Electro-hydraulic winches do not call for special mention as they use a continuous running motor, which can be either a.c. or d.c. They can be operated either singly or in groups from one pump. Many electrical deck auxiliary schemes make use of contactors for control purposes and where these are of such size and numbers as to warrant it they can be accommodated in a separate contactor deckhouse instead of in the winch assembly. This increases the amount of cabling but on the other hand it economises deck space in the vicinity of the winch, making for cleaner lines and unobstructed viewing by the operator. It also facilitates maintenance work which in any case is not always opportune to carry out when the ship is in port and when the winches are in use. While at sea maintenance can be carried out under protection from the elements. In every winch, etc., in which the load is lowered while the motor is mechanically coupled such as in systems employing power lowering it is essential to prevent the load taking charge and lowering at a speed which will damage the motor armature. To safeguard against this contingency centrifugal brakes are provided in some cases and they are so set as to enable heavy loads to be lowered with an assurance that the safe speed cannot be exceeded. Provision must also be made to stop the load running back if the power supply should fail or the overload relay operate and in this event the winch, etc., must not restart when power is restored unless the controller has been returned to the starting point, usually the "off " position.
With d.c. auxiliaries a compound-wound electromagnetic brake is recommended whenever appropriate as the series winding assists in giving quick release if the starting current passes through the winding. The shunt coil must be proportioned to hold off the brake when the winch is running light, i.e. with low series current. If the scheme involves regeneration it is possible for a compound-wound brake to float on when the current in the series coil reverses, causing it to oppose the shunt. Great care must therefore be taken to ensure correct proportions in such cases. For windlasses and capstans a quick release is not so important and a shunt brake is usually satisfactory. A type of brake is shown in Fig. 19-1.
The brake linings are pressed home by the action of four spiral springs. When the brake coil is energised the armature plate is attracted to the face of the magnet, thus releasing the pressure between the brake linings. Copper shims are provided so that the armature plate does not touch the magnet and thus the armature plate is prevented from sticking due to residual magnetism. Warping is carried out on a drum of larger diameter and different shape from those used for handling cargo. These drums are of such a shape that the hawser will always slip towards the waist of the drum. They also have a flange which will prevent the hawser running over the rim and at the inner side a projection on the frame prevents the hawser becoming jammed between the frame and the warp end. A warping winch is not usually fitted with a footbrake because when warping the ship several warps may be sharing the load and if one should be braked that hawser would take all the load and probably break. Exceeding a predetermined torque can be prevented by automatically inserting a resistance in the motor circuit or by other means according to the control system. If a footbrake should be fitted its power should be limited to the full load torque of the winch. Capstan barrels are normally mounted on a vertical driving shaft, the barrel having whelps cast on. The driving motor is frequently mounted below deck in order to leave as much free space as possible for handling lines. The commonest method of operation is for one end of the line to be attached to a fixed bollard on the wharf or on another adjacent vessel and the line is then given three or four turns round the capstan barrel with the free end held by the operator. When the free end is pulled so as to tighten the grip of the line on the barrel the line between the capstan and the fixed end is wound in by the friction on the barrel. If the operator slackens or surges his line it loses its grip on the barrel and the line is freed. There are, of course, other more complex uses for capstans. Anchor windlasses are vital to the safety of the ship, its crew and cargo and there is no standby; they are subject to Classification and Governmental requirements. The cable lifter is specially shaped to fit the links of the cables and will normally fit four or five links around its circumference which is then known as four or five snug although actually only two links are engaged at any one time. The lifter runs freely on a shaft or can be disengaged and has on the outer edge a rim to take a brake band and on the end there are jaws comprising a dog clutch for engaging with corresponding jaws on a gearwheel driven by the motor. The lifter is de-clutched for lowering, the paying-out speed being controlled by the brake or if paid out under electrical power the speed is controlled electrically.
Undue stress must not be applied to the cable and a slipping clutch is incorporated between the motor and magnetic brake and the driving shaft and set to slip at approximately 133 per cent of full load torque. Otherwise excessive stress could be applied to the cable by the momentum of the motor armature, by the magnetic brake, or by a sudden obstruction when heaving or when bringing the anchors into the hawsepipe. The electrical equipment should give a crawl speed to enable the anchor to be housed safely and to allow the motor to stall when it is fully home.
The hauling speed is usually regulated to about 25 ft./min. while the anchor is still holding; after the anchor has broken out and the load thereby reduced the characteristic of d.c. windlasses is arranged to give an average speed of about 50 ft./min. In some waters where there is a fast-running current or stream the speed at which the cable can be recovered is important.
When dropping anchor it is lowered as speedily as possible, usually in bursts of 5 fathoms at a time while the anchor is free to drop. The friction brake is applied to the lifter by means of tightening screws and is used to control the lowering. A bow- stopper is normally fitted at the head of the hawsepipe and relieves the windlass from strain when riding at anchor. This bow- stopper is in the form of a jaw holding a link by a wedge action. When it is necessary to take in cable, when riding at anchor, the jaw holding the link is self-releasing. To break out anchor the vessel is moved into a position directly above the anchor thereby reducing the strain of breaking out. It is usual practice for windlasses to perform warping duties in addition, so the extra refinements required, which differ from windlass duty, must also be combined in the control scheme. These involve a pull of approximately one-third of that of the cable lifters but at a faster speed and a still faster speed when recovering warping lines after they have been cast off from the quayside bollards.
Mooring winches although similar to warping winches have a fundamental difference in that, the rope is fixed to a main barrel and is not operated off a warp end. This type of winch is becoming increasingly in demand owing to the St. Lawrence Seaway regulations. There are two main classes—one which will stall for short periods for use while navigating the locks and the other for constant tensioning against rising and falling tides or lock waters and during rapid loading or unloading. For the former requirement the usual torque limiting provisions (to be described later) are suitable. Ward-Leonard systems can also be adapted, if combined for dual service as a cargo winch, by providing a two-way switch, one way for cargo and the other for mooring. On the mooring side the generator field is weakened, providing a reduced torque on all steps and consequently reduced heating under stalled conditions. For the second class of duty a more expensive type of winch is necessary because it must hold the vessel in position for indefinite periods. The Ward-Leonard system is generally recognised as suited for this condition. Also suitable is a winch known as the Almon Johnson (Laurence Scott). In this the drum is driven through epicyclic gearing with a third or oscillating member moving only as the tension in the cable varies. This oscillating member is used to operate the automatic control which starts, stops and reverses-the motor. A handwheel operated "clutch- brake" band compressed round this oscillating member, acts as a slipping clutch under shocks and prevents cable breakage by limiting the tension. With this brake-band released there is no positive drive to the drum. Cargo cranes are being used to an increasing extent and many leading cargoship owners are now using cranes to replace derricks and winches. The hoist requirements and control systems are to a certain degree similar to those of winches, and in addition there will usually be two extra motors. The luffing drive is straightforward and does not require an elaborate control system.
A controlled lowering system is generally employed and the equipment can be similar to that used for the topping unit on derrick, i.e. for raising or lowering the derrick to the position required except that as luffing is an essential feature of crane operation a speedier movement becomes necessary. For the slewing motion special consideration is necessary. The mass in motion involves not only the load on the hook but that of the crane itself. It must be accelerated and decelerated gently and without jolting. Dynamic braking is appropriate and where the drive is taken from a squirrel-cage motor this can be arranged by injecting d.c. into the stator winding via a transformer and semi-conductor rectifier just prior to switching off. This has the effect of producing a static (as opposed to rotating) field in the machine.
Rotation of the rotor induces an e.m.f. in the rotor windings and the interaction of the stator field and the rotor current produces a torque which opposes the rotation, braking power being dissipated in the form of heat in the rotor winding and in rotor iron losses. The effectiveness of this method can be adjusted to the requirements of the crane by transformer tappings or by an adjustable series resistance. When the crane comes to rest the magnetic brake is applied and the d.c. fed to the motor is switched off. Luffing and slewing controls of cranes are frequently combined in one controller with a single operating lever having motion in two planes. The operator thus has only two levers to handle for the three motions on the crane.
- Материал на блоге⇒ Весь материал предоставляется исключительно в ознакомительных целях! При распространении материала используйте пожалуйста ссылку на наш блог!
- Ошибки⇒ Если вы обнаружили ошибки в статье, то сообщите нам через контакты или в комментариях к статье. Мы будем очень признательны!
- Файлообменники⇒ Если Вам не удалось скачать материал по причине нерабочих ссылок или отсутствующих файлов на файлообменниках, то сообщите нам через контакты или в комментариях к статье.
- Правообладателям⇒ Администрация блога отрицательно относится к нарушению авторских прав на www.electroengineer.ru. Поэтому, если Вы являетесь правообладателем исключительных прав на любой материал, предоставленный на ресурсе, то сообщите нам через контакты и мы моментально примем все действия для удаления Вашего материала.