JPH09119451A - Resinous friction member and damper disk assembly using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily obtain a high coefficient of friction in a resinous friction member. SOLUTION: This resinous friction member (bush) is equipped with a main body 15a, a snap projection 15c and a friction adjusting member 20. The main body 15a is resinous, and has a friction surface brought in contact with the side surface of a subplate 5. The snap projection 15c is integrally formed in a part of the main body 15a, and locked with a retaining plate 4. The friction adjusting member 20 is embedded in the friction surface of the main body 15a by mold forming, and is a member for adjusting the coefficient of friction of the friction surface.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a resin-based friction member,
In particular, the present invention relates to a resin friction member which is arranged between an input side member and an output side member of a damper disk assembly and which makes sliding contact with one of the members when the input side member and the output side member rotate relative to each other. The invention according to another aspect relates to a damper disk assembly, and more particularly to a damper disk assembly having a resin friction member between an input side member and an output side member.

[0002]

2. Description of the Related Art For example, a damper disc assembly used for a clutch disc assembly of a vehicle has a disc-shaped input side plate, an output side hub integrally having a flange on the outer periphery, and an input side plate and a flange. A torsion spring elastically coupled in the direction and a hysteresis torque generating mechanism arranged between the input side plate and the output side hub are provided.

The hysteresis torque generating mechanism has a friction member that is pressed against the flange of the output hub and a cone spring that presses the friction member against the flange.

[0004]

In order to stabilize the hysteresis torque, it is desirable that the friction member of the hysteresis torque generating mechanism is prohibited from rotating relative to either the input side plate or the flange. . Further, considering the assembling workability, it is desirable that the hysteresis torque generating mechanism is integrally attached to the input side plate or the output side hub.

Therefore, the friction member is provided with an engaging portion which engages with the input side plate and prohibits relative rotation of each other, and the friction member and the cone spring are integrated with the input side plate by utilizing this engaging portion. Have already been proposed. With this structure, the friction member always rotates relative to the output hub, so the hysteresis torque is stable, and the friction member and cone spring can be carried together with the input plate to improve workability during assembly. To do.

The engaging portion of the friction member as described above is configured so that the fitting portion which has elasticity and projects in the axial direction fits into the hole of the input side plate. Therefore, the friction member having such a structure is formed by resin molding. However, when the friction member is made of resin, the friction coefficient is generally small, so that a sufficient friction force cannot be obtained, and in particular, a shock during acceleration / deceleration cannot be effectively absorbed. Also, since the contact with the input side plate is not uniform in the initial stage,
The friction coefficient becomes unstable and the desired hysteresis torque cannot be obtained.

An object of the present invention is to easily obtain a high friction coefficient in a resin friction member. Another object of the present invention is to obtain a stable friction coefficient in a resin friction member. Still another object of the present invention is to obtain a damper disk assembly having the resinous friction member as described above.

[0008]

A resin friction member according to a first aspect of the present invention is disposed between an input side member and an output side member of a damper disk assembly, and the input side member and the output side member are relatively rotated. It is a resin-based friction member that comes into sliding contact with any one of the members when it is moved, and includes a main body portion, a locking portion, and a friction adjusting member. The main body is made of resin and has a friction surface that comes into contact with the friction surface of either the input side member or the output side member. The locking portion is integrally formed with a part of the main body portion and locks with a member different from the member with which the friction surface comes into contact. The friction adjusting member is a member that is provided on the friction surface of the main body and adjusts the friction coefficient.

Here, since the friction coefficient of the friction surface can be adjusted by the friction adjusting member, the friction coefficient can be easily increased, and the friction coefficient can be easily stabilized by applying a film or the like to the friction surface. . A resin friction member according to a second aspect of the present invention is the resin friction member according to the first aspect, wherein the friction adjusting member is a high friction member embedded in the friction surface of the main body by molding and having a higher friction coefficient than the surface of the main body. is there.

Here, since the friction adjusting member is integrally embedded by molding, it is easily and firmly mounted. Further, the hysteresis torque can be easily increased, and when used in the damper disk assembly, the shock during acceleration / deceleration can be effectively absorbed. A resin friction member according to a third aspect of the present invention is the resin friction member according to the first aspect,
The friction adjusting member is a film for adjusting the friction coefficient formed on the friction surface of the main body. With this film, the initial friction coefficient can be adjusted arbitrarily. In addition, it can be stabilized.

A resin friction member according to a fourth aspect of the present invention includes the first to third aspects.
In any of the resinous friction members of No. 3, the locking portion is locked to the input side member. A resin-based friction member according to a fifth aspect of the present invention is the resin-based friction member according to the fourth aspect of the present invention, wherein the locking portion has a retaining portion that prevents the engagement with respect to the input side member. Here, the resin-based friction member can be transported together with the input-side member, and the assembling workability is improved.

A resin friction adjusting member according to a sixth aspect of the present invention is the resin friction member according to the fifth aspect, wherein the engaging portion is an elastic fitting portion that is fitted into a hole formed in the input side member while elastically deforming. have. A damper disk assembly according to a seventh aspect of the present invention includes an input side member to which torque is input, and an output side member that is rotatable relative to the input side member and has a flange facing at least a part of the input side member. ,
An elastic connecting member that elastically connects the input side member and the output side member in the circumferential direction, a friction member, and a spring member are provided. The friction member is provided on the friction surface, a friction surface that contacts one of the input-side member and the output-side member, a locking portion that locks on a member different from the member that the friction surface contacts. A resin member having a friction adjusting member for adjusting the friction coefficient of the friction surface. The spring member is a member for press-contacting the friction surface of the friction member with one of the both members.

The damper disk assembly according to the eighth aspect of the present invention is
In the damper disk assembly of the seventh aspect of the present invention, the friction adjusting member is a high friction member having a higher coefficient of friction than other portions embedded in the friction surface by molding. In the damper disk assembly according to a ninth aspect of the present invention, the friction adjusting member is a film for adjusting the friction coefficient formed on the friction surface.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A clutch disc assembly 1 according to an embodiment of the present invention shown in FIG. 1 has torque from an engine (not shown) arranged on the left side of FIG. 1 arranged on the right side of FIG. It is a device for transmitting and shutting off to a transmission (not shown). In FIG. 1, OO is the rotation axis of the clutch disc assembly 1.

The clutch disc assembly 1 is mainly composed of
The hub 2 as an output side member, the clutch disc 3 and the retaining plate 4 as an input side member, the sub plate 5 as an intermediate member, the sub plate 5 and the hub 2
A small torsion spring 6 for elastically connecting the two in the circumferential direction, a large spring unit 7 for elastically connecting the plates 3, 4 and the sub-plate 5 in the circumferential direction, and the plates 3, 4 And a hysteresis torque generating mechanism 8 that generates a predetermined frictional force when relative rotation occurs between the hub 2 and the hub 2.

The hub 2 is arranged at the center of the clutch disc assembly 1 and is connected to a transmission shaft (not shown). The hub 2 has a cylindrical boss 2 extending in the axial direction.
a and a flange 2b integrally formed on the outer periphery of the boss 2a
It is composed of As shown in FIG.
As shown in FIG. 4, a plurality of protrusions 2c are formed at equal intervals in the circumferential direction. As shown in FIG. 3, the flange 2b
Two notches 2d for receiving both ends in the circumferential direction of the small coil spring 6 to be described later are formed at two locations opposed to each other in the radial direction. Further, on the inner peripheral side of the boss 2a, there is a spline hole 2e which is spline-engaged with the shaft of the transmission.
Are formed.

The sub-plate 5 is a disc-shaped plate and is arranged on the outer periphery of the flange 2b of the hub 2. As is apparent from FIG. 2, the sub-plate 5 has four protrusions 5a extending radially outward. Each protrusion 5a
A window hole 5b extending in the circumferential direction is formed in the. Outer notches 5c are formed between the protrusions 5a.
On the inner peripheral side of the sub-plate 5, an inner protrusion 5d is formed at a portion corresponding to the space between the protrusions 2c of the hub 2. Protrusion 2
A predetermined circumferential gap is secured between c and the inner protrusion 5d, which allows the hub 2 and the sub plate 5 to rotate by a predetermined angle. Inner notches 5e are formed at two locations on the inner peripheral side of the subrate 5 corresponding to the notches 2d of the hub 2. A small torsion spring 6 is arranged in the cutout 2d and the inner cutout 5e. In the neutral state shown in FIG. 2, the protrusion 2c is arranged on the rotation direction R2 side between the inner protrusions 5d. That is, the hub 2 makes a predetermined angle R2 with respect to the sub plate 5.
Twisted to the side.

The clutch plate 3 and the retaining plate 4 are arranged on both sides of the sub plate 5. The clutch plate 3 and the retaining plate 4 are a pair of substantially disk-shaped members, and are rotatably arranged on the outer peripheral side of the boss 2 a of the hub 2. The clutch plate 3 and the retaining plate 4 are fixed to each other by a contact pin 11 on the outer peripheral portion. The contact pin 11 is inserted through an outer cutout 5c formed in the sub plate 5.
Since a predetermined gap is secured in the circumferential direction between the contact pin 11 and the outer notch 5c, the plates 3, 4 and the sub-plate 5 can be relatively rotated.

A friction coupling portion 10 is arranged on the outer periphery of the clutch plate 3. The friction coupling portion 10 mainly includes a cushioning plate 12 and an annular friction facing 1 that is fixed to both sides of the cushioning plate 12.
And 3. Cushioning plate 12
Has an annular portion 12a continuous in an annular shape and a plurality of cushioning portions 12b formed so as to project from the annular portion 12a to the outer peripheral side. A part of the annular portion 12a projects to the inner peripheral side, and the projecting portion is fixed to the clutch plate 3 by the contact pin 11.

The clutch plate 3 and the retaining plate 4 are formed with window holes 3a and 4a at positions corresponding to the window holes 5b of the sub-plate 5, respectively.
The large coil spring unit 7 is arranged in the window holes 3a and 4a. On both sides in the radial direction of each of the window holes 3a and 4a, a pressing portion 3a cut and raised outward in the axial direction,
The pressing portion 4b is formed.

The large spring units 7 each include two sets of a first spring assembly 7a and a second spring assembly 7.
b. First spring assembly 7a
Are arranged in the window holes 5b of the sub-plate 5 facing each other in the radial direction, and are composed of a large-diameter torsion spring and a small-diameter torsion spring arranged inside thereof. The circumferential ends of the first spring assembly 7a are circumferentially opposite to the window holes 5b of the sub-plate 5, circumferentially opposite the window holes 3a of the clutch plate 3, and the circumferential direction of the window hole 4a of the retaining plate 4. It is in contact with both ends in the direction. Further, the second spring assembly 7b is arranged in the window hole 5b of the sub-plate 5 opposed to each other in the radial direction, and is composed of the torsion spring and the flow travers 7c arranged inside thereof. The circumferential ends of the torsion spring of the second spring assembly 7b have circumferential ends of the window hole 5b of the sub-plate 5, circumferential ends of the window hole 3a of the clutch plate 3 and the window hole 4a of the retaining plate 4. Are in contact with both ends in the circumferential direction. However, the length of the flow travers 7c is shorter than the circumferential ends of each window hole. That is, the flow travers 7c are movable in the circumferential direction between the circumferential ends of each window hole.

Next, the hysteresis torque generating mechanism 8 will be described. The hysteresis torque generating mechanism 8 is shown in FIG.
Further, as shown in FIG. 5, it is arranged on the outer peripheral side of the boss 2a between the inner peripheral portion of the clutch plate 3 and the inner peripheral portion of the retaining plate 4. FIG. 6 is a view of the hysteresis torque generating mechanism 8 viewed from a different direction from FIGS. 1 and 5.
The hysteresis torque generating mechanism 8 is provided in the first bush 1
4, the second bush 15, the first cone spring 16, the second cone spring 17, and the third bush 18. The first to third bushes 14, 15 and 18 are each made of nylon resin.

The first bush 14 is a resin disk plate. As shown in FIGS. 5 to 7, the first bush 14 has an inner peripheral end close to the boss 2 a and one side surface of the first bush 14.
Of the flange 2b and the projection 2c on the transmission side. The first bush 14 includes the disc portion 14
a and an annular protruding portion 14b protruding from the inner peripheral side of the disk portion 14a toward the transmission side. An annular cutout groove 14c is formed on the transmission side of the annular protrusion 14b. Further, four protrusions 14d extending outward in the radial direction are formed on the outer circumference of the disc portion 14a.

A first cone spring 16 is arranged between the first bush 14 and the retaining plate 4. An outer peripheral end of the first cone spring 16 is supported by the retaining plate 4, and an inner peripheral end of the first cone spring 16 is in contact with an annular cutout groove 14c of the first bush 14. The first cone spring 16 is arranged in a compressed state and urges the first bush 14 toward the flange 2b and the protrusion 2c of the hub 2. As shown in FIG. 11, the first cone spring 16 is formed with a plurality of notches 16a on the outer peripheral side. The notch 16a is formed to reduce a change in biasing force of the first cone spring 16 when the posture of the first cone spring 16 changes due to wear of the first bush 14.

The second bush 15 is a disk-shaped member, as is apparent from FIGS. 8 to 10, and on the outer peripheral side of the first bush 14 is substantially flush with the plane on which the first bush 14 is arranged. It is located concentrically. Second bush 15
Is composed of a disc-shaped main body portion 15a and an annular protruding portion 15b which protrudes toward the transmission side on the inner peripheral side of the main body portion 15a. The engine side end surface of the main body portion 15 a is in contact with the inner peripheral end surface of the sub plate 5. This body 1
The end surface of 5a on the engine side is a friction surface, and as shown in an enlarged view in FIG. A friction adjusting member 20 such as ceramic is integrally formed by molding. On the transmission-side end surface of the annular protrusion 15b, 4 are evenly spaced in the circumferential direction.
One notch 15e is formed. This notch 15e
The protrusion 14d of the first bush 14 is engaged therein so as to be relatively non-rotatable in the circumferential direction and movable in the axial direction. A predetermined gap is secured between the protrusion 14d and the recess 15e in the axial direction. The annular projection 15b has a recess 15
Four protrusions are formed between e and serve as locking portions extending toward the transmission side. This projection is composed of two snap projections 15c and two rod-shaped projections 15d. Here, the protrusions of the same type are arranged so as to face each other in the radial direction. The snap projection 15c is divided into two by a slit extending in the axial direction, and is elastically deformable in the radial direction. Further, the tip of the snap projection 15c has a hook-shaped snap (retaining portion) 15f. The snap projection 15c is inserted into the hole 4c formed in the retaining plate 4. The second bush 15 is hard to come off from the retaining plate 4 in the axial direction by the snap 15f formed on the snap projection 15c. The rod-shaped projection 15d is inserted into the other hole 4c of the retaining plate 4.

The second cone spring 17 is arranged between the second bush 15 and the retaining plate 4. As shown in FIG. 12, the second cone spring 17 is formed with a plurality of notches 17a on the inner peripheral side. The notch 17a is formed to reduce the change in the urging force of the second cone spring 17 when the posture of the second cone spring 17 changes due to the wear of the second bush 15. Further, the notch 17 a portion of the second cone spring 17 is formed by the snap projection 15 of the second bush 15.
c, the rod-shaped projection 15d, and the recess 15e, so that the second cone spring 17 and these members do not interfere with each other. Further, the respective projections 15c, 15e pass through the notches 17a and extend toward the transmission side, whereby the relative rotation between the second bush 15 and the second cone spring 17 is prohibited. The second cone spring 17 is arranged in a compressed state, and the outer peripheral end abuts on the retaining plate 4, and the inner peripheral end, that is, the protrusion 17b abuts on the transmission side surface of the annular protrusion 15b of the second bush 15. ing. In this way,
The second cone spring 17 urges the second bush 15 to the side surface of the sub plate 5 on the transmission side.
The biasing force of the second cone spring 17 at this time is set to be larger than the biasing force of the first cone spring 16.

As shown in FIGS. 1 and 14, the third bush 18 is arranged between the inner peripheral portion of the clutch plate 3 and the flange 2b of the hub 2 and the inner peripheral end of the sub plate 5, and has a circular shape. It has a plate-shaped main body portion 18a and a plurality of snap projections 18b axially protruding from one side surface of the main body portion 18a. The main body 18a has a friction side surface on the transmission side, and contacts the side surface of the flange 2b and the side surface of the inner peripheral end of the sub plate 5. The opposite side surface is in contact with the clutch plate 3. On the friction surface on the transmission side that abuts the side surface of the sub-plate 5, for example, a rubber-based or glass-based mixed-spun or impregnated molded product for friction coefficient adjustment, and a friction adjusting member 21 such as ceramic are integrally formed by molding. Has been done. The snap projection 18b is engaged in the hole 3c formed in the clutch plate 3. This snap projection 18b
Is divided into two by a slit. Therefore, elastic deformation is possible in the radial direction. An annular protrusion 18c extending axially toward the engine is formed on the inner peripheral portion of the third bush 18. The inner peripheral end of the clutch plate 3 is in contact with the outer peripheral side of the annular protruding portion 18c.

Next, the operation of the clutch disc assembly 1 will be described. When the friction facing 13 is pressed against a flywheel (not shown) on the engine side, the torque of the flywheel on the engine side increases the clutch plate 3.
And to the retaining plate 4. This torque is applied to the large coil spring unit 7, the sub plate 5,
It is transmitted to the hub 2 via the small coil spring 6 and further transmitted to a shaft (not shown) on the transmission side.

Flywheel on the engine side (not shown)
When torsional vibration having a small displacement angle is transmitted from the clutch disk assembly 1 to the clutch disk assembly 1, relative rotation occurs between the plates 3 and 4, the sub plate 5 and the hub 2. At this time, the small torsion spring 6 repeatedly expands and contracts in the circumferential direction,
The first bush 14 and the third bush 18 slide on the flange 2b and the protrusion 2c of the hub 2. At this time, due to the characteristics of low rigidity and small frictional force (low hysteresis torque), torsional vibration with a small displacement angle is effectively damped.

When a torsional vibration having a large displacement angle is transmitted to the clutch disc assembly 1, the small torsion spring 6 is compressed and the sub-plate 5 and the hub 2 rotate integrally with each other. Relative rotation occurs between and. At this time, the large spring unit 7 repeatedly expands and contracts, the first bush 14 slides on the flange 2b of the hub 2, the second bush 15 slides on the inner peripheral side surface of the sub-plate 5, and the third bush 18 further moves. Slides on the flywheel side surface of the flange 2b of the hub 2 and the flywheel side surface of the inner peripheral side of the sub plate 5. Here, the second
Since the biasing force of the cone spring 17 is set to be larger than the biasing force of the first cone spring 16, a large frictional force is generated. Also, the friction surface of the second bush 15 with the sub-plate 5 and the sub-plate 5 of the third bush 18
The friction adjusting members 20, 21 having a high friction coefficient
Is embedded by molding, the frictional force becomes larger. Due to such characteristics of large rigidity and large friction force (high hysteresis torque), the torsional vibration having a large displacement angle is effectively damped.

As described above, since appropriate characteristics can be obtained depending on the type of torsional vibration, the clutch disc assembly 1
Is effective in damping the torsional vibration. When assembling the clutch disc assembly 1, the first bush 14, the second bush 15, the first cone spring 16 and the second cone spring 17 of the hysteresis torque generating mechanism 8 are assembled to the retaining plate 4 in advance,
Keep it sub-assy. This assembling work can be easily performed only by inserting the snap projection 15c and the rod-shaped projection 15d of the second bush 15 into the hole 4c of the retaining plate 4. Since the second friction washer 15 is engaged with the retaining plate 4 by the snap 15f of the snap projection 15c, each member does not fall off.
When the assembly is made into a sub-assembly in this way, the work can be facilitated because the sub-assembly can be managed before the entire assembly.
Further, even when the whole is assembled, the work efficiency is significantly improved because it is a sub-assembly.

[Other Embodiments] In the above embodiment, a friction adjusting member such as a sheet material having a high friction coefficient is embedded in the friction surface of each bush by molding, but the structure for adjusting the friction coefficient is the same as that of the above embodiment. It is not limited to the form. For example,
As shown in FIG. 15, coatings 25 and 26 formed by, for example, thermal spraying of zinc, phenolic resin or the like are provided on the friction surfaces of the first bush 14 and the third bush 18 with the respective flanges 2b. The coefficient of friction is made smaller than the other parts, and conversely, the second bush 15 and the third bush 18 are
Similarly, coatings 27 and 28 formed by thermal spraying or the like may be provided on the friction surface with each of the sub-plates 5 to make the friction coefficient larger than other portions.

As a result, a smaller hysteresis torque can be obtained for a torsional vibration with a small amplitude and a larger hysteresis torque can be obtained for a torsional vibration with a large amplitude. Can be absorbed. A member that stabilizes the friction coefficient may be used as the friction coefficient adjusting member. In this case, a film such as a copper-based lubricating member may be formed on the friction surface.

[0034]

As described above, according to the present invention, since the friction adjusting member is provided on the friction surface of the resin friction member, the friction coefficient of the friction surface can be easily adjusted. Further, in the resin-based friction member, a stable friction coefficient can be easily obtained. Further, in the present invention, since the friction coefficient of the friction surface of the resinous friction member can be adjusted, the shock during acceleration / deceleration can be effectively absorbed.

[Brief description of the drawings]

FIG. 1 is a schematic vertical cross-sectional view of a clutch disc assembly according to an embodiment of the present invention.

FIG. 2 is a partially cut-away view taken along the arrow II in FIG.

FIG. 3 is a plan view of the hub.

FIG. 4 is a sectional view taken along line IV-IV of FIG. 3;

5 is an enlarged view of a circle A portion of FIG.

FIG. 6 is a plan view of a first bush.

7 is a sectional view taken along line VI-VI in FIG.

FIG. 8 is a plan view of a second bush.

FIG. 9 is a sectional view taken along line IX-IX of FIG. 8;

10 is a sectional view taken along line XX of FIG.

FIG. 11 is a plan view of a first cone spring.

FIG. 12 is a plan view of a second cone spring.

FIG. 13 is an enlarged partial view of FIG.

FIG. 14 is an enlarged partial view of FIG.

FIG. 15 is a view corresponding to FIGS. 13 and 14 of another embodiment.

[Explanation of symbols]

 1 Clutch Disc Assembly 2 Hub 3 Clutch Plate 4 Retaining Plate 5 Sub Plate 6 Small Torsion Spring 7 Large Spring Unit 8 Hysteresis Torque Generation Mechanism 14 1st Bush 15 2nd Bush 15c Snap Projection 15d Rod-like Projection 16 1st Cone Spring 17 Second cone spring 20, 21 Friction adjusting member 25-28 Film

Claims (9)

[Claims] 1. A resin friction member which is arranged between an input side member and an output side member of a damper disk assembly, and which is in sliding contact with either one of the input side member and the output side member when the input side member and the output side member are relatively rotated. A member, which is a resin-based main body having a friction surface that comes into contact with the friction surface of one of the input-side member and the output-side member, and the friction surface formed integrally with a part of the main-body portion. A resin-based friction member, comprising: a locking portion that is locked to a member different from a member that comes into contact with the friction surface; and a friction adjusting member that is provided on a friction surface of the main body and adjusts a friction coefficient thereof. 2. The resin friction member according to claim 1, wherein the friction adjusting member is a high friction member which is embedded in the friction surface of the main body portion by molding and has a higher friction coefficient than the surface of the main body portion. 3. The resin friction member according to claim 1, wherein the friction adjusting member is a film for adjusting a friction coefficient formed on a friction surface of the main body portion. 4. The resin friction member according to claim 1, wherein the engagement portion is engaged with the input side member. 5. The resin friction member according to claim 4, wherein the engaging portion has a retaining portion that prevents the retaining member from coming off the input side member. 6. The resin friction member according to claim 5, wherein the engaging portion has an elastic fitting portion that fits into a hole formed in the input side member while elastically deforming. 7. An input side member to which torque is input, an output side member having a flange that is rotatable relative to the input side member and faces at least a part of the input side member, and the input side member. And an elastic coupling member that elastically couples the output side member in the circumferential direction, a friction surface that contacts one of the input side member and the output side member, and a member that is different from the member that the friction surface contacts. A resin-based friction member having a locking portion that is locked to, and a friction adjusting member that is provided on the friction surface and that adjusts the friction coefficient of the friction surface; A damper disk assembly comprising: a spring member for press-contacting either one of them. 8. The damper disk assembly according to claim 7, wherein the friction adjusting member is a high friction member embedded in the friction surface by molding and having a higher friction coefficient than other portions. 9. The damper disk assembly according to claim 7, wherein the friction adjusting member is a film for adjusting a friction coefficient formed on the friction surface.