This invention relates to a drive system useful as a vehicle propulsion system or stationary equipment drive, combining mechanical and hydraulic power systems.
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.
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's available power due to the discrete speed ratios, while a purely hydraulic drive has inherently poor efficiency at higher operational speeds.
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.
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.
It is a further object of this invention to provide a transmission system for optimizing use of combined drive systems.
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.
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.
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 HMCVT may also include a 2-speed planetary clutch system to expand the operating parameters of the vehicle or stationary equipment.
The HMCVT may further include a planetary steering system that works with or without the 2-speed planetary clutch system.
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.
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.
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.
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.
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:
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.
The HMCVT is based on a planetary gear set 10 as shown in
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: 1st letter designates which part of planetary gear set 10 is connected to the hydraulic drive pump 22; 2nd 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.
A full HMCVT system in a RSC configuration with all optional components connected is shown in
A full HMCVT system in a SCR configuration is shown in
A full HMCVT system in a SRC configuration is shown in
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.
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.
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.
To prove: define the following terms: h-hydraulic, m-mechanical, i-input, specific speed (Ox) is ratio of x (x=h,m,i) gear speed to input (i) gear speed.
Define a constant R as the speed of the m-gear when the h-gear is not turning: R=Om|Oh=0. Then define Om=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.
Since Om is linear in S, Oh must also be linear with S, as a function of (1−S), since Oh=0 when S=1. At S=1/R, Om=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
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 Oh=(R/R−1) (1−S).
The power split then becomes Ph=1−S and Pm=S. This also means that two forms of power recirculation can occur: “overdrive” when S>1 and “reverse” when S<0.
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.
A considerable advantage of the HMCVT lies in the unique ability of the configured systems as shown in
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.
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.
Two-Speed Transmission
An additional modification to optimize the use of the HMCVT is a two-speed planetary clutch system as shown in
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.
Planetary Steering System
Another useful modification for the HMCVT is a planetary steering system as shown in
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.
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.
Launch Assist Device (LAD)
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.
To solve this problem, an energy absorber, called a Launch Assist Device (LAD) 70 is attached to the pump 22 as shown in
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.
Lockup Brake
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.
This problem can be solved by using a lockup brake 80 as shown in
Anti-Recirculatory Reverser
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.
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.
One potential reverser subsystem 90 as shown in
An alternative subsystem for the reverser shown in
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.
Number | Date | Country | Kind |
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2,363,653 | Nov 2001 | CA | national |
Filing Document | Filing Date | Country | Kind |
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PCT/CA02/01789 | 11/19/2002 | WO |