AUTOMATICALLY ADJUSTING FRICTION CLUTCH
WITH OVER ADJUSTMENT PROTECTION
FIELD OF THE INVENTION
This invention relates to the field of friction clutches and more
particularly to friction clutches having automatic adjustment
mechanisms.
BACKGROUND OF THE INVENTION
Known friction clutches provide a releasable torsional connection
between a motor vehicle engine flywheel and an associated
transmission. Repeated clutch disengagement and engagement cycles
wear the friction material of the clutch driven disc. The wear results
in an axial shift of the pressure plate's position toward the flywheel in
the engaged condition. The axial shift commonly decreases the clutch
engagement force or clamping load.
Some clutches are commonly provided with an adjustment
mechanism which automatically compensates for driven disc wear and
maintains the clutch engagement force. Some automatic adjustment
mechanisms are disposed between the pressure plate and an
associated biasing member such as a diaphragm spring. The biasing
member acts against the adjustment mechanism to bias the pressure
plate toward the flywheel. The clutch engagement force developed by
the biasing member prevents adjustment of the adjustment
mechanism in the engaged condition. When the clutch is released, the
clutch engagement force is eliminated, enabling automatic adjustment
of the clutch. However, over adjustment can occur.
It is known to provide adjustment limiters in the form of finger
elements connected to the pressure plate. The fingers limit the travel
of the adjusting feature relative to the pressure plate. As the driven
disc wears, the fingers are forced further out, allowing the adjustment
mechanism to further adjust. However, the finger elements have
several disadvantages. One disadvantage is that the effectiveness of
the fingers depends on the dimensions of the fingers, the dimensions
of the pressure plate, and the dimensions adjustment mechanism
components. The interdependence of these parts makes it difficult to
provide assemblies with optimally compatible component dimensions.
Another disadvantage of finger systems is that the fingers may
not properly compensate for wear of the driven disc. The fingers
engage the adjustment mechanism at a location radially offset from
where the adjustment mechanism is engaged by the spring. Any axial
deflection or wear of the adjustment mechanism which changes the
axial distance between the spring and finger engagement locations will
resuitantly either decrease the pressure plate departure distance or
decrease the clutch engagement force.
Another disadvantage of the finger system is that the flywheel
surface engaged by the ends of the fingers which pass through the
pressure plate must be a flat, controlled surface. Otherwise, the
fingers will not be displaced equally, and may allow the adjusting
mechanisms to cock, or may result in an incorrect adjustment.
Yet another disadvantage of a finger system is that the fingers
are in the immediate vicinity of the frictional engagement surface of
the pressure plate where a significant amount of heat is generated
during clutch engagements. This heat may affect the operation of the
fingers.
It is desired to provide a clutch having an adjustment
mechanism with a travel limiter relatively insensitive to dimensional
variations of the pressure plate and the adjustment mechanism.
It is desired to provide a clutch having an adjustment
mechanism with a travel limiter which is relatively insensitive to
deflection or wear.
It is desired to provide a clutch having an adjustment
mechanism with a travel limiter which is relatively insensitive to
variations of the flywheel surface characteristics.
It is desired to provide a clutch having an adjustment
mechanism with a travel limiter which is relatively insensitive to
temperature increase of the pressure plate.
SUMMARY OF THE INVENTION
An adjustment mechanism for a frictional clutch includes a
pressure plate, a first annular cam member, a second annular cam
member, a cam spring and an adjustment limiting mechanism. The
pressure plate includes an axis of rotation and an engagement surface
substantially normal to the axis of rotation. The first annular cam
member is concentric with the axis of rotation and has a plurality of
first ramped cam surfaces. The first annular cam member is rotatably
fixed with respect to the pressure plate. The second annular cam
member is rotatable relative to the first annular cam member and has
a plurality of second ramped cam surfaces in engagement with the first
ramped cam surfaces. The second annular cam member, together
with the first annular cam member defines an effective pressure plate
thickness relative to the engagement surface. The cam spring is
connected with the cam member and rotatably biases the cam
members toward an increased cam height. The adjustment limiting
mechanism includes a first plurality of pins, an annular adjusting ring,
and a first plurality of pin receiving members. The first plurality of
pins extends axially from a side of the pressure plate opposite the
engagement surface. The annular adjusting ring is concentric with the
axis of rotation. The first plurality of pin receiving members is fixed to
the adjusting ring and slidably receives the first plurality of pins.
A frictional clutch for a motor vehicle includes a cover, a
pressure plate, a diaphragm spring, an adjusting mechanism, and an
adjustment limiting mechanism. The cover has an axis of rotation.
The pressure plate is coupled to the cover for rotation therewith about
the axis of rotation and has a frictional engagement surface
substantially normal to the axis. The diaphragm spring is interposed
between the cover and to the pressure plate. The diaphragm spring
is selectively moveable between engaged and disengaged positions.
In the engaged position, the diaphragm spring biases the pressure
plate to an engaged position. The adjusting mechanism includes a first
annular cam member, a second annular cam member and a cam
spring. The first annular cam member is centered about the axis and
is axially disposed between the pressure plate and the diaphragm
spring. The second annular cam member is centered about the axis
and has a plurality of second ramped cam surfaces engaging the first
ramped cam surfaces. The engaged cam members are axially
disposed between the pressure plate and the diaphragm spring. The
engaged cams define an effective thickness of the pressure plate from
the frictional engagement surface to an engagement feature of the
second annular cam. The effective thickness increases with the
relative rotation of the cam members in a first direction. The cam
spring is connected with the cam members and induces relative
rotation therebetween in the first direction. The annular fulcrum is
concentric with the axis and is axially disposed between the adjusting
mechanism and the diaphragm spring. The adjustment limiting
mechanism includes a first plurality of pins, an annular adjusting ring
and a first plurality of pin receiving members. The first plurality of
pins extends axially from side of the pressure plate opposite the
engagement surface. The annular adjusting ring is concentric with the
axis of rotation. The pin receiving members are fixed to adjusting ring
and slidably receive the first plurality of pins. A plurality of
engagement tips is disposed on the adjusting ring. The engagement
tips are substantially aligned with the annular fulcrum.
A frictional clutch for a motor vehicle includes a clover, a
pressure plate, a diaphragm spring, an adjusting means, and an
adjustment limiting mechanism. The cover has an axis of rotation.
The pressure plate is coupled to the cover for rotation therewith about
the axis of rotation. The pressure plate has a frictional engagement
surface substantially normal to the axis. The diaphragm spring is
interposed between the cover and the pressure plate and is selectively
moveable between engaged and disengaged positions. The diaphragm
spring, when in the engaged position, biases the pressure plate to an
engaged pressure plate position. The adjusting means maintains a
constant spring engagement force against the pressure plate in an
engaged condition and is axially disposed between the pressure plate
and the diaphragm spring. The adjustment limiting mechanism
includes a first plurality of pins, an annular adjusting ring, a first
plurality of pin receiving members and a plurality of engagement tips.
The first plurality of pins extend axially away from a side of the
pressure plate opposite the engagement surface. The annular
adjusting ring is concentric with the axis of rotation. The pin receiving
members are fixed to the adjusting ring and slidably receive the first
plurality of pins. The engagement tips are disposed on the adjusting
ring and operably engage the diaphragm spring.
A travel limiter for a clutch is disclosed which is relatively
insensitive to dimensional variations of the pressure plate and the
adjustment mechanism.
A travel limiter for a clutch adjustment mechanism is disclosed
which is relatively insensitive to deflection and wear.
A travel limiter for a clutch adjustment mechanism is disclosed
which is relatively insensitive to flywheel surface characteristic
variations.
A disclosed travel limiter for a clutch adjustment mechanism is
relatively insensitive to temperature increases of the pressure plate.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is a sectional side view of an exemplary clutch.
Figure 2 is an axial end view of the clutch of Fig. 1.
Figure 3 is a sectional view of the clutch of Fig. 2 in the direction
arrows 3.
Figure 4 is an enlarged sectional view of a portion of the clutch
of Fig. 1 in the circle 4 in which the clutch is in a first condition.
Figure 5 is a view of the portion of the clutch shown in Fig. 4 in
which the clutch is in a second condition.
Figure 6 is a view of the portion of the clutch shown in Fig. 4 in
which the clutch is in a third condition.
Figure 7 is a view of the portion of the clutch shown in Fig. 4 in
which the clutch is in a fourth condition.
Figure 8 is a view of the portion of the clutch shown in Fig. 4 in
which the clutch is in a fifth condition.
Figure 9 is a view of a portion of the clutch shown in Fig. 7, yet
further enlarged.
DESCRIPTION OF PREFERRED EMBODIMENTS
A frictional clutch 10 for a motor vehicle is shown in Fig. 1 and
Fig. 2. Clutch 10 rotates about axis 12. A flywheel 14, best shown
in Fig. 1, is rotatably fixed to an output shaft of a motor vehicle engine
(not shown). A driven disc 16 centered with respect to axis 12, has a
hub which slidably engages an input shaft 17 of the motor vehicle
transmission. A friction element 18 of driven disc 16 is engaged by an
engagement surface 20 of pressure plate 22 on one side and by an
engagement surface 24 of flywheel 14 on the other side.
A cover 26 is disposed over pressure plate 22 and is fixed to
flywheel 14. A diaphragm spring 28 with a plurality of radially
inwardly extending fingers 30 serves as a biasing member, and is
disposed between cover 26 and pressure plate 22. Spring 28 forces
pressure plate 22 against driven disc 16 which in turn is pressed
against flywheel engagement surface 24 in an engaged condition.
Fingers 30 of diaphragm spring 28 have their inner tips axially
engaged by a release assembly 32. Clutch 10 is selectively moved
between engaged and released conditions by axially displacing release
assembly 32 which resultantly deflects diaphragm spring 28. A
potential alternative to diaphragm spring 28 is a plurality of clutch
levers disposed between cover 26 and pressure plate 22, with either
a diaphragm spring without fingers, or a plurality of coil springs, acting
against the levers to bias pressure plate 22 toward flywheel 14. A
plurality of straps or leaf springs 34 connect pressure plate 22 and
cover 26, biasing plate 22 toward cover 26. An automatic adjustment
mechanism 36 is disposed between pressure plate 22 and diaphragm
spring 28.
Diaphragm spring 28 engages cover 26 indirectly by contact with
a first or outer diameter pivot ring 38. First pivot ring 38 is disposed
inside cover 26. At a location disposed radially inwardly of ring 38,
diaphragm spring 28 engages a fulcrum 40 of adjustment mechanism
36.
Adjustment mechanism 40 includes a first or stationary annular
cam 42 formed as one piece and integral or unitary with pressure plate
22. Alternatively, stationary annular cam 42 may be formed as a
separate piece and disposed against pressure plate 22. Stationary
annular cam 42 is concentric with axis 12. Cam 42 has a plurality of
first ramped cam surfaces 44 on a side opposite pressure plate
engagement surface 20. A second or rotating annular cam 46 is
disposed over stationary annular cam 42. Rotating annular cam 46 is
also concentric with axis 12. Rotating annular cam 46 has a plurality
of second ramped cam surfaces 48 in engagement with first cam
surfaces 44 as best seen in Fig. 3. Fulcrum 40 is formed as a single
r—————
integral one piece unit with rotating annular cam 46. An effective
pressure plate thickness H from engagement surface 20 to a top of
fulcrum 40 is controlled by adjustment mechanism 36 as shown in
Figs. 3 and 6.
A plurality of cam alignment pins 50 are disposed radially within
rotating annular cam 46 for engagement therewith to maintain cam 46
in concentric alignment with axis 12. Cam 46 is additionally or
alternatively provided with a pilot shoulder 52 as shown in Figs. 1 and
4-8 which helps maintain cam 46 concentric with respect to pressure
plate 30.
A cam spring 54 is formed of flat wire like that used for clock
springs. A first end (not shown) of cam spring 54 engages a radially
inwardly extending spring hook (not shown) on rotating annular cam
46. A second end 56 of spring 54 is disposed in a notch 58 in a ring
portion 60 of pressure plate 22, best shown in Fig. 1. As driven disc
16 wears, spring 54 unwinds, biasing cam 46 to a position of increased
thickness H. Alternatively cam spring 54 is formed of round wire (not
shown), and similarly disposed between stationary annular cam 42 and
rotating annular cam 46, biasing the cam 46 in the direction tending
to increase effective pressure plate thickness H from engagement
surface 20 to fulcrum 40.
A mechanism 62 for limiting adjustment of automatic
adjustment mechanism, and preventing over adjustment, 36 includes
an adjusting ring 64, a bracket portion 66 of cover 26, and alignment
pins 50.
Adjusting ring 64 has an engagement surface 66 which contacts
a bracket portion 68 of cover 26 under certain conditions. Bracket
portion 68 is a radial inward extension of cover 26. Radially outwardly
disposed spring engagement tips 70 of ring 64 extend from ring 64
into contact with the annular portion of diaphragm spring 28. Spring
engagement tips 70 engage diaphragm spring 28 substantially
opposite and in radial alignment with fulcrum 40. Notches 72 formed
on the inner diameter of bracket 68 to allow spring engagement tips
70 to extend past cover 26. A ring shoulder portion 74 extends axially
from engagement surface 66 toward pressure plate 22.
Adjusting ring 64 has three cup portions, each of which receive
one of the axially fixed alignment pins 50. A wrap spring 78 is
disposed over each of alignment pins 50 within cup portion 76.^
Springs 78 exert a frictional force on pins 50. The interior of'tup
portion 76 is sufficiently large so that wrap springs 78 do not engage
the interior. However, such clearance is not critical to the operation
of the invention. A first end 82 of cup portion 76/\through which
alignment pin 50 passes, is sized to permit the passage of pin 50, but
not wrap spring 78.
Erosion of the pins and/or springs could undesirably increase and
axial force necessary to axially displace springs 78 axially along pins
50. A relatively close fit between first end 82 and pin 50 prevents dirt,
debris and other contaminants from entering cup portion 82. Wrap
spring 78 is disposed against first end 82. A cap 84 is disposed over
a second end 86 of cup portion 76. Cap 84 prevents the intrusion of
dirt, clutch debris and other contaminants from entering^up portion
76\ at second end 86. Pins 50 are preferably made of a corrosion
resistant material such as stainless steel.
The invention operates in the following manner. In a new or no-
wear condition, as shown in Figures 1 and 4, rotating cam 46 is
oriented to a first position such that thickness or height H is at a
minimum. Friction elements 18 of driven disc 16 are in an unworn
condition. Spring engagement tips 70 are in contact with the annular
portion of diaphragm spring 28. Wrap spring 78 is in a first position on
pin 50, holding ring 64 at a corresponding first position on pins 50.
Engagement surface 66 is pressed against bracket 68.
A vehicle operator selectively disengages and reengages clutch
10 by axially displacing release assembly 32 via a pedal operated
clutch linkage (not shown). After multiple disengagements and
reengagements friction elements 18 wear, decreasing the axial
thickness of driven disc 16.
Clutch 10 is shown in Figure 5 in a partially worn condition in
which no adjustment of adjustment mechanism 36 has yet occurred.
Adjustment mechanism 36 is still at the same height H as shown in
Figure 4. Engagement surface 66 is in contact with bracket 68. Pins
50 have slipped relative to springs 78 and cup portion 76 an amount
approximately equal to the wear of driven disc 16. Additionally, there
is an axial space or gap between spring engagement tips 70 and
diaphragm spring 28 approximately equal to the wear of driven disc
116.
Figure 6 shows the clutch of Figure 5 immediately after release
assembly 32 has been axially displaced to a released position.
Pressure plate 22 has moved away from driven disc 16. There is now
a gap between engagement surface 66 and bracket 68 which is equal
to the axial displacement of pressure plate 22 in moving from the
engaged to the released positions. The annular portion of diaphragm
spring 28 is now in engagement with tip 70. There is a gap between
fulcrum 40 and diaphragm spring 28 which, presuming that cam 46
has not yet begun to move from its position as shown in Figure 5, is
equal to the gap in Figure 5 between diaphragm spring 28 and tip 70.
Responsive to the biasing force of spring 54, cam 46 rotates,
with the inclined cam surfaces 44 and 48 causing fulcrum 40 to engage
diaphragm spring 28 as shown in Fig. 7. It is at this point in the
operation of the clutch adjustment mechanism 36 and adjustment
limiting mechanism 62 that the limiting mechanism 62 fulfills its
purpose. It should be appreciated that this sequence of figures does
not necessarily represent the exact sequencing of the operation of the
elements. For instance, it is anticipated that cam 46 would begin
rotating before release assembly 32 reaches a final released position.
The position of wrap spring 78 on pin 50 does not change with
the shift to the released condition, even though a force which causes
pressure plate 22 to be axially displaced away from flywheel 14 and
driven disc 16 is transmitted through the interface between wrap
springs 78 and pin 50. Wrap springs 78 and alignment pins 50 have
certain controlling parameters such as diameters, lengths, and spring
rates selected so that the force required to axially displace springs 78
along pins 50 is within an optimal range. The optimal range of force
is greater than the maximum anticipated release load applied against
tips 70 by diaphragm spring 28, but is less than the amount of force
applied against fulcrum 40 by diaphragm spring 28 in the engaged
condition. The above-described optimal forces associated with pins 50
and wrap springs 78 prevents over adjustment of adjustment
mechanism 36. Over adjustment is prevented by causing pressure
plate 22 to travel with diaphragm spring 28. Limiting the displacement
of diaphragm spring 28 relative to pressure plate 22 during
disengagement limits the amount of adjustment of thickness H which
can be provided by cam 46 during a single disengagement.
When release assembly 32 is released, allowing clutch 10 to
return to an engaged condition as shown in Figure 8, engagement
surface 66 is again in contact with bracket 68. Pin 50 slips slightly, if
at all, relative to wrap spring 78, and only in an amount attributable
to the wear of driven disc 16 attributable to the reengagement which
occurs in that particular reengagement cycle. Diaphragm spring 28 is
in engagement with both fulcrum 40 and engagement tips 70.
It should be appreciated that there are readily apparent
alternative embodiments to the above-described clutch components
For example, the rotating annular cam and the stationary annular cam
could be transposed so that it is the stationary annular cam which
engages the diaphragm spring. Or, the cams could rotate relative to
each other without either cam being stationary relative to the pressure
plate or the clutch cover. Also, the cam spring could take the form of
a round wire torsional spring instead of a flat wire torsional spring. Or,
a tension coil spring could be employed to rotatively bias the rotating
cam.
The embodiments disclosed herein have been discussed for the
purpose of familiarizing the reader with the novel aspects of the
invention. Although preferred embodiments of the invention have
been shown and disclosed, many changes, modifications and
substitutions may be made by one having ordinary skill in the art
without necessarily departing from the spirit and scope of the
invention as described in the following claims.
Wc Claim:
1. An adjustment device for a frictional clutch (10) comprising:
a pressure plate (22) having an axis of rotation (12) and an engagement surface
(66) substantially normal to the axis of rotation (12);
a first annular cam member (42) concentric with the axis of rotation (12) and
having a plurality of first ramped cam surfaces (44) and being rotatively fixed
with respect to the pressure plate (22);
a second annular cam member (46) rotatable relative to the first annular cam
member (42) and having a plurality of second ramped cam surfaces (48) in
engagement with the first ramped cam surfaces (44) and together with the first
annular cam member (42) defining an effective pressure plate thickness (H)
relative to the engagement surface;
a cam spring (54) connected with the cam member (42, 46) and rotatively biasing
the cam members (42, 46) toward an increased cam height; characterized in that
an adjustment limiting means (62) being connected to the pressure plate (22), the
travel limiting means (62) having:
a first plurality of pins(50) axially extending from a side of the pressure plate
opposite the engagement surface (20),
an adjusting ring (64) concentric with the axis of rotation (12) , and
a first plurality of cup portir/ns (76) fixed to the adjusting ring (64)and slidably
receiving the first plurality of pins (50).
2. An adjustment device as claimed in claim 1 wherein wrap springs (78) are coiled
around and in frictional engagement with each of the pins (50) and the wrap
springs (78) are axially disposed within the cup portions (76), and wherein the
wrap springs (78) resist axial displacement of the adjusting ring (64) relative to
the pins (50).
3. An adjustment device as claimed in claim 1 wherein a plurality of engagement
tips (70) are disposed on the adjusting ring (64).
4. A frictional clutch (10) arrangement for a motor vehicle comprising:
- a cover having (26) having an axis of rotation (12);
- a pressure plate (22) coupled to the cover (26) for rotation therewith about
the axis of rotation (12) and having a frictional engagement surface (20)
substantially normal to the axis(12);
- a diaphragm spring (28)interposed between the cove (26) and the pressure
plate (22) selectively movable between engaged and disengaged positions
and in the engaged position biasing the pressure plate to an engaged
pressured plate (22)position;
- an adjusting device having:
a first annular cam member (42) centered about the axis and axially
disposed between the pressure plate (22) and the diaphragm spring (28),
a second annular cam member (46) centered about the axis (12) having a
plurality of second ramped cam surfaces (48) engaging the first ramped
cam surfaces (44) and the engaged cam members (42, 44) being axially
disposed between the pressure plate (22) and the diaphragm spring (28),
wherein the engaged cams (42,46) define an effective thickness (H) of
the pressure plate (22) from the frictional engagement surface (20) to an
engagement feature (40) of the second annular cam (46) increasing with
relative rotation in a first direction,
a cam spring(54) connected with the cam members (42, 46) and inducing
relative rotation therebetween in the first direction, and
an annular fulcrum (40) concentric with the axis (12) and axially disposed
between the adjusting device and the diaphragm spring (48); and
an adjustment limiting means (62) having a first plurality of pins (50)
axially extending from a side of the pressure plate opposite the
engagement surface (20);
an adjusting ring (64) concentric with the axis of rotation (12), and
a first plurality of cup portions (76) fixed to the adjusting ring (64)and
slidably receiving the first plurality of pins (50), and a plurality of
engagement tips (70) disposed on the adjusting ring (64) and substantially
aligned with the annular fulcrum(40).
5. A frictional clutch (10) arrangement for a motor vehicle as claimed in claim 4,
wherein wrap springs (78) are coiled around and in frictional engagement with
each of the pins (50) and axially disposed within the cup portions (76), and
wherein the wrap springs (78) resist axial displacement of the adjusting ring (64)
relative to the pins(50).
6. A frictional clutch (10) arrangement for a motor vehicle as claimed in claim 4,
wherein an engagement surface (66) of the adjusting ring (64) engages a radially
inwardly extending bracket (68) fixed to the cover (26) when the clutch is in an
engaged condition.
7. A frictional clutch (10) arrangement for a motor vehicle comprising:
- a cover(26) having an axis of rotation;
a pressure plate (22) coupled to the cover(26) for rotation therewith about
the axis of rotation (12) an having a frictional engagement surface (20)
substantially normal to the axis (12);
a diaphragm spring (28) interposed between the cover (26) and the
pressure plate (22) selectively moveable between engaged and disengaged
positions and in the engaged position biasing the pressure plate (22) to an
engaged pressure plate position;
an adjusting device (36) for maintaining a constant spring engagement
force against the pressure plate (22) in an engaged condition axially
disposed between the pressure plate (22) and the diaphragm spring(28);
and
an adjustment limiting means (62) having a first plurality of pins (50)
axially extending from a side of the pressure plate (22) opposite the
engagement surface (20), an adjusting ring (64) concentric with the axis of
rotation (12) , a first plurality of cup portions (76) fixed to the adjusting
ring and slidably receiving the first plurality of pins (50), and a plurality of
engagement tips (70) disposed on the adjusting ring (64) in operable
engagement with the diaphragm spring (28).

An adjustment device (36) for a frictional clutch (10) comprises a pressure plate (22), a First annular cam member (42), a second annular cam member (46), a cam spring (54) and an adjustment limiting means (62). The pressure plate (22) having an axis of rotation (12) and an engagement surface (20) substantially normal to the axis of rotation (12). The first annular cam (42) member is concentric with the axis of rotation (12) and has a plurality of first ramped cam surfaces (44). The first annular cam member (42) is rotatably fixed with respect to the pressure plate (22). The second annular cam member (46) is rotatable relative to the first annular cam member (42) and has a plurality of second ramped cam surfaces (48) in engagement with the first ramped cam surfaces (44). The second annular cam member (46), together with the first annular cam member (42) defines an effective pressure plate thickness (H) relative to the engagement surface (20). The cam spring (54) is connected with the cam member (42, 46) and rotatably biases the cam members (42, 46) toward an increased cam height (H). The adjustment limiting means (62) comprises a first plurality of pins (50), an adjusting ring (64), and a first plurality of pin receiving members (76). The opposite the engagement surface (20). The adjusting ring (64) is concentric with the axis of rotation (12). The first plurality of pin receiving members (76) is fixed to the adjusting ring (64) and slidably receives the first plurality of pins (50).