on MACHINERY. 177 



the face of the teetli, where they are in contact, is too much inchned to the 

 radius, their mutual friction is not much affected, but a great pressure on their 

 axes is produced ; and this occasions a strain on the machinery, as well as a4» 

 increase of the friction on the axes. 



If it is desired to produce a great angular Velocity with the smallest possible 

 quantity of wheel work, the diameter of each wheel must be between three 

 and four times as great as that of the pinion on which it acts. Where the 

 pinion imj)els the wheel, it is sometimes made with three or four teeth only; 

 but it is much better in general to have at least six or eight; and considering 

 the additional labour of increasing the number of wheels, it may be advisable 

 to allot more teeth to each of them than the number resulting from the calcu- 

 lation ; so that we may allow 30 or 40 teeth to a wheel acting on a pinion of 6 

 or 8. In works which do not require a great degree of strength, the wheels 

 have sometimes a much greater number of teeth than this ; and on the other hand, 

 an endless screw or a spiral acts as apinion of one tooth,since it propels the wheel 

 through the breadth of one tooth only in each revolution. For a piiiion of 

 six teeth, it would be better to have a wheel of 35 or 37 than 36; for each 

 tooth of the wheel would thus act in turn upon each tooth of the pinion, and 

 the work would be more equally worn than if the same teeth continued to 

 meet in each revolution. The teeth of the pinion should also be somewhat 

 stronger than those of the wheel, in order to support the more frequent recur- 

 rence of friction. It has been proposed, for the coarser kinds of wheelwork, 

 to divide the distance between the middle points of two adjoining teeth into 

 30 parts, and to allot 16 to the tooth of the pinion, and 13 to that of the 

 wheel, allowing 1 for freedom of motion. 



The wheel and pinion may either be situated in the same plane, both being 

 commonly of the kind denominated spur wheels, or their planes may form an 

 angle: in this case one of them may be a crown or contrate wheel, or both of 

 them may be bevilled, the teeth being cut obliquely. According to the rela- 

 tive magnitude of the wheels, the angle of the bevil must be different, so 

 that the velocities of the wheels may be in the same proportion at both ends 

 of their oblique faces: for this purpose, the faces of all the teeth must be di- 

 rected to the point where the axes would meet. (Plate XV. Tig. 193, 194.) 



VOL. I. A a 



