12 



RESISTANCE OF MATERIALS 



n 



O 



24. Four -in. steel cables are used with a block and tackle on the hoist of a 

 crane whose capacity is rated at 6000 Ib. What is the factor of safety ? (Use tables 

 for ultimate strength of rope.) 



25. A vertical hydraulic press weighing 100 tons is supported by four 2^-in. 

 round structural-steel rods. Find the factor of safety. 



26. A block and tackle consists of six strands of flexible ^-in. steel cable. 

 What load can be supported with a factor of safety of 5 ? 



27. A vertical wooden bar 6 ft. long and 3 in. in diameter is found to lengthen 

 .013 in. under a load of 2100 Ib. hung at the end. Find the value of E for this bar. 



28. A copper wire ^ in. in diameter 

 and 500 ft. long is stretched with a force 

 of 100 Ib. when the temperature is 80 F. 

 Find the pull in the wire when the tem- 

 perature is F., and the factor of safety. 



29. An extended shank is made for 

 a ^|-in. drill by boring a |-iri. hole in 



FIG. 3 the end of a piece of -in. cold-rolled 



steel, fitting the shank into this, and 



putting a steel taper pin through both (Fig. 3). Standard pins taper 1 in. per foot. 

 What size pin should be used in order that the strength of the pin against shear 

 may equal the strength of the drill shank in compression around the hole ? 



30. The head of a steam cylinder of 12-in. inside diameter is held on by ten 

 wrought-iron bolts. How tight should these bolts be screwed up in order that 

 the cylinder may be steam tight under a pres- 

 sure of 1801b./in. 2 ? 



31. Find the depth of head of a wrought- 

 iron bolt in terms of its diameter in order that 

 the tensile strength of the bolt may equal the 

 shearing strength of the head. 



32. The pendulum rod of a regulator used in 

 an astronomical observatory is made of nickel 

 steel in the proportion of 35.7 per cent nickel to 

 64.3 per cent steel. The coefficient of expansion 

 of this alloy is approximately 0.0000005. 



The rod carries two compensation tubes, A 

 and B (Fig. 4), one of copper and the other of 

 alloy, the length of the two together being 10 cm. 

 If the length of the rod to the top of tube A is 

 1m., find the lengths of the two compensation 

 tubes so that a change in temperature shall not 

 affect the length of the pendulum. 



33. Refer to the Watertown Arsenal Reports 

 (United States Government Reports on Tests of 



Metals), and from the experimental results there tabulated draw typical strain dia- 

 grams for mild steel, wrought iron, cast iron, and timber, and computed in each case. 



34. A steel wire ^ in. in diameter and a brass wire in. in diameter jointly 

 support a load of 1200 Ib. If the wires were of the same length when the load 

 was applied, find the proportion of the load carried by each. 



FIG. 4 



