Since this is independent of x, the position of the load on the platform does not 

 affect the result. 



SIMPLE STRUCTURES 



197 



Let the dimensions be so proportioned that - = , and also a = 10 6. Then 



' = A scale so arranged is called a decimal scale. 



295. In the toggle-joint press shown in Fig. 139, the length of the hand lever is 

 = 3^ ft., and 1 2 = 4 in. If the pull P = 100 lb., find the pressure between the 



jaws of the press when the toggle is 

 inclined at 10 to the vertical. 



296. In the crab hook shown in Fig. 

 140, assume that the load Q = 300 lb. 

 and the coefficient of friction /* = .5, 

 and determine the kind and amount 

 of strain in the members ED, AD, and 

 AB for a = 30, ft = 90, a = 6 in., 

 6 = 3 in., and I = 18 in. Show also 

 that in order to hold the weight with- 

 out slipping, the condition which must 

 be satisfied is 



/ I 



2 a sin a 2 a 



297. A steam cylinder is 20 in. 

 in diameter, the steam pressure is 



150 lb./in. 2 , the crank is 18 in. long, and the connecting rod is 5 cranks long. 

 Find the stress in the connecting rod, pressure on cross-head guides, and tangen- 

 tial pressure on crank pin when the crank makes an angle of 45 with the horizontal 

 on the n in end " of the stroke. Find also the maximum tangential pressure on the 

 crank pin. 



298. Calculate the stresses in all 

 the members of the dockyard crane 

 shown in Fig. 141 when carrying a 

 load of 40 tons. 



299. Calculate analytically the 

 stresses in the members of the jib 

 crane shown in Fig. 142 when lift- 

 ing a load of 28 tons., the dimen- 

 sions being as given in the figure. 



D 



300. In the locomotive crane 



shown in Fig. 143, calculate the stresses in boom, mast, back stays, hoisting line, 

 and boom line when the boom is in its lowest position, the dimensions for this 

 case being as given in the figure. 



301. Calculate the maximum stresses in all the members of the stiff-leg derrick 

 shown in Fig. 144 when lifting a load of 10 tons, the dimensions being as follows : 

 mast = 25 ft., boom = 38 ft., each leg = 40 ft. It is customary to load the sills with 

 stone to give the necessary stability. Find the amount of stone required on each 

 sill when lifting the 10-ton load, 



