Wet friction material and manufacturing method therefor

Details Wet friction material and manufacturing method therefor Wet friction material and manufacturing method therefor

2002-12-11

2004-08-24

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

With coating after drying

C427S345000, C427S350000, C523S149000

Reexamination Certificate

active

06780283

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wet friction material used for a clutch, a brake and the like in a friction engaging apparatus, and a manufacturing method therefor.
2. Description of the Related Art
FIG. 19
shows one example of the basic construction of a wet friction clutch. Torque is transmitted when drive plates
2
fitted into a spline
51
of a hub
5
fitted around an input shaft
6
comes into contact with driven plates
1
fitted into a spline
41
of a retainer
4
. In this drawing, reference numeral
3
designates a pressure plate, and
7
is a pressure piston.
FIG. 16
is a perspective view showing the driven plates and the drive plates, and
FIG. 17
is a side cross-sectional view showing the driven plates and the drive plates in combination. The driven plate
1
is made up of a steel plate portion
11
and a spline projection
12
, whereas the drive plate
2
is made up of a steel plate portion
21
, a spline projection
22
, and a wet friction material
23
bonded to both sides of the steel plate portion
21
.
FIG. 18
is a plan view of the drive plate
2
, and a groove
24
which also acts as an oil reservoir is formed in the friction plate
23
bonded to the surface of the steel plate portion
21
.
In view of the current energy and environmental issues, there is a demand for a compact and light-weight friction clutch which reduces operating shock and removes self-induced oscillation such as occurred shudder so as to provide good ride comfort as well as having a large torque capacity. The friction clutch is also required to cope with high energy resulting from increased rotational speed and output of an automobile engine. Thus, the demand is extremely great.
A conventional friction clutch employs many sophisticated controls in order to reduce fuel consumption and operating shock by increasing a continuous sliding state of a clutch during driving of a car, changing a duty efficiency, and controlling an engine so as to reduce the ratio of an input torque to a clutch capacity when a clutch is engaged.
The wet friction material comprises a fibrous base material such as natural pulp fiber and organic synthetic fiber, a filler such as diatomaceous earth, a friction adjustment agent such as cashew resin and a binder such as thermosetting resin. Conventionally, the binder contained in the inside of the friction material forms a high-concentration impregnated layer (a solid binder layer) on the front and rear layers (i.e., both sides) of the friction material.
The thermosetting resin, which is one example of binder, is commonly used as material which forms a wet friction material (composite fibrous paper). The resin of this type comprises phenol resin, epoxy resin, urea resin, melamine resin, silicon resin, or the like. A wet friction material produced by a paper-making method is widely known as the wet friction material. This friction material is manufactured by mixing a fibrous base material such as natural pulp fiber or organic synthetic fiber as fibrous base material with an agent for controlling friction, producing raw paper, and impregnating the raw paper with a diluted thermosetting resin solution, and evaporating the diluted solution in a drying process, and heating the paper to set the thermosetting resin.
The processes from the process of impregnating raw paper with binder to the drying process will be further explained. When the raw paper is impregnated with binder, the binder is diluted with an organic solvent to a predetermined concentration. After the raw paper is sufficiently impregnated with the diluted binder, the organic solvent is evaporated in a drying process. However, the binder is captured by surface tension, whereby an excess binder coating and a high-concentration binder layer are formed along the outermost layer (about 100 &mgr;m) of the friction material surface. It has been impossible to prevent the high-concentration binder layer from being formed along the outermost layers of the front surface (a front layer) and the rear surface (a rear layer). The rear surface and the rear layer are the side of the friction material which is not bonded to the friction surface but bonded to the steel plate. The binder is thermally set in the thermosetting process, as a result of which the binder coating and the high-concentration binder layer formed along the friction material surface are fixed.
It is found that the influence of the excess binder coating and the high-concentration binder layer formed along the outermost layer of the surface by the physical properties (surface tension) of the binder brings about the following problems:
(1) When in an initial state, the binder coating formed over the fibrous base material of the outermost layer is hard and less flexible and forms minute projections. Hence, the binder coating is not necessarily smooth, and only the projections of the binder coating form a sliding surface in view of a macroscopic point when the friction material is in contact with the driven plate (a corresponding sliding surface). For this reason, since a small contact area between the driven plate and the binder, and small original coefficients of friction of the binder, the driven plate result in a small coefficient of friction in the initial state. The binder wears out through the repetition of sliding action, as a result of which a soft fibrous base material is uncovered. This increases the contact area and makes the sliding surface smooth, thereby resulting in an initial running-in state in which the thus uncovered fibrous base material having a large coefficient of friction increases the coefficient of friction between the binder and the driven plate.
(2) The surface of the friction material has a high concentration of binder and lacks flexibility and smoothness, and hence contact between the friction material and the driven plate becomes uneven, thereby bringing about a microscopic wedge effect of an oil film. This wedge effect causes increased operation shock and shudder occurs.
(3) Since the surface of the friction material has a high concentration of binder, the friction material is prone to turn into plastic as a result of a sharp increase in temperature.
Through this running-in process, a torque capacity of the friction material changes from its value which is originally set when the friction value was new during a very short period of time. For this reason, the running in process is considered as a significant quality problem.
FIG. 8
is an explanatory view schematically showing the construction of a surface area of a conventional friction material, and
FIG. 9
is a surface contour line R showing the enlarged surface of the friction material. In these drawings, A is a binder (resin) part, B is a fiber part, and C is a filler. As can be seen from the drawings, the surface contour line R is not smooth (this conventional example is designated by L
2
).
As one example,
FIG. 5
shows the distribution of binder L
2
in a thicknesswise direction inside of a common friction material after it has set. In view of a product, the problem is that if the capacity of a clutch is designed based on a small friction coefficient of new friction material, a torque capacity increases as the friction coefficient varies in time sequence as a result of a running-in process, thereby bringing about operating shock. For an expensive luxury car, an extra learning function might be added for control. With a low friction coefficient of new friction material, the surface temperature of the friction material increases by frictional heating resulting from extension of a slid time under harsh driving environments. This, in turn, promotes the turning of the binder (thermosetting resin) into plastic because many binders are distributed around the surface layer a phenomenon in which the thermosetting resin around the surface of the friction material resets or becomes carbonized by frictional heating, so that a frictional surface becomes a mirror surface). The changing of the binder to plastic brings about a

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