Abstract:
The invention comprises a hydro-mechanical continuously variable transmission (HMCVT) that uses a planetary gear system to provide a combination of hydraulic and mechanical power for a vehicle or stationary equipment. The invention further comprises various ancillary elements to improve the performance of the HMCVT.

Description:
FIELD  
       [0001]     This invention relates to a drive system useful as a vehicle propulsion system or stationary equipment drive, combining mechanical and hydraulic power systems.  
       BACKGROUND  
       [0002]     Hydraulic drive systems are commonly used for large vehicles or stationary equipment. However, as the output speed increases at a given gear setting, the efficiency of the hydraulic drive is correspondingly reduced. This makes it inefficient to run hydraulic drives at the upper half of the gear setting. This problem may be overcome by having multiple gear settings, but the complexity of the resulting transmission negates the benefits of using a hydraulic drive.  
         [0003]     An alternative to a hydraulic drive system is a mechanically driven system. However, conventional mechanical drive systems are limited to discrete gear ratios, which do not allow for infinite speed ratios as found in hydraulic drives. A great deal of power management between the engine and the transmission at all output speeds is necessary for transmission effectiveness. A purely mechanical drive is inadequate to ensure the efficient use of the engine&#39;s available power due to the discrete speed ratios, while a purely hydraulic drive has inherently poor efficiency at higher operational speeds.  
         [0004]     With the increasing costs of fuel and more stringent emissions requirements, there is a need for more efficient drive systems for large and small vehicles, as well as stationary equipment, to replace traditional hydraulic and mechanical drive systems.  
         [0005]     It is an object of this invention to provide a more efficient drive system for large and small vehicles and stationary equipment by combining hydraulic and mechanical power systems.  
         [0006]     It is a further object of this invention to provide a transmission system for optimizing use of combined drive systems.  
         [0007]     It is a still further object of this invention to provide a combined drive system with a dual or multiple speed, shift-on-the-fly gearbox for extended speed and torque ranges.  
         [0008]     It is a still further object of this invention to provide an improved steering system for combined drive systems when applied to differential output speed requirements.  
       SUMMARY  
       [0009]     The invention comprises a hydro-mechanical continuously variable transmission (HMCVT) that uses a planetary gear system to provide a combination of hydraulic and mechanical power for a vehicle or stationary equipment.  
         [0010]     The HMCVT may also include a 2-speed planetary clutch system to expand the operating parameters of the vehicle or stationary equipment.  
         [0011]     The HMCVT may further include a planetary steering system that works with or without the 2-speed planetary clutch system.  
         [0012]     The HMCVT may also include a launch assist device to limit torque applied to the drive pump when the ratio of hydraulic pump displacement to hydraulic motor displacement is small.  
         [0013]     The HMCVT may additionally include a lockup brake coupled to the hydraulic branch input, operative to lock out the hydraulic branch and force all power through the mechanical branch when the transmission output is operating at a pre-selected percentage of its maximum speed. The lockup brake may be combined with the launch assist device into a single device.  
         [0014]     The HMCVT may further include an anti-recirculating reverser device operative to allow the transmission output to operate in a reverse direction of motion without developing a recirculating power flow through the mechanical branch.  
         [0015]     The 2-speed planetary clutch, planetary steering system, launch assist device, lockup brake and anti-recirculating reverser device may be used individually or in any combination of two or more in any given HMCVT. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention itself, both as to organization and method of operation, as well as additional objects and advantages thereof, will become readily apparent from the following detailed description when read in connection with the accompanying drawings:  
         [0017]      FIG. 1  shows a planetary gear set with multiple planetary gears;  
         [0018]      FIG. 2  shows a block diagram of an HMCVT in a RSC configuration with all optional components;  
         [0019]      FIG. 3  shows a block diagram of an HMCVT in a RSC configuration;  
         [0020]      FIG. 4  shows a block diagram of an HMCVT in a SCR configuration;  
         [0021]      FIG. 5  shows a block diagram of an HMCVT in a SRC configuration;  
         [0022]      FIG. 6  shows a block diagram of an HMCVT in a RSC configuration with a 2-speed clutch;  
         [0023]      FIG. 7  shows a block diagram of an HMCVT in a RSC configuration with a planetary steering system;  
         [0024]      FIG. 8  shows a block diagram of an HMCVT in a RSC configuration with a launch assist device;  
         [0025]      FIG. 9  shows a block diagram of an HMCVT in a RSC configuration with a lockup brake;  
         [0026]      FIG. 10  shows a block diagram of an HMCVT in a RSC configuration with a spur gear reverser;  
         [0027]      FIG. 11  shows a block diagram of an HMCVT in a RSC configuration with a bevel gear reverser;  
         [0028]      FIG. 12  shows a block diagram of an HMCVT in a RSC configuration with a mechanical disconnect reverser; 
     
    
     DETAILED DESCRIPTION  
       [0029]     The hydro-mechanical continuously variable transmission (HMCVT) is designed to split input power between a hydraulic drive branch, using a hydraulic pump and motor, and a parallel mechanical drive branch, using shafts and/or gears, recombining the power from each branch into a single output.  
         [0030]     The HMCVT is based on a planetary gear set  10  as shown in  FIG. 1 . A planetary gear set  10  consists of four parts: a carrier gear  12 , a number of planetary gears  14 , a ring gear  16  and a sun gear  18 . The ring gear  16  and the sun gear  18  are connected through the planetary gears  14 . The planetary gears  14  are also connected to the carrier gear  12 . In  FIG. 1 , three planetary gears  14   a - c  are used, more may be used if necessary.  
         [0031]     The planetary gear set  10  is connected to the hydraulic drive pump  22 , the main shaft  26  and the transmission input  40 . A 3 letter code (R=ring gear; S=sun gear; C=carrier gear) has been adopted for purposes of this discussion to describe how the planetary gear set  10  is connected within the transmission: 1 st  letter designates which part of planetary gear set  10  is connected to the hydraulic drive pump  22 ; 2 nd  letter designates which part of planetary gear set  10  is connected to the main shaft  26 ; the last letter designates which part of planetary gear set  10  is connected to the input  40  from the engine.  
         [0032]     A full HMCVT system in a RSC configuration with all optional components connected is shown in  FIG. 2 . In the RSC configuration, a hydraulic drive pump  22  is connected to the ring gear  16  of the planetary gear set  10  and the sun gear  18  is connected to the combiner gear  20  via the main shaft  26 . The combiner gear  20  is also connected to a hydraulic drive motor  24 . The input  40  from the main engine (not shown) to the HMCVT is received by the carrier gear  12 . A detailed view of the RSC configuration is shown in  FIG. 3 .  
         [0033]     A full HMCVT system in a SCR configuration is shown in  FIG. 4 . In the SCR configuration, a hydraulic drive pump  22  is connected to the sun gear  18  of the planetary gear set  10  and the carrier gear  12  is connected to the combiner gear  20  via the main shaft  26 . The combiner gear  20  is also connected to a hydraulic drive motor  24 . The input  40  to the HMCVT is received by the ring gear  16 .  
         [0034]     A full HMCVT system in a SRC configuration is shown in  FIG. 5 . In the SRC configuration, a hydraulic drive pump  22  is connected to the sun gear  18  of the planetary gear set  10  and the ring gear  16  is connected to the combiner gear  20  via the main shaft  26 . The combiner gear  20  is also connected to a hydraulic drive motor  24 . The input  40  to the HMCVT is received by the carrier gear  12 .  
         [0035]     In theory, the carrier gear  12 , ring gear  16  and sun gear  18  may be connected to the input  40 , drive pump  22  and main shaft  26  in any combination. However, the above three configurations have tested as the most practical for application as transmissions for large vehicles.  
         [0036]     Mathematically, it can be shown that in the HMCVT, the power is split such that the power from the hydraulic system (including the drive pump  22  and the drive motor  24 ) combines with the mechanical system (including the main shaft  26 ) to equal 100% of the total powerless efficiency losses. It can further be shown that the percentage of mechanical power increases as the output speed increases, with a corresponding decrease in hydraulic power. The result is a more efficient use of the input energy  40  than in a strictly hydraulic or strictly mechanical transmission.  
         [0037]     It can also be shown that the torque ratio between the ring gear  16  and the sun gear  18  is only dependent on the gear ratio between the ring gear  16  and the sun gear  18 . This means that the final gear ratio of the HMCVT can be set by the choice of ring gear  16  and sun gear  18 .  
         [0038]     To prove: define the following terms: h-hydraulic, m-mechanical, i-input, specific speed (O x ) is ratio of x (x=h,m,i) gear speed to input (i) gear speed.  
         [0039]     Define a constant R as the speed of the m-gear when the h-gear is not turning: R=O m |O h =0. Then define O m =RS, where S reflects the actual speed of the output (as a value from 0 to 1). R and S are used to make the equations independent of the actual configuration of the planetary gear set  10 .  
         [0040]     Since O m  is linear in S, O h  must also be linear with S, as a function of (1−S), since O h =0 when S=1. At S=1/R, O m =1. This means that at S=1/R the i-gear and m-gear are turning at the same speed. Considering the planetary gear model in  FIG. 1 , this means that the ring gear  16  and sun gear  18  are turning at the same speed. For this to occur, the planetary gears  14  must not be turning, meaning that the carrier gear  12  is also turning at the same speed as the ring gear  16  and sun gear  18 .  
         [0041]     More generally, when any two of the gears of the planetary gear set  10  are moving at the same speed, so is the third gear. Using this result, we then get O h =(R/R−1) (1−S).  
         [0042]     The power split then becomes P h =1−S and P m =S. This also means that two forms of power recirculation can occur: “overdrive” when S&gt;1 and “reverse” when S&lt;0.  
         [0043]     In the physical HMCVT, the combiner gear  20  and planetary gear set  10  are responsible for controlling the distribution of power between the drive pump  22  and the drive motor  24 . As the output speed changes, the power split between the drive pump  22  and drive motor  24  is also changed as described above. When the output is motionless (speed=0), main shaft  26  is also motionless (0 rpm). As the output moves, the drive pump  22  must pump fluid and, initially, all the power is derived from the drive pump  22  and drive motor  24 . As the output speed increases, the main shaft  26  and the connected drive motor  24  (through the combiner gear  20 ) must turn faster. As a result, the drive pump gear (in the RSC configuration, ring gear  16 ) turns slower, due to the effect of the planetary gear system  10  and the need to maintain a constant torque ratio.  
         [0044]     A considerable advantage of the HMCVT lies in the unique ability of the configured systems as shown in  FIGS. 2-5  to enable a driven output on both ends of the transmission via a common shaft  26 . This is particularly useful in vehicles or stationary equipment that require duplicated output shafts to two drives such as tracks and/or differentials.  
         [0045]     Furthermore, one or both of the outputs can be engaged or disengaged eliminating the need for a transfer case when configured for multiple output drives.  
         [0046]     The HMCVT speed can be controlled in any conventional manner, however an electronic control system is preferred to best optimize the power splitting in connection with the output speed. Furthermore, the electronic control system can also include control means for the two-speed transmission system, planetary steering system, launch assist device, lockup brake and anti-recirculatory reverser discussed below.  
         [0047]     Two-Speed Transmission  
         [0048]     An additional modification to optimize the use of the HMCVT is a two-speed planetary clutch system as shown in  FIG. 6 . The output  26  is connected to the input of an additional 2-speed shifting planetary  30 . Power enters the shifting planetary through the shifting planetary sun gear  31  and exits through the shifting planetary carrier gear  35 . In low speed operation, the shifting planetary ring gear  33  is held fixed by a low-speed clutch or brake  32 , creating a reduction in the gear ratio. Shifting to high gear for high-speed operation is accomplished by releasing the brake  32  and applying a high-speed clutch  34  to effectively give a 1:1 gear ratio.  
         [0049]     The 2-speed planetary clutch system provides an extended range of available speeds and torques to the operator. The result is an increased operating envelope for the vehicle or stationary equipment.  
         [0050]     Planetary Steering System  
         [0051]     Another useful modification for the HMCVT is a planetary steering system as shown in  FIG. 7 . As shown, power is sent from the main input  40  to two steering planetaries  52 , one for each output drive on either side of the transmission. The steering system also includes a closed-loop hydraulic pump  54  that is driven in relation to the current engine RPM. The output of the pump  54  is connected to a hydraulic motor  56  that drives a cross shaft assembly referred to as a zero shaft  58 .  
         [0052]     The zero shaft  58  is connected to the sun gear  60  of the left and right steering planetaries  52  and the left and right sun gears  60  are driven by the motor  56  in opposite directions. Therefore, when the zero shaft  58  turns, the speed of the inside drive of the vehicle decreases and the speed of the outside drive increases.  
         [0053]     The result of the planetary steering system is a high-precision steering system that provides quick reaction times while maintaining good driving characteristics during straight-ahead motion.  
         [0054]     Launch Assist Device (LAD)  
         [0055]     One characteristic of the HMCVT is that at low output speeds, the pump  22  is set to a very low displacement and the motor  24  is set to a high displacement. In theory, this could create a very large torque multiplication through the hydraulic branch of the HMCVT. However, in that scenario, the hydraulic pressures generated would exceed those that can be withstood by the system. Therefore, the hydraulic ratio must be reduced to limit pressure to acceptable levels. Unfortunately, this corrective measure also reduces the output torque at very low speeds.  
         [0056]     To solve this problem, an energy absorber, called a Launch Assist Device (LAD)  70  is attached to the pump  22  as shown in  FIG. 8 . The LAD  70  provides an initial resistance to the gear element of the planetary  10  driving the hydraulic branch. This resistance limits the torque available to the pump  22  and allows the mechanical branch to reach its full torque output at very low speeds. As a result, the torque is available at the output of the HMCVT.  
         [0057]     The LAD  70  is only required at very low speeds and should be gradually phased out as the speed increases. As shown, the LAD  70  is a modulated brake assembly. However, other devices, such as a fluid coupling or a torque converter, could be used.  
         [0058]     Lockup Brake  
         [0059]     At the upper limit of the HMCVT operating range, the displacement ratio between the motor  24  and the pump  22  decreases to the point where the amount of torque available to the pump  22  is insufficient to keep it turning. With the speed of the hydraulic pump  22  at zero, all the power is transferred exclusively through the mechanical branch. Unfortunately, most currently available pump and motor designs include some degree of internal leakage, preventing the HMCVT from reaching a pure 100% mechanical state.  
         [0060]     This problem can be solved by using a lockup brake  80  as shown in  FIG. 9  to prevent the pump  22  from turning at the end of the operating range. The lockup brake  80  is attached to one of the gears  10  driving the pump  22 . When the upper end of the operating range is reached, the lockup brake  80  is activated, forcing the HMCVT into a pure 100% mechanical mode. The lockup brake  80  can then be deactivated when the HMCVT operating range falls out of the upper regions. The activation/deactivation point for the lockup brake  80  will be determined by the operating conditions and parameters for the vehicle or stationary equipment using the HMCVT.  
         [0061]     Anti-Recirculatory Reverser  
         [0062]     The simplest way to reverse the direction of the final output of the transmission is to reverse the drive motor. When this happens, the power in the mechanical branch is negative and the power in the hydraulic branch is greater than the input power. What is effectively happening is that the drive motor must reverse the direction of the mechanical output of the split speed, feeding power upstream through the mechanical branch. To balance out the power equations, the hydraulic branch must transfer an amount of power equal to the input plus the recirculated power from the mechanical branch.  
         [0063]     In order to accommodate the increased power levels, both the hydraulic and the mechanical branches must have increased component strength and/or size, which is not always practical or desirable. Therefore, an additional reverser subsystem that avoids the need to reinforce the hydraulic is desirable.  
         [0064]     One potential reverser subsystem  90  as shown in  FIGS. 10 and 11  involves the installation of a reversible gear at the input. The input  40  is connected to a sliding clutch/synchronizer assembly  90 . Depending on which gear is engaged with the clutch/synchronizer  90 , the HMCVT components, including the drive pump  22  and motor  24 , will rotate in either the forward or reverse direction. Either spur gears ( FIG. 10 ) or bevel gears ( FIG. 11 ) can be used in the clutch/synchronizer  90 , however, bevel gears allow for a 90-degree change between the engine power output and the main axes of the transmission (the final power output). In either case, if steering planetaries  52  (see  FIG. 7 ) are present, they are connected to the power output through a preset gear ratio and a constant direction so that the steering will function regardless of the forward/reverse direction of the clutch/synchronizer  90 .  
         [0065]     An alternative subsystem for the reverser shown in  FIG. 12  disconnects the mechanical branch during reverse hydraulic operation and locks the mechanical output to the transmission housing (not shown). In this case, the input shaft always rotates in the same direction. The mechanical output may be selectively coupled to either the main shaft  26  or the transmission housing by means of a clutch/synchronizer assembly  112 . When the mechanical output is connected to the main shaft  26 , forward output speeds result. When the mechanical output is connected to the housing, the mechanical branch is locked out of the transmission. Reverse output speeds can then be achieved by reversing the direction of the drive motor. Power flows exclusively through the hydraulic branch and cannot be recirculated back through the mechanical branch.  
         [0066]     Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention.