SAE 750739 STA CO ae 400 Commonwealth Drive, Warrendale, Pa. 15096 Design and Development of the Caterpillar 7155 Semi- Automatic Heavy-Duty Truck Transmission Philip S. Webber and Harry B. Newman Caterpillar Tractor Co. SOCIETY OF AUTOMOTIVE ENGINEERS West Coast Meeting Seattle, Washington 750729 August 11-14, 1975750729 Design and Development of ‘ME HEAVY-DUTY TRUCK INDUSTRY has expe- rienced rapid expansion in recent years. This ‘expansion has necessitated training many new drivers and uechanics. Reducing the effort, skill, and attention required of the starting and gear change process while retaining posi- tive control inproves the beginning, unskilled, and skilled driver's performance. Several innovations in the clutch-tranemiesicn portion of the pover train have been introduced to the industry. Some of these ideas have gained wide acceptance and others have been discon- tinued, are used in special applications, or continually reappear but have Limited success. ENGINE OBJECTIVES ‘The engineering objectives were to find ‘an improved method for converting the output of the prime mover to the demands of the vehicle. These objectives included a target cost between the planetary, multi-clutehed, torque converter type and the more conven- tonal uneynchronized, constant mesh type of the Caterpillar 7155 Semi Automatic Heavy-Duty Truck Transmission Philip S. Webber and Harry B. Newman Caterpllar Tractor Co. tranomissions. Also desired was an effi- ciency greater than 95%, possibly even greater than the conventional transmissions. Driver effort and skill requirements vere to be reduced while convenience was to be improved. Further objectives were to be able to handle the higher horsepower engines planned aad to have improved startability to match the larger, heavier vehicles of the future. The 7155 Transmission achieves these objectives. DESCRIPTION OF OPERATION The 7155 Transmission is a 16 forward, two reverse ratio, semiautomatic, quick shifting mechanical drive transmission. Figure 1 shovs the major groups within the transmission and Figure 2 shows the cab mounted selector control. The driver controls the shift initiation and gear ratio selection by moving the selec~ tor lever in the truck cab. The shift is made automatically when an air pressure sig- nal is sent from the ratio selector to the transmission control group. The heavy-duty truck industry has seen the need for a change in the concept of transmission design for many years. Several improvements have been made and others attempted, but greater improvement is needed to mateh the engine's delivery to the vehicle's demand. Driver performance can be improved ‘ABSTRACT and fatigue reduced by lowering the effort and skill required to make emooth, consistent starts end ratio changes. This paper discusses a solution to this need in the design and development of a semi-automatic, pneumatically controlled, constant mesh transmission.Fig. 1-7155 Transmission groups 1, Transmission control. 2. Gear group. 3. Input clutch. 5. input brake. 7. O11 pump. Fig. 2-7155 Ratio selector The transmission control perforns all shifts in the following sequence: 1, The gear group of the transmission 4s temporarily disconnected From the engine by the input clutch and disconnected from the drive Line by the output clutch. 2. The rotation of the gear group is 4. Output clutch, 6. Output brake. stopped by the input and output brakes. 3. ‘The collars in the gear group are noved to the new gear ratio. 4. The brakes are released and the collars engaged. 5. The gear group is reconnected to the engine by the input clutch and to the drive line of the truck by the output clutch. The shift is now complete. ‘This method of shifting, referred to as "stop and go”, takes place in less than one second. . When starting the truck, the input clutch Is automatically engaged by centrif- ugal force from a torus of ofl within the clutch as the engine speed increases. The of] to £111 the torus, to cool the clutches and brakes, and to cool and Lubri- cate the gears and bearings is circulated by the oil pump driven continuously by the fly- wheel, DESCRIPTION OF GROUPS INPUT GROUPS INPUT CLUTCH ~ The input clutch (Fig. 3) connects the engine flywheel to the transmis= sion input shaft. This clutch uses ten clutch discs with a fibrous friction materialand ten steel clutch plates. The clutch plates and discs are oi] cooled. ALL the components of the input clutch except the output hub and the discs are con nected to and tum with the engine flywheel. The discs and the hub axe connected to the transmission input shaft. At low idle, the start springs hold the start piston against the adapter and washers on the bolts prevent the pressure plate from contacting the clutch plates and discs. The diverter valve is positioned to alloe a small quantity of ofl to fill the space between the piston and adapter and to lubricate the disengaged clutch. 9) ‘1, Engine Flywheel. 4. Clutch dise, 5. Clutch plate. 10. Adapter. 12. Start piston. 13. Output hub. 49. Input shaft. 29. Washer, 30. Bolt. 36, Diverter valve. 3. Start spring. ‘As the engine speed 4e increased above approxinately 775 rpm (Fig. 4), centrifugal force causes the of1 in the space between the piston ad the adapter to increase in pressure. This moves the piston to the right and causes the compression of the start springs. The piston moves until it makes contact with the hub at approximately 1000 rpm. The start piston moves the diverter valve to the right, alloving maximum oi1 flow to cool the clutch. Fig. 4-Input clutch engaged 2. Input hub. 3. Start spring. 4. Clutch disc. 5. Clutch plate. 10. Adapter. 12. Start piston. 24. Load spring. 26, Pressure plate. 36. Diverter valve. ‘As the piston is moved to the right, the pressure plate is moved toward the clutch plates and discs. After the pressure plate makes contact with the clutch stack, ay more movement of the piston compresses the load springs. The compression force holds the clutch plates and discs together. In this position, the cluteh is fully engaged and power from the engine is eent to the input shaft of the transmission. While making a shift (Fig. 5), the input clutch must be disengaged. Before moving the shift collars in the gear group, the tranemis- sion control sends air pressure to a space behind the clutch piston. This moves the piston to the left, pushing the pins toward the pressure plate. Movement of the plate closes the diverter valve. This reduces of1 to the clutch during a shift and compresses the load springs, releasing the input clutch. ‘The guide pins and springs keep the thrust bearings in contact with their races when the clutch piston i released. Rollover springs hold the rollover plate in contact with the clutch plates and discs during a shift. This causes a small anountat : hy a \ ® | aicis L Fig. S-Input clutch released during a shift 4, Clutch disc. 5. Clutch plate. 17. Clutch piston, 18. Thrust bearing. 19. Input shaft. 20. Guide pin. 21. Spring. 22. Thrust bearing. 23. Pin. 24. Load spring. 26. Pressure plate. 27. Rollover spring. 28, Rollover plate. 36. Diverter valve. of torque to be delivered to the transmission input shaft to assist the collar engagements during a shift, by rolling the gear group slowly after the input brake has been released. ‘After the gear couplings are engaged, the transmission control exhausts the air pressure from behind the clutch piston. The force of the load springs causes the pressure plate to ove to the right, engaging the eluteh. INPUT BRAKE - The input broke (Fig. 6) 4s connected to the input shaft of the trans~ mission, This brake uses two brake discs with a fibrous friction material and one steel plate. The dises and plate are cooled by a continuous flow of ofl. The brake hub and discs curn with the input shaft while the plates are held sta~ tionary. The retraction springs are trapped between the reaction plate and the pressure Rig. 6-Input brake applied 7. Reaction plate. 9. Pressure pla 1k, Brake disc. 25. Brake hub. 8, Brake plate. 11. Brake piston. 19. Input shaft. 33. Retraction spring. plate holding the brake piston to the right. In this position, the dises and the input shaft are free to rotate. While making a shift, the input brake must be applied. Before moving the collars, the transmission control sends air pressure to a space behind the piston. This air pressure moves that piston to the left until the dises and plates are pushed together, stopping the gear group. After the gear couplings are moved, the transmission control exhausts the alr pres~ sure from behind the piston. The force of the springs causes the discs and plates to move apart, and the input shaft is free to turn again. LUBRICATION - An of1 pump (Fig. 7) (32), driven by a gear on plate (6), tums with the engine Flywheel. The output of the ofl pump is 29.0 U.S. gpm (109.8 1it/min) at 2000 rpm engine speed. At approximately 1200 rpm engine speed or 12 psi (82.7 kPa) oil pressure, a priorityDESEGN - The gear group has two arrange~ ELSES ments. The direct drive arrangement gives _- 17-23 to 1 reduction in Ist gear with a one . to one tatio in 16th gear, and the overdrive | arrangement gives a 14.48 to 1 reduction in | Ist gear with a .825 to 1 ratio in J6th gear. Both arrangements look and operate the sane and have approximately 21% step ratios, The only differences are the number of teeth on some of the gears and the sequence of shift Mf fork movement for a given gear selection. The gear group (Pig. 8) is a three comtershaft design. The front section of the transmission has a set of three identical countershafts and the rear section of the tranemission has a set of three identical coutershafts with the same distance between each countershaft ond the main shaft. The three main shaft gears in each sec— tion of the transmission have redial support. by their location between the countershafts. These gears (Fig. 9) get axial support from (typical) side plates (21 and 23) that make contact with the eides of the coutershaft gears. The six countershafts are supported in the transmission case by roller bearings. Asset of three main shaft gears (19, 22, and 25) is engaged with the front set of comtershafts. A set of three main shaft nec A = oy a y ay Fy 2 se 1) As engaged a he | fear set f comearshafes. A main ahaft gear yikes | @7) ie engoged with the reverse idler gear, which i cantilevered fron the rear comter~ shaft. Fig. 7-Lubrication of input clutch and brake 6. Plate. 15. Deflector. 19. Input shaft. 25. Brake hub. 26. Pressure plate. 31. Mounting plate. 32. O11 pump. 34. Hole. 35. Relief valve. 36. Diverter valve. valve opens to increase flow to the gear group. ‘The normal of] pressure in the systen ts approximately 25 psi (172 kPa). A relief valve will open at 35 to 40 psi (241 to 276 kPa) co prevent cold of] from causing exces sive of pressure. The ofl for lubricating and cooling the input clutch and brake is seat from the pump through @ passage in the mounting plate to a hole in the center of the input shaft. The oil for the input clutch goes through or by the diverter valve into a reservoir made by the deflector. When the reservoir is full, oil flovs to holes in the pressure plate to cool the clutch. ‘The oi] for the input brake goes through two smali holes in the input shaft and out through holes in the brake hub to cool the Fig. 8-Gear group (front brake. 1. Front countershaft. 2. Rear countershaft. GEAR GROUP 3. Main shaft.Fig. 9-Gear group 2. Front countershaft..6. Rear countershaft. 8. Roller bearing. 11. Main shaft gear. 13. Main shaft gear. 15. Hain shaft gear. 19. Main shaft gear. 21. Side plate. 23. Side plate. 25. Main shaft gear. 27. Main shaft gear. 30. Intermediate shaft. C-D. Shift fork. E-F. Shift fork. OPERATION - Five shift forks (Fig. 9) are moved by air cylinders in the transmission control. These shift forks move sliding col- lars which are connected to the main shafts with splines. By moving a sliding collar into’ engagement with a main shaft gear, power is directed either to the countershafte from the main shaft or to the main shaft from the countershafts. All shift forks can move to either the right or the left. Shift fork (C-D) can also be held in the center position. Figures 10 and 11 show the position of each of the five shift forks for each gear ratio avallable. Power comes through the input clutch to the input shaft. Shift fork (A~B) slides collar (20) to engage with either gear (19) or gear (22) and paver is sent to and divided between the three front countershafts. Shift fork (C-D) slides collar (24) to engage with either gear (22) or gear (25) and pover is sent from the countershafts to the interme~ 10. RN collar. 12. EF collar. 1h. GH collar. 47. Input shaft. 20. A-B collar. 22. Main shaft gear. 24. C-D collar 26, Output shaft. 28. Reverse idler gear. ‘A-B. Shift fork. RON. Shift Fork. G-H. Shift fork. ‘ORECT_DRVE_ARRANGENENT pemueTon] #70 Pa Fig. 10:Position of shift forks for available gear ratios in direct drive arrangenent diate shaft (30), The intermediate shaft then turns in the sane direction as the inputshaft. When sliding collar (20) is engaged with gear (22) and collar (24) also is engaged with gear (22), pover goes straight through the front section of the transmission to the Intermediate shaft. ‘The rear section of the gear group is similarly arranged and will provide four ratios. The rear section output is sent through the output clutch to the output. shaft. In reverse, shift fork (C-D) is held in its center position and shift fork (R-N) is moved to the left to engage collar (10) with gear (27). That gear is in engagenent with the three reverse idler gears which are in engagement with the front countershafts which are tuming in the opposite direction to the input shaft. Pover is sent from the front countershaft through the revere idler gears that are turning in the sane direction as the input shaft. From the reverse idler gears, power is sent through main shaft gears (27) and through the coller (10) to the interme diate shaft. The Jatermediate shaft is now turning in the opposite direction as the input shaft. ‘The gear connections for shift forks (&-¥) and (G-H) are the same as in the earl{er explanation except the direction of rotation is reversed. ‘OVERORIVE_AfANGENENT RencTON | Fao + Fig. 11-Position of shift forks for available gear ratios in overdrive arrangement LUBRICATION (Fig. 12) - The pump sends of1 through paseages in the front transmis- sion case to tvo tubes that are installed parallel to the main shaft of the tranemis— sion. Bach tube has elght small radial holes drilled in it. When pressure oil is in the tube, Lt will flow through these holes and lubricate the gears and bearings of the gear Broup. ‘The priority valve in of1 tube (33) reduces flow to the gear group and supplies extra oil to the input clutch during starting Fig. 12-Gear group lubrication 32, O11 tube, 33. O11 tube. 34. Priority valve. at low engine speeds. After the priority valve has opened, oil pressure is in both tubes. oureur Groups OUTPUT CLUTGH GROUP - The output clutch (Pig. 13) connects the gear group to the output shaft. This clutch uses 13 discs with a fibrous friction material and 12 or ‘nore (for the adjustment of clutch thickness) Steel plates. The clutch discs and plates are cooled by a continuous flow of ofl. ‘The input shaft, input hub, clutch discs, and pressure plate are connected to and turn with the gear group. The clutch plates and output hub are connected to the output shaft. The clutch is engaged by air pressure (sent from the transmission control) working between the mounting plate and the clutch piston. As the clutch piston moves to the Fight, Lt moves the rotating pressure plate through a thrust bearing. This plate moves to the right until the plates and discs of the clutch are held together. Now the output hub is connected to the input hub and pover con go from the gear group to the output shaft of the transmission. When making a shift, the output clutch must be released. Before moving the collars, the transmission control removes the alraeealeay a | ink " [ins Fig. 13-Output clutch and brake 1. Mounting plate. 5. Clutch plate. 7. Brake piston. 9. Brake disc. 10. Output hub.” 11. Pressure plate. 12. Clutch piston. "13. Output shaft. 15. Input shaft. "16, Thrust bea! 17. Input hubS. 18. Clutch rel 19. Retraction spring. 20. Holes. 21, Hole. 3. Brake plate. 6. Clutch dise. Pressure from the clutch piston. With no Pressure on the piston, the clutch release ‘springs move the pressure plate to the left, away from the clutch plates and dises, and the clutch is released. OUIPUT BRAKE ~ The output brake works ia coubination with the input brake to stop the rotation of the gear group. The output brake uses two brake discs with a fibrous friction material and one steel plate. The discs and plate are oi1 cooled. While making a shift, the output brake Bust be applied. Before noving the collars, the transmission control sends air pressure to a space behind the piston. This pressure moves the piston to the left util the discs and plates are pushed together, stopping the gear group. After the collars are noved, the transmission control removes the pressure from behind the piston. The force of the retraction springs causes the discs end plates to separate as the piston moves back to its original position. Now the gear group is free to rotate again. LUBRICATION - O11 for lubrécation and cooling of the output clutch and brake is carried through the center of the transmission shaft to the center of the output shaft. Two holes (20) in the output shaft provide oi1 for the lubrication of the ball, roller, and thrust bearing as well as to cool the output clutch and brake. The additional hole in the output shaft delivers oil for the lubrication of the rear bearing. CONTROL GROUPS COMPONENTS - The complete transmiseion (clutches, brakes, and gear section) is con- trolled by air pressure from the normal 5; tom of the truck. The air is controlled by { two position, three way valves and single and 4 double check valves. The valves are connected } to passages, orifices, air cylinders, and closed volunes which provide the controls for the transmission. Two basic types of three way valves are used, pilot operated and mechanically oper ated. “The pilot operated type has two dif- ferent configurations ... normally closed (Fig. 14) and normally open (Fig. 15), These figures indicate the flow paths for both types of valves with and without pressure applied to the pilot passage. When the pilot Pressure is not present, the differential areas of the seat and piston hold the valve closed. ZF wo. | NO PROT PRESSURE ‘WH PCT PRESSURE Fig. 1-Normally closed valve 1. Pilot passage. 2. Supply passag 3. Delivery passage. 4. Supply passage. ‘The mechanically operated valve (Fig. 16) is nomally closed. The figure indicates the flow paths in the operated and moperated Positions. The spring helps the valve follow the cam during operation,NO PLOT PRESSURE. Fig, 15-Normal 1. Pilot passage. 3. Delivery passage. 2. WH PLOT PRESSURE ly open valve Exhaust passage. 4. Supply passage. a UNOPERATED Fig. 16-Mechani 1. Supply passage. 3. Delivery passage. 5. Stem. 6. Cam plat Double check valves (Fig. 17) and single check valves (Fig. 18) ae used. The double check valve directs flow to the delivery Passage from either supply passage while the Single check valve directs flow from the supply passage and blocks floy from the delivery passage. AIR SYSTEM FOR SHIFT FORKS - Two air cylinders are used to move each shift fork (Pig. 19). Piston (19) within cylinder (16) moves shift fork (20) toward the rear, and piston (21) within cylinder (17) moves the fork toward the front. Ax explanation of the air system for shift forks (A-B), (E-#), and (GH) is given below. The alr system for OPERATED cally operated valve 2, Ball. 4, Exhaust passage. te. shift forks (C-D) and (R-N) is similar to that for the other forks with some added controls to provide centering for (C-D) as well ae forvard-reverse inhibiting. Bach afr cylinder has ite om supply valve. One valve (7) is normally open while the other valve (10) is normally closed. Both supply valves are pilot operated by a mechanically operated valve (4) in the ratio selector. When the mechanically operated valve is closed, no pilot air pressure {s sent to the supply valves. Ar pressure in the supply line goes through the normally open valve, to the piston. This pressure moves the piston and the shift fork to the10 (GK K iy yy Fig. 17-Double check valve 1, Supply passage. 3. Supply passage. 2. Delivery passage. 4 Fig. 18-Single check valve 1. Area, 2. Delivery passage. 3. Supply passage.a (8), (EF), ig. 19-Air system for shift forks and (6-H) with mechanically operated valve closed Pilot supply line, Cam plate. 4. Mechi Sensing circuit. 6. 7. Normally open valve. 3. Exhaust. 10. Normal 11. Exhaust. 12. Hole. 1H. Sensing hole. 15. 16. Air cylinder. 17. 18. Supply line. 19. Pi 20. Shift fork. 21. Pi Fight (tovard the rear of the tranemieeion). The other air cylinder is connected to exhaust through the normally closed valve. After the pigton has moved completely to the right, tt opens the sensing hole which alloys alr to Flow eround the double check valve to the sensing circuit. The air pressure fron the sensing hole indicates that the shift fork hhas moved coupletely and the sliding collar in the gear group has engaged correctly. When the mechanically operated valve is opened by the cam plate, pilot pressure is sent around the double check valve (2) to the supply valves. Operation of the supply valves moves the pistons and shift fork to the left (toward the front of the transmission). The air from the normally closed valve noves the check valve up to prevent alr from going back to the mechanically operated valve and holds the supply valves in their present positions. The supply air to the mechanically operated 2. Double check valve. janically operated valve. Line. 8. Double check valve. ly closed valve, 13. Hole. Sensing hole. Air cylinder. ston. ston. valve can now be removed. ‘The air to the supply valves is exhausted during the start of each chift @ycle, allow ing both valves to retum to their relaxed position for the next comand from the ratio selector, illustrated by valve (i). This arrangenent allows one air passage to control two positions of the shift fork and collar. SENSING CIRCUIT - The sensing circuit receives information from all actuator air cylinders, Signals are only allowed to be sent when the associated fork and collar have moved far enough to have made a positive engagement within the gear group. Double check valves and pilot operated valves are used in suffictent quantity to know when the five coupling collars are in their correct positions. The output of the sensing circuit is used by the transmission control to release the brakes and engage the clutches. ‘PRESSURE REGULATOR - The output clutcha2 a Fig. 20-Pressure regulator 3. Transmission supply line. 4. quick Fill valve. 5. Line to output clutch. 6. Regulator valve. 7. Regulator piston. 8. Orifice. 9. Valve. 12. Spring. 13. Load piston. 15. Load piston. 17. Load piston. is engaged by air pressure controlled to higher levels for higher torques by a pressure regulator (Fig. 20). The pressure regulator reduces the load on the output clutch compo- nents, increasing their life. Air pressure flows’ from the transmission supply line, through the orifice, to the regulator valve. When the air pressure to the output clutch is less than the pressure setting, the regulator apring force moves the regulator piston up and opens the valve. If the air pressure to the ourput clutch is too high, the regulator piston and valve move down to stop supply air from flowing through the regulator, and if necessary the piaton moves further than the valve, relieving the overpressure. ‘The pressure regulator receives signals from the actuator cir system to adjust the load on the regulator spring by moving the load pistons, Valve (9) combines two of these signale for the waximum alr pressure. ‘QUICK FILL VALVE ~ The quick fi11 valve allows ait pressure from the transmission supply line to flow around the ball directly to the output clutch piston. This lets the output clutch piston fill more rapidly than Af the flow cane through the orifice and regulator. When the pressure in the output clutch piston becomes approximately 6 psi. (41.4 kPa), the force on the quick £111 valve overcones the spring force and the valve closes. With the valve closed, air flow through the orifice and pressure regula~ tor modulates the output clutch engagement. SHIFT CYCLE CTRCUIT - The shift cycle circuit is composed of pilot operated valves (Fig. 14 and 15), double check valves (Fig. 17), single check valves (Fig. 18), orifices, and volumes. The valves control the direction of flow of air and the orifices and volumes determine tine delays for sequenc~ ing the fumetions of the control. Exhausting the shift signal in the ratio selector by noving the shift lever to the right (Fig. 2) initiates the sequence of the shift cycle. The first operation is to 111 the input clutch piston (#ig. 5), exhaust the output clutch piston (Fig. 13), and exhaust the actuator supply clreuit (Fig. 19, line 18). After sufficient delay for filling and exhausting the clutch pistons, the input (ig. 6) and output (Pig. 13)'brake pistons are pressurized. ‘The cam plate (Fig. 19, item 3) deter- mines the mechanically operated valves that will be actuated for the ratto selected and applies air preseure to the pilot pistons of ; the appropriate actuator valves. The second delay determines when the Amput brake piston (Fig. 6) will be exhausted and the actuator supply reapplied (Fig. 19, Line 18). The actuator valves (Fig. 19)direct air to the correct air cylinders, moving the shift fork and collars in their correct direction. With the iaput brake released, the input clutch applies low level torque to the gear group (Fig. 9), rotating the collars and main shaft gears until the teeth engage (when necessary). When all collars have been engaged, determined by the actuator pistons having uncovered the sensing signal holes of all air cylinders, the output brake is released, and the input and output clutches are engaged. Oa the rare occasions when all the col- Jars do not engage in a short period of tise, 2 third timer releases the air behind the output brake piston alloying the whole gear group to rotate. This final rotation may Finish the collar engagement, but occasion~ ally the control must be recycled. This third timer also communicates with the ratio selector preparing it for the next shift. DEVELOPMENT COMPONENT TESTING - Laboratory evalua tions of potential components vere begun long Defore a complete transmission was designed. Most of that work centered around development of the pneumatic controls, gear couplings, and means to connect and disconnect the out- put. From that work, design paraneters were established and a complete transmission was built for lab and vehicle evaluations. The 3 i a3 present design then evolved as experience was Gained. After performance and durability tests in the lab and in proprietary trucks had demonstrated the reliability of the design, evaluations of units in customeromed trucks were begin. Concurrently, changes and improve- ments which were suggested by our field expe- rience were evaluated in the lab. ‘There have been many laboratory perfor~ mance and durability tests of components. A few of the major tests which supplement the explanation of how the unit functions will be discussed. ‘START TESTS - Much of the shife perfor~ mance and durability testing was performed on the test setup shown in Figure 21. That setup Included a 270 HP engine, a 7155 Tranomissioa, and an eddy current absorption dynamometer. One test was performed to evaluate the dura~ bility of the centrifugally engaged clutch on a start. A 72,000 Lb. (33,000 Kg) gue truck was instruented and data was recorded during starte from a standstill on the level in various gears. The results indicated that the input clutch absorbed 340 BTU (359 kJ) of energy during a start in 8th gear. That vas 4 severe test for the input clutch because 4th gear is recomended for starting a 72,000 Lb. (33,000 Kg) vehicle on the level. In’ the lab, 12th gear vas used to provide effective inertia at the input. equivalent to that of a Joaded truck in 8th gear and the dyno load was adjusted to simulate the rolling resis~ Fig. 21-Performance and durability test setup4 tance. After 5000 starts, the input clutch was in excellent condition with no measurable ‘A typical oscl1logram of the input clutch engagement when the governor control was advanced rapidly to the full-open position is show in Figure 22. Note that the engine oped of the ofl in the torus lagged the fly wheel speed during the rapid engine acceler~ atioa. In @ truck, the overshoot can be avoided and the rate of torque bulldup can be controlled by how rspidly the driver accelerates the engine. GOVERNOR CONTROL RAPIOLY ADVANCED TO FULL OPE? DOO RP ENGINE SPEED TRANSMISSION INPUT SPEED DRIVE SHAFT TORQUE TIME Fig. 22-Centrifugal clutch engagement SHIFT CYCLE ~ Another test was 2 500,000 shift run on an 8-9-8 cycle. That cycle was selected because all collars (except reverse) were shifted. A typical oscillogran of an 5-9 shift ig shom in Figure 23. As dic~ cussed earlier, all of the shift cycles are the same except for the selection of the proper collar or collars to be engaged to provide the selected ratio. A shift vill be Giecussed with reference to Figure 23. When the shift lever is moved out of a notch, rotated, and alloved to move back into another notch, a shift cycle ie initiated. The input clutch pressure rises and the rear clutch pressure exhausts to release those clutches. After a timed interval, both the input and output brakes are applied to bring the gears to a stop. A second timer (controlled pres~ sure rise rate within an appropriate volume being supplied through a properly sized orifice) then causes actuator supply pressure to direct air to the actuators required to provide the selected ratio. In the example (8-9 shift), all four collars are shifted. ALL other shifts are either one, two, or three collar shifts. The rise of actuator supply pressure then turns off the input brake. The release of the input brake allows “WEUT- CLUTCH \ PRESSURE INPUT BoE PRESSURE [OUTPUT CLUTCH PRESSURE Nea UTPUT BRAKE PRESSURE are COLL ar F———Ee DISPLACEMENTS on Fig, 23-Oscillogram of 8-9 shift cycle the built-in drag of the input clutch (zeferred to as rollover) to rotate any butted couplings Into engagement. The output brake remains applied to prevent rotation of the output side of those couplings. In the example, all four collars were shown to have butted. The occurrence of butts is random. Sometimes the collar teeth line up with the internal spaces of the mainshaft gear splines and the couplings engage when they move seross. When all of the selected collars ave engaged, an alr pressure signals the cycle to progress, The output brake pressure then exhausts to release that brake, the input clutch pressure exhausts to reengage that clutch and recomneet the gears to the engine, and the output clutch pressure begins to rise at a controlled rate to reconnect whe gears to the drive shaft with a gradual torque buildup for a smooth shift feel. ‘The shift times vary slightly but are alvays less than 1.0 sec., depending upon how many collars fare shifted as well as hoy many of them butt. DRIVE TRAIN STMULATOR - The test setup shown in Figures 24 and 25 can be thought of as an indoor proving ground where precise tests of components or systems can be con ducted under closely controlled repetitive conditions. The 1693 engine without the fan and accessories is equivalent to a 450 HP engine in a truck. The 7155 Transmission is mounted to the engine flywheel housing and a truck drive shaft connects the output to the simulator. The drive system mechanically simulates the mass-elastic system of a loaded truck and its motion resistance as reflected35 i EDDY CURRENT DYNO INERTIA FLYWHEEL [es © Fig. 24-Orive train simulator test setup IDYNO TORQUE Fig. 25-Control console for drive train simulator16 to the transmission output shaft. Physically, the transmission drives into a series of specially designed rubber couplings, a slip clutch, an inertia flywheel, and finally an absorption dynamometer. The rubber couplings simulate the axle shaft and tire stiffness as well as damping inherent in wheeled vehicles. ‘The slip clutch simulates the traction limit of the drive wheels. On a vehicle, tire elippage effectively limits torque peaks, particularly during shifts. The inertia flywheel simulates the mass of a loaded truck. ‘The eddy-current absorption dynamometer equipped with a special field-forcing control is progranned to simulate motion resistance ses rolling, grade, and wind. ‘The setup includes a ratio selector which is positioned by cylinders to simulate driver's notions. Major features of the con trol console (Fig. 25) include the progranmer, indicators for speeds, pressures and tempera— tures, torque indicators for both drive shaft and dynamometer torques, and the safety shut~ dom annunciator. The programer has one analog channel which controls dynamometer load as well as space for 25 digital channels to control such functions as engine speed settings md transmission gear selections. Automatic safety shut-down circuits protect the factlity and the first-out annunclator Indicates which condition shut the test down. ‘SIMULATOR TEST CYCLE - The test cycle was developed from observations and measure~ ments recorded during operation of @ loaded truck. Every attempt vas made to provide realistic loads ... only time was compressed. ‘The cycle is now programmed to operate the Fig. 26-Fleld Installation-"'Michigan Tra urban hauler transmission on a 1.6 mile (2.57 km) course in 4 minutes with grades varying from 0.4 to 13.7%. The transmission shifts through 10 of the 16 geare, skipping 1, 2, 3,5, 7, and 9 uch as an operator might normally do. The cycle begins with the engine at lov idle, the trmsnission in neutral, and the dynanometer load equivalent to truck rolling resistance. The transmission is then shifted to 4th and the engine is accelerated to engage the cen- trifugally operated clutch. During the acceleration, load is rapidly applied by the dyno and then slowly decreased to allow the engine to accelerate to the upshift speed. After the 4&6 shift, load continues to drop gradually to allow the engine to reaccelerate. This sequence continues witil the transmission gets to 16th when the load begins to increase gradually to lug the engine to the domnshife epeed, Single step dovnshifts occur until the vait gets to 12th where again the load begins to decrease and permit upshifts back to 16th. The domashift sequence is then repeated and continues until the transmission is back to neutral, Essentially, the truck is going up grades, coming to a stop sign, and then going up the grades again. This is'a severe cycle for the transmission. We consider that 50,000 shifts are equivalent to over 100,000 miles (260,900 ka) of operation in = line haul truck va many transmission components. Two production unite have coupleted 300,000 shifts - 600,000 equivalent miles (965,400 km) ~ each on this cycle with no indications of pending problens. FIELD TESTING - Anyone who ts familar with the introduction of new products knowsthat all of the laboratory testing imaginable does not necessarily prove-out a product. The final test 1s how well it performs ia actual applications when subjected to all of the elements which cannot possibly be antici- pated and how it is accepted by users who make their "bread and butter" with the product. That phase of testing began with an extensive program which included the installation of 12 experimental units in custoner-omed trucks. Applications were selected to include the extremes in both terrain and climatic condi- tLons. uv The 12 experimental units accumulated over 3 million miles (4,83 million km) in various spplications from an 11 axle "Michigan Train" rig (Pig. 26) hauling sand and gravel into the Detroit area to line haul wits operating primarily. on Interstate highways (Fig. 27). The transmission in the truck shown accumulated approximately 500,000 niles (804,500 km) behind a 375 HP engine over about a 21/2 year period. Driver and ovner reactions were enthusi~ astic. Drivers liked the 7155 because there was little effort required with the fingertip Fig. 27-Field installat: interstate operation Fig. 28-7155 Installed in a freightliner chassiscontrol and nc clutch pedal to operate, they sould shift sroothly and dependably in’ all types of operation, they could concentrate ore on driving and less on shifting, they rould determine when a shift occurred (no surprises), and they had better trip times im urban and rural areas where driving condi— Hons were less than ideal, Omers liked the mits because there was no sacrifice in fuel ronsumption ead a minimum of maintenance vas required. The truck Factory contacts vere impressed vith the simplicity of the installation ... no clutch pedal or associated linkage, no gear shift mechanism with the complex tower on COE nodels, no heat exchanger or assoctated of] ad water lines. To install the 7155, the irive ring is bolted to a standard flywheel and the tranexiseion ie mounted on an SAF No. I flywheel housing (Fig. 28). The ratio selector is then installed in the cab with a Flexible control Line assembly connected fron the ratio selector to the transmission control. air supply line and a standard rear support spring are provided and the installation is complete. Provisions have also been made for oil pressure, temperature, and reverse pressure pickups. The reverse pressure signal is used for back-up lights or warning devices. sUIOMARY The Caterpillar 7155 Seml-Autonatic Heavy-Duty Truck Tranemission 1s a new concept in the evolutionary process of converting the output of a prime mover to the demand of che vehicle. The diesel engine is the current prime mover and appears well intrenched for the heavy-duty truck industry into the future. No drastic changes in the vehicle's general configuration appear in the making st this tine. Therefore, the 7155 Transmis— sion is ready to install'tn any current and future heavy-duty truck, Since the gears do not dip in the Iubri- ting ofl and ofl pressure is not used for the controls, the only loss associated wich the ofl is the small lubricating pump. with this small loss level, the mechanical effi- “This pape: abject wo eon. Sutera al epaons ‘ices papers of un tao ag re ‘respon, be Sect however theppe ae seer eed by SAE for wsorsging tnd oma Di vt Be pte’ wi te paper HH pubes ciency 1s greater than 95%, eliminating the cost and installation of an cil cooler and Lines. The design for use with ongine of1 ‘and “top of frame" of1 checking and filling improves the serviceability and maintain ability of the 7155 Transmission. ,Addition of to SAE heavy-duty PTO's, left and right, increases the versatility t0 cover most auxiliary drive requirements of the trucking industry. ‘The incorporation of "fingertip" shift ing controls and "no clutch pedal” starting has increased the useability of the Cransnis- sion by unskilled drivers and reduced the effort and concentration requirement of the accomplished drivers. To further help all drivers, the ratio selector lever movenent has an inhibiting mechanism that allows up~ shift movenent from aeutral to fourth (the normal starting gear for @ loaded vehicle on level, hard surface), in two step increments from fourth to eleveath (for ascolerating the toad), and single stepped fron eleventh to sixteenth. The dowushift inhibiting ts similarly arranged, but any racio can be used in either erection to match particular Supply-demand situations. Lifting the shite lever releases the inhibitor allowing gross ratio changes necessitated by ornering, stops, and unusual condi tions. ‘The mechanically inhibited ratio selec tor, modulated clutch engagements, and 21% progressive steps serve to reduce strain and abuse on the engine and drive train thus reducing the maintenance requirements of the vehicle. Gears, bearing, and shafts have been designed for the 450 horsepower range with an anticipated "over the road” life of 500,000 mites (804,500 km). The overall reduction of 17.23 to 1 is intended to start the heavier vehicles of today and the near Future in all normal and most abnormal cond{- ttons. ‘The 7155 Transmission was designed and developed to increase the heavy-duty truck performance by improving the rétio change technique and reducing the driver's required effort and skill. It 1s ready to take ite place in the industry. ‘5 SAE Teaectont. For perminon to path te paper lori pr inact he SAE Pubcon Ds, Tene we to strat pent be comidred fx peetaton ox pale si though SAE thal ed he anasto a 380 wad baal sp ood maar Sete. Empreinte Bos, SAE 20 pase booke Pineda USA,