Transactions

ANDRIOLA—ELECTRIC-SLIP COUPLINGS FOR USE W ITH DIESEL ENGINES 575D i s c u s s i o nM. R. L o r y . 5 Mr. Andriola is to be congratulated on hisinteresting and informative paper. The application of electriccouplings on a large scale has come so recently in this countrythat little has been written about them. This paper is a valuableaddition to the literature.It is fortunate that the author has included some informationfrom his associates at A.S.E.A. That company’s actual experiencein building couplings antedates our own by several yearsbecause geared-Diesel drive has been popular in Europe for sometime, while its use on a large scale is quite recent here. However,‘ Westinghouse Electric & M anufacturing Company, East P ittsburgh,Pa.we are now making rapid strides, as indicated by the fact thatthe writer’s company alone now has more total horsepower incouplings built or under construction than have been built orare on order abroad, based on the latest published information.*We are now building the largest electric couplings in the world,rated 4375 hp at 180 rpm, for use with Sun-Doxford engines onfour Maritime Commission CP-3 cargo and passenger vessels.Of the 62 motorships already built or on order on June 1, 1941,for the Maritime Commission, 38 have geared drives of which 30are equipped with electric couplings and 8 with hydraulic couplings,while 24 are direct drive.The principal difference between the electric couplings for theseMaritime Commission ships and foreign-built couplings is in theamount of torque available at high slip for maneuvering. Figs.3 and 4 of the paper show a coupling with 40 per cent torque at100 per cent slip. The Maritime Commission engineers recognizedthat, if more torque than this were provided, the couplingscould be used extensively as an aid in maneuvering. Consequently,their specifications required a minimum of 75 per centtorque up to 140 per cent slip.The Mormacpenn, first of four C-3 cargo ships built by theSun Shipbuilding and Drydock Company, and the first geared-Diesel ship to be completed under the Commission program, haselectric couplings. Each of these ships has four Busch-Sulzerengines rated 2230 hp at 240 rpm, driving through Westinghouseelectric couplings and Falk gears. The couplings on these shipshave proved very satisfactory in service. The ships are exceptionallyeasy to maneuver. The engine and coupling controlsare centralized in a control desk. The right-hand lever on thedesk controls the operation of all four couplings. The usual procedureis to warm up two engines ahead and two astern at the“stand-by” signal. Then the operator can carry out any maneuverexcept “full ahead” or “full astern” by means of thecoupling control and engine-speed levers. No starting air isconsumed and the ease of operation is comparable to Dieselelectricdrives. The writer has observed response to six bells in1 min when docking.The couplings on these vessels have about 100 per cent torqueat high values of slip. This enables a crash-stop reversal to bemade by disconnecting the engines from the propeller and reversingthem at no load. The couplings are then energized andreverse the propeller while the engines run on fuel. This methodof reversal is very fast and uses little starting air. The Mormacpennwas forced to make a crash stop in New York harbor toavoid a collision in a fog. The propeller was turning at higherthan full speed astern in less than 1 min. While this time wasshortened by the fact that the ship was not up to full speed aheadwhen the reversal was started, the quick reversal was creditedwith avoiding a crash. It is doubtful if a coupling with torque,as shown in the curves of the paper, would be able to reverse thepropeller from full speed.The writer was greatly interested in the mathematical analysisof the characteristics of the coupling which affect torsional vibration.When his company first studied electric couplings, someengineers recognized the torsional characteristics and worked upcurves similar to the author’s Fig. 5 for the Navy. Discussionsof torsional characteristics led up to the extensive tests made byMr. Dashefsky which were mentioned in the paper. It is impossibleto compare the formulas in the paper exactly with thosepublished4 by the writer and others because differences in constructionmodify the analysis. The A.S.E.A. couplings are builtof solid iron in the secondary core and the analysis must takecare of eddy currents induced in the iron parts. We use laminatedironcores and these eddy currents have negligible effect and were8 ^.iS.B.A. Journal, March, 1940.