Abstract:
A power split transmission concept for two or three or four modes is provided with two variable hydrostatic units, an engine mechanically connected to the row  1  sun of a planetary gear set with at least two planetary rows, the second row of the planetary rows being connected to a row  2  sun, whereby both hydrostatic units stay connected with the same planetary element through all modes, one hydrostatic unit being connected to the sun of row  2  and the other hydrostatic unit being connected to a ring gear of planetary row  2.

Description:
BACKGROUND OF THE INVENTION 
     The Invention relates to a power split transmission, which is preferably used in agricultural tractors, wheel loaders, fork lift trucks, self driving agricultural and industrial vehicles etc. These types of transmissions are advantageous when having infinitely variable ratios, so that the vehicle speed can be adjusted independently on the engine speed. This allows great control of the engine power, for example to use maximum engine power for maximum profitability in time or for best fuel economy. Vehicles like tractors need a high efficiency of the trans-mission to get low fuel consumption. This drives the transmission concept to a power split design, which is a combination of a mechanical and a hydrostatic transmission. The concept of the transmission thereby is a key factor to achieve high fuel efficiency and low cost. The proposed concept gives a simple planetary design with high efficiency for dual yoke hydrostatic units, which for example are disclosed in the German Patent Application DE 10 2006 025 347. In these hydrostatic units the cylinder blocks of a pump and a motor of bent axis type are coupled by a common yoke. When the pump is varied from zero to maximum flow rate the motor is forced from maximum to minimum flow rate. 
     STATE OF THE ART 
     The U.S. Pat. No. 6,761,658 (Stettler) discloses a four mode transmission concept with 2 variable hydrostatic units. This is a concept, which is suitable for production since it has moderate complexity with regard to a four mode transmission. Disadvantage arise from the two independently variable hydrostatic units, which is required from the transmission concept. The complexity of 2 variable units is seen in U.S. Pat. No. 6,945,041 (Fleming). US 2003 166 430 (Folsom) shows a transmission concept with 2 variable hydrostatic units and only one simple planetary. This requires relatively large hydrostatic units, which are requiring lots of space and are expensive. DE 101 28 076 (Fischer) shows a four mode transmission using one variable and one fixed displacement unit. This is a simple hydrostatic unit. The concept requires four clutches, three brakes and four rows of planetaries, which gives high complexity for manufacturing. DE°19°52°966 (Orshansky) shows a four and five mode power split transmission with a very complex planetary system. The carrier holds six planets, which all have two gears (12 gears in total). 
     Therefore, a primary object of the present invention is the provision of transmission concept suited for dual yoke hydrostatic units, which is of low complexity for manufacturing. 
     A further object of the present invention is to provide a simple planetary design with high efficiency for dual yoke hydrostatic units. 
     A further object of this invention is to provide a cost efficient transmission with at least two forward and one mode in reverse. 
     A further object of the present invention is to provide a cost efficient transmission with three modes and two reverse. 
     These and other objects will be apparent from the drawings, the description and the claims which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Transmission Concept with 2 Modes in Forward+1 Mode in Reverse 
         FIG. 2 : Speed Diagram for Planetary Elements in the 2 Mode Concept 
         FIG. 3 : Pressure, Power &amp; Flow for the 2 Mode concept 
         FIG. 4 : Transmission Concept with 3 Modes Forward+2 Reverse (w/idler gear) 
         FIG. 5 : Speed Diagram for Planetary Elements in the 3 Mode Concept 
         FIG. 6 : Pressure, Power, Flow for the 3 Modes Concept 
         FIG. 7 : 3 Modes Forward w/Reversing Planetary at Transmission Output Shaft 
         FIG. 8 : 3 Modes Forward w/Reversing Planetary at Transmission Input Shaft 
         FIG. 9 : Transmission Concept with 4 Modes in Forward+2 Modes in Reverse 
         FIG. 10 : Speed Diagram for Planetary Elements in the 4 Mode Concept Using the 2 Mode Planetary Concept (2 full modes+2 half modes) 
         FIG. 11 : 4 Modes Forward w/Reversing Planetary at Transmission Output Shaft using the Ring Gear as Mode 1 Output 
         FIG. 12 : Speed Diagram for Planetary Elements in the 4 Mode Concept using the Ring Gear as Mode 1 Output 
         FIG. 13 : As  FIG. 11 , but the Planetary optimized for Manufacturing 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the transmission concept with two variable hydrostatic units VU 1 , VU 2 . The VU 1   1  is connected via the gears  2 ,  3  to the carrier  4  of the planetary row  2 , the carrier  4  holding two rows  1 ,  2 . The VU 2   5  connects via the gears  6 ,  7  to the sun  8  of planetary row  2 . The engine  22  is driving the sun  10  via shaft  9 . This transmission concept has two speed ranges (called modes) in forward and one speed range (mode) in reverse. For mode 1 forward the sun  8  connects via gear  7 , gear  13  and clutch  14  to the transmission output shaft  12 . The output shaft is driving into the rear axle and to the front axle. For reverse is the sun  8  drives via gear  7 , the idler gear  15 , gear  16  and clutch  17  to the output shaft  12 . The idler  15  reverses the speed for driving reverse. In mode 2 forward, the carrier  18  drives via gear  19 , gear  20  and clutch  21  into the output shaft  12 . The output shaft connects to the differential  24  of the rear axle and to the shaft  25 , which connects to the front axle. 
       FIG. 2  shows the speeds of the planetary elements. It is very advantageous to connect the engine to the sun of the row  1  planetary. This helps sizing the planetary gears in a way, so that the second sun (sun  2  in  FIG. 2 ) is not getting to extremely high speeds. The ratio K 2  is designed to have an output speed at second sun between 0 and a max. value. This allows to connect the first mode to the second sun. The planetary ratio K 1  and the gear ratios of the gears  19 ,  20  and gears  7 ,  13  are calculated to have the same output shaft  12  speed at the max mode 1 speed and the min mode 2 speed. This gives a mode 1 to 2 change with no change in vehicle speed. 
     At vehicle start the VU 2  is connected to the output shaft, which then is not turning. VU 1  connects to the ring gear, which is at max. speed (dashed line). The vehicle accelerates with VU 1  increasing the angle from 0 to max. At the same time VU 2  increases in speed and VU 1  decreases in speed. At the max speed of mode 1, VU 1  is with min speed at max displacement and VU 2  in max speed at min displacement. The dotted line shows this operating point. After the shift into mode 2 further acceleration is by increasing the displacement of VU 2  (while decreasing speed) and by decreasing displacement of VU 1  (while increasing speed). 
     This planetary concept is very good in efficiency since it does not have counter rotating speeds. All elements speeds are positive and have the same direction as the engine. 
     The other positive aspect is, that the speeds of VU 1  and VU 2  are always positive and the displacements follow the opposite sequence of mode 1. This allows the use of the dual yoke concept shown in DE 10 2006 025 347. This is a big advantage in cost and reliability as described in the patent. 
       FIG. 3  shows the operating conditions of the hydrostatic module. At the x-axis is vehicle speed, on the y-axis is engine power and the pressure, flow and power of the hydrostatic module (VU 1  and VU 2 ). At very low vehicle speeds, the power is mostly transferred hydrostatic. At about ¼ of mode 1 speed the hydro power is maximum (˜80% of engine power) and it reduces to 0 at the end of mode 1. In mode 2 the hydro power only gets to about 30% of the engine power. The low power at the hydrostatic module gives high efficiency for the transmission. The best points of transmission efficiency are in the high end of mode 1 and 2 due to low hydrostatic power level and low pressure. In the very low vehicle speed the power in the transmission is limited by the pressure setting of the hydrostatic module. Assuming a ratio of 3 for the second mode, then the mode 1 to 2 shift is at 20 km/h for a vehicle having 60 km/h as max. speed. A ratio of 2 to 4 is practical, depending on the vehicle needs. 
       FIG. 4  shows an extension of the transmission concept shown in  FIG. 1 . A third row of planetary  26  is added. This gives a third sun  27 , which drives the output shaft  12  via gears  28  and  29  and clutch  30 . Since a three mode transmission gives about 15-20% speed of the max vehicle speed, the 1 mode reverse is typically not enough. So a three mode transmission requires more than 1 mode for reverse.  FIG. 4  shows a second mode for reverse by adding the idler gear  31 , gear  32  and the clutch  33 . 
       FIG. 5  shows the planetary speeds with the added 3rd mode. Via proper sizing of the sun  3 , a full 3rd mode is added over the 2 mode speeds of  FIG. 2 . 
       FIG. 6  shows the operating conditions of the hydrostatic module in a 3 mode concept. The hydrostatic power is positive in mode 1 (parallel to the mechanical power), negative in mode 2 (power flows backwards) and positive again in mode 3. The mode 3 in addition allows to reduce the size of the hydrostatic modules VU 1  and VU 2 , so that less power is transferred via hydrostatics. This gives improved efficiency for the total transmission. The best points for efficiency are at the end of each mode (11 km/h, 26 km/h and 60 km/h). 
     Without reducing the size of the hydrostatic module, the additional 3rd mode allows to use the components of the 2 mode in a higher horsepower transmission. This gives a modular design and improved usage of the same parts. 
       FIG. 7  shows a 3 mode with full reverse speed. The shaft  42  drives the carrier  36 . In forward the carrier is connected via the clutch  37  to the output shaft  39 . In reverse the carrier  36  drives the planet  35 , which connects to planet  40 . The carrier also drives planet  40 , which connects to the ring gear  41 , which is locked by the brake  34 . In this way the reverse speed can be as fast as the forward speed. This is typically used in wheel loaders. 
       FIG. 8  shows an alternative reverse to  FIG. 7 . The reversing planetary is at the transmission input shaft. The engine  22  drives the carrier  43 . The output is the sun  44 , which drives sun  10  of the planetary row  1 . The function of the planetary is the same as described in  FIG. 7 . This position of the reversing planetary is advantageous since this shaft has less torque and less speed. Losses are less and parts are smaller in size. 
       FIG. 9  shows a possibility for 4 modes out of the basic planetary for a 2 mode system from  FIG. 1 . The sun  8  drives another set of gears  48 ,  49  to the output shaft  12 , which gives a mode 3. The carrier  18  drives a forth mode through the gear set  45 ,  46 . 
       FIG. 10  shows the speeds of the concept of  FIG. 9 . The modes  2  and  3  are modes with reduced ratios. They do not utilize the full capability since Mode 3 has to start with the dotted/dashed line. Otherwise the mode 3 output speed would be zero or very small, which gives very high pressure levels in the hydrostatic module. 
       FIG. 11  shows an alternate solution for a 3 or 4 mode concept. Here the mode 1 comes from the ring gear  51  through gear  52  to the shaft  42 . In consequence the mode 2 is from the sun  3  through gears  56  and  57 . Mode 3 is from the carrier  53 . The planetary can be designed so that the carrier has less output speed compared to the sun of planetary row  2 . This reduces the gear ratio for the gears  52  and  54 . The potential mode 4 is as mode 2 from the sun  3 , but has separate gear ratios  58  and  59 . 
       FIG. 12  shows the element speeds of the transmission concept of  FIG. 11 . Key is to reduce the planetary output speed for mode 1 since the output shaft is very slow in mode 1. This can be done by decreasing K 1  ratio and increasing K 2 . The high sun  2  output speed matches nice with the high speed of VU 2 . 
       FIG. 13  shows a modified planet design, which is easier to machine. The original planetary row  3  is changed with row  2 . This improves the spline machining.