Now for a difinitive answer...March 11 2009 at 11:42 PM
SeaBat (Login SeaBat)
Response to LST 325 Bow Doors
from Mr. Alferi's compendium on LST's....
"BOW DOOR EQUIPMENT:
The bow door is handled by two 3-horsepower bow door drive units, each consisting of a gear motor which drives a screw through open gearing. This screw is engaged by means of a taper and wedge to a rack traveling in a rack guide and mating with a segment made fast to the bow doors. Motors are operated by means of push button stations marked OPEN, CLOSE and STOP. Four of these push buttons are provided per vessel, two for each motor. Two limit switches are provided, one at either end of the travel of the rack. These limit switches are actuated by a switch stop. Adjustment of these two limit switches must be made after assembly so as to provide for the proper arc of travel of the bow doors. Clearance is provided between the hub flanges of the screw gear, and the bedplate washers. This clearance is such that when the doors are almost closed, and the bow door limit switch has operated, the doors may be jacked together and locked by means of turn-buckles. The limit switch which operates to shut off the motor when the bow door closes should be so set that after the door has come to rest it may be completely closed by means of the above mentioned turn-buckles, making use of the clearance specified previously.
Extreme care must be taken during installation of the bow door equipment to insure that the rack and rack guide are properly aligned with the bow door segment and that the gear motor base is properly shimmed to correspond with this alignment. If the rack guides are not sufficiently rigid, deflection will be permitted and the screw may be so loaded as to cause severe deflection, unduly increasing the load on the electric motor.
The bow door equipment is designed for a rack thrust of 14,000 pounds at a rack speed of 1.98 feet per minute. Care should be taken that this equipment is not allowed to be operated at extreme overload or without proper lubrication.
RAMP GATE EQUIPMENT:
The ramp gate equipment consists primarily of a main shaft mounting two wildcats at the extreme ends and driven through a double reduction set of spur gears by a 10-horsepower gear motor. The equipment operates in both directions by means of push button stations marked, HOIST, LOWER and STOP.
A limit switch is provided which is operated by a sprocket of the low speed shaft and which actuates the arm of a track type limit switch. This limit switch is arranged to operate in the lower or open position of the ramp gate only. An additional limit switch is provided which is to be mounted so as to be operated by the ramp gate itself when being closed. This limit switch is to operate in the upper or closed position of the ramp gate only. The adjustment of this limit stop should be made carefully so as to shut off the motor at the proper time and prevent the ramp from damaging the ship structure. The ramp gate gear motor is provided with regenerative braking in the lowering direction, in order to prevent the ramp from lowering at excessive speed.
The ramp may also be lowered without the use of power by disengaging the sliding pinion, and locking it in the disengaged position. Before do doing it is necessary that the hand brake wheel be tightened in order to prevent the ramp from falling after the pinion is disengaged. After release of this pinion the ramp may be lowered by releasing the hand brake a sufficient amount to permit the ramp to start to lower. Care should be taken not to allow the ramp to fall at too great a speed. When operating under these conditions it should be borne in mind that as the ramp lowers, its load increases progressively and will be more difficult to hold the nearer it approaches a horizontal position.
If difficulty is experienced in engaging or disengaging the sliding pinion it may be readily shifted by operating the hand release on the electric brake, at the same time rotating the high speed gear slightly by hand, bringing the teeth of the sliding pinion and the teeth of the low speed gear into alignment in order that they may properly mesh.
If one chain is tighter than the other during installation or if by some reason excessive sag develops in one or the other of the chains, it may be brought back into alignment by disengaging the coupling bolts and rotating the coupling in the proper direction a sufficient amount to engage with the next set of holes, thereby bringing the chain into the same tension.
This ramp gate machinery is designed for a load on the chains of 17,250 pounds and a speed of rotation of the wildcat shaft of 2.62 RPM. Care should be taken that this equipment is not allowed to be operated at extreme overload or without proper lubrication."