US3756354A

US3756354A - Diameter brake and locking device for a rotatable shaft

Info

Publication number: US3756354A
Application number: US00132517A
Authority: US
Inventors: W Clark
Original assignee: Individual
Current assignee: Landis Tool Co
Priority date: 1971-04-08
Filing date: 1971-04-08
Publication date: 1973-09-04
Anticipated expiration: 1990-09-04

Abstract

A rotatable shaft (40) of a machine tool is clamped by an apparatus which stops rotation and locks the shaft (40) against movement. This apparatus may be applied to lock the shaft (40) of a rotary-type truing mechanism or dresser (10) which includes a diamond-impregnated roller (11) for dressing a grinding wheel (12). The hydraulic brake assembly (57) is comprised of a circular split-brake collar (58) having angularly disposed or tapered end portions (63) which are confined by a locking plunger (61) having complementarily angled end portions (62). The plunger (61) is advanced by hydraulic fluid which compresses the brake collar (58) around the shaft (40) to form a friction brake to stop and lock the rotation of the shaft (40). A light truing pass may then be made across the face of the grinding wheel (12) to perform a fine finishing operation. Thereafter, the plunger (61) is retracted by spring forces when the fluid pressure is released, and the brake collar (58) is opened to allow rotation of the shaft (40) to be resumed.

Description

United States Patent 91 Clark 1 51 Sept. 4, 1973 DIAMETER BRAKE AND LOCKING DEVICE FOR A ROTATABLE SHAFT [75] Inventor: Wilbur R. Clark, Waynesboro, Pa.

[73] Assignee: Landis Tool Company, Waynesboro,

22 Filed: Apr. 8, 1971 211 App]. No.2 132,517

[52] US. Cl. 188/77 R, 188/75 [51] Int. Cl. Fl6d 49/00 [58] Field of Search 188/75, 77 R, 77 W,

188/336, 339, 343; l25/1l R, 11 CC, 11 TP, 11 CD, 11 H, 11 N Primary ExaminerEvon C. Blunk Assistant Examiner-J0hnny D. Cherry Att0rney-Joseph R. Spalla RANGE OF CONTACT [57] ABSTRACT A rotatable shaft (40) of a machine tool is clamped by an apparatus which stops rotation and locks the shaft (40) against movement. This apparatus may be applied to lock the shaft (40) of a rotary-type truing mechanism or dresser (10) which includes a diamondimpregnated roller (11) for dressing a grinding wheel (12). The hydraulic brake assembly (57) is comprised of a circular split-brake collar (58) having angularly disposed or tapered end portions (63) which are confined by a locking plunger (61) having complementarily angled end portions (62). The plunger (61) is advanced by hydraulic fluid which compresses the brake collar (58) around the shaft (40) to form a friction brake to stop and lock the rotation of the shaft (40). A light truing pass may then be made across the face of the grinding wheel (12) to perform a fine finishing operation. Thereafter, the plunger (61) is retracted by spring forces when the fluid pressure is released, and the brake collar (58 is opened to allow rotation of the shaft (40) to be resumed.

11 Claims, 6 Drawing Figures PAIENIEDscr 4 ma SHEEI 1 0F 2 INVENTOR WILBUR R. CLARK PAIENIEIIsnr 4 Ian SHEEI 2 [If 2 FIG. 4

APPLY BRAKE 81 REVERSE INVENTOR WILBUR R. CLARK DIAMETER. BRAKE AND LOCKING DEVICE FOR A ROTATABLE SHAFT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to an improved apparatus for braking and locking a rotatable shaft, and more particularly, to an apparatus for braking and locking the shaft of a truing mechanism for dressing grinding wheels. This brake enables a truing roller to be fixed against rotation and held during the reverse pass across the operative face of the grinding wheel including the rounded corners thereof. By holding the truing or dressing roller stationary, only a few of the diamonds will contact the cylindrical face of the grinding wheel and a very light truing cut will produce a fine finish on the grinding wheel.

2. Description of the Prior Art Prior to this invention, the rotary truing roller was locked by a small locking plunger which is mounted in alignment with an annular flange carried by the shaft of the dressing roller and with which the extreme end of the locking plunger is engaged when the locking plunger is projected under fluid pressure. This apparatus for locking the truing roller has several deficiencies. First, the locking of the truing roller by the small locking plunger is not positive. Secondly, because the locking plunger engages the annular flange at one point only, the rotary shaftwas easily deflected, causing inaccuracies in the dressing operation. Further, the locking plunger is subject to extreme wear due to the single small surface area thereof in contact with the rotating annular flange.

In a prior US. Pat. No. 3,547,230, issued Dec. 15, 1970, the shaft of a truing mechanism is locked to prevent rotation, by applying equal pressure to opposite sides of a brake drum or shaft. Pivotally mounted brake members are mechanically interlocked through a cam device to clamp a brake disc with an equal amount of clamping pressure from each brake member to prevent I deflection of the shaft. While this apparatus is satisfac- SUMMARY OF THE INVENTION In accordance with the invention, improved means are provided for locking a rotatable shaft against movement, such as the shaft of a truing mechanism having a motor-driven, diamond impregnated roller for dressing grinding wheels. This diameter brake and locking mechanism enables a truing roller to be fixed against rotation and held during the reverse pass across the operative face of the grinding wheel. The diameter brake and locking mechanism includes a split brake collar wrapped around the shaft and having spaced apart end portions. The collar is fixed against rotation and is normallyout of engagement with the shaft. A device is provided for closing the collar around the shaft to apply braking forces thereto while applying substantially equal and opposite forces against the shaft itself so that the axis of the shaft is not displaced during the braking operation. V I

In the preferred embodiment, the split-brake collar includes angular end portions which are confined within matching angular surfaces of a plunger which is slidably mounted within a housing. The plunger is advanced by hydraulic pressure which closes the collar to provide a friction brake to stop and lock rotation of the shaft which carries the truing roller. By holding the truing roller stationary, only a few of the diamonds contact the cylindrical face of the grinding wheel, and a very light finish cut is produced during the reverse pass across the operative face of the grinding wheel, which may include the rounded corners thereof. The plunger is retracted by spring pressure when the hydraulic pressure is released and the split-brake collar is opened to allow rotation of the shaft and the truing roller to be resumed.

It is, therefore, an object of the present invention to brake and lock the shaft of a truing mechanism by applying a frictional force against majorportions of the periphery of the shaft in substantially equal and opposite directions, to prevent the axis of the shaft from being deflected in any direction.

Another object is to provide the split-brake collar with a large surface area which is sealed against contaminants, such as dirt and abrasive particles.

Another object is to provide a diameter brake and locking device which uses a simple novel construction which is actuated rapidly.

Another object is to provide a diameter brake and locking device which is self-centering without any need for adjustment.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claims, and the several views illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view showing the operational environment of the brake mechanism of this invcntion, and certain hydraulic controls therefor;

FIG. 2 is a fragmentary enlarged side elevational view of the grinding wheel of FIG. 1 and showing specifically the-details of the truing mechanism and the associated brake mechanism;

FIG. 3 is an elevational view of the brake mechanism of FIG. 2 as viewed from the left side thereof with portions broken away and shown in section;

FIG. 4 is a fragmentary horizontal sectional view taken along the line 4-4 of FIG. 3, and shows specifically the details of the brake mechanism;

FIG. 5 is a detailed cross-sectional view similar to FIG. 4, of a modified brake collar having a relieved area; and

FIG. 6 is a schematic showing the relative movement of the truing roller as compared with a profile for controlling the movement thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in detail, reference is made first to FIG. 1 wherein the general environmental structure of the brake and truing mechanism is illustrated. It will be seen that the truing mechanism, which is generally referred to by the numeral 10 includes a diamond-impregnated truing roller 11, which is illustrated in engagement with a grinding wheel 12 to perform a truing or dressing operation thereon. The movement of the truing roller 11 relative to the grinding wheel 12 is controlled by a fixed profile 13 having a contoured surface which corresponds to the desired shape of the grinding wheel 12. The profile 13 is engaged by a follower assembly 14 including a follower 15 which is in direct sliding contact with the profile 13 and which is of the same width and contour as the truing roller 11.

The manner in which the truing mechanism 10 is operated is more specifically disclosed in the commonly assigned U.S. Pat. No. 3,121,423 of Ralph E. Price, et al., issued Feb. 18, 1964. Accordingly, there will be herein only such operation of the mechanism for shifting and positioning the truing mechanism 10 as will be necessary to understand the instant invention.

The truing mechanism 10 is mounted on a truing bar 16 which is carried by a slide 17 for movement toward and away from the axis of the grinding wheel 12. The slide 17, in turn, is slidably mounted on a carriage 18 which is mounted for traverse movement parallel to the axis of the grinding wheel 12. The carriage 18 is mounted on a suitable portion of the bed (not shown) of the grinding machine, of which the grinding wheel 12 is a part.

The truing mechanism 10 is advanced and retracted by means of a cylinder 20 which is secured to the slide 17 and in which there is positioned a piston 21. A piston rod 22 is coupled to the carriage 18. The piston 21 and the cylinder 20 normally function to hold the follower l5 in engagement with the profile 13. The actuation of the cylinder 20 is controlled by means of a valve 23 which is connected to a suitable pressure pump 24. The valve 23 is, in turn, actuated by

suitable solenoids

25 and 26.

Traversing movement of the carriage 18 is effected by means of a piston 27 which is mounted within a cylinder 28. The piston 27 has a piston rod 29 connected to the carriage 18. The movement of the piston rod 29 is controlled by means of a conventional valve 30 which is connected to the pump 24 for receiving fluid under pressure therefrom. The valve 30 is actuated and positioned by means ofa pair of

solenoids

31 and 32 in a conventional manner.

Referring now to FIGS. 2 and 4 in particular, it will be seen that the truing mechanism 10 includes a rigid frame 33 of a suitable configuration. The frame 33 is removably secured to the truing bar 16 by means of a plurality of fasteners 3434 (FIGS. 2 and 3).

As is best shown in FIGS. 3 and 4, the lower portion of the frame 33 supports a spindle assembly 35 which includes a housing 36, having mounting ears 3737. The mounting ears 3737 have slotted openings 3838 which receive fasteners 3939 to secure the spindle assembly 35 to the frame 33. The spindle assembly 35 which is mounted within the housing 36, includes a shaft 40 which is mounted for rotation within

bearings

41 and 42. The

bearings

41 and 42 are axially spaced by a tubular sleeve 43, a spacer 44, and a thrust collar 46. The shaft 40 is positioned within the housing 36 by means of a nut 47, which is adjustably secured against a spacer 48 which is pinned to the shaft 40. One end of the shaft 40 has mounted thereon the truing roller 11, while the other end of the shaft 40 is provided with a dual-grooved pulley 49 for driving the shaft 40.

A power unit 51 which is preferably in the form of an electric motor, but which may be in the form of any desired type of power unit, is mounted to a motor bracket 54 which is secured to the housing 36. The power unit 51 is mounted in a manner wherein it may be vertically adjusted, as shown in FIG. 3. The power unit 51 is provided with a dual-grooved pulley 52 which is aligned with the pulley 49 and which is drivingly connected thereto by means of belts 53-53. It is to be noted that the

pulleys

49 and 52 are encased within the motor bracket 54, which forms a housing.

The frame 33 also has secured thereto a coolant head 55 (FIG. 2) for distributing coolant on the truing roller 11 and the grinding wheel 12 during a dressing operation. A coolant line 56 is suitably connected to the coolant head 55 to supply an adequate supply of coolant.

Referring now to FIGS. 3 and 4, there are illustrated the details of a brake assembly 57 which is the subject of the instant invention. The braking assembly 57 includes a circular split-brake collar 58 which may be drawn against the periphery of the sleeve 43 to brake and lock the shaft 40 against rotation. This movement is effected when hydraulic pressure is directed to the head end of a cylinder 59, which is formed within the housing 36, to advance a plunger 61. The plunger 61 includes inclined or angular end portions 62-62, which are oppositely inclined and engaged with complementary inclined or angular end portions 6363 on the brake collar 58. The end portions 6363 of the brake collar 58, are separated by an opening 64 which enables the brake collar 58 to be contracted by the plunger 61 to frictionally brake and lock rotation of the sleeve 43 and the shaft 40. The brake collar 58 includes a relief 60 that provides a single deflection point so that the sides of the brake collar 58 engage the periphery of the sleeve 43 with approximately equal pressure when the plunger 61 is advanced. A small clearance 58. is provided between the brake collar 58 and the housing 36 so that the brake collar 58 will seek a self-centering position within the housing 36 which enables the brake assembly 57 to be adjustment-free.

The end portions 6363 of the brake collar 58 are formed with a desired angle and with sufficient thickness and rigidity to enable the side portions of the brake collar 58 to be drawn against the periphery of the sleeve 43 without applying a downward force against the shaft 40 which could cause vertical displacement. In the preferred embodiment, the approximate contact area includes an arc of on each side of the brake collar 58 as shown by the legends on FIG. 5 relating to the alternate embodiment. It is to be understood that this is by way of example, and that the invention also contemplates greater or smaller arcs of contact.

In an alternate arrangement, as shown in FIG. 5, the brake collar 58 is provided with an arcuate relief 76 to insure that each side portion of the brake collar 58 is drawn inwardly about the single deflection point provided by the relief 60, without engaging the shaft 40 on the upper portion thereof. As mentioned previously, contact by the surfaces 7777 of the brake collar 58 would produce a downward force on the shaft 40 which could cause an undesirable deflection thereof. The relief 76 tends to reduce the possibility of contact by the surfaces 7777, although the same result can be obtained, as in the first embodiment, by making the end portions 6363 relatively rigid and the area of the brake collar 58 under the relief 60 relatively flexible. As before, a major portion of each side of the brake collar 58 provides a large surface area of contact to brake and lock the shaft 40 against rotation. The are of contact is about 90, as shown by the legends. Horizontal deflection of the shaft 40 is avoided since the braking forces are of substantially equal pressure and opposite to one another.

As shown in FIGS. 4 and 5, the brake collar 58 in both embodiments, extends around the shaft 40 for nearly 360 degrees. It is to be understood that the brake collar 58 could still function if it extended only slightly more than 180 degrees around the shaft 40. In such a design, the plunger 61 would have to be larger in order to urge the brake collar 58 into braking contact with the shaft 40.

The plunger 61 is normally retained in a nonoperative, raised position by a compression spring 66 which is housed within a bore 66A in the plunger 61. Therefore, when fluid pressure is not applied to the plunger 61, the plunger 61 will be raised by spring pressure. The brake collar 58 is also urged into an open po sition by the spring 66 which applies a spreading force against the brake collar 58. This enables the sleeve 43 and the shaft 40 to rotate, when the hydraulic pressure which was directed to the head end of the cylinder 59 is released. The natural resiliency of the brake collar 58 also acts to raise the plunger 61 and release the shaft 40. It is to be understood that the sleeve 43 could be eliminated and the braking forces would then be applied directly to the shaft 40.

The truing roller 11 is readily removable from the shaft 40 by the removal of a retaining nut 65 which is shown in FIG. 3. It will be readily apparent from FIGS. 3 and 4 that the truing roller 11 is in the form of a relatively thin wheel. In actual practice, a typical wheel has a width of 0.1875 inches and a diameter on the order of 3 inches. In addition, it is to be noted that the truing roller 11 has rounded corners. These rounded corners not only facilitate the proper truing of the rounded corners of the grinding wheel 12, but also facilitate the movement of the follower 15 around the contours of the profile 13, as is best shown in FIG. 6. It is to be noted that the contour of the follower 15 corresponds to that of the truing roller 11.

Referring once again to FIG. 1, it will be seen that the operation of the brake assembly 57 is controlled by means of a control valve 67. The valve 67 is suitably connected to the pump 24 and the actuation thereof is controlled by a solenoid 68.

The actuation of the valve 30 for controlling the traversing of the carriage 18 may be controlled by suitable limit switches, such as a limit switch 69 which is operable by means of an abutment 71 carried by an extension of the carriage 18. In addition, it is to be noted that the limit of retraction of the slide 17 may be controlled by a limit switch 72 which is engaged by an abutment 73 carried by the slide 17.

OPERATION Referring now generally to FIGS. 1 and 6, it is to be understood that the truing roller 11 is normally disposed in a retracted position at one side of the grinding wheel 12. At this time, the follower 15 is out of contact with the profile 13. When it is desired to initiate the dressing operation, the solenoid 31 is actuated to shift the valve 30 to a position where fluid is directed into the left hand end of the cylinder 28, thereby moving the carriage 18 to the right until the follower 15 is aligned with the profile 13 adjacent a corner portion thereof. Then, with the power unit 51 energized and and the truing roller 11 rotating, the valve 23 directs fluid into the head end of the cylinder 20 so as to urge the slide 17 toward the grinding wheel 12 with the result that the follower 15 comes into contact with the profile 13 and the truing roller 11 engages the grinding wheel 12 at the left hand corner thereof. Then, while the slide 17 is still urged toward the grinding wheel 12, the valve 30 is actuated to move the carriage 18 to the right with the result that the follower 15 moves around the left hand corner of the profile 13 to enable the truing roller 11 to dress the left hand comer of the grinding wheel 12. The carriage 18 is then moved further to the right with the peripheral face of the grinding wheel 12 being trued. Thereafter, as the carriage 18 continues to move to the right, the right hand corner of the grinding wheel 12 is trued. When the carriage 18 and the slide 17 reach a predetermined point in the truing of the right hand corner of the grinding wheel 12, the movement of the carriage 18 is reversed.

At this time, the power unit 51 is deenergized, and the solenoid 68 is energized, which shifts the valve 67 to direct hydraulic fluid from the pump 24 through the valve 67 and to the head end of the cylinder 59. The plunger 61 is lowered, and the end portions 62-62 apply a force to cam or wedge the end portions 6363 to draw the side portions of the brake collar 58 inwardly. The side portions of the brake collar 58 are deflected about the relief and apply equal and opposite forces against the sleeve 43. This movement not only brakes rotation of the shaft 40 and the truing roller 1 1, but also holds them in a fixed or locked position once rotation has stopped.

The carriage 18 moves back toward the starting position thereof until the follower 15 is in contact with the profile 13 at the right corner thereof, after which the truing roller 11 is moved to the left, truing the right hand corner, the peripheral face, and the left hand cor ner of the grinding wheel 12, after which the truing mechanism 10 is deenergized.

It is to be understood that the truing roller 11 is rotating during the first pass across the grinding wheel 12 which effects a rough dressing operation. However, since the truing roller 11 is fixed against rotation when making the return pass across the grinding wheel 12, it will be seen that the second pass performs a light dress ing operation. Thus, the grinding wheel 12 is reshaped to the desired contour and, at the same time, provided with the desired finish without undue wearing away of the truing roller 11.

It is to be understood that in view of the specific configuration of the truing roller 11, and the fact that it rotates about an axis parallel to the axis of the grinding wheel 12, it is possible for the truing roller 11 to be fixed against rotation during the reverse pass thereof across the grinding wheel 12, so that not only may the peripheral face of the grinding wheel 12 be dressed while the truing roller 11 is fixed against rotation, but also the two corners thereof.

In another embodiment (not shown), using similar parts, the end portions 63-63 of the brake collar 58 could be tapered outwardly instead of inwardly, as shown in the drawings. The tapered surfaces on the plunger 61 would be complementary-shaped. In this arrangement, the braking action would occur by moving the plunger 61 radially outwardly rather than inwardly, as presently shown, but the overall braking action would be the same. An arcuate relief could be provided on either side of the relief 60 to prevent the adjacent surfaces of the brake collar 58 from engaging and deflecting the shaft 40 in an upward direction when the plunger 61 is raised.

It is to be understood that mechanisms other than tapered or angular surfaces could be utilized to close the brake collar 58, such as other forms of camming devices or plungers. Alternatively, in applications where large braking forces are not required, the brake collar can be designed to grip and brake the shaft by its own natural resiliency. In this case, the plunger 61 or other mechanism would act to open the brake collar when braking forces are not desired.

Although only a preferred embodiment of the invention has been specifically illustrated and described herein, it is to be understood that other variations may be made in the brake mechanism and the operation thereof, without departing from the spirit of the invention.

I claim:

1. A brake mechanism for a rotating shaft comprising a split brake collar selectively configured for placement about the shaft, said brake collar including spaced end portions,

means for preventing the rotation of said brake collar relative to the shaft, and

plunger means including camming means for engaging said spaced end portions,

said spaced end portions being selectively configured so that advancement of said plunger means from a first position to a second position will slidingly displace said plunger means along said spaced end portions and cam said spaced end portions together thereby reducing the spacing existent therebetween and applying a braking force to the shaft.

2. A brake mechanism according to claim 1, wherein said collar extends around the shaft for more than 180", but less than 360.

3. A brake mechanism according to claim 1 wherein said collar end portions include projections having oppositely inclined surfaces thereon, and

said plunger means include complementary oppositely inclined surfaces for mating with the inclined surfaces of said end portions whereby advancement of said plunger means cams the collar end portions together.

4. A brake mechanism according to claim 3 wherein said plunger means further comprises a recess between said complementary inclined surfaces, and a spring positioned in said recess for normally urging said plunger into a retracted position and said collar into an open position.

5. A brake mechanism according to claim 1 further comprising:

a housing for supporting the shaft, said brake mechanism being internally mounted therein to seal and protect both the braking surfaces of the shaft and the brake mechanism against contaminants; and

a power unit supported by said housing for rotating the shaft.

6. A brake mechanism according to claim 5, wherein there is provided a small clearance between said collar and said housing so that said collar seeks a self centering position within said housing and the brake mechanism is substantially adjustment-free.

7. A brake mechanism according to claim 1, wherein said collar includes a relief to form a single deflection point for said collar end portions.

8. In combination with a truing roller for dressing a grinding wheel, a spindle assembly including a shaft. said truing roller being carried by the shaft, and said shaft having a braking surface thereon, a brake mechanism in which the improvement comprises:

a split-brake collar wrapped around said shaft and having spaced end portions with oppositely inclined surfaces thereon, said collar being fixed against rotation and having a natural tendency to spring into a larger arc out of contact with said shaft;

a plunger having complementary inclined surfaces in engagement with the inclined end portions of said collar; and

means for advancing said plunger to urge the end portions of. the collar together so that the collar contacts the shaft and applies a braking force thereto.

9. The combination as recited in Claim 8, wherein said plunger includes a recess between said inclined surfaces, and which further includes:

a spring positioned in said recess for normally urging the plunger into a retracted position and for urging the collar into a larger arc.

10. The combination as recited in Claim 9, wherein said collar includes a first relief on the inside periphery thereof immediately adjacent said plunger so that the thrust force of the plunger is not transmitted to said shaft, and said collar further includes, a second relief on the inside periphery thereof disposed approximately 180 degrees from said plunger for forming a single deflection point for said collar end portions so that opposite sides of said collar intermediate said reliefs apply substantially opposite and equal braking forces to said shaft to minimize axial deflection thereof.

11. The combination as recited in Claim 10, wherein said collar extends around said shaft slightly less than 360 degrees, and upon advancement of said plunger said collar engages said shaft over opposite and equal acrs of approximately degrees each.

Claims

1. A brake mechanism for a rotating shaft comprising a split brake collar selectively configured for placement about the shaft, said brake collar including spaced end portions, means for preventing the rotation of said brake collar relative to the shaft, and plunger means including camming means for engaging said spaced end portions, said spaced end portions being selectively configured so that advancement of said plunger means from a first position to a second position will slidingly displace said plunger means along said spaced end portions and cam said spaced end portions together thereby reducing the spacing existent therebetween and applying a braking force to the shaft.

2. A brake mechanism according to claim 1, wherein said collar extends around the shAft for more than 180*, but less than 360* .

3. A brake mechanism according to claim 1 wherein said collar end portions include projections having oppositely inclined surfaces thereon, and said plunger means include complementary oppositely inclined surfaces for mating with the inclined surfaces of said end portions whereby advancement of said plunger means cams the collar end portions together.

5. A brake mechanism according to claim 1 further comprising: a housing for supporting the shaft, said brake mechanism being internally mounted therein to seal and protect both the braking surfaces of the shaft and the brake mechanism against contaminants; and a power unit supported by said housing for rotating the shaft.

6. A brake mechanism according to claim 5, wherein there is provided a small clearance between said collar and said housing so that said collar seeks a self-centering position within said housing and the brake mechanism is substantially adjustment-free.

8. In combination with a truing roller for dressing a grinding wheel, a spindle assembly including a shaft, said truing roller being carried by the shaft, and said shaft having a braking surface thereon, a brake mechanism in which the improvement comprises: a split-brake collar wrapped around said shaft and having spaced end portions with oppositely inclined surfaces thereon, said collar being fixed against rotation and having a natural tendency to spring into a larger arc out of contact with said shaft; a plunger having complementary inclined surfaces in engagement with the inclined end portions of said collar; and means for advancing said plunger to urge the end portions of the collar together so that the collar contacts the shaft and applies a braking force thereto.

9. The combination as recited in claim 8, wherein said plunger includes a recess between said inclined surfaces, and which further includes: a spring positioned in said recess for normally urging the plunger into a retracted position and for urging the collar into a larger arc.

11. The combination as recited in claim 10, wherein said collar extends around said shaft slightly less than 360 degrees, and upon advancement of said plunger said collar engages said shaft over opposite and equal arcs of approximately 90 degrees each.