Lash adjuster for valve actuator

A lash adjuster is provided which can maintain stable valve lift. An adjuster screw is mounted into a threaded hole formed in the bottom of the end plate of a lifter body. An elastic member is mounted in the threaded hole to bias the adjuster screw axially downwardly. The female threads of the threaded hole and the male threads of the adjuster screw are serration-shaped. A plurality of axial grooves are formed in the inner periphery of the threaded hole to circumferentially divide the female threads into many separate portions, and satin-finished rough surfaces are formed on the pressure flanks of the male threads by shot-peening to enable quick expulsion of oil film disposed between the opposed pressure flanks, thereby stabilizing the valve lift.

BACKGROUND OF THE INVENTION

This invention relates to a lash adjuster for automatically adjusting the valve clearance in a valve actuator of an internal combustion engine.

A valve actuator for opening and closing intake or exhaust valves (hereinafter simply “valves”) by rotating its cam includes a lash adjuster mounted between the cam and the valve to automatically adjust the valve clearance therebetween.

U.S. Pat. No. 4,548,168 discloses such a lash adjuster which includes a lifter body having an end plate kept in contact with a cam and formed with a blind threaded hole in its bottom surface. An adjuster screw is in threaded engagement with the threaded hole and is biased axially by an elastic member mounted between the top end of the screw and the closed end of the threaded hole. The female threads of the threaded hole and the male threads of the adjuster screw are serration-shaped so that the pressure flank to which a push-in load applied to the adjuster screw is applied has a larger flank angle than the clearance flanks.

This lash adjuster is mounted between a cam and the stem of a valve. A valve spring biases the valve toward the cam to press the end of the valve stem against the bottom end of the adjuster screw. As the cam rotates with the valve stem pressed against the adjuster screw, the valve stem is moved up and down between its open position and closed position of the valve.

If a valve clearance forms between the top end of the valve stem and the adjuster screw due e.g. to thermal expansion of the cylinder head, under the force of the elastic member, the adjuster screw will move axially downward while turning in one direction with the clearance flanks of the screw sliding along the clearance flanks of the nut until the valve clearance disappears.

Conversely, if the pressure from the valve stem is applied to the adjuster screw, the adjuster screw is pushed up or retracts until the axial play between the pressure flanks of the female threads and the male threads disappears. Once the play disappears, the adjuster screw cannot be pushed up any further because the frictional force between the pressure flanks is large.

But if, for example, the valve seat is worn, when the engine is started, large force will be momentarily applied to the adjuster screw from the stem when the stem rises and abuts the screw, thus pushing up the screw against the large frictional force between the pressure flanks, until the valve face is completely seated on the valve seat. Thus, it is possible to completely shut the valve when the base circle of the cam contacts the end plate of the lifter body, even if the valve seat is worn. This prevents pressure leakage. In this case, the adjuster screw is pushed up until the axial gaps between the pressure flanks disappear after the force from the valve stem has disappeared.

On the other hand, in a situation where no valve clearance adjustment is necessary while the engine is running, the adjuster screw scarcely turns and moves axially within the gap or play between the female threads of the threaded hole and the male threads of the adjuster screw.

That is, the pressure flanks of the male threads of the adjuster screw repeatedly collide against and move away from the pressure flanks of the female threads.

In the valve assembly, lubricating oil such as engine oil is present. Such lubricating oil inevitably flows into between the pressure flanks and forms an oil film. When the adjuster screw undergoes an axial load, the pressure flanks tend to expel such oil film when they move toward each other. The oil film produces a pressure against the pressure from the pressure flanks. The oil film has a load-bearing limit. If the pressure applied from the pressure flanks to the oil film exceeds this maximum load limit, the oil film will break up and will be discharged. The pressure flanks of the male and female threads thus directly contact each other. Since the friction between the pressure flanks is large, it prevents the adjuster screw from turning.

On the other hand, if the pressure applied to the oil film from the pressure flanks balances with the load-bearing limit of the oil film, the oil film will remain therebetween. That is, the pressure flanks are practically separated from each other by the oil film. Thus, the adjuster screw tends to retract toward the closed end of the threaded hole while turning due to reduced friction between the pressure flanks. This reduces the valve lift amount.

Generally it is known that the smaller the total area of the pressure flanks, the smaller the load-bearing limit of the oil film. Also, by dividing the pressure flanks to the greater number of sections for a given area of the pressure flanks, it is also possible to reduce the load-bearing limit of the oil film.

JP patent publication 03-501758 proposes a lash adjuster including an adjuster screw having a plurality of circumferential grooves formed in the pressure flanks to reduce the load-bearing limit of oil film present between the pressure flanks, so that the oil film can be expelled smoothly and quickly, thereby stabilizing the lift of the valve.

JP patent publication 2000-130114 discloses a lash adjuster in which a plurality of axial grooves are formed in the inner periphery of a threaded hole formed in the lifter body at circumferential intervals to circumferentially divide the pressure flank of the female thread into many small sections, thereby expelling the oil film smoothly and quickly.

The lash adjuster disclosed in either of the abovementioned Japanese publications has one problem that when the pressure flanks of the female and male threads are abraded and get worn, the contact surfaces tend to become smooth. This reduces friction between the pressure flanks to such an extent as not to be able to check the rotation of the adjuster screw.

One way to avoid this problem is to roughen the pressure flanks of the female and male threads. But ruggedness formed by such rough surfaces are not sufficient to efficiently discharge oil film. Especially in a low-temperature condition in which the viscosity of lubricating oil becomes high, it takes time to expel oil and thus the adjuster screw can slip and turn, so that the valve lift decreases.

The load-bearing limit of the oil film varies with the distance between the opposed pressure flanks, their area, shape and speed at which they move toward each other, viscosity of lubricating oil, etc.FIGS. 11A and 11Bare graphs showing the relationship between the distance between the opposed pressure flanks and the ambient temperature and the load-bearing limit of the oil film.

The graph ofFIG. 11Ashows the results for a lash adjuster in which the female threads of the threaded hole formed in the lifter body and the male threads of the adjuster screw have pressure flanks and clearance flanks provided alternating with the pressure flanks so that the pressure flanks have a greater flank angle than the clearance flanks. This lash adjuster has no axial grooves as used in the lash adjuster disclosed in JP patent publication 2000-130114. The graph ofFIG. 11Bshows the results for the same lash adjuster as used inFIG. 11Aexcept that it has the axial grooves as used in JP publication 2000-130114.

The graph ofFIG. 11Ashows that the load-bearing limit of the oil film increases sharply with increase in the viscosity of the oil film, which in turn increases with reduction in the temperature. Even while the distance between the pressure flanks of the female and male threads is relatively large, the pressure-bearing force of the oil film may balance with the axial load transmitted from the cam to the valve through the lash adjuster at low temperature.

FIG. 11Bshows that the shorter the distance between the pressure flanks, the greater the load-bearing force of the oil film. Thus, if the pressure flanks become smooth due to wear, the distance therebetween before they contact decreases. Thus, even if the pressure flanks are circumferentially divided into small sections, the load-bearing force of the oil film can grow rather large.

An object of this invention is to provide a lash adjuster in which oil film disposed between the opposed pressure flanks can be expelled smoothly and quickly from when the distance between the opposed pressure flanks is large to the instant they contact, and even after the pressure flanks have been worn due to long use, friction sufficient to keep the adjuster screw from turning is maintained between the opposed pressure flanks, so that stable valve stroke is maintained.

SUMMARY OF THE INVENTION

According to this invention, there is provided a lash adjuster comprising a lifter body including an end plate having a top surface and a bottom surface formed with an axial blind threaded hole having female threads, the lifter body axially slidably mounted between a cam and a stem of a valve with the top surface in contact with the cam, an adjuster screw having male threads on its outer periphery which are in threaded engagement with the female threads of the threaded hole, and an elastic member mounted in the threaded hole so as to bias the adjuster screw axially, the female threads of the threaded hole and the male threads of the adjuster screw being serration-shaped and having pressure flanks which receive pressure applied to the adjuster screw in such a direction as to push the adjuster screw into the threaded hole, and clearance flanks arranged between the adjacent pressure flanks, the pressure flanks having a greater flank angle than the clearance flanks, one of the inner periphery of the threaded hole and the outer periphery of the adjuster screw being formed with a plurality of axial grooves that divide the female threads or the male threads into a plurality of separate sections in a circumferential direction, and the pressure flanks of the female threads or the male threads divided by the axial grooves being formed with rugged surfaces.

From another aspect of the invention, there is provided a lash adjuster comprising a lifter body including an end plate having a top surface and a bottom surface formed with an axial blind threaded hole having female threads, the lifter body axially slidably mounted between a cam and a stem of a valve with the top surface in contact with the cam, an adjuster screw having male threads on its outer periphery which are in threaded engagement with the female threads of the threaded hole, and an elastic member mounted in the threaded hole so as to bias the adjuster screw axially, the female threads of the threaded hole and the male threads of the adjuster screw being serration-shaped and having pressure flanks which receive pressure applied to the adjuster screw in such a direction as to push the adjuster screw into the threaded hole, and clearance flanks arranged between the adjacent pressure flanks, the pressure flanks having a greater flank angle than the clearance flanks, one of the inner periphery of the threaded hole and the outer periphery of the adjuster screw being formed with a plurality of axial grooves that divide the female threads or the male threads into a plurality of separate sections in a circumferential direction, and the pressure flanks of the female threads or the male threads that are not circumferentially divided by the axial grooves are formed with rugged surfaces.

In either invention, the rugged surfaces may be formed by helical grooves formed along the lead of the threads or satin-finished surface formed by shot-peening.

In either invention, oil film can be quickly expelled from between the opposed pressure flanks from the time when a distance is present between the pressure flanks to the time when they contact each other.

The rugged surfaces formed on the pressure flanks divided circumferentially into small sections keep sufficient roughness and thus a frictional coefficient sufficient to prevent the adjuster screw from being pushed into the threaded hole while turning, even if the pressure flanks get worn with prolonged use. This stabilizes the valve lift.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawings, the lash adjuster embodying the present invention will be described. First referring toFIG. 1, the lash adjuster A embodying this invention is mounted between a cam1of a direct type valve actuator and a valve stem2.

The valve stem2carries a spring retainer3at its top end. A valve spring4biases the spring retainer3and thus the valve stem2upwardly to keep a valve head5pressed against a valve seat6.

As shown inFIG. 2, the lash adjuster A includes a lifter body11. As seen inFIG. 1, the lifter body11is slidably mounted in a guide hole7formed in a cylinder head B. The lifter body11has an end plate12that is kept in contact with the cam1. A recess13is formed in the bottom surface of the end plate12. The lash adjuster A further includes a nut14having its upper portion received in the recess13of the lifter body11. In this embodiment, the nut14is made integral with the lifter body11by caulking the edge of the recess13.

But instead, the nut14may be fixed to the lifter body11with any other means such as by brazing.

The nut14has a threaded hole15having its top end closed by the end plate12. An adjuster screw16is in threaded engagement with the threaded hole15of the nut14and is biased axially downwardly by an elastic member17mounted in the threaded hole15between the end plate12and the adjuster screw16. A cap18(FIG. 2) formed by pressing is fitted on the nut14. Together with the nut14, the cap18is also fixed to the lifter body11by caulking the edge of the opening of the recess13. At its bottom, the cap18has a flange18athat abuts the bottom of the nut14, and also prevents the fall of the adjuster screw16from the threaded hole15.

Referring toFIG. 3, the female threads15aof the threaded hole15and the male threads16aof the adjuster screw16both have pressure flanks19a,19b, which receive pressure applied to the adjuster screw16in such a direction that the screw16is pushed into the nut14, and clearance flanks20a,20bprovided alternately with the pressure flanks19a,19band having a smaller flank angle than the pressure flanks19a,19b. Thus, as a whole, the female threads15aand the male threads16aare serration-shaped. Further, the threads15aand16ahave such a lead angle that under the pressure applied to the adjuster screw16from the elastic member17, the adjuster screw16can move axially downward while turning.

With the lash adjuster A mounted in a valve actuator, if a valve clearance exists between the top end of the valve stem2and the adjuster screw16e.g. due to thermal expansion of the cylinder head B, under the pressure of the elastic member17, the adjuster screw16moves axially downward along the clearance flanks20a,20bwhile turning in one direction until the valve clearance disappears.

Conversely, if the pressure from the valve stem2is applied to the adjuster screw16while the engine is at a stop, the adjuster screw16is pushed up until any axial gaps between the pressure flanks19of the female threads15aand the male threads16adisappear. Once the gaps disappear, the adjuster screw16cannot be pushed up any further because the frictional force between the pressure flanks19a,19bis large.

But if, for example, the valve seat6is worn, so that the distance between the top end of the valve stem2and the cam1decreases, the adjuster screw16will be pushed by a variable load applied from the valve stem2so as to retract while turning. This prevents the valve head5from being closed incompletely when the base circle1aof the cam1gets into contact with the end plate12of the lifter body11. Thus, it is possible to completely shut the valve when the base circle1aof the cam1contacts the end plate12of the lifter body11, even if the valve seat is worn.

On the other hand, in ordinary operating situations where no valve clearance adjustment is necessary while the engine is running, the adjuster screw16scarcely turns, and repeatedly displaces axially only within the gaps between the female threads15aof the threaded hole15and the male threads16aof the adjustor screw16.

That is, the pressure flanks19bon the male threads16aof the adjuster screw16repeatedly collide against and move away from the pressure flanks19bon the female threads15a. In the valve assembly, lubricating oil is present. Such lubricating oil inevitably flows into between the pressure flanks19a,19b. It is necessary to expel such lubricating oil as quickly as possible when the pressure flanks19a,19bmove toward each other. If lubricating oil has not been expelled from between the pressure flanks19when the pressure flanks contact each other, the lubricating oil will reduce the frictional force between the pressure flanks19a,19b, thereby allowing the adjuster screw16to move axially upwardly relative to the lifter body11while rotating in one direction. This reduces the lift of the valve.

In order to expel lubricating oil from between the pressure flanks19a,19bas quickly as possible when the pressure flanks19a,19bmove toward each other, the female threads15aof the nut member14are configured as shown inFIGS. 5A-5Cand/or the male threads16aof the adjuster screw16are configured as shown inFIGS. 6A-6D.

In the example ofFIG. 5A, a plurality of axial grooves21are formed in the inner periphery of the nut14so as to circumferentially divide the female threads15into a plurality of separate portions.

InFIG. 5B, a plurality of helical grooves22are formed in the pressure flanks19aof the female threads15aof the nut14to form roughened surfaces.

The nut14shown inFIG. 5Chas both the axial grooves21shown in FIG.5A and the helical grooves22shown in FIG.5B.

The helical grooves22shown inFIGS. 5B and 5Care formed by tapping and are preferably 0.1-0.5 mm deep.

InFIG. 6A, the pressure flanks19bof the male threads16aof the adjuster screw16are subjected to shot-peening to form satin-finished surfaces23thereon.

FIG. 7is a profile of the satin-finished surface23formed by shot-peening, in which the vertical axis indicates the surface roughness while the horizontal axis indicates the length in a circumferential direction.

InFIG. 6B, a plurality of helical grooves24are formed in the pressure flanks19bof the male threads16ato roughen them. The helical grooves24may be formed by rolling or cutting. Like the helical grooves22ofFIGS. 5B and 5C, the helical grooves24are preferably about 0.1-0.5 mm deep.

InFIG. 6C, a plurality of axial grooves25are formed to circumferentially divide the male threads16aof the adjuster screw16into a plurality of separate portions.

Table 1 shows possible combinations of threaded structures of the nut14shown inFIG. 5with threaded structures of the adjuster screw16shown in FIG.6.

FIG. 4shows the combination No. 3 shown in Table 1.

In combinations No. 1 and No. 2, in one of the female threads15aof the nut14and the male threads16aof the adjuster screw16, a plurality of axial grooves21,25dividing the thread in a circumferential direction are formed and helical grooves22,24are formed in the pressure flanks19aor19bof the thread divided by the axial grooves21,25. In combinations No. 3 to No. 5, in one of the female threads15aand the male threads16a, a plurality of axial grooves21,25are formed whereas in the other of them, a satin-finished surface23or helical grooves22,24are formed.

With any of these combinations, it is possible to smoothly and reliably expel lubricating oil present between the pressure flanks19a,19bwhen they move toward each other.

With these arrangements, in a situation where the adjuster screw16reciprocates relative to the nut14within the gaps between the female threads15aand the male threads16a, when the pressure flanks19bof the male threads16aand the pressure flanks19aof the female threads15amove toward each other, lubricating oil therebetween is smoothly discharged through the axial grooves21or25until the pressure flanks19a,19bcome close to each other. When the pressure flanks19a,19bcome into contact with each other, the lubricating oil still remaining therebetween will flow through the satin-finished surface23or the helical grooves22or24into the axial grooves21or25and then will be discharged therethrough.

Thus, lubricating oil between the pressure flanks19a,19bcan be quickly and reliably expelled from space between the pressure flanks19a,19b. The pressure flanks19a,19bare thus pressed against each other with substantially no lubricating oil disposed therebetween, so that the friction therebetween is kept high when they are pressed against each other. The high frictional force therebetween prevents the adjuster screw16from turning and being pushed into the nut14due to the axial pressure transmitted through the pressure flanks19a,19b.

Another feature of these arrangements is that the helical grooves, axial grooves and/or satin-finished surface keep a sufficient roughness of the pressure flanks19a,19beven when the pressure flanks19a,19bhave worn considerably, thereby keeping a sufficiently high frictional force therebetween. This also prevents the adjuster screw16from turning and being pushed into the nut14. Thus, it is possible to provide a stable valve lift.

For a lash adjuster including the nut14shown inFIG. 5C, i.e. a nut having both the axial grooves21and the helical grooves22, and a lash adjuster including the nut14shown inFIG. 5A, i.e. a nut having only the axial grooves21, and the adjustor screw16shown inFIG. 6A, i.e. an adjuster screw having the satin-finished surface23formed by shot peening on the pressure flank19bof the male thread16a, a low-temperature characteristics test was conducted after using them in a harsh environment. The results of the test are shown inFIGS. 8A and 8B.

In the graphs ofFIGS. 8A and 8B, the lower lines A1show the number of revolutions of the crankshaft. As shown, the crankshafts were rotated at a constant speed.

The upper curves B1are lift curves for the valve5showing closed positions and fully open positions of the valve. While only part of each lift curve is shown, it is to be understood that closed positions and fully open positions of the valve alternate along the upper chain line and the lower chain line, respectively, in the time axis direction.

FIGS. 9A and 9Bshow the results of sweep tests for a lash adjuster including a nut14having only the axial grooves21formed in the inner periphery of the threaded hole15for the number of revolutions of the crankshaft.FIGS. 9A and 9Bshow the results before and after the endurance test.

FIGS. 10A and 10Bshow the results of sweep tests for a lash adjuster including a nut14having only the helical grooves22formed in the pressure flank19aof the female thread15afor the number of revolutions of the crankshaft.FIG. 10Ashows the test results at normal temperature whileFIG. 10Bshows the test results at a lower temperature.

InFIGS. 9A,9B and10A and10B, lower curves A2and A3show the number of revolutions of the crankshaft which linearly increased from 800 rpm idling to a maximum of 6000 rpm and then linearly reduced to the 800 rpm idling speed.

Similar to the lift curves B1ofFIGS. 8A and 8B, the lift curves B2and B3show how the closed position and fully open position of the valve changed.

From these results, it will be apparent that with a lash adjuster including the nut14shown inFIG. 5Cwith the axial grooves21in the threaded hole15and the helical grooves22on the pressure flank19aof the nut, or a lash adjuster including the nut14ofFIG. 5Awith the axial grooves21in the threaded hole15and the satin-finished surface23on the pressure flank19bof the male threads16a, it is possible to reliably expel oil film even at low temperatures, and also keep high friction coefficiency between the pressure flanks19a,19beven after long use. Thus, stable valve lift characteristics are maintained.

In the embodiment ofFIG. 1, the lash adjuster according to this invention is mounted in a direct type valve actuator to automatically adjust the valve clearance. But it can also be used with an end pivot type valve actuator. Further it may be used in an auto-tensioner or chain tensioner.

With this arrangement, when the opposed pressure flanks on the threads repeatedly move toward and away from each other, it is possible to quickly and reliably expel oil film from between the pressure flanks.

Thus, the pressure flanks can be reliably brought into contact with each other without oil film disposed therebetween. The friction therebetween is thus kept high. This prevents the adjuster screw from being pushed into the nut while turning. Thus, a stable valve lift is maintained.

Even if the pressure flanks get worn, the rugged surface formed on one of the pressure flanks keeps sufficient roughness and thus frictional resistance to keep the adjuster screw from turning. Thus, a stable valve lift is maintained for a long period of time.