Note: Descriptions are shown in the official language in which they were submitted.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
Title: Automatic Valve Clearance Adjuster
Description of Invention
This invention relates to an automatic valve clearance adjuster for a
valve operating mechanism as may be used, for example, in an internal
combustion engine; the adjuster comprising an internally screw threaded
housing, an externally threaded screw extending within the housing, and force
exerting means acting axially on the screw to cause it to rotate and advance
outwardly of an open end of the housing to lengthen the adjuster; the co-
operating thread forms of the screw and the housing being so configured that
the screw will rotate freely and advance out of the housing under the bias of
the
said axially directed force.
When such a valve clearance adjuster is placed in a space between rivo
relatively-movable components of a valve operating mechanism, with the
housing engaging one component and the screw engaging the other component,
the adjuster operates to take up clearances in the valve operating mechanism
by
extending, by advancement of the screw member out of the housing, until it
fully occupies the space between the components. By way of example, the
adjuster may be interposed directly or indirectly between a cam and a valve or
a
valve operating member. In another awangement the adjuster may provide a
fulcrum for a lever type of valve-operating member, the position of the
fulcrum
being varied by the adjuster to take up operating clearances.
The background to the present invention including certain examples of
prior art, and aspects of the present invention itself, will now be described
by
way of example with reference to the accompanying drawings, of which:-
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
2
Figures 1, 2 and 3 illustrate possible dispositions of automatic valve
clearance adjusters in valve operating mechanisms for internal combustion
engines;
Figure 4 is a cross-sectional view, on an enlarged scale, of the screw-
threads of an adjuster which may be in accordance with the invention;
Figure 5 shows an automatic valve clearance adjuster in accordance
with the invention, a portion of cam follower and a diagrammatic
representation of the relevant cam position, where the adjuster is required to
take up clearances in the valve tl-ain;
Figure 6 shows the valve clearance adjuster of Figure 5, and an
indication of the relevant cam position, where the adjuster has just
eliminated
clearances in the valve train;
Figure 7 shows the valve clearance adjuster of Figures 5 and 6, a
portion of the cam follower and a diagrammatic indication of the relevant cam
position, where the valve train is just beginning to open a valve;
Figure 8 shows an automatic valve clearance adjuster in accordance
with the prior al-t, the adjuster having just taken up clearance which may
previously have existed at one or more positions such as A, B, C, and D in the
engine valve train;
Figure 9 shows the adjuster of Figure 8, in its condition when the valve
is being opened;
Figure 10 shows an automatic valve clearance adjuster according to a
ful-ther embodiment of the prior al-t;
Figure 11 shows the valve clearance adjuster of Figures 8 and 9, with
the screw displaced laterally wlthm the housing of the adjuster;
Figure 12 shows the adjuster of Figures 8 and 9, with the screw
angularly displaced within the housing;
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
3
Figure 13 is a fragmentary view of contacting flanks of mating screw-
threads having a relatively low flank angle;
Figure 14 is a fragmentary view of contacting flanks and mating screw-
threads having a relatively high flank angle;
Figure 15 is a fragmentary view of mating screw-threads illustrating the
relationship between axial and radial thread clearance;
Figure 16 illustrates a position an adjuster in accordance with the
invention may assume prior to assembly into an engine;
Figure 17 shows an automatic valve clearance adjuster in accordance
with a further aspect of the invention, ready for assembly into an engine.
In all the claims, corresponding reference numbers are used for
coiTesponding parts.
Referring firstly to Figures l, 2 and 3, in each of these figures there is
depicted an automatic valve clearance adjuster comprising a housing 1, an
internal screw thread 2 extending into the housing, and a screw member 3
having an external screw thread 4 co-operating with the internal thread 2 of
the
housing. Reference numeral 5 indicates an axial force applied by spring means
to the screw member at the end 6 thereof which is in the housing: preferably
such force is applied by a compression spring engaging the screw member as
described hereafter. Reference numeral 7 indicates the end of the screw
member which protc-udes outwardly from the housing 1 and which engages a
co-operating pan of the valve operating mechanism in order to t1-ansmit a
valve-
operating force or provide a reaction force, depending on the nature of the
valve operating mechanism, which is necessary when the valve is to be opened
against the force of its closing spring by the operation of a cam on a
camshaft.
In the example shown in Figure l, the adjuster is self contained and is
stationary in a socket in the body of the engine. The protl-uding valve
actuating
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
4
end 7 of the screw member 3 has a domed end with a spherical surface, said
domed end fits inside a cavity 9 in a cam follower 8. The cavity 9 has an
ogive
or conical cross section and therefore makes a narrow circular band of contact
with the domed end 7. Rotation of the cam 10 makes the cam follower 8
oscillate about the fulcrum provided by the domed end 7 and thus actuates the
engine valve via the valve stem, l 1.
In Figure 1 the housing 1 is shown with an open lower end which
would be suitable if the force 5 were provided by oil pressure. For the force
5
to be provided by a spring the housing would have a closed end, as described
hereafter.
Referring to Figure 2, this shows an adjuster incorporated in a "bucket"
type of cam-follower; the housing I is preferably integral with the bucket 13.
The valve actuating end 7 of the screw member 3 is shaped to make with the
valve stem 14 an area of circular, annular or conical contact. In this design
of
valve actuator mechanism the bucket 13 is slidably mounted in a bore in the
body of the engine. The cam 10 operates directly via the bucket 13 and the
adjuster to impart downward valve operating movement to the valve 14.
In the example shown in Figure 3 the adjuster is self contained and acts
as a moveable push-rod beriveen the cam 15 and a rocker-arm 17. The adjuster
is slidably mounted in a bore in the body of the engine. The adjuster
transmits
motion from the cam 15 to the rocker-arm 17 which can pivot about the axis 41
and so impart downward valve operating movement to the valve 18. The
adjuster is fitted with a pressure pad 16 between the rocker-ann 17 and the
valve actuating end 7 of the screw member 3. The pressure pad can slide
axially within the housing 1 but is restrained against rotation therein.
In addition to the designs described above, the adjuster could be
incorporated in other types of valve train. For example the adjuster housing 1
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
could be an integral part of a rocker-arm in a rocker-arm/push-rod mechanism.
The known prior art will now be discussed by reference to Figures 8, 9,
and 10, each of which shows a valve clearance adjuster which mainly
comprises a screw within an internally threaded housing as previously
mentioned. In each case the valve clearance adjuster is shown with a schematic
representation of one type of engine valve train mechanism with which said
adjuster is in working relationship. The reasoning to be put forward applies
to
any other valve h~ain mechanism.
In one category of prior atrt there are several variations on a simple
working principle in which the aforementioned screw is not acted upon by an
axial force but is rotated and advanced out of the housing under the action of
a
torsion spring. This is explained by reference to Figure 10 which shows the
cam 10 of an engine, said cam being shown in an angular position which would
allow the engine valve to close as shown at 39. Figure 10 also shows that the
torsion spring 29 has rotated the screw 30 so that the said screw has advanced
out of the housing 38 to eliminate any clearance which may previously have
existed at one or more positions such as A, B, C and D in the engine valve
ri~ain. Moreover, Figure 10 shows that there is an unbroken path of
compressive
force transmission from the base 31 of the housing 38 through the contacting
screw threads (as shown for example at 33) and then through positions A, B, C
and D to the top of the stem 34 of the engine valve. Therefore, differential
thermal expansions of the various engine pants can produce a situation in
which
the valve is prevented from closing, (as shown for example at 40 in Figure 9)
thus leading to component wear and reduced engine performance. This is a
serious disadvantage in all valve clearance adjusters which function under the
action of a torsion spring.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
6
In another category of prior ant (GB-A-2033472, EP-T-0032284,
GB-2160945, GB- 2211263, WO-A1-89/05898, WO-90/10786, WO-90/10787)
the aforementioned internal and external threads are of buttress formation;
this
is a common feature throughout this category. Such a configuration is depicted
in Figure 8 in which the screw 3 and the housing 1 are shown in axial cross
section. The buttress thread flanks 24H and 24S which are shown with an
inclination GR to a line perpendicular to the axis are tetuned "running
flanks".
The bum-ess tlwead flanks 25H and 25S which are shown with an inclination GL
to a line perpendicular to the axis are termed "locking flanla". The helix
angle
is Z.
Figure 8 depicts the engine cam 10 in an angular position which would
allow the engine valve 11 to close as shown at 39. It also shows that the
compression spring 22 has pushed the running flanks 24S of the screw into
contact with the running flanla 24H of the housing and has advanced the screw
3 out of the housing 1 to eliminate any clearance which may have existed
previously at one or more of positions such as A, B, C, D in the engine valve
tt-ain. The advancing of the screw 3 out of the housing is hereinafter
refetTed to
as "take-up" movement.
Figure 9 depicts a situation in which the engine cam 10 has turned into
an angular position in which it exerts a force which is reacted by the stem 11
of
the valve and by the screw 3 of the clearance adjuster. The reaction on the
valve stem has opened the valve as shown at 40; the reaction on the screw has
forced the locking flanks 25S of the screw through the clearance (26 Figure 8)
and into contact with the locking flanks 25H of the housing. In theory, the
assembly of screw 3 and housing 1 should then behave as a solid in resisting
the said reaction because rotation of and consequent retraction of the screw
into
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
7
the housing should be prevented by the enhanced frictional resistance which
results from the relatively high angle of inclination GL of the locking
flanks.
Summarising, the use of buttress threads should, in theory, ensure that
under the action of axially applied forces there is a high resistance to the
screw
3 being pressed into the housing and a low resistance to the screw being
advanced out of the housing;
Practical trials have shown that attempts to put the above described
principle of operation into practice are attended by several difficulties,
which
are described as follows:
It is essential that there should be a certain predetermined axial
clearance between the threads on the screw and the threads in the housing
because, as show at 26 in Figure 8, it ensures that there is no obstruction to
valve closure. To ensure that the screw nuns freely within the housing it is
also
necessary to have radial clearance between thread crests and adjacent thread
roots, as shown at 19 in Figure 4. It is therefore possible for the screw 3 to
be
displaced laterally and so become eccentric to the housing 1 as shown in
Figure
1 l; or to become tilted relative to the housing as shown in Figure 12. With
the
screw in either of these positions, when the running flanks of the screw
threads
and the running flanks of the housing threads are pushed towards one another
under the action of the spring 22, there is little tendency for the screw to
be
returned to the concentt-ic position if the generator 35 (Figure 11) of the
surfaces of the running flanks is too near to peipendicularity to the line of
action of the force exerted by the spring 22. An elementary illush~ation of
this
is given in Figures 13 and 14. Figures I3 alld I4 are enlargements of the zone
labelled F in Figures 11 and 12. In the example shown in Figure 13 the flank
angle Gk is drawn with a small angle of inclination. Figure 14 shows another
example and is drawn with GIZ having a larger angle of inclination. The
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
8
reference numerals 36 and 37 indicate elements of the running flanks 24S of
the
screw and the running flanks 24H in the housing respectively. Under the same
frictional conditions in each of the two examples, a force P could produce the
movement Q more readily in the example shown in Figure 13 than in the
example shown in Figure 14. The movement Q is necessary to re-centralise
said screw within the housing 1.
Thus, as observed in practical tt-ials, when the angle of inclination of
the running flanks is low the screw 3 is likely to remain in an unfavourable
position in the housing l, as shown in Figure 11 or Figure 12. in these
unfavourable positions, as shown in Figures 11 and 12, the running flank of
the
thread on the screw makes only line or point contact with the running flank of
the thread in the housing, and contiguous "surface-to-surface" contact through
an oil film (i.e. boundary lubrication conditions) cannot take place and
consequently there is a high frictional resistance which opposes, and in most
cases prevents, "take-up" movement. Although it is common practice to
lubricate the threads with oil under pressure from the engine's lubrication
system, under the aforementioned point or line contact conditions the required
contiguous oil film lubrication cannot be maintained, friction therefore
becomes excessive, and so "take-up" movement is prevented or at best becomes
sluggish.
Conversely, when valve opening forces bring together the locking
flanks 25S of the threads on the screw and the locking flanks 25H of the
threads in the housing, see Figure 9, the angularity of the locking flanks is
favourable for cenh~alising the screw within the housing. Contiguous oil film
lubrication conditions can then exist alld a movement (teamed "back-off
movement) of the screw 3 into the housing 1 can take place momentarily, that
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
9
is until the compressive force acting between the engaging locking flanks
breaks down the oil film and allows metal-to-metal contact to take place.
Summarising, in spite of the theoretical high friction/low friction
properties of the locking flanks/running flanks respectively, the "back-off
movement can be too high and/or the "take-up" movement can be too low. If, in
each valve closing/valve opening cycle, the "back-off movement exceeds the
"take-up" movement, the screw 3 retracts progressively into the housing 1 and
contact is lost between the members of the valve train mechanism.
As well as the shortcomings in performance, as just described, there are
difficulties in manufacture and there are problems concerning service
durability
when bum-ess threads are used. These problems are described as follows:
The aforementioned axial clearance (26 & 9 in Figures 8 and 9
respectively) has to be held within close limits by close control in the
manufacture of the internal and external threads. If said clearance is too
high,
valve operation will be unsatisfactory and noisy. Refemin~ to Figures 8, 9,
and
15, typical values of GL and GR are 75° and 15° respectively.
Giving the radial
clearance (KM in Figure 15) the symbol C, the axial clearance is therefore: -
JK + KL = (C x tangent IS) + (C x tangent 75)
i.e. C multiplied by 4. This means that, for example, a tolerance of O.lmm on
axial clearance would require the combined diametral tolerancing on the
internal and external threads to be set at about .OSmIll. This is an
exceedingly
difficult requirement in mass production.
A similar situation exists in regard to wear of the tlu-ead surfaces during
service. A small increase in radial clearance due to surface erosion or
flattening of surface asperities on the sloping flanks will cause a fourfold
addition to the axial clearance and will again lead to unsatisfactory and
noisy
valve operation.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
The provision of grooves and ridges on the locking flanks of the screw
was disclosed in GB-2211263. This modification in the design was introduced
in an attempt to keep the "back off movement within acceptable limits; the
intention of the design was that the ridges, having a reduced surface area,
would more easily break down the oil film between the engaging locking flanks
and would therefore effect an earlier metal-to-metal condition, thus reducing
the magnitude of the "back-off movement. There are disadvantages with this
design:-
a. The manufacture of locking flanks with grooves and ridges is
difficult.
b. As a result of the decreased area of contact, the rate at which
wear occurs on the engaging locking flanks is increased with a
consequent increase in axial clearance as explained in the
previous paragraph.
The close diametral tolerances on the threads of the components of an adjuster
(i.e. on the screw and in the housing) necessitate even closer tolerances in
the
manufacture of the tools which are used in the production of said components.
This applies particularly to the formation of the threads on the taps which
produce the internal threads in the housings. Also, the proportions of the
buth~ess cross-section require the teeth of the taps to be unusually wide,
this can
cause undesirably high tapping torques.
To t1y to increase the efficiency of engines which have automatic valve
clearance adjusters, emphasis is being placed on reducing friction between the
contacting surfaces of the cam 10 and cam-follower 8.
One object of the present invention is to provide an improved
construction of mechanical valve clearance adjuster, with particular reference
to:
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
11
a. improving the reliability of "take-up" movement;
b. reducing friction between cam and cam follower;
c. avoiding the unfavourable ratio of radial clearance / axial clearance
which occurs between internal and external mating threads when
the said threads have a buttress formation, and which therefore
necessitates very close manufacturing tolerances and also causes a
rapid increase in axial clearance between the mating threads.
The valve clearance adjuster, according to one aspect of the present
invention, comprises an internally threaded housing and, within said housing,
a
screw member having an external thread with a form which is generally
complementary to the internal thread form of the housing and fits therein with
a
predetermined axial clearance; the thread being trapezoidal in form,
symmetrical in axial cross section and exhibiting equal frictional resistance
against movement in either axial direction; the flank angles, helix angle and
number of starts in the screw thread being determined to ensure that the screw
member will rotate and advance axially out of the housing solely under the
influence of an axial force on its non-emerging end; the emerging end of said
screw member being configured to work in conjunction with an adjacent
component of, for example, the valve train of an IC engine and to receive from
said adjacent component a frictional resistance to rotation; the non-emerging
end of said screw being configured so that it assists axial movement of the
screw member when it is acted upon by the aforesaid axially directed force.
One possible reason for sluggish "take-up" movement is the tendency
for the screw 3 to move into a position of IlOrl-C011Cent1'lclty with the
housing l,
as shown in Figures 11 and 12. In the present invention this tendency is
reduced by increasing the angle G,~ of the slope of the running flanks as
already
discussed with reference to Figures 1 s and 14.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
12
In order to avoid the need for very close tolerances on the diameters of
the screw tIu-eads, it is very desirable to effect a substantial reduction in
the
ratio:-
Axial clearance + Radial clearance = tangent GL + tangent GR
as already discussed with reference to Figure 15. As an example an increase in
GR from 15° to 30° and a reduction in GL down to a value of
30° from the value
of GL = 75° which has been taken as an example, the ratio axial
clearance
radial clearance is reduced from (tangent 15° + tangent 75°) to
(2 x tangent
30°), i.e. from 4 to 1.15.
Thus, in the present invention, the flanks of each thread on the screw
member and in the 1-~ousing are preferably inclined at an angle of 30°
on each
side of a perpendicular to the axis of the screw thread when viewed in axial
cross section. These flank angles are found in modern threads which are used
in standard bolts and nuts etc., and are commonly termed 60° threads.
This has
the advantage that for inspection proposes commonly available devices can be
used; for example, microscope graticules, thread measunng needles, thread
measuring machines, gap gauges and optical projector screens.
Thus the angle Gk of the running flanks eduals the angle GL of the
locking flanks, i.e. G = GR = GL = 30°. Giving the helix angle the
symbol Z
and the coefficient of friction the symbol y, the condition for the screw to
rotate and advance axially undea axial load is given by the formula: -
tangent Z> y = cosine G
this assumes negligible fl'ICtloI1 UIl the end of the screw which end receives
the
axial force. As approximate examples, a screw with an outside diameter of
8mm and a lead of 4mm, would rotate and advance under axial load if the
coefficient of friction had a value less than approximately 0.14. Increasing
the
lead to Smm would enable satisfactory operation to be achieved with an even
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
I3
higher coefficient of friction, viz. 0.17. The design of screw threads
according
to these examples could be achieved satisfactorily by the use of two-start
threads.
Thus it is possible to have a screw member which has a thread of
symmetrical form with flank angles G of 30° and which can rotate and
advance
axially solely under the influence of axial force. In addition, the 30°
flank
angle enables the screw member to remain concentric relative to the housing
and so avoid the point contact conditions which occur when lower flank angles
are used, as described earlier. "Take-up" movement can thereby be achieved
under the influence of an axial force. Such an axial force may be provided,
for
example, by the pressure of oil from the engine's lubrication system or by
means of a small spring preferably acting on the end of the screw through the
medium of a ball-ended plunger.
Practical h~ials have shown that when a screw member is made with a
symmetrical "V" foam, said screw member can be given sufficient resistance to
"back-off' movement by providing an external frictional torque resistance at
the
end of the screw which end contacts a co-acting member of the engine valve
train.
According to the design of the engine valve train, the zone of contact
between the end of the screw member and the co-acting member of the valve
h~ain can be in the form of, for example:-
a. a circular line
b. an annular area
c. an area of the curved surface of a thin conical frustum
d. an area of the curved surface of a thin slice of a sphere.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
14
In the case of b. c. or d. the width dimension may be small enough to
justify the assumption that the contact condition is a line of circular
contact. In
all cases the axis of the circle, perpendicular to its plane, coincides with
the
axis about which the screw member could rotate. In each of the four examples
a value D could be assigned to represent the diameter of the circle to which
frictional resistance acts tangentially. Giving the mean diameter of the screw
thread the symbol d, external frictional resistance to "back-off movement is
largely dependant upon the ratio D = d and can therefore be controlled by
design of:-
I. mean diameter of the screw
II. the valve actuating end of the screw member
III. the part of the member of the valve train which part contacts the
screw member.
By way of example, engine tests have shown that an adequate control of
"back off ' movement of an adjuster is obtained when a ratio of D = d = 2 is
used in conjunction with a screw thread having an helix angle of 10°.
This in
turn gives a reliable and responsive "take up" movement.
The mode of operation of the adjuster in accordance with the invention
will now be described with reference to Figures 5, SA; 6, 6A; and 7, 7A which
by way of example correspond with Figure 1. Figure 5 shows the screw
member 3 in a notional position within the housing 1 when clearance has
developed between members of the engine valve train. For ease of description
the entire amount of clearance is shown as a single gap 42 between the end 7
of
the screw member 3 and the cavity 9 in the cam follower 8. In this situation
the
cam 10 can be in any angular position in which its constant radius portion is
contacting the cam follower 8, this is indicated diagrammatically in Figure
SA.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
Owing to the relatively high helix angle and to the low frictional torque
resistance between the ball 29 and the spherical depression in the end 6 of
the
screw member, the spring 22 is able to produce "take-up" movement, i.e. to
advance the screw member 3 (upward as illusri-ated) out of the housing 1 until
the spherical end 7 contacts the suuface of the cavity 9 in the cam follower
8, as
shown in Figure 6.
With regard to the situation shown in Figure 6, tlu-ee points should be
noted:-
a) The cam 10 can still be regarded as being in any angular position
in which any point on its constant radius portion is contacting the
cam follower 8, as shown diagrammatically in Figure GA.
b) A clearance gap 43 is maintained below (as illustrated) the
threads of the screw member 3 as was the case in the situation
shown in Figure 5.
c) The force exerted by the spherical end 7 on the surface of the
cavity 9 in the cam follower 8 is small. Said force is limited to the
force in the spring 22 minus the effort required to produce the
"take-up" movement.
Figures 7 and 7A show the situation where the cam 10 has turned into a
position where it has pushed the cam follower 8 just sufficiently to move the
screw member 3 downwards through the previously existing clearance gap 43.
That is to say the clearance gap is now above (as illustz~ated at 44) the
threads
of the screw member 3. At this instant between the mating threads there will
be
a continuous oil film and consequently there will be from within the housing 1
a low resistance to upward "back-off movement of the screw member 3.
Resistance to "back-off' movement is therefore provided externally by friction
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
16
at the contact between the end 7 of the screw member and the surface of the
cavity 9 in the cam follower 8.
Subsequent slight rotation of the cam 10 induces a rapidly increasing
load throughout the entire valve train in order to overcome the force with
which
the engine valve is held against its seating. The aforementioned oil film
between the mating threads is broken down and friction at the threads
contributes an internal resistance to "back-off movement.
With a given thread geomeriy and frictional. conditions the
aforementioned external resistance to "back-off movement is largely
dependant upon the ratio D = d,
where D = effective diameter of the circle of frictional contact
beriveen the end 7 of the screw and the surface of the
cavity 9 in the cam follower 8.
d = mean diameter of the screw thread.
The required resistance to "back-off movement is therefore obtained by
design of I the screw thread (in terms of its effective diameter),
II the valve actuating end 7 of the screw member 3
III the part of the member of the valve train which part
contacts the end 7 of the screw member 3.
Also, increased resistance to "back-off movement can be obtained by
increasing the angle W (Figure 6), i.e. the angle between a tt-ansverse plane
and
a common tangent to the contacting sunnces of the cavity 9 and the spherical
end 7.
Further rotation of the cam 10 opens the engine valve, then allows it to
close, and then brings the valve train back to a situation corresponding to
Figure 5 or Figure 6.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
17
When an automatic valve clearance adjuster in accordance with the
invention is not incorporated in a valve operating mechanism, i.e. not
assembled in an engine, and hence is not constl-ained by the components it
engages in the mechanism, the force exerting means may cause the screw
member to advance outwardly of the housing to its maximum extent. In this
condition the adjuster cannot be assembled into the engine, and requires to be
shortened by retraction of the screw member into the housing by rotational
movement relative thereto, until the overall length of the adjuster is
sufficiently
reduced for it to be placed between the relevant components. For assembly of a
plurality of such adjusters into their respective positions in the individual
parts
of the valve operating mechanism for an engine having a plurality of valves,
clearly it is desirable that there should be some means for temporarily
holding
the adjusters in their contracted condition so that such assembly is
facilitated.
It is broadly the object of another aspect of the present invention to
meet the above described requirement for temporarily holding an adjuster in
its
contracted condition.
According to another aspect of the invention, therefore, I provide a
valve clearance adjuster comprising a housing having an internal screw thread,
a screw member extending into the housing from an open end thereof and
having an external screw thread engaging the thread within the housing, and
spring means acting on the screw member in the direction of its longitudinal
axis; the co-operating screw threads of the screw member and housing being of
such a configuration that the screw member will rotate and advance out of the
housing under the influence of the spring means; wherein there is provided
abutment means operable between the screw member and the housing when the
screw member is screwed into the housing to an inner position, whereby
frictional forces can be established beriveen the screw member, housing and
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
18
abutment means sufficient to retain the screw member in said inner position
against the action of the spring means.
In a valve clearance adjuster in accordance with this aspect of the
invention, it is possible for the screw member to be screwed into the housing
to
its inner position at which the abutment means operable between the screw
member and the housing is engaged. Then if the screw member is tightened to
cause an increased force to be exerted between the screw member, abutment
means and housing, frictional forces will be established therebetween of
sufficient magnitude to resist the action of the spring means which tend to
advance the screw member outwardly of the housing. The dimensions of the
valve clearance adjuster should be aiTanged so that when the screw member is
in its inner position the overall length of the adjuster is sufficiently small
to
enable it to be easily assembled in the reduired position in the operating
mechanism for a valve of an engine: hence an engine with a plurality of the
valve clearance adjusters can easily be assembled. Tests have shown that on
starting the engine, the shock induced in each adjuster by impact from the cam
which operates its particular valve is sufficient to free the frictional lock
between the screw and housing, so that the screw member immediately takes up
the correct working position relative to the housing of the adjuster.
The screw member of the adjuster may be adapted to be engaged by a
tool by which it can be screwed into the housing to cause the abutment means
to be bought into operation as aforesaid. lIl etl7bOdlIneIltS Of adjuster
described
hereafter, wherein the screw member has a head with a pant-spherical surface
for engagement with a complementary pant-spherical socket formation in a
rocker amn or the like, a tool for engagement with the screw member may
comprise a socket or recess with a surface configuration able to establish
frictional engagement with the head of the screw member sufficient to enable
it
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
19
to be tightened to engage the abutment means when the screw is in its inner
position.
The abutment means may comprise a shoulder formation provided
inside the housing and engageable by an innermost end face of the screw
member when the latter is in its inner position. Such a shoulder may be
afforded by the housing itself, or a separate member, e.g. a sleeve, affording
such a shoulder may be inserted in the housing.
Alternatively, the screw member may be provided with an abutment
formation which is engageable with an end face of the housing. Such a
formation may comprise a collar provided on the screw member beneath a head
pan thereof. Such a collar may also be usable for holding the screw member
to screw it into the housing and tightening it when the collar has engaged the
housing, rendering it unnecessary for a separate tool to be used in this case.
Preferably the valve clearance adjuster is in accordance with the first
aspect of the invention. However, it may alternatively be an adjuster
incorporating a buttress tlwead such as refen-ed to in the above discussion of
the
prior art.
Figure 16 shows an adjuster such as illustc~ated in Figure 5, 6 or 7, in
the condition it can assume when not assembled into an engine. The screw
member has advanced out of the housing. so the overall length of the adjuster
will be substantially greater than the space available to be occupied by the
adjuster in an engine.
For assembly of the adjuster into an engine, therefore, the screw
member 3 has to be screwed into the housing 1 to the extent that the overall
length of the adjuster is reduced to a magnitude permitting easy assembly.
Such overall length of the adjuster will be slightly less than the typical
operating lengths of the adjuster Sh0wI1111 Figures 6 and 7.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
If the characteristics of the screw thread operative between the screw
member and housing of the adjuster are such that the screw member will not
stay in such a position by friction, in accordance with the invention there
may
be provided abutment means operable between the screw member and housing
when the screw member is in an innermost position similar to that in which it
is
shown in Figure 5. One form of such abutment means is shown in Figure 17,
afforded by a cylindrical sleeve 50 disposed within the housing and
surrounding the spring 22. One end of the sleeve 50 engages the closed lower
end 51 of the housing, while the other end of the sleeve abuts end face 6 of
the
screw member in the annular region surrounding the recess therein.
When the screw member is screwed into the housing and tightened into
engagement with the abutment sleeve 50, forces are established between the
screw member and sleeve and between the cooperating threads of the screw
member and housing for the screw member frictionally to be held in such an
innermost position, despite the force exerted thereon by the spring 22. This
enables the adjuster easily to be installed. However, when the engine in which
the adjuster is installed is started, impact from the cam is sufficient to
free the
frictional lock between the screw member and housing, so the screw
immediately takes up the con -ect working position as above described.
Instead of the sleeve 50, the housing could be provided with an integral
abutment formation with which the screw member is engageable when it is in
its innermost position. Such a formation could be provided, for example, by
having the interior of the housing of a smaller diameter in its part occupied
by
the spring 22, beneath the lower end of its internal screw thread. Yet a
further
possibility is that the screw member could be provided with an external collar
beneath its head, engageable with outer end face 55 of the housing. Such a
CA 02399059 2002-08-O1
WO 01/57367 PCT/GBO1/00410
21
collar may be used as a means for tinning the screw when it is to be screwed
into the housing until the collar abuts the latter.
If the screw member is not provided with such a collar or other
formation enabling it to be screwed into the housing in this way, a tool such
as
that indicated at 52 in Figure 17 may be used to engage the part-spherical
head
end 7 of the screw member to turn it. Tool 52 has a tapering socket 53 at one
end whose angle is such that when pressed into engagement with the head of
the screw member by hand it will fi-ictionally grip the latter with sufficient
force to screw the screw member into the housing against the action of spring
22 and engage the abutment sleeve 50. Tool 52 has a grip portion 54 enabling
it to be held and turned easily by a person using it.
As above referred to, in some designs of adjuster incorporating spring
means for advancing the screw member axially out of the housing, there may
not be a need for the expedient above described to facilitate assembly of the
adjuster into an engine. As above refen-ed to, engine tests have shown that
adequate control of adjuster "back off' movement is obtained with a ratio of
D/d of 2 is used, in conjunction with a screw tlwead having an helix angle of
10° which in turn gives a reliable and responsive "take up" movement of
the
adjuster. However in cases where overall engine design considerations impose
constraints which necessitate lower values of D/d, a lower thread helix angle
has to be considered. For example, an adjuster which has to have a helix angle
of 6°, which should produce adequate "take up" movement when it is
subject to
effective lubrication in an engine, should have sufficient thread friction to
keep
the screw in an inner position within the housing without the need for the
assembly device.
In the present specification "comprise" means "includes or consists of
and "comprising" means "including or consisting of'.
CA 02399059 2002-08-O1
WO 01/57367 PCT/GB01/00410
22
The features disclosed in the foregoing description, or the following
claims, or the accompanying drawings, expressed in their specific forms or in
terms of a means for performing the disclosed function, or a method or process
for attaining the disclosed result, as appropriate, may, separately, or in any
combination of such features, be utilised for realising the invention in
diverse
forms thereof.
