An overhead travelling crane conversion study

Abstract

At Malta Drydocks, in the lathe workshop, three overhead travelling cranes (otcs) move on the same gantries along the length of the workshop, one of which is the otc no.457 which works from a de supply and is manually controlled. A feasibility study was made as regards the conversion of this crane from :- a) floor b) c) logic. a cabin controlled .. otc into one that can be remotely operated from level using a pendant push button control station. a 440V de mains operated supply to a 3 phase 415V 50Hz supply. manually operated drum controllers to controllers using contactor /relay This involved i) The study of the actual electrical, physical and functional layout of the crane from schematic diagrams of the motor control circuits and their protective equipment as well as from observations made on sight. ii) Testing of motors and control equipment. iii) Planning and design of several schemes for the realization of the con version mentioned. iv) Obtaining quotations/budget prices for (iii). (i) proved difficult initially since no schematic diagram for the control system could be found at first. So diagrams of an almost identical otc to otc no.457 were used as well as photographs of the crane motors and hoisting drum unit. Several observations naturally had to be made on sight (on crane), while the crane was operating so as to determine how various components worked. The original crane setup is as follows:- The travelling crane travels consists of two bridge girders, on top of which, the trolley consisting of the self-contained hoisting unit and traverse travelling motor, travels on rails. The girders are supported at the ends by trucks having 4 wheels in all and which travel along the rails of the gantry which is the structure supporting the crane. Suspended from the girders on one side, is the operator's cabin, containing the drum controllers, mains isolator, protection and warning devices. The trolley wheels are coupled to the traverse motor through the gear reducer, in the same manner, the bridge girder wheels are coupled to the longitudinal travelling motor. A shoe type brake in series with the motor armature arrests any motion produced by the two motors just mentioned, by acting on the coupling drum whenever the armature supply is cut off. The hoisting machinery consists of the hoisting motor coupled to an oil filled enclosed reduction gear box which in turn is connected to the load brake. The latter drives the rope drum, from which the hook is connected by two rope windings. At first the function and operation of the load brake could not be understood since it was not even known that it was a load brake, however from direct observations and tests its operation was deduced. In brief it is a mechanism whereby it allows normal hoisting for all loads (up to 10 tons) and lowering of light loads but restrains overhauling loads. Its operation is purely mechanical whilst the other frictional shoe type brake is connected in shunt with the hoisting motor armature. [...]