Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
`~ Mechanical switching apparatus 2 0 7 6 6 7 3
The invention refers to a mechanical switching apparatus
with a housing cont~; n; ng a retainer supporting a ball which
has a control lever projecting radially from it on the input
side and said lever is rigidly fi~ed so that, together with
the ball, two rotaional movements can be made along axes which
are perpendicular to each other and pass through the mid-point
of the ball. Such mechanical switching apparatus is used, for
example, to switch gearboxes in construction plant, in
particular wheel loaders, excavators, and similar equipment.
Such construction plant and associated vehicles have gearboxes
for driving and operating which possess several forward and
several reverse gears so that the operator must have the
possibility of, first of all, being able to select the general
direction of travel, forwards or backwards, in order that,
from this first position, for example in forward travel,
he/she is then able to select the appropriate forward gear by
changing up or down without there being the danger that a
reverse gear is accidentally selected during this procedure.
A mechanical switching apparatus of the above-mentioned
type is also known for private vehicle applications for
tilting the external mirror about its two swivelling axes to
enable the external mirror to be adjusted to the desired
spatial angle. In this case the switching apparatus has a
housing cont~;n;ng a retainer supporting a bail which can
swivel or rotate in all directions. A control lever is rigidly
connected to the ball, whereby the possibilities for movement
of the control lever and therewith the ball too, are limited
by a shift gate corresponding to its construction. With this
known switching apparatus for adjusting the external mirror,
the control lever can be swivelled from a neutral middle
position into two planes perpendicular to each other which
intersect at the axis of the control lever. Four individual
positions can be attained in order to bring about the four
movements necessary for the external mirror. Here, the two
positions in each of the two planes are allocated opposing
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movements. In order to finally cause the appropriate swivel
movement of the external mirror, electrical contacts are
located in the area of the control lever and, therewith, the
housing which are actuated by the control lever in the
respective swivel position. The electrical contacts can be
provided on the side of the control lever where this projects
radially from the ball. However, it is also possible to
provide an extension piece on the other side of the ball, in
line with the axis of the control lever, which in turn,
actuates the four switch contacts concerned. A disadvantage of
this switching apparatus is that the electrical switch
contacts are located in the immediate vicinity of the control
lever and thus within the housing for the mechanical switching
apparatus. Therefore,the electrical switch contacts are not
separated from the mechanical parts of the switching
apparatus. If the gear shift gate is, for example, in the
shape of a cross, then each of the four switching positions
can only be reached from the neutral middle position, i.e. it
is not possible, for example, with a driving gearbox, to first
preselect the general direction of travel, forward or reverse,
and then from this position, to select an appropriate
different forward or reverse gear.
The invention is concerned with the aforementioned
problem and attempts to provide a switching apparatus which
initiates movement via a control lever on the input side which
is can then be taken up or transferred on the output side,
whereby two types of movement then ensue on the output side
which are independent from each other. To illustrate this the
first type of movement should be allocated, for example, to
the forward and the reverse travel, while the second type of
movement is to be the changing up or changing down of the
respective gear in the forward or reverse direction. In this
case an obvious movement of the control lever, for example,
when changing up a gear, should thus be possible in the same
direction regardless of whether forward or reverse travel-was
already selected as the general direction of travel. It is
also important to transmit the movements on the output side so
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that s~ubsequent electrical switches, electronic control
components, etc. are kept separate from the mechanical parts
of the switching apparatus and can, thereby, be accommodated
in a special housing which is additional to the housing for
the mechanical switching apparatus.
According to the invention, this is achieved in the
mechanical switching apparatus described above in that two
separate transmission elements are provided on the outputside,
which are formed from pins and the axes of which are arranged
parallel to each other, and that the two transmission elements
are each coupled with the ball via a free-wheel mechanism,
whereby the two free-wheel mechanisms are so constructed and
positioned in relation to each other that, with the first
rotational movement of the control lever, the first free-wheel
mechanism has a catch function for the first transmission
element and the other free-wheel mechanism has a free-wheel
function for the other transmission element, while with the
second rotational movement of the control lever the situation
is reversed. On the input side, only a control lever makes
contact with the ball, the movements of which are, in
connection with a gearshift gate, so restricted that it can
make two rotational movements in two planes perpendicular to
each other and passing through the mid-point of the ball.
Here, there is the possibility that the control lever can make
two swivel movements, i.e. rotational movements around two
such axes perpendicular to each other neither of which
coincides with the axis of the control lever itself. In this
case the two axes around which rotation is possible and the
control lever axis together form a three-dimensional axis
system. However, it is also possible that one of the two
rotational movements is actually the rotation of the control
level about its own axis. In this case the gear shift gate is
an elongated hole, slotted version. The gear shift gate can be
formed between the control lever and the housing for the
control facility or also from parts of the retaineriin wh~ch
the ball is supported. It is essential to allocate to the
control lever on the input side with its two movement
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possibilies two transmission elements located on the output
side, whereby the first transmission element only transfe~s
movement corresponding to the first rotational movement of the
control lever, and the other transmission element only
movement-of the other rotational movement of the control
lever. Furthermore, this transferring must be carried out in
an independent fashion. This means, for example, that a
rotational movement at the control lever, which changes down a
gear, for example, should be possible regardless of whether,
in the travel situation concerned, forward or reverse travel
is in operation. In both cases changing down a gear must take
place.
The arrangement of the two separate transmission
elements on the output side with their a~es parallel to each
other renders it possible to construct these transmission
elements relatively long as pins, slides or similar, and to
lead away from the ball parallel to each other and in such a
direction so that they can pass through the wall of the
housing for the switching apparatus and also through the side
of a second housing, whereby a simple seal in the region of
the penetrations in the two housing walls is possible. This
has the advantage that subsequent electronic switching and
control components in the second housing can be accommodated
separated and protected from the mechanical parts in the first
housing, i.e. the housing for the switching apparatus.
A free-wheel mechanism is located between the ball and
each of the two transmission elements on the output side. The
construction of the free-wheel mechanisms, their arrangement
and the arrangement of the transmissons elements relative to
the ball are all matched to each other in that the two free-
wheel mechanisms have, alternateiy, free-wheel and catch
functions related to the two rotational movements of the
control lever on the input side. This means that with the
first rotational movemen~ of the control lever, the first
free-wheel mechanism has a catch function and the other free-
wheel mechanism has a free-wheel function so that only one of
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the two transmission elements on the output side performs or
transmits any movement, while the other transmission element
shows no movement and thereby r~mA; n~ stationary. With the
second rotational movement of the control lever the situation
is then reversed. This allocation reflected in the
construction of the gate in such a way that an unambiguous
se~uence is possible starting from the neutral middle
position. Firstly, for example, only the first rotational
movement is possible starting from the neutral middle position
and this selects the general direction of travel, forwards or
backwards. Then, when one of these two general directions of
travel has been selected, the second rotational movement of
the control lever is possible, while at the same time, the
first rotational movement is blocked. It is then possible to
change up or change down a gear. During this second rotational
movement there is no chance of changing the general direction
of travel, forwards or backwards.
The two individual transmission elements located on the
output side perform different lllovell~ents. The first
transmission element can carry out a rotational movement'and
the other transmission element a translation movement. The
first transmission element is arranged with its axis in line
with the mid-point of the ball; the free-wheel mechanism of
this transmission element is located in the region of the mid-
point of the ball. This transmission element performs a
rotational movement but no translation movement. The
rotational movement is than carried out when the axis, around
which the control lever rotates, coincides with the axis of
this transmission element. The free-wheel mechanism which is
allocated to the other transmission element must then have a
free-wheel function. The free-wheel mechanisms described'in
this application can also be designated as clutches. The
clutches provided must have, on the one hand, a catch function
and, on the other hand, a free-wheel function regardless of
around which axis the control lever is turned or swlvelled'.~If
it is a rotational movement then'this'always means a limited
swivel or rotational movement around its own axis,
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consequently, never a rotational movement of more than 360
but rather always a swivelling movement within a relatively
narrow range of angles of, perhaps, up to 45 or 90 mAx; mum.
The free-wheel mechanism of the first transmission
element can be constructed as a joint, torsion spring, folding
bellows or similar. These construction possibilities mean that
this free-wheel mechanism, or rather the clutch, can be formed
in a very simple fashion. This is also the objective of the
present invention. Such a joint, torsion spring, folding
bellows or similar has a catch function with rotational
movement around its own axis or the axis of the transmission
element, while a rotational movement around an a~is
perpendicular to this causes a free-wheel function.
The other transmission element must then be located with
its axis outside the mid-point of the ball; the associated
free-wheel mechanism for this transmission element is switched
between a surface of the ball and the transmission element. In
this case it can be the surface of the ball in the retainer
directly or a piece of a further surface of, for example, a
ball surface with a larger radius which is rigidly connected
with the ball on which the control lever sits. This second
free-wheel mechanism or clutch must be so constructed that it
only has a catch function when the free-wheel mechanism or
clutch of the first transmission element has a free-wheel
function or vice versa.
This construction can be achieved in a very simple
manner in that the free-wheel mechanism of the second
transmission element is formed from an arc-shaped rib on the
surface of the ball and a groove in the second transmission
element, whereby the axis of the arc-shaped rib, i.e. the axis
around which the arc-shaped rib is located relative to the
ball, coincides with the axis of the first transmission
element. Therefore, if the first transmission element performs
a rotational movement, then the arc-shaped rib merely slides
along in the groove of the second transmission element without
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the second transmission element performing any movement.
The arc-shaped rib can be a special part coupled with
the ball and may be arranged on a larger radius than the inner
side of the retainer. Only if one of the transmission elements
is arranged with its axis in line with the mid-point of the
ball and the other transmission element is arranged with its~
axis parallel to this does the possibility arise of achieving
two movements independent of each other on the output side
without using an elbow joint or gear wheels, whereby the first
movement is a rotational movement and the other movement is a
translation movement. In fact it is possible to so arrange the
two transmission elements that neither is provided with irts
axis in line with the mid-point of the ball. In such a case,
however, the axes of the two transmission elements must be
perpendicular to each other so that these two movements can
only then be in turn further transmitted in the same direction
into a second housing by using a right-angle drive, gear
wheels or similar.
The ball of the switching apparatus may have two
extension pieces for the selective connection with ~he control
lever, the axes of which are offset by g0C to each other.
Therefore, it is possible to use essentially identical parts
of the switching apparatus and still fix the control lever
alternatively on one or the other extension piece of the ball.
In one case the control lever performs two rotational
movements around two axes which are both vertical in relation
to the control lever axis. In the other case one of the two
rotational movements is a turning movement about the control
lever axis. Despite these two possibilities, nothing alters on
the output side.
The firs~ transmission element can perform a rotational
movement and the second transmission element can perform a
translation movement. Of equal importance is the opposite
arrangement because, in particular, the establishment of the
designation for the first and second transmission elements is
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not compulsory. However, it is also possible that both
transmission elements perform rotational movements.
It is advisable if both transmission elements can be
sealed as they pass through the wall of the housing to the
switching apparatus. The sealing can then be carried out in a
very simple manner. For example, it is sufficient to arrange
the appropriate o-rings. Despite this in-line arrangement and
the projection of the transmission elements through the wall
of the housing and, if necessary, one wall of the second
housing, the-rotational movements of the control lever in the
two planes perpendicular to each other on the input side are
not hindered by this.
It is useful if the ball is supported by a spring whose
end not adjacent the ball is moved and guided into catch
indents on the housing. This results in the advantage that the
supporting of the ball is essentially free of any play because
it is always pressed by the spring from one side into the
retainer. Another advantage is that the catch indents show the
operator that the correct position has been attained.
Several embodiment examples of the mechanical switching
apparatus are illustrated in the drawings and are explained in
the following.
Figure 1 shows a vertical cross-section through a first
embodiment example,
Figure 2 shows a cross-section II-II as indicated in
figure 1,
Figure 3 shows a cross-section III-III as indicated in
figures 1 and 2,
Figure 4 shows a horizontal cross-section through a
second embodiment example of the switching apparatus,
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Figure 5 shows a cross-section v-V as indicated in
figure 4,
Figure 6 shows a cross-section VI-VI as indicated in
figures 4 and 5,
Figure 7 shows a separate view of a joint which operates
as a free-wheel mechanism, and
Figure 8 shows a plan view of figure 7.
The switching apparatus illustrated in figures 1 to 3
has a housing 1 in which all mechanical parts of the switching
apparatus are accommodated. It is understood that an
associated control lever 2 with knob 3 leads outwards from the
housing 1 in order to enable operation on the input sidë.
Located inside the housing 1 is a retainer 4 which has
perforations in several locations, as shown, and forms a
circular inner surface in which a ball 5 is supported. The
ball 5 is essentially constructed as a hollow ball and also
possesses perforations in various locations. The ball 5 and
the retainer 4 have a common mid-point 6. The control lever 2
has an axis 7 which passes through the mid-point 6 of ball 5.
The housing 1 and/or retainer 4 has an H-shaped opening 8
which forms a gear shift gate for the possible movements of
control lever 2. The opening 8 is covered and protected by a
flexible gaiter 9. A connecting piece 10, which serves for the
fixing of the control lever 2 to the ball 5, is coupled to the
ball 5.
As is particularly obvious from figure 2, coupled to the
ball 5 is an extension piece 11 which leads out from the ball
S on the opposite side to the control lever 2 and has an arc-
shaped rib 12 with an essentially circular cross-section
projecting into space and, therefore, also reproduces the
surface of the ball 5 but at a larger radius. Instead of the
arc-shaped rib 12 it would also be possible to directly
utilize the surface of the ball 5 but this results in
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difficulties in the constructional arrangement because of the
small amount of space.
The arc-shaped rib 12 is allocated to a first
transmission element 13 which can best be seen in figure 3.
The transmission element 13 is constructed as a pin is and so
supported in the housing 1 that it can perform a translation
movement in the direction of its axis 14. The various
positions are indicated in figures 1 and 3 by means of double
chain dot lines. A neutral middle position 15, a first
translation position 16 and a translation position 17 are
indicated,~whereby the first translation position 16 could be
allocated to the forward travel and the second translation
position 17 to the backward travel. From figures 1 to 3 it can
be seen that the control lever 2 is swept through a first
plane I, corresponding to the plane of the drawing in figure
1, in the direction of arrow 18 making a rotational movement
around an axis 19 perpendicular to plane I and passing through
the mid-point 6 of the ball 5. In the opposite swivel
direction, as shown by arrow 20, around the same axis 19, the
second translation position 17, which could be allocated to
the backward travel, is attained. It is to be understood that
the translation positions 16 and 17 are associated with
appropriate electric switches or other electronic control
components which are not illustrated here. The electrical or
electronic components are accommodated in a housing 21,
protected and separated from housing 1. The two housings 1 and
21 are connected to each other via flanges and the
transmission element 13 is passed through the two adjacent
walls of housings 1 and 21 with the help of an O-ring 22.
However, not only the first transmission element 13 is
provided on the output side; there is also a further
transmission element 23 which is also constructed in pin or
bolt form and h-as an axis 24. The axis 24 of transmission
element 23 is in line with the mid-point 6 of the ball 5. The
axes 14 and 24 run parallel to each other. The two
transmission elements 13 and 23 are thus aligned in the same
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-- 11 --
direction when seen from the ball 5 so that their ends away
from the ball finish in the second housing 21 and,
therefore, lead out of the housing 1.
A first free-wheel mechanism 25, which can also be
designated as a clutch, is switched between the transmission
element 13 and the ball 5 (and thus the control lever 2 as
well). This free-wheel mechanism 25 is essentially formed by
the arc-shaped rib 12 and a groove 26 which spans over a
section of the perimeter of the pin-type transmission
element 13 as illustrated (figures 2 and 3). There is some
play between the rib 12 and the groove 26 which either
remains constant, increases or decreases according to the
direction of movement. It can be seen that when the control
lever is moved through plane I according to arrows 18 and
20, the play increases because the transmission element 13
is carried along by the rib 12 into the translation position
16 or 17. If, however, there is a rotational or swivelling
movement of the control lever 2 about axis 24 which,
perpendicular to the plane of the drawing, passes through
the mid-point 6, then the transmission element 13 remains in
its position and the rib 12 merely slides along into the
groove 26. With this direction of mo~ement the free-wheel
mechanism 25 has a free-wheel function while with a
swivelling movement of the control lever 2 through the plane
I, i.e. about axis 19, it has a catch function.
A free-wheel mechanism or clutch 27 is also switched
between the transmission element 23 and the ball 5 or rather
the control lever 2. This free-wheel mechanism 27 is
arranged at the mid-point 6 of the ball 5. It can be made
from a torsion spring 28 as shown in Figure 3. The form of
the free-wheel mechanism 27 passing through an articulated
joint 29 is shown clearly in figures 7 and 8. One of the
ends of the torsion spring 28 (designated only in figure 3)
is coupled to the transmission element 23. Its other end is
supported by and connected to an insert 30. The insert 30 is
fixed, for example, screwed, into the perforated ball 5.
The free-wheel mechanism 27 has a catch
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function when turned about the axis 24 so that via the torsion
of the ball 5 about axis 24, the transmission element 23
performs a rotational movement. However, with a rotational
movement of the ball 5 around axis 19 the torsion spring 28
merely kinks so that the free-wheel mechanism 27 has a free-
wheel function. The transmission element 23 then performs no
rotational movement.
Anyway, the transmission element 23 is only constructed
and arranged to perform, if necessary, rotational movements,
and not translation movements. The transmission element 23,
through a recess in ball 5 and an opening in retainer 4, is
also passed through the two walls of housings 1 and 21 facing
each other and finishes in the interior of housing 21. An 0-
ring 32 serves as the seal. A switch arm 33 is fi~ed on the
free end of the transmission element 23 which successively
actuates a series of electric switches 34, only indicated in
figure 1, upon rotation around axis 24 which are allocated to
the individual gears which are to be switched in the forward
or reverse travel. The housing 21 can have a corresponding
number of LEDs 35 in order to indicate to the operator the
actual gear.
The control lever 2 not only performs the rotational or
swivelling movements around the axis 19 corresponding to the
arrows 18 and 20 in the plane of the drawing shown in figure
1, but also a second rotational or swivelling movements around
the axis 24, i.e. in the plane of the drawing shown in figure
2 according to the arrows 36 and 37. This movement is also
limited by the opening 8 in the housing 1. Under such a
turning movement, the clutch 27 has a catch function for
transmission element 23, while the clutch 25 has a free-w~eel
function for transmission element 13. In doing this, the set
first or second translation position 16 or 17 of transmission
element 13 is not left but rather the transmission element 23
turns around the axis 24 so that the switch arm 33
successively actuates the individual switches 34. Then the
sweep direction according to arrow 36 can be allocated to
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changing up the gears, i.e. for example,-from 2nd gear to 3rd
gear, etc.,while the sweep movement according to arrow 37 is
allocated to changing down, i.e. for example, from 6th gear
down to 5th gear or similar.
It should also be mentioned that the gear shift gate is
so formed by the opening 8 that the sweep movement about axis
24 is only possible from the first or second translation
position 16 or 17, but not from the neutral middleposition 15.
Beneath ball 5, i.e. on the opposite side to the control
lever 2, is a spring 39 which is switched between the
extension piece 11 supporting rib 12 and a socket 38, whereby
the socket 38 is supported by a ball catch 40. This is
illustrated in a partial cross-section. The other partial
cross-section shows a somewhat modified, however, functional
similar construction. The ball catch works in conjunction with
a slip surface 41 in which indented grooves 42 are provided
corresponding to the neutral middle position 15 and the two
transmission positions 16 and 17. The ball 5 is always pressed
against the retainer 4 by a spring 39 so that the parts are
always in a defined, relative position to each other. As the
ball catch clicks into the respective groove 42, this shows
the operator that the desired correct position has been
reached.
The operation and switching sequence of the mechanical
switching apparatus can be as follows, and can be assigned to
switching the gearbox of an item of construction plant, for
example, a wheel loader, as follows: From the neutral middle
position according to figures 1 to 3, the control lever 2 is
first swivelled about axis 19 according to arrow 18. The
transmission element 23 thus remA; n~ untriggered while
transmission element 13 takes on the translation position 16.
Starting from this position, the control lever 2 can now
perform a second rotational movement according to arrow`36
(figure 2), i.e. around axis 24. The ~ree-wheel mechanism 25
thereby functions as a free-wheel, the rib 12 merely slides
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through the groove 26, but the transmission element 13 remains
in the translation position 16, while the transmission element
23 is turned via the clutch 27 because of its catch function.
Therefore, the switch arm 33 also turns and an upward gear
change takes place as desired. This is how, for example, a
gear change from first to second gear is accomplished. If the
driver wishes to change up further, for example, from 2nd to
3rdgear, the driver keeps the control lever 2 in the swivelled
position according to arrow 36 until the actual gears change
up in succession is indicated to the driver by the LEDs 35.
If, for example, the fifth gear is reached, as desired by the
driver, then the driver can bring back the control lever 2, in
the opposite direction to arrow 36, to the position shown in
figure 2 so that the fifth gear r~m~;~s in operation. The
switching sequence can, however, be realized in a different
manner. It is, for example, possible that from a swivelled
position, the control lever 2 is brought back au~tomatically
according to arrow 36, and then a new swivelling movement
according to arrow 36 is necessary for change up to a further
gear. The same applies to changing down. With this second
switching concept, the control lever 2 - still with a first
transmission position 16 in operation - is in a sort of
initial position between the two sweep possibilities according
to arrows 36 and 37 so that the operator has the choIce,
during forward travel, to change up or down a gear. Whether
the choice of gear actually occurs depends on other factors.
The same is true in principle for travel in rèverse.
This means a simple, clear and logical switching possibility
for the operator resulting from the particular design of the
mechanical switching apparatus.
Figures 4 to 6 illustrate a second version of the
mechanical switching apparatus in three cross-sections related
to each other, whereby the drawings do not correspond to those
of figure 1 to 3 with respect to the sequence, but show that
this second version is almost identical with the first version
or rather, the majority of the individual parts can be used
~ - 15 - 207~b73
for both versions. ~ere, only the control lever 2 is not
attached to extension piece 10 but, on the contrary, to insert
30 which is provided in a suitably modified form to the insert
30 in embodiment example of figures-1 to 3 so that the control
lever 2 can be fixed to it. The relative position and the
construction of the two transmission elements 13 and 23 on the
output side are identical. Equally, only control lever 2
catches in a different position of the ball and in such a way
that the axes 7 and 24 coincide. The sweep movement according
to arrows 18 and 20 about axis 19 shown in figure 1 is now
presented in figure 6. Owing to the different arrangement of
control lèver 2 in relation to ball 5, the second rotational
movement of control lever 2 now becomes a movement around its
own axis 7 or 24. This is illustrated in figure 6 and figure
4. Consequently, the construction of the opening 8 is reduced
to an elongated slot according to figure 6. This also results
in a logical and easily comprehensible switching possibility
for the operator in that, for example, the rotational movement
is allocated to forward travel according to arrow 18, to
backward travel according to arrow 20, to changing up
according to arrow 36, and to changing down according to arrow
37. Understandably, in this case the construction of the free-
wheel mechanisms or clutches 25 and 27 is always executed in
such a way that if the first clutch has a catch function the
other clutch has a free-wheel function, and vice versa.
A different form of construction for the free-wheel
mechanism 27 is shown in figures 7 and 8. Here, only the
second transmission element 23 and the insert 30 are shown in
relation to the mid-point of the ball. It can be seen that
instead of a torsion spring 28, an articulated joint 29 is
realized which consists of a projecting disc at the insert 30
and a corresponding slot in transmission element 23. Upon
rotational movement according to arrow 36 the free-wheel
mechanism 27 has a catch function, while a rotational movement
in direction of arrow 18 or 20 is followed by a free-wheel
function. Instead of the torsion spring 28 or the articulated
joint 29, a segment of a rubber tubing could be incorporated
in order to realize the clutch 27.
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i s t o f p a r t s
1 = Housing 34 = Switch
2 = Control lever 35 = LED
3 = Knob 36 = Arrow showing movement
4 .= Retainer 37 = Arrow showing movement
5 = Ball 38 = Socket
6 = Mid-point 39 = Spring
7 = Axis 40 = Ball catch
8 = Opening. 41 = Slip surface
9 = Flexible gaiter 42 = Groove
10 = Connec~ing piece
11 = Extension piece
12 = Rib
13 = Transmission element
14 = Axis
15 = Neutral position
16 = Transmission position
17 = Transmission position
18 = Arrow showing direction
of movement
19 = Axis
20 = Arrow showing direction
of movement
21 = Housing
22 = O-ring
23 = Transmission element
24 = Axis
25 = Free-wheel mechanism
(clutch)
26 = Groove
27 = Free-wheel mechanism
(clutch)
28 = Torsion spring
29 = Articulated joint
30 = Insert
31 = ~pening
32 = O-ring
33 = Switch arm
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