OPERATING MACHINERY AND SIMILAR PARTS. 49. The parts shall be simple in design, and easily erected, inspected, adjusted, and taken apart. The fastenings shall securely hold the parts in place after they have been set. Kind of Material. 50. Rolled or forged steel shall be used for bolts, nuts, keys, cotters, pins, axles, screws, worms, piston rods, trunnions and crane hooks, if any. 51. Trunnions, pins and shafting over 41⁄2 in. in diameter shall be of forged structural steel. Shafting 41⁄2 in. or less in diameter may be of cold-rolled steel. 52. Forged or cast steel shall be used for levers, cranks, and connecting rods. 53. Cast steel or forged steel shall be used for couplings, end shoes, racks, toothed wheels, brake wheels, drums, sheaves, and hangers where supported weight will cause tensile stresses. Large sheaves may be built of structural steel. 54. Pinions shall be made of forged steel and cut from the solid metal, unless pinions are too large for forgings. 55. Sockets used for holding the ends of wire ropes shall be forged without welds from the solid steel. The equalizing levers connecting the ropes to the counterweights, or moving span, shall be of forged steel. Cast-Iron. 56. Cast-iron may be used in boxes for shafts, 2 in. or less in diameter, and which obviously carry light loads. Other boxes shall be of cast steel. 57. Cast-iron may be used in eccentrics, cylinders, pistons, fly wheels, and parts of motors which are usually made of cast-iron. Castiron shall not be used for any trunnion or axle supports. Metal for Bushings. 58. Phosphor bronze, brass, and babbitt metal shall be used for the bushing or lining of journal bearings and other rotating or sliding surfaces to prevent seizing. 59. Phosphor bronze only shall be used for bushing for the trunnions of bascule and lift bridges, or in any large bearing carrying heavy loads. 60. The bushings for large bearings, such as for trunnions and similar parts, shall be held from rotating in their casings. The force tending to cause rotation shall be taken as one-eighth of the load on the trunnion or bearing, and as acting tangent to the surface between the back of the bushing and casing; this force shall not be considered as counteracted by any frictional resistances between bushing and casing. It shall be practicable to take out the bushing when the trunnion is slightly lifted. Castings. 61. Castings which are to be attached to rough unfinished surfaces shall be provided with chipping strips. The outer unfinished edges of ribs, bases, etc., shall be rounded off and inside corners filleted. Bolts and Nuts. 62. Bolts and nuts up to 11⁄2 in. in diameter shall have U. S. Standard V-threads. Nuts and exposed bolt heads shall be hexagonal in shape, and each nut shall be provided with a washer. If the nut will come on an inclined surface, a special seat, whose top surface is at right angles to the bolt, shall be cast or built up to receive the nut. Bolt heads which are countersunk in castings shall be square. 63. Nuts which are subject to vibration and frequent changes of load shall have locking arrangements to prevent the gradual unscrewing of the same. If double nuts are used for that purpose, each nut shall be of the standard thickness. Nuts subject to vibration shall be further secured by split pins through the bolt. Screws. 64. Screws which transmit motion shall have square threads. Tap Bolts, Set Screws, Etc. 65. Tap bolts and stud bolts shall not be used, except by special permission. 66. Set screws shall not be used for transmitting torsion to shafts or axles. Collars. 67. Collars shall be used wherever necessary to hold the shaft from moving horizontally. Each collar shall have at least two set screws at an angle of 120 degrees. Shaft Couplings. 68. Shaft couplings, unless of the flexible kind, shall be of the flange type, or split muff with bolt heads and nuts countersunk. 69. For large shafts, couplings such as are used for rolling mill shafting may be used. 70. Couplings shall be keyed to shaftings. Keys-Approximate Dimensions. 71. If practicable, hooked and tapered keys shall be used. The taper shall be -in. per ft. The approximate width of the key shall be onefourth of the diameter of the shaft. The height at mid-section of tapered length shall be three-fourths of the width. The length of the hook, measured parallel to the shaft, shall be equal to the width of the key. 72. If tapered keys are not practicable, parallel faced keys of about the above proportions shall be used. 73. Tapered keys shall bear on top, bottom, and sides; parallel faced keys shall bear on sides only. 74. The length of a key shall be not less than that of the hub. The key, when driven into its final position, shall bear on the full length of the hub. 75. The foregoing dimensions are approximate. The shape of the key must be such as to have unit stresses in shear and bearing not exceeding those allowable in the table. (159). 76. If practicable, the keys and grooves shall be made so that the keys may be backed out. 77. Keys shall be sunk in grooves in both hub and shaft. The depth of a groove shall be such that the bearing will not exceed the allowable unit stress. Set Screws, Etc. 78. Keys shall be held by set screws or equivalent means. In vertical shafts, bands clamped about the shaft, or other devices, shall be placed below the key. Hub. 79. If practicable, the length of the hub shall not be less than two diameters of the shaft; its thickness not less than one-third of the diameter of the shaft. The hub shall have a light driving fit on the shaft. 80. The groove in the hub shall be made on the center line of an arm. 81. Hubs shall be bored truly at the center of the wheel. Keys in Trunnions. 82. For trunnions and similar parts, which are designed chiefly for bending and bearing, the keys, key-ways, and bolts shall be designed to hold the trunnions from rotating. The force tending to cause rotation shall be taken at one-fifth the load on the trunnion, and shall be taken as acting at the circumference of the trunnion. Journals. 83. Journals shall be proportioned to resist, not only the various stresses to which they are subjected, without exceeding the permissible fiber and bending stresses, but also to prevent a tendency to heat and seize. 84. Divided journal and trunnion bearings shall be used, and the cap shall be fastened to the base with turned bolts recessed into the base. The nuts and heads shall bear on finished bosses cast on the bearing. There shall be -in. clearance between the lining of the base and the cap or its lining to allow for expansion. Bushings. 85. Steel bearings carrying steel shafts or journals shall be lined with bronze or brass. If shafts are 3 in. or less in diameter and of a slow motion, babbitt metal may be used. Bearings of steel on steel for moving surfaces will not be allowed. Bearings. 86. In cast-iron bearings carrying light shafts, no lining is needed. 87. The bearings of shafts shall be placed as near to the points of loading as possible. 88. The footsteps of vertical shafts shall be of tool steel and run on bronze discs. Lubrication. 89. Provision shall be made for the effective lubrication of journals, or any other sliding surfaces. Closed oil or screw compression grease cups shall be used. Grooves shall be cut in the surface of the trunnion to provide for the proper distribution of grease or oil. Grease and oil cups must hold the lubricant for any position of the moving parts. Grease Grooves. 90. The grooves in large trunnions shall approximate to a U shape; the size shall be such that a wire 5-16-in. in diameter may lie wholly within the groove. The edge of the U shall be rounded to a 14-in. radius. 91. The grooves shall be straight, running parallel to the axis of the trunnion. They shall be so located, not less than three in number, that all parts of the bearing surface of the bushing will be swept by the contained lubricant in an opening and in a closing of the bridge. The grooves must allow of being cleaned with a wire. Grease Cups. 92. In any trunnion bearings, or similar heavy bearings, strong screw compression grease cups shall be used for the grooves. 93. Oil and grease ducts shall be so located, if practicable, that the lubricant will flow by gravity toward the bearing surface. Housing of Sheaves. Dust Covers. 94. Counterweight sheaves shall be housed to protect from the weather. Dust covers shall be provided for principal bearings where practicable. Shaft Supports and Couplings. 95. Line shafts, extending from the center of the bridge to the end, shall not be continuous, but shall be connected with claw couplings. Each length of shafting shall rest in not more than two bearings with the couplings close to the bearings. 96. If shaft supports are connected to the floor beam in bridges having long panels, intermediate supports shall be used. These shall be adjustable, and are intended merely to prevent the shaft from sagging. Equalizing Gears. 97. Equalizing gears or devices shall be used to insure equal action at the pinions and operating racks. Unsupported Length of Shafts. 98. The unsupported length of shafts shall not exceed L = 80 d2 for shafts supporting their own weight only; L = 50d for shafts carrying pulleys, gearing, etc., where L= length of shaft between center of bearings, in inches, and d diameter of shaft, in inches. Speed of Shafting. 99. Line shafts connecting machinery at the center to that at the ends shall run at fairly high speed. The speed reduction shall be made in the machinery near the end. Formulas for Shafts. 100. In designing circular shafting, trunnions, and axles, the greatest unit fiber stress in tension or compression due to bending and twisting shall be calculated by the following formula: ΙΟΙ. The maximum unit shear shall be calculated by the following formula: S = 16 π d3 102. In these formulas, f= unit fiber stress in tension or compression; Sunit shear; d= diameter of shaft; M = the simple bending moment; and T=the simple twisting moment. 103. If a shaft, trunnion, or axle has one key-way cut at the section where the maximum stresses occur, f and S shall be increased onesixth; if two key-ways are cut, increase by one-fourth. is enlarged through the hub, this does not apply. Minimum Shafting. If the shaft 104. Shafting transmitting power for the operation of the bridge, and shafting 4 ft. or more in length forming part of the operating machinery of the rail locks and bridge locks, shall not be less than 22 in. in diameter. Distance Between Shaft Supports. 105. In figuring the bending moment on shafts, trunnions, and journals, the distance center to center of bearings shall be taken. Style of Gear Teeth. 106. Gear teeth shall be of the involute type with an angle of obliquity of 20 degrees. The roots below the clearance line shall be filtered. 107. The width of the teeth may be as great as four times the pitch, but not more, except for wheels running at a very high velocity, as in motors where abrasion is to be considered. Strength of Beveled Gear Teeth. 108. In estimating the strength of teeth in beveled wheels, the pitch at the middle section shall be taken. Pitch Circle. 109. For the purpose of accurately setting gear teeth in the field. erection, the pitch circle shall be scribed on the ends of the teeth. Worm Gearing. IIO. Worm gearing, for transmitting power, shall have an angle of thread not less than 20 degrees. The worm shall run in oil. A bronze or brass collar shall be used at the end of the worm and at the end of the wheel axle to take care of the end thrust. The wheel shall be of bronze. If a nut engages the worm, the nut shall be of bronze. |