Toothed gear transmission having two partial transmissions disposed parallel to each other in the power flow

The invention is based on a toothed gear transmission in which an input shaft is connected to an output shaft by a first and a second partial transmission, which are disposed parallel to each other in the power flow and which both have a frictionally engaged load shifting clutch and an intermediate shaft, in which each of the intermediate shafts can be effectively connected to the input shaft through the load shifting clutch, and with at least one first transmission constant. It is proposed that for starting, the first load shifting clutch can be placed in effective connection with the output shaft through a transmission constant and the second load shifting clutch can be placed in effective connection with the output shaft through a transmission constant, each at a gear ratio of the lowest gear speed.

FIELD OF THE INVENTION
 The invention relates to a toothed gear transmission having two partial
 transmissions disposed parallel to each other in the power flow as well as
 a process for shifting such toothed gear transmissions.
 BACKGROUND
 The manual transmissions with a friction clutch which are usually used in
 motor vehicles are simple and sturdy, but they have the disadvantage that
 an interruption in tractive power occurs during gear changes. Conventional
 automatic transmissions with torque converters avoid an interruption of
 tractive power during gear changes, but due to the torque converter they
 are less efficient than manual transmissions. Moreover, they are heavier
 and more cost-intensive.
 In toothed gear transmissions having two frictionally engaged load shifting
 clutches and two partial transmissions disposed parallel to each other in
 the power flow, it is possible to change gears under load, which is done
 with an overlap control of the load shifting clutches. This makes it
 possible to achieve an inexpensive automatic transmission having good
 efficiency. Generally speaking, the even gear speeds are allocated to one
 partial transmission and the odd gear speeds are allocated to the other
 partial transmission, so that shifting can be done sequentially under
 load, although individual gear speeds cannot be skipped without further
 action, so-called double upshifting or double downshifting.
 A generic toothed gear transmission is known from U.S. Pat. No. 4,658,663.
 In the toothed gear transmission, an input shaft is connected to an output
 shaft through a first and a second partial transmission, which are
 disposed parallel to each other in the power flow and which both have a
 frictionally engaged load shifting clutch. The load shifting clutches can
 be used to connect concentrically arranged intermediate shafts of the
 partial transmissions with the input shaft. The intermediate shafts can be
 effectively connected with the output shaft through transmission
 constants, through a joint countershaft disposed parallel to the input
 shaft, and through toothed gear pairs. The toothed gear pairs for the
 forward gear speeds I, III, and IV, as well as the toothed gear pair for
 the reverse gear speed, belong to the first partial transmission. The
 fourth gear speed is designed as a direct gear speed and can be selected
 by engaging a shifting component between the output shaft and the
 intermediate shaft of the first partial transmission.
 The second partial transmission comprises the toothed gear pairs of forward
 gear speeds II and V, as well as an additional toothed gear pair, whose
 gear ratio is equal to that of the third gear speed, whose toothed gear
 pair belongs to the first partial transmission. In the first partial
 transmission, a toothed gear clutch for the third gear speed and the
 direct clutch for the fourth gear speed are combined into a change clutch
 having a joint synchronizer sleeve, which can be coupled with an idler
 gear of the toothed gear pair for the third gear speed or with the output
 shaft.
 The load shifting clutches of the partial transmissions are alternately
 allocated to the gear speeds. The first load shifting clutch of the first
 partial transmission is always used to start from the first gear speed,
 which causes greater wear to occur on it than on the second load shifting
 clutch.
 SUMMARY OF THE INVENTION
 The aim of the invention is to further develop a generic toothed gear
 transmission and in particular to achieve longer maintenance intervals,
 less wear, and greater security. It is fulfilled in accordance with the
 invention by the features of the independent claims. Additional
 embodiments are contained in the subclaims.
 The invention is based on a toothed gear transmission in which an input
 shaft is connected to an output shaft by a first and a second partial
 transmission, which are disposed parallel to each other in the power flow.
 The partial transmissions each have a frictionally engaged load shifting
 clutch and an intermediate shaft, and each of the intermediate shafts can
 be effectively connected to the input shaft through the load shifting
 clutch. In addition, the toothed gear transmission has at least one first
 transmission constant.
 It is proposed that for starting, the first load shifting clutch can be
 brought in effective connection with the output shaft through a
 transmission constant, and the second load shifting clutch can be brought
 in effective connection with the output shaft through a transmission
 constant having the gearing of the lowest gear speed. Both load shifting
 clutches can be used for the process of starting from the lowest or first
 gear speed. The stress resulting from the starting process can be
 distributed to both load shifting clutches. An excess temperature can be
 avoided in spite of inexpensive dimensioning, wear can be reduced, and
 maintenance intervals can be extended. Moreover, if one load shifting
 clutch fails, the second load shifting clutch can be used to start from
 the first gear speed, increasing mobility security. It is also possible to
 construct the load shifting clutches differently for different starting
 processes.
 The solution in accordance with the invention can be used in a toothed gear
 transmission in which the intermediate shafts can be connected to an
 output shaft through at least one countershaft and through at least one
 toothed gear pair. In addition, the solution in accordance with the
 invention can also be used in a toothed gear transmission in which the
 intermediate shafts having at least one transmission constant are directly
 connected to at least one output shaft. Toothed gear transmissions with a
 countershaft are generally used in motor vehicles having rear-wheel drive,
 and toothed gear transmissions without countershafts are generally used in
 motor vehicles having front-wheel drive.
 It is particularly advantageous for it to be possible to connect the first
 and the second load shifting clutches simultaneously to the output shaft
 with the gear ratio of the lowest gear speed. This allows the load
 shifting clutches to be dimensioned so they are particularly inexpensive
 and light.
 The solution in accordance with the invention can be achieved by various
 embodiments appearing appropriate to those skilled in the art, for example
 with two equal transmission constants, where one transmission constant is
 allocated to the first intermediate shaft and one transmission constant is
 allocated to the second intermediate shaft, and where output to an output
 shaft can take place through the transmission constants, through at least
 one countershaft, and through a common toothed gear pair or through two
 equal toothed gear pairs, or where output can take place through two equal
 transmission constants directly onto at least one output shaft. Moreover,
 with different transmission constants it is possible to create two first
 gear speeds using different toothed gear pairs.
 However, it is particularly advantageous for it to be possible to place at
 least the transmission constant allocated to the lowest gear speed in
 effective connection with the first and the second load shifting clutches.
 Both load shifting clutches can be connected to the output shaft through
 the same transmission constants and through the same toothed gear pair.
 Additional toothed gear pairs, weight, and construction space, as well as
 assembly effort and costs, can be saved, and greater efficiency can be
 achieved due to low weight and a low rotating mass. Moreover, the load
 shifting capabilities can be increased and, with a small number of toothed
 pairs, in addition to sequential shifting at least the most important
 double upshifts and double downshifts under load are made possible. Double
 upshifts and double downshifts are particularly necessary before and after
 a passing maneuver.
 For various reasons, it may be useful to start from the second or an even
 higher gear speed instead of from first gear, for instance when slowly
 starting with a high-power internal combustion engine, when starting on
 slippery ground, on slopes, and/or when starting with little or no loaded
 weight, etc.
 High stresses can occur on a load shifting clutch, particularly when
 starting from a higher gear speed. In order to avoid the resulting
 one-sided load and increased wear, it is advantageous for it to be
 possible to place the first load shifting clutch in effective connection
 with the output shaft through a transmission constant and the second load
 shifting clutch in effective connection with the output shaft through a
 transmission constant to start with a gear ratio of the second-lowest gear
 speed or of the second gear speed. It is fundamentally also conceivable
 that it be possible to connect both load shifting clutches to the output
 shaft to start with a gear ratio of an even higher gear speed.
 If the first and second load shifting clutches can be simultaneously
 connected to the output shaft with the gear ratio of the second gear
 speed, the load shifting clutches can be dimensioned to be particularly
 inexpensive and light.
 Simultaneous connection can advantageously be made possible in that a
 toothed gear of the first transmission constant is disposed on the first
 intermediate shaft and a toothed gear of at least a second transmission
 constant is disposed on the second intermediate shaft, and in that the
 first intermediate shaft can be placed in effective connection with the
 output shaft through a shifting component disposed between the toothed
 gears and through the second transmission constant. Additional toothed
 gear pairs, construction space, and costs can be saved.
 If one toothed gear pair is allocated to the first and the second gear
 speed, one toothed gear pair can be used to start from the first and to
 start from the second gear speed, and it can be dimensioned accordingly.
 Moreover, particularly in a neutral position, the shifting components for
 the second gear speed can be preselected, so that after starting from the
 first gear speed it is possible to shift from the first to the second gear
 speed solely by an overlap control of the load shifting clutches. The
 change of gears from the first to the second gear speed, which usually
 occurs under high load, can be completed with particular comfort.
 In one embodiment of the invention, it is proposed that it be possible to
 connect the intermediate shaft of the first partial transmission through
 the first transmission constant to a second countershaft which is parallel
 to the input shaft and which can be effectively connected to the output
 shaft through at least one toothed gear pair allocated to the first
 partial transmission, and that the first transmission constant can
 optionally be brought in effective connection with the first, the second,
 or both load shifting clutches. In addition to using both load shifting
 clutches to start from the first gear speed through the first transmission
 constant, it is possible to achieve an advantageous transmission
 arrangement having a small number of toothed gear pairs and a large number
 of load shifting capabilities in which, in addition to sequential
 shifting, the most important double upshifts and double downshifts are
 possible under load.
 If the output shaft can be connected to the intermediate shaft of the first
 partial transmission by an engageable and disengageable shifting component
 to form a direct gear speed, three torque paths between the input and
 output shafts can advantageously be created by the connectable second
 countershaft. The torque paths can be used alternately and in particular
 can make possible double downshifting from the direct gear speed and
 double upshifting to the direct gear speed under load without an
 additional toothed gear pair.
 Moreover, a space-saving toothed gear transmission having few toothed gear
 pairs and a high number of load shifting capabilities, in particular with
 double upshifting and double downshifting capabilities under load, can be
 achieved in that, in addition to the first transmission constant on the
 first intermediate shaft, at least a second transmission constant
 different from it is disposed on the second intermediate shaft and at
 least one transmission constant can be placed in effective connection with
 the output shaft through both countershafts. It is possible to start from
 multiple gear speeds optionally with the first, the second, or both load
 shifting clutches. It is particularly advantageous that each transmission
 constant can be effectively connected to each countershaft, so that each
 toothed gear pair can be placed in effective connection with each
 transmission constant. Each toothed gear pair can have at least two gear
 speeds allocated to it, or, with more than two transmission constants,
 more than two gear speeds can be allocated to it.
 An advantageous embodiment for a broad range of applications has, in
 addition to two transmission constants, three toothed gear pairs for six
 different forward gear speeds and one toothed gear for at least one
 reverse gear speed. One toothed gear of the toothed gear pair allocated to
 the fourth and sixth gear speeds is disposed on the second countershaft,
 and one toothed gear of the toothed gear pair allocated to the third and
 fourth gear speeds and one toothed gear of the toothed gear pair allocated
 to the first and second gear speeds are disposed on the first
 countershaft.
 An advantageous variant also consists of having a one toothed gear of the
 toothed gear pair allocated to the third and sixth gear speeds disposed on
 the second countershaft and one toothed gear of the toothed gear pair
 allocated to the second and fourth [gear speeds] and one toothed gear of
 the toothed gear pair allocated to the first and second toothed [gear
 speeds] disposed on the first countershaft. A first gear speed having a
 high gear ratio can advantageously be achieved, for example for use in an
 internal combustion engine with a turbocharger, in an internal combustion
 engine with low power, in an all-terrain vehicle, etc.
 The load shifting clutches are advantageously used individually or together
 for starting as a function of at least one operating parameter. In one
 embodiment, it is proposed that the difference in rotational speed on at
 least one, advantageously both load shifting clutches and the power to be
 transferred be recorded, and that the load shifting clutches be used
 individually or together to start, depending on the difference in
 rotational speed and the power to be transferred. If starting is to be
 done with a high difference in rotational speed with high power, it is
 advantageous to use both load shifting clutches. If starting is done with
 a low difference in rotational speed, it is possible to start solely with
 the first or with the second load shifting clutch, which can be designed
 differently for different loads. The load shifting clutches can be
 dimensioned to be particularly inexpensive and light and to be operated
 with low wear.
 Moreover, it is advantageous for the weight of the motor vehicle, including
 cargo, and/or the slope of a route driven by the vehicle to be recorded,
 and for the load shifting clutches to be used individually or together for
 starting depending on the weight and/or slope. When starting on a steep
 slope and/or with heavy cargo, it is advantageous to use both load
 shifting clutches for starting.
 As the temperature of the load shifting clutch rises, wear generally
 increases. To reduce wear, in accordance with the invention the
 temperature on at least one, advantageously on both load shifting clutches
 is recorded. Depending on the temperature of the load shifting clutches,
 they are used individually or together for starting. If the temperature of
 one load shifting clutch rises over a specific value, for example due to
 frequent starting in traffic at a high ambient temperature, the load
 shifting clutches can be used alternately or jointly. The heat energy can
 advantageously be dissipated through both load shifting clutches, an
 increased temperature in only one load shifting clutch can be avoided, and
 wear can be reduced.
 To achieve the longest possible maintenance intervals, it is proposed that
 the wear on the load shifting clutches be recorded and that the load
 shifting clutches be used individually or together for starting as a
 function of the wear, with the load shifting clutches being used to start
 in such a way that at least to a certain degree, equal wear occurs in both
 load shifting clutches. Replacement is not necessary until both load
 shifting clutches have been mostly worn. Particularly long maintenance
 intervals can be achieved. Moreover, during maintenance, both load
 shifting clutches can advantageously be replaced at the same time, saving
 effort and cost.
 To avoid having both load shifting clutches simultaneously become
 completely worn and simultaneously fail, it is advantageous for only one
 of the two load shifting clutches to be used for starting after a certain
 amount of wear. This creates a type of safety buffer, so that after
 failure of one load shifting clutch a driver can safely proceed to the
 closest shop using the clutch that has not yet been fully worn. A display
 advantageously shows the driver that only one load shifting clutch is
 being used for the starting process. The driver can then adapt his driving
 practices and get to a shop in sufficient time.
 Additional advantages are indicated in the following description of the
 drawings. The drawings show a sample embodiment of the invention. The
 description and the claims contain numerous features in combination. Those
 skilled in the art will also find it beneficial to consider these features
 individually and to combine them into useful additional combinations.

DETAILED DESCRIPTION
 FIG. 1 shows a toothed gear transmission in accordance with the invention
 in which an input shaft 10 is connected to an output shaft 20 through a
 first partial transmission 22 and a second partial transmission 24, which
 are disposed parallel to each other in the power flow. Partial
 transmissions 22, 24 each have a frictionally engaged load shifting K1, K2
 and an intermediate shaft 12, 14. The intermediate shafts 12, 14 are
 disposed concentric to each other and coaxially to input shaft 10, and can
 each be effectively connected through load shifting clutches K1, K2 to
 input shaft 10.
 First intermediate shaft 12 of first partial transmission 22 can be
 effectively connected, on the one hand to output shaft 20 by a toothed
 gear clutch S2, which can be engaged and disengaged to form a direct fifth
 gear speed V, and on the other hand by toothed gear clutch S2 through a
 first transmission constant C1 to a countershaft 18 which is constructed
 as a hollow shaft and which is parallel to output shaft 20.
 Countershaft 18 of first partial transmission 22 is connected to output
 shaft 20 by a toothed gear pair ZIV/VI to form the highest and the
 second-highest gear speeds IV, VI. Toothed gear pair ZIV/VI has a fixed
 gear 52 which is connected to output shaft 20 in such a way that it cannot
 rotate and which meshes with idler gear 30. Idler gear 30 is disposed
 concentrically on countershaft 18 so that it can rotate, and can be
 coupled to countershaft 18 by toothed gear clutch S5, which can be engaged
 and disengaged. Intermediate shaft 12 is supported in fixed gear 56, which
 can in particular save construction length.
 Moreover, countershaft 18 of first partial transmission 22 can be
 effectively connected to output shaft 20 through a toothed gear pair
 ZIII/IV to form the third and the fourth gear speeds III, IV. Toothed gear
 pair ZIII/IV has a fixed wheel 36, which is disposed concentrically on
 output shaft so that it cannot rotate, and an idler gear 32, which is
 disposed concentrically so that it can rotate on an additional
 countershaft 16 parallel to output shaft 20. Idler gear 32 can be coupled
 to countershaft 18 through engageable and disengageable toothed gear
 clutch S5, and to countershaft 16 through toothed gear clutch S6.
 Second intermediate shaft 14 of second partial transmission 24 is
 constructed as a hollow shaft and is connected through a second
 transmission constant C2 to countershaft 16, which passes through
 countershaft 18 of first partial transmission 22 with play. Second
 transmission constant C2 has an idler gear 28 which is disposed
 concentrically so that it can rotate on intermediate shaft 14 and which
 can be connected through a toothed gear clutch S1 to intermediate shaft
 14, and a fixed gear 38 which is connected concentrically to countershaft
 16 so that it cannot rotate. Countershaft 16 belonging to second partial
 transmission 24 is connected to output shaft 20 through toothed gear pair
 ZIII/IV for the third and fourth gear speeds III, IV, through a toothed
 gear pair ZI/II for the first and second gear speeds I, II, and through a
 toothed gear pair ZR for two reverse gear speeds R.
 Toothed gear pair ZI/II for first and second gear speeds I, II has an idler
 gear 40 which is disposed concentrically on output shaft 20 so that it can
 rotate and which can be coupled to output shaft 20 with toothed gear
 clutch S3, and a fixed gear 34 disposed concentrically on countershaft 16
 so that it cannot rotate.
 Toothed gear pair ZR for reverse gear speed R has an idler gear 42 which is
 disposed concentrically on output shaft 20 so that it can rotate and which
 can be coupled to output shaft 20 through engageable and disengageable
 toothed gear clutch S3, and a fixed gear 44 that is connected
 concentrically with countershaft 16 so that it cannot rotate. A toothed
 gear for reversing the direction of rotation, not shown in greater detail,
 is disposed between idler gear 42 and fixed gear 44 of toothed gear pair
 ZR.
 First transmission constant C1 has a fixed gear 46 which is disposed
 concentrically on countershaft 16 so that it cannot rotate and which
 meshes with an idler gear 26 disposed concentrically so that it can rotate
 on intermediate shaft 12 of first partial transmission 22. Idler gear 26
 can be connected by the engageable and disengageable toothed wheel clutch
 S2 to intermediate shaft 12 of first partial transmission 22 and through
 toothed gear clutch S1 to intermediate shaft 14 of second partial
 transmission 24. Second load shifting clutch K2 or second intermediate
 shaft 14 is thereby effectively connectable to both second transmission
 constant C2 and first transmission constant C1. First transmission
 constant C1 can optionally be placed in effective connection with the
 first, the second, or both load shifting clutches K1, K2 by toothed gear
 clutches S1 and S2.
 Moreover, intermediate shaft 14 of second partial transmission 24 can be
 effectively connected to fixed gear 46 of transmission constant C1 and
 therefore to countershaft 18 of first partial transmission 22 through
 second transmission constant C2 by an engageable and disengageable toothed
 gear clutch S4, and to output shaft 20 through toothed gear pair ZIV/VI
 allocated to first partial transmission 22. Intermediate shaft 12 can
 advantageously be effectively connected to countershaft 16 through first
 transmission constant C1 by engageable and disengageable toothed gear
 clutch S4, and to output shaft 20 through toothed gear pairs ZIII/IV,
 ZI/II, ZR allocated to second partial transmission 24. Each transmission
 constant C1 and C2 can be used to output through each toothed gear pair
 ZI/II, ZIII/IV, ZIV/VI, ZR to output shaft 20. Transmission constants C1
 and C2 are different, so that each toothed gear pair ZI/II, ZIII/IV,
 ZIV/VI, ZR is allocated to two gear speeds.
 FIG. 2 shows the toothed gear transmission during the process of starting
 from first gear speed I with first load shifting clutch K1. Load shifting
 clutch K1 is closed, toothed gear clutch S2 couples idler gear 26 of first
 transmission constant C1 to intermediate shaft 12, toothed gear clutch S4
 couples transmission constant C1 with countershaft 16, and toothed gear
 clutch S3 couples idler gear 40 of toothed gear pair ZI/II for first and
 second gear speeds I, II to output shaft 20. The power flow runs from
 input shaft 10 through load shifting clutch K1, intermediate shaft 12,
 toothed gear clutch S2, transmission constant C1, toothed gear clutch S4,
 countershaft 16, toothed gear pair ZI/II, and through toothed gear clutch
 S3 to output shaft 20.
 Idler gear 28 of second transmission constant C2 is coupled through toothed
 gear clutch S1 with intermediate shaft 14. Second gear speed II is
 preselected and can be shifted with particular comfort merely by an
 overlap control or by closing second load shifting clutch K2 and by
 opening first load shifting clutch K1 under load.
 FIG. 3 shows the toothed gear transmission during the process of starting
 from first gear speed I with second load shifting clutch K2. Load shifting
 clutch K2 is closed, toothed gear clutch S1 couples intermediate shaft 14
 with idler gear 26 of first transmission constant C1, toothed gear clutch
 S4 couples transmission constant C1 to countershaft 16, and toothed gear
 clutch S3 couples idler gear 40 of toothed gear pair ZI/II to output shaft
 20. The power flow runs from input shaft 10 through second load shifting
 clutch K2, intermediate shaft 14, toothed gear clutch S1, transmission
 constant C1, toothed gear clutch S4, countershaft 16, toothed gear pair
 ZI/II, and through the toothed gear clutch S3 to output shaft 20.
 FIG. 4 shows the toothed gear transmission in the process of starting from
 the first gear speed I with first and second load shifting clutches K1,
 K2. Load shifting clutches K1, K2 are closed, toothed gear clutch S1
 couples intermediate shaft 14, and toothed gear clutch S2 couples
 intermediate shaft 12 to idler gear 26 of transmission constant C1.
 Toothed gear clutch S4 couples transmission constant C1 to countershaft 16
 and toothed gear clutch S3 couples idler gear 40 of toothed gear pair
 ZI/II to output shaft 20. The power flow runs from input shaft 10 through
 load shifting clutches K1, K2, through intermediate shafts 12, 14, through
 toothed gear clutches S1, S2, transmission constant C1, toothed gear
 clutch S4, countershaft 16, toothed gear pair ZI/II, and through toothed
 gear clutch S3 to output shaft 20. If the difference in rotational speed
 in the load shifting clutches K1, K2 drops to zero, load shifting clutch
 K2 is opened.
 For starting, both load shifting clutches K1 and K2 can be effectively
 connected to output shaft 20 through the same transmission constant C1 and
 through the same toothed gear pair ZI/II. As a function of operating
 parameters, the first and second load shifting clutches K1, K2 can
 optionally be used individually or together to start from first gear speed
 I. If second load shifting clutch K2 is used alone for starting from first
 gear speed I, it is possible to shift into second gear speed II without
 interruption of tractive force by transferring to load shifting clutch K1,
 or to shift directly with interruption of tractive force.
 FIG. 5 shows the toothed gear transmission in the process of starting from
 second gear speed II with both load shifting clutches K1, K2. Both load
 shifting clutches K1, K2 are closed, toothed gear clutch S2 couples idler
 gear 26 of first transmission constant C1 to intermediate shaft 12,
 toothed gear clutch S1 couples idler gear 26 of first transmission
 constant C1 and idler gear 28 of second transmission constant C2 to
 intermediate shaft 14. Toothed gear clutch S3 couples idler gear 40 of
 toothed gear pair ZI/II to output shaft 20. The power flow runs from input
 shaft 10 through load shifting clutches K1, K2, intermediate shafts 12,
 14, toothed gear clutches S1, S2, transmission constant C2, countershaft
 16, toothed gear pair ZI/II and through toothed gear clutch S3 to output
 shaft 20. Toothed gear clutch S1 comprises two parts 48, 50 that are
 connected together and can slide into each other. Moreover, various
 constructions are possible that those skilled in the art will recognize as
 useful, in which the two idler gears 26, 28 of transmission constants C1,
 C2 can effectively be connected to intermediate shaft 14.
 FIG. 6 shows a summary of possible gear changes under load, in each case
 from a source speed Q to a target speed Z. The possible gear changes are
 marked with X. The respective shifted toothed gear clutches are listed
 under GS. If, in addition to a neutral position of the toothed gear
 clutch, two shifting positions are possible, the coupled transmission
 constant or the coupled toothed gear pair is specified for each toothed
 gear clutch. With two transmission constants C1 and C2 and three toothed
 gear pairs ZI/II, ZIII/IV, ZIV/VI, six different forward gear speeds I,
 II, III, IV, V, VI can be achieved, in which the first gear speed
 I.sub.1,2 occurs twice, third gear speed III.sub.1,2 occurs twice, and the
 fourth gear speed IV.sub.1,2,3 occurs three times.
 From gear speeds I.sub.1, III.sub.1, and V, it is possible to shift with an
 overlap control from load shifting clutch K1 to K2 sequentially under load
 respectively into gear speeds II, IV.sub.1, VI and vice versa. Moreover,
 important double upshifts and double downshifts under load are possible,
 in the form of double downshifting from VI to IV.sub.2, from V to
 III.sub.2, and from IV.sub.2 to II and double upshifting from II to
 IV.sub.2, from III.sub.2 to V, and from IV.sub.2 to VI. In addition,
 further multiple shifts under load are possible, which are marked with X.
 Before shifting from fourth gear speed IV.sub.1,2,3 to a target speed, the
 fourth gear speed IV.sub.1,2,3 under load is always selected, from which
 shifting to target speed under load is possible. The same applies to the
 third gear speed III.sub.1,2.