629,725. Controlling reversing-gearing; positive clutches. SINCLAIR, H. Sept. 16, 1947. No. 25324. [Class 80 (ii)] The engagement of forward and reverse driveselecting clutches in reversing-gearing having a driving member capable of exerting substantial driving torque in only one direction of rotation is facilitated by the provision of auxiliary driving means capable, while the driven member is running, and on disengagement of the engaged selecting clutch, of accelerating the driving member in the reverse direction to a speed sufficient to synchronize the other selecting clutch. As shown, the selecting clutches are of the "synchro - self - shifting " type described in Specifications 513,974 and 534,470, but balked jaw clutches, magnetically-actuated friction clutches or, according to the Provisional Specification, torque-assisted friction clutches may be employed. As used in one form of marine power plant having a main driving elastic-fluid turbine 20, Fig. 1, suitable for use in combination with gas generators as a source of power gas, the turbine rotor shaft 21 and a layshaft 26 are geared together through gears 29, 30 to rotate oppositely. Gears 24, 25, both meshing a large output gear 31 on the pro pellor shaft 32 are alternately connectable to their shafts 21, 26 by ahead and astern drive - selecting clutches. The ahead clutch comprises a nut 33 slidable on right-handed helical splines on the shaft 21 and having teeth 35 engageable with teeth 36 on the gear 24. Sets of balking pawls 39, 40 on the nut 33 operate in the manner described in the above-mentioned Specifications to register the teeth 35, 36 for clean meshing when the nut moves into engagement from the right and left respectively. A control sleeve 41 is provided having straight splines 42 slidingly engaging splines 43 on the nut 33, and dogs 44 drivingly co-acting in one axial position of the sleeve with dogs 45 on the shaft 21, the dogs having considerable mutual circumferential play when thus aligned. The astern clutch 35A, 36A is generally similar to the ahead clutch 35, 36, but its nut 33A runs on lefthanded helical splines on the shaft 26. The ahead and astern control sleeves 41, 41A are actuated by yokes 48, 48A, Fig. 3, operated from a preselector handle 53 through oppositely reciprocable rods 51, 51A having spring lost-motion connections with the yokes. These connections are similar and, in the case of the rod 51 and yoke 48, comprise collars 57, 58 having pin and slot co-action with the rod and which are urged apart by a preselector spring 51 to engage bosses 50, 49 on the yoke 48. The yokes 48, 48A are linked by an indicator lever 60 and carry abutments 72, 72A co-acting with a piston stop 64 when the latter is thrust out by the action of pressure fluid thereon. An auxiliary turbine 62, Fig. 1, driving in the opposite direction to the turbine 20, is connected positively to the shaft 21 and has a valve 63 for controlling the supply of working fluid to it. The valve 63 and a valve 68 controlling the supply of pressure fluid to the piston 64 are linked by a common operating member 71 so as to open and close together. When ahead gear is in operation, the mechanism is in the position shown in Figs. 1 and 3. The direction of rotation is such that both the nuts 33, 33A are thrust to the left by their helical splines. Under these conditions, the nut 33A has moved to the left to a position where both its sets of pawls are completely free of the teeth 36A. Due to the position of the control sleeve 41, however, the dogs 44, 45 co-act and prevent the nut 33 moving to the left to disengage the teeth 35, 36. To change gear the turbine 20 is cut out, the turbine 62 cut in, and the lever 53 moved fully to the other side of its central position. Slowing of the shaft 21 relatively to the gear 24, due to the way on the ship, causes the nut 33 to move to the right to a position where the teeth 36 disengage the teeth 35 and ratchet with the pawls 39. On removal of the torque load from the dogs 44 the preselector spring 59 moves the sleeve 41 to a "free" position, as shown in Fig. 8, where it is arrested by the piston stop 64 which is now thrust out. The sleeve 41A is moved to a corresponding " free position by its preselector spring or by the piston 64, pushing the nut 33A into initial ratcheting engagement with the teeth 36A. Further movement of the sleeve 41A is prevented by frontal abutment of its dogs with the dogs on the shaft 26. When the turbine 62 has stopped the shafts 21, 26 and accelerated them in the reverse direction sufficiently to synchronize the astern clutch 35A, 36A, the nut 33A will move to the right until the teeth 35A have passed the teeth 36A and the latter are ratcheting with the lefthand set of pawls. Such movement of the nut helically on the shaft 26 rotates the dogs on the control sleeve 41A clear of the dogs on the shaft 26 and the astern preselector spring moves the sleeve 41A into a position corresponding to that of the sleeve 41, in Fig. 1. The resulting movement of the pointer 61 tells the operator to cut out the turbine 63 and cut in the turbine 20. The piston stop 64 is no longer thrust out, and the spring 59 therefore completes the movement of the sleeve 41 to the position shown in Fig. 10. Torque reversal thrusts the nut 33A to the left to engage the teeth 35A, 36A, the nut being retained in this position by the sleeve 41A and, when the propellor shaft 32 reverses, the nut 33 is moved to the left to the position shown in Fig. 10 where it is completely free of the teeth 36. In neutral, the lever 53 is in its central position. To engage either gear, the turbine 63 is energised and the nuts 33, 33A pass to the position where their left-hand pawls ratchet. Subsequent movement of the lever 53 to the selected position followed by cutting-out of the turbine 63 and cutting-in of the turbine 20 establishes the appropriate speed in a manner similar to that described above. According to the Provisional Specification, both the ahead and astern clutches with their associated gears may be mounted on the main turbine rotor shaft, one gear meshing the output gear directly and the other through a layshaft train or a number of layshaft trains in parallel. The main turbine may be provided with a speed limiting governor and the auxiliary turbine may be connected to the main turbine rotor shaft through a slip clutch. De-energization of the auxiliary turbine and reenergization of the main turbine when changing speed is effected automatically by a trip actuated by the movement of the nut of the selecting clutch to be engaged, upon synchronization thereof. In an alternative form applicable to cases in which it is inconvenient to stop the prime mover when changing gear, the driving member may be the driven part of a preferably controllable hydraulic turbo-coupling or like electric slip coupling the driving part of which is connected to the prime mover. The auxiliary driving means may then be a power take-off from the prime mover arranged in parallel with the coupling and including an auxiliary slipable coupling. In a mechanism of this kind as shown in Figs. 11 and 12, the reversing-gear is basically the same as that shown in Figs. 1 and 3. The main driving shaft 121, Fig. 11, is, however, hollow and rigid with an input flange 173 connected by a flexible coupling 175, Fig. 12, with a flange 174, integral with the hollow runner shaft 176 of a scoop-controlled hydraulic turbo-coupling of the kind shown in Fig. 4 of Specification 493,703, [Group XXIX]. The coupling impeller 177 is bolted to a flange 178 on the crankshaft of a governed I.C. engine (not shown) and houses a self-aligning bearing 179 supporting the shaft 176. An auxiliary drive shaft 181 passes through the shafts 121, 176 and connects the flange 178 to a clutch 183, 187, the two driving members 183 of which are slidably splined on the shaft 181 and co-act to form a chamber 185, to which pressure fluid can be admitted through a pipe 186 so as to force the members 183 apart into frictional engagement with two driven clutch members 187 forming the body of a gear 189 arranged to drive, through an idler 190, a gear 191 fast on the layshaft 126. The pipe 186 branches from a pipe 170 connecting the control mechanism piston stop 164 and its operating valve 168, so that on disengaging the main coupling and operating the valve 168 the clutch 183, 187 is engaged to drive the shafts 126, 121 backwardly through the idler 190 and so allow synchronization of a chosen selecting clutch as previously described. According to the Provisional Specification, the clutch and idler train may be replaced by a planetary bevel reverse gear, having its sun wheels connected to the main and auxiliary driving shafts respectively and its planet carrier either stationable by a friction brake or merely retardable by a fluid turbobrake or magnetic eddy current brake. Gearing according to the invention may be used in turbine driven locomotives to provide engine braking by engaging reverse drive during forward downhill motion. Reference has been directed by the Comptroller to Specification 624,180.