A Treatise On The Principles And Practice Of Dock Engineering

5 22 DOCK ENGINEERING. be obtained by varying the résistance in the shunt circuit, and so affecting the exciting current. The starting torque is about three times the normal amount. Compound-wound motors may have their field coils wound either difierentially, with the series coils in opposition to the shunt coils, or curaulatively, with the series coils assisting the shunt coils. Where great regularity of speed is required the differentially-wound motor is probably the better, but it has not met with any great measure of success. One objection to it is the liability to start in the wrong direction, owing to the reversed series winding. The most important feature of the cumulatively- wound motor is the increased torque at starting, due to the series coil. It combines, in fact, to a certain extent, the starting power of the series motor with the speed regulation of the shunt motor. In this last respect, how- ever, it is not so good as the shunt motor. This type of motor is sometimes fitted to cranes where the motor is allowed to run constantly, and in such situations has given good results. Applications of Power. The various types of appliances, which it is proposed to briefly describe, may be classified under the following heads : — Gate machinery, Sluice machinery, Capstans, Wharf and floating cranes, Jiggers and transporters, Slipway machinery, Ooal tips and hoists, Grain elevators, Pumps, Miscellaneous apparatus, such as moorings, &c. Dock Gate Machinery.—Dock gates may be worked by means of chains or of arms or struts. The chains may be wound on barrels or drums in gear with rotary shafts driven by steam, hydraulic or electric power, or they may pass over sheaves at the ends of the cylinder and ram respectively of a hydraulic machine. An example of the former class is that given in figs. 518 and 519, which show the plan and section of a gate crab or winch worked by hydraulic power. The ram system has already been exemplified in figs. 516 and 517. Where space is restricted and long chains are necessary,. a cupped drum grasping the links of the chain will be used in preference to a barrel, which is less compact. There are drawbacks, however, to this arrangement, in that special links are required, and that a corresponding adjustment must be made whenever stretching occurs. Chains, in fact, call for constant attention. They must be frequently