PREFACE TO TRIBOLOGY SERIES The data concerning tribology@ in common with those relating to many other phenomena of concern to engineers in industry@ are widely scattered and when located are of variable quality and relevance to industrial application. Engineering designers do not@ generally speaking@ have the facilities or time required to collect and evaluate all the data available which may be applicable. However they require to view designs not against some locally derived data@ but against what can be demonstrated to be the best data and they need to know that these data are up-to-date and vouched for by a qualified cross-section of the engineering and scientific community. It is ESDU's purpose to provide data of this nature relating to tribology as in many other fields. ESDU has prepared this information for both the designer and the designer's customer and@ by serving the needs of many@ accomplishes the task at much lower cost than could apply to the individual performing alone. A qualified staff@ with the assistance of many individuals and establishments throughout the world in research@ industry and teaching@ collects and evaluates the existing data. The staff discards that which is irrelevant or unreliable@ correlating and distilling what remains into an authoritative working tool for engineering application. Throughout@ their work is guided and monitored by expert groups and ad hoc working parties. ESDU Data Items are organised in Series. This Series on Tribology has been compiled from Data Items prepared up to 1969@ under the guidance of the I. Mech. E. Bearings and Lubrication Committee or Sub-committees or Working Parties appointed by that Committee. The individuals responsible for the presentation of each Item are listed on the inside of the covers in each case. Work on Data Items for the Tribology Series@ apart from necessary amendments@ was suspended between 1969 and 1973.