Netv Engine for Riding. 81 



of this paper preclude any detailed statement concerning the manner 

 in which these investigations were carried out. 



3.— The Thrust Plate. 



The thrust plate or ]:)earing surface against which the screw 

 rotates is of great importance, in that any imperfection or weakness 

 of its surface or any diversion from a strictly normal position, of 

 even a small area of its surface, may result in some error or slight 

 disturbance of the screw which is almost inevitably communicated 

 directly to the ruling. Hence a thrust plate must be made of 

 material combining the properties' of compactness, hardness, tough- 

 ness and durability in a high degree. This material, whether 

 natural or artificial, must also be such as is capable of receiving 

 the highest possible optical finish, that is, with respect to the 

 perfection of its working or bearing surface. The importance of 

 these requirements will be appreciated when it is borne in mind 

 that errors resulting from imperfection of a thrust bearing will be 

 periodic and if so an error of amplitude less than .0000005 in. would 

 be quite appreciable by its effect on the finished grating. Thus in 

 the selection of a substance suitable for a thrust surface we are 

 restricted to an extremely limited choice of material. Naturally, to 

 those familiar with the limitations and requirements involved, the 

 diamond is at once suggested as the most likely substance to ade- 

 quately fulfil all the demands made upon it. Unfortunately a 

 diamond of a suitable size and shape for the screw thrust of a 

 ruling engine would be difficult to procure, extremely costly 

 and almost impossible to work, without professional aid, to 

 the requisite perfection of surface demanded. Of other substances 

 we only propose to mention those of which we have had actual 

 experience, and know wherein they failed to meet requirements. 

 Naturally, of the substances experimented with, only the most 

 perfect examples obtainable were used. These included the hardest 

 steel, crystalline quartz, agate, topaz and sapphire. 



The steel thrust plat« was promptly discarded; though hard and 

 tough and worked to a highly finished surface, it required lubrica- 

 tion to prevent seizure and this at once introduced instability, 

 owing probably to the varying thickness of the oil film. Moreover, 

 with steel against steel (the screw terminal face also being steel) 

 signs of wear wei'e soon apparent. The two examples of quartz 

 which were tried, afforded, for a time, some promise of success and 

 permanence; eventually, how^ever, minute circular scratches or 



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