Abstract:
A balanced clutch system is provided for use in a transmission. The balanced clutch system includes a clutch assembly that has a clutch piston and a balance piston that cooperate with a clutch piston cavity and a balance piston cavity. Each of the clutch piston cavity and balance piston cavity receives a continuous supply of fluid and establishes a centrifugal head pressure therein, during use. The centrifugal head pressure established in the balance piston cavity contributes to a force that opposes a self-engaging tendency of the clutch assembly that is created by the centrifugal head pressure established within the clutch piston cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to clutch systems for industrial transmissions an more specifically to an industrial transmission that has a balanced clutch system that resists self-engaging. 
     2. Discussion of the Related Art 
     It is known that industrial transmissions are suitable for numerous power transmitting applications. Recently, industrial transmissions have been implemented more frequently in the well drilling and other energy production industries. For example, in light of concerns regarding hydrocarbon availability, efforts have been made to increase productivity of hydrocarbon wells, in terms of efficiency and overall production. One known technique for increasing productivity of a hydrocarbon well is to stimulate the well by hydraulically (or using other fluids for) fracturing the subterranean geological formation of the well in a manner that increases permeability and therefore flow rate of the well, increasing its productivity. 
     Many efforts have been made to provide machines and systems that incorporate transmissions and are suitable for such well-stimulating fracturing tasks. Accordingly, machines and systems (referred to hereinafter as “fracturing machines”) have been introduced for pumping fluids into hydrocarbon wells. Such machines typically have an internal combustion engine that drives a transmission which, in turn, drives a hydraulic pump to generate pressures that are large enough to fracture the underground formations, increasing well productivity. As power requirements for fracturing procedures have increased over time, so have the demands on the transmissions in the fracturing machines. 
     Use of these fracturing machines can require them to be operated for extended periods of time. During such extended periods of use, the transmission may be required to selectively engage or disengage various transmission or PTO (power take off) outputs, so as to direct power from the engine toward one or more well-drilling or pumping related tools or accessories. 
     Typical transmissions used in fracturing machines include hydraulically actuated clutches. The clutches are mounted on shaft assemblies that, at times, rotate when the clutch is disengaged. The clutch itself, or components thereof, may also rotate when disengaged. With the shaft and/or clutch rotating while the clutch is disengaged, oil volume captured in a clutch piston cavity establishes a centrifugal pressure head that is proportionate to shaft rotational speed. This centrifugal pressure acts on the clutch piston, actuating the clutch piston toward its engaged position and thereby self-engaging or auto-engaging due to the centrifugal oil pressure head. Self-engagement of clutches can be problematic whether they are fully engaged or partially engaged so as to create a rotational drag or slipping condition within the clutch that can create heat and reduce the use life of various clutch components. 
     Some efforts have been made to alleviate clutch self-engagement that is caused by centrifugal head pressure. Some attempts are only able to prevent clutch self-engagement at certain narrowly defined operational (rotating) speeds. Typical clutch self-engagement attenuating configurations evacuate pressure behind the clutch piston, for example, by draining oil from the clutch piston cavity that is behind the clutch piston. Draining the clutch piston cavity in this way can cause actuation delays when trying to move the clutch piston. For example, prior art clutches can experience a clutch disengagement delay because it takes time to evacuate oil from the clutch piston cavity. These prior art clutches can also experience a clutch engagement delay because the clutch piston cavity has to be sufficiently replenished before a hydraulic pressure can build therein, to an extent needed to hydraulically push the clutch piston. 
     The prior systems fail to provide a solution to the problem of preventing clutch self-engagements while maintaining quick actuation response characteristics of clutches. 
     Accordingly, there was a need for a clutch system that can mitigate tendencies of clutch self-engagement across a wide range of operating speeds, while having quick actuation, engagement and disengagement characteristics. A solution which minimizes complexity without compromising integrity was preferred. 
     SUMMARY OF THE INVENTION 
     The present invention provides a balanced clutch system for use in a transmission and that provides equal and opposite balancing forces to a clutch piston, preventing the clutch piston from self-engaging due to a centrifugal oil pressure head. The balanced clutch system includes a clutch assembly that has a clutch piston and a balance piston that cooperate with a clutch piston cavity and a balance piston cavity. Each of the clutch piston cavity and balance piston cavity receives a continuous supply of fluid, for example, oil that is shared with the transmission, and establishes a centrifugal head pressure therein during use. The centrifugal head pressure established in the balance piston cavity contributes to a force that opposes a self-engaging tendency of the clutch assembly that is created by the centrifugal head pressure established within the clutch piston cavity. In other words, centrifugal head pressure within the clutch piston cavity biases the clutch piston toward its engaged position; however, centrifugal head pressure within the balancing piston cavity balances such self-engaging biasing force so as to mitigate the centrifugally generated self-engaging tendencies of the clutch system. 
     In one embodiment of the invention, a balance piston cavity provided between a clutch piston and balanced piston remains full of oil. Oil is continuously supplied to this balance piston cavity by an orifice that conveys transmission lubricating oil, the same lubricating oil that supplies all transmission components such as gears and bearings with forced lubrication. 
     In another embodiment of the present invention, the balanced clutch system includes a shaft being rotatable and having a lubricating fluid passage and a clutch fluid supply passage. A clutch assembly is provided concentrically around the shaft and includes a clutch input accepting power into the clutch assembly and a clutch output conveying power out of the clutch assembly. A clutch piston is also provided and is movable for (i) engaging the clutch assembly so as to translate rotation of the clutch input to rotation of the clutch output, and (ii) disengaging the clutch assembly so as to isolate the clutch input from the clutch output. The clutch assembly may further include a clutch piston cavity provided within the clutch assembly and accepting fluid from the clutch fluid supply passage of the shaft, and a balance piston cavity provided within the clutch assembly and accepting fluid from the lubricating fluid passage of the shaft. During use of the balanced clutch system, fluid in the clutch piston cavity and balance piston cavity establishes respective centrifugal head pressures that contribute to forces applied to opposing sides of the clutch piston such that when the clutch assembly is disengaged, the clutch piston remains in a substantially constant position during variations in such centrifugal head pressures. 
     According to another aspect of the invention, the centrifugal head pressures in the clutch piston cavity and balance piston cavity vary at the same rate of change with respect to each other. An outermost portion of the clutch piston cavity and an outermost portion of the balance piston cavity may be substantially equally spaced from the shaft in a radial direction. 
     According to another aspect of the invention, a balance piston is spaced from the clutch piston, the balance and clutch pistons being on opposing sides of the balance piston cavity. A pin may be provided that abuts the clutch piston and extends through the balance piston. The pin moves in unison with the clutch piston and applies an engaging force to a clutch pack of the clutch assembly when the clutch piston is biased toward the clutch pack. 
     The balance piston may be provided between the balance piston cavity and a clutch pack of the clutch assembly and a seal may extend between the balance piston and the shaft. Furthermore, the balance piston may be fixed with respect to movement along a longitudinal axis of the shaft. The balance piston may also have a surface area that is larger than a surface area of the clutch piston. The balance piston surface area can be defined at an interface between the balance piston and the balance piston cavity, and the clutch piston surface area can be defined at an interface between the clutch piston and the clutch piston cavity. 
     In another embodiment, the clutch piston and balance piston have dissimilar cross-sectional profiles. A portion of the balance piston can be housed inside of the clutch piston. 
     According to yet another embodiment, a smaller surface area of a first side of the clutch piston interfaces the clutch piston cavity and a larger surface area of a second side of the clutch piston interfaces the balance piston cavity. An outermost portion of the clutch piston cavity is located radially closer to the shaft than is an outermost portion of the balance piston cavity. 
     In another embodiment of the invention, the volume of the balance piston cavity varies inversely with respect to the volume of the clutch piston cavity, such that the volume of the clutch piston cavity increases when the volume of the balance piston cavity decreases, and the volume of the clutch piston cavity decreases when the volume of the balance piston cavity increases. The volumes of the clutch piston cavity and balance piston cavity remain substantially constant when the clutch pack is disengaged and while the centrifugal head pressures of the clutch piston cavity and the balance piston cavity vary, negating self-engaging tendencies of the clutch system. 
     In yet another embodiment of the invention, a force provided by the centrifugal head pressure within the balance piston cavity substantially mitigates the force provided by the centrifugal head pressure of the clutch piston cavity. 
     Another embodiment of the invention is directed to a balanced clutch system having a first balanced clutch assembly provided on a first rotatable shaft and a second balanced clutch assembly provided on a second rotatable shaft. Each of the first and second balanced clutch systems has a clutch piston and balance piston and a corresponding clutch piston cavity and balance piston cavity. The clutch piston cavity and the balance piston cavity of the first balanced clutch assembly define at least one of substantially analogous volumes and substantially analogous radial distances from a longitudinal axis of the first rotatable shaft. However, the clutch piston cavity and the balance piston cavity of the second balanced clutch assembly have different volumes and are located at different radial distances, at least at their outermost portions, from a longitudinal axis of the second rotatable shaft. 
     These and other aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a pictorial view of a transmission incorporating a balanced clutch system in accordance with the present invention; 
         FIG. 2  is a cross-sectional side elevation of the transmission shown in  FIG. 1 ; 
         FIG. 3  is a close-up cross-sectional view of a portion of a first clutch assembly, taken at the dashed circle labeled “ 3 ” in  FIG. 2 ; and 
         FIG. 4  is a close-up cross-sectional view of a portion of a second clutch assembly, taken at the dashed circle labeled “ 4 ” in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the drawings, and particularly to  FIGS. 1 and 2 , there is shown a pictorial and a cross-sectional view of a transmission  5  that incorporates a balanced clutch system  10  that is shown as having three clutch assemblies  12 ,  14 , and  16  ( FIG. 2 ), each being configured to provide equal and opposite balancing forces to a clutch piston, preventing the clutch piston from self-engaging due to a centrifugal oil pressure head. It is noted that for multiple clutch embodiments of the balanced clutch system  10  the various clutch assemblies  12 ,  14 ,  16  can have different configurations while achieving substantially the same self-engaging mitigation functions. As seen best in  FIG. 2 , clutch assemblies  14  and  16  are analogous to each other and their particular configuration is different from that of clutch assembly  12 . The particular exemplary configurations are described in greater detail elsewhere herein. 
     Still referring to  FIGS. 1 and 2 , transmission  5  is preferably an industrial transmission such as, e.g., a model TA90-7500 transmission available from Twin Disc, Inc. Racine Wis., USA. Transmission  5  includes an input flange  6  that couples to and receives power from a prime mover, such as a crankshaft of an internal combustion engine. By way of the balanced clutch system  10 , specifically by controlling the three clutch assemblies  12 ,  14 , and  16 , which are mounted upon shafts  22 ,  24 , and  26  respectively, an operator may selectively direct power through the transmission  5  and/or to the various outputs of the transmission  5 . The outputs include an output flange  32  and a pair of PTO&#39;s (power take offs)  34  and  36 , each of which can transmit power from the prime mover to a driven device or accessory. 
     Referring now to  FIGS. 2 and 3 , one end  22 A of shaft  22  is supported by a pair of tapered roller bearings B 1  and is splined to the input flange  6 . A second end  22 B of shaft  22  is supported by another pair of tapered roller bearings B 2  and is connected to the output flange  32 . A hydraulic collector  23  is provided concentrically around the second end of shaft  22 , between the bearings and the output flange  32 . Fluid passages  122  extend through the length of the shaft  22  and convey transmission lubricating oil, some of which is also used as the clutch actuating fluid. In this regard, the shaft  22  fluidly connects the collector  23  and the clutch assembly  12  to each other. A control valve CV 1  can be provided along this fluid connection, preferably at the second end  22 B of the shaft  22  which is in the collector  23 , for controlling fluid flow through the fluid passages  122  or other passages of the shaft  22 . 
     As best seen in  FIG. 3 , clutch assembly  12  is configured to provide equal and opposite balancing forces to a clutch piston, preventing the clutch assembly  12  from self-engaging due to a centrifugal oil pressure head. Namely, clutch assembly  12  includes a clutch basket  100  and a clutch hub  110  that house a clutch pack  120  therebetween. Clutch pack  120  has multiple friction and smooth discs that are stacked against each other in an alternating series, with some being locked into rotational unison with the basket  100  and the others being locked into rotational unison with the hub  110 . Axially compressing or squeezing the clutch pack  120  is what engages the clutch assembly  12  so that rotation of the basket  100  and hub  110  can be transmitted between each other. Such compressive or squeezing forces, that are used to engage the clutch pack  120  and thus clutch assembly  12 , are provided by hydraulic actuation of clutch piston  130 . 
     Still referring to  FIG. 3 , clutch piston  130  is has a seal engaging its outer and inner perimeters or circumferential surfaces, and is movable toward and away from the clutch pack  120  to engage and disengage it, respectively. A fluid filled clutch piston cavity  140  is adjacent a first side of the clutch piston  130  and a balance piston cavity  150  is adjacent a second side of the clutch piston  130 . Each of the clutch piston cavity  140  and the balance piston cavity  150 , by way of a respective orifice, receives a continuous supply of fluid, for example, oil that is shared with the transmission and is conveyed through the shaft  22 . For example, the clutch piston cavity  140  receives fluid from a clutch fluid supply passage  142  which is connected to one or more of the fluid passages  122  extending through the shaft  22 , which is configured for conveying fluid for actuating the clutch piston  130 . The balance piston cavity  150  receives fluid from a lubricating fluid supply passage  152  which is connected to one or more of the fluid passages  122  extending through the shaft  22 . 
     Still referring to  FIG. 3 , the volumes of each of the clutch piston cavity  140  and the balance piston cavity  150  are variable, although they vary inversely with respect to each other. In this regard, the volume of the clutch piston cavity  140  increases when the volume of the balance piston cavity  150  decreases, and the volume of the clutch piston cavity  140  decreases when the volume of the balance piston cavity  150  increases. However, the volumes of the clutch piston cavity  140  and balance piston cavity  150  only change when an operator purposefully actuates the clutch assembly  12 . Correspondingly, the volumes of the clutch piston cavity  140  and balance piston cavity  150  remain substantially constant when the clutch pack  120  is disengaged, even if the centrifugal head pressures within the clutch piston cavity  140  and the balance piston cavity  150  vary, for example, when the rotational speed of shaft  22  changes. 
     Referring yet further to  FIG. 3 , an end boundary of the balance piston cavity  150  that is furthest from the clutch piston  130  is defined by a side surface of a balance piston  160 . In this embodiment, balance piston  160  is adjacent but spaced from the clutch pack  120 . The balance piston  160  is bolted, near its outer perimeter, to a gear  165  that is provided between a pair of clutch packs  120 , with the clutch packs  120  being mirror images of each other as reflected about the gear  165  ( FIG. 2 ). An inner perimeter, namely an inner circumferential surface  162 , of the balance piston  160  is sealed against the clutch hub  110 , by way of a seal S 1  that is seated in an undercut in the outer surface of the clutch hub  110 . 
     Still referring to  FIG. 3 , an opening extends through the balance piston  160  and accepts a pin  132  therethrough. Pin  132  abuts and moves in unison with the clutch piston  130 , extends through the balance piston  160 , and engages the clutch pack  120  so as to translate movement of the clutch piston  130  to the clutch pack  120  without moving the balance piston  160 . A compression spring (not shown) is provided between the clutch piston  130  and balance piston  160 , typically being provided concentrically around the pin  132 , to help urge the clutch piston  130  away from the clutch pack  120  during disengagement of the clutch pack  120 , again without moving the balance piston  160 . 
     Still referring yet further to  FIG. 3 , it is apparent that the dimensions and configurations of the clutch piston  130  and the balance piston  160  directly influence the dimensions and configurations of the clutch piston cavity  140  and the balance piston cavity  150 . For example, below the interface between the balance piston  160  and the gear  165 , a surface of balance piston  160  that faces shaft  22  is radially spaced from the shaft  22  by substantially the same distance as an outer circumferential surface of the clutch piston  130 . Accordingly, in this embodiment, an outermost portion of the clutch piston cavity  140  and an outermost portion of the balance piston cavity  150  are substantially equally spaced from the shaft  22  in a radial direction. 
     Referring now to  FIGS. 2 and 4 , clutch assembly  14  is described, while noting that such description is equally applicable to the analogous clutch assembly  16  which is disposed substantially symmetrically to shaft  22 . Description of shaft  26 , upon which clutch assembly  16  is mounted, is likewise omitted. Clutch assembly  14  is concentrically mounted upon shaft  24 . Shaft  24  is supported at a first end  24 A by a roller bearing B 3  and at a second end  24 B by a pair of tapered roller bearings B 4 . A portion of the end  24 B of shaft  24  that is supported by the pair of tapered roller bearings B 4  extends past the bearings B 4  and is housed in a hydraulic collector  25 . As with shaft  22 , shaft  24  includes fluid passages  124 , extending longitudinally therethrough, which convey transmission lubricating oil, some of which is also used as the clutch actuating fluid, and a control valve CV 2  is preferably provided at an end of the shaft  22 , for example end  24 B, influencing the fluid flow through the shaft  22 . 
     Referring now to  FIG. 4 , the clutch assembly  14  is also configured to provide equal and opposite balancing forces to a clutch piston, preventing the clutch assembly  14  from self-engaging due to a centrifugal oil pressure head. Clutch assembly  14  includes a clutch basket  200  and a clutch hub  210  that house a clutch pack  220  therebetween, the same as clutch pack  120 . An input gear  202  is provided at an outer surface of the clutch basket  200 , for driving the clutch assembly  14 . 
     Still referring to  FIG. 4 , a clutch piston  230  is movable toward and away from the clutch pack  220  to engage and disengage it, respectively. A fluid filled clutch piston cavity  240  is adjacent a first side of the clutch piston  230  and a balance piston cavity  250  is adjacent a second side of the clutch piston  230 . Each of the clutch piston cavity  240  and the balance piston cavity  250 , by way of a respective orifice, receives a continuous supply of fluid, for example, oil that is shared with the transmission and is conveyed through the shaft  24 . For example, the clutch piston cavity  240  receives fluid from a clutch fluid supply passage  242  which is connected to one or more of the fluid passages  124  extending through the shaft  24 , which is configured for conveying fluid for actuating the clutch piston  230 . The balance piston cavity  250  receives fluid from a lubricating fluid supply passage  252  which is connected to one or more of the fluid passages  124  extending through the shaft  24 . As with the previously described embodiment of  FIG. 3 , the volumes of each of the clutch piston cavity  240  and the balance piston cavity  250  are variable and vary inversely with respect to each other. 
     Referring further to  FIG. 4 , balance piston  260  defines an end boundary of the balance piston cavity  250 . The clutch piston  230  and balance piston  260  have dissimilar cross-sectional profiles and a portion of the balance piston  260  is housed inside of the clutch piston  230 . Unlike the previously described embodiment, the inner diameter or inner circumferential surface of the balance piston  260  is not sealed against the shaft  24 . However, the other perimeter of the balance piston  250  is sealed against an inwardly facing shoulder-like projection  232  of clutch piston  230 , by way of seal  234 . A compression spring (not labeled) is provided between the clutch piston  230  and the balance piston  260 , concentrically around the shaft  24 , to help urge the clutch piston  230  away from the clutch pack  220  during disengagement of the clutch pack  220 . This can be done without moving the balance piston  260  by providing a retaining ring  262  on the shaft  24  that serves as a mechanical stop that prevents the balance piston  260  from sliding axially toward the clutch pack  220 . 
     Still referring to  FIG. 4 , the clutch piston  230  and balance piston  260  are configured so that the clutch piston cavity  240  is smaller than and positioned radially closer to the shaft  24  than the balance piston cavity  250 . Furthermore, since the clutch piston  230  separates the clutch piston cavity  240  and the balance piston cavity  250  from each other, the clutch piston  230  has a smaller surface area on its side that interfaces the clutch piston cavity  240  and a larger surface area on its opposing side that interfaces the balance piston cavity  250 . In this embodiment, the balance piston cavity  250  is a void with an L-shaped perimeter having a triangular projection extending toward the input gear  202  from the upright segment of such L-shaped perimeter. Accordingly, the balance piston cavity  250  is asymmetrical about both longitudinal and transverse axes defined through axial and radial midpoints of the balance piston cavity  250 . 
     In light of the above, the particular way in which the clutch assembly  12 ,  14 ,  16  balances the clutch piston  130 ,  230  depends on the particular configurations of and cooperation between the clutch piston cavity  140 ,  240  and balance piston cavity  150 ,  250 . Referring to  FIGS. 2 and 3 , when shaft  22  and/or the clutch assembly  12  itself rotates, centrifugal head pressures of the clutch piston cavity  140  and balance piston cavity  150  will be largely analogous with respect to their common reference pressure P o  that is defined within the collector  23 . Stated another way, the centrifugal head pressures of clutch piston cavity  140  and balance piston cavity  150  will measure approximately the same in terms of their oil column value with respect to the oil column value of defined in the collector  23 . Accordingly, balancing forces are applied from the clutch piston cavity  140  and balance piston cavity  150  toward the opposing sides of the balance piston  130 , in opposite directions, holding the balance piston  130  in place despite variations in rotational speed of the system. 
     Referring now to  FIGS. 2 and 4 , when shaft  24  and/or the clutch assembly  14  itself rotates, the centrifugal head pressures of the clutch piston cavity  240  and balance piston cavity  250  are dissimilar with respect to their common reference pressure P o  that is defined within the collector  25 . Stated another way, the centrifugal head pressures of clutch piston cavity  240  and balance piston cavity  250  will measure differently or have different magnitudes in terms of their oil column value with respect to the oil column value defined in the collector  25 . However, since the overall forces applied from the clutch piston cavity  240  and balance piston cavity  250  are functions of their respective radial distances from the shaft  24 , and since the balance piston cavity  250  is further from the shaft  24 , since the balance piston  260  surface area is larger than the clutch piston  230  surface area, the overall force applied to the clutch piston  230  from the clutch piston cavity  240  and balance piston cavity  250 , in opposing directions, is substantially the same. Accordingly, in this embodiment also, when the clutch assembly  14  is disengaged, the balance piston  130  is held place despite variations in rotational speed of the system. 
     Regardless, it is noted that many changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of some of these changes is discussed above. The scope of others will become apparent from the appended statements of invention.