. Machine design. wo or three ropes in excess of the number actuallynecessary. On the other hand the American systemhas the advantage of a uniform regulation of the ten-sion on all the plies of rope. The guide pulley, whichguides the last slack turn of rope back to the startingpoint, is usually also a tension pulley and can beweighted to secure any desired tension. The English STRENGTH OF MANILA ROPES. 191 method is most used for heavy drives from engines tohead shafts ; the American for lighter work in dis-tributing power to the different rooms of a grooves in the pulleys for mani
. Machine design. wo or three ropes in excess of the number actuallynecessary. On the other hand the American systemhas the advantage of a uniform regulation of the ten-sion on all the plies of rope. The guide pulley, whichguides the last slack turn of rope back to the startingpoint, is usually also a tension pulley and can beweighted to secure any desired tension. The English STRENGTH OF MANILA ROPES. 191 method is most used for heavy drives from engines tohead shafts ; the American for lighter work in dis-tributing power to the different rooms of a grooves in the pulleys for manila or cotton ropesusually have their sides inclined at an angle of about45°, thus wedging the rope in the groove. The Walker groove has curved sides as shown inFig. 83, the curvatureincreasing towards thebottom. As the ropewears and stretches itbecomes smaller and sinksdeeper in the groove ; thesides of the groove beingmore oblique near thebottom, the older rope isnot pinched so hard as thenewer and this tends to. Fig. 83. throw more of the work on the latter. 97. Strength of Manila Ropes. The formulas fortransmission by ropes are similar to those for belts,the values for S and <£ being changed. The ultimatetensile strength of manila and hemp rope is about10000 lb. per sq. in. To insure durability and efficiency it has been foundbest in practice to use a large factor of safety. R. Jones in his book on Machine Designrecommends a maximum tension of 200 d2 poundswhere d is the diameter of rope in inches. This cor-responds to a tensile stress of 255 lb. per sq. in. or afactor of safety of about 40. The value of / for manila on metal is about ,but as the normal pressure between the two surfaces 192 MACHINE DESIGN. is increased by the wedge action of the rope in thegroove we shall have the apparent value of /: f1 = f -^sin-^-where a=angle of groove, For a=45° to 30° f1 varies from to and these values should beused in formula (110). (1—e~ J ) in this
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