CRANES 



1009 



this part are disposed as represented in Jig. 592 (a plan of the top plates), and known 

 us ' chain riveting ' ; B B is tho pillar, which is firmly secured by a base plate p, to a 

 stone foundation B ; and fits at top into a cup-shaped bearing c', which is so firmly 

 secured to the side plates of the jib, at or near to the point where tho curvature com- 

 mences, and on which bearing the jib is free to revolve. Fig. 590 is a transverse verti- 

 cal section of the lower part of the jib, showing the manner of fitting the bearings 

 for tho chain-barrel (which is placed in the interior), and the spindles and shafts of 

 the wheel gearing, by which the power is applied there to D, the chain-pulley, 

 which is inserted in an aperture formed in the top of the jib. The chain passing over 

 this pulley enters tho interior of the crane and is continued down to the chain- 

 barrel. E is a pulley or roller, which is interposed about half- way between the chain- 

 pulley and the chain-barrel, for the purpose of preventing the chain rubbing against 

 the plates. Fig. 591 is a plan of the lower plates. 



Fig. 593 is a vertical section of another crane constructed iipon the same principle 

 as that just described, but calculated for lifting much greater weights (say 20 tons) ; 

 it differs in having the lower or concave side A A, of the jib strengthened by means of 

 three additional plates B is B, whereby the interior is divided into one large and threes 



503 



smaller cells, ns shown in figs. 594 and 595, which are cross-sections upon the lines a b, 

 and c d of fig. 593. This arrangement of the cells to strengthen the lower or concave 

 side is advisable, in order to obtain sufficient resistance to the compression exerted by 

 the load lifted, without unnecessarily increasing the weight of the other parts. 

 The tension exerted upon the upper or convex plates does not require so much ma- 

 terials to withstand it; c, is the toe of the jib, which rests in a step formed in the bot- 

 tom of the cylindrical castings D, which is built into the masonry forming the basis of 

 tho machine. E E are two of a set of pulleys, which are mounted between two rings F F. 

 Voi, I. 3 T 



