LOAD ON DERRICKS

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purchase two single blocks, the standing end of the fall being at the upper
block.

Find (1) The stress on the shackle of the upper gin block; (2) the
thrust on the derrick; (3) the tension on the span; (4) the stress on the
leading block; (5) the stress on the span block.

Fig 18 —A Single Purchase Fall
XY = 20 ft. XA = 23ft,YA = 13 ft.
In parallelogram ABDC, AB = 10 tons, AC = 5 tons, AD = 14 tons
In parallelogram ADEF, AD = 14 tons, AF « 9£ tons, AE = 17* tons
In parallelogram XGKH, XG = XH = 5 tons, XK = 7J tons.
, In parallelogram YLNM, YL = YM = 9J tons, YN = "l4J tons.
Construction.—(1) Draw the mast, derrick, span, eta, to a convenient
scale. The pull on the hauling part of the fall will be approximately
half the weight as the power gained by the guntackle purchase is two.
Make AB=W tons; AC—5 tons; draw BD parallel to the derrick and
CD parallel to the fall; join AD, which will be the diagonal of the
parallelogram ABDC\ AD represents the resultant of the two forces
acting at A, viz., the load of 10 tons and the power of 5 tons required
to hold it in suspension. AD is the stress on the shackle (14 tons).