GB2033472A - Automatically adjusting valve clearance - Google Patents

Abstract

An automatic tappet adjuster for a valve operating mechanism has two components 12, 14 with co-operating buttress thread form screw threads 32. The axial free play in the threads 32 sets the valve clearance. Excess clearance 56 is taken up by rotational movement of one component, 14 acted on by a spring 40, relative to the other component 12. When component 14 is loaded by the cam 24 during the valve opening movement, component 14 first moves to take up the clearance and then wedges in component 12, due to the flank angle of the threads so that the cam thrust is transmitted to the valve. When excessive clearance exists, spring 40 acts downwards on component 14 while it is not loaded by the cam and it rotates to reduce the clearance. An embodiment is described in which the adjuster is located within a bucket-type tappet of an overhead camshaft engine. <IMAGE>

Description

SPECIFICATION Valve clearance adjuster This invention relates to an automatic valve clearance adjuster for a valve operating mechanism.

It is well known to provide a mechanical valve clearance adjuster for an internal combustion engine valve gear with a screw thread which must be manually adjusted at regular intervals to give the correct clearance in the valve mechanism. It is also well known to use a hydraulic tappet in a valve mechanism to provide self adjustment by means of pressurised oil located between two parts which move relative to each other, changes in the quantity of pressurised oil compensating for any wear. In a hydraulic tappet, movement between the two parts of the tappet control a hydraulic connection to the interior of the tappet and thereby controls the volume of trapped oil and hence the valve clearance.

It is also known from UK patent specification No. 510864 to provide a hydraulic tappet which can be modified to use a coarse pitch screw thread for controlling the hydraulic connection to and the volume of oil in the interior of the tappet. The thread has clearance representing the desired clearance in the valve operating mechanism. It also has a steep flank on one side of the threadform to produce low friction and allow adjustment by relative rotation of the threaded parts in response to an axial spring load when the valve operating load is removed. The opposite flanks are steeply angled to provide a wide flat surface to accomodate the ends of oil passages which can be closed off at the screw thread by taking up the thread clearance in the direction of valve operation.As the oil passage become closed, a hydraulic lock develops within the tappet and the pressure of the oil in the closed interior of the tappet transmits the valve operating load between the two parts of the tappet.

An object of the invention is to provide a simple mechanical (as opposed to hydraulic) system for adjusting clearance in a valve operating mechanism for an internal combustion engine.

The invention provides a valve operating mechanism for a valve of an internal combustion engine, the mechanism including an automatic clearance adjuster between two components of the mechanism, the components having co-operating serew threads of a kind which exhibit higher friction in one direction of loading than in the opposite direction and which exhibit a pre-determined axial free play, the components being spring loaded with respect to each other in a sense opposite to the transmission of valve operating forces between the components, the arrangement being such that valve operating forces are transmitted between the screw threads in the higher friction direction to prevent relative rotation between the members and such that when no valve operating force is being transmitted the spring loading urges the threads into engagement in the low friction direction, the spring loading causes relative rotation of the components so that they take up rotational positions such that the clearance in the mechanism is equal to the axial play in the screw thread.

A fundamental difference between the present invention and the above mentioned prior art is the high friction developed between the screw threads of the present invention during valve actuation to ensure that correct adjustment is maintained and that positive valve operation is achieved. This contrasts with the prior art where both the large volume of oil supplied direct to the screw thread and the oil pressure within the tappet prevents firm seating between the two parts of the thread.

Preferably the axial spring loading acts on one of the components through a further screw thread to tend to restrict rotation of that component with respect to the other component in a direction opposed to the relative rotation of the components caused by the spring.

Preferably the co-operating screw threads have buttress thread forms. That is each thread form has a sloping ramp face and a steeper face (which may be perpendicular to the axis of the thread).

Preferably when the buttress thread forms are rotated in one direction relative to each other high friction is produced by the ramps of the buttress thread forms contacting each other to give a wedging action, and in the other direction low friction is produced by the steeper faces of the buttress thread forms coming into contact with each other.

Valve clearance adjustment is generally more difficult to achieve with an overhead camshaft layout than with a push rod layout due to the lack of space available with an overhead camshaft arrangement. This makes an automatic clearance adjuster particularly desirable but also causes problems in the design of a suitably compact automatic clearance adjuster.

According to a further feature of the present invention there is provided a valve operating mechanism for an overhead camshaft operated valve of an internal combustion engine, the mechanism including a bucket-type tappet and an automatic clearance adjuster between an adjuster sleeve bearing against the tappet and the stem of the valve, the stem and sleeve having the co-operating screw threads, the sleeve being spring loaded with respect to the stem in a sense opposite to the transmission of valve operating forces between the sleeve and stem.

Preferably access holes are provided in the edge of the tappet to permit manual rotation of the sleeve for setting up the mechanism.

Preferably the spring acts on the sleeve through a ball member in order to prevent the spring from affecting relative rotation between the sleeve and stem.

Preferably the engagement between the cam and the tappet is such as to tend to rotate the tappet and the adjuster sleeve in a direction to tend to increase valve clearance.

This rotational tendency should preferably be provided only when the cam is in a position corresponding to a closed valve and may be provided by an off-set of the cam surface which engages the tappet. This off-set may be provided by a chamfer to remove part of the cam surface.

It is desirable to provide for a main valve stem to extend as far as possible into the tappet in the interests of reducing the overall length of the valve mechanism to a minimum and thereby keeping the height of the engine to a minimum. The existence of the adjuster mechanism within the bucket interferes with the normal extension of the spring to a reaction point well within the tappet. In accordance with a still further feature of the present invention a valve spring reaction sleeve is secured to the valve stem at a position nearer to the valve head than the adjuster sleeve, extends around the sleeve into the tappet and has an external flange within the bucket to provide a reaction point for the main valve spring.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a cross sectional elevational view of an apparatus according to the invention Figures 2 to 4 are schematic representations of the positional relationship of the thread forms of the two components to each other.

Figure 5 is a diagrammatic cross-section through a further valve mechanism in accordance with the present invention; and Figure 6 is a view in the direction of arrow II of Fig. 1 showing part of the mechanism.

Fig. 1 shows a valve operating mechanism 10 which comprises two components 1 2 and 14 in screw threaded engagement with each other at 1 6. The component 14 has a ball 1 8 which locates in the socket 20 of a push rod 22. Oscillatory movement of the push rod 22 is provided by the action of a cam 24 positioned on a cam shaft (not shown).

This oscillatory movement of the push rod 22 is transmitted via the screw threaded engagement 1 6 of the component 14 to the component 1 2. The component 1 2 is a rocker arm which is pivoted about an axis 26 and is free to move in one plane only in a direction parallel to the axis of the push rod 22 about its own axis 26. The abutment 28 of the component 1 2 abuts the valve stem (not shown) of the valve of an internal combustion engine valve. The valve has a conventional valve spring (not shown).

The component 14 can conveniently be described in three separate parts. One part 1 8 abuts the socket 20 of the push rod 22 as previously described. The part next to it 30 is a threaded part which engages at 1 6 with the component 1 2. The thread 32 of the part 30 is of buttress thread form and its action will be described subsequently.

The other part 34 of the component 14 is also screw threaded with a fine, but preferably standard thread form. Component 34 is located in a body 36 with an internal screw thread 38. A spring member 40 is secured to the body 36, preferably by welding. The spring member 40 acts between the body 36 and the component 1 2 to which it is secured at 42 by fastening means 44.

The adjusting mechanism is used to automatically adjust the valve gear mechanism of an internal combustion engine to take up any excess clearance. The mode of operation will now be described with reference to Figs. 2-4.

These show a portion of the buttress thread form of both the component 1 2 and the component 14. For convenience component 1 2 will be referred to as the nut and component 14 as the screw.

When the cam is in the rotational position shown in Fig. 2 there is no valve operating load on the screw 30. The spring means 40 therefore ensures that the faces 46 of the screw 30 and 48 of the nut 1 2 are in contact.

Between the face 50 of the screw 30 and the face 52 of the nut 1 2 there is a clearance in an axial direction which is the required clearance in the valve mechanism. To illustrate that there is no other clearance, the valve mechanism is shown in contact with the cam 24.

When the cam rotates it applies a load via the push rod 22 to the screw 14, which load takes effect at the junction 1 6 of the components. The screw moves parallel to its axis, in this case vertically upwards, giving a clearance 54 between the faces 46 and 48 as shown in Fig. 2. The faces 50 and 52 come into contact and they are wedged securely due to the particular shape of the buttress thread form. Rotational movement of the two components relative to each other is prevented by this wedging action of the buttress thread form. Consequently load can be transmitted from the push rod 22 via the components 1 2 and 14 to the abutment 28 and thence to the valve of the internal combustion engine.

While the screw is being moved upwards to give the clearance 54 between the faces 46 and 48 there is an interim period when the two thread forms are not in contact. If, for instance, the cam 25 is acting eccentrically on the push rod 22 this may give rise to torque which would tend to rotate the screw upwards thus increasing the clearance at the valve.

This rotation is constrained by the action of the spring means 40 on the thread 38 in the body 36.

Fig. 4 shows a notional position when wear in the mechanism has occurred but no adjust ment has taken place. This wear may, for example, take place at the interface 56 of the mechanism and the cam 25 and is ilustrated by a gap at this interface in Fig. 4. In this situation the total clearance in the valve mechanism is the desired clearance at the junction 1 6 plus the additional clearance at interface 56.

In this situation the force of spring means 40 is acting in a downward direction on component 14 holding it in firm contact through the low friction faces of the screw threads 32. This friction is sufficiently low in conjunction with the coarseness of the thread 32 to cause the component 1 4 to rotate and move in a downward direction under the influence of the spring force. This movement continues until the whole of the gap at the interface 56 has been taken up and in that situation the configuration of the valve mechanism corresponds to that shown in Fig. 2.

Thereafter the valve mechanism operates as described with reference to Figs. 2 and 3 until such time as the clearance again increases as a result of further wear. In practice the adjustment takes place gradually as wear occurs with the result that no substantial excess clearance as shown at 56 ever occurs. In this way the valve mechanism is self adjusting and compensates for wear.

During the adjusting operation it is of course necessary for the component 14 to be able to rotate and this requires a relatively low friction in threads 38. In contrast to this, it is desirable to have some friction at threads 38 to prevent inadvertant rotation of component 14 during normal operation of the mechanism as the clearance is being taken up. In practice a compromise between high and low frictin is required at thread 38 and this compromise can be met by a conventional thread form and a fine pitch thread.

Figs. 5 and 6 show an embodiment of the invention applied to the valve gear of an overhead camshaft internal combustion engine.

A valve 61 has a head 62 and a stem 63 and is guided in a cylinder head casting 64 in the usual way. The cylinder head carries a tappet guide 65 within which a bucket-type tappet 66 is slideable. A cam 67 carried on an overhead camshaft 68 is arranged in the usual way to operate the tappet 66 and thereby operate the valve 61. A main valve spring 69 serves the usual purpose of returning the valve to a closed condition when rotation of the cam 67 allows this closure.

Further details of the reaction points of the valve spring 69 will be discussed subse quently.

As thus far described the mechanism is conventional and the invention is concerned with an adjuster mechanism between the valve stem 63 and the tappet 66 to provide automatically a limited clearance in the valve mechanism.

An internally screw threaded adjuster sleeve 71 co-operates with a screw thread 72 on the exterior of the valve stem 63 near the top of the valve stem. These screw threads correspond to the threads described in detail with reference to Figs. 2, 3, and 4 and in particu lar they incorporate an axial clearance, higher friction in one direction of relative rotation and low friction in the opposite direction of relative rotation.

The upper end of the valve stem 63 incorporates a bore 73 within which an adjuster spring 74 is located. The adjuster spring acts in compression between the base of the bore 73 and a ball 75 which reacts on an end closure 76 of the sleeve 71. The spring thus tends to urge the sleeve 71 downward in relation to the stem 63 to urge the screw threads into mutual contact in the low friction direction and to take up the clearance in the screw threads.

The end closure 76 of sleeve 71 bears against the tappet 66 and incorporates extensions 77 to which access is available through access holes 78 in the tappet to enable the sleeve 71 to be rotated manually when setting up the valve mechanism.

A main valve spring reaction sleeve 79 surrounds the adjuster sleeve 71 and is secured at its lower end to the valve stem 63 by conventional collets 81. Sleeve 79 extends up within the tappet 66 and at its upper end incorporates an outwardly extending valve spring reaction flange 82. The main valve spring 69 operates between the flange 82 and a seat on the cylinder head. In this way, the normal length of the valve spring 69 is substantially maintained without adding to the height of the valve mechanism as a whole.

As best seen in Fig. 6, the face of the cam is chamfered at 83 so that if the tappet 73 is in contact with the cam 67 with the cam in the rotational position shown, the cam bears on the tappet at a position off-set from its centre. Due to this, rotation of the cam tends to induce some rotation of the tappet.

The operation of the adjuster mechanism in taking up excess clearance is substantially as described in relation to Figs. 1 and 4 and will only be explained briefly. Initially, the mechanism is set up with an excess clearance and with the cam in the position shown, i.e. with the valve seated. Spring 74 moves the adjuster sleeve 71 in an upward direction, the sleeve rotating relative to the valve stem by the effect of the low friction of the screw thread to permit this movement. This movement occurs until the tappet 66 comes into contact with the cam 67 so that the only clearance in the mechanism is the clearance within the screw threada between the stem 63 and sleeve 71.On normal operation of the valve mechanism, the threads are loaded in the high friction direction so that axial movement can be transmitted from the tappet through the screw thread to the valve to lift the valve in the usual way. If excess clearance tends to develop, this is automatically taken up by the adjuster mechanism by relative rotation between the sleeve 71 and valve stem 63.

The mechanism shown in Figs. 5 and 6 is also capable of providing an increased clearance if the clearance of the valve mechanism should reduce below a minimum requirement.

This effect is achieved by the provision of chamfer 83 which tends to cause the cam 67 to rotate the tappet 73 and with it the adjuster sleeve 71 in a direction to increase the clearance in the mechanism. This rotational tendency occurs at a time when the valve is fully seated and the force of engagement between the tappet 73 and cam 67 is merely that of the adjuster spring 74. This slight tendency to rotation during each revolution of the cam produces a sufficient bias towards an increase in clearance to prevent the clearance from becoming too small. The clearance cannot become excessively large because when the clearance becomes equal to the clearance between the screw threads, there is no further contact between the cam 67 and tappet 73 as the chamfer 83 rotates past the tappet.

Claims (10)

1. A valve operating mechanism for a valve of an internal combustion engine, the mechanism including an automatic clearance adjuster between two components of the mechanism, the components having co-operating screw threads of a kind which exhibit higher friction in one direction of loading than in the opposite direction and which exhibit a predetermined axial free play, the components being spring loaded with respect to each other in a sense opposite to the transmission of valve operating forces between the components, the arrangement being such that valve operating forces are transmitted between the screw threads in the higher friction direction to prevent relative rotation between the members and such that when no valve operating force is being transmitted the spring loading urges the threads into engagement in the low friction direction, the spring loading causes relative rotation of the components so that they take up rotational positions such that the clearance in the mechanism is equal to the axial play in the screw thread.

2. A mechanism according to Claim 1 in which the axial spring loading acts on one of the components through a further screw thread to tend to restrict rotation of that component with respect to the other component in a direction opposed to the relative rotation of the components caused by the spring.

3. A mechanism according to Claim 1 or Claim 2 in which the co-operating screw threads have buttress thread forms.

4. A mechanism according to Claim 3 in which when the buttress thread forms are rotated in one direction relative to each other high friction is produced by the ramps of the buttress thread forms contacting each other to give a wedging action, and in the other direction low friction is produced by the steeper faces of the buttress thread forms coming into contact with each other.

5. A valve operating mechanism according to Claim 1 for an overhead camshaft operated valve of an internal combustion engine, the mechanism including a bucket-type tappet and an automatic clearance adjuster between an adjuster sleeve bearing against the tappet and the stem of the valve, the stem and sleeve having co-operating screw threads the sleeve being spring loaded with respect to the stem in a sense opposite to the transmission of valve operating forces between the sleeve and stem.

6. A mechanism according to Claim 5 wherein access holes are provided in the edge of the tappet to permit manual rotation of the sleeve for setting up the mechanism.

7. A mechanism according to Claim 5 or Claim 6 wherein the spring acts on the sleeve through a ball member in order to prevent the spring from affecting relative rotation between the sleeve and stem.

8. A mechanism according to any one of Claims 5 to 7 wherein the engagement between the cam and the tappet is such as to tend to rotate the tappet and the adjuster sleeve in a direction to tend to increase valve clearance.

9. A mechanism according to Claim 8 wherein the rotational tendency provided only when the cam is in a position corresponding to a closed valve by an off-set of the cam surface which engages the tappet.

10. A mechanism according to anyone of Claims 5 to 9 wherein a valve spring reaction sleeve is secured to the valve stem at a position nearer to the valve head than the adjuster sleeve, extends around the sleeve into the tappet and had an external flange within the bucket to provide a reaction point for the main valve spring.