Gear axial thrust force optimization for high efficiency vehicle transmission

A transmission selectively coupled to an engine crankshaft of an internal combustion engine arranged on a vehicle includes an input shaft, a mainshaft, an output shaft, a first countershaft and a second countershaft. A first gear set includes a first mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the first countershaft and a first countershaft gear arranged on the second countershaft. A second gear set includes a second mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the second countershaft and a second countershaft gear arranged on the second countershaft. The gears of the first gear set all have a first helix angle. The gears of the second gear set all have a second helix angle. The first and second helix angles are selected to provide gear constant leading whereby thrust forces directed onto the first and second countershafts are balanced.

FIELD

The present disclosure relates generally to a transmission having an improved bearing configuration and a related method that minimizes bearing frictional losses by reducing the bearing thrust loads that are generated by the gears of the transmission during operation.

BACKGROUND

Conventional automotive vehicle powertrains typically have multiple-ratio transmission mechanisms that establish power delivery paths from an engine to vehicle traction wheels. In order to deliver power efficiently, the transmission gearing must be designed to balance and reduce thrust loads carried by the transmission shaft support bearings. It is also desirable to minimize the axial thrust loads generated by helical gearing to improve overall efficiency of a vehicle transmission.

SUMMARY

A transmission selectively coupled to an engine crankshaft of an internal combustion engine arranged on a vehicle includes an input shaft, a mainshaft, an output shaft, a first countershaft and a second countershaft. The first and second countershafts are offset from the mainshaft and drivably connected to the input shaft and the mainshaft. A first gear set includes a first mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the first countershaft and a first countershaft gear arranged on the second countershaft. The gears of the first gear set are meshingly engaged. A second gear set includes a second mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the second countershaft and a second countershaft gear arranged on the second countershaft. The gears of the second gear set are meshingly engaged. The gears of the first gear set all have a first helix angle. The gears of the second gear set all have a second helix angle. The first and second helix angles are selected to provide gear constant leading whereby thrust forces directed onto the first and second countershafts are balanced.

According to additional features, the transmission further includes a first and second countershaft bearing that rotatably support the first countershaft. A third and fourth countershaft bearing rotatably support the second countershaft. Axial loads into the first, second, third and fourth bearing are mitigated based on the gear constant leading. The axial loads are less than 10 Newtons. In one configuration the axial loads are zero. At least one of the first, second, third and fourth countershaft bearings comprises a cylinder roller type bearing. In one arrangement, all of the first, second, third and fourth countershaft bearings comprise cylinder roller type bearings.

According to other features, the first mainshaft gear creates a first force. The first countershaft gear of the first countershaft creates a second force. The first countershaft gear of the second countershaft creates a third force. The second and third forces are equal and opposite to the first force. The second mainshaft gear creates a fourth force. The second countershaft gear of the first countershaft creates a fifth force. The second countershaft gear of the second countershaft creates a sixth force. The fifth and sixth forces are equal and opposite to the fourth force. The first force is equivalent to the fourth force.

In other features, the helix hands are elected for gears of the first and second set such that forces generated by the gears are directed toward at least one bearing of the transmission having a reduced slip speed relative to remaining bearings of the transmission. The transmission further includes a first mainshaft bearing, a second mainshaft bearing and a pocket bearing that all support the mainshaft. The pocket bearing is arranged between the first and second mainshaft bearings. Forces generated by the gears are directed toward the pocket bearing while forces experienced at the first and second mainshaft bearings are zero.

A transmission selectively coupled to an engine crankshaft of an internal combustion engine arranged on a vehicle and constructed in accordance to another example of the present disclosure includes an input shaft, a mainshaft, an output shaft, a first countershaft and a second countershaft. The first and second countershafts are offset from the mainshaft and drivably connected to the input shaft and the mainshaft. A first gear set includes a first mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the first countershaft and a first countershaft gear arranged on the second countershaft. The gears of the first gear set are meshingly engaged. A second gear set includes a second mainshaft gear arranged on the mainshaft, a first countershaft gear arranged on the second countershaft and a second countershaft gear arranged on the second countershaft. The gears of the second gear set are meshingly engaged. A plurality of bearings support the input shaft, the mainshaft, the output shaft, the first countershaft and the second countershaft. Thrust forces directed onto the first and second countershafts are balanced. Gears of the first and second set have helix hands that generate forces directed toward at least one bearing of the plurality of bearings having a reduced slip speed relative to remaining bearings of the plurality of bearings.

According to additional features, the gears of the first gear set all have a first helix angle. The gears of the second gear set all have a second helix angle. The first and second helix angles are selected to provide gear constant leading whereby thrust forces directed onto the first and second countershafts are balanced. The plurality of bearings further comprises a first, second, third, and fourth countershaft bearing. The first and second countershaft bearings rotatably support the first countershaft. The third and fourth countershaft bearing rotatably support the second countershaft. Axial loads on the first, second, third and fourth countershaft bearings are zero. At least one of the first, second, third and fourth countershaft bearings comprises a cylinder roller type bearing.

According to other features, the first mainshaft gear creates a first force. The first countershaft gear of the first countershaft creates a second force. The first countershaft gear of the second countershaft creates a third force. The second and third forces are equal and opposite to the first force. The second mainshaft gear creates a fourth force. The second countershaft gear of the first countershaft creates a fifth force. The second countershaft gear of the second countershaft creates a sixth force. The fifth and sixth forces are equal and opposite to the fourth force. The first force is equivalent to the fourth force. The plurality of bearings further comprises a first mainshaft bearing, a second mainshaft bearing and a pocket bearing that all support the mainshaft. The pocket bearing is arranged between the first and second mainshaft bearings. Forces generated by the gears are directed toward the pocket bearing while forces experienced at the first and second mainshaft bearings are zero.

A method for selecting helical gears in a transmission for minimizing thrust forces within the transmission includes selecting at least two gear sets. Each gear set has a mainshaft gear, a first countershaft gear and a second countershaft gear. Torque transmitting gears of the gear sets are determined for each power path within the transmission. A first helix angle for a first gear set of the at least two gear sets is selected. A second helix angle for a second gear set of the at least two gear sets is determined based on the first helix angle to balance axial forces experienced between the first and second gear sets.

According to additional features, a matrix of linear equations is prepared. The matrix has data related to (Y) a sum of axial forces for the gears of the first gear set, (M) having the tangential force transfer function from helix angle to force, and (X) helix values. The matrix is prepared in the form Y=MX. The second helix angle is determined by solving for X using a linear squares matrix solver.

DETAILED DESCRIPTION

With initial reference toFIG. 1, a multiple-speed, change-gear transmission constructed in accordance to one example of Prior Art is shown and referred generally to at reference10. The multiple-speed, change-gear transmission10is a heavy duty transmission selectively driven by a fuel-controlled engine (such as a diesel engine or the like, not shown) through an input shaft18. The multiple-speed, change-gear transmission10may be of the compound type comprising a main transmission section connected in series with a splitter and/or range-type auxiliary section. Transmissions of this type, especially as used with heavy duty vehicles, typically have 9, 10, 12, 13, 16 or 18 forward speeds. The particular example used in this disclosure has 12 forward speeds and therefore 12 power paths. A transmission output shaft20extends outwardly from the multiple-speed, change-gear transmission10and is drivingly connected with vehicle drive axles22, usually by means of a prop shaft.

The multiple-speed, change-gear transmission10has a mainshaft collectively identified at reference30and made up of a first mainshaft38and a second or intermediate mainshaft40. The mainshaft30is coaxial with the input shaft18. The transmission10has a first countershaft42and a second countershaft44. The countershafts42and44are offset from the input shaft18and the mainshaft30. The countershafts42and44are illustrated as being offset from one another, however in some examples the countershafts42and44may be coaxial with each other. The output shaft20may be coaxial with the mainshaft30.

The first mainshaft38is supported for rotation in a housing46of the transmission10by a first mainshaft bearing38A. The second mainshaft40is supported in the housing46of the transmission10by a front and rear second mainshaft bearings40A and40B. A pocket bearing30A further supports the mainshaft30. The first countershaft42is supported for rotation by the housing46of the transmission10by first and second countershaft bearings42A and42B. The first countershaft42of the transmission14has countershaft gears50,52,54,56and58. The second countershaft44is supported for rotation by the housing46of the transmission10by third and fourth countershaft bearings44A and44B. The second countershaft44of the transmission10has countershaft gears60,62,64,66and68. The mainshaft30of the transmission10has mainshaft gears70,72,74,76and78. A master clutch can selectively communicate torque into the transmission10. A headset clutch84, a first sliding dog clutch88and a second sliding dog clutch90can move left and right as viewed inFIG. 1to connect various mainshaft gears70-78and countershaft gears50-58and60-68for attaining a desired drive gear and torque path within the transmission10.

The right hand end of the mainshaft30is drivably connected to a sun gear110. A planetary carrier112is connected to or is integral with the output shaft20, which is connected drivably through the drive axle22to vehicle traction wheels. A ring gear118engages planet pinions120carried by the carrier112.

As used herein the term “hand” is used to denote a direction that the gear teeth slope on the gear. When looking from the side of the gear, top to bottom is right handed. Bottom to top is left handed. Hand in combination with power flow direction determines thrust direction. The term “helix angle” is used to denote an angle between any helix of a helical gear and an axial line on its right, circular cylinder. As is known, the angle of teeth on helical gears create a thrust load on the gear when they mesh. These trust loads must be accommodated with the bearings identified herein.

As will become appreciated from the following discussion, the present teachings provide a transmission with gears that are optimized with helix angles and hands that minimize thrust forces within the transmission. With thrust forces optimized, bearing losses can be reduced allowing the transmission to incorporate more cost effective bearing options. In general, during operation of the transmission10, the gears mainshaft gears70-78and countershaft gears50-58and60-68are under load and generate forces that cause the respective shafts38,40,42,44to thrust in different directions. These forces are a factor of the helix angles of the respective gears and an amount of torque coming into the gear. According to the present disclosure, helix angles (and the direction they generate axial thrust force) are selected so all of the resulting forces balance out so that a net thrust on any individual shaft is zero.

With the heavy duty transmission10shown inFIG. 1, a torque path must travel through at least two gear layers to be communicated from the input shaft18to the output shaft20. The transmission10generally has a first gear set150, a second gear set152, a third gear set154, a fourth gear set156and a fifth gear set158. The first gear set150can include the mainshaft gear70, and the countershaft gears50,60. The second gear set152can include the mainshaft gear72, and the countershaft gears52,62. The third gear set154can include the mainshaft gear74and the countershaft gears54,64. The fourth gear set156can include the mainshaft gear76and the countershaft gears56,66. The fifth gear set158can include the mainshaft gear78and the countershaft gears58,68.

According to the present disclosure a method of optimizing the transmission includes determining which gears are active for given speeds of the transmission. For example, in first gear four gear sets (layers) may be active. In second gear a different four gear layers may be active. A matrix is built that identifies the gears being used for each power path (forward speed) of the transmission. An input torque can be set to a certain condition. Thrust generation can then be determined for each gear depending on which helix angle is used for the gears.

If the gear helix angles and hands are chosen so that axial forces generated by all active gears sum to zero, the transmission has constant lead. To generate a constant lead design, active gears (torque transmitting) are determined for each power paths within the transmission. The direction (fore or aft) of axial thrust forces for the active layers are determined from the gear helix hands, power flow directions, and rotational directions. A systematic method is used to adjust the helix angles for all gear layers until the total net forces on the individual shafts are as close to zero as possible.

Gear thrust forces for any gear within the transmission can be calculated using:

Fx=Axial gear thrust force in [N]. τ=Gear input torque in [Nm]. dw=Gear pitch diameter in [m]. β=Gear helix angle in [rad].

Helix angles are optimized by arranging the force equations for all of the power paths into a matrix of linear equations in the form of Y=MX. Y is an array that contains the sum of axial forces for the gears with known helix angles. M is a matrix that contains the tangential force transfer function for all of the gears with unknown helix angles. M contains the transfer function from helix angle to force. The tan β is determined by the helix hand and direction of rotation. X is an array of variables that contain the helix information (equal to tan(β)). Once formulated, X can be solved using a linear squares matrix solver to optimize the design. It will be appreciated that a helix angle must be known for one gear and the helix angles for the remaining gears can be solved. The gears in each gear set will have a common helix angle. However, each gear set will not necessarily have the same helix angle as another gear set. For example, if the helix angle is known for the gears in the first gear set150, the helix angles can be solved for the remaining gear sets.

After solving for X, the helix angles for each unknown gear within the transmission can be calculated by taking the arctan(X). Using this approach, the thrust forces that are generated by the individual gear layers within the transmission can be balanced for all shafts and power paths simultaneously. In some examples, as described herein, where axial forces can be completely eliminated on a shaft, the bearing efficiency can be further improved by changing the shaft support bearings to a more efficient type since there are not axial loads to support on these shafts.

FIG. 1shows a torque path utilizing the first and second gear sets150and152. Helix angles are arbitrarily chosen for each of the gear sets150,152,154,156and158. A force F1A is created by the mainshaft gear70. A equal and opposite force F1B is created by the countershaft gears50,60. A force F2A is created by the mainshaft gear72. An equal and opposite force F2B is created by the countershaft gears52,62. The forces F1A, F1B are not equal to the forces F2A, F2B. In the example provided F1A and F1B are 2.44 kN whereas F2A and F2B are 3.20 kN.

The input shaft18and the output shaft20are thrusted toward the outer wall of the transmission10. In other words, the input shaft18is being urged leftward inFIG. 1while the output shaft20is being urged rightward. These loads need to be accommodated by the transmission10as a whole and specifically by the bearings38A and40B. In the example provided, the force acting on the bearing38A is 4876N when rotating at 1000 RPM and having a bearing loss of 77.5 W. The force acting on the bearing40B is 6409 N when rotating at 1306 RPM and having a bearing loss of 111.5 W. The difference between the forces acting on the bearings38A and40B is realized at the bearings42A and44A. Specifically, bearings42A and44B must accommodate a force of 766 N when rotating at 1368 RPM and having a bearing loss of 72.5 W. The bearing40A accommodates 0 N force when rotating at 306 RPM and has a bearing loss of 0 W. Those skilled in the art will understand that the values given above are merely exemplary and others may be used. As can be appreciated bearing losses are realized throughout the transmission10.

Turning now toFIG. 2, a transmission210that incorporates constant leading and LH main shaft helix hands according to one example of the present disclosure will be described. The transmission210comprises like components of the transmission10described above and identified with reference numerals increased by 200. In the transmission210, shaft thrust forces are minimized through gear constant leading. If the gear helix angles and hands are chosen so that axial forces generated by all active gears sum to zero, the design has constant lead. Forces directed onto the countershafts242and244are balanced and axial loads are eliminated into the countershaft bearings242A and242B. For purposes of this disclosure, eliminated may mean an inconsequential load such as 10 N or less and preferably 0 N.

In the transmission210, the helix angle for the gears in gear set350is adjusted to 25.56 degrees. As a result, the countershaft forces are completely balanced and axial loads are eliminated into the countershaft bearings242A and244A. Since the countershaft bearings242A and244A no longer are required to support axial loads in the transmission210, bearings44A,44B,42A and42B can be changed to more efficient cylinder roller type bearings (as opposed to taper roller bearings required in the transmission10,FIG. 1). The reduced load in combination with the more efficient bearing type reduces the total bearing losses for the transmission210to 315.2 W as compared to 412.6 W for the transmission10above.

A force F3A is created by the mainshaft gear270. A equal and opposite force F3B is created by the countershaft gears250,260. A force F4A is created by the mainshaft gear272. An equal and opposite force F4B is created by the countershaft gears252,262. The forces F3A, F3B are equal to the forces F4A, F4B. In the example provided F3A, F3B, F4A and F4B are all 3.20 kN. The force acting on the bearing238A is 6409N when rotating at 1000 RPM and having a bearing loss of 102.1 W. The force acting on the bearing240B is 6409 N when rotating at 1306 RPM and having a bearing loss of 111.5 W. The force acting on the bearings242A and244A is 0 N when rotating at 1368 RPM and having a bearing loss of 26.4 W.

Turning now toFIG. 3, a transmission410that incorporates constant leading and RH main shaft helix hands according to one example of the present disclosure will be described. The transmission410comprises like components of the transmission10described above and identified with reference numerals increased by 400. In the transmission410, gear helix hands are changed (LH to RH) such that the forces generated by the gears are directed toward bearings that have lower slip speeds. Bearing power loss is reduced since the losses are a function of the differential rotation speed between inner and outer races of the bearing in combination with the load.

Returning to the transmission210(FIG. 2), the mainshaft bearings284and240B that carry the mainshaft axial load have high slip speeds since their outer races are attached to ground and their inner races are attached to the shafts. The pocket bearing240A that separates the two main shafts238and240has a much lower delta speed. Therefore, it is able to carry the load more efficiently.

Referring again to the transmission410(FIG. 3), the mainshaft gear helix hands are switched from LH to RH. The axial loads are removed from the bearing484and440B. All of the mainshaft axial forces are directed toward the pocket bearing440A. Since the slip speed of the bearing430A (300 RPM) is lower than the slip speeds of the bearing484(1000 RPM) and bearing440B (1306 RPM) the total power loss is reduced if the coefficients of friction are similar for all three bearings. When comparing a power loss of bearing430A (53.0 W) in the transmission410with the power loss of bearing238A (102.1 W) and bearing240B (115.5 W) in the transmission210, the overall power loss for the three bearings430A,438A and440B. Since the two gear sets550and552generate equal and opposite axial forces due to the constant leading (F5A, F5B, F6A and F6B are equal), no axial force needs to be carried by bearing438A or bearing430A and the countershaft bearing axial loads remain 0 N. In the configuration of the transmission410, a total reduction in bearing power loss over the transmission10is 61.8% (412.6 W compared to 157.7 W).

The foregoing description of the examples has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular example are generally not limited to that particular example, but, where applicable, are interchangeable and can be used in a selected example, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.