spans opening in from one (1) to one and one-half (12) minutes, the acceleration should take place in from ten (10) to twenty (20) seconds and the retardation in from ten (10) to fifteen (15) seconds. Where the time allowed for opening and closing is greater, the period of acceleration and retardation should be increased correspondingly. In lifting-decks, the time for opening should vary from one-half (1⁄2) to one (1) minute, and the time of acceleration should be decreased in due proportion. Except when the span and the counterweight are at mid-height of lift, the counterweight ropes themselves are unbalanced. This condition may be overcome by special balancing chains; but when this is not done, the weight of the unbalanced rope must be taken care of by the operating equipment. The force necessary to overcome the effect of the unbalanced cables is where R is the weight of the unbalanced cables at any point in the travel of the span. It must be remembered that for the first half of the operation in either direction this unbalanced load acts against the force tending to move the span, whereas in the latter half thereof it acts with that force and against the braking action. For normal operation a wind load of two (2) pounds per square foot shall be assumed as acting against the exposed area of the span as it is seen in vertical projection. The friction on the guides due to this wind load must be overcome by the operating ropes. This friction shall be taken as fifteen (15) per cent of the said wind load. For normal operation of from one (1) to one and one-half (12) minutes the operating equipment must be capable of overcoming the above forces. It must also be capable of moving the span for all wind loads of less than fifteen (15) pounds per square foot, although the time of operation under such a condition shall be increased accordingly. The span-locks for lift bridges shall, as a rule, be operated by hand, when the operator is located in the machinery-house. However, when mechanical operation is required therefor, it shall be designed to meet the case in hand. 89. Bascule Spans For bascule bridges the power equipment will depend on the type of bascule used; and, in general, it will be governed by the preceding specifications for lift bridges. For rolling bascules the coefficient of rolling friction shall be taken at eight (8) per cent. The operating equipment on all types must be capable of holding the span in any position for a wind load of fifteen (15) pounds per square foot on the exposed surface as seen in vertical projection, and of moving it in the specified time against a wind load of two (2) pounds per square foot thereon. POWER EQUIPMENT 90. Hand Operation When hand power is used on the span, the number of men required and the time to operate shall be based on the assumption that one man can exert a force of 40 pounds on a lever with a speed of 160 feet per minute, developing about one-fifth of a horse-power. For figuring the strength of the machinery a man shall be assumed to exert a starting force of 125 pounds. 91. Electric Operation The motors for performing the various operations necessary to open and close the movable span shall be of either A. C. (alternating current) or D. C. (direct current) construction, depending upon the kind of current available at the bridge site. For alternating current, crane or milltype slip-ring induction, or railway motors shall be used; and for direct current, railway, crane, or mill type; series-wound motors with slotteddrum armatures and form-wound armature coils. All motors shall be weatherproof or protected by weatherproof housings so arranged as to permit easy access for inspection and repairs. The housings shall be tapped for conduits so as to avoid exposing the motor-leads. Standard commercial motors in common use shall be selected so that duplicate parts can be readily obtained. Motors entirely enclosed shall be provided with openings for inspecting commutator and brushes. The motor shall either have the armature shaft extended and key-seated for a coupling, or shall be provided with back gears. In the latter case a forged steel pinion shall be keyed and locked to the armature shaft and shall engage a cast-steel gear keyed to a secondary shaft with bearings in the motor frame. The secondary shaft shall be key-seated for a coupling, and the back gearing shall be properly housed. Both gears shall have machine-cut teeth. For light spans one motor shall generally be employed to operate the span; but for heavy spans two motors, although one motor only may be used, if the engineer so decides. Where two motors are employed to operate the span in the normal time, provision shall be made for operating it in a longer time with one of the motors alone. With D. C. equipment the two motors shall be operated in series parallel. A separate motor shall be used to raise the ends of swing spans, and to operate the locks and gates where mechanical power is used therefor. The motors shall be capable of developing the necessary torque and horse-power required for performing the various operations within the times specified. Motors shall be rated on the one-half (12) hour basis, according to the Standard Rules of the A. I. E. E., viz.: After one-half hour's run at the rated loads, the temperature of any part of the motor windings shall not exceed by more than fifty (50) degrees C. that of the surrounding air, if the temperature of the surrounding air is twenty-five (25) degrees C. The permissible rise in temperature shall be increased or decreased one-half of one per cent for each degree centigrade that the surrounding air is less than or greater than twenty-five (25) degrees C. The normal running and starting torques and the maximum running and starting torques of the motors shall be obtained from the company or companies manufacturing the motors selected. For normal operation, the sum of the normal starting torques of the motors shall be slightly in excess of the starting torque needed to move the span, and the sum of the normal running torques at maximum speed required shall be slightly in excess of the running torque required at the end of the accelerating period. Where two motors operate the span, the maximum starting and running torques of each motor shall be well in excess of the total starting and running torques required. Under all conditions of operation there shall be no injurious heating or sparking of the motors. The speed of the motors throughout the operations shall be such as to open or close the span in the required time. All motors shall be equipped with standard solenoid brakes with a braking torque that will stop operation in the required time. These brakes shall be set by springs or other mechanical means, and released by solenoids operating only when the motors are drawing current, except as hereinafter provided. The solenoids shall have ample capacity for all currents passing through the motors without exhibiting injurious heating. The friction surfaces shall be of materials not affected by moisture. To make coasting possible, a release shall be provided for each solenoid. brake, allowing it to draw current when the motors are shut off at the will of the operator. Weatherproof motors shall be provided with weatherproof solenoids. Motors shall be mounted so as to afford easy access for inspection and repairs. They shall be supported on good, substantial brackets or foundations. For each size of motor there shall be furnished the following extra parts: one armature, one set of field coils, one set of brushes, and one pinion and one split gear (if the latter two are supplied with the motor) fitted and ready for quick installation. Controllers shall be of the reversing-drum type with contacts protected by blowout magnets, except where the currents are too large for the ordinary controller or where remote control is necessary, in which cases there shall be magnetic switches on the switchboard operated by master controllers. All controllers shall be of ample carrying capacity to operate the motors under all conditions without injurious sparking. They shall be capable of varying and maintaining the speed from zero at the start to the maximum running speed without injurious sparking or shock due to sudden variation in speed. Sufficient steps shall be provided on the controller so that the torque of the motor will vary approximately as the torque required. The controllers shall be so wired that the solenoid brake will be released on the first notch and the motors started on the second. Where two D. C. motors are used, they shall be connected by one series parallel type controller, capable of operating both motors or either motor alone. Separate controllers shall be used for the end lifting, locking, or gate motors where electrical operation is provided. All apparatus shall be so interlocked that all operations can be performed only in their proper sequence. In railway bridges, emergency switches shall be provided so as to release the various motors from interlocking in case the interlocking system becomes deranged or in case there is not sufficient time to set the signals without great risk of a boat's striking the span. These switches shall be placed in sealed glass cases on the switchboard. Cast grid resistances shall be used in the motor circuits, designed so as to carry the currents required without destructive heating. They shall be properly mounted so as to avoid serious vibration and so as to give proper access for ventilation and inspection. In addition to the solenoid brakes, hand brakes shall be used for the main operating motors when the operator is located in the machineryhouse; otherwise an electric brake shall be employed. The hand brakes shall be of the band type, and shall be operated by a lever located near the controllers. The brake shall have a braking torque equal to the normal starting torque of the motors. Hard maple blocks attached to· steel bands shall bear on a cast-steel brake-wheel. The coefficient of friction between the blocks and the cast steel brake-wheel shall be taken at twenty (20) per cent. The brakes shall be of the type shown in Fig. 78a. O is the centre or support of the brake wheel; B and E are the supports of the bell cranks ABD and DEF, respectively; A and C are the points of connection of the brake-band to the bell crank ABD; H is the support of the lever LII, and G is the connection point for the link FG. Let K Force applied on the brake-lever. P R = Force at the circumference of the brake-wheel to be overcome. e = Base of the Naperian logarithms 2.71828. f = Coefficient of friction between the brake-wheel and the bandblocks. Then λ = Angle of contact between the brake-band and the brake-wheel in radians. ef. (See Table 78d.) P = T1 T, = (λ − 1)T ̧. Ꭲ (Tt + T2) × R2 × R3 × a R1X RX RS Where an electric brake is used, it shall be set by a spring and released. by a solenoid. The brake will always be set except when the span is to be opened, when it will be released. If it is needed during the operation, it will again be set by cutting current off of motor. It shall be so designed that no injury will result if released indefinitely. There shall be a shunt circuit controlling the solenoid, and it shall be so arranged that the brake cannot operate while the motor is drawing current. A mechanical release |