Friction unit for synchronization device of transmission

The friction surfaces (4) of two essentially annular friction elements (1) which, through relative motion, can be arranged to transmit torque, are each approximately formed like the surface of a spherical layer whose center (M) is situated on the axis (5) of the annular frictional elements (1) outside the bottom plane of the spherical layer. This is a simple way to ensure a whole-surface, interlocking contact and a good friction performance over a large number of cycles even during a minor tilt of the two cooperating friction surfaces (4).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a friction unit, especially for synchronization 
devices of vehicle manual transmissions, which include two essentially 
annular friction elements, which, through relative motion, can be arranged 
to transmit torque through their contacting friction surfaces. 
2. The Prior Art 
Friction units of this kind are known and, for instance in couplings or 
brakes, or particularly in the aforementioned synchronization devices of 
gear boxes, enable a controlled friction-tight torque transmission. Thus, 
friction elements with essentially disk-shaped friction surfaces are 
known, but these present the drawback of a relatively high power 
requirement in order to press together the friction surfaces for the 
transmission of a specific torque. For this reason, especially in 
compactly built arrangements, frictional elements have become common in 
which the friction surfaces are conical, whereby, through appropriate 
selection of the cone angle, transmission of the required torque can be 
ensured even by means of a relatively minor coupling force. 
In the case of the aforementioned manual transmissions, annular frictional 
elements are used as so-called synchronization rings for synchronization 
of the revolutions in the conical arrangement described. These frictional 
elements generally consist of a ring-shaped substrate or support made of 
brass or steel and a friction lining made of abrasive material, like for 
instance organic friction coatings or sintered friction coatings. During 
actual synchronization, the frictional element, having an interior and/or 
exterior friction surface, is pressed against one or two opposing conical 
surface(s), whereby the revolutions of the two gear shafts to be 
synchronized are brought into phase. With a manual transmission this is 
necessary during each shift from one gear to another. 
Commensurate with the applicable state of the art, the designs of such 
frictional elements within the various manual transmissions differ very 
widely. However, with regard to their conical designs the friction 
surfaces show a commonality which, as mentioned, has proved advantageous 
due to space limitations in the gear boxes. 
By analysis of a great number of friction units of the type mentioned that 
were inadequate with regard to a satisfactory synchronization effect, it 
was found that the required friction moments for adequate torque 
transmission often could no longer be attained after a relatively small 
number of shift or synchronization cycles, and up to now there has been no 
logical explanation for this. 
SUMMARY OF THE INVENTION 
Taking as a basis the known problems addressed above, the present invention 
starts out with the assumption that the conical design of the friction 
surfaces acting together suffers from the fundamental defect that, during 
the shifting process or the initiation of synchronization (which also 
holds true for couplings, brakes and the like), the friction cone itself, 
which is short because of a relatively narrow annular friction element, 
must be pushed onto the cone which is its counterpart, and it cannot be 
guided during that process. As such, a slight tilt of the two cones in 
relation to each other may occur. This results in the two friction 
surfaces not lining up surface to surface, so that relatively small, 
basically sickle-shaped friction surfaces will be stressed 
disproportionally or will even be overstressed. The consequence is 
increased wear at the overstressed parts. The synchronization device, or 
generally, the friction device, due to the poor contact pattern, does not 
attain the required friction moments and finally fails. As a result of the 
unequal contact, the contacting friction areas are finally thermally 
overstressed and damaged. With friction elements operating within oil 
having a high additive content, reactions occur at the thermally 
overstressed friction areas, which all told, goes along with an ever 
diminishing friction result. 
Beginning with the stated problems or the above-mentioned considerations, 
the present invention provides a solution to the challenge of enabling an 
even, friction-tight contact of the actual friction surfaces, or to 
exclude an uneven contact. Its construction design and the manufacturing 
process should be appropriately easy to realize. 
In accordance with the invention, the above-mentioned task involving a 
friction unit of the aforementioned type is accomplished by having the 
friction surfaces in each case shaped at least approximately like the 
surface of a spherical layer whose center is situated on the axis of the 
annular frictional element outside the bottom plane of the spherical layer 
(the bottom plane is defined, both here and in the following, as the 
imaginary plane defined by the end of the spherical layer at the side 
nearest to or coincident with the larger opening of the annular friction 
element). The cooperating friction surfaces therefore are always 
essentially shaped like parts of a spherical surface, as a very simple way 
to prevent any tilt during their engagement for friction-tight 
cooperation. This simple solution to the problems discussed above is all 
the more astonishing when the friction behavior results are examined. With 
immersed friction units, the friction moment achieved could, for instance, 
be kept constant from one oil to another, while the oil type had made a 
difference before, and over at least 10,000 cycles. The performance with 
regard to wear, which was gauged by the gradually occurring axial shift 
during cooperation of two partnered-up friction surfaces over a period of 
time, and which remained practically constant after 2,000 cycles, was even 
more astonishing. 
It must be pointed out here that the above directions for the friction unit 
design or actual embodiment according to the invention must not be 
interpreted in a strict geometrical sense, because by its very nature, 
deviations within the scope of the usual tolerances or rather, tolerance 
pairings, can readily be tolerated. Besides, the friction pairs used these 
days always exhibit some elasticity, so that deviations from the strict 
geometrical relation mentioned for the spherical layer formation of the 
friction surfaces, within a certain scope, will have no influence on the 
friction performance of the friction unit. The point is that the friction 
surfaces operating together during the beginning of their friction-tight 
contact may be tilted considerably in relation to one another with regard 
to their shaft angles, but this still allows for practically whole-surface 
positive locking of the two friction surfaces. Only a minor portion of the 
friction surfaces, which then does not contact due to tilting, is lost for 
torque transmission, but this is at most a small percentile. 
The mentioned geometrical relation shall further be thus understood that, 
with the embodiment of the friction unit in accordance with the invention, 
naturally the focus must always be on the friction system itself, that is, 
on the two friction surfaces actually working together, or their absolute 
size and angle alignment, because the primary task is to ensure that the 
desired torque may be transmitted under the given conditions. For this 
purpose an appropriate cone angle generally is determined, together with 
the diameter and width of the friction surfaces initially assumed to be 
conical. Afterward the center of the surface of the relevant spherical 
layer is placed in such a way that, in relation to this spherical layer, 
it lies outside of its bottom plane (usually the lateral border of the 
ring), or in any case just barely within this bottom plane, on the 
imaginary central axis of the friction elements, as this is the only way 
to ensure that the two cooperating friction surfaces can indeed be axially 
moved in relation to one another. If it should occur that, due to a 
disproportionate ring radius or diameter vis-a-vis the ring width and the 
cone angle, the center of this spherical layer comes to be within the ring 
width or spherical layer, then, by correcting the aforementioned 
condition, a correction can be made to ensure the geometrical relationship 
with regard of the position of the center of the spherical layer. 
During experiments with synchronization devices of vehicle drives embodied 
according to the invention, friction moments with constant frictional 
values, a considerable oil independence and an only slight change 
(increase) in axial moveability over many thousand shift cycles resulted. 
No overstressed friction surface areas of the kind initially described in 
relation to drawbacks with the state of the art were found. 
According to the invention, the process for manufacturing essentially 
annular friction elements, specifically for synchronization devices for 
vehicle manual transmissions, is characterized by first producing the 
friction surfaces as conical surfaces with a cone angle chosen for torque 
transmission, and in a subsequent calibration step at least approximately 
develop the friction surfaces as the surface of a spherical layer with its 
center on the axis of the annular friction elements outside the bottom 
plane of the spherical layer. 
In accordance with advantageous embodiments of the process described, the 
calibration step may be carried out either by grinding the friction 
surface, or, in case of sintered friction coatings, by stamping with a 
stamping tool having the opposite surface of the surface of the spherical 
layer. In case of a friction ring with a powder metal sintered coating 
working together with a metallic opposite surface, the metallic friction 
surface can be ground accordingly and the sintered friction surface be 
stamped as described, and it is unimportant whether the basically conical 
shape is produced before calibration and before or after sintering of the 
frictional coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the friction unit includes an outer annular friction 
element 1 and a cooperating inner annular friction element 1'. The outer 
annular friction element 1 includes a support ring 2, for instance made of 
steel or brass, which on its inner surface is provided with a powder metal 
sintered friction layer 3, and the inner annular friction element 1' 
includes a support ring 2', also made of, e.g., steel or brass, and a 
powder metal sintered friction layer 3' on an outer surface thereof. The 
actual friction surface of element 1 is found at the free inner surface of 
friction layer 3 and has a generally conical arrangement in relation to 
axis 5 of friction element 1, whereby one angle Y is chosen or calculated 
according to known and, in connection with such arrangements, routine 
calculation methods for torque transmission, taking into account the width 
Z and radius X of friction surface 4. The outer friction surface of 
friction layer 3' is similarly configured. 
Friction surface 4 deviates from a pure cone surface, as exhibited by 
support 6 at the transition toward support ring 2, and is at least 
approximately shaped like the surface of a spherical layer whose center M 
here lies at the intersection of axis 5 of friction elements 1,1' with a 
radius, R, on the cone angle Y selected for torque transmission, of half 
of the width of the friction surface (Z/2). As can be seen from FIG. 1, 
the here present proportions of ring radius X to ring width Z to cone 
angle Y result in locating the spherical layer center point M further from 
the lower end 3A of the friction ring 3 than its upper end 3B and at the 
penetration point of axis 5 through the bottom plane B of the friction 
layer, which here coincides with the upper plane of the upper end of the 
illustrated friction element 1. It would also be possible to locate M 
above the upper plane of friction element 1, within the illustration 
according to FIG. 1. If M were to lie within the width Z of the friction 
element 1 or the friction surface 4 (between upper plane B and lower plane 
A of the friction layer), then the two cooperating friction elements 1 
could no longer be axially moved relative to one another. In that case the 
proportion of X:Z:Y would have to be appropriately adapted or changed. 
As seen in the figures, the cooperating friction element 1' includes an 
inner ring 2' which supports a friction layer 3' providing a similar 
opposing outer surface in relation to friction surface 4 which, during 
initiation of the friction-tight torque transmission, is pressed 
predominantly concentrically and in the direction of axis 5 relative to 
friction element 1, and may readily tilt vis-a-vis friction element 1, 
whereby nevertheless a large-surface, interlocking contact of the two 
cooperating friction surfaces 4 will occur. 
The friction moment in each case and in this embodiment (just as with the 
purely conical arrangement of the cooperating friction surfaces known up 
to now) also results from the ring diameter or ring radius X, the ring 
width Z and cone angle Y, which is easy to understand, considering that 
(as illustrated in FIG. 2), the actual "cone angle" in the upper region of 
the friction element is smaller, but conversely in the lower part oE the 
friction ring correspondingly larger than the cone angle Y ascribed 
tangentially in the center. 
In a particular example of the embodiment a single cone ring 1 with a 
friction diameter of x=72 mm is sintered with a metallic friction coating 
and provided with a cone angle Y of 6.50.degree.. At a ring height Z of 
7.9 mm the design is realized in such a way that the radius of curvature 
of the friction surface is 36 mm. The opposite cone is given the same 
radius of curvature, which is produced by grinding. Both parts, the 
friction ring and the opposite cone, during this design solution and 
within a certain angle range, can operate in a friction-tight manner 
without tilting. Dynamic friction values of 0.11 to 0.12 resulted no 
matter whether mineral oil (such as SAE 75W-G14) or synthetic oil (such as 
EGL284) were used. After 10,000 cycles an axial deviation of 0.08 to 0.12 
was determined. 
In another example a double cone friction ring with a friction diameter of 
X=100 mm in the shape of a flat part was sintered with a friction coating 
and subsequently reshaped, for instance, according to Austrian Patent No. 
385,826. The design of the friction ring was realized with a ring height Z 
of 9.8 mm and a radius of curvature R of 50.015 mm on the outside and 
45.004 mm on the inside (with 2.times. the ring thickness subtracted). 
With this friction ring, the tangent angle Y was 7. 
The insertion point M for the radius of curvature was established at the 
intersection of the tangent vertical and ring axis 5. The opposing cones 
on the outside and inside were provided with the same radius of curvature. 
The friction ring was tested for 10,000 cycles in wholly synthetic oil 
(such as BOT 72/94) and in mineral oil (such as SAE 74W-GL4). Both tests, 
independently from the specific axial force of 2 to 6 N/mm.sup.2, yielded 
a friction value of 0.105 to 0.115 and an axial deviation of 0.1 mm. The 
axial deviation occurred within the first 2,000 cycles and after that, no 
change could be determined. The friction ring showed an even contact 
pattern, inside and outside.