PREFACE TO MECHANISMS SERIES The data concerning mechanisms@ 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 that 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 mechanisms as in many other fields. ESDU prepares 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 would 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 Mechanisms has been compiled from Data Items prepared@ up to 1969@ under the guidance of the I.Mech.E. Kinematics Committee or Sub-committees or Working Parties appointed by that Committee. The individuals responsible for the preparation of each Item are listed within each Item. Work on Data Items for the Mechanisms Series@ apart from necessary amendments@ was suspended between 1969 and 1975.