluminum) and nickel alloys, volume changes occur during the reciprocal transition from the hardened state to the solution annealed state (Ill. 16.2.2.9-9). In these materials, however, the effects are usually limited to blanks or semi-finished parts. For this reason, the dimensional changes are not noticed as they are in hardened steel parts.
If, during the hardening of steels, the cooling process does not result in the complete transformation of austenite into hardened structure, the structural conditions will be unstable. This can result in a later volume increase when the material is heated (below 300 °C, bottom diagram) or cooled (about -25 °C). If the inner part of slip joints with very tight tolerances is affected, they can become jammed. Evidently, this process led to the failure of piston engines in fighter aircraft in World War II. Components of the injection system jammed at very low surrounding temperatures (top diagram). A similar process is described in Example 16.2.2.9-2. In this case, the metering piston of a helicopter engine fuel regulator jammed during testing in a cooled chamber. In this extremely tightly tolerated slip joint, a dimensional increase of only a few m was sufficient to cause damage (middle diagrams).
In roller bearings at high operating temperatures, a 200 mm diameter was observed to grow by 0.01 mm.

The diagram shows an expected dimensional change of up to 1.6 parts per thousand in connection with the annealing temperature for the typical roller bearing material 100 Cr 6 (Ref. 16.2.2.9-6). The lower the austenitizing temperature, the smaller the expected dimensional changes in this case.

Illustration 16.2.2.9-9: Hardenable nickel alloys experience permanent volume changes. These are related to the g' hardening phase. If it goes into solution, it will result in expansion of the lattice and an increase in volume. In contrast, hardening leads to a volume reduction. This means that the behavior is the opposite of that in martensitic steels (Ill. 16.2.2.9-8). The volume proportion of the hardening phase should play an important role. It increases along with the heat strength of the materials, i.e. the proportion of g' (left diagram).
If the entire structure has transformed, dimensional changes of between 0.5 and 1 parts per thousand can be expected, depending on the alloy.

Usually, this type of change will not cause problems during the finishing process or operation. This is because heat treatments are usually done on raw parts with sufficient oversize. Therefore, problems should be limited to unique situations.

It is possible that problems could occur during diffusion welding of parts with different heat treatment states or made from different materials. If there are very different volume proportions of the g'-phase, it will threaten the exact dimensional accuracy required by the welding process. The typical very high temperatures of this welding process are sufficient to cause corresponding structural changes (Ill. 16.2.2.9-7).

Illustration 16.2.2.9-10: The most common causes of dimensional problems (warping) during finishing are residual stresses and/or thermal strain. Residual stresses can occur during raw part or finished part production. In many cases, thermal strain causes plastic deformations that in turn create undesired residual stresses. A typical example is welds.

Residual stresses can change during later operation. This leads to warping, which can have unallowable consequences as shown in the following examples:

Compressor housings of a turboshaft engine with a longitudinal assembly design were made from forged parts with a turned inner side and milled outer side (bottom diagrams). After the installation of new engines, heavy rubbing of the compressor rotor occurred in several cases. During this process, the compressor housings were irreparably damaged by embrittlement resulting from oxygen absorption during overheating. An investigation revealed that the housings evidently permanently ovalized during the first startup, i.e. the first heating to operating temperature. The warping occurred primarily along the axial flange. The deformations were traced back to residual stresses from the finishing process. Apparently, they were related to plastic suface deformations caused by milling. The solution was a suitable shot peening of the threatened area in the parts that had already been produced. The residual stresses from the shot peening overlayed with the residual stresses from the finishing process in a favorable way that did not result in any unallowable warping. For the current series, optimization of the machining process should be a preferred solution.
A sudden shifting of residual stresses with corresponding warping can also occur under the influence of weak external forces. One example is the “jar lid effect” (top middle and right diagrams, Ill. 16.2.2.4-1). In this case, a thin-walled membrane (lid, disk) enclosed by a stiff ring zone suddenly deforms in an umbrella-like manner.
Disk-shaped structures without reinforcing rings can experience S-shaped deformations (8-shape, top left diagram).

These processes are reversible. They can be observed in thin-walled rotor disks. Destabilizing residual stresses can be created during raw part production (forging stresses, casting stresses) or by intensive machining. This indicates that the machining parameters and/or clamping of the part were not sufficiently optimized.
A solution is to suitably break down the residual stresses. This can be done through stress relief annealing in stabilizing equipment. In the case of rotor disks, it may be possible to pre-spin them until allowable local plastification occurs.

Thermal stresses with disrupting warping of the part can occur during finishing steps that involve heating. Typical examples include heat treatments, welding, and soldering. The cause is thermal strain differences resulting from uneven temperature distribution or differences in strain behavior resulting from the joining of different materials. These dimensional changes can be very problematic and cause bonding flaws in soldered joints or EB welds, which require exact maintenance of the joining gap.

Even small structurally-dependent volume changes can cause problems in case of very tight tolerances. They can occur during finishing (e.g. during diffusion welding; Ill. 16.2.2.9-9) or during operation (e.g. during the jamming of sliding systems; Ill. 16.2.2.9-8).
If parts with considerable porosity, such as cast or sintered parts, are subjected to an HIP process (Ill. 15.3-8) that presses pores together and closes them, dimensional changes can be expected. In cast parts, these are zones with collections of porosity, i.e. cavity nests. In these areas, the surface will sink. This must be taken into consideration if very tight tolerances are necessary for optimal functioning of a surface exposed to a flow.
In sintered parts, the shrinking during the sintering process is a process-specific problem. In this case, as well, locally concentrated residual porosity can be the cause of deformations during an HIP process.

In some materials, creep as a relaxation of residual stresses (Ill. 16.2.2.4-5.1) can lead to warping of thin-walled parts even at room temperatures. One example is titanium alloys that exhibit warping a few hours or days after having sufficiently high residual stresses induced in them (e.g. through shot peening or machining).

Inhomogeneous parts and materials: This refers to parts with design- or construction-specific inhomogeneities that result in differences in physical properties. These properties include stiffness (modulus of elasticity), strength/flow limit, and thermal strain. Typical examples of this are parts with fiber reinforcement in the form of an integrated fiber layer or bandage. This problem has become significant with the use of ceramic-fiber-reinforced metals such as SiC-fiber-reinforced titanium. In flat structures it is important that the fiber layers are positioned symmetrically in order to prevent unallowable warping.

Illustration 16.2.2.9-11: Rework to save valuable and/or deadline-related parts is not unusual in finishing processes. Although it is trivial, it is often not realized that reworking cannot be repeated as frequently as desired. This situation usually occurs in the case of repairs with dimensional changes on parts that have been run. These processes are most often related with the removal of a seemingly insignificant amount of material. However, this cannot be repeated multiple times on parts with tight tolerances. The processes are primarily etching and abrasive blasting, which are mainly used to remove coatings (stripping, top diagram).

No significant abrasive material removal occurs during shot peening, but the roughness increases considerably relative to a smooth original surface, which “raises” the surface a minimal amount (bottom diagram).

References

16.2.2.9-1 P. Adam, “Fertigungsverfahren von Turboflugtriebwerken”, Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, page 188.

16.2.2.9-2 A. Schäffler, “Funktion der Fluggasturbine und ihrer Hauptkomponenten”, self-published CD, (March 2003), pages 3.5-55 to 3.5-57.

16.2.2.9-3 B.Syren, H. Wohlfahrt, E.Macherauch, “Zur Entstehung von Bearbeitungseigenspannungen”, periodical “Archiv Eisenhüttenwesen” 48 (1977), No 8, August, pages 421-426.

16.2.2.9-4 B.-K. Choi, J. Lentz, A.J. Rivas-Guerra, M.P. Mignolet, “Optimization of Intentional Mistuning Patterns for the Reduction of the Forced Response Effects of Unintentional Mistuning: Formulation and Assessment”, ASME-Paper 2001-GT-393 of the “International Gas Turbine and Aeroengine Congress and Exhibition”, New Orleans, LA, June 4-5, 2001 and periodical “Journal of Engineering for Gas Turbines and Power”, January 2003, Vol. 125, No. 4, pages 131-140.

16.2.2.9-5 J.J. Marra, “Tuned-Mass Damper for Turbine Blades”, periodical “NASA Tech Briefs”, October 1993, pages 99 and 100.

16.2.2.9-6 J. Grosch, “Schadenskunde im Maschinenbau”, Expert Verlag, Volume 308, Kontakt&Studium, Maschinenbau, 2nd Edition, ISBN 3-8169-1202-8, 1995, pages 37 and 38.

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