JP2002161981A - Seal ring and manufacturing method of seal ring and oil hydraulic seal device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To contribute to an improvement of fuel consumption of automobiles by means of reducing friction torque as much as possible on sliding contact portions without losing sealing performance even under the interaction of high pressure oil and in the case that a counterpart is non-ferrous metal such as aluminum alloy and the like. SOLUTION: In the seal ring made of fiber reinforced synthetic resin, not only a tip of fiber which protrudes in a sealing surface and causes wear acceleration of the counterpart is removed but also solid lubricant is made to adhere on the sealing surface or a coat is made to form on it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seal ring mainly used for sealing hydraulic oil of a relative rotating portion in an automatic transmission of an automobile or the like, a method of manufacturing the seal ring, and a hydraulic seal device provided with the seal ring. It is.

[0002]

2. Description of the Related Art Conventionally, an automatic transmission used for a vehicle such as an automobile includes a torque converter, a gear, a brake, and a multi-plate clutch. It is necessary to adopt a configuration in which this clutch engagement is performed by hydraulic pressure. For this reason, the hydraulic circuit requires a seal in a part that rotates relatively, a seal ring is installed in one annular groove between the members, and the seal ring is pressed against the outer periphery (or the inner periphery) of the mating member by hydraulic pressure. It comes into sliding contact with the side wall surface of the ring groove. The seal ring has conventionally been made of cast iron, but in recent years has been replaced by a synthetic resin seal ring which is more easily adhered to a mating member and has excellent sealing properties.

[0003]

In such conventional synthetic resin seal rings, glass fibers or fibrous fillers are generally added in order to improve their mechanical strength and wear resistance. However, when a fiber-reinforced synthetic resin seal ring is manufactured by injection molding, the fiber tip also protrudes on the sealing surface of the seal ring, and the protruding fiber tip wears a mating material to be in sliding contact with, thereby deteriorating the sealing performance. Resulting in. In particular, when the mating member to be in sliding contact is a non-ferrous metal such as an aluminum alloy and glass fiber is added to the synthetic resin, the abrasion of the mating member (non-ferrous metal) due to the tip of the fiber protruding from the sealing surface becomes remarkable. .

[0004] Further, since the synthetic resin seal ring has a good adhesiveness and a large abrasion torque on a sliding contact surface with a mating member, various friction torque reduction techniques have been used. Specifically, for example, as shown in FIG. 12, there is a type in which an oil reservoir pocket is set on the inner peripheral side of the side surface of the seal ring. In such a seal ring, the operating oil is supplied to the sliding contact surface from the oil reservoir pocket, so that an oil film is easily formed on the sliding contact surface, and the friction torque is reduced. However, in the seal ring as described above, the seal area on the side surface of the seal ring is reduced in the oil reservoir pocket setting portion, so that the sealability under high oil pressure is lower than that of a seal ring having a flat side surface shape. In addition, when contaminants are present in the hydraulic oil or when abrasion powder is generated on the sliding surface, the foreign matters are easily taken into the oil reservoir pockets, and the taken foreign matters are maintained in the rotation direction. Therefore, there is a problem in that the sliding contact surface of the oil sump installation portion is abnormally worn, and the sealing performance is deteriorated.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such problems of the prior art. In a fiber reinforced synthetic resin seal ring, the seal ring protrudes from the seal surface of the seal ring, which causes abrasion of a mating member. Along with removing the fiber tip,
By applying a solid lubricant on the sealing surface or forming a coating of lubricant, the sliding contact portion can be maintained without impairing the sealing performance even under high hydraulic pressure and even when the mating member is a non-ferrous metal such as an aluminum alloy. It is intended to contribute to improving the fuel efficiency of automobiles by reducing the friction torque of the vehicle as much as possible.

[0006]

According to a first aspect of the present invention, there is provided a seal ring, wherein a seal ring mounted in an annular seal ring groove set on an outer periphery of a shaft member has a radial direction. A ring inner peripheral surface facing inward, a ring outer peripheral surface parallel to the ring inner peripheral surface and facing outward in the radial direction, and a pair of ring side surfaces facing both sides in the axial direction,
When hydraulic pressure is applied, the outer peripheral surface of the ring is pressed against the inner peripheral surface of the housing of the housing that is rotatably fitted to the outer periphery of the shaft member, and the seal side holds the hydraulic pressure by pressing the side surface of the ring against the side surface of the seal ring groove. In the, the seal ring body is made of a fiber reinforced synthetic resin, and at least one of the seal surface of the ring outer peripheral surface and the ring side surface has a fiber tip protruding from the ring surface removed, and a lubricant adheres to the surface, Alternatively, it is characterized in that a lubricant film is formed.

In the seal ring according to the present invention, the thickness of the lubricant adhered on the seal surface of the seal ring or the formed lubricant film is 50 μm. It can be:

Further, in the seal ring according to the present invention, as described in claim 3, the lubricant having a coating or a film formed on the sealing surface of the seal ring is a solid lubricant. Things.

Furthermore, the seal ring according to the present invention can be used for a hydraulic seal of a relative rotating portion of an automatic transmission used for a vehicle such as an automobile. .

[0010] Furthermore, the hydraulic seal device according to the present invention,
As described in claim 5, the seal ring can be in sliding contact with a nonferrous metal counterpart material.

Further, in the method for manufacturing a seal ring according to the present invention, the solid lubricant powder is shot peened on the seal surface of the seal ring, and the seal surface is formed on the seal surface. At the same time as removing the protruding fiber tips, a solid lubricant can be attached to the sealing surface or a film of the solid lubricant can be formed.

Further, according to the method for manufacturing a seal ring according to the present invention, powder of a solid lubricant having an average particle diameter of 100 μm or less is formed on the seal surface of the seal ring. A method for manufacturing a seal ring that performs shot peening by spraying at an injection speed of 100 m / sec or more can be provided.

[0013]

FIG. 2 is an enlarged view showing a use state of a hydraulic seal portion of an automatic transmission having a seal ring according to the present invention.

In FIG. 2, if oil pressure is supplied from the oil passage 17 of the shaft member 16 to the oil passage 19 of the housing 18, the oil passage is also transmitted to each seal ring groove 20 and the inner peripheral surface of each seal ring 10 is provided. In order to act on the inner side surface 21a and the inner side surface 12i, the seal ring 10 has its outer peripheral surface 21a pressed against the inner peripheral surface 22 of the housing 18 and the outer side surface 12o, that is, the sealing surface pressed against the sealing side surface 23 of the sealing groove 20. A sealing effect is exhibited. In such a state, the housing 1
8 rotates with respect to the shaft member 16 so that each seal ring 1
0, the friction torque generated between the outer peripheral surface 21b and the inner peripheral surface 22 of the housing 18 is larger than the friction torque generated between the seal side surfaces (12o, 23) of the ring 10 and the seal ring groove 20. The seal ring 10 rotates with the housing 18 to cause relative rotational movement on both sides of the seal.

When the seal ring body is made of synthetic resin having excellent sealing properties in the sealing device, a fibrous filler is added to the synthetic resin from the viewpoint of improving the mechanical strength and wear resistance of the seal ring. However, in the case of a seal ring made of fiber-reinforced synthetic resin, the tip of the fiber having high rigidity also protrudes on the sealing surface, and the tip of the fiber wears the sealing side surface of the seal ring groove serving as the mating member, and the sealing is performed. The nature will deteriorate. Therefore, in the seal ring of the present invention, the aggressiveness of the fiber-reinforced synthetic resin seal ring on the mating member is reduced by removing the fiber tips protruding on the seal surface of the seal ring, and the good sealability is maintained for a long time. I do.

In addition, the seal ring made of the fiber-reinforced synthetic resin of the present invention has a lubricant adhered or coated on the seal surface in order to reduce a friction torque generated between both seal side surfaces of the ring and the seal ring groove. ing. In the initial stage of sliding, the lubricant is transferred to the seal side surface of the seal ring groove, which is the mating member, and the sliding portion of the seal ring and the seal side surface of the seal ring groove slides relative to each other, causing friction. The torque is reduced.

Since the purpose of transferring the lubricant, which is adhered or formed on the sealing surface of the seal ring to the sealing surface of the seal ring, to the sliding surface of the mating member in the initial stage of sliding to reduce the friction torque, is to reduce the friction torque. The thickness of the lubricant is set to 50 μm or less since there is only a minimum thickness that can be transferred to the lubricant. When the thickness of the lubricant is made larger than this, the lubricant is easily separated from the surface of the seal ring, so that the transfer of the lubricant to the partner member is less likely to occur, and the wear of the lubricant itself is promoted. As a result, a continuous friction torque reduction effect cannot be obtained.

Further, since the seal ring is basically used in oil, the lubricant attached to or coated on the seal surface of the seal ring should not be removed by the oil so that the lubricant is not removed. Use solid lubricant. Further, some seal rings for automatic transmissions for automobiles include PV =
Some are used in the 40 MPa · m / s region. In this case, the surface temperature of the sliding surface of the seal ring becomes abnormal at 200 ° C., so that the solid lubricant has a stable friction torque reduction even in the region of 200 ° C. abnormality. It is desirable to select one that shows an effect.

The seal ring made of the fiber-reinforced synthetic resin of the present invention can suppress the wear of the mating member as described above. It is most suitable as a seal ring for a seal device made of non-ferrous metal. At the same time, since the friction torque of the sliding contact portion is reduced, the seal ring of the present invention is also effective for improving the fuel efficiency of an automobile.

In the seal ring of the present invention, the removal of the fiber tips protruding from the seal surface and the attachment of the solid lubricant to the seal surface or the formation of the coating film are performed by shot peening the solid lubricant on the seal surface of the seal ring. At this time, the average particle size of the solid lubricant is set to 100 μm or less, and the injection speed is set to 100 m / s.
Above.

As described above, in order to remove the tips of the fibers protruding from the sealing surface of the seal ring and to adhere the solid lubricant or to form a film by shot peening of the solid lubricant, the solid lubricant must be 100 m thick. It is necessary to inject at an injection speed of at least / s. When the injection speed of the solid lubricant is lower than 100 m / s, the collision energy of the solid lubricant is reduced, so that the removal rate of the fiber tip protruding from the seal surface is reduced. In addition, the solid lubricant adheres or forms a coating on the seal surface of the seal ring by being heated during collision with the seal ring. Therefore, when the collision energy is reduced, the adhesion of the solid lubricant to the seal ring also deteriorates. I do.

In order to inject the solid lubricant powder at an injection speed of 100 m / s or more using air as a medium, it is necessary to make the solid lubricant powder easy to get on the air. For this purpose, it is effective to make the particle size as small as possible. Injection speed of 100 m / p of solid lubricant powder with compressed air with air pressure of about 5 kgf / cm ² which is commonly used
In order to inject at s, the average particle size of the solid lubricant powder needs to be 100 μm or less. Average particle size is 100μ
In order to inject a solid lubricant exceeding 100 m / m at an injection speed of 100 m / s, an air pressure of 5 kgf / cm ² or more is required. For this purpose, a compressor having a large capacity must be used.

Further, when the solid lubricant is subjected to shot peening on the sealing surface of the seal ring, a minute gap existing between the synthetic resin as the base material and the fibrous filler is caused by collision of the solid lubricant. The effect of burying and increasing the adhesion between the base material and the fibrous filler is also obtained. When the close contact between the base material and the fibrous filler is increased on the sealing surface of the seal ring, the fibrous filler is prevented from falling off the seal ring, which causes abrasion of the mating member in the initial stage of sliding, and the mating member is worn. Decrease.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on examples and comparative examples. Example 1 A polyetheretherketone resin (hereinafter referred to as PEEK) was used as a base material of a seal ring, and carbon fiber was added to the resin at 20% by weight, and a straight joint 15 having a size shown in FIG. A seal ring was formed. Then, using an air direct pressure type shot peening apparatus shown in FIG. 4, oxybenzoyl polyester powder having an average particle size of 15 μm was applied to the sealing side sealing surface.
Injection pressure: 6 kgf / cm ² , injection distance: 150 mm,
Injection was performed under the conditions of injection time: 60 seconds to obtain a seal ring of Example 1 as shown in FIG.

The injection nozzle is fixed at the time of injection, and the seal ring fixed on the turntable 26 is rotated at a rotation speed of 1 rotation / 5 seconds. Benzoyl polyester powder is injected.

The PEEK resin used as the base material of the seal ring of this embodiment has the highest level of load deformation temperature among thermoplastic resins and is excellent in wear resistance.
Further, the oxybenzoyl polyester used as the powder of the solid lubricant has a molecular structure close to that of the PEEK resin as the base material, so that a high adhesion to the base material can be obtained.
It has heat resistance of at least 00 ° C.

Incidentally, the oxybenzoyl polyester has the same effect of reducing the wear torque as shot peening even when compounded in the base material.

[0028] After molding the sealing ring in Comparative Example 1 Example 1, the other without shot peening were obtained in the same manner the seal ring of Comparative Example 1.

Comparative Example 2 In Example 1, after forming the seal ring, the average particle size was 7
Using 0 μm silicon carbide (SiC) 2 powder,
The sealing ring of Comparative Example 1 was obtained in the same manner except that shot peening was performed on the sealing side sealing surface.

Evaluation method The seal rings obtained in Example 1 and Comparative Examples 1 and 2 were evaluated as follows. (1) Surface Condition of Seal Ring The surface of each seal ring was observed with an electron microscope (SEM). (2) Wear test In order to confirm the effect of improving the slidability of each seal ring, FIG.
The wear test was performed in a hydraulic fluid for automatic transmission (ATF) using a vertical pin-on-disk type friction wear tester shown in FIG.

An aluminum alloy (ANDC-13), which is being used for the shaft member 17 of the automatic transmission or the like due to recent demands for weight reduction, is selected as a mating member to be brought into sliding contact, and has a diameter of 60 mm.
The test piece was mounted on a test device in the form of a 38-mm, 10-mm thick disk 38 test piece. The surface roughness of the sliding contact surface of the disk 38 was Ra = 0.6 μm. This abrasion tester has a ring holder 34 at the top, and the ring holder 34 has a spring force of a snap ring 35 installed on the inner peripheral surface 21a of the seal ring so that the seal ring 10 does not move in the radial direction when sliding. The seal ring outer peripheral surface 2
1b is pressed into the holder groove and fixed. On the other hand, the disk holder 3 connected to the rotating shaft 40 is located below the tester.
When the disk 38 is fixed to the disk holder 39 with bolts, the disk 38 becomes rotatable with respect to the seal ring 10. Next, the seal ring 10 is brought into sliding contact with the disk 38 by lowering the ring holder 34, and a pressure P is further applied from the axial direction of the ring holder 34 to bring the seal ring 10 and the disk 3 into contact.
8 is pressed. At this time, the sliding portion between the seal ring 10 and the disk 38 is provided with an automatic transmission hydraulic fluid (ATF) 41.
Is immersed in.

Evaluation Results FIG. 5 shows the surface condition of the sealing surface 12o of the PEEK seal ring 10 obtained in Example 1. FIG. 6 shows the surface condition of the seal face 12o of the unprocessed shot peened product of Comparative Example 1 which was implemented as a comparison.

5 and 6, the number of fiber tips (white short fibers) appearing on the sealing surface 12o of the seal ring 10 is smaller in the shot peened product than in the unprocessed product. Thus, the effect of removing the fiber tip by shot peening can be confirmed. FIGS. 7 and 8 show enlarged surface states of FIGS. 5 and 6, respectively. In the shot-peened product, it is recognized that spherical fine oxybenzoyl polyester adheres to the sealing surface 12o of the seal ring 10.

From the above observation results, by subjecting the seal surface 12o of the seal ring 10 to shot peening with a solid lubricant, the fiber tips protruding on the seal surface 12o are removed, and the solid surface is left on the seal surface 12o. It can be confirmed that the lubricant has adhered or the film was formed.

Further, using the above-mentioned abrasion tester, pressure contact pressure:
FIGS. 10 and 11 show the results of a wear test performed under the test conditions of 5 MPa, a friction speed of 10 m / sec, and a test time of 6 hours.
Shown in

FIG. 10 shows the change over time in the coefficient of friction. In Examples of the present invention (shot peening using oxybenzoyl polyester), the friction coefficient was reduced by 10 to about 20% as compared with Comparative Example 1 (unprocessed shot peened), and the effect of reducing friction torque was recognized. . On the other hand, in Comparative Example 2 (SiC shot peened product), S adhered to the seal surface 12o of the seal ring 10
It is recognized that iC becomes a foreign matter on the sliding contact surface, and the friction torque is increased rather than in Comparative Example 1. From the above results, it can be confirmed that the oxybenzoyl polyester as the solid lubricant adheres to the seal surface 12o of the seal ring 10, thereby reducing the friction torque of the sliding contact surface.

FIG. 11 shows the amount of wear of the seal ring 10 and the disk 38 after the wear test.
The shot-peened products of Example 1 and Comparative Example 2 of the present invention were compared with those of Comparative Example 1 with respect to the disk 3 as the mating material.
The wear of No. 8 is reduced. As described above, in the shot peened product, since the wear of the disk 38 as the mating material is suppressed, the fiber tip projecting on the sealing surface 12o of the seal ring 10, which is a factor for promoting the wear of the disk, is shot peened. It can be confirmed that it has been removed. However, also in the shot peened product, in the first embodiment of the present invention in which the solid lubricant is shot peened on the sealing surface 12o of the seal ring 10, the solid lubricant reduces the friction torque of the sliding contact portion to thereby reduce the sliding contact portion. While the calorific value is reduced and self-wear of the seal ring is suppressed, in Comparative Example 2 in which SiC (not a solid lubricant) is shot-peened, the frictional torque of the sliding contact portion is increased and the calorific value is reduced. , Self-wear of the seal ring is promoted. Thus, performing the shot peening of the solid lubricant on the seal surface of the seal ring is also effective for suppressing self-wear of the seal ring.

From the above wear test results, it was found that the seal ring 1
In the seal ring 10 of the present invention in which the solid lubricant is shot-peened to the zero seal surface 12o, the friction torque of the sliding contact surface is reduced, the self-wear of the mating member and the seal ring is suppressed, It can maintain good sealing properties and is most suitable as a seal ring for an automatic transmission.

[0039]

As described above, according to the present invention, the structure is such that the seal ring mounted on the annular seal ring groove set on the outer periphery of the shaft member has a ring inner peripheral surface facing inward in the radial direction, It comprises a ring outer peripheral surface facing inward in the radial direction and parallel to the inner peripheral surface of the ring, and a pair of ring side surfaces facing both sides in the axial direction. The ring outer peripheral surface is rotatable relative to the outer periphery of the shaft member during hydraulic operation. A seal ring that is pressed against the inner peripheral surface of the housing of the housing to be fitted and holds the oil pressure by pressing the ring side surface against the inter-seal surface of the seal ring groove, wherein the seal ring main body is made of a fiber-reinforced synthetic resin, and At least one of the seal surface of the ring outer peripheral surface and the ring side surface has a fiber tip protruding from the ring surface removed and a lubricant adhered to the surface or a lubricant By using a seal ring with a coating formed thereon, it is possible to reduce the aggressiveness of the seal ring to the mating material, maintain good sealing performance, and reduce the friction torque of the sliding contact part as much as possible. The effect can be obtained. At this time, the thickness of the lubricant adhered on the seal surface or the thickness of the lubricant forming the film is 50 μm or less, and the solid lubricant is used, so that the above effects can be exerted to the maximum. It becomes possible. In addition, the removal of the fiber tips protruding from the seal surface of the seal ring of the present invention and the attachment of the solid lubricant to the seal surface, or the formation of the coating film, are performed by subjecting the solid lubricant powder to shot peening on the seal surface. However, in order to maximize the effect of shot peening, it is necessary to perform shot peening of a solid lubricant powder having an average particle diameter of 100 μm or less at an injection speed of 100 m / s or more.

Further, in addition to the above effects, the present invention can improve the sliding characteristics of the seal ring only by using the shot peening device without changing the shape and material of the seal ring. It is not necessary to change the basic design of the ring, and it is possible to obtain an excellent effect that the investment for improving the slidability is inexpensive.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a seal ring of the present invention.

FIG. 2 is a diagram illustrating a use state of the seal ring of the present invention.

FIG. 3 is a diagram illustrating a seal ring used for a wear test.

FIG. 4 is a diagram illustrating a shot peening apparatus.

FIG. 5 is a diagram illustrating a surface state of a seal ring after shot peening according to the present invention.

FIG. 6 is a diagram illustrating a surface state of a seal ring before shot peening.

FIG. 7 is a diagram illustrating an enlarged surface state of FIG. 5;

FIG. 8 is a diagram illustrating an enlarged surface state of FIG. 7;

FIG. 9 is a view for explaining a vertical pinion disk type wear tester used for a wear test.

FIG. 10 is a diagram illustrating a change in a friction coefficient in a wear test.

FIG. 11 is a diagram illustrating a comparison of wear amounts in a wear test.

FIG. 12 is a diagram illustrating a seal ring provided with an oil pocket.

[Explanation of symbols]

Reference Signs List 10 seal ring 11 fiber-reinforced synthetic resin 12i, 12o seal ring side surface 13 fibers appearing on seal surface of seal ring 14 lubricant attached or coated on seal surface of seal ring 15 ... Seal ring abutment 16 ... Shaft member 17 ... Oil passage (shaft member) 18 ... Housing 19 ... Oil passage (housing) 20 ... Seal ring groove 21a ... a Inner peripheral surface of seal ring 21b ... ... b Outer peripheral surface of seal ring 22 ... Inner peripheral surface of housing 23 ... Seal side surface of seal ring groove 24 ... Cabinet 25 ... Injection nozzle 26 ... Turntable 27 ... Hopper 28 ... Injection tube 29 ... Powder flow controller 30 …… Recovery pipe 31 …… Recovery tank 32 …… Dump valve 33 …… Powder tan 34 ...... ring holder 35 ...... snap ring 36 ...... load cell 37 ...... torque detector 38 ...... disk 39 ...... disc holder 40 ...... rotary shaft 41 ...... automatic transmission hydraulic fluid (AT
F) 42 ... Oil pocket

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C10N 40:02 C10N 40:02 40:34 40:34 50:08 50:08 F term (Reference) 3J043 AA17 BA04 CB07 CB24 DA01 DA11 4H017 AA04 AA13 AA20 AA31 AA39 AB13 AB17 AC16 AD01 AE02 4H104 CB13A EA08A LA03 PA01 PA19 QA11

Claims (7)

[Claims] A seal ring mounted on an annular seal ring groove set on an outer periphery of a shaft member has a ring inner peripheral surface facing inward in a radial direction, and a ring inner surface facing in parallel with the ring inner peripheral surface and facing radially outward. A ring outer peripheral surface and a pair of ring side surfaces facing in both axial directions are provided. During hydraulic operation, the ring outer peripheral surface is pressed against an inner peripheral surface of a housing of a housing which is rotatably fitted to the outer periphery of the shaft member. A seal ring that holds oil pressure by pressing a ring side surface against a seal ring groove side surface, wherein the seal ring body is made of a fiber-reinforced synthetic resin, and at least one of the ring outer peripheral surface and the ring side surface has a ring surface. A seal ring characterized in that a fiber tip protruding from the surface is removed, and a lubricant is adhered to the surface or a lubricant film is formed on the surface. 2. The seal ring according to claim 1, wherein the thickness of the lubricant adhering on the seal surface of the seal ring or the thickness of the formed lubricant film is 50 μm or less. 3. The seal ring according to claim 1, wherein the lubricant adhered or coated on the seal surface of the seal ring is a solid lubricant. 4. The seal ring according to claim 1, wherein the seal ring is used for a hydraulic seal of a relative rotating portion of an automatic transmission used for a vehicle such as an automobile. 5. A hydraulic seal device wherein the seal ring according to claim 1 is in sliding contact with a nonferrous metal counterpart. 6. A shot peening of a solid lubricant powder on a seal surface of a seal ring to remove a tip of a fiber protruding on the seal surface, and at the same time, attach a solid lubricant on the seal surface, or apply a solid lubricant. A method for producing a seal ring, comprising forming a film of: 7. An injection speed of 100 g of solid lubricant powder having an average particle diameter of 100 μm or less on the seal surface of the seal ring.
7. The method for manufacturing a seal ring according to claim 6, wherein shot peening is performed at a rate of m / sec or more.