Abstract:
Vehicle gearbox with a control shaft rotatably and axially displacebly mounted transversely to the axial direction of the vehicle gearbox, the rotational movement of the control shaft from a neutral position effecting, via motion-transmitting elements, engagement of a gear speed. A lock element cooperating with the control shaft is coordinated with a sensor sensing the position of a clutch connected to the gearbox, and locks the control shaft in the neutral position when the clutch pedal is not depressed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a vehicle gearbox, comprising an input shaft mounted in a housing, at least one countershaft driven by the input shaft, a main shaft, pairwise interengaging gears supported by the countershaft and the main shaft, at least one gear in each pair being freely rotatably mounted on its shaft and lockable to said shaft by means of an engaging sleeve axially displaceably but non-rotatably mounted on said shaft, said engaging sleeve being coordinated with synchronizing arrangements, a shift control, the movement of which in one direction from a neutral position effects via motion transmitting elements, displacement of an engaging sleeve to an engagement position, and the movement of which from the neutral position in another direction effects displacement of another engaging sleeve to an engagement position, and lock means comprising a lock element with control means, which, during predetermined operating conditions, hold the lock element in a locked position, in which it impedes movement of the shift control. 
     It is previously known to provide a gearbox of the above described type, which has the main shaft coupled to a subsequent range gearbox of planetary type, for example, in accordance with SE 9306735-5, with locking means to prevent mis-shifting. A lock function which prevents sing in the basic gearbox from the initiation of shifting in the range gearbox until this shifting step has been completed, i.e. when the shift selector mechanism has returned to its rest position, uses a sensor which cooperates with the control cylinder of the range gearbox and senses the position of its piston rod. When the driver preselects a new range (high to lower, low to high), the shifting is initiated by air being supplied to the control piston of the range gearbox, as soon as the gearbox shift lever reaches the neutral position. During initial movement of the control piston, a valve, under the influence of the sensor, directs compressed air to a control cylinder connected to a locking element. The control cylinder moves the locking element to a position in which it locks the control shaft in the neutral position. When the shifting in the range gearbox is completed, the control cylinder of the locking element is depressurized and spring means cooperating with the locking element move the locking element out of its locked engagement so that a new gear can be engaged in the basic gearbox. 
     Another locking function which prevents shifting from high range to low range at speeds above a predetermined speed, uses a speed sensor which controls a magnetic valve in the compressed air supply line to the control cylinder of the range gearbox. When the vehicle speed exceeds the predetermined speed limit, the magnetic valve closes and prevents compressed air supply to the control cylinder. 
     In gearboxes for heavy trucks, both those with and without a range gearbox with lockout function of the above described type, there are at times so-called servo-synchronizing devices. Such synchronizing devices use the rotational energy of the rotating components of the gearbox itself, to reduce the shift lever force required for the synchronizing work. In principle, shifting can occur after manual initiation of the synchronization (a light movement of the shift lever from the neutral position towards a gear speed position) completely by the action of the servo system. This involves, however, a risk. If a driver carelessly places his hand on the shift lever with the lever in the neutral position, the weight of the hand and part of the arm can be enough to move the lever to the synchronizing position without it being noticed. If the clutch pedal is depressed and the engine is disengaged from the gearbox, this will not be a problem because the shifting will be carried out normally. If, however, the clutch is not disengaged and the engine drives the input shaft of the gearbox, the synchronizing cones movable relative to each other will frictionally engage one another and soon be worn out, since it is not only the shafts, gears and discs in the gearbox which must be synchronized, but the entire engine. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is in general to achieve a gearbox of the type described by way of introduction which is equipped so that unintended mis-shifting is made impossible, to thereby prevent initiation of the synchronizing work, especially in a gearbox with servo-synchronization, which can rapidly result in wearing out of the frictional surfaces of the synchronizing cones. 
     This is achieved according to the invention by virtue of the fact that the control means of the lock element comprise means which are affected by the position of a clutch pedal and which hold the lock element in the locked position when the clutch pedal is not actuated. This prevents shift lever movement from the neutral position as long as the clutch is engaged. Only after the clutch pedal has been depressed, which in practice is not done unintentionally, can the lever be moved from the neutral position for shifting. Therefore there does not have to be any risk of unconscious or unintentional initiation of the synchronizing work in a gearbox with servo-synchronization. 
     In a preferred embodiment of a gearbox according to the invention comprising a basic gearbox and a range gearbox and with a locking device which prevents shifting in the basic gearbox before the shifting in the range gearbox has been completed, uses a single locking device both for the locking function in the shifting in the range gearbox and for the locking function when the clutch is engaged. Instead of using, as in the previously known and described locking device, a single-acting control cylinder with spring means biasing the locking element in the direction away from the locking position, a double-acting piston cylinder device is used with spring means which bias the locking element towards the locked position. This guarantees the locking function in the neutral position even if there should occur a pressure failure in the vehicle compressed air system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail with reference to examples shown in the accompanying drawings, where FIG. 1 shows a partially cut-away side view of a gearbox according to the invention with a basic gearbox and a range gearbox, FIGS. 2 a  and  2   b  show cross-sections through a neutral position lock for the gearbox in FIG. 1 together with schematically represented control means, and FIG. 3 shows a longitudinal section through a portion of the gearbox range mechanism comprising a control cylinder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1,  1   a  designates the housing of a basic gearbox, in which an input shaft  2 , a countershaft  3  and a main shaft  4  are mounted. The input shaft  2  drives the countershaft  3  via a pair of gears  5  and  6 , respectively, fixed to the respective shafts. Five additional gears  7 ,  8 ,  9 ,  10  and  11  are fixed to the countershaft  3 . The gears  7 ,  8  and  9  engage gears  12 ,  13  and  14  which are freely rotatably mounted on and lockable to the main shaft  4  with the aid of axially displaceable engaging sleeves  15  and  16 , which cooperate with synchronizing devices of a type which is known per se and which are not shown in more detail here. These can, for example, be of the type shown and described in SE-A-502042, i.e. so-called servo-synchronizers. The first, second and third gear speeds, respectively, are obtained by locking the free gear  13 , the free gear  14  or the free gear  12 , respectively, onto the main shaft  4  by displacing the respective engaging sleeve  15 ,  16  with the aid of the gear selector mechanism of the gearbox. This selector mechanism comprises a pair of shifting rods  17 ,  18 , each having a notch  19  into which a finger  20  on a control shaft  21  is insertable. The shifting rod  17  is pivotally joined to a shifting fork  22  pivotally mounted in the gearbox housing  1 a, said shifting fork  22  engaging the engaging sleeve  15 . The shifting rod  18  is pivotally joined to a shifting fork  23  mounted in the gearbox housing, and engages the engaging sleeve  16 . The control shaft  21  is disposed transversally to the axial direction of the gearbox and is rotatable and displaceable in this direction. When the control shaft  21  is displaced axially, the finger  20  is moved laterally out of engagement with the notch  19  in a shifting rod  17  or  18  and into a corresponding notch in an adjacent shifting rod  18  or  17 . By turning the control shaft  21 , one or the other of the shifting rods is displaced, thereby displacing the associated shifting fork  22  or  23  to engage the first, second or third gear speed. 
     At an end of the main shaft  4  protruding from the housing  1   a , there is fixed a sun gear  30  in a planetary gear set generally designated  31 . The planetary gear set is enclosed in a housing  1   b , screwed securely to the housing  1   a . The sun gear  30  engages planet gears  32 , which are carried on the planet gear carrier  33 , which is solidly joined to an output shaft  34 . With the aid of an axially displaceable engaging sleeve  35 , a ring gear  36  engaging the planet gear  32  can be joined either to the housing  1   b , thereby reducing (low range) the r.p.m. of the of the output shaft  34  relative to the r.p.m. of the main shaft  4 , or with the planet gear carrier  33 , where-upon these shafts are locked together for direct drive (high range) of the output shaft  34 . The gear ratio in the range gear set formed by the planetary gear set  31  is selected so that, when high range is engaged in the range gearbox, the first gear speed position in the basic gearbox will result in the fourth gear speed, the second gear speed position will provide the fifth gear speed and the third gear speed position will provide the sixth gear speed. This means that the fourth gear speed has the same gear lever position as the first gear speed, the fifth gear speed has the same position as the second gear speed, and the sixth gear speed has the same position as the third gear speed. 
     The gearbox has a neutral position lock generally designated  40  with double-lock functions, which will be described in more detail with reference to FIGS. 2 a ,  2   b  and  3 . The neutral position lock comprises a cam element  41 , which, in the example shown, is made in one piece with the shifting finger  20  on the control shaft  21  and which is consequently fixed relative to the control shaft. The cam element  41  is made with a V-shaped groove  42 , which receives an end  43  of a lock bolt  44 . The cam element  41  and the groove  42  have sufficient extent in the axial direction of the control shaft  21 , that a portion of the groove  42  in the neutral position is always directly opposite the end of the lock bolt regardless of the lateral position of the control shaft  21 . With the end  43  of the lock bolt inserted in the groove  42  of the cam element  41 , the control shaft  21  is locked against rotation, which means that the basic gearbox selector lever (not shown) joined to the control shaft cannot be moved from the neutral position without great resistance. 
     The lock bolt  44  is solidly joined to a control piston  45  in a compressed air cylinder  46 , the cylinder chambers  47 ,  48  of which are connected, on either side of the control piston  45  via pneumatic valves  49 ,  50 , to a compressed-air circuit (not shown). A helical spring  51  in the cylinder chamber  48  biases the control piston  45  in the direction towards the cam element  41 , and this means that the spring  51  will hold the end  43  of the lock bolt inserted in the cam groove  42 , when the cylinder chambers  47 ,  48  are depressurized and the control shaft  21  is in the neutral position with the cam groove  42  lying directly opposite the lock bolt (FIG. 2 b ). As can be seen in the figures, the lock bolt  44  has bevelled end edges, which in combination with the V-shape of the groove  42  means that the control shaft  21  is not completely locked in this position. If sufficient force is applied to the shift lever (not shown), the control shaft can be turned pressing at the same time the lock bolt  44  out of the cam groove  42 . This is, however, not normally required unless there is a pressure failure in the vehicle compressed-air circuit. The spring-biased lock bolt thus opposes the turning of the control shaft and increases the force which the driver must apply to the shift lever. This eliminates the risk of unintentional initiation of the synchronizing work. 
     As is indicated schematically in FIG. 2 a , the cylinder chamber  47  of the compressed-air cylinder  46  is connected to the vehicle compressed-air system via a valve  49 , which is actuated by the movement of the vehicle clutch pedal  61 . When the clutch pedal  61  is let up, the valve  49  depressurizes the cylinder chamber  47  and the lock bolt  44  is kept in the locked position by the spring, thus preventing unintentional shifting. When the clutch pedal  61  is depressed for intentional shifting, compressed air is supplied to the cylinder chamber  47 . The piston area in the cylinder chamber  47  is selected so that the force on the piston exerted by the pressure is greater than the force of the spring  51 , so that the lock bolt  44  is moved out of engagement with the cam element and shifting can be effected unimpeded by the neutral position lock. 
     As is also indicated schematically in FIG. 2 a , the opposite cylinder chamber  48  of the compressed-air cylinder  46  can be supplied with compressed air via the valve  50 . This valve is actuated mechanically by a piston rod  62  joined to a piston  63  in a compressed-air cylinder  64 . The piston rod  62  is joined to a shift fork  65  engaging the engaging sleeve  35  of the range gearing (see FIG.  3 ). The piston rod  62  is made with a pair of depressions  66 ,  67 , representing the two shift positions of the range gearing, i.e. low-range and high-range. When a sensor  68  cooperating with the valve  50  protrudes into one of the depressions  66  or  67 , the valve  50  is in a position in which the cylinder chamber  48  of the cylinder  46  is depressurized. As soon as the piston rod  62  starts to be displaced to complete shifting in the range gearbox, the sensor  68  is pressed upwards, thereby opening the valve  50  and supplying compressed air to the cylinder chamber  48 , so that the lock bolt  44  is moved to the locked position to lock the control shaft  21  in the neutral position even if the driver depresses the clutch pedal to shift the basic gearbox in connection with shifting in the range gearbox, thereby supplying compressed air at the same time to the opposite cylinder chamber  47  via the clutch controlled valve. This is because the sum of the spring force and the compressed-air force in the locking direction is greater than the compressed-air force in the opposite direction. In this manner, shifting in the basic gearbox is prevented prior to completed shifting in the range gearbox. As soon as the shifting in the range gearbox has been completed and the sensor  68  has reached one of the depressions  66  or  67 , the cylinder chamber  48  of the lock cylinder  46  is depressurized and the lock bolt  44  is moved out of its locked engagement by the pressure in the cylinder chamber  47 .