Page:The Kinematics of Machinery.djvu/459

 CONSTRUCTIVE ELEMENTS. 437

reached a very advanced stage that it attempted to decompose materials supposed to be elementary; and similarly it has been necessary that kinematic science should be cleared of many erroneous prejudices before it could attempt to analyse the separate pieces from which the machine is formed in the workshop, and make their nature really intelligible.

Wherever the designing of machinery has been made a systematic study, it has to a certain extent been recognised that the machine consists of only a limited number of different parts occurring in it over and over again. Different writers have given to these different names, such as " details," " elements," " simple-parts," etc. ; I myself have for many years called them the "constructive elements," (bauliclie elemente) of machinery.

The constructive elements have formed the subject of many text- books. 54 In these, however, it has not been proposed that this sub- division should be taken absolutely, or indeed without very con- siderable limitation. It is not assumed, as in the case of the "simple machines," that all machines are simply combinations of these " elements," but only that the latter occur with special frequency in machine construction. Some idea of this sort has always existed below the surface ; the want of exact ideas as to the nature of the machine has, however, prevented its clear enuncia- tion, so that as the art of machine construction has advanced there has been a somewhat suspicious uncertainty as to which and what these " elements " were. Neither a very clear enumeration nor satisfactory definitions of them have been given. Only by instinct, as it were, their number has been more or less distinctly limited ; or at least they have in general been treated as if some such limita- tion did exist.

The following enumeration of constructive elements therefore makes no pretension to absolute completeness. It is simply a list of those parts which different writers on machine design have in- cluded under the head of constructive elements, or some equivalent title, and fairly represents the details supposed usually to belong to that class. These are considered to be :

Screws and screwed joints, Eivets and riveted joints,

Keys, cutters, gibs, and keyed Plummer blocks, "bearings, joints generally, pedestals,