LASH ADJUSTER

Abstract

A lash adjuster is provided of which the return spring is never compressed excessively when the adjusting screw is pushed into the nut member, and of which the external thread on the outer periphery of the adjusting screw can be formed easily by rolling. The lash adjuster 1 includes a nut member 12 having an internal thread 13, an adjusting screw 15 having an external thread 14 on its outer periphery which is in threaded engagement with the internal thread 13, and a return spring 17 biasing the adjusting screw 15 in the direction to protrude from the nut member 12. The adjusting screw 15 pivotally supports an arm 7 of a valve gear at its end protruding from the nut member 12. The adjusting screw 15 has at its portion protruding from the nut member 12 a stopper portion 23 having a larger diameter than the inner diameter of the nut member 12. By supporting the stopper portion 23 on the open end of the nut member 12, it is possible to restrict the movement of the adjusting screw 15 when the adjusting screw 15 is pushed into the nut member 12.

Description

TECHNICAL FIELD

This invention relates to a lash adjuster mounted in an engine valve gear.

BACKGROUND ART

Known valve gears for moving a valve provided at an intake port or an exhaust port of an engine include one comprising an arm pivotable about one end thereof and adapted to be pushed down at its central portion, thereby pushing down a valve stem at the other end (swing arm type valve gear), one comprising an arm pivotable about its central portion and adapted to be pushed up by a cam at one end thereof, thereby pushing down a valve stem at the other end (rocker arm type valve gear), and one comprising a valve lifter vertically movably supported and adapted to be pushed down by a cam, thereby pushing down a valve stem (direct type valve gear).

In these valve gears, gaps between their component parts may change due to differences in thermal expansion between component parts, which may cause noise and compression leakage. Also, when the sliding parts of the valve gear become worn too, gaps between component parts of the valve gear may change, which may also cause noise.

In order to prevent such noise and compression leakage, ordinary valve adjusters include a lash adjuster for absorbing gaps between component parts of the valve gear.

One known lash adjuster used in a swing arm type valve gear comprises a nut member inserted in a mounting hole formed in the top surface of a cylinder head, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude upwardly from the nut member, the adjusting screw pivotally supporting the arm of the valve gear with its end protruding from the nut member (Patent documents 1 and 4).

One know lash adjuster used in a direct type valve gear comprises a lifter body vertically slidably inserted in a guide hole formed in a cylinder head, a nut member fixed to the lifter body so as to be vertically movable together with the lifter body, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude downwardly from the nut member, the adjusting screw pressing the valve stem of the valve gear with its end protruding from the nut member (Patent document 2).

One known lash adjuster used in a rocker arm type valve gear comprises a nut member inserted in a mounting hole formed in the bottom surface of the arm which pivots as the cam rotates, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude downwardly from the nut member, the adjusting screw pressing the valve stem of the valve gear with its end protruding from the nut member (Patent documents 3 and 4).

In Patent documents 1-3, the return spring is a coil spring that applies an axial force to the adjusting screw that tends to move the adjusting screw in the direction to protrude from the nut member. In Patent document 4, the return spring is a torsion spring that applies torque to the adjusting spring that tends to move the adjusting screw in the direction to protrude from the nut member.

In these lash adjusters, as a cam rotates and a load that tends to push the adjusting screw into the nut member is applied, the pressure flank of the external thread of the adjusting screw is supported on the pressure flank of the internal thread of the nut member, so that the adjusting screw is axially fixed in position.

If the relative position between the arm and the valve stem changes due e.g. to thermal expansion of the valve gear, according to the degree of change in the relative position, the adjusting screw axially moves in the nut member while rotating, thereby absorbing gaps between component parts of the valve gear.

Patent document 1: JP Patent Publication 2005-273510A
Patent document 2: JP Patent Publication 2003-227318A
Patent document 3: JP Patent Publication 2006-132426A
Patent document 4: JP Utility Model Publication 64-34407

DISCLOSURE OF THE INVENTION

Object of the Invention

With these types of lash adjusters, in an ordinary situation, only slight slip occurs between the external thread of the adjusting screw and the internal thread of the nut member. But if abnormal loads or vibrations act on the adjusting screw due to over-speed of the engine, if the frictional resistance between the pressure flanks decreases due to wear of the external thread and the internal thread, or if the adjusting screw is suddenly and rapidly pushed in due to sudden thermal expansion of the valve gear, unnecessarily large slip may occur between the external thread of the adjusting screw and the internal thread of the nut member.

As a result, the adjusting screw is markedly pushed into the nut member, which may cause breakage of the return spring due to over-compression or separation of the arm.

One way to prevent the return spring from being excessively compressed and broken when the adjusting screw is pushed into the nut member would be to provide a tubular extension at the end of the adjusting screw located in the nut member so as to surround the return spring. The extension restricts the movement of the adjusting screw by abutting the bottom of the nut member. But with this arrangement, since the tubular extension of the adjusting screw is low in rigidity, when the external screw is formed on the outer periphery of the adjusting screw by rolling, the external screw tends to be formed incompletely.

An object of the present invention is to provide a lash adjuster of which the return spring is never compressed excessively when the adjusting screw is pushed into the nut member, and of which the external thread on the outer periphery of the adjusting screw can be formed easily by rolling.

Means to Achieve the Object

In order to achieve this object, the adjusting screw is provided with a stopper portion at its portion protruding from the nut member, the stopper portion having a diameter larger than the inner diameter of the nut member. The stopper portion abuts an open end of the nut member, thereby restricting movement of the adjusting screw when the adjusting screw is pushed into the nut member.

The stopper portion may have a tapered surface on its outer periphery which is configured to abut a tapered surface formed on the open end of the nut member. With this arrangement, when the adjusting screw is pushed into the nut member, the tapered surfaces contact each other, thereby preventing inclination of the adjusting screw. This helps the adjusting screw to protrude smoothly thereafter.

Alternatively, the stopper portion may have a flat surface on its outer periphery which extends perpendicular to the axial direction and configured to abut a flat surface formed on the open end of the nut member so as to extend perpendicular to the axial direction. With this arrangement too, when the adjusting screw is pushed into the nut member, the flat surfaces contact each other, thereby preventing inclination of the adjusting screw, which helps the adjusting screw to protrude smoothly thereafter.

The stopper portion may be integrally provided at the portion of the adjusting screw protruding from the nut member, or may be a separate member from the adjusting screw. In the latter case, such a separate stopper member may be a snap ring fitted on the outer periphery of the portion of the adjusting screw protruding from the nut member.

This invention is applicable to a lash adjuster wherein the return spring is a compression spring that applies an axial force to the adjusting screw that tends to push the adjusting screw out of the nut member. In this case, the external thread and the internal thread may have a serration-shaped section with their pressure flanks having a larger flank angle than their clearance flanks.

This invention is also applicable to lash adjuster wherein the return spring is a torsion spring that applies torque to the adjusting screw that tends to push the adjusting screw out of the nut member. In this case, the external thread and the internal thread may be serration-shaped threads, trapezoidal threads or triangular threads. The torsion spring may be one of a torsion coil spring, a spiral spring and a volute spring.

This invention is applicable e.g. to the following lash adjusters.

1) A lash adjuster wherein the nut member is inserted in a receiving hole formed in a top surface of a cylinder head, and wherein the adjusting screw pivotally supports an arm of a valve gear at its end protruding from the nut member.
2) A lash adjuster of any wherein the nut member is fixed to a lifter body vertically slidably inserted in a guide hole formed in a cylinder head, and wherein the adjusting screw presses a valve stem of a valve gear at its end protruding from the nut member.
3) A lash adjuster wherein the nut member is inserted in a receiving hole formed in a bottom surface of an arm that pivots as a cam rotates, and wherein the adjusting screw presses a valve stem of a valve gear at its end protruding from the nut member.

The adjusting screw may comprises a pivot member axially slidably inserted in the nut member, an externally threaded member supporting an end of the pivot member located in the nut member and having the external thread on its outer periphery, and an elastic member disposed between the pivot member and the externally threaded member.

In this case, the pivot member preferably has a square hole formed in its end located in the nut member, and the externally threaded member preferably has a square shaft engaged in the square hole, whereby the pivot member and the externally threaded member are rotatable together due to the engagement of the square shaft in the square hole. With this arrangement, since the externally threaded member can be rotated by rotating the pivot member, the externally threaded member can be easily mounted in the nut member.

ADVANTAGES OF THE INVENTION

With the lash adjuster according to this invention, when the adjusting screw is pushed into the nut member, the stopper portion abuts the open end of the nut member, thereby restricting movement of the adjusting screw. This prevents over-compression of the return spring.

With this lash adjuster, since the stopper member for restricting the movement of the adjusting screw is provided at the portion of the adjusting screw protruding from the nut member, it is not necessary to provide a tubular extension for restricting the movement of the adjusting screw by abutting the bottom member at the end of the adjusting screw located in the nut member. Thus, the external thread on the outer periphery of the adjusting screw can be formed easily by rolling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve gear including a lash adjuster according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of the lash adjuster shown in FIG. 1, showing the state in which its adjusting screw is pushed in.

FIG. 3 is an enlarged sectional view of a modification of the lash adjuster shown in FIG. 2.

FIG. 4 is an enlarged sectional view of a further modification of the lash adjuster of FIG. 2.

FIG. 5 is an enlarged sectional view of a still further modification of the lash adjuster of FIG. 2.

FIG. 6 is a front view of a valve gear including a lash adjuster according to a second embodiment of the present invention.

FIG. 7 is a front view of a valve gear including a lash adjuster according to a third embodiment of the present invention.

FIG. 8 is an enlarged view of a modification of the return spring shown in FIG. 2.

FIG. 9 is an enlarged view of a modification of the return spring shown in FIG. 6.

FIG. 10 is an enlarged view of a modification of the return spring shown in FIG. 7.

FIG. 11 is an enlarged sectional view of a lash adjuster according to a fourth embodiment of the present invention.

FIG. 12 is an enlarged sectional view of a modification of the return spring shown in FIG. 11.

FIG. 13 is an enlarged sectional view of a further modification of the return spring shown in FIG. 11.

FIG. 14 is an enlarged sectional view of a still further modification of the return spring shown in FIG. 11.

FIG. 15 is a sectional view taken along line XV-XV of FIG. 14.

DESCRIPTION OF THE NUMERALS

• 1. Lash adjuster
• 2. Cylinder head
• 7. Arm
• 11. Receiving hole
• 12. Nut member
• 13. Internal thread
• 14. External thread
• 15. Adjusting screw
• 15A. Pivot member
• 15B. Externally threaded member
• 15C. Spring washer
• 17. Return spring
• 18. Pressure flank
• 19. Clearance flank
• 23. Stopper portion
• 24, 25. Tapered surface
• 26, 27. Flat surface
• 28. Snap ring
• 31. Lash adjuster
• 32. Cylinder head
• 35. Valve stem
• 39. Guide hole
• 40. Lifter body
• 41. Nut member
• 42. Internal thread
• 43. External thread
• 44. Adjusting screw
• 45. Return spring
• 51. Stopper portion
• 61. Lash adjuster
• 65. Valve stem
• 67. Arm
• 71. Cam
• 73. Nut member
• 74. Adjusting screw
• 75. Return spring
• 76. Receiving hole
• 77. Internal thread
• 78. External thread
• 82. Stopper portion
• 91. Lash adjuster
• 94, 95. Flat surface
• 96. Square hole
• 97. Square shaft

BEST MODE FOR EMBODYING THE INVENTION

FIG. 1 shows a valve gear including the lash adjuster 1 according to the first embodiment of the present invention. This valve gear includes a valve 4 for an intake port 3 formed in a cylinder head 2 of an engine, a valve stem 5 connected to the valve 4, and an arm 7 that pivots as a cam 6 rotates.

The valve stem 5 extends upwardly from the valve 4 and is slidably inserted through the cylinder head 2. An annular spring retainer 8 is fixed to the outer periphery of the valve stem 5 at its upper portion. A valve spring 9 is mounted between the bottom surface of the spring retainer 8 and the top surface of the cylinder head 2. The valve spring 9 biases the valve stem 5 upwardly through the spring retainer 8, thereby seating the valve 4 on a valve seat 10.

As shown in FIG. 2, the lash adjuster 1 comprises a tubular nut member 12 received in a receiving hole 11 formed in the top surface of the cylinder head 2, an adjusting screw 15 having an external thread 14 on its outer periphery at its lower portion which is in threaded engagement with an internal thread 13 formed on the inner periphery of the nut member 12, a bottom member 16 fixed to the bottom end of the nut member 12, and a return spring 17 mounted between the adjusting screw 15 and the bottom member 16.

The external thread 14 and the internal thread 13 each have a pressure flank 18 for receiving an axial load that tends to push the adjusting screw 15 into the nut member 12, and a clearance flank 19, and have a serration-shaped section with the pressure flank 18 having a larger flank angle than the clearance flank 19.

The return spring 17 is a compression coil spring having its bottom end supported on the bottom member 16 and applying at its top end an axial force to the adjusting screw 15 through a spring seat 20 that tends to protrude the adjusting screw 15 upwardly from the nut member 12. The end portion of the adjusting screw 15 that is inserted in the nut member 12 has a tubular shape, and receives the spring seat 20.

The end of adjusting screw 15 protruding from the nut member 12 is engaged in a recess 21 formed in the bottom surface of the arm 7 at one end thereof. The arm 7 is thus pivotally supported about the protruding end of the adjusting screw 15. The bottom surface of the arm 7 at the other end is in contact with the top end of the valve stem 5. The arm 7 carries at its mid-portion a roller 22 that is in contact with the cam 6, which is located over the arm 7.

The portion of the adjusting screw 15 protruding from the nut member 12 has a stopper portion 23 having a diameter larger than the inner diameter of the nut member 12. As shown in FIG. 2, the stopper portion 23 has a tapered surface 24 on its outer periphery which is adapted to abut a tapered surface 25 formed at the open end of the nut member 12 when the adjusting screw 15 is pushed into the nut member 12, thereby restricting the movement of the adjusting screw 15.

The operation of the lash adjuster 1 is now described.

When the cam 6 is rotated by the engine and the cam lobe 6a of the cam 6 presses down the arm 7, the valve 4 separates from the valve seat 10, thus opening the intake port 3. At this time, force acts on the adjusting screw 15 that tends to push in the adjusting screw 15. But due to the frictional resistance between the external thread 14 of the adjusting screw 15 and the internal thread 13 of the nut member 12, the adjusting screw 15 is prevented from rotating, so that the adjusting screw 15 is axially fixed in position.

When the cam 6 further rotates and the cam lobe 6a moves past the roller 22, the valve stem 5 rises under the biasing force of the valve spring 9, until the valve 4 is seated on the valve seat 10 and the intake port 3 is closed.

In a strict sense, when the cam lobe 6a of the cam 6 presses down the arm 7, slight slip occurs between the pressure flank 18 of the external thread 14 and the pressure flank 18 of the internal thread 13. But after the cam lobe 6a has moved past the roller 22 and until the cam lobe 6a again contacts the roller 22, since a load tending to push in the adjusting screw 15 is removed, the adjusting screw 15 returns to the original position under the biasing force of the return spring 17.

When the distance between the cam 6 and the arm 7 increases due to differences in thermal expansion between component parts of the valve gear such as the cylinder head 2, valve stem 5 and arm 7 while the engine is running, the adjusting screw 15 moves in the protruding direction while rotating under the biasing force of the return spring 17. Thus, a gap never forms between the base circle 6b of the cam 6 and the roller 22.

Conversely, when the contact surfaces of the valve 4 and the valve seat 10 become worn, even while the base circle 6b of the cam 6 is in contact with the roller 22, the biasing force of the valve spring 9 continuously acts on the adjusting screw 15 as a load tending to push in the adjusting screw 15. Thus, due to slight slip that occurs between the external thread 14 and the internal thread 13 every time the cam lobe 6a contacts the roller 22, the adjusting screw 15 is gradually pushed into the nut member, and the valve stem 5 gradually moves up, thus preventing formation of a gap between the contact surfaces of the valve 4 and the valve seat 10.

As discussed above, in an ordinary situation, only slight slip occurs between the external thread 14 of the adjusting screw 15 and the internal thread 13 of the nut member 12. But if abnormal loads or vibrations act on the adjusting screw 15 due to over-speed of the engine, if the frictional resistance between the pressure flanks 18 decreases due to wear of the external thread 14 and the internal thread 13, or if the adjusting screw 15 is suddenly and rapidly pushed in due to sudden thermal expansion of the valve gear, unnecessarily large slip may occur between the external thread 14 of the adjusting screw 15 and the internal thread 13 of the nut member 12.

As a result, the adjusting screw 15 is markedly pushed into the nut member 12. But as shown in FIG. 2, when the adjusting screw 15 is pushed in a certain distance, the stopper portion 23 abuts the open end of the nut member 12, thus preventing any further movement of the adjusting screw 15, and thus preventing breakage of the return spring 17 due to over-compression or separation of the arm 7.

When abnormal loads or vibrations or sudden thermal expansion of the valve gear disappears, the adjusting screw 15 is moved in the protruding direction under the biasing force of the return spring 17, and the lash adjuster returns to normal.

With this lash adjuster 1, since the adjusting screw 15 has a stopper portion 23 at its portion protruding from the nut member 12, it is not necessary to provide a tubular extension for restricting the movement of the adjusting screw 15 by abutting the bottom member 16 at the end of the adjusting screw 15 located in the nut member 12. Thus, the external thread 14 on the outer periphery of the adjusting screw 15 can be formed easily by rolling.

With this lash adjuster 1, when the adjusting screw 15 is pushed into the nut member 12, the tapered surface 24 on the outer periphery of the stopper portion 23 abuts the tapered surface 25 formed at the open end of the nut member 12, thereby preventing inclination of the adjusting screw 15 by the contact of the tapered surfaces 24 and 25. This helps the adjusting screw 15 to protrude smoothly thereafter.

As shown in FIG. 3, the stopper portion 23 may have on its outer periphery a flat surface 26 perpendicular to the axial direction and configured to abut a flat surface 27 formed on the open end of the nut member 12 so as to be perpendicular to the axial direction when the adjusting screw 15 is pushed into the nut member 12. With this arrangement too, it is possible to prevent inclination of the adjusting screw 15 when the adjusting screw 15 is pushed into the nut member 12 by the contact of the flat surfaces 26 and 27, thus helping the adjusting screw 15 to protrude smoothly thereafter.

In the above embodiment, the stopper portion 23, which has a larger diameter than the inner diameter of the nut member 12, is integrally provided at the portion of the adjusting screw 15 protruding from the nut member 12. But instead, the stopper portion 23 may be a separate member from the adjusting screw 15. For example, such a separate stopper member 23 may be a snap ring 28 fitted around the portion of the adjusting screw 15 protruding from nut member 12, as shown in FIG. 4.

In the above embodiment, the portion of the adjusting screw 15 having the external thread 14 on the outer periphery is integral with its portion protruding from the nut member 12. But as shown in FIG. 5, the adjusting screw 15 may comprise a body 15b having the external thread 14 on the outer periphery, and a head 15a separate from the body 15b and protruding from the nut member 12. In this case, the head 15a may be slidably inserted in the nut member 12, and an engaging member 29 may be provided at the open end of the nut member 12 to prevent separation of the head 15a from the nut member 12 by engaging the head 15a.

FIG. 6 shows a valve gear including the lash adjuster 31 according to the second embodiment of the present invention. As with the first embodiment, this valve gear includes a valve 34 provided at an intake port 33 of a cylinder head 32, and a valve stem 35 connected to the valve 34. The valve stem 35 extends upwardly from the valve 34, and has a spring retainer 36 fixed to its upper portion. A spring retainer 36 biases the spring retainer 36 upwardly, thereby seating the valve 34 against a valve seat 38.

The lash adjuster 31 comprises a lifter body 40 vertically slidably inserted in a guide hole 39 formed in the cylinder head 32, a nut member 41 vertically movable together with the lifter body 40, an adjusting screw 44 having an external thread 43 on the outer periphery thereof which is in threaded engagement with an internal thread 42 formed on the inner periphery of the nut member 41, and a return spring 45 biasing the adjusting screw 44 in the direction to protrude downwardly from the nut member 41.

The lifter body 40 comprises a tubular portion 46 and an end wall 47 closing the top end of the tubular portion 46. A hard shim 48 is fixed to the top surface of the end wall 47. A cam 49 is in contact with the shim 48. The nut member 41 is integrally formed at the central portion of the end wall 47, and has its top end closed by the shim 48.

The external thread 43 and the internal thread 42 each have a pressure flank for receiving an axial load tending to push the adjusting screw 44 into the nut member 41, and a clearance flank, and have a serration-shaped section, with the pressure flank having a larger flank angle than the clearance flank.

The return spring 45 is a compression spring having its top end supported by the shim 48 and applying at its bottom end an axial force to the adjusting screw 44 through a spring seat 50 that tends to push the adjusting screw 44 downwardly out of the nut member 41. The end of the adjusting screw 44 protruding from the nut member 41 is pressed against the top end of the valve stem 35. The end of the adjusting screw 44 located in the nut member 41 has a tubular shape in which the spring seat 50 is received.

The adjusting screw 44 has at its portion protruding from the nut member 41 a stopper portion 51 having a larger diameter than the inner diameter of the nut member 41. When the adjusting screw 44 is pushed into the nut member 41, the stopper portion 51 restricts the movement of the adjusting screw 44 by abutting the open end of the nut member 41. The stopper portion 51 can be formed by rolling simultaneously with the external thread 43.

As with the first embodiment, with this lash adjuster 31, in an ordinary situation, only slight slip occurs between the external thread 43 of the adjusting screw 44 and the internal thread 42 of the nut member 41. But if abnormal loads or vibrations act on the adjusting screw 44 due to over-speed of the engine, or if the frictional resistance between the pressure flanks decreases due to wear of the external thread 43 and the internal thread 42, unnecessarily large slip may occur between the external thread 43 of the adjusting screw 44 and the internal thread 42 of the nut member 41.

As a result, the adjusting screw 44 is markedly pushed into the nut member 41. But when the adjusting screw 44 is pushed in a certain distance, the stopper portion 51 abuts the open end of the nut member 41, thus preventing any further movement of the adjusting screw 44, and thus preventing breakage of the return spring 45 due to over-compression.

With this lash adjuster 31, since the stopper portion 51 for restricting the movement of the adjusting screw 44 is provided at the portion of the adjusting screw 44 protruding from the nut member 41, it is not necessary to provide a tubular extension for restricting the movement of the adjusting screw 44 by abutting the shim 48 at the end of the adjusting screw 44 located in the nut member 41. Thus, the external thread 43 on the outer periphery of the adjusting screw 44 can be formed easily by rolling.

In this embodiment, the nut member 41 and the lifter body 40 are integrally formed. But the nut member 41 and the lifter body 40 may be separate members with the nut member 41 fixed to the lifter body 40. In either case, it is important that the nut member 41 move vertically together with the lifter body 40 when the lifter body 40 is vertically moved.

FIG. 7 shows a valve gear including the lash adjuster 61 according to the third embodiment of the present invention. This valve gear includes a valve 64 provided at an intake port 63 of a cylinder head 62 of an engine, a valve stem 65 connected to the valve 64, and an arm 67 pivotally supported about a pivot shaft 66. The valve stem 65 extends upwardly from the valve 64, and has a spring retainer 68 fixed to its upper portion. A valve spring 69 biases the spring retainer 68 upwardly, thereby seating the valve 64 on a valve seat 70.

The arm 67 has its central portion pivotally supported by the pivot shaft 66. The arm 67 carries at one end thereof a roller 72 kept in contact with a cam 71 so that the arm 67 pivots as the cam 71 rotates. The lash adjuster 61 is mounted to the other end of the arm 67.

The lash adjuster 61 comprises a nut member 73, an adjusting screw 74 and a return spring 75. The nut member 73 is inserted in a hole 76 extending vertically through the arm 67. The adjusting screw 74 has an external thread 78 on its outer periphery that is in threaded engagement with an internal thread 77 formed on the inner periphery of the nut member 73.

The nut member 73 has a top end protruding from the top surface of the arm, and a tubular cap 79 having a closed top end is fitted on and fixed to the protruding top end portion of the nut member 73. The cap 79 engages the top edge of the hole 76, thereby preventing the nut member 73 from separating downwardly from the hole 76. The nut member 73 has a flange 80 at its bottom end which is in abutment with the bottom surface of the arm 67, and configured to receive upward force applied to the nut member 73.

The external thread 78 and the internal thread 77 each have a pressure flank for receiving an axial load tending to push the adjusting screw 74 into the nut member 73, and a clearance flank, and have a serration-shaped section, with the pressure flank having a larger flank angle than the clearance flank.

The return spring 75 is a compression spring having its top end supported by the cap 79 and applying at its bottom end an axial force to the adjusting screw 74 through a spring seat 81 that tends to push the adjusting screw 74 downwardly out of the nut member 73. The end of the adjusting screw 74 protruding from the nut member 73 is pressed against the top end of the valve stem 65.

The adjusting screw 74 has at its portion protruding from the nut member 73 a stopper portion 82 having a larger diameter than the inner diameter of the nut member 73. When the adjusting screw 74 is pushed into the nut member 73, the stopper portion 82 restricts the movement of the adjusting screw 74 by abutting the open end of the nut member 73. The stopper portion 82 can be formed by rolling simultaneously with the external thread 78.

As with the first embodiment, with this lash adjuster 61, in an ordinary situation, only slight slip occurs between the external thread 78 of the adjusting screw 74 and the internal thread 77 of the nut member 73. But if abnormal loads or vibrations act on the adjusting screw 74 due to over-speed of the engine, or if the frictional resistance between the pressure flanks decreases due to wear of the external thread 78 and the internal thread 77, unnecessarily large slip may occur between the external thread 78 of the adjusting screw 74 and the internal thread 77 of the nut member 73.

As a result, the adjusting screw 74 is markedly pushed into the nut member 73. But when the adjusting screw 74 is pushed in a certain distance, the stopper portion 82 abuts the open end of the nut member 73, thus preventing any further movement of the adjusting screw 74, and thus preventing breakage of the return spring 75 due to over-compression.

With this lash adjuster 61, since the stopper portion 82 for restricting the movement of the adjusting screw 74 is provided at the portion of the adjusting screw 74 protruding from the nut member 73, it is not necessary to provide a tubular extension for restricting the movement of the adjusting screw 74 by abutting the cap 79 at the end of the adjusting screw 74 located in the nut member 73. Thus, the external thread 78 on the outer periphery of the adjusting screw 74 can be formed easily by rolling.

In the first embodiment, a compression spring is used as the return spring 17 for biasing the adjusting screw 15 in the direction to protrude upwardly from the nut member 12, a torsion coil spring may used instead as shown in FIG. 8.

In FIG. 8, the return spring 17 has its bottom end engaged in an engaging hole 83 formed in the bottom member 16, and its top end engaged in an engaging hole 84 formed in the adjusting screw 15. Thus this return spring applies torque to the adjusting screw 15 that tends to push the adjusting screw out of the nut member 12 by its torsional deformation.

In the second embodiment too, as shown in FIG. 9, a torsion coil spring may be used as the return spring 45 for biasing the adjusting screw 44 in the direction to protrude downwardly from the nut member 41.

In FIG. 9, the return spring 45 has its top end engaged in an engaging hole 85 formed in the shim 48 and its bottom end engaged in an engaging hole 86 formed in the adjusting screw 44. Thus this return spring applies torque to the adjusting screw 44 that tends to push the adjusting screw out of the nut member 41 by its torsional deformation. The external thread 43 on the outer periphery of the adjusting screw 44 and the internal thread 42 on the nut member 41 are both vertically symmetrical trapezoidal thread.

In the third embodiment too, as shown in FIG. 10, a torsion coil spring may be used as the return spring 75 for biasing the adjusting screw 74 in the direction to protrude downwardly from the nut member 73.

In FIG. 10, the return spring 75 has its top end engaged in an engaging hole 87 formed in the cap 79 and its bottom end engaged in an engaging hole 88 formed in the adjusting screw 74. Thus this return spring applies torque to the adjusting screw 74 that tends to push the adjusting screw out of the nut member 73 by its torsional deformation. The external thread 78 on the outer periphery of the adjusting screw 74 and the internal thread 77 on the nut member 73 are both vertically symmetrical triangular thread.

FIG. 11 shows a valve gear including the lash adjuster 91 according to the fourth embodiment of this invention. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

The adjusting screw 15 comprises a pivot member 15A axially slidably inserted in the nut member 12, an externally threaded member 15B supporting the end of the pivot member 15A located in the nut member 12 and having an external thread on its outer periphery, and a spring washer 15C disposed between the pivot member 15A and the externally threaded member 15B. The spring washer 15C may be a disk spring washer or a wavy washer.

An annular anti-separation member 92 is fitted on the open end of the nut member 12 through which the pivot member 15A extends. The pivot member 15A is formed with an annular groove 93 at its portion extending through the open end of the nut member 12. The anti-separation member 92 is engaged in the annular groove 93, thereby preventing separation of the pivot member 15A from the nut member 12.

At its portion protruding from the nut member 12, the pivot member 15A has a stopper portion 23 having a diameter larger than the inner diameter of the nut member 12. The stopper portion 23 has a flat surface 94 formed on its outer periphery to extend perpendicular to the axial direction. When the pivot member 15A is pushed into the nut member 12, the flat surface 94 is configured to abut a flat surface 95 formed on the anti-separation member 92 provided at the open end of the nut member 12 to extend perpendicular to the axial direction, thereby restricting the movement of the pivot member 15A.

The pivot member 15A is formed with a square hole 96 in the center of its end inserted in the nut member 12. The externally threaded member 15B is formed with a square shaft 97 engaged in the square hole 96. Thus, when the pivot member 15A is rotated, the pivot member 15A and the externally threaded member 15B rotate together due to the engagement of the square shaft 97 in the square hole 96.

The return spring 17 is a torsion coil spring having its bottom end engaged in an engaging hole 98 formed in the bottom member 16, and its top end engaged in an engaging hole 99 formed in the externally threaded member 15B. The return spring 17 thus applied torque to the externally threaded member 15B that tends to push the pivot member 15A out of the nut member 12 due to its torsional deformation.

With this lash adjuster 91, when the adjusting screw 15 is pushed into the nut member 12, the stopper portion 23 abuts the anti-separation member 92 at the open end of the nut member 12, thus restricting the movement of the adjusting screw 15. This prevents breakage of the return spring 17 due to over-compression.

With this lash adjuster 91, since the stopper portion 23 for restricting movement of the adjusting screw 15 is provide at the portion of the pivot member 15A protruding from the nut member 12, it is not necessary to provide a tubular extension at the end of the externally threaded member 15B located in the nut member 12 for restricting the movement of the adjusting screw 15 by abutting the bottom member 16. Thus, the external thread 14 on the externally threaded member 15B can be formed easily by rolling.

With this lash adjuster 91, when the engine is stopped in a high-temperature state, and differences in shrinkage appear between component parts of the valve gear when the engine cools down subsequently, the spring washer 15C between the externally threaded member 15B and the pivot member 15A is compressed, thereby absorbing such differences in shrinkage. Thus, when the engine is restarted, no gap forms between the valve 4 and the valve seat 10 due to differences in shrinkage. This prevents compression leakage.

With this lash adjuster 91, since the pivot member 15A and the externally threaded member 15B are adapted to rotate together due to the engagement of the square shaft 97 in the square hole 96, it is possible to rotate the externally threaded member 15B by rotating the pivot member 15A. Thus, compared to an arrangement having no square shaft 97 and the square hole 96, the externally threaded member 15B can be easily mounted in the nut member 12.

In this embodiment, the elastic member disposed between the pivot member 15A and the externally threaded member 15B is the spring washer 15C. But the spring washer 15C may be replaced by a different elastic member (such as a compression coil spring).

If a torsion coil spring is used as the return spring 17, it may be a tubularly wound member as shown in FIG. 11, or may be a conically wound member as shown in FIG. 12.

Also, the return spring 17 may be a torsion spring other than a torsion coil spring (such as a spiral spring or a volute spring).

The return spring 17 in FIG. 13 is a spiral spring comprising a spirally wound thin sheet. This return spring 17 has its radially outer end rotationally fixed to the bottom member 16 of the nut member 12 and its radially inner end engaged in a slit formed in a protrusion 100 formed on the end of the externally threaded member 15B located in the nut member 12. The return spring 17 thus applies torque to the externally threaded member 15B that tends to push the pivot member 15A out of the nut member 12 due to its torsional deformation. The external thread 14 on the outer periphery of the externally threaded member 15B and the internal thread 13 on the inner periphery of the nut member 12 are vertically symmetrical triangular threads.

The return spring 17 in FIGS. 14 and 15 is a volute spring comprising a helically wound thin sheet. This return spring 17 has its radially outer end rotationally fixed to the bottom member 16 of the nut member 12 and its radially inner end engaged in a slit formed in a protrusion 101 formed on the end of the externally threaded member 15B located in the nut member 12. The return spring 17 thus applies torque to the externally threaded member 15B that tends to push the pivot member 15A out of the nut member 12 due to its torsional deformation. The external thread 14 on the outer periphery of the externally threaded member 15B and the internal thread 13 on the inner periphery of the nut member 12 are vertically symmetrical trapezoidal threads.

Claims

1. A lash adjuster comprising a nut member (12) having an internal thread (13) on its inner periphery, an adjuster screw (15) having an external thread (14) on its outer periphery which is in threaded engagement with the internal thread (13), and a return spring (17) biasing the adjusting screw (15) in a direction to protrude from the nut member (12), wherein an axial load that tends to push the adjusting screw (15) into the nut member (12) is received by pressure flanks (18) of the external thread (14) and the internal thread (13),

characterized in that the adjusting screw (15) has a stopper portion (23) at its portion protruding from the nut member (12), said stopper portion (23) having a diameter larger than the inner diameter of the nut member (12), wherein the nut member (12) has an open end configured to support the stopper portion (23), thereby restricting movement of the adjusting screw (15) when the adjusting screw (15) is pushed into the nut member (12).

2. The lash adjuster of claim 1 wherein the stopper portion (23) has a tapered surface (24) on its outer periphery which is configured to abut a tapered surface (25) formed on the open end of the nut member (12).

3. The lash adjuster of claim 1 wherein the stopper portion (23) has a flat surface (26) on its outer periphery which extends perpendicular to an axial direction and configured to abut a flat surface (27) formed on the open end of the nut member (12) so as to extend perpendicular to the axial direction.

4. The lash adjuster of claim 1 wherein the stopper portion (23) comprises a snap ring (28) fitted on the outer periphery of the portion of the adjusting screw (15) protruding from the nut member (12).

5. The lash adjuster of claim 1 wherein the return spring (17) is a compression spring that applies an axial force to the adjusting screw (15) that tends to push the adjusting screw (15) out of the nut member (12), and wherein the external thread (14) and the internal thread (13) have a serration-shaped section with their pressure flanks (18) having a larger flank angle than their clearance flanks (19).

6. The lash adjuster of claim 1 wherein the return spring (17) is a torsion spring that applies torque to the adjusting screw (15) that tends to push the adjusting screw (15) out of the nut member (12), and wherein the external thread (14) and the internal thread (13) are serration-shaped threads, trapezoidal threads or triangular threads.

7. The lash adjuster of claim 6 wherein the torsion spring is one of a torsion coil spring, a spiral spring and a volute spring.

8. The lash adjuster of claim 1 wherein the nut member (12) is inserted in a receiving hole (11) formed in a top surface of a cylinder head (2), and wherein the adjusting screw (15) pivotally supports an arm (7) of a valve gear at its end protruding from the nut member (12).

9. The lash adjuster of claim 1 wherein the nut member (41) is fixed to a lifter body (40) vertically slidably inserted in a guide hole (39) formed in a cylinder head (32), and wherein the adjusting screw (44) presses a valve stem (35) of a valve gear at its end protruding from the nut member (41).

10. The lash adjuster of claim 1 wherein the nut member (73) is inserted in a receiving hole (76) formed in a bottom surface of an arm (67) that pivots as a cam (71) rotates, and wherein the adjusting screw (74) presses a valve stem (65) of a valve gear at its end protruding from the nut member (73).

11. The lash adjuster of claim 1 wherein the adjusting screw (15) comprises a pivot member (15A) axially slidably inserted in the nut member (12), an externally threaded member (15B) supporting an end of the pivot member (15A) located in the nut member (12) and having said external thread (14) on its outer periphery, and an elastic member (15C) disposed between the pivot member (15A) and the externally threaded member (15B).

12. The lash adjuster of claim 11 wherein the pivot member (15A) has a square hole (96) formed in its end located in the nut member (12), and wherein the externally threaded member (15B) has a square shaft (97) engaged in said square hole (96), whereby the pivot member (15A) and the externally threaded member (15B) are rotatable together due to the engagement of the square shaft (97) in the square hole (96).