The derricks that are most commonly used in building construction are of the stiff-leg, guy, and breast or house types. Some of these types are subject to slight modifications, and they may then be known by different names; but the general classification here given is comprehensive enough for ordinary purposes. Each type may be arranged to be operated either by hand or by mechanical means, - the method of operating usually being determined by the weight and quantity of the material to be handled.

35. Stiff-Leg Derricks

These are usually employed in excavating for building foundations and for doing other work at or near the ground level, although they are also used to some extent in the more advanced stages of building construction. It is particularly important to see that the timbers that are used in stiff-leg derricks are sound and of adequate size for the work to be performed, and that the several members are properly jointed and fitted. Selected Oregon fir or yellow pine is most suitable for derrick construction. All irons, such as the goose-necks and the connection plates at the heels of the stiff legs, should be of the proper sizes. All bolts should be of the best material, and provided with adequate heads and a sufficient number of threads. Washers should be placed on both ends of the bolts, where necessary, and the nuts should be drawn up tightly. Lock-washers or jam nuts should be used, to prevent the nuts from working loose, or the same object may be attained by checking the threads on the bolts after the nuts have been tightened. The pins securing the connection plates to the bed sills should be of adequate size, and fitted with cotter pins at both ends. Inspections often show that the connection-plate pins have been lost, and that ordinary bolts, frequently of too small a size, have been substituted. Such a practice should be prohibited.

See that the goose-necks are fitted to the stiff-legs in a proper manner, so that there will be no undue friction on the gudgeon pin. If the goose-necks are improperly fitted, or the heels of the stiff-legs are incorrectly set on the bed sills, the holes in the goosenecks will be worn into an elliptical shape, making it more difficult to handle the derrick, and also causing it to jar when the boom is swung around. Furthermore, it is quite possible that the gudgeon pin will become so worn that it will fracture and cause an accident. A collar should be placed on the gudgeon pin, above the goose-necks, and a hole should be drilled through this collar and the gudgeon pin, through which a bolt should be passed to hold the collar in position.

See that all of the derrick sheaves are of suitable size, that they are secured in place by pins of the proper diameter, and that cotters are placed in both ends of every pin. Pay particular attention to the recess for the sheaves at the end of the boom, because if this is too wide the sheaves will wabble or work along on the pin, and this may cause the cables to run off the sheaves. If the recess is found to be too large, place filler plates on both sides of the sheaves, to make them run true.

Fig. 47. Diagram of a Stiff-leg Derrick.

Fig. 48. Diagram of a Guy Derrick.

(See the explanatory note on page 77, in connection with the diagram for a stiff-leg derrick. For convenience, only one guy rope is shown, but in practice there should be at least six, as described on page 81. The boom should fit loosely in its seat, and the pin in the boom seat should never be omitted).

Give particular attention to the weighting of stiff-leg derricks, because this is an item of extreme importance. The necessary weight should be calculated by a competent engineer, in every case. The material used for the weighting should always be inclosed in well-constructed boxes, so that the weights will remain in the proper position and will not be dislodged or shifted about by the vibration of the derricks, or in any other way.