Switchable cam follower

Details Switchable cam follower Switchable cam follower

1999-12-14

2001-03-06

Lo, Weilun (Department: 3748)

Internal-combustion engines

Poppet valve operating mechanism

With means for varying timing

C123S090500, C123S090550, C123S19800E

Reexamination Certificate

active

06196176

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a switchable cam follower for a valve train of an internal combustion engine, which valve train can be actuated indirectly by tappet push rods, said cam follower having the following features:
the cam follower can be installed in driving relationship between a camshaft and an end of the tappet push rod, said cam follower comprising an outer and an inner section as well as coupling means,
the outer section can be inserted with its outer peripheral surface into a reception of the internal combustion engine, the inner section being mounted in a recess of the outer section while being movable relative to the outer section,
one of said sections has a support for the end of the tappet push rod and the other of said sections has an axially opposite contacting surface for a cam of the camshaft,
said sections can be coupled to each other by the coupling means so that on coupling, a high lift of a valve train gas exchange valve loadable by the tappet push rod can be effected and, on uncoupling of the sections by the coupling means, a low lift or a zero lift of the gas exchange valve is obtainable.
BACKGROUND OF THE INVENTION
A cam follower of the pre-cited type is known from U.S. Pat. No. 5,361,733. A drawback of this cam follower is that no means are provided for excluding switching errors of its coupling means. Since, as a rule, such a cam follower is switched in a base circle phase of the cam in which its two sections bear uniformly against the base circle of the cam and the receptions for the coupling means are aligned to each other during this base circle phase, it is possible, for example at high speeds of rotation or due to other influences such as pressure medium fluctuations and the like, that, when leaving the base circle of the cam, i. e. at the beginning of the run-on flank, the coupling means have not yet reached their coupling or uncoupling position. If in such a transition state, the coupling means concerned overlaps an annular surface between the sections only slightly, the cam lift can be interrupted resulting in a considerable loading of the components and noise generation. With a too gradual servo medium pressure build-up in front of the coupling means for the desired coupling, especially at high rotational speeds, it is likewise possible that the coupling means do not reach their full coupling position.
OBJECT OF THE INVENTION
It is therefore an object of the invention to create a switchable cam follower of the pre-cited type in which the aforesaid drawbacks are eliminated and, particularly, switching errors of the coupling means are avoided by relatively simple constructional measures.
SUMMARY OF THE INVENTION
The invention achieves these objects by the fact that
locking means are associated to the coupling means, and
the locking means are configured so as to permit a displacement of the coupling means during a contact of a, as seen in rotating direction, first portion of a base circle of the cam and to block a displacement of the coupling means during a contact of a last portion or an end of a base circle of the cam and during contact of an adjoining first portion of a run-on flank of the cam.
Due to the locking means of the invention, the at least one coupling means which is made preferably as a piston is fixed in one of its end positions (uncoupled state—coupled state) till a sufficiently large or small servo medium pressure for its displacement or return displacement is formed. The initially described switching errors caused by insufficient servo medium pressure in front of the at least one coupling means are thus effectively eliminated. It is further pointed out that even undesired servo medium pressure fluctuations during the rotation of the cam can no longer cause an undesired displacement of the coupling means.
An important aspect is that the coupling means are fixed directly at the end of the base circle in one of their end positions. Advantageously, the fixed state continues to exist during a contact of a first portion of a run-on flank of the cam.
It is possible to use differently configured locking means and also such that function independently of a track on the cam. Advantageously, however, as proposed by the invention, the locking means is made as a slide which, at one end scans a signal track on the loading cam and cooperates at the other end with the coupling means. Depending on the configuration of the locking means, the signal track can extend groove-shaped or as an elevation on the outer peripheral surface of the loading cam. Advantageously, the blocking of the coupling means by the locking means is realized already shortly before the end of the base circle of the cam. This guarantees with a high degree of certainty that, with starting cam lift, the coupling means has taken a defined end position.
If the end position of the coupling means has not been fully reached, the proposed wedge shape of the catch recesses causes a forced displacement of the coupling means during the inward movement of the locking means.
In the extremely rare case that the other end of the locking means is not situated opposite a catch recess of the coupling means, according to the invention, the locking means possesses a certain elasticity. This prevents wear and destruction of the components concerned.
In place of the aforesaid pin-like slide, it is also possible to use a slide which, in the region of the coupling means, surrounds this in the manner of a ring. This also enables a locking position of the locking means in which this is displaced towards the cam and extends, for example, in a groove on the outer peripheral surface of the cam. In contrast, in the case of the locking means with a pin-like configuration, the locking position is realized with the help of an elevation of the signal track on the outer peripheral surface of the cam.
Although the locking means of the invention are intended particularly for cooperating with coupling means configured as slides or pistons, the use of coupling means with other configurations is also conceivable. Possible alternatives are wedges, balls, snap projections and the like.
According to a further proposition of the invention, for uncoupling, the inner section is pivoted away from the outer section. However, the invention is equally applicable to configurations in which the two sections are axially displaceable relative to each other as described in the generic prior art document.
As a returning means for displacing the inner section from its pivoted position into its unpivoted position, the invention proposes a torsion spring. This can be configured at the same time as an axle for the mounting of the inner section in the recess of the outer section.


REFERENCES:
patent: 5203289 (1993-04-01), Hara et al.
patent: 5558052 (1996-09-01), Schwarzenthal et al.
patent: 5934232 (1999-08-01), Greene et al.
patent: 5950583 (1999-09-01), Kraxner et al.

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