Operational Experience of the S.S.S. (Synchro-Self-Shifting) Clutch Particularly in Naval Propulsion Machinery

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1 72-GT-81 $3. PER COPY $1. TO ASME MEMBERS The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation. Full credit should be given to ASME, the Professional Division, and the author (s). Copyright 1972 by ASME Operational Experience of the S.S.S. (Synchro-Self-Shifting) Clutch Particularly in Naval Propulsion Machinery H. A. CLEMENTS Managing Director, S.S.S.Gears Limited, Acton Town, London, England The S.S.S. (Synchro-Self-Shifting) overrunning clutch with its pawl actuated helical sliding motion is well known, particularly in the gas turbine field. This paper reviews briefly the clutch operating principle, then outlines some of the service experience since 1958 in naval main propulsion drives in COSOS, COSAG, CODOG, CODAG and COGOG plant. Extra features are then described such as a lock-out control as is usually necessary for turbine applications to permit turbine testing, e.g., when in harbor; also a lock-in control as is essential when the clutch has to transmit power in both directions of rotation. Various clutch mounting respective advantages. The paper concludes with information regarding reliability during many years of service experience. Contributed by the Gas Turbine Division of the American Society of Mechanical Engineers for presentation at the Gas Turbine and Fluids Engineering Conference & Products Show, San Francisco, Calif., March 26-3, Manuscript received at ASME Headquarters, December 28, Copies will be available until January 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y. 117

2 Operational Experience of the S.S.S. (Synchro-Self-Shifting) Clutch Particularly in Naval Propulsion Machinery H. A. CLEMENTS 1 BRIEF HISTORICAL BACKGROUND Fo many years, the S.S.S. Clutch was applied to relatively low-power applications, mainly in diesel shunting locomotive change-speed gearboxes, in association with Vulcan-Sinclair fluid couplings. Such an application, which involved many hundreds of clutch engagements each day, provided much valuable operating experience. So the proven design principles of the S.S.S. Clutch were available for the much higher power applications arising upon the introduct on of the gas turbine for marine propulsion, and for a-c generator drives. In 1958, the Royal Navy ordered the first S.S.S. Clutches for their COSAG "County" Class GM Destroyers, and "Tribal" Class Frigates, wherein each S.S.S. Clutch was required to transmit 75 hp from an A.E.I. G6 gas turbine. Since then, the S.S.S. Clutch has been selected by 13 of the World's navies for 21 different classes of vessels. In the industrial field, S.S.S. Clutches transmitting 1, hp each at 3 rpm, have been in service on gas turbine driven generator sets for approximately six years. Similar designs of clutches transmitting 4, and 7, hp at 3 rpm, have been in service for the same period. By the end of 1971, a total of 334 S.S.S. Clutches had been supplied and ordered, for naval propulsion machinery, to transmit 4,23,2 hp. For mounting between a gas turbine and a generator, 116 S.S.S. Clutches had been supplied and ordered to transmit a total of 5,, hp. The largest S.S.S. Clutches in service at present are about 6 ft in diameter and are mounted between the hydraulic turbine and motor/generator in pumped-storage plant in South Germany. Four of such clutches have been in service since 1969, and each transmits 136, hp at 6 rpm. These particular clutches have now completed many thousands of engagements, and, as a result of their highly satisfactory operation, larger pumped-storage plants now being constructed will incorporate S.S.S. Clutches, each to transmit 34, hp at 6 rpm. The horsepower transmitting capacity of the largest S.S.S. Clutch in service at the end of each year over the past decade is illustrated in Fig. 1. Fig. 2 shows the aggregate of the total horsepower transmitting capacity of all the S.S.S. Clutches delivered up to thb end of each year. ACTION OF THE S.S.S. CLUTCH With a normal tooth-type clutch, the problem is to mesh the teeth at the instant when the speeds of the two shafts are synchronized. This is achieved precisely and without the possibility of error, by means of the pawl actuated helical sliding motion, invented specifically for the S.S.S. Clutch. When the speeds of the shafts pass through synchronism, the pawls on one clutch element engage with ratchet teeth on the other element and move the sliding component along the helical splines, thereby engaging the driving and driven clutch teeth smoothly and positively. The relative angular positions of the pawls, ratchet teeth, and clutch teeth are such that the driving teeth enter the spaces between the driven teeth precisely. The pawls are only required to overcome the small force that is necessary to accelerate the lightweight helical sliding component into partial tooth engagement. The pawls cannot transmit any driving torque because they move out of contact with the ratchet teeth by either relative axial or rotational movement before the clutch teeth have shifted into full driving engagement. For this reason, the S.S.S. Clutch can be engaged at high rates of relative acceleration between the input and output shafts. A most important aspect of the basic S.S.S. Clutch is that it is completely automatic in operation, and its engagement and disengagement is not dependent on servo-mechanisms or interlocks. Since the drive is transmitted through gear teeth, there is practically no limit to the torque capacity for which an S.S.S. Clutch can be designed. 2

3 34 IN Ui o. 6 O 4 5 a X O 8. 3 O O O 2 2 I YEAR Fig.l Horsepower transmitting capacity of largest S.S.S. Clutches in service in past decade YEAR Fig.2 Aggregate total horsepower transmitting capacity of S.S.S. Clutches delivered It is suitable for high rotational speeds. Naturally, S.S.S. Clutches, operating on the foregoing basic principles, incorporate various additional features to overcome problems which arise in drives involving very high power and/or high speeds. These features include ways of avoiding sustained pawl operation with the driven machine in rotation while the clutch input elements are at rest, also the "cushioning" of clutch engagement under high relative acceleration conditions, and furthermore mechanical locking of the clutch in the engaged condition when it is required to transmit astern power. These important design features, again originated specifically for the S.S.S. Clutch, have been published elsewhere. The various marine applications of the S.S.S. Clutch are listed in Fig. 3; Fig. 3 also gives information regarding the clutch mounting arrangement, maximum operating speed, etc. Clutches are in service in both bi-directional and uni-directional drives. In uni-directional marine gas turbine applications, such as when the c.r.p. propeller is used, the S.S.S. Clutch design can be of the simplest construction, since it acts simply as an overrunning clutch. Clutch engagement takes place automatically at the instant the speed of the input shaft commences to overtake that of the output shaft. Conversely (if not locked into engagement), the clutch will commence to disengage as soon as the gas turbine slows down relative to the propeller shaft. 3 MARINE OPERATIONAL EXPERIENCE OF THE S.S.S. CLUTCH About 2 years ago, the Royal Navy decided upon the design of propulsion machinery for their "Whitby" twin screw vessels and "Blackwood" Class single screw vessels. This machinery consisted of a pair of main steam turbines for high power propulsion and a single cruising steam turbine for each propeller shaft. The cruising turbine required an automatic clutch for disconnection while the vessel was being propelled by the main turbines. This clutch was also required to re-engage the cruising turbine at speed when the vessel was steaming in the cruising turbine power range. Maneuvering of the vessel ahead and astern was carried out on the main turbines only, with the cruising turbine declutched. The Napier design of overrunning clutch selected for this installation was incorporated in the high-speed shaft rotating at a cruising turbine speed of about 85 rpm. When disengaged, the clutch output members were required to rotate at a maximum speed of 12,5 rpm. This Napier clutch had friction elements in order to shift the clutch sliding components along helical splines to engage or disengage the clutch teeth. A complete set of machinery was then constructed and tested at PAMETRADA, and, during these trials, the cruising turbine acceleration 3

4 MACHINERY ENGINE No. PROPS DRIVEN BY ENGINE TOTAL & TYPE OF PROPS CLUTCH SPEED CLUTCH INPUT ROTATION CLUTCH OUTPUT ROTATION CLUTCH MOUNTING FIG. No. TYPE OF VESSEL COSOS STEAM TURBINE I 2 FP 85 UNIDIRECTIONAL BIDIRECTIONAL 6 FRIGATE CODOG P & W F CRP 1 UNIDIRECTIONAL 6 HYDROFOIL CODAG FRANCO TOSI / AEI G6 2 FP 14 BIDIRECTIONAL 5 DESTROYER CODOG RR PROTEUS I 3 FP 5 6 FAST PATROL BOAT COSAG A E I G6 I 2 FP 14 BIDIRECTIONAL 5 DESTROYER COSAG A E I G 6 I I FP 14 5 FRIGATE COSAG R.R. OLYMPUS I 2 FP 14 5 DESTROYER COGOG R R OLYMPUS I I CRP 56 UNIDIRECTIONAL UNIDIRECTIONAL 6 R R PROTEUS I I CRP 1 5 FRIGATE CODOG R R OLYMPUS 2 2 CRP FRIGATE COGOG P & W F 74 I 2 CRP 36 5 PEW FTI2 I 2 CRP 11 5 DESTROYER CODOG GE L M CRP 1 5 PATROL GUNBOAT CODOG R R OLYMPUS I 2 CRP 56 6 DESTROYER CODOG PEW F T 4 I 2 CRP 36 5 PATROL VESSEL COGOG COGOG I R R OLYMPUS 2 CRP 56 7 DESTROYER AND R R TYNE I 2 CRP 35 7 FRIGATE LY CO MIND TF HOVERCRAFT Fig.3 Summary of some maring applications of the S.S.S. Clutch rate was adjusted so that the clutch functioned perfectly. Following installation of the machinery in the first vessels, clutch trials were carried out and while the clutch engagements were satisfactory under smooth weather conditions, the clutch engaged with considerable impact in rough sea conditions -- such that it sometimes rebounded out-ofengagement a number of times before finally remaining in engagement. This violent clutch engagement was due to the severe propeller retardations, which can occur due to pitching of the vessel in rough seas, at the same time as the cruising turbine was accelerating to engage the clutch. A number of modifications were made to the Napier clutch, but these were not successful in overcoming these severe problems that were not brought to light during shore machinery trials. In 1958, the Royal Navy requested that an S.S.S. Clutch be designed for incorporation in the "Whitby" Class cruising turbine drive, and a pair of clutches were designed and installed initially in the twin screw vessel H.M.S. "Scarborough." Trials were carried out in 1959 when the clutches were proved to be entirely satisfactory in operation. During rough sea trials, when the clutch was purposely engaged under exaggerated relative acceleration conditions by superimposing large rudder movements, the clutch engagement (even in these circumstances) was hardly perceptible, even when standing near the gear casing. Following these trials, the clutches completed many thousands of hours of operation, both in H.M.S. "Scarborough," and later in a single screw vessel of the "Blackwood" Class (H.M.S. "Keppel"). The Royal Navy, however, reached the decision that the use of cruising steam turbines was not fully justified for this class of vessel, so the cruising turbines were removed from all "Whitby" and "Blackwood" Class vessels, thus providing valuable additional space in the engine room. The next vessels considered by the Royal Navy were the "County" Class Guided-Missile Destroyers, and "Tribal" Class General Purpose Frigates, having COSAG propulsion machinery. S.S.S. Clutches were specified for connection and disconnection of the gas turbines. In view of the foregoing experience with the 4

5 2 IS %- FULL LL di _J 4 D O a a 5 S o TIME (SECONDS) Fig.4 S.S.S. Clutch conditions during maneuver from full ahead to full astern Napier type clutch, the Royal Navy fully appreciated the difficulty in proving the value of an automatic clutch in a shore test installation; hence, an elaborate testing program was carried out on the special shore trials machinery for the first "County" Class destroyer. These trials are summarized in a paper presented to the Institute of Marine Engineers entitled "Machinery Installations of Guided Missile Destroyers and General Purpose Frigates" by Commander J. M. Dunlop, R.N. and Commander E. B. Good, R.N. From this paper, it will be appreciated that for the clutch trials, one gas turbine was used to drive through its S.S.S. Clutch and reduction gearing, to the water brake, then the second gas turbine was accelerated; just prior to the second clutch engagement taking place at synchronism, the power of the first (driving) gas turbine was reduced to idling, and full water braking was applied to decelerate the propeller shaft system rapidly at the time of clutch engagement. The S.S.S. Clutch was successful and completed many engagements under this severe, but realistic, condition. As a result of this satisfactory operation, S.S.S. Clutches were installed for the gas turbines in all the "County" Class naval vessels. In this case, the S.S.S. Clutches included a mechanical locking control, as is necessary to transmit astern as well as ahead power from the gas turbine reversing gear through selectively OUTPUT SHAFT R.P.M. (Input at rest) Fig.5 Viscous drag torque when S.S.S. Clutch overrunning filled ahead and astern hydraulic couplings. Maneuvering when on steam turbine propulsion is, of course, carried out by the ahead and astern steam turbines, and, therefore, the S.S.S. Clutches, following automatic disengagement, are shifted by a servo-cylinder to a bi-directionally free "locked-out" condition to ensure that the clutches do not engage and rotate the gas power turbine backward when the vessel is being propelled astern under steam power. When such a "locked-out" condition is used, precautions have to be taken to ensure that the clutch is not shifted back to the ratcheting condition while the clutch input shaft is rotating in a forward direction relative to the clutch output members; otherwise instant failure of the pawl mechanism will result. In the vessels concerned, correct operation of the clutch controls is ensured by good watch-keeping. Nevertheless, on rare occasions, during new crew training periods, the clutches have been damaged due to such control mal-operation. It is important to note that where gas turbines are driving a c.p. propeller (and also in any other such cases where the clutch is only subjected to one direction of rotation), the servooperated "locked-out" condition is not generally required, and the clutches cannot then be damaged due to mal-operation as described in the foregoing. The S.S.S. Clutches installed in the "County" Class and "Tribal" Class vessels are reported to have completed a total of more than 4,, hr of operation and have required negligible main- 5

6 DISENGAGED PRIMARY PINION SECONDARY PAWLS MAIN CLUTCH TEETH (STRAIGHT) PRIMARY PAWLS DASHPOT LUBRICATING OIL SUPPLY 417, MOMMUF Input shaft [): LOCK-OUT RELAY HELICAL MECHANISM SLIDING COMPONENT MAIN HELICAL SLIDING COMPONENT MAIN HELICAL SPLINES ENGAGED Fig.6 Typical quill shaft mounting of S.S.S. Clutch tenance. In 1966, S.S.S. Clutches were selected by the Royal Canadian Navy for use in the D.D.H. Destroyers, which, although subjected to successful testing within one set of gearing at NAVSEC, Philadelphia, have not as yet been subjected to seagoing experience. Also in 1966, the Royal Navy decided to install an "all gas turbine" plant in place of the conventional steam turbines in a "Blackwood" Class vessel, H.M.S. "Exmouth." In this case, the S.S.S. Clutches, which are incorporated in the Proteus cruising gas turbine drives and in the Olympus main gas turbine drives, have been subjected to about three years seagoing experience. S.S.S. Clutches have also been supplied to the Italian Navy for a number of Destroyers and Patrol Boats, having CODAG and CODOG propulsion machinery. S.S.S. Clutches were also selected for use within the A.E.I. gearing supplied to Tacoma Boatbuilding Co. and to Peterson Shipyard for the 1 latest P.G. Gunboats Nos. PG92-PG11. These vessels have a single gas turbine, geared to drive both propeller shafts, and each shaft set of gearing incorporates an S.S.S. Clutch. S.S.S. Clutches are also provided in the diesel drive of these vessels. Considerable seagoing service has been experienced, and the S.S.S. Clutches have proved to be highly satisfactory. About 5 S.S.S. Clutches are in operation in Vosper-Thornycroft type Fast Patrol Boats. Each clutch transmits the drive from the Rolls-Royce Proteus gas turbine, developing 425 hp at 5 rpm. S.S.S. Clutches are also used in the Vosper- Thornycroft Mark V Fast Destroyers. These vessels have twin shafts with CODOG propulsion machinery, and the S.S.S. Clutch for the Olympus gas turbine drive transmits 25, hp at 566 rpm. A number of these vessels have successfully completed commissioning trials and are operational. S.S.S. Clutches have been selected for use in the latest Type 21 Frigates, and Type 42 Destroyers being delivered to the Royal Navy; these clutches are incorporated in both the Rolls-Royce Tyne cruising gas turbine and the Olympus boost gas turbine drives. Similar clutches will also be used in destroyers ordered by the Argentines, Netherlands, Brazil, and other navies. S.S.S. Clutches have also been supplied for the 25,-hp gas turbine drives for the three 6

7 DISENGAGED LUBRICATING OIL SUPPLY SECONDARY PAWLS MAIN CLUTCH PRIMARY PAWLS TEETH (STRAIGHT) MAIN HELICAL SPLINES LOCKING SLEEVE Input from gas turbine through flexible coupling PRIMARY PINION INTERNAL JOURNAL RELAY HELICAL G LOCATION BEARINGS SLIDING COMPONENT MAIN HELICAL LOCKING TEETH SLIDING COMPONENT (STRAIGHT) ENGAGED S LOCKED Fig.7 Typical shaft end mounted S.S.S. Clutch latest twin screw CODOG vessels for the U.S. Coastguard. 4 CLUTCH "LOCK-OUT" CONTROL In many marine applications of the S.S.S. Clutch, it is desirable to lock the clutch in the disengaged condition under some circumstances, so that automatic engagement does not take place. This may be necessary, for instance, to permit the gas turbine to be tested in harbor without driving the gearing and propeller shaft system. For this purpose, the S.S.S. Clutch can be provided with a manually operated control, which is operated with the machinery at rest. In some applications, it is necessary for the S.S.S. Clutch to be shifted to the "locked-out" condition by remotely operated control. For instance, in cases where a fixed pitch propeller is used with CODOG machinery, and the propeller is subjected to both directions of rotation with the clutch disengaged, a locked-out condition is necessary so that the S.S.S. Clutch does not engage and rotate the power turbine backward when the propeller shaft is driven astern by the diesel engine. All S.S.S. Clutches listed in Fig. 3, as having a bi-directional output shaft, incorporate the remotely operated lock-out feature. It should be borne in mind that when an S.S.S. Clutch is in the "locked-out" condition, it is essential that the clutch is not shifted back to the normal overrunning condition (pawls ratcheting), when the output shaft connected to the propeller is rotating backward relative to the turbine shaft. Such clutch control movement would result in immediate damage to the pawl and ratchet mechanism, as the pawls are then rotating in the opposite direction relative to the ratchet teeth. Bearing in mind that such an S.S.S. Clutch is normally incorporated in a boost gas turbine drive, the clutch is only normally selected for engagement with the boost gas turbine at rest and with the propeller shaft already rotating in the ahead direction, i.e., being driven by the cruising engine. After shifting the clutch control to the "pawls ratcheting" condition, the boost gas turbine is started, and the clutch is automatically engaged at synchronism. Movement from the locked-out setting into the normal overrunning condition when incorrect conditions of relative rotation are present is usually avoided by good operating drill. However, in some latest designs now being supplied, the clutch incorporates an internal baulking mechanism to prevent the possibility of mal-operation. 5 S.S.S. CLUTCH "LOCK-IN" CONTROL If the S.S.S. Clutch is required, after automatic engagement, to transmit power in an astern as well as in an ahead direction of rotation, it is provided with a locking sleeve to lock the clutch in the engaged position. This locking sleeve is pre-selected for engagement before 7

8 CLUTCH DISENGAGED CLUTCH ENGAGED MAIN HELICAL SLIDING COMPONENT DASHPOT SECONDARY PAWLS PRIMARY PAWLS HELICAL SPLINES CLUTCH TEETH HELICAL SLIDING COMPONENT RELAY HELICAL SLIDING COMPONENT Input Erase boost Input from got turbine cruising gas turbine MAIN HELICAL SPLINES RELAY CLUTCH TEETH MAIN CLUTCH TEETH RELAY HELICAL SPLINES PRIMARY PINION SECONDARY PAWLS PRIMARY PAWLS DASH POT CLUTCH ENGAGED CLUTCH DISENGAGED Fig.8 Typical mounting of S.S.S. Clutches when two turbines are driving a single pinion starting the gas turbine, but the design is such that it is inherently prevented from shifting until the clutch teeth shift fully into engagement at synchronism. Only then can locking of the clutch in engagement take place. It is important to note that the locking sleeve is not loaded by torque when the clutch is transmitting ahead torque, but only loaded when transmitting astern torque. The locking sleeve is always shifted out of engagement while the clutch is transmitting ahead torque, in which circumstances the locking teeth are completely free of load. Hence, there is no possibility of damage to the ends of the teeth, as they are shifted out of engagement, such as may arise in other types of toothed clutch. When unlocked, the S.S.S. Clutch automatically disengages as soon as the turbine slows down relative to the gearing. If the S.S.S. Clutch is incorporated in a unidirectional drive from a turbine to a c.r.p. propeller, such a locking sleeve is not essential, but has been fitted in many cases when it has been thought to be undesirable for the clutch to temporarily disengage and then re-engage during the somewhat brief transient negative torque conditions which may occur in such machinery when maneuvering from ahead to astern. Fig. 4 shows propeller speed and torque conditions which are likely to occur in a naval vessel with gas turbine driven c.r.p. propeller during a crash astern maneuver. When proceeding at full speed, a crash astern order is given, and immediately the gas turbine fuel rate is reduced to idling so the complete power turbine propeller shaft system decelerates according to Curve "A." After a short time delay of, say, 1 sec, the propeller pitch commences to shift from ahead pitch toward zero pitch, and this causes the propeller shaft system to accelerate slightly (Curve "B") due to the forward motion of the ship through the water acting on the propeller in the finer pitch setting. At this moment, the torque in the shaft system (with the propeller at the finer ahead pitch position) is slightly negative. As the propeller shifts further toward zero pitch and then astern pitch, the shaft system is reduced rapidly in speed, as shown on Curve "C," and the propeller speed may fall to 3 percent of full speed. After, say, 4 sec, the 'turbine power is increased, and the shaft system is accelerated according to Curve "D" to stop the ship and drive it astern. When the S.S.S. Clutch in such propulsion machinery does not have a mechanical locking sleeve, it incorporates a powerful double-acting dashpot in order to slow down the rate of clutch movement toward tooth disengagement during the brief negative torque period of such a maneuver. Should the clutch disengage, the conditions will arise as shown dotted in Fig. 4. Referring to this graph, it is assumed that the clutch dis- 8

9 DISENGAGED PAWLS MAIN CLUTCH TEETH (STRAIGHT) HELICAL SLIDING COMPONENT LOCKING SLEEVE DASH POT ( Input from primary reduction gearwheel STRAIGHT SPLINES LUBRICATING OIL SUPPLY HELICAL SPLINES LOCKING TEETH (STRAIGHT) SECONDARY PINION RATCHET TEETH ENGAGED t LOCKED Fig.9 Typical mounting of S.S.S. Clutch on an intermediate shaft engages at "Y," so that the power turbine immediately commences to decelerate relative to the propeller shaft system as shown in Curve "E." As the propeller pitch shifts further toward zero and then astern pitch, there is a rapid deceleration of the propeller shaft so that the clutch re-engages at "Z," whereupon the complete propeller shaft/power turbine shaft system decelerates according to Curve "F." Trials have been carried out in a number of gas turbine driven vessels, and some have experienced clutch disengagement/re-engagement in accordance with the dotted curve in Fig. 4, although, in these cases, clutch movement was not audible, even when standing near the gearing. Measurements have been taken of the relative acceleration rate across the clutch and it did not exceed 25 rpm/ sec. This is well within the acceleration capacity of the S.S.S. Clutch fitted. Trials of this nature have also been carried out in H.M.S. "Exmouth," and, in this case, the dashpot was sufficiently stiff to prevent disengagement during a crash astern maneuver. However, H.M.S. "Exmouth" is a single-shaft vessel, and the magnitude and duration of the negative torque can be substantially higher in a conventional twin shaft vessel if maneuvering is accompanied by larger rudder movements. Clutch disengagement, followed by automatic re-engagement, can be prevented even in the previously mentioned circumstances by applying oil pressure within the clutch dashpot to one side of the dashpot piston only. 6 CLUTCH OVERRUNNING "VISCOUS" DRAG When an S.S.S. Clutch is overrunning at high speed, the pawls ratcheting motion ceases because of the effect of the oil immersed ratchet teeth. The fact that the pawl carrier is rotating within a rim of oil does result in a small overrunning drag which can tend to maintain the gas power turbine in slow speed rotation, having regard to the very low friction of the power turbine bearings. Fig. 5 shows the relatively low overrunning viscous drag torque of a 25,-hp S.S.S. Clutch at various output speeds and with the input at rest. 7 CLUTCH MOUNTING ARRANGEMENTS In most applications of the S.S.S. Clutch to marine main propulsion drives, the clutch is incorporated most conveniently in the main gearing high-speed shaft system. A number of typical 9

10 mounting arrangements of the clutch in the highspeed system are described in sections 7.1, 7.2, and 7.3, while section 7.4 describes clutches mounted in the shaft system rotating at an intermediate speed. 7.1 Quill Shaft Mounted, Fig. 6 This is the preferred arrangement, and as the clutch is generally mounted over the secondary gearing, it does not result in an increase in the overall length of the gearbox. 7.2 Shaft End Mounted, Fig. 7 This arrangement has been successfully used in many applications, particularly where the gas turbine is mounted forward of the gearing and the primary gearing is aft of the secondary gearing, in which case the clutch is again positioned over the secondary gearing and does not result in an increase in the overall length of the gearbox. In the arrangement shown, the clutch incorporates an internal journal bearing which is subjected to relative rotation only when the clutch is disengaged. However, such a braring can result in some degree of increased unbalance during a most unusual high-speed/low-power operation, such as when driving a c.p. propeller at high speed in fine pitch. For details of such operation, see reference (6). 1 ] The internal bearing can be avoided by extending the clutch input shaft toward the turbine (or in the other direction, through the pinion shaft), to a second gear case-supported bearing. 7.3 Two Turbines Driving a Single Pinion, Fig. 8 This particular requirement has been stipulated for a number of COGOG naval vessels currently building. Such an arrangement does ensure that when driving on either boost or the cruising engine, none of the main gearing is rotating without a load, as could be a source of some extra noise and losses. However, in such an arrangement, it is not generally practicable to use the shaft end mounted type of S.S.S. Clutch, as shown in Fig. 7, due to the difficulty in feeding oil into the high-speed shaft system to lubricate the internal bearing; other arrangements would result in excessive overall length. Avoidance of all internal bearings without excessive overall length have been realized by the use of S.S.S. Clutch designs as shown in simplified form in Fig. 8. As is the case in most designs of S.S.S. Clutches, this particular design of clutch does not 1 Numbers in parentheses designate References at end of paper. need a special high-pressure oil Supply, but uses oil from the normal gearbox lubricating oil supply system at, say, 15 psi. This oil for lubricating the clutch pawl, ratchet mechanism, clutch teeth, etc., passes from a gear case mounted oil jet into a rotating oil catcher, thence flowing centrifugally into the clutch. 7.4 Intermediate Shaft Mounted Clutches, Fig. 9 It is often advantageous to incorporate the clutch in a shaft system rotating at an intermediate speed, rather than in the high-speed shaft. The lower speed clutches have the following advantages: 1 In general, when propelling on one engine, the high-speed gearing associated with any stationary engine is at rest, so reducing the noise level and power loss. 2 Although the S.S.S. Clutch has accurately controlled internal clearances, such clearances must result in a slight degree of unbalance, particularly when under high-speed, light-load conditions. By mounting the clutch in the intermediate shaft, the effects of such unbalance is considerably decreased due to the reduced rotational speed. 3 As the high-speed gearing is disconnected by the clutch, this gearing can remain at rest, if damaged, while the vessel can continue to be propelled by another engine. This, of course, could be important particularly in a single shaft ship. 4 Although the viscous drag at the S.S.S. Clutch, when overrunning, is small, it is lower in intermediate shaft mounted clutches as compared with high-speed clutches. This means that the power turbine is more likely to come to rest together with the primary gearing, after shutdown, while the vessel continues to be propelled by another engine. The gearing between the turbine and the clutch, when mounted on the intermediate shaft, provides some extra friction, hence increasing the probability that the turbine will come to a standstill. Of course, an intermediate shaft mounted clutch must transmit higher torque than a highspeed clutch, thereby entailing a modest increase in the clutch size. However, the lower speed clutch can often be simpler in construction, as compared with a high-speed clutch and, hence, not much higher in cost. A disadvantage is that the intermediate speed clutch arrangement does often result in some increase in the overall length of the gear casing, (although not necessarily the overall distance from the gearbox input flange to the output flange face). If locked train gearing is used in conjunction with intermediate shaft mounted clutches, two 1

11 clutches must be incorporated, viz, one in each layshaft. In this case, only one of the S.S.S. Clutches has pawls which partially interengage the teeth of this clutch when passing through synchronism, whereupon the shifting element of the clutch comes into contact with a pivoted lever interconnecting both clutches. Further axial movement of the first clutch then causes a similar motion to be transferred to the second clutch, so this is also shifted precisely into tooth engagement. During the final self-shifting travel of both clutches into full driving engagement, the interconnecting lever becomes relieved of load. 8 RELIABILITY OF S.S.S. CLUTCHES The basic S.S.S. overrunning clutch has no parts subject to wear when it is engaged, or when it is overrunning at high speed, i.e., the condition with the output at high speed and the input at rest. Under clutch engagement conditions, the forces on the pawl and ratchet mechanism are very small, even when the clutch automatically engages under high relative acceleration between the input and output shafts. Furthermore, as has been mentioned earlier, the basic S.S.S. Clutch is not dependent on servomechanism, with controls and interlocks, for its satisfactory operation. These very important aspects ensure a highly reliable operation, and this has been confirmed by extensive naval propulsion experience, as well as in many high power gas turbine industrial drives. In the previously mentioned field, no cases have been experienced where any parts of the pawl and ratchet mechanism, or the clutch components themselves, have had to be replaced upon account of wear in service. In point of fact, the only times that replacement parts have had to be fitted to naval propulsion S.S.S. Clutches has been to clutches of the type having a remotely operated locked-out condition. This has occurred on very few occasions when inadvertent movement of the control to shift the clutch from a locked-out condition to the ratcheting condition has taken place while the clutch input shaft, driven by the turbine, was rotating in a forward direction relative to the output shaft connected through the gearing to the propeller. Such mal-operation will cause instant damage to the pawl and ratchet mechanism, but the parts are easily replaced once the clutch is removed from the gearbox. However, such mal-operation is not feasible in uni-directional installations, e.g., with a c.r.p. propeller drive where such a remote lock-out control is unnecessary. As previously mentioned, for future installations, where the S.S.S. Clutch is incorporated in machinery wherein the propeller shaft is subject to both directions of rotation, so that the clutch requires a remotely operated lock-out control, the clutch will incorporate an internal baulk mechanism to prevent any possibility of such a maloperation. REFERENCES 1 Dunlop, J. M., and Good, E. B., "Machinery Installations of Guided Missile Destroyers and General Purpose Frigates," Transactions of the Institute of Marine Engineers, Vol. 75, No. 1, London. 2 Weaving, P.D.V., and Sampson, W. H., "Progress and Development in Naval Propulsion Gears ," Transactions of the Institute of Marine Engineers, Vol. 75, No. 3, London. 3 Raper, R. G., "Steam in the Royal Navy," Paper given at the Convention on "Treads in Steam Plant," May 2-4, 1963, arranged by the Steam Plant Group to the Institution of Mechanical Engineers held in Harrogate, England. 4 Benn, D. H., "Gas Turbines Versus Steam Reliability Analysis for a Warship Propulsion Plant," ASME Paper No. 68-GT-9. 5 Sachs, R. M., "Description of Propulsion Systems for DDH-28G Class Gas Turbine Destroyers," ASME Paper No. 69-GT Beale, G. B., and Gowans, B. J., "Transmission Design for Warships of the Royal Navy," Transactions of the Institute of Marine Engineers, Vol. 82, No. 7, London. 1 1