Abstract:
A throttle valve for throttling the flow of combustion air to an internal combustion engine is disclosed having a throttle body made of a first plastic and defining a throat for passing combustion air and an intersecting passageway to rotatably support a throttle shaft. The throttle shaft is made of a second plastic that is filled with a solid lubricant and extends through the intersecting passageway supported on each end by a boss that is molded concurrently with the throat. The inside surfaces of the bosses support the throttle shaft and are molded in the same molding process that forms the throat. A butterfly is mounted to the shaft for rotation in the conduit to throttle the flow of combustion air. Boss to throat volumetric ratios of 0.5 or less are provided by this construction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/342,590, filed Jun. 29, 1999, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to valves for throttling the flow of combustion air to an internal combustion engine. More particularly, the invention is related to such valves made from plastic. 
     BACKGROUND OF THE INVENTION 
     Throttle valves for large automotive and industrial internal combustion engines have traditionally been made from light metal alloys such as magnesium and aluminum. These materials are resistant to the elevated temperatures in the engine compartments of modern vehicles. They can also be molded to the tight dimensional tolerances required for accurate and repeatable control of air flow over long lifespans of these engines. They also have the strength and stiffness required to support large diameter butterfly valves (typically 50-85 mm) needed for the large volumetric flow rates of such engines. 
     Automotive throttle valves typically are typically butterfly valves. Butterfly valves have a roughly circular plate called butterfly that is disposed in the circular throat of a throttle body to pass or to block flow, depending upon its rotational position. The butterfly is typically fixed to a throttle shaft that extends across the throat of the throttle body generally perpendicular to the direction if combustion air flow. The throttle shaft extends through at least one the wall of throttle body and is connected to a throttle lever disposed on the outside of the throttle body to rotate the shaft and thereby open and close the throttle valve. 
     Recently, throttle valves have been proposed and made using a variety of plastic components. For example, the Rover K-3 employs a plastic throttle body with a metal shaft and a plastic bearing insert. U.S. Pat. No. 5,769,045 discloses a plastic throttle body with steel shaft and a metal needle roller bearing insert in the throttle body. U.S. Pat. No. 5,304,336 shows a throttle valve having a plastic throttle body and a simultaneously molded throttle body, shaft, and butterfly. U.S. Pat. No. 5,666,988 discloses a plastic throttle shaft. U.S. Pat. No. 5,098,064 discloses a plastic throttle body with metal bearing inserts for supporting a metal throttle shaft. An example is shown in FIGS. 4A-B. All of these constructions have problems. 
     There are several problems faced by a manufacturer of throttle valves with plastic components. Among others, throttle body wear is significant. To reduce wear, bearings or bushings are typically into a plastic throttle body to support the throttle valve shaft. High temperatures in engine compartments also pose problems. Creep at elevated temperatures may cause a plastic throttle shaft and butterfly to deform, requiring frequent vehicle tune-ups. If both the throttle body and the throttle shaft are made of plastic, the problems with wear and creep are enhanced since both parts may creep and wear, increasing the probability of misalignment, leakage and failure. 
     The very manufacture of throttle valves having plastic throttle bodies and plastic shafts is fraught with difficulties. Unless plastic components are carefully designed and made with critical attention paid to wall thicknesses, material selection, cooling rates, and plastic injection pressure to mention but a few factors, they are prone to shrink and deform, which causes misalignment, leakage around the shaft or the butterfly, and even accelerated wear. 
     The patents cited above suggest several solutions to some of the problems presented by the use of plastics. Unfortunately, the solutions themselves generate their own problems. For example, plastic or metal bearings or bushing may be inserted into bosses extending from the throttle body to support the throttle shaft. This, however, requires large diameter plastic bosses extending from the surface of the throttle body into which the bearing or bushing can be inserted to support the throttle shaft and reduce wear. Unfortunately, these large bosses cause distortion of the throttle body throat, and prevent the throttle valve butterfly from fitting properly into the throat of the throttle body. This distortion is due largely to the increased volumetric ratio of the bosses in relation to the throat itself. The large bosses draw a substantial amount of plastic away from the primary task of filling out the throat of the throttle body. This delays packing out the throat of the throttle body (with pressure) and results in distortion of the throttle body throat, as well as sinks at the bosses themselves. These distortions and sinks prevent a (separately molded) butterfly from properly sealing the throat closed when the butterfly is later inserted into the throat and attached to the throttle shaft. At the very least, these problems require significant throttle body mold alterations and tuning, as well as a precise control of the molding process itself to insure a proper fit between the butterfly and throat. 
     Another method of avoiding the distortion, sealing and wear problems is to simultaneously mold both the throttle body and the throttle shaft, as shown in U.S. Pat. No. 5,304,336. In this process, the throttle shaft and throttle body are molded in a single manufacturing process, first the throttle body, and then the throttle shaft and integral butterfly. Once the body is molded, the pins that form the interior of the throat and those that form the holes in the bosses that support the throttle shaft are partially withdrawn and molten plastic is injected into the void that is thereby created. In this manner, the just-molded hollow bosses and the interior walls of the throttle body become part of the “mold” and themselves form the throttle shaft and integral butterfly. Distortion and warping are less of a problem in this process, since the throttle shaft and butterfly are formed by the just-molded throttle body itself, rather than being separately molded and later inserted into the throttle body. 
     While this last process improves sealing by, in effect, custom mating each plastic throttle shaft and butterfly to the plastic throttle body, it requires the use of two quite different thermoplastics: a higher melting point plastic to form the throttle body, and a significantly lower melting point plastic to form the shaft and butterfly. It has also required that manufacturers mold the throttle body using a structural plastic mixed with PTFE (polytetrafluoroethylene). The PTFE acts as a mold release agent and insures that the molded-in-place shaft and butterfly do not stick to the throttle body when it is formed. Unfortunately, it also requires complex molding equipment and precise timing and sequencing of the two-stage injection molding process. 
     What is needed, therefore is an improved throttle valve construction that may employ a smaller boss to minimize throat distortion, yet does not require complex multistage molding operations, and provides an accurately dimensioned throat that seals satisfactorily to a subsequently inserted and separately molded butterfly. 
     It is an object of this invention to provide such a throttle valve. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the present invention, an improved throttle valve for controlling the flow of combustion air to an internal combustion engine is provided having a throttle body, and a throttle shaft made of a plastic filled with solid lubricant, and a butterfly attached to the throttle shaft. 
     In accordance with another embodiment of the invention, a throttle valve for throttling the flow of combustion air to an internal combustion engine is provided including, a plastic throttle body that has a throat for passing combustion air and an intersecting passageway that supports the throttle shaft, a plastic throttle shaft filled with a solid lubricant that extends through the intersecting passageway, and a butterfly attached to the shaft for rotation in the throat to throttle the flow of combustion air. The passageway is generally perpendicular to the throat and is defined by two bosses that are molded integral with the throttle body and have an inner molded surface that contacts and supports the throttle shaft. The throttle shaft is integrally molded with a throttle arm that engages a throttle shaft rotating means. The throttle shaft may include at least one boss to which the butterfly is fixed. 
     Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a throttle valve in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of the throttle valve of FIG. 1 taken at Section  2 — 2  in FIG.  1  and through the length of the throttle shaft showing the fitment of the throttle shaft and its butterfly in the throat of the throttle body; 
     FIG. 3 is a partial cross-sectional view of the throttle valve of FIG. 1 taken at Section  3 — 3  in FIG. 1 perpendicular to the length of the throttle shaft; 
     FIGS. 4A and 4B are partial cross sectional views of a prior art throttle body and throttle shaft showing the use of bearings and large diameter bosses to support a throttle shaft; and 
     FIG. 5 is a fragmentary cross-section of a throttle shaft and supporting bosses showing circumferential sealing ridges disposed about and extending from the shaft where it passes into the throat of the throttle valve. 
    
    
     Before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-3, a throttle valve  10  is shown having a throttle shaft  12  extending across combustion air passageway (or throat)  14  of throttle body  16 . A butterfly  18  is fixedly mounted to throttle shaft  12  and is located in combustion air passageway  14 . Throttle lever  22  is fixed to the throttle shaft, and a cable (not shown) extends from the throttle lever. In operation, throttle lever  22  is pulled by the cable causing the throttle shaft to rotate and the butterfly to open. 
     A throttle valve return spring  20  is disposed about the periphery of throttle shaft  12  having one end engaged with the throttle shaft and another engaged with the throttle body. Return spring  20 , which is coiled around the throttle shaft, rotates the throttle shaft to a substantially closed position when the cable is released. In the substantially closed position throttle lever  22  abuts throttle stop  24  that is screwed into throttle body  16 . In this manner, return spring  20  pulls throttle lever  22  against throttle stop  24  when the cable is released. Throttle lever  22  is preferably integrally molded with shaft  12 . 
     An aperture  26  is provided in lever  22  where the lever abuts throttle stop  24 . This aperture permits throttle stop  24  to be adjusted when the throttle valve is closed. Throttle stop  24  has a recess  28  that is engaged by a screwdriver, a hex wrench, a Torx driver or similar rotating tool for screwing the throttle stop into or out of throttle body  16 . 
     To adjust the throttle stop (which typically regulates minimum engine airflow) the tool is inserted through aperture  26  and into recess  28  that it engages to transmit rotational forces. As throttle stop  24  is rotated, it advances into or out of throttle body  16 . Since throttle lever  22  is held against throttle stop  24  by return spring  20 , the adjustment of throttle stop  24  also adjusts the rotational position of throttle lever  22 , throttle shaft  12 , and butterfly  18 . 
     Throttle shaft  12  extends across combustion air passageway  14  and is supported for rotation at either end by bosses  30  and  32 . These bosses are sized to permit throttle shaft  12  to rotate with respect to throttle body  16  but to limit the leakage of air into or out of passageway  14  between throttle shaft  12  and bosses  30 ,  32 . The bosses are located on opposing sides of throttle body  16  and are disposed to locate throttle shaft  12  substantially in the middle of throttle body  16  and substantially perpendicular to the longitudinal axis of passageway  14  (i.e. the direction of air flow). Shaft  12 , in the region where it is supported for rotation by throttle body  16 , has a diameter of 9-13 mm, and preferably between 10 and 12 mm. 
     Two bosses, throttle shaft  17 ,  19  extend from throttle shaft and through corresponding holes in butterfly  18 . These are mushroomed on the ends to attach butterfly  18  to shaft  12 . 
     As shown in FIG. 2, boss  30  supports a length of the throttle shaft between throttle lever  22  and butterfly  18 . Given the preferred 2 to 4 millimeter average wall thickness of the combustion air passageway  14 , this additional support provides an extended bearing surface for throttle shaft  12 . Boss  30  preferably has a wall thickness of between 2 and 4 mm. 
     In prior art separately assemblable throttle valves, the bore diameter of a first boss (item  30 ′ in FIG. 4B) is substantially greater due to the need to insert plastic or metal bushings or bearings into the first boss to surround and support the subsequently inserted throttle shafts. As a result, for a boss with a typical wall thickness of 2-6 millimeters, the ratio of boss wall thickness to overall boss diameter is preferably between 1:4 and 1:8, or more preferably between 1:5 and 1:7, and the ratio of boss wall thickness to combustion air passageway wall thickness is preferably between 1:1 and 1:2. The inside diameter of boss  30  is between 9 and 13 mm, and preferably between 10 and 12 mm. 
     The practical effect of eliminating the bearing or bushing is to reduce the volume of plastic required to form the boss since the inside diameter of the boss is reduced. By reducing the volume of plastic required to form the first boss, there is a subsequent reduction or elimination of sinks and voids in the combustion air passageway wall where that wall adjoins the first boss  30 . 
     As best seen in FIGS. 2 and 3, a portion. T of the passageway wall surrounds butterfly  18  when butterfly  18  is in a substantially closed position. When butterfly  18  is in this substantially closed position, a small gap between the edge of the butterfly  18  and the passageway wall is provided to permit the flow of a very small quantity of air. This gap is carefully controlled by adjusting throttle stop  24 . 
     Quite small adjustments of throttle stop  24  that cause very small movements of butterfly  18  toward or away from its substantially closed position can cause dramatic changes in the performance of an internal combustion engine to which throttle valve  10  is attached. Sinks, voids and distortion in the inner passageway wall in region T similarly cause dramatic changes in the performance of the engine even though they are quite small. This is particularly true for throttle bodies in which butterfly  18  is formed in separate process and subsequently attached to throttle shaft  12 . In a worst case scenario, it may be impossible to close butterfly  18  sufficient to permit the engine to operate at an idle. Eliminating the use of a bearing, bushing or insert to support throttle shaft  12  permits the inside diameter of boss  30  to be reduced. This permits the outside diameter of boss  30  to be reduced, which thereby reduces the volume of plastic that is diverted into the mold cavity that forms boss  30 . 
     The degree to which the boss volume is reduced can be expressed as a volumetric ratio: the ratio of the volume of plastic in bosses  30  and  32  (to be discussed below) to the volume of plastic forming the throat of the throttle body in region T. Region T is defined as that portion of the throttle body passageway extending between the upper edge of butterfly  18  and the lower edge of butterfly  18  when the butterfly is closed. In the embodiment of FIGS. 1-3, boss volume to throat volume ratios of 0.5 or less can be achieved. With careful design, boss to throat volume ratios of 0.3 or less can be provided. 
     Second boss  32  is provided on the diametrically opposite wall of the throttle body to receive and support the free end of the throttle shaft. The second boss has the same ratios of thickness and diameter as described above regarding the first boss. As shown in FIG. 4A, prior art bosses  32 ′ that support the free end of the throttle shaft also had larger inner and outer diameters due to the need to provide a bearing (as shown), bushing or insert to support the free end of the throttle shaft. 
     Throttle body  16  is preferably comprised of plastics such as polyamide (PA), polybutylene terepthalate (PBT), high temperature nylon (HTN), syndiotactic polystyrene (SPS), polyphenylene sulphide (PPS), polyetherimide (PEI), polyethersulphone (PES), polyamide-imide (PAI), polypthalamide (PPA), polypropylene (PP) or polyethylene terepthalate (PET). Of these, PA, PBT, HTN, SPS, PPS and PEI are preferred. Of these, PA, PBT, HTN and SPS are particularly preferred. If the throttle body is molded integral with an intake manifold, the preferred materials for forming this integrated unit would be PA, PBT, PET, SPS and PP. 
     Throttle shaft  12  is comprised of a resin/solid lubricant blend where the resin is preferably a high temperature nylon (HTN), polyetherimide (PEI), polyamide (PA), polypthalarnide (PPA), polyphenylene sulphide (PPS), polyethersulphone (PES), liquid crystal polymer (LCP, “Zenite” by DuPont), polyetherketone (PEK) and polyamide-imide (PAI). The solid lubricant is preferably a fluorinated hydrocarbon, graphite or molybdenum disulphide. The preferred fluorinated hydrocarbon is a fluoroethylene polymer, most preferably polytetrafluoroethylene (PTFE). The mass percentage of solid lubricant in the resin/solid lubricant blend is preferably between 8% and 20%. More preferably it is between 12% and 18%. The addition of a solid lubricant to the traditional throttle shaft resins is disadvantageous since it significantly reduces the strength and the elastic modulus of throttle shaft  12 . As an example, a 100% polyamide plastic has been identified as having a tensile strength of 12,000 psi whereas the same material with 20% by weight of PTFE has a strength of 9,000 psi. In addition, the flex modulus of 100% polyamide is 410 ksi, whereas the 80/20 polyamide blend is only 350 ksi. Thus, providing a PTFE blended throttle shaft would appear to substantially reduce its strength and toughness. 
     In practice throttle shafts with added solid lubricants, such as PTFE, do not fail as often as the statistics would suggest. Indeed, tests simulating the particular environment suggest that, contrary to expectations, throttle shafts filled with solid lubricants such as PTFE get tougher with abuse. 
     During typical testing, throttle body  16  is cycled between −40 C. and +140 C. as testing is performed. The testing includes snap testing in which the throttle lever  22  is pulled away from throttle stop  24  until the valve is entirely open, and then the lever is released, at which point return spring  20  causes throttle shaft  12  and butterfly  18  to accelerate toward the closed position. When throttle lever  22  suddenly abuts throttle stop  24 , the lever/shaft/butterfly subassembly is subject to a severe deceleration. As a result, a severe oscillating torsional load is applied to shaft  12  as butterfly  18  oscillates back and forth. This test is typically performed for a thousand cycles. The testing also includes cyclical testing, in which the valve is rapidly opened and closed for two million cycles. In both of these tests, a vacuum is applied to the outlet side of the butterfly to simulate engine vacuum and hence the bending and shear forces applied to the throttle shaft by that vacuum. 
     This testing process has been designed to reflect the type of loads that are placed on the throttle shaft during a lifetime of operation. These oscillating torsional loads and bending moments have the additional effect of strengthening a PTFE-filled throttle shaft in a manner not suggested by published strength and elongation figures. 
     The oscillating torsional loads cause the shaft to elastically deflect in torsion and subsequently relax to an unstressed state. This torsional deflection and relaxation causes the PTFE to migrate from the stressed fibers to the surface of the shaft, increasing the strength of the shaft in the stressed layers (the outer circumferential layers) and depositing a layer of PTFE on the surface of the shaft. The more the shaft is cycled, the more PTFE migrates to the surface, the higher the shaft strength becomes and the better the lubrication. This strengthening is different from work hardening, since work hardening would require plastic deformation of the shaft—a condition that should be avoided. Plastic deformation of the shaft and butterfly may cause misalignment and accelerated wear. 
     By employing a solid lubricant, the contamination of motors and resistors coupled to the shaft is also reduced, since the PTFE will not migrate along the shaft surface as oil does and contaminate a resistor or motor attached to the shaft. 
     FIG. 5 shows an alternative embodiment of throttle shaft  12  in which a plurality of outwardly extending circular ribs or ridges  50  are disposed along its length to seal against the inner surface of the first and second bosses. These ridges preferably have a height of 0.1 to 2.0 millimeters, but are shown here out of proportion to their actual size for clarity. The PTFE migration is significant enough in the throttle shaft that these ridges center the shaft in the bore of the bosses. 
     To make and assemble the valve, throttle body  16  (including bosses  30 ,  32 ) are preferably molded of plastic. Throttle shaft  12  and butterfly  18  are preferably molded of plastic. Shaft  12 , once formed, is inserted into the passageway defined by bosses throttle shaft  30 ,  32 . Butterfly  18  is then inserted into throat  14  and located on bosses throttle shaft  17 ,  19 . These bosses, throttle shaft are mushroomed to secure butterfly  18  on shaft  12 . Alternatively, butterfly  18  may be secured to shaft  12  by threaded fasteners or rivets. 
     Thus, it should be apparent that there has been provided in accordance with the present invention a plastic throttle body that fully satisfies the objectives and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, it is evidence that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.