ON A NEW INSTRUMENT FOR DRAWING ENVELOPES. 



625 



revolve in opposite directions. The bevel wheels can be thrown in or out 

 of gear, to either enable this to be done or the main spindle to be coupled 

 up in one piece, so as to revolve in the ordinary manner. 



It may also be pointed out that not only cycloidal but involute 

 and other classes of rolling curves may be drawn by means of this 

 instrument. 



Examples of the simplest forms of applying this instrument are 

 shown in figs. 10, 11, and 12. The upper part of the figure shows an out- 

 line selected, and the lower part the result of tracing in a number of 

 consecutive positions the outline of the selected figure, the result being 

 an envelope representing the profile of the curve or ' gear ' which would 

 engage with it. 



Fig. 10. 



In fig. 10, in which the selected form is a radial square tooth of rect- 

 angular section, it is clear that a portion of the envelope required for 

 contact as the tooth is coming into gear is swept away or removed as the 

 selected profile is coming out of gear ; hence it would be impossible to find 

 an envelope corresponding with the selected form which would work 

 smoothly in practice, or avoid considerable 'back- lash.' 



The next case chosen for the profile is that of the standard American 

 screw thread. In this case the resulting envelope gives a tooth which 

 would work quite smoothly, but the normal of the surface in contact 

 nearly always is in a direction which would result in considerable pres- 

 sure upon the bearings of the two shafts. 



Fig. 12, in which an ordinary cycloidal tooth has been taken for the 

 profile, gives, as a result, another tooth of cycloidal form, and shows that 

 the mutual action would not only be perfectly smooth, and that there 



1898. s s 



