In this chapter a machine element is recognized as a one piece or assembled component, which is not scheduled to be disassembled and can be exchanged as a whole. From this definition the author deviated in single cases, when it seemed to him suitable. In an aeroengine a high number of so called machinene elements, are used mostly multiple. This chapter deals with a selection. These machine elements are removed, assembled and assessed during maintenance and repair. For this the condition of the parts must be evaluated for serviceability, further use or reparability. Not at least they are often connected with failures and safety problems. It is necessary to knowigly avoid such influences, to identify and evaluate risks, as well as carrying out pointed measures and remedies. This demands the knowledge and identification of specific failure modes. For this, the function must be understood, part specific failures as well as deteriorating influences and its typical locations at the machine element must be known.
The experience shows, that sufficient expertise about the operation behaviour of the machine elements and the evaluation of its conditions, is essential in the use of specifications, maintenence manuals, assembly instructions etc. To this belongs a certainty of the definition. For example,if an indication at a part must be allocated sufficient sure a description or a picture. So a foreign object damage requires other evaluations and/or measures than an external similar small fatigue out break.
With this the persponnel in maintenance, respectively repair has a very demanding task. This reflects also in the job description for technicians and assemblers of the aeroengines (chapter 19.1.2, Fig. "Job descriptions of repairers"). To meet this, sufficient practical trainings are necessary. These must highlight all necessary aspects. Unfortunately for this, the available literature and training material is often unsufficient. Either they are kept very simple. Therefore important background information are missing. However these are necessary for the understanding, correct decisions and the motivation. Or concerned is specialist literature, which deals certain aspects sufficently in detail for experts (e.g., laboratory activities, trouble shooting). Such literature is usually scattered in different publications. Its complexity and detail informations are too extensive for the content, needed in the practical work. So, for the practician the danger of confusion and uncertainty exists.
Here will be tried to supply the practician with helpful informations of selected machine elements. For this, specialist literature, failure reports and procuct descriptions are analysed and summarised in motivating illustrations. It is pointed at applied, generally available literature (e.g. frequently through the internet) in the describing illustration texts. This literature is specified at the end of every chapter.
Highlighted are, as far as possible, the following aspects:

• Operation loads.
• Influences of maintenance and assembly.
• Production problems.
• Failure mechanisms.
• Failure modes/appearances.
• Failure causes.
• Weak points.
• Examples of failures (case studies).
• Remedies.

Problems and failures at machine elements can depend from the aeroengine type and/or the operation profile in its frequency, formation and temporal sequence. This will also show in instructions and manuals.
In other chapters of this book and the other volumes, also machine elements are discussed in detail aspects about the particular theme. For example

• in the oil system (chapter 22.3) anti friction bearings and O-rings.
• Fuel system, gear wheels of pumps (Fig. "Fuel influencing fuel pumps") or
• Auxilary media for bolt connections (chapter 22.3, volume 4, Ill. 16.2.1.8.3-7).

Concluding it should be pointed at the special problem for the machine elements as SUP's (chapter 20.2.1). For this, just expensive parts like anti friction bearings or „bulk goods“ like bolts offer itself. The identification of such parts is in a high degree left to the experience and the „right feeling”.