Abstract:
A transmission having one or more clutches and brakes wherein only one clutch or brake need be engaged to connect the input to the output is equipped with a two-step actuator valve arrangement in which two valves, arranged in series, must be opened before any clutch or brake is engaged to power the output shaft. A single first-step actuator valve is arranged in series with a plurality of second-step actuator valves. Each of the second-step valves controls a clutch or brake.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to an electro-hydraulically controlled transmission, and in particular to a transmission having one or more releasable coupling mechanisms, such as a clutch or a brake, wherein only one such mechanism needs to be engaged for a transmission output shaft to be drivingly coupled to the input shaft. A two-step actuator valve arrangement is provided in the hydraulic control circuit so that two valves must be moved to open positions to engage a releasable coupling mechanism. 
     It is common in vehicle transmissions, such as a power shift transmission, to have a number of clutches and/or brakes that are selectively engageable in various combinations to route power through the transmission to produce different gear ratios. In a typical power shift transmission, multiple clutches and/or brakes must be engaged to provide power to the transmission output shaft. To engage multiple clutches or brakes, multiple valves in the hydraulic circuit need to be opened. Multiple valves are needed because different combinations of brakes and clutches must be engaged to produce different transmission gear ratios. A dedicated valve controls each clutch and brake. The need for multiple valves to be opened to provide power to the output shaft significantly reduces the likelihood of inadvertently doing so due to a valve sticking in an open position. 
     The present invention is useful in a transmission where only a single releasable coupling mechanism, such as a clutch or brake, needs to be engaged to provide power to the output shaft. The present invention provides a two-step actuator valve arrangement in which two valves arranged in series must be opened before any releasable coupling mechanism is engaged to power the output shaft. In the present invention, a single first-step actuator valve is arranged in series with a plurality of second-step actuator valves. The second-step actuator valves are arranged parallel with one another. In the resulting control circuit, both the first-step actuator valve, and one of the second-step actuator valves, must be opened before power is provided to any one of the clutches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a hydro-mechanical transmission having the two-step clutch actuator valve of the present invention. 
     FIG. 2 is a schematic of the hydraulic circuit operable to engage the releasable coupling mechanisms of the transmission. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A hydro-mechanical transmission having the two-step clutch actuator valve of the present invention is shown schematically in FIG.  1  and designated generally at  10 . The transmission  10  is only an example of one transmission that may utilize the two-step clutch actuator valve of the present invention. The transmission  10  has an input shaft  12  that is driven by an engine  14  or other source of rotational power. The transmission has a hydrostatic unit  16  including a variable displacement pump  18  and a fixed displacement motor  20 . The pump and motor are coupled hydraulically as shown by the lines  22  to form a hydrostatic transmission unit. Those skilled in the art will recognize that both the pump and the motor may be variable displacement components. The pump has an input shaft  24  that is driven by the transmission input shaft  12  through a pair of gears  26  and  27 . 
     The transmission includes a combining mechanical transmission  30  having planetary system  32 . The planetary system  32  includes three planetary gear sets  34 ,  36 , and  38 . The planetary gear sets have a common planet gear carrier  28  that carries the integral planet gears P 1  and P 2  of planetary gear sets  34  and  36  respectively. The carrier  28  also carries the planet gears P 3  and P 4  of the reversing planetary gear set  38 . The planet gears P 1  and P 2  are integrally formed and thus rotate together. The planet gears P 2  mesh with a ring gear R 2 . The ring gear R 2  is formed integrally with a gear  40 , coaxial with the transmission input shaft  12 . The drive gear  42  drives the gear  40  on a hydrostatic unit output shaft  44 . Thus the ring gear R 2  serves as a hydrostatic power input element to the combining mechanical transmission. 
     The transmission input shaft  14  also drives a sun gear S 1  of the first planetary gear set  34  whereby the sun gear S 1  is the mechanical power input element to the combining mechanical transmission. Sun gear S 1  meshes with the planet gear P 1 . The planetary gear set  36  includes a sun gear S 2  meshing with the planet gears P 2 . 
     In the transmission  10 , the input shaft  12  extends through the entire transmission to drive a power take off, not shown, and/or to drive other vehicle components, such as a hydraulic pump. The output shaft  46  is a sleeve shaft that surrounds the input shaft  12 . Two clutches, a low range clutch CL and a high range clutch CH, selectively couple elements of the planetary system to the transmission output shaft  46 . The low range clutch CL is engagable to couple the carrier  28  to the output shaft  46  for a low speed forward range. The high range clutch CH is engagable to couple the sun gear S 2  to the output shaft  46  for a high-speed forward range. 
     The output shaft  46  is fixed to the sun gear S 3 . Ring gear R 3  is selectively grounded by the reverse brake  48 . This stops the rotation of the ring gear R 3  and causes the sun gear S 3  to rotate in the reverse direction for a reverse speed range. When the reverse brake  48  is applied, both the low and high range clutches CL and CH are disengaged, whereby the sun gear S 3  drives the output shaft  46 . 
     The transmission  10  operates in three ranges, a reverse range, a low speed forward range and a high-speed forward range. Each range uses a separate path through the mechanical transmission to the output shaft  46  resulting in unique gear ratios for each range. The low range clutch CL, the high range clutch CH and the reverse brake  48  are all releasable coupling mechanisms that are selectively engageable to couple the output shaft  46  to the input shaft  12 . 
     With reference to FIG. 2, a fluid power circuit is shown for providing fluid power to engage the clutches and brake. The fluid power circuit provides hydraulic power to the transmission, only the portion of the fluid power circuit relevant to the invention is shown in FIG.  2 . The circuit includes a pump  60  supplying hydraulic fluid under pressure to lines  62  and  64 . Line  62  is a control circuit that is used to operate the actuator valves while the line  64  provides the fluid power to engage the clutches and brake. The circuit  58  includes a single first-step actuator valve  68  that is arranged in series with three second-step actuator valves  70 ,  72  and  74 . The valve  70  operates the low range clutch CL, the valve  72  operates the high range clutch CH and the valve  74  operates the reverse brake  48 . The valves  70 ,  72  and  74  are arranged parallel with one another. Each of the valves  68 ,  70 ,  72  and  74  has a solenoid pilot stage and a main stage. 
     The first-step valve  68  includes a solenoid pilot valve  78  and a main stage valve  86 . An orifice  76  is placed in the control line  62  before the pilot valve  78 . The pilot valve  78  is normally opened, and leads to the reservoir  80 . When the solenoid  82  is energized, the pilot valve  78  is closed, and a pressurized fluid is directed through the line  84  to move the spool of the main stage valve  86  to the open position. This allows pressurized fluid from the line  64  to flow through the valve  86  and to each of the valves  70 ,  72  and  74 . 
     Each of the valves  70 ,  72  and  74  are similarly constructed and have a pilot valve  90  that receives fluid from the control line  62  through an orifice  92 . When any one of the solenoids  94  are energized, the pilot valves  90  are closed, providing fluid through the line  96  to open the respective main stage valve  98 . The circuit  58  requires that two valves must be opened to engage any one of the low range clutch, the high range clutch or the reverse brake. Each clutch and brake requires that the first-step valve  68  and one of the second-step valves be opened. Thus, the probability of inadvertent clutch or brake engagement due to a valve remaining open is reduced. It will be appreciated by those skilled in the art that any number of second-step valves can be arranged with the first-step valve  68  depending on the number of clutches and brakes in the transmission. 
     The valve  68  is a modulating valve allowing the fluid flow through the valve to be modulated. This valve is designed for operation with a vehicle clutch pedal to allow the operator to partially engage a clutch by partially releasing the clutch pedal. 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.