Abstract:
A roller finger follower assembly for variably activating a valve in an internal combustion engine includes at least one vertical recessed channel formed inside an outer arm and a shaft. The channel includes a machined upper surface closing the channel. The shaft engages with the recessed channel, the shaft reciprocates within the recessed channel, and the upper surface stops an upward movement of the shaft.

Description:
TECHNICAL FIELD 
     The present invention relates to mechanisms for altering the actuation of valves in internal combustion engines; more particularly, to a switchable rocker arm such as a roller finger follower capable of changing between high and low or no valve lifts; and most particularly, to a stop for limiting the upward travel of an inner arm of a switchable rocker arm. 
     BACKGROUND OF THE INVENTION 
     Variable valve activation (VVA) mechanisms for internal combustion engines are well known. It is known to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such valve deactivation or valve lift switching can substantially improve fuel efficiency. 
     A Roller Finger Follower (RFF), as a type of rocker arm, acts between a rotating eccentric camshaft lobe and a pivot point on the engine, such as a Hydraulic Lash Adjuster (HLA), to open and close an engine valve. Switchable RFFs may be a “deactivation” type or a “two-step” type. The term switchable deactivation RFF, as used herein, means the switchable RFF is capable of switching from a valve lift mode to a no lift mode. The term switchable two-step RFF, as used herein, means the switchable RFF is capable of switching from a first valve lift mode to a second and lesser valve lift mode that is greater than no lift. When the term “switchable RFF” is used herein, by itself, it includes both types. 
     A typical switchable RFF includes an outer arm and an inner arm. The inner arm is movably connected to the outer arm. It can be switched by a locking member, from a coupled mode wherein the inner arm is immobilized relative to the outer arm, to a decoupled state wherein the inner arm can move relative to the outer arm. Typically, the outer arm of the switchable RFF is pivotally supported at a first end by the HLA. A second end of the outer arm operates against an associated engine valve for opening and closing the valve by the rotation of an associated eccentric cam lobe acting on an inner arm contact surface which may be a roller. The inner arm is connected to the outer arm for pivotal movement about the outer arm&#39;s second end with the contact surface of the inner arm disposed between the first and second ends of the outer arm. Typically, the locking member includes a locking pin disposed in a bore in the first end of the outer arm, the locking pin being selectively moved to engage the inner arm to thereby couple the inner arm to the outer arm when engaged, and decouple the inner arm from the outer arm when disengaged. 
     In a switchable two-step RFF, the outer arm typically supports a pair of rollers carried by a shaft. The rollers are positioned to be engaged by associated low-lift eccentric cam lobes that cause the outer arm to pivot about the HLA, thereby actuating an associated engine valve to a low-lift. The inner arm, in turn, is positioned to engage an associated high-lift eccentric cam lobe sandwiched between the aforementioned low-lift lobes. The switchable two-step RFF is then selectively switched between a coupled and a decoupled mode by the locking member. In the coupled mode, with the inner arm locked to the outer arm, the rotational movement of the central high-lift lobe is transferred from the inner arm, through the outer arm to cause pivotal movement of the RFF about the HLA, which, in turn, opens the associated valve to a high-lift. In the decoupled mode, the inner arm is no longer locked to the outer arm and is permitted to move relative to the outer arm against a lost motion spring that biases the inner arm away from the outer arm. In turn, the rollers of the outer arm engage their associated low-lift lobes. The rotational movement of the low-lift lobes is transferred directly through the outer arm, and the associated valve is reciprocated by the outer arm to a low-lift. 
     A switchable deactivation RFF typically includes an outer arm and an inner arm. The inner arm supports a roller carried by a shaft. The roller is engaged by an eccentric lifting cam lobe for actuating an associated engine valve. Like the switchable two-step RFF, the switchable deactivation RFF is selectively switched between a coupled and a decoupled mode by a movable locking member. In the coupled mode the inner arm of the switchable deactivation RFF is locked to the outer arm and the rotational movement of the associated lifting cam lobe is transferred from the inner arm, through the outer arm to cause pivotal movement of the RFF about the HLA, which, in turn, opens the associated valve to a prescribed lift. In the decoupled mode, the inner arm becomes unlocked from the outer arm and is permitted to pivot relative to the outer arm against a lost motion spring. In the decoupled mode, the rotational movement of the lifting cam lobe is absorbed by the inner arm in lost motion and is not transferred to the outer arm. Thus, the associated valve remains closed when the switchable deactivation RFF is in its decoupled mode. 
     In a first switchable deactivation RFF design, the inner arm makes contact with an associated cam lobe while the outer arm does not. The lost motion spring biases the inner arm away from the outer arm and, with the outer arm supported by the HLA, serves to load the inner arm against its associated cam lobe in the decoupled mode. In a switchable deactivation RFF having a lost motion spring with an effective force exerted on the HLA that is higher than the opposing force of an associated HLA spring, the opposing forces must be properly managed to prevent reactive pump-down of the HLA induced by the force of the lost motion spring. For this purpose, an expansion travel limiter is incorporated in the switchable deactivation RFF to limit the movement of the inner arm relative to the outer arm. Thus, when the switchable deactivation RFF is in its decoupled mode, and the inner arm of the RFF follows the cam lobe, the lost motion spring will push the outer arm until the expansion travel limiter is engaged. At that point, further movement of the outer arm relative to the inner arm ceases, HLA pump-down is prevented and HLA leak-down recovery is initiated. Moreover, at that point, since the effective preload force of the lost motion spring is greater than the expansion force of the HLA, pump-up of the HLA is prevented. Note also that, when the inner arm of the RFF follows the base circle of the associated cam lobe, the expansion travel limiter further serves to set a clearance gap or mechanical lash between the locking pin and the inner arm to assure proper alignment of the locking pin with the inner arm when the RFF switches between its decoupled and coupled modes and to define the total mechanical lash in the valve train. 
     In an alternate switchable deactivation RFF design, in order to increase its resistance to HLA pump-up beyond that provided by the installed load of the lost motion spring, null pads may be added on the outer arm of the switchable deactivation HLA for contacting zero-lift, constant radius null lobes disposed on either side of the associated lifting lobe. In this design, when the inner arm contacts the expansion travel limiter, the inner arm of the RFF is prevented from contacting the base circle of its associated cam lobe by the null pads first contacting with the zero-lift null lobes. Since the inner arm is held away from the base circle of the cam by the expansion travel limiter, the force of the lost motion spring cannot pump-down the HLA. By the null pads making contact with the zero-lift null pads, pump-up of the HLA is prevented by the opposing installed load of the associated valve closing spring. The expansion travel limiter establishes the mechanical lash between the locking pin and the inner arm; as well as the clearance (lash) between the inner arm and the base circle of the cam. The pin lash plus the cam lash establishes the total mechanical lash of the valve train. 
     Various lost motion expansion limiters used in switchable RFFs are known in the art. For example, in U.S. Pat. No. 6,532,920, a switchable two-step RFF is shown wherein the roller shaft of the outer arm contacts a throughbore in the inner arm to limit inner arm travel. As shown in U.S. Pat. Nos. 5,544,626, 5,653,198 and 6,314,928, bumper pads or projections formed at the lower end of the inner arm are used to limit inner arm travel of the switchable RFFs. The disadvantage of these devices in the prior art is that the stop position cannot be precisely controlled resulting in sometimes too small or too large of a mechanical lash between the locking pin and the inner arm or, in the case of a switchable deactivation RFF with null pads, resulting in a clearance between the inner arm and base circle of the associated cam lobe that is too too small, or even non-existing. A mechanical lash that is too small may result in the locking pin being unable to reliably engage the inner arm. A lash that is too large may permit excess pump-down of the HLA thereby delaying the opening point, decreasing the lift and advancing the closing point of the associated valve in the coupled mode which is known to contribute to engine roughness at idle and/or emission problems. A cam clearance that is too small (in the case of a switchable deactivation RFF with null pads), between the inner arm and its base circle, increases total lash when the inner arm is allowed to contact the cam base circle and may similarly affect the opening, closing and lift characteristics of its associated valve. 
     What is needed in the art is a device that precisely limits the amount of upward pivotable movement of the inner arm relative to the outer arm caused by the force of the lost motion spring. 
     It is a principal object of the present invention to provide an inner arm stop to precisely position the inner arm relative to the outer arm thereby controlling mechanical lash and HLA pump-down. 
     SUMMARY OF THE INVENTION 
     Briefly described, a switchable RFF includes a pivotable and therefore decoupleable inner arm positioned central to an outer arm. In one aspect of the invention, a roller is carried by a shaft that is supported by the inner arm. The shaft may be free to axially rotate relative to the inner arm. A lost motion spring acts between the inner arm and the outer arm. In one embodiment, the shaft is a stepped shaft that includes a major diameter for carrying the roller and a reduced diameter portion at each end. Each of the shaft ends reciprocates in recessed channels formed inside the outer arm, under the force of the lost motion spring, when a latching mechanism is in a disengaged position and the inner arm is decoupled and, therefore, in lost motion. The movement of the shaft within the channels, and thus the roller and the inner arm, is limited when the ends of the shaft contact an end surface of the recessed channels. 
     The recessed channels and the end surface of each channel may be formed in many ways. For example, the recessed channels and end surfaces may be cast in the case of an investment cast outer arm, machined, stamped, cast and coined, cast and machined, or formed using electrical discharge machining. In one aspect of the invention, a cast channel includes a transverse hole formed at the upper end of the channel such as by machining or punching, so that the stopped position of the shaft, and thus the roller and inner arm, is located simply and accurately by the formed hole. 
     In one aspect of the invention, the channel width is less than the major diameter of the shaft and greater than the diameter of the end portion. Shoulders formed between the major diameter of the shaft and the reduced diameter portions are in close alignment with inside surfaces of the outer arm such that the axial shaft position is limited by contact between the shoulders and the inside surfaces of the outer arm. In the case where the shaft is free to axially rotate relative to the inner arm, this holds the shaft in a relatively centered position. In one aspect of the invention, it also prevents the reduced diameter portions of the shaft from getting caught on the through holes formed in the outer arm. 
     Since it is desired to precisely control a final stopped position of the inner arm roller relative to the outer arm, the tolerance variation of the stopped position of the roller is minimized by the construction in accordance with the invention. In one aspect of the invention, by controlling the size and position of the outer arm through holes, the stopped position of the roller can be maintained with precise control. 
     In accordance with the invention, in the case where the roller shaft is free to rotate relative to the inner arm, the invention requires no staking or clips to assemble the shaft to the inner arm. This reduces manufacturing cost, minimizes distortion caused by the staking, and reduces wear on all shaft surfaces by distributing the shaft loads over greater areas of contact. 
     In an alternate embodiment in accordance with the present invention, the reduced diameter portions of the shaft may be designed to be temporarily collapsible into the major diameter portion of the shaft during assembly, such that the inner arm can be readily assembled to the outer arm from the top. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which: 
         FIG. 1  is an isometric view of a deactivation roller finger follower assembly in accordance with the invention; 
         FIG. 2  is a cross-sectional view of the deactivation roller finger follower taken along line  2 - 2  in  FIG. 1  in accordance with the invention; 
         FIG. 3  is an isometric view of a cast outer arm of the deactivation roller finger follower in accordance with the invention; and 
         FIG. 4  is an isometric view of a cast and machined outer arm of the deactivation roller finger follower in accordance with the invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 and 2 , a deactivation RFF assembly  10  in accordance with the invention is illustrated. While this invention is described in the context of a switchable deactivation RFF, it should be understood that the inner arm stop as described below may be applied to a switchable two-step RFF as well. 
     Switchable deactivation RFF assembly  10  includes an inner arm  12  that is pivotably and therefore deactivateably disposed in a central opening in an outer arm  14 . Inner arm  12  pivots within outer arm  14  about a pivot shaft  16 . Inner arm  12  includes a contact surface. The contact surface may be a roller  18  carried by a shaft  30  that is supported by the inner arm  12 . A bearing  22  may rotatably support roller  18  on shaft  30  for following a cam lobe of a lifting cam of an engine camshaft (not shown). Bearing  22  may be, for example, a roller or needle bearing. Shaft  30  may or may not be fixed from rotation with inner arm  12 . In the case where the contact surface does not include a roller, shaft  30  may be pins extending from either side of the inner arm. Outer arm  14  includes two inside walls  144  positioned parallel to each other. A pair of recessed channels  40  is formed in inside walls  144 . Channels  40  are positioned across from each other. Each channel  40  includes a dimensionally controlled end surface  42  closing channel  40  at one end. End surface  42  stops movement of shaft  30  within channels  40 . Channels  40  may be open at an end opposite the described closed end. A lost motion spring  24  acts between inner arm  12  and outer arm  14  to pivot the inner arm away from the outer arm. A socket  26  for pivotably mounting RFF assembly  10  on an HLA (not shown) is included at a first end  141  of outer arm  14 . A pad  28  for actuating a valve stem (not shown) is included at a second end  142  of outer arm  14 . A latching mechanism  20  disposed within outer arm  14  at the first end  141  thereof selectively couples or decouples inner arm  12  to or from outer arm  14 . 
     The switchable deactivation RFF assembly  10  is selectively switched between a coupled and a decoupled state. In the coupled state inner arm  12  and, therefore shaft  30 , is coupled to outer arm  14 , and rotation of the lifting cam is transferred from roller  18  through shaft  30  to pivotal movement of outer arm  14  about the HLA which, in turn, reciprocates the associated valve. In the decoupled state, inner arm  12  and, therefore shaft  30 , is decoupled from outer arm  14 . Thus, shaft  30  does not transfer rotation of the lifting cam to pivotal movement of outer arm  14 , and the associated valve is not reciprocated. Rather, shaft  30  is reciprocated within recessed channels  40  formed inside outer arm  14 . Channels  40  retain and guide reciprocation of shaft  30 . 
     Referring to  FIG. 2 , in one aspect of the invention, shaft  30  may be a stepped shaft that is of a generally elongated cylindrical shape that transitions towards both ends in a step in accordance with a preferred embodiment of the invention. Shaft  30  includes a major diameter center portion  32  and a reduced diameter end portion  34  at both ends of major the diameter center portion  32 . At each intersection of center portion  34  with an end portion  32 , and therefore at both ends of center portion  32 , a face  36  is formed. Both end portions  34  preferably have substantially the same length and diameter. The diameter of end portions  34  is smaller than a diameter of center portion  32  thereby defining face  36 . Center portion  32  supports bearing  22  and roller  18  and is supported by inner arm  12 , as shown in  FIG. 2 . Each of the reduced diameter portions  34  reciprocates in vertical recessed channels  40  due to the force of lost motion spring  24  when latching mechanism  20  is in disengaged position and inner arm  12  is decoupled from outer arm  14  and, therefore, in lost motion. The movement within channels  40  of shaft  30 , and thus roller  18 , is limited when reduced diameter portions  34  of shaft  30  contact end surface  42  of recessed channels  40 . Shaft  30  may be made from bearing steel and may be hardened throughout. In one aspect of the invention, shaft  30  may be a solid shaft formed as an integral part whereby the shaft is installed into the assembly by first installing the roller and shaft into the inner arm, then positioning the shaft ends into the channels of the outer arm from the bottom of the outer arm. The inner arm, with the lost motion spring in place, is then attached to the outer arm at its pivot point. 
     In an alternate embodiment, reduced diameter end portions  34  are formed as separate pieces from major diameter center portion  32 . End portions  34  may be formed to be collapsible within center portion  32  enabling assembly of inner arm  12  to outer arm  14  from the top. Collapsible end portions  34  may be configured by using a hollow straight shaft as center portion  32  and smaller solid straight shafts as end portions  34 . End portions  34  may be slideably inserted into both ends of hollow center portion  32 . A spring inserted between the slideable end portions  34  serves to expand the end portions  34  outward after assembly of inner arm  12  to outer arm  14 . When expanded, collapsible end portions  34  will engage channels  40  inside the outer arm  14 . 
     Referring to  FIG. 3 , recessed channels  40  and end surface  42  of each channel  40  may be formed integral with outer arm  14  during a casting process. To accurately position the end surface  42  at the upper end of each channel  40 , in one aspect of the invention, a transverse through hole  44  may be formed into outer arm  14 , such as by machining or punching, following the casting process, as illustrated in  FIG. 4 . The upper inside surface of through hole  44  forms end surface  42 . Thus, the position of the upper inside surface of the through hole limits the upward movement in the vertical direction of the shaft and precisely controls the final stopped position of the inner arm roller relative to the outer arm. Forming through hole  44  as described enables the upward travel of the inner arm to be more precisely controlled than in the as cast embodiment. A machined hole, for example, provides a rounded circumference so that shaft  30  rests against a curved surface when making contact with end surface  42 , which is known in the art to resist wear between the contact points. By setting the size and/or position of through hole  44 , the stopped position of shaft  30  and roller  18  can be easily set in accordance with assembly requirements. 
     Thus, in accordance with the invention, end surfaces  42  may be formed by machining or punching or also, for example, by stamping, by casting and coining, or by electrical discharge machining. 
     Channel  40  has a width that is preferably less than the major diameter of center portion  32  of shaft  30  and greater than the diameter of end portion  34  such that at least a portion of face  36  is able to contact inside wall  144 . Shaft  30  is guided by channels  40 . Faces  36  in proximity with inside walls  144  of outer arm  14  hold an axially free shaft  30  in a relatively centered position within RFF  10 . In the case of the embodiment shown in  FIG. 4 , faces  36  and inside walls  144  also prevent reduced diameter end portions  34  of shaft  30  from entering into and getting caught on through holes  44 . 
     In the configuration described above where shaft  30  is free to rotate relative to the inner arm, retainers such as clips or staking to assemble shaft  30  to inner arm  12  are not needed. This reduces cost, minimizes shaft distortion from staking, and reduces wear on shaft  30  by enabling the bearing or roller loads to be distributed over a greater circumferential area of shaft  30 . 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.