Abstract:
A planar type of over-running clutch including first and second confronting plates is disclosed herein. The second plate itself includes movable struts and associated biasing mechanisms for cooperative engagement with cooperating shoulder members of the first plate under certain circumstances. An arrangement forming part of said second plate and cooperating with each strut and biasing mechanism of said second plate is provided for preventing the struts from moving to certain biased first positions when said second plate rotates in a particular way.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to one-way clutches, which are common components in rotary mechanical power transmission systems. More specifically, the invention is an improvement on the planar “strut” type of one-way clutch as first seen in U.S. Pat. No. 5,670,978 and subsequent improvements sold under the Trademark, “Mechanical Diode” (®).  
           [0002]    One-way clutches or OWC&#39;s as they are commonly known provide a variety of different functions in rotary power transmission systems such as safety devices for helicopter auto-rotation, hold-backs for conveyor systems and as shift components in automotive transmissions to name a few.  
           [0003]    All OWC&#39;s including the planar type, require significant lubrication in high-speed applications to prevent wear and damage due to friction. The presence of fluid is more critical in the planar type of OWC as the lubrication serves as an active component in stabilizing the behavior of the strut when overrunning in excess of the maximum dry speed. This speed varies according to the actual geometry of the clutch but has been experimentally established at ˜2,000 RPM for the general example shown later in this discussion. In very high speed applications, all OWC&#39;s require a copious amount of lubricant flow to carry off waste heat generated as a consequence of fluid shear.  
           [0004]    One problem currently not well addressed in all OWC&#39;s is in the rare occasions where system failure, contamination or momentary inertial forces cause a momentary cessation of lubricant flow. While damaging to all types of OWC&#39;s, this event can cause a rapid, catastrophic failure in a planar OWC.  
           [0005]    In the example of helicopter auto-rotation this is very serious since the reason this safety feature might be needed is in the event of sudden loss of oil and the subsequent seizing of the engine and gear train.  
           [0006]    Refinements have been made on the original strut geometry of planar OWC&#39;s that improve this situation so as to give a longer survivable time in an oil starved condition such as in U.S. Pat. No. 5,597,057 and U.S. Pat. No. 6,116,394. While this material shows an improvement, the techniques disclosed do not address the lack of stability of the strut but merely seek to restrain its resultant poor behavior. U.S. Pat. No. 6,116,394 describes the problem where unconstrained and deprived of fluid, the rear portion of the strut can enter the space of a notch and Impact with high force causing damage. U.S. Pat. No. 5,597,057 treats this by elongating the ears on the strut so that they protrude past the notch and will impact against the face of the notch plate rather than on a ramp of a notch. U.S. Pat. No. 6,116,394 shows a different strategy. It attempts to trap one edge of the strut between its pocket and the face of the notch plate so as to constrain its rotation in the event of oil loss.  
           [0007]    In the particular case of an OWC with one member stationary and the other rotating, there is a simple solution that allows high-speed over running in the absence of fluid without strut failure. It is one purpose of this invention to show such a method.  
           [0008]    Another purpose of this invention is to address the root cause of this failing in planar OWC design and remove the stimulus for bad behavior in those situations where the clutch is deprived of operating fluid. This is done by biasing the strut out of contact with the notch plate during overrun by utilizing the outward force generated by the strut carried by its pocket plate and to force a reaction with a cooperating feature on the pocket plate to counteract the bias of the engagement spring.  
           [0009]    It is important to describe the sequence of events that cause catastrophic strut failure during high-speed, no-oil overrun in these prior art devices. FIG. 1 shows the general construction of a strut type planar clutch comprised of notch plate  7 , pocket plate  2 , strut  3  and spring  9 . FIGS. 2 through 4 show sequential cross-sectional views of a single strut  3  according to the prior art during over run with no oil.  
           [0010]    First, according to FIG. 2, the strut  3  is biased upward into a passing notch  10  by its spring  9 . Next, the strut tip  11  is struck a glancing blow by the passing ramp of notch  10 , imparting a rotational moment about the strut  3  center of mass and also generating a downward thrust to the strut  3 . It is important to note that this initial impact is relatively small in magnitude. Now looking to FIG. 3, the strut impacts the bottom of its pocket  13 , rebounding upward and pivoting about the point of contact as can be seen in FIG. 4. The rear of the strut  3  continues to rise into an adjacent notch  10 . Finally, the rear of the strut  12  is struck smartly by that notch  10  ramp, imparting a large shock to the strut  3 .  
           [0011]    It is this last impact in the series that imparts the damaging forces and velocities to the strut  3 . Since this last impact requires the strut  3  to be in an orientation contrary to the bias of the spring  9 , it does not happen normally but only as a consequence of the entire sequence of FIGS.  2  to  4  as described above and only in the absence of surrounding fluid. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a cutaway perspective view of the prior art clutch.  
         [0013]    [0013]FIG. 2 is a cross section view of the prior art clutch taken along lines  2 , 3 , 4 , of FIG. 1 approximately through the center of one strut and normal to centrifugal force acting on the strut.  
         [0014]    [0014]FIG. 3 is a similar view of the prior art clutch to FIG. 2 showing the components later in the sequence.  
         [0015]    [0015]FIG. 4 is a similar view of the prior art clutch to FIG. 3 showing the components still later in the sequence.  
         [0016]    [0016]FIG. 5 is an exploded perspective view showing the components of the invention.  
         [0017]    [0017]FIG. 6 is a cutaway perspective view.  
         [0018]    [0018]FIG. 7 is an enlarged detail of the area noted in FIG. 6.  
         [0019]    [0019]FIG. 8 is a side cutaway view corresponding to the lines  8 - 8  of FIG. 6 and sectioning one strut  17  parallel to the centrifugal force  24  (F 1 ).  
         [0020]    [0020]FIG. 9 is a sketch showing the forces and geometry of the embodiment.  
         [0021]    [0021]FIG. 10 is a schematic gear train diagram showing the proposed invention in its location in a typical application.  
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIG. 5 is an exploded view of the assembly consisting of notch plate  15 , pocket plate  16 , a plurality of struts  17  carried by the pocket plate  16 , an outer strut support ring  20  and a retaining ring  18 . FIG. 6 shows these components in their assembled configuration. FIG. 7 is an enlarged view of the cutaway portion showing the components in more detail.  
         [0023]    In operation, the outer component, the notch plate  15  is held stationary and the pocket plate  16  is connected to the desired element to be controlled. During over-run, the pocket plate  16  rotates clockwise  25 , in this example, and at high speed carrying the struts  17 , which are forced outward by momentum , F 1  indicated by the arrow  24  which is a function of the speed of rotation of the pocket plate  16 . This force  24  pushes angled strut faces  21  against the angled surfaces  23  of the outer strut support  20  thereby generating a force perpendicular to force  24  and counter to the bias of the spring  19  as seen in FIG. 8 thus preventing the strut  17  form tilting away from its pocket  16  and towards the notch plate  15 . When rotation of the pocket plate  16  exceeds a designed “sleep” speed, rotation of the strut  17  in any axis is inhibited by the forces generated by the cooperating angled strut edge  21  and the angled surface  23 , thereby keeping the strut  17  out of contact with the notch plate  15 .  
         [0024]    At the point in time when the clutch is about to engage and lock (direction reversal), the rotational speed of the pocket plate  16  must match to the stationary notch plate  15  before reversing. Before this point is reached, the velocity drops below the calculated point of balance, the centrifugal force  24  on the strut  17  subsides and the spring  19  overcomes the forces generated by the angled surfaces  21  and  23 . Once this speed threshold is crossed, the strut  17  then behaves normally e.g. as the prior art devices operate. This normal behavior is only allowed at velocities below the critical limiting speed for dry over-running. Above the sleep speed, the struts  17  are inhibited from interacting with the notch plate  15  in any way, regardless of fluid condition.  
         [0025]    [0025]FIG. 10 is a schematic drawing showing an example of this OWC invention  25  in use in a typical application involving speed reduction using a generic planetary gear set  24 . In this example, input rotation  27  is present at the sun gear  31  of the gear set  24  and the ring gear  29  becomes the output  28 . This common application can only function if the planet gear carrier  32  is constrained from rotating e.g. “is grounded”. Interposing the previously described OWC  25  between the planet carrier  32  and ground  26  will provide for an over-running output  28  that will function properly even in the intermittent absence of oil supply such as in the case desirable for Helicopter auto rotation.  
         [0026]    When the input  27  is driving the output  28  at the designed ratio of the gear set  24 , the carrier  32  is forced in an absolute rotational direction e.g. relative to ground, similar to that rotational direction of the sun gear  31 . Constraining the carrier  32  to not rotate in this direction, via the lock function of the OWC assembly  25 , allows the gear set  24  to function and thereby drive the output  28  at the required ratio.  
         [0027]    In the case where the input rotation  27  ceases, or in other general cases where output  28  over-running of the input  27  prescribed speed is desired, The output  28  is now pulling the input  27  rather than being pushed by it and therefore all forces in the assembly reverse. This force reversal urges the carrier  32  to rotate in a direction, relative to ground  26 , opposite to the driving case above and the one-way clutch assembly  25  unlocks in response to this direction reversal allowing the free over-run of the output  28  at a velocity greater than that prescribed by the input  27 .  
         [0028]    As previously described, the grounded member of this OWC  25 , is the notch plate  34 , similar to that described as  15 . The rotating member connected to the carrier  32  is the pocket plate previously described as  16 . When the output  28  described above over-runs the input  27 , the pocket plate  33  of OWC assembly  25  is forced to rotate, relative to ground  26 , in its over-running direction. In the case where this rotational speed becomes too fast for safe, oil free, operation, the inventive features previously described come into play to inhibit contact of the orbiting struts carried by pocket plate  33  with the stationary notch plate  34 .  
         [0029]    Going back to FIG. 8, the strut  17  behavior is controlled as a function of the rotational speed of the pocket plate  16 , which happens to be clockwise  24  in this example. Different rotational directions or switching of the pocket plate  16  and notch plate  15  as inner and outer members are obvious and does not avoid the invention herein. Similarly, the notch plate  15  is not required to be stationary so long as the absolute velocity of the pocket plate  16  controls the behavior of the struts  17  within the bounds of an acceptable “dry” rotational speed difference between pocket plate  16  and notch plate  15  and as long as the point of relative rotation reversal between the two members allows an absolute pocket plate  16  rotational velocity below the sleep threshold.  
         [0030]    As an actual example, A clutch having struts  17  radially positioned at 2.5 inches from the axis of rotation will retract its struts  17  and not interact with the stationary notch plate  15  at approximately 790 RPM speed of the pocket plate  16  if the geometry defined in FIG. 9 is used with a strut  17  mass of 0.08 ounces. This “sleep” speed can be tuned by varying the mass and geometry of the strut  17 , as well as the spring  19  force in accordance with the equation provided below and according to FIG. 9.  
         ω   2     =         (         F   s          D   s       -       F   2          D   2         )        g         D   1        W                 r                             
 
         [0031]    Where:  
         [0032]    ω—Clutch pocket plate angular velocity  
         [0033]    F S —Spring force in the strut down position  
         [0034]    D S —Distance from strut inner angled edge to spring force  
         [0035]    D 1 —One-half of the strut thickness  
         [0036]    D 2 —Planar distance between the wedge side strut edges  
         [0037]    F 2 —Cam down force exerted by the wedge  
         [0038]    W—Strut weight  
         [0039]    r—Distance from MD axis to strut center of mass  
         [0040]    g—Gravitational Constant