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Aviation History
1942
1942 - 2323.PDF
NOVEMBER 5TH, 1942 FLIGHT 503 CUN-TURRET OPERATION simple hydraulic rams, but for V the traversing it is essential that the turret should be unlimited as to rotation, and, therefore, a rotary motor is necessary. For this hydraulic motor the first design comprised a type rather similar to a gear-type oil pump, but leakage at low speeds showed its inherent weakness. After an in vestigation which covered multi-cylinder plunger and other types, the multi-vane oil motor was decided upon, and the develop ment of this to the present stage of precision is a story which cannot be told fully in the present circumstances. In a vane-pump the speed is high, pressures are generally low, and inter-cell leakage is of little importance. The stator has generally a plain eccen tric bore, and the vanes are spring-loaded to take up the discrepancies of the changing "diameter" on which the \anes operate. In this vane-type oil motor, pressures ' j*,e fairly high, the speed at times may be very low and none of the comparative crudities of the pump permissible. Owing to the degree of pressure employed, it is very desirable that the vanes should not have to slide during the pressure stroke. To satisfy this condition, the vane tips have to move on an arc concentric with the rotor spindle during the working stroke. This then becomes entangled with another consideration: spring loading is out of the question in case a vane should stick, for at low speeds where accuracy is of utmost importance, there is none of the helpful centrifugal force that is to be found in the high-speed pump. In order to control the sliding of the vanes, therefore, the in-sliding vane is used to actuate the out-sliding \ane by means of light tappet rods which, staggered for the three pairs of vanes, pass diametrically through the rotor. This feature therefore controls the internal profile of the stator: first, there is the concentric arc of the working stroke, and diametrically opposite that is a concentric arc of shorter radius, as demanded by the constant-diameter is The control handles have a two-way rock ing movement which gives a naturally " in stinctive " action. Turret cupola under loading test. consideration. (Left) The special gear-milling machine designed and made by Parnalls for cut- ting turret gear- rings. (Right) The six-vane oil motor, developed • expressly by Parnalls for their power- operated gun turrets. These two arcs are joined by what— developed on a straight-line basis—is a serpentine curve, and here again extreme accuracy is necessary in grinding the cam-profile of the stator-bore because, on the constant- diameter basis, one-half of the cam-ramp must be com plementary to the other. The cam-profile is, therefore, obviously of basic impor tance, and as soon as the design was stabilised, Parnalls designed and built their own boring and grinding machines, which deal very effectively with the stator bore. An in teresting feature is the oscillation of the work-carrying head instead of the grinding head, so that the latter—with a wheel running at very high speed—has the utmost rigidity. Certain details, such as the shaping of the blade-tips in order to obtain the best seal under line-contact conditions, needed a good deal of tedious experiment. So did the hydraulic balancing of the blades, but eventually the design was so stabilised for production that practically no modifications have been necessary despite the heavier duties put upon it from time to time. For instance, turret torques at the outbreak of war were already twice the 1932 figures ; present-day figures cannot be quoted. As an indication of the flexibility of its performance, the vane oil-motor gives accurate and smooth action oyer a speed range of o to 1,000 r.p.m. Without modification the present unit has given satisfaction at pressures up to 1,500 lb. per sq. in.
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