28 MACHiM DRAWING \M> DBHOB 



j secured t tip <\\<. \M..U:/|H iron or 



keys, both dri\en tV..m the sanu end .f the box. \ ! nmce 

 space is left between the head of the forward k. \ .n..| the 

 point of the hind one, to facilitate the driving of them out, as 

 tin -n only one key needs to be started at a time. Sometimes 

 a single key the whole length of the box is used, in \\ hi.-h case 

 it is necessary that the key ways in the shafts be of exactly 

 the same depth. 



The half- lap coupling, introduced by Sir William Kairl.airn, 

 is shown in tig. *JM. In this form of box coupling the ends of 

 the shafts overlap wit liiii thr 1><,\. It is evid.-nt that ODfl -hai't 

 cannot rotate without the other BO long as the box remains over 

 the lap. To keep the box in its place it is fitted with a saddle 



It will I)e noticed that the lap joint is sloped in sm-h a 

 way as to prevent the two lengths of shaft from 1>< -in^ 

 pulled asunder by forces acting in tin- direction of their length. 



Half-lap couplings are not used for shafts al>m< ; inches 

 in diameter. 



It may here be pointed out that tin- half lap ronplinLf is 

 expensive to make, and is now not much used. 



^hafte are weakened by cutting key ways in thru, 

 often the ends which carry couplings are enlarged in diameter, 

 as shown in fig. 22, by an amount equal to the thickness of tin 

 key. An objection to this enlargement is that wheels and 

 pulleys require either that their bosses l>e bored out large 

 enough to pass over it, or that they be split into halves. \\hirh 

 are bolted together after being placed on the shaft. 



I)t mention* of liu.r ( ',!/>!, 



I) diameter of shaft. 



I t hick ness of metal in box. 



L -length of box for butt couj 



L, = length of box for lap coupl 



i length of lap. 



D, = diameter of shaft at lap. 



