Abstract:
A power take off unit includes an output gear having a hydraulic circuit. The hydraulic circuit is an internal hydraulic circuit that is in fluid communication with an internal clutch assembly. The hydraulic circuit includes a flow-restrictive passage that modulates the flow of hydraulic fluid in order to provide a soft start clutch engagement that reduces shock loads associated with loads produced when starting torque is applied to the attached equipment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/003,920, filed May 28, 2014, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates in general to power transfer assemblies. In particular, this invention relates to a power take off unit that is configured to provide a clutch engagement rate that reduces shock loads. This invention further relates to a power take off unit having a clutch engagement rate that can be generally matched to the inertia characteristics of a range of driven equipment or a particular piece of driven equipment. In addition, this invention relates to a power take off unit having a clutch engagement system where the rate of clutch actuation is adjustable. 
         [0003]    Power take off (PTO) units transfer power from a power source, such as a transmission or engine, to a driven piece of equipment, such as a pump, generator, or other rotating component, typically by way of meshing gear trains. In order to selectively operate the driven equipment, PTO units typically include a mechanism to engage and disengage the gear train in order to start or stop the flow of power to the driven equipment. Often, PTO units rely on clutches to selectively couple the power source to the driven equipment. Clutches operate by compressing one or more driven discs, coupled to the power source, against one or more driving discs, coupled to the driven equipment. The one or more driven discs or driving discs may be configured as friction plates that are compressed against the other of the one or more driven or driving discs, configured as steel plates to aid in power transfer and permit controlled engagement of clutch assembly. Controlling the engagement of the clutch assembly is important to prevent damage to the PTO unit, the driven equipment and interconnecting shafts, and the power source. The damage can occur if shock loads are imparted to the system as power is abruptly transferred from the power source to the driven equipment. 
         [0004]    Controlled engagement of the clutch assembly includes consideration, in part, of two system characteristics—inertia and actuation acceleration. Inertia is a function of the operating system, such as the type of driven equipment, including parameters such as the system rotating mass and loads imparted by the end use device and the power source being used. There is little that can be adjusted to change the inertia characteristics of the driven equipment once the system has been selected. The actuation acceleration controls how fast or “hard” the clutch engages and transfers power from the power source to the driven equipment. Too fast of an actuation speed creates a torque spike in the system that can damage components. Too slow of an actuation speed causes the slippage within the clutch which will damage the clutch assembly. 
         [0005]    It would be advantageous to provide a PTO unit having a clutch system that could control the actuation acceleration of the clutch assembly to prevent inertia damage and excessive clutch wear. It would further be advantageous to provide a clutch actuation system that can be adjusted to tailor the clutch actuation acceleration to the specific inertia of the attached driven equipment system. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention relates to a power take off unit that is configured to provide a clutch engagement rate that reduces shock loads. This invention further relates to a power take off unit having a clutch engagement rate that can be generally matched to the inertia characteristics of a range of driven equipment or a particular piece of driven equipment. In addition, this invention relates to a power take off unit having a clutch engagement system where the rate of clutch actuation is adjustable. 
         [0007]    In a first aspect of the invention, a power take off unit includes a housing and an input gear train rotatably supported by the housing. A clutch assembly includes a driving housing, a driven housing, and an actuation piston. The clutch assembly is supported within the housing. The driving housing is connected to the input gear train. An output shaft has a hydraulic circuit that includes a primary feed that is in fluid communication with a secondary feed. The primary feed is configured to receive a flow of fluid from a hydraulic fluid source. The secondary feed is in fluid communication with the actuation piston. At least one of the primary and secondary feeds has a diameter configured to provide a clutch engagement rate that is proportional to a driven equipment moment of inertia to substantially prevent shock loading. 
         [0008]    In a second aspect of the invention, a power take off unit includes an input gear train connected to an external power source. A clutch assembly has a driving collar, a driven collar, and a clutch pack positioned between the driving and driven collars. The clutch pack includes a plurality of driving plates engaging the driving collar and a plurality of driven plates interposed between the plurality of driving plates and engaging the driven collar. The driving collar is connected to the input gear train. An actuation piston is oriented to compress the clutch assembly. An output shaft is connected to the driven collar and includes a hydraulic circuit having a primary feed that is in fluid communication with a secondary feed. The primary feed receives a flow of fluid from a hydraulic fluid source. The secondary feed is in fluid communication with the actuation piston. The primary feed has a diameter in a range of about 0.38 inches (9.65 mm) to about 0.06 inches (1.52 mm) and the secondary feed having a diameter that is smaller than the primary feed diameter. The secondary feed diameter is directly proportional to a driven equipment moment of inertia to define a clutch engagement rate. 
         [0009]    A feature of the second aspect of the invention defines the secondary feed diameter to be in a range of about 0.09 inches (2.29 mm) to about 0.016 inches (0.406 mm) which produces a clutch engagement rate of about 100 milliseconds for the driven equipment moment of inertia in a range of 0.006 LB-FT 2  to about 20 LB-FT 2 . 
         [0010]    In a third aspect of the invention, a vehicle-mounted power take off unit includes an input gear train connected to a vehicle-mounted power source. A clutch assembly is driven by an actuation piston to selectively engage the input gear train to an output shaft. A driven equipment load is coupled to the output shaft and has a moment of inertial in a range of about 0.006 LB-FT 2  to about 20 LB-FT 2 . A hydraulic circuit receives a flow of fluid from a hydraulic fluid source and delivers the flow of fluid to the actuation piston such that the clutch assembly engages the input gear train to the output shaft in about 100 milliseconds. 
         [0011]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross sectional view of a power take-off (PTO) unit having a clutch actuation control circuit in accordance with the invention. 
           [0013]      FIG. 2A  is the PTO clutch support shaft of  FIG. 1 , in partial cross section, showing a first embodiment of a clutch actuation control circuit. 
           [0014]      FIG. 2B  is an exploded view of a second embodiment of a clutch actuation control circuit. 
           [0015]      FIG. 2C  is an enlarged view of a portion of the clutch actuation control circuit of  FIG. 2B . 
           [0016]      FIG. 3A  is an exploded view of a third embodiment of a portion of a clutch actuation control circuit. 
           [0017]      FIG. 3B  is an end view of a control orifice of the clutch actuation control circuit of  FIG. 3A . 
           [0018]      FIG. 4  is a chart of test results of PTO shaft torque versus clutch control orifice diameter for a driven auxiliary piece of equipment having a mass moment of inertia. 
           [0019]      FIG. 5  is a chart of estimated clutch engagement times for the orifice tests of  FIG. 4 . 
           [0020]      FIG. 6  is a chart of confirmation tests of PTO shaft torque versus clutch control orifice diameter for a driven auxiliary piece of equipment having a mass moment of inertia, similar to  FIG. 4 . 
           [0021]      FIG. 7  is a chart of confirmation estimated clutch engagement times for the orifice tests of  FIG. 6 , similar to  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Referring now to the drawings, there is illustrated in  FIG. 1  a power take-off (PTO) unit, shown generally at  10 . Though shown and described in the context of a PTO unit, the various embodiments of the invention described below may be used in conjunction with any hydraulically actuated clutch system having a through-shaft hydraulic clutch actuation control circuit. The PTO unit  10  is configured to accept power input from a primary drive component, such as an engine or a transmission, and produce an output sufficient to drive an auxiliary device such as, for example, a hydraulic pump, air compressor, or electric generator. In the illustrated embodiment, the PTO unit  10  includes an input gear train, shown generally at  12 , an output shaft  14  terminating in an output spline or keyway  14   a  and further having a drive gear or spline  14   b , and a clutch assembly, shown generally at  16 , that is connected between the input gear train  12  and the output shaft  14 . The input gear train  12  includes an input gear  12   a  that connects to a power source, such as for example a transmission. An intermediate gear  12   b  is connected to the input gear  12   a . Though illustrated as being integrally connected to the input gear  12   a , the intermediate gear  12   b  may be directly or indirectly meshed or engaged to the input gear  12   a . The gears are arranged such that the teeth are in a meshing engagement to transfer rotary motion and power from the input gear  12   a  to the output spline  14   a . It should be understood that the gears may be provided in any number and in any mounting arrangement other than depicted and remain within the scope of the invention. 
         [0023]    The clutch assembly  16  includes a driving housing  18  and a driven housing  20 . The driving housing  18  includes a driving gear  18   a , shown meshed to the intermediate gear  12   b , and a driving collar  18   b  connected to the driving gear  18   a . Though illustrated as being integrally connected to the driving housing  18 , the driving gear  18   a  may be directly or indirectly meshed or engaged to the driving housing  18 . The driving gear  18   a  and driving housing  18  are arranged such that they cooperate to transfer rotary motion and power from the driving gear  18   a  to driving housing  18  and to a clutch pack, as will be explained below. It should be understood that the gears and plates may be provided in any number and in any mounting arrangement other than depicted and remain within the scope of the invention. The driven housing  20  includes a driven gear  20   a , shown engaging the drive gear  14   b  of the output shaft  14 , and a driven collar  20   b . Though illustrated as being integrally connected to the driven housing  20 , the driven gear  20   a  may be directly or indirectly meshed or engaged to the driven housing  20 . A clutch pack, shown generally at  22 , includes driving plates  22   a  and driven plates  22   b  that are arranged in an alternating pattern. The driving collar  18   b  engages the driving plates  22   a , typically by way of teeth formed on the outer surface of each driving plate, that engage corresponding longitudinal spline teeth  18   c  formed on the inner surface of the driving collar  18   b . The driven collar  20   b  includes longitudinal teeth  20   c  formed on an outer surface that engage corresponding teeth formed around an inner diameter of the driven plates  22   b . The driving and driven plates  22   a  and  22   b  are able to slide along the longitudinal teeth  18   c  and  20   c  of the respective driving and driven collars  18   b  and  20   b  as the clutch pack  22  is compressed and released. During compression, the clutch pack  22  transfers rotary motion and power from the driving housing  18  to the driven housing  20 . A release spring  24  maintains the clutch pack  20  in a free spinning condition such that no torque or power is transferred from the driving housing  18  to the driven housing  20  until the spring force is overcome by a clutch actuation force. 
         [0024]    Referring to  FIGS. 1 and 2A , the output shaft  14  of the PTO unit  10  includes a hydraulic actuation circuit, shown generally at  26 . The output shaft  14  may, alternatively, be an intermediate shaft coupled to a gear set or an output shaft. The hydraulic circuit  26  of the output shaft  14  provides a fluid connection between the clutch assembly  16  and a source of hydraulic fluid pressure  28 , which may be an external source connected to the PTO unit  10 . The source of hydraulic fluid pressure  28  provides the force necessary to compress the clutch pack  22  against the force of the release spring  24  in order to energize the clutch and transfer power from the input gear  12   a  to the output spline  14   a . The hydraulic circuit  26  includes a primary feed  30 , illustrated as a conduit extending along the centerline or central axis of the output shaft  14 . The primary feed  30  is illustrated having a diameter of D 1 . In one embodiment, the diameter D 1  may be in a range of about 0.38 (⅜) inches to about 0.06 ( 1/16) inches. The diameter D 1  is selected to provide sufficient fluid flow to engage the clutch pack  22  over a broad range of operating conditions and to accommodate a range of output shaft lengths. In one embodiment, the range of output shaft lengths is from about 5 inches to about 32 inches. As output shaft length increases, the fluid resistance through the primary feed increases. Additionally, because the viscosity of hydraulic fluid varies based on ambient versus operating temperature conditions along with fluid type, diameter D 1  is selected to permit sufficient fluid flow, over a temperature range of about −40° F. to about +250° F. In a specific embodiment of the primary feed  30 , the diameter D 1  is about 0.125 (⅛) inches. 
         [0025]    The primary feed  30  connects to a secondary feed  32  having a diameter D 2  that is smaller than the primary feed diameter D 1 . In one embodiment where the primary feed  30  has a diameter of about 0.125 inches, the secondary feed  32  may have the diameter D 2  in a range of about 0.09 ( 3/32) inches to about 0.016 ( 1/64) inches. In another embodiment, the diameter D 2  may be in a range of about 0.06 ( 1/16) inches to about 0.025 inches. In yet another embodiment, D 2  is about 0.03 ( 1/32) inches. In one aspect of these embodiments, a consideration may be made to maintain the secondary feed diameter D 2  larger than a hydraulic system bleed hole (not shown). In one embodiment, the diameter D 2  may be about 0.03 ( 1/32) inches and the bleed hole diameter may be about 0.025 inches, having a ratio of about 1.24. In another embodiment, the ratio of secondary feed diameter D 2  to bleed hole diameter may be about 1.50 to about 1.10. Prior art secondary feeds have been known to have the same diameter, D 1 , as the primary feed  30 . However, such a large diameter secondary feed can support a clutch engagement rate in a range of about 0.01 seconds to about 0.05 seconds. This clutch engagement rate has been found to be too fast to permit smooth start-up of equipment mounted downstream of the PTO unit  10 . Particularly, if the downstream equipment has a high inertia, or resistance to motion, as the clutch engagement speed becomes faster, the driveline and PTO unit  10  are subjected to larger torque spikes. For example, a small hydraulic pump may have a moment of inertia of 0.006 LB-FT 2 , while a large PTO driven blower may have a moment of inertia of over 20 LB-FT 2 . This large difference in inertia values creates shock loading conditions and clutch plate wear issues for a PTO having a fixed clutch engagement rate that may operate such a variety of driven equipment. 
         [0026]    Referring still to  FIGS. 1 and 2A , the secondary feed  32  is illustrated extending from the primary feed  30  at a generally perpendicular angle, though the secondary feed may extend at any suitable angle desired. The secondary feed  32  supplies hydraulic fluid and the attendant clutch actuation force to a clutch actuation piston  34 . The clutch actuation piston  34  may abut a stop plate  36  that is connected to the output shaft  14 . As shown in  FIG. 1 , seals  38  are disposed between the output shaft  14  and the actuation piston  34  and stop plate  36 . The seal  38  positioned between the piston  34  and the output shaft  14  permits axial movement of the piston along the shaft  14  in response to the actuating fluid pressure. The stop plate  36  is prevented from axially moving by a snap ring  40  seated on the output shaft  14 . Thus, the fluid pressure is directed to the piston  34  against the clutch pack  22 . The hydraulic fluid flow from the secondary feed  32  is directed to a crown  34   a  of the actuation piston  34 . The crown  34   a  seats against the stop plate  36  in response to forces from the release spring  24 . 
         [0027]    Referring now to  FIGS. 2B and 2C , there is illustrated another embodiment of an output shaft, shown generally at  100 , that may be used in the PTO unit  10 , described above, in place of the output shaft  14 . The output shaft  100  includes a primary feed  110 , similar in configuration to primary feed  30  described above. The primary feed  110  may have a diameter D 1  within the ranges of diameter D 1  of the primary feed  30 . A secondary feed  120  is illustrated intersecting the primary feed  110  to provide fluid communication between the source of hydraulic fluid and the clutch assembly  16 . The secondary feed  120  has a diameter D 3 , that may be larger, smaller, or the same size as diameter D 1 . The diameter D 3  is sized to provide a flow of hydraulic fluid that is the same as D 1  or at least is greater than the flow necessary to provide a soft start clutch engagement. The soft start clutch engagement is defined as the clutch engagement speed that substantially reduces or eliminates a torque spike in the powertrain system to which the PTO unit is installed by balancing the inertia of the driven equipment against the torque available from the engine or transmission that is used to drive the driven equipment. In addition, the clutch engagement speed takes into account the amount and extent of relative slip motion between driving and driven plates and the clutch facing material to provide adequate operating life. 
         [0028]    Diameter D 3  is also sized to permit an orifice  130  to be inserted to restrict the flow of hydraulic fluid to the clutch assembly  16  to an appropriate level to provide the soft start capability. In a specific aspect of this embodiment, the outer diameter of the orifice  130  is sized to be a press fit or interference fit such that, once installed, the orifice  130  cannot be easily removed from the secondary feed  120 . As illustrated, the orifice  130  has an inner diameter D 2  that is within the same general ranges as the diameter D 2  of the secondary feed  32  described above. The ability to produce a single output shaft  100  that can be adapted to flow-restrict the hydraulic circuit to permit a soft start clutch engagement for a wide variety of driven equipment configurations helps standardize manufacturing processes and tooling to minimize machining costs and tooling setups. 
         [0029]    Referring now to  FIGS. 3A and 3B , there is illustrated another embodiment of a PTO unit, shown generally at  200 , having an output shaft  210  that includes a hydraulic circuit, a portion of which is shown generally at  220 . The hydraulic circuit  220  of the output shaft  210  includes a primary feed  230 . The PTO unit  200  is illustrated as an exploded view showing the output shaft  210  removed. The PTO unit  200  includes a rear cover assembly  240  having a solenoid valve  242 , an output shaft support member  244  and a hydraulic circuit input  246 . The primary feed  230  of the output shaft  210  includes a receiving pocket  232 , illustrated as a threaded aperture, that accepts a metering plug  234  having an orifice  236 . The orifice  236  may have a diameter D 2  that is similar in size range to the diameter D 2 , described above. The metering plug  234  is oriented at the end of the output shaft  210  and in close proximity to the rear cover assembly  240  in order to provide easy access thereto. In the illustrated embodiment, the rear cover assembly  240  may be removed and the metering plug  232 , which includes a torque transmitting feature  238  (shown as a slot for a screwdriver) to permit easy removal. This provides the ability to optimize the clutch engagement speed for the specific piece of driven equipment, particularly if modifications are required in the field. Thus, the metering plug  234  may be changed to more closely match the equipment requirements, even where those requirements change over time. 
         [0030]    As driven equipment inertias increase, a corresponding delay or slowing of equipment acceleration speeds results in reduced torsional impact loads being generated and transmitted to driveline and PTO unit components. Additionally, if driven equipment inertias are in the lower ranges, above, higher clutch engagement rates provide acceptable torsional resultant forces and improved clutch life. The secondary feed diameter ranges (identified in the respective embodiments, above, as D 2  or D 3 ), above, are adjusted such that a timed delay in clutch lock-up or full engagement results in a reduced torsional impact load generated at full clutch lock-up. It has also been found that an upper limit to this timed lock-up delay (i.e., longer time to engage) is clutch slippage resulting in a temperature rise in excess of the driving and driven plate materials, which equates to greater than expected clutch wear and reduced clutch life. While some amount of clutch slippage is the mechanism that permits full engagement delay, excessive slippage results in a temperature rise that damages friction plates and mating driving or driven plate surfaces. Such damage may be associated with galling of mating clutch surfaces, localized surface welding, and glazing resulting in a reduced frictional interface between driving and driven members. Thus, as driven equipment inertias increase, for a given PTO unit  10  having a given output shaft length L, the diameter D 2  (or D 3 ) is reduced from the diameter of the primary feed D 1 . Correspondingly, as the driven equipment inertias become smaller in magnitude, the diameter D 2  (or D 3 ) may become larger, approaching diameter D 1 . In addition to the inverse correspondence of secondary feed diameter to driven equipment inertia, considerations of fluid impedance in the primary feed line due to the length of the output shaft may also be applied. For example, as the length of the primary feed increases, a larger secondary feed diameter may be used to provide a desired clutch engagement speed for a particular driven equipment inertia. 
         [0031]    Referring now to  FIGS. 4-7 , there are shown various test results relating orifice size to torque spikes at the output shaft  14  of the PTO unit when driving an auxiliary device, such as a 185 CFM rated air compressor. As shown in  FIGS. 4 and 6 , as the orifice diameter increases, the clutch engagement torque spikes become larger. In addition, the clutch engagement times become shorter, as shown in  FIG. 5 . It is noteworthy that the clutch engagement time estimates do not sharply change, for a given equipment inertia value, until the orifice size nearly doubles. Additionally, for the given inertia of the driven auxiliary equipment, the clutch engagement time is similar for a standard production secondary feed diameter of about 0.125 inches. The clutch engagement torque spikes are shown in  FIGS. 4 and 6  to be minimized, thus reducing the torsional load spikes that cause damage to PTO components. As shown in  FIG. 7 , clutch engagement times of about 100 milliseconds are sufficient to provide adequate clutch life. 
         [0032]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.