Background Art As disclosed in Korean U. M. Laid-open Publication No. 2000-14730, a tread brake unit is a pneumatic brake system which has been typically used in railway vehicles to brake a wheel of the vehicle by a frictional braking action produced by a brake shoe that presses against a wheel tread when the brake unit is activated.

FIGS. 1,2,3,4A and 4B are views showing the construction of a conventional tread brake unit. As shown in the drawings, the conventional tread brake unit is pneumatically operated, and includes a brake casing 10, a self-adjusting device 20, and a brake shoe assembly 40. The brake casing 10 has a piston 14 with a wedge surface 13. The piston 14 is installed in a cylinder chamber 12 that is defined in the upper portion of a housing 11. When compressed air is introduced into the chamber 12 through an air inlet 16 of the cylinder control socket 15, the piston 14 is moved downward in the chamber 12. When the piston 14 is released from the compressed air, the piston 14 is elastically moved upward to its original position by a restoring force of a piston spring 17. The self-adjusting device 20 is installed in the housing 11 such that the self-adjusting device 20 is nearly perpendicular to the piston 14. The self-adjusting device 20 includes a yoke 21 with a yoke bearing 22. When the piston 14 is moved downward in the cylinder chamber 12 due to compressed air, the yoke

bearing 22 of the self-adjusting device 20 comes into contact with the wedge surface 13 of the piston 14, thus pushing a push sleeve 23, an adjusting nut 24 and an adjusting screw 25 of the self-adjusting device 20 in one direction. The self-adjusting device 20 thus automatically compensates for an excessive clearance between the brake shoe of the shoe assembly 40 and the wheel tread caused by frictional abrasion of the brake shoe, and maintains a desired clearance between the brake shoe and the wheel tread.

The brake shoe assembly 40 is placed in the brake unit such that the assembly 40 is brought at its rear end into close contact with the front end of the adjusting screw 25, and presses its brake shoe against a wheel tread, thus coming into frictional contact with the wheel tread to produce a braking action.

Such conventional tread brake units may be designed such that they selectively use pistons 14 with wedge surfaces 13 having various wedge angles capable of producing various braking forces. Commercially-designed braking forces of the conventional tread brake units typically range from 1326.5 kgf ((E) 13 KN) to 3571.4 kgf (35 KN) at pressure of 3.88 kgf/cm2 ((E) 380 kPa = 3. 8 bar), so that it is possible for the tread brake units to produce various braking forces.

In FIG. 2, the reference numerals 26 and 26a denote a rear bearing and a bearing pin, respectively, which are arranged in parallel to the yoke bearing 22 around the wedge surface 13 of the piston 14; the reference numeral 27 denotes a spring sleeve; 28 and 29 denote a control socket and a leader nut, respectively; 31 and 32 denote a clutch spring and a lock washer, respectively; 33 and 34 denote a compression ring and a rear guide bushing, respectively; and 35, 36 and 37 each denote a compression coil spring acting as a return spring. In a normal state of the brake unit, the leader nut 29 is in contact with the spring sleeve 27 at splined surfaces 1 Oa thereof, and the adjusting nut 24 is in contact with the guide bushing 34 at splined surfaces lOb thereof. The control socket 28 is held on the housing 11 by a stopper S.

During a braking operation of the tread brake unit with an excessive clearance between the brake shoe and the wheel tread due to an abrasion of the brake shoe, the self-adjusting device 20 automatically adjusts the excessive clearance. The automatic clearance adjusting operation of the self-adjusting device 20 will be described herein

below with reference to FIGS. 4A and 4B. That is, during a braking operation of the brake unit, the push sleeve 23 is moved forward by a distance corresponding to a normal clearance"A"determined to allow a disengagement of a clutch of the brake unit during a braking or releasing operation of the brake unit, as shown in FIG. 4B. In such a case, the control socket 28 stops a flange 27a of the spring sleeve 27. Upon further movement of the push sleeve 23 in the forward direction, the spring sleeve 27 is released from contact with the leader nut 29, so that the leader nut 29 is rotated. Such a rotation of the leader nut 29 is continued until the brake shoe of the brake shoe assembly 40 presses against the wheel tread to produce a braking action.

When the brake shoe of the shoe assembly 40 presses against the wheel tread to produce a braking action as described above, the clutch spring 31 is compressed between the lock washer 32, compression ring 33 and the rear guide bushing 34.

Therefore, the leader nut 29 is locked to the lock washer 32, so that the leader nut 29 stops its rotation. However, since the push sleeve 23 is further moved forward in such a case, the compression coil spring 35 is compressed.

During a releasing operation of the tread brake unit, the push sleeve 23 moves backward by a distance determined by a restoring force of the compressed spring. In such a case, the leader nut 29 is continuously kept locked to the lock washer 32, and there is no rotation in the elements of the self-adjusting device 20. The spring sleeve 27 moves backward to the control socket 28 by a distance corresponding to the clearance"A", thus returning to its original position.

In such a case, the leader nut 29 stops the movement of the adjusting screw 25, so that the adjusting screw 25 is not further moved backward. Therefore, the adjusting nut 24 is forcibly rotated during the releasing operation of the brake unit. In such a case, the compression coil spring 36 acting as a return spring provides a precise returning force to the push sleeve 23, thus always precisely returning the push sleeve 23 to its original position during such a releasing operation.

However, the above-mentioned tread brake unit is problematic, as follows.

That is, the brake shoe of the shoe assembly 40 is frictionally abraded due to repeated operation of the brake unit, so that it is necessary to periodically check the brake shoe

so as to adjust the clearance between the brake shoe and the wheel tread or change the existing shoe with a new one when the shoe is excessively abraded. To replace the existing brake shoe of the shoe assembly 40, the brake shoe must be forcibly spaced apart from the wheel tread using a separate tool to create a substantial space between the brake shoe and the wheel, prior to changing the shoe with a new one. Therefore, the conventional tread brake unit is inconvenient to a worker while replacing the existing shoe.

In a detailed description, during a process of changing an existing brake shoe of the conventional tread brake unit with a new one, it is necessary to space the brake shoe of the shoe assembly 40 from the wheel tread by a predetermined distance in an effort to allow a worker to easily replace the existing brake shoe. To space the brake shoe of the shoe assembly 40 from the wheel tread, the worker must forcibly rotate the adjusting screw 25 using a separate tool. However, the adjusting screw 25 of the conventional tread brake unit is designed such that it is very difficult for a worker to rotate the screw 25, and the tread brake unit has limited space which does not allow the worker to easily or conveniently access or manipulate the screw 25. Therefore, the conventional tread brake unit is problematic in that it is very difficult to change an existing brake shoe with a new one.

Disclosure of the Invention Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a tread brake unit, which is provided with a resetting means in a self-adjusting device so as to move a brake shoe assembly backward from a wheel tread to a desired position capable of forming a substantial space between the brake shoe and the wheel tread, and thus allowing a worker to easily manipulate an adjusting screw during a process of changing the abraded brake shoe with a new one, and which thus simplifies the brake shoe changing process and is convenient to the worker during the brake shoe changing process.

In order to accomplish the above object, the present invention provides a tread brake unit pneumatically operated to press a brake shoe thereof against a wheel tread to produce a braking action, comprising a brake casing having a piston with a wedge surface, the piston being installed in a cylinder chamber defined in an upper portion of a housing of the brake casing, such that the piston is moved downward in the cylinder chamber in response to compressed air introduced into the chamber through an air inlet, and being elastically moved upward in the chamber to return to an original position thereof by a restoring force of a piston spring when the compressed air is released from the piston ; a self-adjusting device installed in the housing of the brake casing such that the self-adjusting device is nearly perpendicular to the piston, the self- adjusting device consisting of a yoke, a push sleeve, an adjusting nut and an adjusting screw, and coming at a yoke bearing of the yoke into contact with the wedge surface of the piston in response to a downward movement of the piston in the cylinder chamber, so that the push sleeve, the adjusting nut and the adjusting screw are moved in one direction, and automatically compensate for an excessive clearance between a brake shoe and a wheel tread caused by frictional abrasion of the brake shoe, thus allowing a predetermined clearance to be maintained between the brake shoe and the wheel tread ; a brake shoe assembly having the brake shoe provided at a front end thereof and placed in the brake unit such that a rear end of the shoe assembly is brought into close contact with a front end of the adjusting screw, and the brake shoe presses against the wheel tread to produce the braking action ; a resetting means cooperating with the self- adjusting device so as to create a predetermined space between the brake shoe and the wheel tread and allow an easy replacement of the brake shoe, the resetting means being moved in a direction toward the wheel tread during a process of replacing the brake shoe, thus simultaneously separating the adjusting nut and a leader nut of the self- adjusting device from a guide bushing and a spring sleeve of the self-adjusting device, respectively, and allowing the adjusting screw to move backward relative to both the adjusting nut and the leader nut in a direction from an advanced position of the adjusting screw to a retracted position due to an elastic restoring force of the shoe assembly, and creating the predetermined space between the brake shoe and the wheel

tread for the replacement of the brake shoe, the resetting means consisting of a pushing force generating unit for generating a pushing force of the resetting means, and a pushing force transmitting unit for transmitting the pushing force of the pushing force generating unit to the leader nut, thus moving the leader nut in a direction opposite to a direction where the restoring force of a torsion spring of the shoe assembly is applied, the pushing force generating unit being designed such that a circumference of the pushing force generating unit varies in a radius from a center of rotation, thus having a cam structure.

In the tread brake unit, the pushing force transmitting unit comprises a control socket, with an inward projection formed around an inner circumferential surface of the control socket to engage with a flange of the spring sleeve; and a reset ring fitted into an opening provided at a rear end of the spring sleeve such that the reset ring is elastically and axially movable relative to the spring sleeve and comes at a front end thereof into contact with a rear end of the leader nut, with a reset spring being interposed between the reset ring and the spring sleeve, the reset ring being closed by a support wall formed in a rear portion of the reset ring; and the pushing force generating unit comprises: a reset plunger positioned against the support wall of the reset ring, with a bearing interposed between the reset plunger and the support wall, such that the reset plunger is rotatable relative to the support wall; and a reset member provided with a cam for pushing the reset plunger, the reset member also having a rod-shaped handle that is supported by a reset housing.

In the tread brake unit, wherein the pushing force generating unit comprises a return spring set in the reset housing, with a first end of the return spring being stopped by a closed inside end of the reset housing and a second end of the return spring being locked to the reset member, the return spring thus elastically returning the reset member to its original position when the reset member is released from an external force after being rotated.

In the tread brake unit, a sealing member is provided around a junction of an open outside end of the reset housing and the reset member inserted into the reset housing through the open outside end, thus sealing the junction.

Brief Description of the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a sectional view of a conventional tread brake unit; FIG. 2 is a sectional view taken along the line A-A of FIG. 1, showing the construction of the conventional tread brake unit; FIG. 3 is a left-side view of FIG. 1, showing the construction of a brake shoe assembly included in the conventional tread brake unit; FIG. 4A is a partially sectioned view of a self-adjusting device included in the conventional tread brake unit of FIG. 1; FIG. 4B is a partially sectioned view of the self-adjusting device included in the conventional tread brake unit of FIG. 1, when the brake unit is in a braking operation; FIG. 5A is a sectional view corresponding to FIG. 1, but showing a tread brake unit in accordance with a preferred embodiment of the present invention; FIG. 5B is a sectional view of the portion"C"of the tread brake unit shown in FIG. 5A ; FIG. 6 is a sectional view taken along the line C-C of FIG. 5A, showing the construction of the tread brake unit according to the present invention; FIG. 7 is a perspective view of the tread brake unit according to the present invention; FIG. 8 is a partially sectioned view taken along the line C-C of FIG. 5A, showing a self-adjusting device included in the tread brake unit of the present invention, when the self-adjusting device operates to adjust for an excessive clearance of the brake unit; and FIG. 9 is a view corresponding to FIG. 8, but showing an operation of the self- adjusting device during a releasing operation of the tread brake unit according to the

present invention.

Best Mode for Carrying Out the Invention Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.

FIG. 5A is a sectional view of a tread brake unit in accordance with a preferred embodiment of the present invention, which is arranged around a wheel of a vehicle.

FIG. 5B is a sectional view of the portion"C"of the tread brake unit shown in FIG. 5A.

FIG. 6 is a sectional view showing the construction of the tread brake unit according to the present invention. FIG. 7 is a perspective view of the tread brake unit according to the present invention. FIG. 8 is a partially sectioned view of a self-adjusting device included in the tread brake unit of the present invention, when the self-adjusting device operates to adjust for an excessive clearance. FIG. 9 is a partially sectioned view, showing an operation of the self-adjusting device during a releasing operation of the tread brake unit according to the present invention.

As shown in the drawings, the tread brake unit 100 according to the present invention includes a brake casing 10, a self-adjusting device 20, and a brake shoe assembly 40. The brake casing 10 has a piston 14 with a wedge surface 13. The piston 14 is installed in a cylinder chamber 12 that is defined in the upper portion of a housing 11. When compressed air is introduced into the chamber 12 through an air inlet 16, the piston 14 is moved downward in the chamber 12. When the piston 14 is released from the compressed air, the piston 14 is elastically moved upward to its original position by a restoring force of a piston spring 17. The self-adjusting device 20 is installed in the housing 11 such that the self-adjusting device 20 is nearly perpendicular to the piston 14. The self-adjusting device 20 includes a push sleeve 23, an adjusting nut 24 and an adjusting screw 25. When the piston 14 is moved downward in the cylinder chamber 12, the push sleeve 23, adjusting nut 24 and the adjusting screw 25 of the self-adjusting device 20 are pushed in one direction, thus

automatically compensating for an excessive clearance between the brake shoe of the shoe assembly 40 and the wheel tread caused by abrasion of the brake shoe due to repeated frictional contact of the brake shoe with the wheel tread. The self-adjusting device 20 thus maintains a desired clearance between the brake shoe and the wheel tread. The brake shoe assembly 40 is placed in the brake unit such that the assembly 40 is brought at its rear end into close contact with the front end of the adjusting screw 25, and presses its brake shoe 46 against the tread Wl of a wheel W, thus coming into frictional contact with the wheel tread to produce a braking action. That is, the tread brake unit is pneumatically operated to press the brake shoe 46 of the shoe assembly 40 against the wheel tread Wl to produce a braking action.

The tread brake unit 100 also has a resetting means 50. The resetting means 50 cooperates with the self-adjusting device 20 so as to create a predetermined space between the brake shoe 46 and the wheel tread Wl and allow an easy replacement of the brake shoe 46. That is, a replacement of the brake shoe 46 is desired due to an excessive abrasion of the shoe 46, the resetting means 50 is moved in a direction toward the wheel tread, thus simultaneously separating the adjusting nut 24 and a leader nut 19 of the self-adjusting device 20 from a guide bushing 34 and a spring sleeve 27, respectively. Therefore, the adjusting screw 25 is move backward relative to both the adjusting nut 24 and the leader nut 29 in a direction from an advanced position of the adjusting screw 25 to a retracted position due to an elastic restoring force of the shoe assembly 40, so that the predetermined space is created between the brake shoe 46 and the wheel tread for the replacement of the brake shoe.

The resetting means 50 is a semiautomatic device that is operated to retract the brake shoe 46 from the wheel tread so as to allow a worker to easily replace the brake shoe when necessary. When the resetting means 50 retracts the brake shoe 46 from the wheel tread Wl for a replacement of the brake shoe 46, the resetting means 50 uses the elastic restoring force provided by a torsion spring 48 installed at a hinged joint between the shoe holder 44 and the shoe holder rink 42 of the shoe assembly 40. The resetting means 50 is provided on the self-adjusting device at a position opposite to the shoe assembly 40, with a control socket 61 arranged such that it is horizontally aligned

with the leader nut 29 of the self-adjusting device 20. The resetting means 50 also has a reset member 73 having a cam 73a. During an operation of the resetting means 50, the reset member 73 is rotated to move the control socket 61 forward, thus pushing the leader nut 29 toward the wheel tread through the control socket 61. Therefore, the leader nut 29 is released from its engagement with the spring sleeve 27 at the splined surfaces lOa thereof. The adjusting screw 25 is thus moved backward from the wheel tread due to the restoring force of the torsion spring 48 of the shoe assembly 40, so that the adjusting screw 25 is retracted into the housing 11 and the distance between the brake shoe 46 and the wheel tread Wl is increased. Therefore, a worker is allowed to easily and conveniently access and manipulate the brake shoe 46 when repairing or replacing the brake shoe 46 or a brake lining.

In such a case, the leader nut 29 is released from the rear guide bushing 34 around the lock washer 32, so that the adjusting nut 24 is rotatable relative to the leader nut 29. The adjusting screw 25 is thus retracted in a direction opposite to the brake shoe 46.

As described above, the resetting means 50 is provided in back of the rear end of the adjusting screw 25 such that the adjusting screw 25 is moved backward from the wheel tread in response to a rotation of the leader nut 29.

The resetting means 50 according to the present invention also has a pushing force transmitting unit 60 and a pushing force generating unit 70. The pushing force generating unit 70 generates a pushing force of the resetting means 50, and the pushing force transmitting unit 60 transmits the pushing force of the generating unit 70 to the leader nut 29, thus moving the leader nut 29 in a direction opposite to the direction where the restoring force of the torsion spring 48 of the shoe assembly 40 is applied.

The pushing force generating unit 70 is designed such that its circumference varies in the radius from the center of rotation, thus having a cam structure.

The pushing force transmitting unit 60 of the resetting means 50 includes a control socket 61 and a reset ring 63. In the control socket 61, an inward projection 61a is formed around the inner circumferential surface of the socket 61 and engages with the flange 27a of the spring sleeve 27. The reset ring 63 is fitted into an opening

provided at the rear end of the spring sleeve 27 such that the reset ring 63 is elastically and axially movable relative to the spring sleeve 27 and comes at its front end into contact with the rear end of the leader nut 29, with a reset spring 53 being interposed between the reset ring 63 and the spring sleeve 27. The reset ring 63 is closed by a support wall 63a integrally formed in a rear portion of the ring 63.

The pushing force generating unit 70 of the resetting means 50 includes a reset plunger 71 and a reset member 73. The reset plunger 71 is positioned against the support wall 63a of the reset ring 63, with a bearing B interposed between the plunger 71 and the support wall 63a, such that the plunger 71 is rotatable relative to the support wall 63a of the set ring 63. The reset member 73 consists of a cam 73a that selectively pushes the reset plunger 71 forward. The reset member 73 also has a rod- shaped handle H that is supported by a reset housing 75.

The pushing force generating unit 70 of the resetting means 50 also preferably includes a return spring 77 which is set in the reset housing 75. A first end of the return spring 77 is stopped by a closed inside end of the reset housing 75, and a second end of the spring 77 is locked to the reset member 73. The return spring 77 thus elastically returns the reset member 73 to its original position when the rod-shaped handle H of the reset member 73 is released from an external force after being rotated.

The reset member 73 is rotatably set in the reset housing 75 that is mounted to the rear end of the tread brake unit 100.

A sealing member P having a bellows structure is provided around the junction of an open outside end of the reset housing 75 and the reset member 73 which is inserted into the reset housing 75 through the open outside end.

Therefore, when a replacement of the existing brake shoe 46 or the brake lining of the shoe assembly 40 is desired, the resetting means 50 is operated to retract the adjusting screw 25 backward from the wheel tread Wl through a multi-threaded screw and multi-threaded nut mechanism, thus creating a substantial space between the brake shoe 46 and the wheel tread W1 and allowing a worker to easily and conveniently replace the brake shoe 46 or the brake lining.

In the drawings, the reference numeral 23a denotes a push ring, and the

reference character"O"denotes an 0-ring. The push ring 23a is set in the tread brake unit 100 such that its rear end is in contact with the front end of the guide bushing 34 and its front end is mounted to the hinged joint between the adjusting screw 25 and the shoe assembly 40. The push ring 23a is thus moved toward the shoe assembly 40 along with the adjusting screw 25, guide bushing 34 and the adjusting nut 24 during a braking operation of the tread brake unit 100. The O-ring"O"is set in the resetting means 50 for accomplishing a sealing effect of the actuating mechanism of the resetting means 50.

The operation and effect of the tread brake unit according to the present invention will be described herein below.

In order to brake the wheel, compressed air is introduced into the cylinder chamber 12 of the brake casing 10 through the air inlet 16 under the state of a released position of the brake unit 100 as shown in FIG. 8, the piston 14 is moved downward in the chamber 12. In such a case, the power generated by the downward moving piston 14 is distributed to the push sleeve 23 and the adjusting screw 25 through a power- distributing device, thus moving the push sleeve 23 and the adjusting screw 25.

Therefore, the brake shoe assembly 40 moves toward the wheel tread Wl by a distance corresponding to an initially preset application stroke"A", thus starting a braking action.

When compressed air is further introduced into the cylinder chamber 12 through the air inlet 16, both the push sleeve 23 and the adjusting screw 25 are further moved as shown in FIG. 8. In such a case, since the guide bushing 34 is not moved, the guide bushing 34 stops the leader nut 29 through the lock washer 32, so that the leader nut 29 is not rotated. Therefore, the adjusting screw 25 is axially moved without being rotated, thus maximizing the braking force of the brake shoe while compensating for an elastic deformation stroke"E"which is generated when the brake shoe of the shoe assembly 40 comes into contact with the wheel tread.

In such a case, the stroke of the adjusting nut 25 becomes A+E.

When there occurs an excessive clearance"Z"in the brake unit due to abrasion of the brake shoe caused by repeated operation of the brake unit, the adjusting screw 25

is continuously rotated along with the adjusting nut 24 until the brake shoe of the shoe assembly 40 is brought into contact with the wheel tread to produce a braking force.

In such a case, the moving stroke of the adjusting screw 25 becomes A+E+Z.

During a releasing operation of the brake unit, the compressed air is released from the cylinder chamber 12, so that the spring sleeve 27 is moved backward to come into close contact with an adjusting ring 39, thus allowing the guide bushing 34 to be spaced apart from the leader nut 29. In such a case, the leader nut 29 is rotated due to the rotating force of the adjusting screw 25.

Therefore, the adjusting screw 25 moves backward in the brake casing of the brake unit 100, thus sufficiently compensating for the excessive clearance"Z". When the excessive clearance"Z"is sufficiently compensated for, as described above, the leader nut 29 comes into close contact with the guide bushing 34, so that the leader nut 29 stops its rotation. The tread brake unit 100 thus maintains the initially preset clearance"A"between the brake shoe of the shoe assembly 40 and the wheel tread.

The process of changing the existing brake shoe 46 of the shoe assembly 40 with a new one will be described herein below with reference to FIG. 9. When it is desired to replace the existing brake shoe 46, the handle H of the resetting means 50 is rotated. The reset member 73 of the resetting means 50 is thus rotated, and the rotating force of the reset member 73 is transmitted to the reset plunger 71 and the reset ring 63, thus moving the reset plunger 71 and the reset ring 63 in a forward direction.

Therefore, the leader nut 29 that comes into contact with the front end of the reset ring 63 is moved in the same direction.

When the reset member 73 returns to its original position, both the reset plunger 71 and the reset ring 63 elastically return to their original positions via a reset spring 53 which is installed between the reset ring 63 and the self-adjusting device 20.

In such a case, the brake shoe assembly 40 also returns to its original position.

Therefore, the spring sleeve 27 and the guide bushing 34 are spaced apart from the leader nut 29 and the adjusting nut 24, respectively, so that both the adjusting nut 24 and the leader nut 29 become rotatable. Therefore, the adjusting screw 25 is rotated to move backward in the brake unit 100 by the torsion spring 48 of the shoe assembly 40.

Due to the backward movement of the adjusting screw 25, the shoe assembly 40 including the shoe holder 44 and the brake shoe 46, except for the shoe holder link 42 of the assembly 40, is retracted from the wheel tread, thus defining a space between the brake shoe 46 of the shoe assembly 40 and the wheel tread, and allowing a worker to easily and conveniently replace the brake shoe.

In the present invention, the resetting means 50 includes the control socket 61, the cam-type reset member 73, and the reset ring 63. The control socket 61 limits a backward movement of the flange 27a of the spring sleeve 27, and rotatably supports the reset member 73. The reset ring 63 comes into indirect contact with the reset member 73 through the needle bearing B, so that the reset ring 63 is moved along with the reset member 73. The reset ring 63 is also brought into selective contact with the leader nut 29 of the self-adjusting device 20, thus selectively moving the leader nut 29.

That is, the resetting means 50 according to the present invention cooperates with the self-adjusting device 20 through a nut and bolt mechanism, thus automatically adjusting the clearance between the brake shoe 46 of the shoe assembly 40 and the wheel tread Wl so as to maintain a normal clearance. When a replacement of the brake shoe 46 is desired, the reset ring 63 is moved toward the wheel tread so as to move the adjusting screw 25 backward, thus enlarging the space between the brake shoe 46 and the wheel tread W 1.

In the present invention, the reset housing 75 of the resetting means 50 is preferably assembled with the self-adjusting device 20 through a screw-type engagement.

Industrial Applicability As described above, the present invention provides a tread brake unit having a resetting means in its self-adjusting device. The tread brake unit is designed such that the resetting means is installed in the back of the self-adjusting device so as to allow the brake shoe assembly of the brake unit to be moved in response to operation of the resetting means. Due to the resetting means, it is easy to create a substantial space

between the brake shoe of the shoe assembly and the wheel tread, thus allowing a worker to easily and conveniently replace the brake shoe with improved work efficiency. The tread brake unit also allows the replacement of the brake shoe to be simply accomplished without forcing the worker to forcibly space the shoe assembly apart from the wheel tread by using a separate tool, thus simplifying the process of replacing the brake shoe and reducing the time consumption during the brake shoe replacing process.