GB2042668A

GB2042668A - Tappet Slack Adjusters

Info

Publication number: GB2042668A
Application number: GB8004247A
Authority: GB
Original assignee: RIV SKF Officine di Villar Perosa SpA; RIV Officine di Villar Perosa SpA
Current assignee: SKF Industrie SpA
Priority date: 1979-02-15
Filing date: 1980-02-08
Publication date: 1980-09-24
Also published as: IT7967325A0; FR2449196A1; DE3005583A1; NL8000932A; JPS55131513A; IT1118357B; SE8001163L

Abstract

A mechanical tappet with automatic take-up of slack comprises a first member (1) arranged to be kept in contact with the stem (10) of a valve (11), and a second member (2) mobile axially relative to the first and arranged to cooperate with the operating cam (17) of said valve, between said two members (1, 2) there being disposed a resilient element (5) arranged to move the first member axially relative to the second in order to constantly keep the first (1) in contact with said stem (10) and the second (2) in contact with said cam (17). The element (5) may be a spiral spring and the members (1, 2) may be held together by a screw thread of trapezoidal cross section. <IMAGE>

Description

SPECIFICATION Mechanical Tappet with Automatic Take-up Slack This invention relates to a mechanical tappet with automatic take-up of slack.

Those members which control the opening and closure of the valves in internal combustion engines, i.e. the cam and tappet, are subject to wear due to the fact that they slide against each other under pressure, and in particular one element of the tappet, namely the cap, which is of a material conveniently more wearable than the cam, becomes worn during operation to give rise to slack which prevents the complete opening of the valve.

An adjustment has therefore periodically to be made using spacers, and possibly the cap has to be replaced.

However there are mechanisms which continuously and automatically adjust the slack as soon as it arises. These are situated on the tappet and are substantially of hydraulic type. In this respect, a cylindrical chamber is provided in this member and is closed lowerly by a piston resting on the valve stem. A bore, provided in it and closed by a lamina valve supported by a cup spring resting on the top of the chamber, connects the interior of this latter to a pressurised oil conduit. When the cam terminates its working arc and no longer presses against the tappet cap, the pressurised oil penetrates into said chamber if there is any slack due to wear, and axially displaces the piston relative to the tappet until this slack is nullified.

When the cam, during its working arc, next cooperates with the tappet by way of the cap, the lamina valve closes under the pressure of the oil inside the chamber and prevents it from flowing out. The piston-tappet assembly therefore behaves as a single axially rigid element able to support the working force.

The oil pressure must obviously be controlled at a suitable value which opens the lamina valve during the described slack take-up stage, but which enables it to close as soon as the cam begins its working arc. If this were not so, then if the pressure were too low the slack would not be taken up, whereas if the pressure were too high the chamber would fill excessively, to give abnormal opening and imperfect closing of the valve.

Whereas the first case leads only to a lack of slack take-up and thus an insufficient aperture of the valve leading to contact collision between the cap and cam, the second case is particularly dangerous because if the valve opens excessively there could be contact between the valve and the piston head, so fracturing it. In either case, the engine operation is incorrect.

Even known improvements of this hydraulic mechanism do not obviate the aforesaid drawbacks, as they relate only to a different method of shutting off the oil, for example by means of a ball valve with a cylindrical spiral return spring, and to improving pressing action of the oil on the ball valve, by making the oil pass through a pre-chamber. It is also apparent that any impurity present in the oil circuit which deposits on the valve, whether this be a lamina or a ball valve, and preventing it from closing, would lead to an escape of oil during the working arc of the cam and thus insufficient valve opening.

The object of the present invention is therefore to take-up the slack between the tappet and cam due to disc wear, without the aforesaid drawbacks.

The present invention therefore provides a mechanical tappet with automatic take-up of slack, comprising at least one first member arranged to be kept in contact with the stem of a valve, and a second member mobile axially relative to the first and arranged to co-operate with the operating cam for said valve, between said two members there being disposed a resilient element arranged to move the first member axially relative to the second in order to constantly keep the first in contact with said stem and the second in contact with said cam.

According to the present invention, said first member is connected to the second by means of a thread, the axis of which is parallel to said axial direction, said resilient element being connected to said members to generate a resilient moment able to rotate the first member relative to the second and thus give rise to said axial movement between them, said resilient element being mounted between said members with a predetermined torsional pre-load when the first is in contact with said stem and the second in contact with said cam.

The present invention will be more apparent from the description of one embodiment given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through the tappet of the present invention; Figure 2 is a longitudinal section through an assembly in which the tappet of the present invention is inserted; Figure 3 is a longitudinal section through part of the tappet, showing one operating state; Figure 4 is a longitudinal section through the same part of the tappet, showing a further operating state; Figure 5 is a longitudinal section through a different embodiment of a tappet according to the invention; Figure 6 is a plan view from above of part of a camshaft and a cam for operating the tappet of the invention.

With reference to Figure 1, the mechanical tappet for automatically taking-up slack, according to the present invention, consists of a bucket member 1 which, in its lower cavity, carries an internally hollow cylindrical mobile element 2 movable axially, and connected to it by a non-reversible screw coupling. In the embodiment illustrated, this coupling comprises threads of trapezium cross-section, with upper sides 3 inclined to the horizontal and lower sides 4 less inclined and almost horizontal.

Inside the element 2 there is a cylindrical spiral spring 5, the ends of which are bent substantially at a right angle, the upper one being fixed into a cavity 6 in the element 2, and the lower one fixed into a further cavity 7 in the bucket member 1. A bore 8 is conveniently provided transversely to the element 2 in an upper side portion thereof, in order to cause it to move axially by rotating it.

With reference to Figure 2, the stem 10 of a valve 11 rests on an axial projection 9 in the lower part of the bucket member 1, this valve being normally closed by the action of springs 1 2 co-operating with a collar 1 3 fixed to the stem 10. A vertical cylindrical seat 14 provided in a casing part 15 guides the bucket member 1, which can slide axially therein by way of its outer cylindrical surface 1 6.

A cam 1 7 co-operates with the element 2, possibly by pressing on a block (not shown in Figure 2) slightly embedded in it.

In the embodiment shown in Figure 5, between the cylindrical mobile element 2 and the member 1 there is disposed a spring 20 which is different from that of Figure 1, and is in fact a volute spring in the form of a metal strip wound as a spiral to form a number of turns, each of which is displaced axially to the adjacent one. The radially outer end of this spring is fixed to the member 1 by a pin 21, and the radially inner end comprises a lug 22 which is inserted into a corresponding slot in a cylindrical projection 23 on the mobile element 2. In this manner, the spring 20, when suitably pre-loaded, is able to transmit both a moment and an axial force to the mobile element 2.

The assembly and operation of the mechanical tappet with automatic take-up of slack, as heretofore described, are as follows: the spring 5 is fixed into the seats 6 and 7 respectively of the element 2 and bucket member 1. The element 2 is screwed into the bucket member 1, using a tool inserted into the bore 8, such that the spring 5 becomes subjected to a pre-load which is substantially torsional, but also obviously provides an axial force. In this manner, the spring 5 tends to rotate the element 2 relative to the member 1 with a predetermined moment. The spring 20 of the embodiment shown in Figure 5 can be assembled in a completely analogous manner.

The bucket member 1 is now inserted by way of its outer cylindrical surface 1 6 into the seat 14 in the casing 15, and the cam 1 7 is mounted so that a part of its rest circle faces the element 2, which, when released, rotates by the action of the spring 5 and slides axially until it rests against the cam. The valve 11 is correspondingly closed. As the screw coupling is non-reversible, any axial force, such as the force exerted by the cam 1 7 during its working arc, will not rotate the element 2 relative to the member 1, and it therefore remains in its initial position. If on termination of the working arc there is no contact between the element 2 and the base circle of the cam 17, the spring 5 rotates the element until it makes contact with it.

Consequently, if slack occurs between the element 2 (or possibly its cap) and the cam 1 7 due to wear, take-up is immediate and automatic, and occurs at the end of the cam working arc. At that moment the spring 5 acts both torsionally and axially, and thus the upper sides 3 of the trapezium cross-section of the threads are in contact, as shown in Figure 3. The lower sides 4 are however separated, and this allows free expansion of the stem 10 of the valve 1 for example due to heat.

During its working arc, the cam 1 7 presses on the element 2, which compresses the spring 5 axially. As the spring shortens, it brings the lower sides 4 of the trapezium cross-section of the threads into contact, as shown in Figure 4, thus transmitting motion to the bucket member 1. It is apparent that if the spring 5 is very rigid, it can shorten only slightly, and thus not bring the lower sides 4 of the trapezium cross-section of the threads into contact, so leaving both the sides 3 and both the sides 4 separated from each other.

According to the invention, suitable means are also disposed for preventing any sticking between the tappet and relative operating cam 1 7 due to length variations in the chain of coupled elements of the tappet produced by temperature changes.

In this respect, if the engine cools, length reductions occur, in particular in the stem 10 (Figure 2) of said chain. Thus, because of this reduction, a certain slack would tend to be generated between the mobile member 2 and cam 1 7, and the tappet of the invention would tend to eliminate this slack by the described operation. Thus when the engine reaches its operating temperature, the chain of coupled elements elongates, giving rise to jamming between the mobile element 2 and cam 1 7.

Said means shown in Figure 6 comprise simply a cam 1 7 mounted on the relative cam shaft 1 7a in such a manner that its central plane, indicated by m in Figure 6, is at a predetermined eccentricity e relative to the diametrical plane perpendicular to the cam axis (indicated by d in Figure 6). By means of this constructional arrangement, the resultant R of the forces exchanged between the cam 17 and mobile member 2 is at a distance e from the tappet axis, and thus a tangential friction force Rf (f being the coefficient of friction) is transmitted to the tappet by the cam, to generate a moment Rfe on the mobile element 2, which tends to rotate it. Said moment is therefore able to rotate the element to a sufficient extent to eliminate said sticking. In this respect, as soon as the element 2 is made to rotate sufficiently to eliminate the sticking, the force R exchange between the cam 1 7 and element 2 diminishes, and the moment Rfe generated by it is no longer able to further rotate the mobile element 2.

The mechanical tappet with automatic take-up of slack, according to the invention, therefore possesses high reliability due mainly to its mechanical structure, and moreover, as the spring 5 (20) is not subjected to high force or fatigue stress, the tappet has good lasting characteristics which make it suitable for continuous use in internal combustion engines. Furthermore, if it should fracture, no trouble results other than a lack of slack take-up. Any sticking between the cam and tappet is also prevented by the presence of the described means for preventing sticking occurring.

Finally, modifications can be made to the described embodiment of the mechanical tappet with automatic take-up of slack, according to the present invention, without leaving the scope of the inventive idea.

In particular, any resilient element other than the spring 5 can be inserted between the element 2 and member 1, provided it is able to generate a resilient moment therebetween. In particular, a spiral spring or one or more springs of any form and structure can be used.

Claims

1. A mechanical tappet with automatic take-up of slack, comprising at least one first member arranged to be kept in contact with the stem of a valve, and a second member mobile axially relative to the first and arranged to co-operate with the operating cam of said valve, between said two members there being disposed a resilient element arranged to move the first member axially relative to the second in order to constantly keep the first in contact with said stem and the second in contact with said cam.

2. A tappet as claimed in claim 1, wherein said first member is connected to the second by means of a thread, the axis of which is parallel to said axial direction, said resilient element being connected to said members to generate a resilient moment able to rotate the second member relative to the first and thus give rise to said axial movement between them, said resilient element being mounted between said members with a predetermined torsional pre-load when the first is in contact with said stem and the second in contact with said cam.

3. A tappet as claimed in claims 1 or 2, wherein said first member is a cylindrical member provided with two cavities, namely a lower cavity arranged to contain the end of said stem, and an upper cavity arranged to house at least part of said second member, said second member being bounded by an outer cylindrical surface provided with a thread arranged to engage with a corresponding thread provided on the inner lateral surface of said upper cavity.

4. A tappet as claimed in one of the preceding claims, wherein said resilient element is constituted by a spiral spring, one of its ends being fixed to said first member and its other end to said second member.

5. A tappet as claimed in claim 4, wherein said spring are substantially bent and are inserted into corresponding bores in said members.

6. A tappet as claimed in one of claims 2 to 5, wherein said thread is a thread of trapezium cross-section.

7. A tappet as claimed in claim 6, wherein the profile of said thread comprises two sides of different inclination to the thread axis.

8. A tappet as claimed in one of claims 3 to 7, wherein axial slack is provided between the sides of said thread of the first member and said thread of the second member.

9. A tappet as claimed in one of claims 2 to 8, wherein comprising means for eiiminating sticking which could occur between said second member and said cam as a result of dimensional variations in said members and said cam produced by temperature changes.

10. A tappet as claimed in claim 9, wherein said means are provided by disposing said cam relative to said tappet such that the central plane through the cam is positioned at a predetermined distance e from the diametrical plane through said tappet perpendicular to the axis of rotation of the cam, such that the cam transmits to said second member a force which is eccentric to the axis of the member, such that when any sticking occurs between the cam and tappet, said force generates a friction moment on said second member which is sufficient to make it rotate, so eliminating the sticking.

11. A mechanical tappet with automatic takeup of slack, substantially as described and illustrated in the accompanying drawing.