Transmission
This application is a continuation-in-part of our copending application Ser. No. 171,920, filed Aug. 16, 1971, entitled &#34;Transmission&#34; and assigned to the assignee of record herein and now abandoned. 
    
    
     This invention relates to transmissions and in particular to a gear shifting arrangement for an in-line transmission. 
     In an in-line transmission, a plurality of gears forming a change-speed gear set are journalled side-by-side on a shaft and are in constant running mesh with corresponding companion gears. The shifting arrangement drivingly couples the change-speed gears to the shaft one at a time to provide the different transmission ratios. In one known type of shifting arrangement for this type of transmission, a key is axially slid in a groove in the shaft on which the change gears are journalled to couple the gears one at a time to the shaft. 
     The present invention contemplates an improvement in the above-described type of transmission gear shifting arrangement. The construction of the improved shifting arrangement permits component parts to be subassembled onto the transmission shaft without using separate attaching parts, the object of the construction being to greatly facilitate the assembly of the component parts into the transmission, reducing the cost and complexity thereof. 
     A further object of the invention is to provide a manually operable shifting arrangement wherein the shift effort, while being easy and smooth, provides sufficient &#34;feel&#34; to enable the operator to positively sense the shift positions. 
     An additional aspect permits a neutral start safety switch to be used with the improved shifting arrangement without the need of additional parts other than the switch itself. 
     Some additional objects of this invention are to provide a transmission with a change-speed gear set and key arrangement of economical manufacture and assembly which can be readily and smoothly shifted into any gear from any other gear or neutral position, whicheliminates backlash or free wheeling of the engaged transmission, is of comparatively economical manufacture and assembly, has a long useful life, and is substantially service and maintenance free. 
    
    
     These and other objects, features and advantages of this invention will be apparent from the following description, appended claims and accompanying drawings in which: 
     FIG. 1 is a horizontal sectional view taken substantially along the transmission case edge flange and with the transmission cover removed. 
     FIG. 2 is a vertical sectional view of the transmission of FIG. 1 taken substantially along the line 2--2 in FIG. 1. 
     FIG. 3 is a vertical sectional view taken along line 3--3 in FIG. 2. 
     FIG. 4 is a vertical sectional view taken along line 4--4 in FIG. 1. 
     FIG. 5 is an exploded view taken in the same direction as the view of FIG. 1 showing the output shaft, two shift keys and the shift collar of the transmission of FIG. 1 shown separately therefrom. 
     FIG. 6 is an axial end view of a typical change gear of the transmission of FIG. 1. 
     FIG. 7 is a top view of one of the two shift keys shown in FIG. 5. 
     FIG. 8 is a horizontal sectional view of a modified form of a transmission embodying this invention taken substantially along the transmission case edge flange with the transmission cover removed. 
     FIG. 9 is a partially sectioned and partly exploded view taken in the same direction as the view of FIG. 8 showing the output shaft, two shift keys and the shift collar of the transmission of FIG. 8. 
     FIG. 10 is an axial end elevational view of a typical change gear of the transmission of FIG. 8. 
     FIG. 11 is a horizontal sectional view taken on line 11--11 of FIG. 10. 
     FIG. 12 is an axial end view of a typical washer which is adapted to be received in the change gear of FIG. 10. 
     FIG. 13 is a sectional view taken on line 13--13 of FIG. 8. 
     FIG. 14 is an enlarged fragmentary view of the output shaft of the transmission of FIG. 8 with one of the keys received in the shaft and two change gears received thereon. 
     FIG. 15 is an enlarged side view of a shift key of the transmission of FIG. 8. 
    
    
     Referring to FIGS. 1 and 2, the transmission 10 of the invention comprises a case 12 whose upper edge 14 mates with the lower edge (not shown) of a cover 16 to enclose the transmission mechanism. A vertical input shaft 18 (FIG. 2) is journalled within a cylindrical bore 20 of an elongated boss 21 in the bottom wall of case 12 by needle bearings 22. The inner splined end of shaft 18 receives a bevel pinion gear 24. Gear 24 is retained on shaft 18 by a retaining ring 26, and bears against a thrust race 28 between itself and case 12. A second retaining ring 30 and washer seal 32 on the external portion of shaft 18 retain the described input arrangement on case 12. 
     A counter shaft 34 and an output shaft 36 (FIG. 1) are journalled in the right-hand and left-hand end walls of the transmission enclosure by means of flanged sealing bushings 38. Four companion spur gears 40, 42, 44 and 46 and a bevel gear 48 are keyed side-by-side on counter shaft 34 for rotation therewith. The five gears 40 through 48 are retained on shaft 34 between a thrust race 50 and a circular shoulder 52 on shaft 34. Bevel gear 48 meshes with bevel pinion gear 24 so that rotation of input shaft 18 rotates all five gears 40 through 48 in unison. 
     Companion gears 40, 42, 44 and 46 are in constant running mesh with four spur change gears 54, 56, 58 and 60 respectively. Change gears 54 through 60 are journalled for free rotation on intermediate diameter cylindrical portion 62 of output shaft 36. Gears 54 through 60 are axially retained as a cluster on portion 62 by a shoulder 63 of shaft 36 and a thrust race 65. Referring also to FIG. 6, each change gear 54 through 60 has four identical arcuately extending teeth 64 which project radially inwardly of identical circular gear bores 66 to journal the gear on shaft 36. Each tooth 64 has a 45° arcuate extent, and the four teeth 64 are symmetrically arranged to define 45° arcuate free spaces 70 between adjacent teeth. As can be seen in FIG. 1, the axial dimension of each tooth 64 is slightly less than one-half the axial dimension of the gear. Therefore, this construction defines an annular free space 68 adjacent teeth 64 and spaces 70 which lies radially between the right-hand portion of each bore 66 and portion 62 of shaft 36. This structure of gears 54 through 60 enables transmission 10 to be properly shifted through four forward speeds as will become apparent. 
     Because input shaft 18 rotates in only one direction to similarly rotate gears 54 through 60, a single reverse speed is provided by a sprocket and chain arrangement located adjacent the right-hand end wall. A sprocket gear 72 is keyed to shaft 34 for rotation therewith and axially retained thereon by a shoulder 74 and a thrust race 76. Sprocket 72 drives a sprocket gear 78 on shaft 36 by means of an endless roller chain 80, only a portion of which is illustrated in FIG. 1. Sprocket 78 is journalled for free rotation on another intermediate diameter cylindrical portion 82 of shaft 36 of the same diameter as portion 62, by means of four inwardly projecting arcuate teeth 84 whose circumferential extent is the same as teeth 64 of gears 54 through 60 but whose axial dimension is the same as that of sprocket 78 itself. This construction defines four 45° arcuate spaces (not shown) similar to spaces 70 of gears 54 through 60. Sprocket 78 is retained on portion 82 by a circular shoulder 86 of shaft 36 and a thrust race 88. 
     The structure of the improved shifting arrangement of the invention can be better understood by referring to FIG. 5 in addition to FIGS. 1 and 2. Output shaft 36 further includes a maximum diameter cylindrical portion 90 between the portions 62 and 82, the axially opposite ends of portion 90 defining shoulders 63 and 86. Two diametrically opposed rectangular grooves 92 and 94 extend axially of shaft 36 all the way through portions 62, 90 and 82 and have their respective flat bottom surfaces 92&#39; and 94&#39; (FIGS. 4 and 5) tangent to the outer periphery of the minimum diameter cylindrical opposite end portions 96-98 and 100 of shaft 36. Hence, the axially opposite ends of each groove 92 and 94 are open and accessible for endwise insertion of two identical shift keys 104 and 106 of rectangular cross-section which are slidable axially of shaft 36 within grooves 92 and 94 respectively. The width of each key 104, 106 is dimensioned to fit snugly within the rectangular cross-section of grooves 92 and 94. The length of each key 104, 106 is slightly less than the length of portion 90 of shaft 36, and the two keys occupy the position illustrated in FIG. 1 when the transmission is in its primary neutral position. A radial projection 108 at the left-hand end of each key (as viewed in FIG. 1) travels within the portions of grooves 92 and 94 which are coextensive with shaft portions 62 when keys 104, 106 are shifted to the left from the illustrated position. Projections 108 project radially outwardly beyond the surface of portion 62 and terminate just short of the inside diameter of bores 66 in gears 54 through 60 to permit projections 108 to be axially shifted through gears 54 through 60. 
     With keys 104, 106 in the primary neutral position of FIG. 1, projections 108 are within the annular free space 68 of gear 60. When gear 60 is rotated relative to shaft 36 to a position wherein projections 108 register with diametrically opposite spaces 70 of gear 60, keys 104, 106 may be shifted to the left from the position illustrated in FIG. 1 to bring projections 108 into diametrically opposed spaces 70 and thus between adjacent pairs of teeth 64. Further rotation of gear 60 causes two diametrically opposed teeth 64 to engage projections 108 to thereby couple gear 60 in rotational driving relation to shaft 36 (first gear). Further leftward shifting of keys 104, 106 disengages projections 108 from teeth 64 of gear 60 so that projections 108 are now situated within space 68 of gear 58. The axial dimension of projections 108 being less than the axial dimension of spaces 68 prevents coupling of more than one change gear at a time to shaft 36. Keys 104, 106 may be further shifted to selectively individually couple gears 58, 56 and 54 to shaft 36 in similar fashion (second, third and fourth gears). When keys 104, 106 are at their far left limit of travel, the left-hand ends of the keys are registered between teeth 64 of gear 54 and flush with thrust race 65. While the illustrated construction of teeth 64 is merely exemplary, such an arrangement provides four possible gear engagement positions per gear revolution and prevents excessive wrap-up of shaft 36. 
     A shift collar 110 is journalled on portion 90 of shaft 36 and encircles keys 104, 106 for axially shifting the keys. The right-hand portion of each key 104, 106 comprises two projections 112 and 114 which project radially outwardly beyond the surface of portion 90 (and the bore 117 of collar 110) and are axially separated so that the collar 110 is snugly embraced therebetween (FIG. 5). The illustrated construction provides smooth shifting performance and facilitates subassembly of keys 104, 106 and collar 110 onto shaft 36. Keys 104 and 106 are first inserted within the collar bore, prior to assembly of these parts to shaft 36, so that projections 112, 114 embrace collar 110 as illustrated by the engagement of key 106 with collar 110 in FIG. 5. With the two keys held manually diametrically opposed relatively to the axis of collar 110, collar 110 is slipped endwise over either end 96-98 or 100 of shaft 36, and rotated until keys 104, 106 are angularly aligned with grooves 92, 94. Collar 110 may then be slid further onto shaft 36, thus inserting keys 104, 106 endwise into grooves 92, 94. The keys are now securely captured on shaft 36 by assembling gears 54-60, sprocket 78, thrust races 65 and 88 and bushings 38 on shaft 36 and dropping this sub-assembly into place in housing 12. 
     It will be noted that projections 114, in addition to retaining collar 110, serve as clutch dogs to engage the internal teeth of sprocket 78 when keys 104, 106 are shifted to the right from the position illustrated in FIG. 1 to thereby couple sprocket 78 in driving relation to shaft 36. 
     Returning to FIGS. 1, 2 and 4, shifter mechanism 118 comprises a vertical shaft 120 journalled in .Iadd.sidewalls such as .Iaddend.the upper and lower walls of the transmission enclosure. A bifurcated shift fork 122 having upper and lower arms 124, 126 is affixed to shaft 120. The radially outer ends of arms 124, 126 receive coaxial pins 128 and 130 which project inwardly thereof to engage a central circular external groove 116 of collar 110. Pins 128 fit snugly within groove 116, providing only enough free play to permit collar 110 to freely revolve on and/or with shaft 36. The upper external end 132 of shaft 120 (FIG. 2) is adapted to be rotated by a crank and actuator arrangement (e.g., a Bowden cable) not illustrated. Counterclockwise rotation of shaft 120 (as viewed in FIG. 1) shifts collar 110 axially to the left and thereby similarly moves keys 104, 106 to successively engage gears 60, 58, 56 and 54. Clockwise rotation of shaft 120 shifts keys 104, 106 to the right to engage sprocket 78 to effect reverse drive. 
     As best seen in FIGS. 1 and 2, a sector plate 134 is affixed to the upper portion of shaft 120 within the transmission enclosure. The outer arcuate edge 136 of plate 134 is concentric with shaft 120 and is provided with a plurality of nine angularly spaced notches or detent seats 138 through 154 which are numbered in increasing order in the clockwise direction as viewed in FIG. 1. As better seen in FIG. 3, seats 138 through 154 are adapted to be engaged by a detent ball 156 which is biased inwardly toward plate 134 by a spring 158. Ball 156 and spring 158 are situated within an opening 160 in cover 16, and the force of ball 156 against plate 134 is set by an adjustment screw 162 which is threaded into opening 160. The engagement of ball 156 with the seats 138 through 154 is correlated to the axial position of keys 104, 106 as follows. 
     With transmission 10 in the position illustrated in FIG. 1, ball 156 engages detent 140, thus establishing the primary neutral position. When shaft 120 is rotated clockwise to engage ball 156 with seat 138, projections 114 of keys 104, 106 drivingly connect sprocket 78 to shaft 36, thus establishing the reverse position. Further clockwise movement of plate 134 is prevented by abutment of the chamfered corner 155 adjacent seat 154 with an internal stop (not shown) on cover 16. When shaft 120 is rotated counterclockwise to engage ball 156 with seat 142, projections 108 of keys 104, 106 drivingly connect gear 60 to shaft 36, establishing first or low forward speed. Further counterclockwise rotation of shaft 120 to engage ball 156 with seat 144 causes projections 108 to disengage gear 60 and to shift into space 68 of gear 58, thus providing an intermediate neutral position between first and second speed. Similarly, engagement of ball 156 with seats 146, 150 and 154 establishes second, third and fourth forward speeds respectively by engaging projections 108 with the teeth 64 of gears 58, 56 and 54 respectively in that order. Movement of plate 134 beyond full counterclockwise position (i.e., engagement of ball 156 with seat 154) is prevented by abutment of the left-hand ends of keys 104, 106 with thrust race 65. Seats 148 and 152 define additional intermediate neutral positions between second/third and third/fourth forward speeds. 
     The transmission thus far described combines an easy, smooth shift effort having sufficient &#34;feel&#34; to enable the operator to sense the individual shift positions with a rugged operational capability. It should be understood, however, that it may sometimes be desirable to modify this arrangement. For example, the intermediate neutral detents 144, 148 and 152 could be removed so that when shifting between first and fourth forward speeds, the operator would &#34;feel&#34; only the positions wherein a drive connection is actually established. The arcuate dimensions of teeth 64 may be varied to enlarge or decrease the size of spaces 70 into which projections 108 may be shifted. Moreover, the number of available gear ratios may be made more or less, and in some arrangements the number of grooves and shift keys required may be changed with corresponding changes in the change gears to receive the revised number of keys. For example, three axial grooves may be provided 120° apart for three shift keys and three teeth may be provided in gears 54 through 60 to define three arcuate key-receiving openings. Furthermore, the change gears and shifting arrangement could be provided on the counter shaft 34 rather than in the output shaft 36. 
     Referring now to FIGS. 1 and 3, four circular holes 163a, b, c and d are provided in plate 134 radially inwardly of detent seats 140, 144, 148 and 152 respectively. Operation of shift mechanism 118 to any neutral position wherein ball 156 engages one of seats 140, 144, 148 and 152 moves plate 134 such that the hole 163 radially inwardly of that one seat is directly below the actuator 164 of a neutral start safety switch 166 which is threaded into a hole 167 in cover 16. Actuator 164 in the form of a sphere, is retained within switch 166 for vertical movement and is spring biased downwardly to assume the position illustrated in FIG. 3 when hole 163a is aligned therewith (primary neutral position of transmission 10). With actuator 164 in this position, switch 166 forms a closed circuit between terminals 168 and 170 which is interlocked with the starting circuit to enable the engine which drives transmission 10 to be cranked. When shift mechanism 118 is operated from a neutral position to one of its gear-engaging positions (first, second, third, fourth or reverse), plate 134 forces actuator 164 upwardly out of the hole 163 with which it was engaged and maintains the actuator in retracted position in switch 166. When actuator 164 is so retracted, the circuit between terminals 168 and 170 opens to prevent the engine from being cranked. In the case of a riding lawnmover, garden tractor or the like, this safety feature prevents accidental lurching of the vehicle should it be attempted to start the engine with the transmission in other than the neutral position. 
     Moveover, should the engine stall when the transmission is in second, third or fourth gear, the provision of holes 163b, c and d permits the engine to be cranked when projections 108 are shifted to the immediately adjacent spaces 68 of one of the gears 54, 56 or 58. Thus, after such a stall, the likely angular misalignment of projections 108 with spaces 70 of the change gears lying between projections 108 and the space 68 of gear 60 which would prevent projections 108 from being conveniently shifted to the primary neutral position is inconsequential. It is to be noted that switch 166 is the only additional transmission part required to provide this feature. The spring biased actuator 164 and holes 163a, b, c and d also provide an additional detent arrangement for the neutral positions. Hence, it can be seen that detent seats 140, 144, 148 and 152 are optional whenever switch 166 is employed, and that various detent arrangements may provide either the same or different detent force for the neutral and gear-engaging positions. 
     Transmission 10 drives an output gear 172 on the left-hand external end of shaft 36. The load to be powered (such as the tractor axle) may be connected to gear 172 by a chain or other drive arrangement. It is also preferable to provide a clutch between the engine and input shaft 18 so that the engine may be disengaged when shifting gears. A circular rotor 174 is slidably keyed to the right-hand external end of shaft 36. If desired, a conventional disc brake assembly (not shown) may be attached to case 12 below shaft 36 so as to support a pair of brake pads on opposite sides of rotor 174. Such a disc brake usually has an actuator lever operated to cam the brake pads into gripping relation with rotor 174 to provide a braking torque on shaft 36, as is well understood in the art. 
     In the transmission thus described, change gears 54 through 60 are preferably constructed of a sintered metal comprising iron powder, which may have nickel content for improved impact strength. For example, one formulation which provides satisfactory results is as follows: 
     MATERIAL 
     NI--MN--MO Pre Alloy having the composition by weight of: 
     NI 0.45 percent 
     MO 0.60 percent 
     MN 0.35 percent 
     S 0.010 percent 
     P 0.010 percent 
     FE Balance 
     C 0.5/0.6 percent 
     Density 6.7 grams/c.c. (min.) 
     HEAT TREAT 
     Harden &amp; Draw 400°-600° F. 
     To RC 32-40 Apparent Hardness 
     Keys 104, 106 are preferably constructed from steel such as SAE 1117 steel and case hardened. Alternatively keys 104, 106 may be forged sintered metal. It has been observed that with this construction there is less chipping of teeth 64 than occurs when gears 54 through 60 are constructed from ordinary case hardened gear steel. Although the edges of teeth 64 and projections 108 are preferably beveled, beveling of teeth 64 may be unnecessary with the above-described preferred construction. Also, the four upright edges 186 of each projection 108 and the two outer upright edges 188 of each projection 114 are preferably beveled to reduce chipping when these edges 186, 188 contact the internal teeth of the change gears. Preferably, the corners are rounded to a radius of between 0.015 and 0.030 inch. It should be noted that the shaft keyways or grooves 92, 94 are entirely internal of the transmission enclosure and hence present no sealing problem between shaft 36 and the transmission housing when shaft 36 is journalled therein. In other words, portions 96 and 100 of shaft 36 are cylindrical and uninterrupted to fit closely within the associated bushings 38 so that grease cannot escape through the bushing. Moreover, because the moving parts, including the shift and safety switch actuating mechanisms, are all internal, they are protected from outside contamination. It can thus be seen that the invention provides a compact and low cost multispeed transmission which is of rugged construction. The transmission interior is preferably packed with lubricating grease after assembly so that minimum wear and long life of all moving parts are achieved. 
     FIG. 8 illustrates a modified transmission 10&#39; embodying this invention which has five forward speeds and one reverse speed, and which is generally similar in construction and operation to transmission 10. Transmission 10&#39; employs the same case 12 and cover 16 as transmission 10 and has a similar input shaft 18&#39; (not shown) and countershaft 34&#39; journalled therein. Five companion spur gears 200, 202, 204, 206 and 208 and a bevel gear 48&#39; integral with spur gear 208 are splined in coaxial side by side relation on countershaft 34&#39; for rotation therewith. Spur gears 200 through 208 correspond generally with spur gears 40 through 46 and bevel gear 48&#39; with bevel gear 48 of transmission 10. Bevel gear 48&#39; meshes with the bevel pinion gear 24 (not shown) fixed to input shaft 18&#39; so that the input shaft rotates all six gears 200 through 208 and 48&#39; in unison. 
     Companion gears 200 through 208 are in constant running mesh with five speed change spur gears 210, 212, 214, 216 and 218. Gears 210 through 218 are journalled in coaxial side by side relation for free rotation on a cylindrical portion 62&#39; of output shaft 36&#39;. Gears 210 through 218 are axially retained as a cluster or portion 62&#39; by a shoulder 63&#39; of output shaft 36&#39; and a thrust race or washer 65. As shown in FIGS. 11 and 12, each change gear 210 through 218 has an axial bore 220 therethrough adapted to receive cylindrical portion 62&#39; of shaft 36&#39; for free rotation of the gear thereon, and a counterbore 222 in one face thereof in which a flat washer 224 is located either by mere seating in the counterbore so as to be readily removable, or being fixed in the counterbore as by brazing or sintering to the gear face. Each change gear has four symmetrically arranged arcuate slots or free faces 228 in bore 220 which extend axially through the gear and terminate circumferentially at generally radially extending opposed end walls 230 and 232. Interposed between slots 228 are four integral teeth 64&#39; which in assembly slidably bear at their peripheral face on output shaft 36&#39;. Each free space or slot 228 has a generally arcuately extending step 234 therein and a shift key retaining slot 236 extending generally radially outwardly of step 234, the junction of step 234 with slot 236 being defined by an abutment wall 238 extending radially and axially of the gear. Washer 224 (FIG. 12) has a bore 239 with an inside diameter equal to that of bore 220 of the speed change gear and thus covers one axial end of each slot 228. 
     Gears 210 through 218 are selectively coupled for rotation with output shaft 36&#39; by two identical shift keys 104&#39; and 106&#39; each of which has a cantilever spring arm 240 yieldably biasing generally radially outwardly of shaft 36&#39;, a projection or lug 108&#39; located adjacent the free end of the key. Lugs 108&#39; each have camming ramps which slidably engage with washers 224 in response to axial movement of the key lug through the change gears 210-218 to selectively and individually connect and disconnect these change gears with shaft 36&#39;. Thus, as best seen in FIG. 15, each lug 108&#39; has two ramps 242 and 244 inclined toward each other, extending generally radially outwardly and convergently terminating in an apex 247. Each lug 108&#39; also has a ramp 246 having a steeper slope than ramp 244 and extending therefrom to a generally radially extending end face 248. Ramp 246 is located to facilitate rapid initial displacement of lugs 108&#39; and prevent end face 248 from butting on the face of a washer 224. 
     Preferably, keys 104&#39; and 106&#39; are machined or die stamped from flat stock with the entire bottom edge or face of the key including the bottom edge of arm 240 lying in one plane. After machining or blanking of the key, its arm 240 is bent at 250 (FIG. 15) to incline the arm with respect to the shank 251 (FIG. 15) of key 104&#39; so that it is inclined outwardly with respect to the axis of output shaft 36&#39; when in assembled relation therewith with shank surfaces 253 seated flat on the bottom surface 92&#39; or 94&#39; of its associated groove. Preferably, bend 250 is axially spaced from projection or lug 112 approximately .[.250.]. .Iadd.0.25 inch.Iaddend..[.inches.].. It has been found that if bend 250 is immediately adjacent projection 112, the key has a tendency to fracture in use, apparently due to localized high stresses produced by bending of the key aggravated by machining marks in the key from the machining of radius 252 adjacent projection 112. 
     Keys 104&#39; and 106&#39; are shifted axially back and forth in grooves 92 and 94 of shaft 36&#39; by a shifter mechanism similar to previously described shifter mechanism 118. Thus, a sector plate 134&#39; is fixed to shaft 120 which is connected to bifurcated shifter 122 operably coupled with collar 110. To correlate the rotary position of shaft 120 with the neutral and gear engaging positions of the shift keys, the outer generally arcuate edge 136&#39; of sector plate 134&#39; has a plurality of detent seats therein sequentially engageable with detent ball 156. Plate 134&#39; has a reverse detent seat 138&#39; but only one neutral detent seat 140&#39;. Unlike plate 134, plate 134&#39; has only one cylindrical hole 163a for actuating the neutral start safety switch 166, and it has five forward speed detents seats 254, 256, 258, 260 and 262 without any neutral detent seats therebetween. To facilitate the shifting of transmission 10&#39; from one gear to another, detent seats 254 through 260 each have two inclined ramps 264 and 266 associated therewith, and detent seat 262 has an inclined ramp 264 associated therewith. Detent ball 156, due to the bais of its spring and the shape of the seats in plate 134&#39; tends to be self-centering in the selected seats. The positions of shift keys 104&#39; and 106&#39; are correlated with the detentretained positions of plate 134&#39; so that when lugs 108&#39; are received in slots 228 of each speed change gear 210 through 218, the lugs are centered so they do not bear on the adjacent washers 224 (as shown in phantom in FIG. 14). This yieldable retention of keys 104&#39;, 106&#39; in centered position by detent ball 156 in the associated seat of plate 134&#39; prevents a wearing away of the lugs or projections 108&#39; which might otherwise occur if they were allowed to have a sliding frictional engagement with washer 224 of the adjacent speed change gear, which rotates relative to the shift key when transmission 10&#39; is in use. 
     In use, transmission 10&#39; is shifted from one gear to another by the rotation of control shaft 120 which shifts keys 104&#39; and 106&#39; axially in slots 92 and 94 respectively of shaft 36&#39; while engaging and disengaging detent 156 with the various detent seats in plate 134&#39;. When detent 156 is received in detent seat 140&#39; of sector plate 134&#39;, transmission 10&#39; is in neutral and shift keys 104&#39; and 106&#39; are in the position shown in FIG. 8. Rotation of shaft 120 clockwise (as viewed in FIG. 8) to engage detent 156 with reverse detent seat 138&#39; moves shift keys 104&#39; and 106&#39; axially to the right (as viewed in FIG. 8) and engages key lug 114 with sprocket 78, thereby coupling sprocket 78 with output shaft 36&#39; to effect reverse drive of the output shaft 36&#39;. 
     Rotation of shaft 120 counterclockwise from reverse or neutral position so that detent 156 engages in detent seat 254 moves shift keys 104&#39; and 106&#39; axially to the left to couple speed change gear 218 with output shaft 36&#39; to effect forward drive of the output shaft in first gear. During this axial movement of keys 104&#39; and 106&#39; to the left, ramps 246 and 244 of each lug 108&#39; will sequentially engage the washer 224 associated with gear 218 and thereby cam the lugs 108&#39; radially inwardly, the resilient spring arms 240 of the associated keys deflecting as they yieldably resist such camming action. After apex 247 has cleared washer 224, arms 240 force lugs 108&#39; radially outwardly into slots 228 thereof if or when the lugs register therewith (as shown in broken line in FIG. 14). In some instances, when lugs 108&#39; are shifted axially into registry with the bore of a gear they will first bear on the peripheral face of teeth portions 64&#39; between slots 228 of the gear and then will spring radially outwardly when the closest slot 228 is brought into angular registry with the lug as the gear rotates clockwise (as viewed in FIG. 10) with respect to shaft 36&#39;. Thus, each lug 108&#39; will be sprung by its associated arm 240 into a slot 228 adajcent end wall 232, strike against and momentarily slidably bear on step 234, and then be sprung further radially outwardly into the associated retaining slot 236 where the lugs are snugly received between generally opposed radial walls 230 and 238 to thereby couple the gear with the output shaft. However, in the event that lugs 108&#39; of the keys are axially shifted into registry with gear 218 precisely when one of its slots 228 is aligned with the keys, they will spring directly into the slots without first bearing on teeth portions 64&#39;. When this occurs, the rate of radially outward movement of lugs 108&#39; into slots 228 is controlled by the engagement of ramps 242 of the lugs with the inner edge of a washer 224 alone or in cooperation with step 234 of the slot. 
     In a similar manner, rotation of shaft 120 so that detent 256 sequentially engages with detent seats 254 through 264 in sector plate 134&#39; moves keys 104&#39; and 106&#39; axially to the left (FIG. 8) to sequentially engage lugs 108&#39; of the keys in an opposed pair of slots 228 of speed change gears 216 through 210 to sequentially couple each of these gears with output shaft 36&#39;, thereby providing the second through fifth forward speeds of transmission 10&#39;. 
     Whenever transmission 10&#39; is shifted from one forward gear to another, complete disengagement of the lugs 108&#39; of the shift keys from one speed change gear before reengagement of the lugs with another speed change gear is assured by the cooperation of ramps 242, 244 and 246 of the lugs with washers 224 of the speed change gears. For example, when transmission 10&#39; is shifted from the first forward speed change gear 218 to the second forward speed change gear 216, ramps 246 and 244 sequentially bear on and slide under washer 224 of gear 216, thus camming lugs 108&#39; of the shift keys radially inwardly to completely disengage them from slots 228 of gear 218 before the lugs can begin to move radially outwardly to engage in the slots of gear 216. Likewise, when transmission 10&#39; is shifted back in the other direction from second gear to first gear, ramps 242 bear on washer 224 of gear 216 to cam lugs 108&#39; of the shift keys radially inward to completely disengage the lugs from slots 228 of gear 216 before the lugs can begin to move radially outward to engage in the slots 228 of gear 218. 
     It is also to be noted that transmission 10&#39;, unlike transmission 10, can always be readily shifted from neutral into any forward gear or from any forward gear to another forward gear regardless of the angular relationship of speed change gears 210 through 218 to each other. This is due to the provision of yieldable spring arms 240 on keys 104&#39; and 106&#39; which permit lugs 108&#39; during shifting of the keys to move radially inwardly to retract and disengage from slots 228, rather than, as in transmission 10, sliding through the slots in the gears which in many instances are not axially aligned with each other and thus block passage of the keys axially through the gears. Thus, transmission 10&#39; can be readily shifted through the forward gears without having to jog or slightly rotate the output shaft of the transmission to align the shift keys with the slots in the gears. 
     To provide adequate time for lugs 108&#39; of shift keys 104&#39; and 106&#39; to move radially outward and become fully seated or bottomed in retaining slots 236 of the speed change gears, the arcuate length of slots 228 is greater than the transverse width or thickness of lugs 108&#39;. Preferably, the arcuate length of slots 228 is in the range of 11/4 to 2 times and preferably substantially 11/2 times the thickness (the vertical dimension of lug 108&#39; as seen in FIG. 13) of the portion of lugs 108&#39; received in the slots. However, so increasing the arcuate length of the slots would introduce excessive backlash between output shaft 36&#39; and the speed change gear driving the output shaft were it not for the provision of step 234 in each slot. Wall 230 and the riser wall 238 of step 234 thus form a shorter retaining slot 236 and provide more closely spaced opposed abutments for receiving lugs 108&#39; therebetween. Preferably, the distance between walls 230 and 238 is just sufficiently greater than the thickness of lugs 108&#39; to provide sufficient clearance for the lugs to slide into retaining slots 236. Hence, any backlash due to relative angular movement between output shaft 36&#39; and the speed change gear is reduced to an imperceptible amount once lugs 108&#39; of the shift keys are received in retaining slots 236. Accordingly, it will now be appreciated that stepped slots 228 provide both adequate time for the lugs to become fully engaged in slots 236 while simultaneously eliminating backlash in transmission 10&#39;. Moreover, steps 234 in slots 228 ease the entry of lugs 108&#39; into retainer slots 236, thereby minimizing the impact and resulting shock with which the lugs bottom in slot 236, which in turn is believed to minimize wear and tear on the lugs of shift keys 104&#39; and 106&#39; and thereby substantially increase their useful life. 
     Another advantage of transmission 10&#39; is the very positive 37 feel&#34; afforded the operator when manually shifting through the forward drive shift points of transmission 10&#39;. This yieldable resistance to shifting is provided either entirely by the action of spring keys 104&#39;, 106&#39; or in conjunction with the ramps 264 and 266 of sector plate 134&#39; which one associated with the speed change gears 210 through 218. These ramps function during shifting as over-center cams tending to initially retard and then subsequently urge shifting mechanism 118&#39; of transmission 10&#39; from one gear to an immediately adjacent gear. This positive feel is provided even in the absence of the detent seats by the camming of projections or lugs 108&#39; of shift keys 104&#39;, 106&#39; over a washer 224 every time transmission 10&#39; is shifted from one forward gear into another, neutral into a forward gear, or a forward gear into neutral. However, if the detent seats are eliminated from plate 134&#39;, it is preferable to modify edge 136&#39; thereof so as to be a smooth segment of a spiral to yieldably bias shift keys 104&#39;, 106&#39; generally axially to the right (as viewed in FIG. 8) so the projections 108&#39; thereof bear against the washer 224 of the engaged gear and, hence, will not bear on the washer 224 of the speed change gear immediately to the left which will be rotating relative to the engaged gear. This modification will thus prevent a wearing away of lugs 108&#39; and/or washers 224 by sliding frictional engagement therebetween. The biasing force thus developed on shift keys 104&#39; and 106&#39; must not, however, produce sufficient force to cam lugs 108&#39; radially inwardly and thereby unintentionally decouple the output shaft 36&#39; from the driving gear. 
     In the transmissions thus described, speed change gears 54 through 60 and 210 through 218 are preferably constructed of a sintered metal comprising iron powder, which may have nickel content for improved impact strength. One exemplary formulation which provides satisfactory results has been set forth previously herein. 
     In accordance with another feature of transmission 10&#39;, speed change gears 210 through 218 are preferably made by powdered metallurgy techniques and counterbore 222 is formed during compacting of the powdered metal compacts from which the sintered metal gears are made. It has been found that, in addition to eliminating subsequent machining, configurating the compacting dies to produce the counterbore in the green compact produces a higher density of the powdered metal in the area of teeth 64&#39; and slots 228 which in turn increases the strength and impact resistance in this area of the sintered metal gear. 
     In addition, when speed change gears 210 through 218 are made by powdered metallurgy techniques, washers 224 can be pre-coated with brazing material and placed as loose inserts directly into the cavity of the gear compacting mold. When the compacts are subsequently sintered, washers 224 will securely adhere to the sintered metal gear. Preferably, the washers 224 received in counterbore 222 of gears 210 through 218 are made of a high carbon steel such as SAE 1075 steel and case hardened. It also has been observed that there is less chipping of teeth 64 of gears 54 through 60 when constructed of sintered metal rather than ordinary case hardened gear steel. Although the edges of teeth 64 and lugs 108 are preferably beveled, beveling of teeth 64 may be unnecessary with the above-described preferred construction. 
     Shift keys 104, 106 and 104&#39; and 106&#39; are preferably constructed from steel such as SAE 1117 steel and case hardened. Alternatively, the shift keys may be forged sintered metal. Also, the two outer upright edges 188 of each projection or lug 114 of shift keys 104&#39; and 106&#39;, like keys 104 and 106, are preferably beveled to reduce chipping when edges 188 engage the speed change gears. Preferably, the corners are beveled by being rounded to a radius of between 0.015 and 0.030 inch. 
     It should be noted that the keyways or grooves 92, 94 in output shaft 36&#39; of transmission 10&#39; are entirely internal of the transmission enclosure and, hence, present no sealing problem between the output shaft and its associated transmission housings when the shaft is journalled therein. Transmission 10&#39; also retains the other advantages of transmission 10, such as protection of the moving parts, including the shift and safety switch actuating mechanisms, from outside contamination, a compact low-cost rugged construction, minimum wear and long life of all moving parts.