MXPA98009499A - System and method for change assisted by mecani transmission fluid - Google Patents

Abstract

The present invention relates to a shift mechanism for a mechanical transmission having a plurality of selectable gear ratios, the gear mechanism comprising: a selector wheel having a plurality of shift forks placed around it, wherein the rotation of the selector arrow selects one of the shift forks and the translation of the selector arrow is operative to move the selected shift fork to link or de-link a selected gear ratio, a control rod dynamically coupled to the selector arrow for positive rotation with it while allowing a predetermined relative axial translation between them, a cylinder assembly operatively associated with the selector arrow and the control rod providing a fluid coupling therebetween, to apply a force on the selector arrow in response to the relative translation between the selector arrow and the rod of contra

Description

SYSTEM AND METHOD FOR CHANGE ASSISTED BY MECHANICAL TRANSMISSION FLUID Technical Field The present invention relates to a system and method for changing mechanical transmissions using fluid under pressure to reduce the operator effort required to change speeds. BACKGROUND ART Conventional mechanical transmissions, particularly those for heavy-duty trucks, typically use a gearshift lever coupled to the transmission via a mechanical link to effect a ratio change or transmission change. In these transmissions it is desirable to minimize the effort required by the driver to change the transmission to reduce driver fatigue while still maintaining an acceptable "feel" to the driver. More recently, automated and semi-automated mechanical transmissions have been developed that use one or more remote actuators to effect a gear change in the transmissions. In these transmissions it is desirable to reduce the force of change so that the associated actuators can be reduced in size and / or required energy, resulting in cost and weight reduction of the overall system of the vehicle.

Various transmission shift mechanisms have been developed and applied to both traditional (manual) mechanical transmissions and automated or semi-automated mechanical transmissions. The change mechanisms that use a single arrow or rail of changes to select and link the ratios of a multi-speed mechanical transmission, for a vehicle, are known in the state of the art, as can be seen by reference to the patents of the United States 4,621,537; 4,532,823; and 4,222,281, the disclosures of which are incorporated herein by reference. Such mechanisms may have advantages relative to multiple-rail change devices such as those disclosed in U.S. Patents 4,445,393; 4,275,612; 4,584,895; and 4,722,237, the disclosures of which are incorporated herein by reference. These advantages may include reduced complexity (fewer parts), the ability to use improved bearings and better surface finishing, since only an arrow, a smaller package, and relatively simple control of an auxiliary transmission are involved, including detection of conditions of transmission such as neutral conditions and gear status. Regardless of the particular shift mechanism used, gear shift transmissions generally use a shift bar housing assembly. Typically, such shift bar housings are manually and operator controlled by a fixed shift finger to a shifter mounted directly or to the cross date of a remotely controlled shifting mechanism. To prevent the selection of more than one gear at any particular time, interlock mechanisms are provided. The shift bar housing assembly contains one or more axially movable shifting bars, one or more shifting rails or one or more shifting rods. Each change rail carries at least one shift fork, which is moved axially to link or disengage a gear from the selected transmission. The shift rail (or shift fork in a single-track shift transmission) is generally selected by a transverse movement (left or right) of an actuator or shift lever. The selected gear is engaged or disengaged with an axial or longitudinal movement (forward or backward) of the shift rail, which causes an axial movement of a clutch member, a gear bearing clutch teeth, or a jaw clutch, as is well known in the state of the art, and can be seen by reference to United States patents 4,445,393; 4,754,665; 4,876,924; and 5,053,961, the disclosures of which are incorporated herein by reference. Gearbox housing assemblies that are not operated directly by the driver are also known in the state of the art. Such assemblies may be driven by pressurized hydraulic fluid, pressurized air, or electric motors, and have suitable controls for them, as can be seen by reference to United States patents 4,428,248; 4,445,393; 4,722,237; and 4,873,881, all assigned to the assignee of the present invention and all incorporated herein by reference. SUMMARY OF THE INVENTION An objective of the present invention is to provide a system and method to change a mechanical transmission that reduces the effort required by the operator while maintaining an acceptable sense of change. A further object of the present invention is to provide a system and method for changing a mechanical transmission using a fluid under pressure to reduce the force of change required by the driver. Still another object of the present invention is to provide a system and method for changing a mechanical transmission, which reduces the effort of change of the operator, but which is applicable to both simple and compound transmissions. Yet a further object of the present invention is to provide a system and method for changing a single-rail mechanical transmission using fluid under pressure to reduce the operator's changing force. Another object of the present invention is to provide a mechanical transmission of a single shift rail having a shift assist system integrated with the gear shift rail. Still another object of the present invention is to provide a system and method for changing a mechanical gear shift transmission that produces an assist force proportional to the force exerted on the gear lever by the operator. In carrying out the above objectives and other objects, aspects and advantages of the present invention, a system for reducing the effort of the operator in the changes of a mechanical transmission includes a control member fluidly coupled to a member of changes for a distance Default displacement via a proportional valve and piston arrangement. The control member is connected to a valve plunger while the shift member is connected to a valve body of the proportional valve. Relative movement between the control member and the shift member provides airflow proportional to the piston to provide an assist force proportional to the force applied to the shift lever. In one embodiment, the shift finger is coupled to the control member via pins placed through slots that are elongated to provide the predetermined distance of travel. In another embodiment, the shift finger is directly connected to the shift member that is connected by pin to the control member using slots as in the first embodiment. Another form of realization uses a proportional valve and piston arrangement for each rail of changes in a mechanical transmission of multiple rails of changes. A method according to the teachings of the present invention is also provided. Several advantages are associated with the present invention. For example, the system and method of the present invention provide a change assist device integrated with the shift mechanism. The present invention does not require electronic detection of a cross gate position or load of the shift lever, but nevertheless provides a change assist force proportional to the load of the shift lever. The present invention provides a limited fluid coupling such that a positive mechanical coupling is used to change the transmission in the event that fluid pressure is lost or excessive force is applied to the shift lever. For multiple rail transmissions having a valve and piston arrangement for each rail of changes, the force for each synchronizer can be varied using different piston sizes without the need for additional valve control. The above advantages, in addition to other advantages, features and objectives of the present invention will be readily apparent to a person skilled in the art from the following detailed description of the invention, when read with the accompanying drawings. Brief Description of the Drawings Figure 1 is a schematic illustration of a composite mechanical transmission having a shifting system in accordance with the present invention; Fig. 2 is a partial cross-sectional view of a simple mechanical transmission, illustrating a system and method for reducing the operator change effort, wherein a finger of changes is pin-connected to a control member according to the present invention; Figure 3 is a partial cross-sectional view of an alternate embodiment for reducing the effort of change of the operator, where a shift finger is coupled to a shift member that is connected by pin to a control member in accordance with the present invention; and Figures 4A-4E provide partial cross-sectional views of a fluid cylinder assembly to provide a proportional force of change assistance in accordance with the present invention. Minor (s) Mode (s) to Bring the Invention to Practice Some terminology will be used in the following description for reference convenience only, and will not be limiting. The words "upwards", "downwards", "to the right", and "to the left" will designate addresses in the drawings referred to. The words "forward" and "rearward" will refer, respectively, to the front and rear ends of the transmission, as conventionally mounted on a vehicle, being respectively from the left and right sides of the transmission, as illustrated in FIG. figure 1. The words "inward" and "outward" will refer to directions towards and away, respectively, from the geometric center of the device and its designated parts. This convention or terminology applies to the words mentioned above in a specific manner, in addition to their derivations and words of similar connotation. The term "composite transmission" is used to designate a gear change transmission or gear shift having a main transmission section with multiple forward speeds and an auxiliary transmission section connected in series with the main transmission section, where the gear ratio selected in the main transmission section can be combined by a gear reduction selected in the auxiliary transmission section. "Synchronized clutch assembly" and words of similar connotation designate a clutch assembly used to non-rotatably engage a selected gear to an arrow by means of a positive clutch, in which the attempted engagement of said clutch is impeded until the Clutch members are in substantially synchronous rotation.

The term "transmission", as used herein, will refer either to a single transmission or to the main transmission section of a composite transmission. Referring now to Fig. 1, a schematic diagram illustrating a transmission 10 having a shift mechanism 12 according to the present invention is shown. The transmission 10 is a composite mechanical transmission having a main section 14 connected in series with an auxiliary transmission section 16, as is well known in the art. The main section 14 provides a plurality of selectable gear ratios, which are selected using the shift mechanism 12 according to the present invention. The main section 14 includes an inlet arrow 18 which is selectively coupled to a crank shaft 20 of a motor E via a friction master clutch C which includes a pulse portion 22 and a driven portion 24. The input arrow 18 of the main section 14 is connected to an input gear 26 which drives countershaft assemblies 28, 30 of the main section 14, which are substantially identical. The counter-shaft 30 is held in the housing H via bearings 32, 34. Several counter-gear gears 36, 38, 40, 42, 44, 46 are fixed for rotation with the counter-shaft 30 and in constant engagement with corresponding gears 26, 48 , 50, 52, 54 and 56, respectively. The traction gears of the main shaft are selectively engageable with the main shaft 64 via sliding clutch collars 58, 60, 62. The gear 56 is the reverse gear and is coupled to the counter shaft engagement 46 of the main section via the intermediate idling gear 66. The clutch collars 58, 60, 62 are axially positioned using corresponding shift forks 68, 70 and 72, respectively, as is well known in the art. The clutch collars 58, 60, 62 may be of the synchronized drive or double jaw type of unsynchronized drive. The shift forks 68, 70, 72 are generally part of the shift mechanism 12 and are operatively associated with a selector arrow 74 using a well-known slit and wedge arrangement. A gear shift lever 76 is connected to a shift finger assembly 78 which cooperates with a selector block 80 to transfer the shift force of the gear lever operator 76 to the selector arrow 74, as explained below with more detail. As is known in the art, a transverse movement of the shift lever 76 rotates the selection arrow 74 to select one of the shift forks 68, 70 or 72. A longitudinal or axial movement of the shift lever 76 moves the selector arrow 74 together with a shift fork selected to link or de-link a selected gear ratio. In one embodiment of the present invention, the selector block 80 is directionally coupled to a control rod 82 such that the selector arrow 74 is rotationally fixed to the control rod 82. However, the directional coupling allows a relative axial translation predetermined between the selector arrow 74 and the control rod 82. In one embodiment (better illustrated in Figure 2), the directional coupling is provided by ducktail pins passing through a bore in the control rod 82 and an axially elongated slit in the selection arrow 74. The axially elongated slit allows a predetermined relative axial translation between the control rod 82 and the selector arrow 74. The shift mechanism 12 also includes a cylinder assembly 84 operatively associated with the selector arrow 74 and control rod 82 to provide a fluid coupling therebetween. For application to composite transmissions, such as transmission 10, the cylinder assembly 84 may be contained within the transmission housing. For simple transmissions, the cylinder assembly 84 may be positioned as illustrated in Figures 2-3, where the range cylinder of the auxiliary section is located in composite drives arranged in a similar manner. Preferably, a proportional valve 86 is used to direct a fluid under pressure, such as air or aulic fluid, to an appropriate side of the piston 88 traveling within the cylinder 90. The fluid under pressure acts on the piston 88 to apply a force to the selection arrow 74 to assist in linking or uncoupling the selected gear ratio via translation of a selected shift fork from the shift forks 68, 70, 72. A more detailed illustration and explanation of the operation of the shift mechanism 12 are provided with reference to Figures 2-4. As also shown in Figure 1, the auxiliary section 16, similar to the main section 14, includes two substantially identical countershaft assemblies 92, 94. Each counter shaft assembly 92 includes a counter shaft 96 supported within the housing H by bearings 98, 100. The auxiliary section counter shaft gears 102, 104 are in constant engagement engagement with the range 106, 108, gears, respectively. The output / range gear 108 is fixed for rotation with the main shaft 54. A two-position synchronized jaw clutch assembly 112 couples the range 106 engagement to the main shaft 64, or alternatively couples the range 108 engagement to the output arrow 110, to provide high range or low range operation, respectively. Of course, the present invention is also applicable to range type transmissions that use a combined auxiliary splitter / range type section. Transmission 10 is typically referred to as having a "four plus one" main section, combined by a two-speed auxiliary section. Although the main section 14 provides five forward gear ratios and a reverse gear ratio, typically, the lowest gear ratio provided by engaging the main shaft gear 54 directly to the main shaft 64 is referred to as a "drive gear". ", which is operated only when the auxiliary section is in the low range. As such, the main section 14 provides four forward relations that are combined by the auxiliary section 16, which operates either in the low range or the high range to provide a total of eight forward gear ratios for normal operation, in addition to a drag gear. Of course, the present invention can be used in any of the various mechanical transmissions to reduce the operator effort required during transmission shifts. Referring now to Figure 2, a partial cross section of a six-speed mechanical transmission having a shift mechanism according to the present invention is shown. The transmission 110 includes a plurality of gears 112 that define a plurality of gear ratios between an input and a transmission output. The engagement ratios are selected via a shift lever (not shown) connected to the shift finger assembly 114 which includes a shift finger 116 positioned for engagement with a selector block 118. A selector arrow 120 is disposed within the shift housing 122 and includes a plurality of associated shift forks 124, 126, 128, 130, each selectable by rotation of the selector shaft 120 using a slit arrangement 130 and wedge 132, as is known in the art. The shift forks 124, 126, 128 are coupled to the associated clutches 134, 136 and 138, respectively, to link or disengage a corresponding gear from the plurality of gears 112 via axial translation of the selector shaft 120. The transmission 110 may include various gear state sensors such as the neutral sensor 140 and the reverse sensor 142. In the embodiment illustrated in figure 2, the force of the engagement lever is transmitted in a normal manner to the shift finger assembly 14 and the shift finger 116 to the selector block 118 which is directionally coupled to the internal control rod 144 by at least one pin 146. Al less an axially elongated slit 148 cooperates with a corresponding pin 146 to provide the directional coupling. The axially elongated slot may be in the control rod 144, the selector arrow 120, or both. This provides positive rotation of the control rod and selector arrow while allowing predetermined relative axial translation between the control rod 144 and the selector arrow 120. In the embodiment of FIG. 2, the movement of the selector block 118 does not translate immediately. the movement of the selector arrow 120. The pins 146 can translate along the selection arrow 120. The length of the slit 148 provides adequate clearance for relative movement between the control rod 144 and the selector arrow 120, which is of about one millimeter in one embodiment. This relative movement is used to operate a proportional valve assembly 150. The control rod 144 is secured or fixed to the valve plunger 152 while the valve body 154 is secured to the selector shaft 120. The piston 156 is secured to the body of the valve. valve 154 and moves inside cylinder 158. Control valve 160 directs a pressurized fluid, such as hydraulic fluid or air, from source 162 to operate piston 156 in any direction, depending on the direction of movement of the valve plunger 152. As the piston 156 moves within the cylinder 158, the pressurized fluid is passed through a controlled discharge 164. As such, the control valve and cylinder assembly is operatively associated with the selector shaft 120 and the control rod. 144 to provide a fluid coupling between them. A fluid under pressure inside the cylinder? 58 applies a force on the piston 156 which is connected to the valve body 154 and the selector arrow 120 in response to the relative translation between the selector shaft 120 and the control rod 144. Referring now to Figure 3, an alternative embodiment of a shift mechanism according to the present invention is shown. In this embodiment, the selector arrow 166 cooperates with the control rod 168 and the cylinder assembly 170 to reduce the effort of changing the operator in a manner similar to the embodiment described and illustrated with reference to Figure 2. In this embodiment, the selector block 172 is connected to the selector arrow 166 in a conventional manner. The control rod 168 is directionally coupled to the selector arrow 166 via a pin 174 and the axially elongated slot 175. Preferably, the control rod 168 is pin-connected to a selector wedge 169 of the selector arrow 166. cylinder 170 includes proportional control valve 178 with valve body 180 (which is attached to piston 182) attached to selector shaft 166. Control rod 168 is secured to valve plunger 184. As illustrated, the shape of embodiment of figure 3 requires a shorter control rod 168 which may be advantageous, depending on the particular application. Referring now to Figures 4A-4E, partial cross-sectional views of a cylinder assembly for a shift mechanism according to the present invention are shown. Cylinder assembly 200 includes a cylinder body 202 and cylinder cover 204 that are secured to the transmission housing H using appropriate fasteners (not shown) that pass through perforations 206. Assembly 200 includes a valve assembly. proportional control 208 including a valve body 210 secured to a piston 212 by appropriate fasteners 214, such as elastic rings or the like, such that the valve body 210 and the piston 212 move as a single unit inside the cylinder body 202. The selector arrow 216 is secured to the valve body 210 so that the force applied by the pressurized fluid to the piston 212 is transferred through the valve body 210 to the selector shaft 216. The control rod 218 is secured to the threaded end of the plunger 220 that contacts the rod piston 222. The bar cylinder 224 cooperates with the rod piston 222 to direct the fluid under pressure through the control valve assembly 208 to an appropriate chamber of the cylinder 202. The discharge control valve 226 cooperates with the rod valve and the plunger 220 to control the discharge of pressurized fluid from the interior of the cylinder body 202 through the discharge gate 228. A selectable discharge orifice 230 is used to control the discharge rate of the pressurized fluid that is supplied through the fluid source 232. Appropriate seals, such as seals 234, are used in various locations of the cylinder assembly 200 to reduce or eliminate fluid leakage. In operation, as the relative axial movement between the selector arrow 216 and the controller 218 increases, the valve plunger 220 opens a clearance between the rod piston 222 and the rod piston valve seat 236 allowing fluid to enter. under pressure through the source 232 to pass through the channel 238 to the inner portion of the cylinder body 202 which applies a force on the piston 212. This force is transmitted through the valve body 210 to the selector arrow 216 for provide a change force in addition to the force exerted on the gear change lever by the operator. As the piston 212 moves within the cylinder 202, the pressurized fluid is allowed to discharge from the opposite cylinder chamber. The flow rate of the discharged fluid is controlled by means of a predetermined selectable orifice 230 before fluid passes through the discharge gate 228. When the force is removed from the shift finger, the valve plunger 220 becomes stationary, such that the valve body 210 closes the bar valve seat 236. The fluid flow under pressure is provided to both piston chambers such that the assist force generated by the cylinder assembly 200 is removed. The force generated by the Cylinder assembly 200 is preferably proportional to relative axial movement between selector shaft 216 and control rod 218. Proportional assistance is governed by the seat clearance of the bar valve during operation. Greater force applied to the shift finger increases the clearance between the bar valve piston 222 and the valve seat 236, allowing greater fluid flow to the cylinder chambers. The valve plunger 220 operates against a bar valve spring 240 to react against the opening force. The spring 240 is pre-loaded so that a minimum level of force is required before any assistance is provided by the cylinder assembly 200. Note that the directional coupling illustrated in Figures 1-3 is such that a fluid coupling is provided. through the cylinder assembly 200 is provided for a relative, axial movement predetermined between the selector arrow 216 and the control rod 218. Preferably, about 1 mm of displacement is provided by the axially elongated slot. Once the relative displacement exceeds this value, the fluid line is transformed to a positive coupling such that any movement of the control rod results in a corresponding movement of the selector arrow, and vice versa. Figures 4B-4E illustrate the relative movements within the cylinder assembly 200 to link and disengage the selected synchronizers. The placement of Figure 4A represents a neutral position. When a force is applied to the control rod 218 to the right, the bar valve piston 222 overcomes the force of the bar valve spring 240 and is moved out of the valve seat 236, allowing pressurized fluid to flow through the channel 228 into the left chamber of the cylinder 202. This creates a force on the piston 212 to assist in the synchronizer linking. In a similar manner, a force FD on the control rod 218 in the opposite direction, as illustrated in Figure 4C, connects pressurized fluid from the source 232 to the channel 242, which fills the right chamber of the cylinder 202 by applying a additional force via the piston 212 to the control rod to help untie the selected synchronizer. Simultaneously, the channel 238 is connected to the discharge gate 228 through the plunger, the bar valve, and the discharge valve. If the FD force continues to cause relative displacement between the control rod and the selector arrow, the cylinder assembly will continue to provide a shift assist through the neutral position to link the next synchronizer, as illustrated in FIG. 4E. A change to neutral results in the position illustrated in Fig. 4D, with the bar valve piston 222 seated and the unloading control valve 226 disengaged, such that the pressurized fluid passes directly from the source 232 through both channels 238 and 242 before being discharged through the discharge gate 228 such that additional force is not generated by the piston 212. Although the best mode contemplated for practicing the invention has been described in detail, it will be understood that various modifications are possible without departing from the spirit and scope of the invention, as defined by the following claims.

Claims (17)

1. CLAIMS 1. A shift mechanism for a mechanical transmission having a plurality of selectable gear ratios, the gear mechanism comprising: a selector arrow having a plurality of shift forks placed around it, where the rotation of the selector arrow selects one of the shift forks and the translation of the selector arrow is operative to move the selected shift fork to link or de-link a selected gear ratio; a control rod coupled directionally with the selector arrow for positive rotation therewith while allowing a relative, predetermined axial translation between them; a cylinder assembly operatively associated with the selector arrow and the control rod providing a fluid coupling therebetween, to apply a force on the selector arrow in response to the relative translation between the selector arrow and the control rod.
2. 2. The change mechanism of claim 1, wherein the cylinder assembly provides a force proportional to the relative translation.
3. 3. The shift mechanism of claim 1, wherein the cylinder assembly comprises a hydraulic cylinder assembly.
4. 4. The change mechanism of claim 1, wherein the cylinder assembly comprises a pneumatic cylinder assembly.
5. The change mechanism of claim 1, wherein the cylinder assembly comprises: a cylinder housing; a piston disposed within the housing; a valve body secured to the selector shaft and the piston and fluidly sealed for axial movement relative to the housing; a valve plunger disposed within the valve body and secured to the control rod; and a control valve disposed within the valve body in contact with the valve plunger to direct fluid under pressure to the cylinder housing to apply force to the selector arrow via the piston.
6. The change mechanism of claim 5, wherein the control valve comprises a bar valve.
7. The change mechanism of claim 5, wherein the control valve comprises: a bar valve cylinder; a rod valve piston that cooperates with the bar valve cylinder; and a spring disposed between the bar valve cylinder and the bar valve piston.
8. 8. The change mechanism of claim 1, further comprising: a finger set of changes; a selector block positively coupled to the selector arrow for rotation and translation with it, the selector block cooperating with the shift finger to transfer the force of changes from a shift lever to the selector arrow.
9. The change mechanism of claim 1, further comprising: a finger set of changes; a selector block positively coupled to the control rod for rotation and translation with it, the selector block cooperating with the shift finger to transfer the change force of a shift lever to the selector arrow.
10. 10. A shift mechanism for a mechanical transmission having a plurality of gears defining selectable gear ratios via a shift lever and a shift finger assembly, the shift mechanism comprising: a shift bar housing; a selector arrow disposed within the shift bar housing, the selector arrow having a plurality of shift forks associated therewith, each selectable by rotation of the selector shaft and axially displaceable to link or disengage a selected gear; a control rod arranged concentrically within the selector arrow and directionally coupled to the selector arrow via at least one pin passing through the control rod and the selector arrow, where a selected component of the control rod and selector arrow it includes an axially elongated slit to provide positive rotation of the control rod and the selector arrow while allowing a relative, predetermined axial translation between them; a proportional valve assembly, operatively associated with the selector arrow and the control rod providing a fluid coupling therebetween to apply a force on the selector arrow in response to relative translation between the selector arrow and the control rod.
11. The change mechanism of claim 10, further comprising: a fixed selector block for "rotation and translation with the selector arrow, the selector block operatively associated with the shift finger assembly to transmit force from the shift lever to the selector shaft 12.
12. The shift mechanism of claim 10, further comprising: a fixed selector block for rotation and translation with the control rod, the selector block operatively associated with the shift finger assembly for transmitting lever force. changes to the selector arrow 13.
13. The change mechanism of claim 10, wherein the selector arrow comprises: a plurality of selector arrows, each associated with a single fork of the plurality of shift forks, each selector arrow having one. control rod and an associated proportional valve assembly 14.
14. The change mechanism of claim 10, of the proportional valve assembly comprises a hydraulic valve assembly.
15. The change mechanism of claim 10, wherein the proportional valve assembly comprises a pneumatic valve assembly.
16. 16. A method for reducing the effort of the operator during the change of a mechanical transmission by having a plurality of gears defining a plurality of selectable gear ratios, the transmission including a shift lever that cooperates with a set of shift fingers that drives a selector shaft having at least one shift fork selected by rotation of the selector arrow, the shift fork being translated in response to the translation of the selector arrow to link or de-link a selected gear ratio, the method comprising: engaging the selector arrow to a control rod such that the selector arrow rotates with the control rod while allowing a predetermined axial displacement between them; and fluidly coupling the selector arrow to the control rod to provide additional force on the selector arrow while the relative displacement of the control rod and selector arrow is between zero and the predetermined axial displacement. The method of claim 16, further comprising: varying the additional force supplied to the selector shaft based on a corresponding force applied to the shift lever.