US2581297A - Elevator safety device - Google Patents

Description

Jan. 1, 1952 .\R. RISSLER ELEVATOR SAFETY DEVICE 2.v SHEETSSHEET 1 Filed July 20, 1949 e Rll O U MB N adm Mmfi Y 8 B L WITNESSES:

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1952 R. RISSLER ELEVATOR SAFETY DEVICE 2 SHEETSSHEET 2 -Fig.3.

. V ,lllloli IIPP.

Filed July 20, 1949 WITNESSES:

I INVENTOR Lennius RRisslef.

ATTORNEY Patented Jan. 1, 1952 215 391 EPEWQQK ETEXDEX Q Lennius R. Risslerg; Hohokus, N;

Ele c qrne ati a E arepar nattachin ssisnQr-.. to..-

tor car guided by. guide rails. is provided with an acceleration responsiveisa'fety device whichis I completely mountedonthe car. When the acceleration of the car. in a downward dlrection.ex ceeds a predetermined value; the. safety device operates to retard movementof the car.

The invention further. contemplates theprovi- 'sionof a speedresponsiveoperator forthe safety device.- If; the speed: of; the: carexceeds-a predeterm-med: value; the speedaresponsive operatorv is efiective. independently? for operating thesafety device.

In .a preferred.embodimentof the invention, the safety device com-prisesapairof: spacedapaltallel rollerswhich. normally-are; out ofycontact witha guide rail, butWhich-..ar,e operablefitovgrasp tlielguide. railctherebetween. In response torol1 ing of: the rollers; in one direction on the guide rail the rollers are. moved: toward each other to, grasp the rail. firmly. Rotation of the rollers ultimately isstonp da o p ovide. an ect ve brake ina action. The ollers; ma e e e b v rse ing the direction of.- rotaiQn thereof-- al ne the guide rail. If the rollers are employed to stop a downwardly travelling elevator can, the resetting of the rollers may be. effected merely by energizing the elevator motor to raise theelevator car.

It is, therefore, .an object of the invention to provide an acceleration responsive safetydevice completely mounted ona movable bod-y.

It is a further object of the invention to pro.- vide a safety device as. defined in the preceding paragraph and a speed responsive operator for independently operating the safety device when the speed of the movable bodyexceedSapredetermined value.

It is also an objectgof the-invention; topr vid a safety device whereina pairor spacedrollers are positioned tograspiasu deraii; therebetweeo u aorernqnbrali r s ns o .rollin of the rollers na meat: terminedtdirection alon thegu d raiL Isis-an additional objec of the nvent on to moi/idea safety device as s t f rth n theme:

ceding, paragraph whereinotherollersar reset to inactive condition by rotating the rollers in a direction. opposite. to said pred te mined. dire tion.

It.;is..a. still;further. object of the invention to provide an..imp 0v method fo p ra in devices.

other. jects of thecinivent on wfllbeap am t from thejcllowinsdescri tio ake i -c jun tionv with the accompan i draw n s. in ich:

Ei ure. '11 is a. viawinside elevation with parts broken. awayand, parts schem i a ly show of an. elevator. system embodying the invention.

Rig. 2- is a view. section taken along the line, II-11; of, Fig. .1. with parts broken away,

Fig. 3.:is .a. view in side elevation of the apparatus :shown. in Fig. 2. with parts omitted, and

Fig. his adetailed view of an inertia operator suitable. forthe. apparatus of- Figs. 1, 2? and 3.

Referring to the drawings, Fig. 1 shows an elevatorsystem-swhich comprises a sling l which ismountedin a conventional. manner by means ofiguideshoes 3...:for movement ina vertical path. In. accordance. with conventional practice, the guidecshoesfii are. secured to the sling and slid.- ingly engage continuous vertical guide rails 5 in order to constrain the sling is for. movement in avertical Path.

Adjacent its lower end-the slingl has a safety device 1 which may be operated tov grasp the guide- 1 ails.75 for therpurpose. of retarding. the descent ofthe sling. 'Such operation of the safety :devlce Ti. may she-effected through ,a governor-ropeSi -which has its ends secured to the sling. The governor-rope 9 passes around the Wheel of--a governor i i and a tensioning wheel 1 :3 lira-conventional manner. I

It fw be understood that the Wheel Ha is mtitea aeerdal ew ih w em n l the sling I rate of movementof thesling ex: ceeds a predetermined value, centrifugal weights tas-eevetm H wsr ta br ke tol jn as'e th sweat: repel This ra e ma be 9 a s s? J bstantial bral'ring force to the on aesp rm ts. t rare 9. li a t mq 'h whenthe force pp ed by .thesling tothe goveri s ake tc vt t sw 91 e z pr ethe v rnora be p vided when e ect ca w tch wh ch. ope when he san i q forhenu rose ter sheave l1 has an elevator rope l9 passing thereabout. One end of the rope i9 is secured to the sling I, whereas the remaining end of the rope is secured to a counterweight (not shown).

Rotation of the traction sheave I1 is efiected by means of an elevator motor 2| which is energized through a conventional motor control 23 from a source of electrical energy 25. The source of electrical energy may be connected to the elevator system through a suitable circuit interrupter 21. The motor control may be of any conventional form suitable for controlling the operation of the elevator car between the floors of the building to be served by the elevator car. Since such motor controls are Well known in the art, a more detailed discussion thereof is unnecessary. A conventional electro-mechanical brake 28 for the motor is illustrated in Fig. 1.

The circuit interrupter 21 may be retained in its closed position by means of a latch 21a. This latch is tripped by means of a spring 21b which urges the latch towards the right, as viewed in Fig. l. A solenoid 21d is energized from a suitable source of electrical energy here represented by a battery 29, to develop a force opposed to the spring 21?). Consequently, if the circuit interrupter is closed and the solenoid 21d is energized, the circuit interrupter remains closed until the solenoid is deenergized. Deenergization of the solenoid may be effected by operation of a manual switch Si or by opening of either of two safety switches 33 and 35. The safety switch 33 is provided in the governor II as previously set forth and is open when the speed of the elevator car exceeds a predetermined value. The safety switch 35 is provided in the safety device 1. This switch is designed to open when the safety device is operated to stop the elevator car. A suitable construction for the safety switch 35 will be discussed below.

The safety device 1 is shown in detail in Fig. 2 and includes two braking units 31 and 39 which are associated respectively with the two guide rails 5. The guide rails are of conventional construction. Thus the lower guide rail 5 has two guiding surfaces 5a and 5b. The braking unit 31 is designed to engage these surfaces 511 and 5?). By grasping the guide rail, the braking unit 31 retards the movement of the elevator car.

The braking unit 31 includes two levers 4| and 43 which are mounted for rotation about a pin 45. As shown more clearly in Fig. 3, the pin 45 is secured to two spaced plates 41 and 49 which are secured to each other by means of channels 5| and 53. The channels 5i and 53 may form part of the sling I.

By inspection of Fig. 2, it will be observed that the levers 4! and 43 together with the pivot pin 45, constitute a pincers having jaws 4| a and 43a. positioned on opposite sides of the guide rail. These jaws are biased towards each other by means of a compression spring 55 which is located between arms M1) and 43b of the lovers. The jaws are prevented from closing on the guide rail by means of a through bolt 51 which projects Lal through openings in the arms 4| 2) and 43b. The head 51a and nut 51b of this bolt serve as stops for the arms of the levers and determine the minimum distance between the jaws Ma and 43a.

The levers 4| and 43 are biased by means of a spring 6| in a clockwise direction about the pin 45, as viewed in Fig. 2-, towards a stop screw 63. The spring has one end secured to the arm MI) and its remaining end secured in any suitable manner to the channel 53. The stop screw 63 is in threaded engagement with the web of the channel 53 and may be adjusted to center the associated guide rail 5 between the jaws 4m and 43a.

Braking forces are transmitted between the jaw 43a and the associated guide rail 5 by means of a roller 65 located within the jaw. This roller is moved between an ineifective position and an effective position by means of an arm 61 which is secured to a cross shaft 69. The cross shaft is mounted for rotation in suitable brackets 1| which are secured to the channel 53. The lever 61 has two lugs 13 and projecting therefrom. The roller 65 is positioned between these two lugs.

Referring to Fig. 3, it will be observed that the jaw 430: has a pocket 11 within which the roller 65 is located. The pocket 11 has an upper end wall Tia, a lower end wall 11b and a side wall 110. Conveniently, the end wall 11b may have the same radius of curvature as the roller 65. The end wall 11a may be substantially perpendicular to the associated guide rail.

It will be noted that the lower part of the side wall 110 is inclined with respect to the guide rail with which the jaw is associated. This lower part of the side wall and the associated guide rail in effect define a tapered race within which the roller 65 may rotate and move vertically to a limited extent. If the roller 65 is raised by rotation of the cross shaft 69, the inclined surface 110 forces the roller towards the associated guide rail until the roller engages the guide rail. After this the safety becomes self-energizing with no more force required from the operating mechanism. If the elevator car is travelling in a downward direction, the guide rail engages and rolls the roller in an upward direction into engagement with the end wall 11a. The upper part of the side wall Tic is substantially parallel to the associated guide rail. Upon engagement of the end wall 110. by the roller 65, rotation of the roller ceases and the roller thereafter slides upon the guide rail to develop a braking force between the guide rail and the jaw 43a.

The roller may be constructed of any suitable material, such as steel. Although the roller may have a smooth cylindrical surface, a knurled or slightly roughened surface preferably is employed therefor.

During its upward movement the roller 65 en ters the constricted portion of its race and forces the jaw 43a to the right, as viewed in Fig. 3. The resulting movement of the lever 53 about its pivot pin (Fig. 2) moves the lever away from the stop screw 63. In addition, the lever if; through the spring transmits a force to the lever 4| which urges the jaw ii-a towards the associated guide rail 5. If the jaw 41a. engages the guide rail, the guide rail is firmly grasped between the jaw and the roller and the downward travel of the elevator car rapidly is retarded.

If the jaw 41a is brought into firm engagement with the associated guide rail, some difiiculty will be encountered in braking the jaw free from the guide rail to permit subsequent operation of the Referring again to Fig. 3, the jaw Ma isp'rovided with a pocket 83 having an upper end wall sea; a lower end wall 33b and aside wall 830. The side wall 830 may be parallel to the associated guide rail, but preferably the lower part is inclined relative totheguide rail to-provide a tapered race for the roller 8! which is somewhat similar'to' the tapered race for the roller EEbut which may be substantially shorter than the latter race. The pocket for the roller 8i maybe closed "by means of a detachable plate- 85.

The roller 8! projects slightly-from the-jaw i is.

towards the associated guide rail but normally is not in engagement-with theguiderail. When the jaw 31a isforced towards the guide rail, the roller 8! engages-the guide rail. Assuming that the elevator car is-travelling in a downward direction, the roller 3! is rolled upwardly by the guide rail until it en ages its upper end wall 83a. Rotation of the roller thereafter ceases and the roller slides on the guide rail to develop a braking force between the jaw 41a and the associated guide rail.

In moving towards their braking positions, the rollers 65and ti spread the jaws 41a and tea and compress the spring 55 (Fig. 2). The braking force developed between-the braking unit Bl and the associated guide rail is dependent on the force exerted on the levers by means of the spring 55. By suitable selection of the-spring this braking force may be adjusted in accordance with the requirements of any elevator system.

Operation of the braking unit 31in response to increase in acceleration of the elevator car in a downward direction above a predetermined value i effected by a spring 69 which is compressed between an arm 9! secured to the cross shaft 69 (Fig. 4) and a washer 93. The washer 93 is adjustably secured to the channel 53 by means of a stud 95 which passes freely through the arm 9!. The spring 88 biasesthe cross shaft 69 in a clockwisedirection, as viewed in Fig. 1 and the bias may be adjusted by rotation of a nut 95a in threaded engagement with the stud 95.

The cross shaft 69 normally i maintained in the position illustrated in Fig. 4by meansof a weight W whichis connected by an arm.Wa to the cross shaft 69. The torque exerted by the weight on the cross shaft 69 is in opposition to the torque exerted on the cross shaft 69 by the spring 89. 53a in the channel 53 and has a lower edge 91 engaging the channel for the purpose of. limiting the downward movement of the weight .W.

The torque applied to the shaft 69by the weight W depends on the acceleration of the elevator car. If the elevator car is acceleratingdownwardly. at a rate equivalent to gravity, the weight W exerts no torque on the shaft 69 and the spring 89 operates the shaft 69 in a. clockwise direction, as

viewed in Fig. 4, to set the braking unit 37 (Fig.

2). By proper selection of the weight W and of the spring 89, and by suitableadjustment of the nut 9511, the cross shaft 69 may be operated to apply a braking effort between the elevator car and its associated guide rail when the acceleration of the car in a downward direction exceeds a predetermined value.

Although the braking unit 31. alone may be employed as a safety device, it is desirable to. apply a brakingfeifort to. bothof the. guide rails; For

The arm Wa extendsthrough a slot 6 this reasonthe brakiriguni-t 39-"is employed-and operates-in aman'nersimilar to theoperation of the braking 'unit t1. The brakingunit 39 has levers Ill! and 163 which correspond respectively to the levers- #1 and- 13 of the'braking unit 31. The levers it! andiiis'have pockets for receiving rollers H35 and I01, respectively, and these rollers correspond to therollers 8| and 65 previously discussed. It will be noted that the roller I8! is moved ina vertical direction by means of an arm H39 which is secured to the cross shaft 59. The

relationship-between the arm I99 and the roller it? issimilar tothat existing between the arm 57 and the roller- 55. Rotation of the'cross' shaft 59, therefore; iseifectivefor simultaneously urging the rollers tdand 1 ill into engagement with their respective guide rails. Following such engagemerit, the rollers 8i and l05-are forced into engagement with their respectiveguide rails.

Itshould be observed that'once a roller engages aguide rail, the guide rail forces the roller to roll into braking position. Because of this servo action large tolerances may be employed for the parts and substantial variations in positions of the parts do not impede. operation of the braking units.

Although the rollers are. self-locking in their braking positions, they may be readily released by a reversal in the direction of movement of the associated elevator car. Afterthe braking units have set elevator motor may be energized to raise the elevator car. Since the rollers are free to rotate away from their braking positionsfor a substantial distance, the upward motion of the elevator car rapidly returns the rollers to the positions illustrated in-the drawings and the elelarly to Figs. 1 and '3, it will be observed that one end of the governor rope is connected to the upper end of a shackle III. This shackle has a rod H3 provided with a square cross section extending through a square hole in the plate 49. The rod I it is snugly received withinthe hole to prevent rotation thereofrelative to the plate 69 but is slidable in a vertical direction with respect to the plate. The shackle is biased in a downward direction relative to the plate 39 by means of a spring I i5 which is compressed between the plate and a washer Ill. The washer may be secured to the shackle in any suitable manner as by means of a nut H9 which is in threaded engagement with an extension of the shackle. If a s'ufiicient force i's'applied to the governor rope to compress the spring l it, the shackle iii may be moved in an upward direction relative to the plate is. The upward motion of the shackle may be limited by means 'of'a sleeve lZi positioned within the spring to'serve as a stop. The remaining end of the governor rope is secured to the lower endof the 'shackle'in any suitable manher as by means of clamps I23.

Upward movement ofthe shackle HI with respect to the plate 591s employed for rotating the cross'shaft S9 in aclockwise'direction, as viewed inFig. 3. --To this end the shackle II I is provided with a cam l25 which is positioned beneath through the cam I25to rotate in a clockwise direction for the purpose of setting the braking units.

The Shauna I: doesinot-impede. independent rotation of. the. .arm. 61 in. a. clockwise direction about the axis of the cross shaft 69. Consequently, the shaft 69 may be operated independently by either the spring 89 (Fig. 4) or the cam I25.

It will be recalled that safety switches may be employed for controlling the energization of the elevator system. One of these switches 35 may be operated by the cross shaft 69 in the manner illustrated in Fig. 4. It.will be noted that the switch 35 includes a bridging bar 350. which is secured to the cross shaft 69. The bridging bar bridges two stationary contacts 35b and 350 which are insulated from each other. The switch in Fig. 4 is illustrated in closed position. When the spring 89 rotates the shaft 89 about its axis, it also moves the bridging bar 35a away from the fixed contacts 35b and 35c to open the switch.

It is believed that the operation of the system as a whole now may be set forth. It will be assumed that the circuit interrupter 21 of Fig. 1 is closed and held in latched position by means of the latch 21a. The switches 3I, 33 and 35 all are assumed to be closed. Let it be assumed first that the elevator car I5 is operating in a downward direction and that the speed of the elevator car gradually approaches an unsafe value. At a predetermined speed value, the governor I l operates to apply a brake to the rope 9. As the elevator car continues downwardly, the spring H5 is compressed and the cam I25 (Fig. 3) lifts the arm Bl to rotate the cross shaft 59 in a clockwise direction. When the stop I2I reaches the plate es, the elevator car increases the force acting on the rope 9 and the rope begins to slip through the brake of the governor. This brake is designed to exert a substantial predetermined force on the governor rope.

The rotation of the cross shaft 69 operates through the arms 61 and I09 to raise the rollers 55 and Iii? (Fig. 2). These rollers enter the more restricted portions of their races and are urged into engagement with the associated guide rails 5. Once the rollers engage the guide rails, the braking units become self-energizing and the rollers are rolled into engagement with the upper end surfaces of their races. The rollers thereafter slide on the guide rail without rotating and exerting a substantial braking force between the guide rails and the elevator car.

As the rollers 65 and IE1 are moved upwardly in their races, they force their associated levers about their pivoting axes to move the rollers BI and I55 into engagement with the associated guide rails. Once such engagement is efiected, the rollers 8i and I05 become self-energizing and are rolled by their associated guide rails into engagement with the upper end walls of their races. Thereafter the rollers BI and I05 slide on the guide rails to exert a substantial braking force thereon.

It will be recalled that the springs 55 determine the forces urging the rollers against the associated guide rails. Once the braking forces required are determined and the friction coefficients are ascertained, the springs 55 may be selected to provide the desired braking efiorts. The maximum braking forces may be applied very rapidly after the governor rope has been stopped. For example, it is entirely feasible to apply the maximum braking effort in response to car travel of less than 2 /1 after the governor rope has been stopped.

If the governor includes a safety switch 33 (Fig. 1), this switch also operates to disconnect the elevator system from its source of electrical energy.

If it is desired to reset the braking mechanism, the source of electrical energy may be reconnected to the elevator system and the motor control may be set for upward travel of the elevator car. Since the braking units engage the guide shafts only through the rollers 55, 8|, I05 and H11, and since these rollers are free to rotate for a substantial distance away from their braking positions during upward travel of the elevator car, excessive power is not required to start the elevator car upwardly after the braking units have operated. The braking units are automatically reset after a very short travel of the elevator car in the upward direction.

Let it be assumed next that the elevator car accelerates in a downward direction at an excessive rate. As a result of such acceleration, the torque exerted by the weight W (Fig. 4) on the cross shaft 69 is substantially reduced. When the acceleration reaches a predetermined value, the torque exerted on the cross shaft 59 by the spring 89 becomes sufficient to rotate the cross shaft in the proper direction for setting the braking units. The governor rope and its shackle (Fig. 3) do not interfere with such rotation of the cross shaft 69. As a result of such rotation, the braking units rapidly retard the elevator car in the manner previously discussed. Also the safety switch 35 opens and the circuit interrupter 2! (Fig. 1) is tripped to deenergize the elevator system. The braking units may be reset completely in the manner previously described.

The ease of resetting eliminates the need for special wrenches or doors providing access to the braking units. This feature is particularly desirable for safeties employed on elevator counterweights. Such counterweights often are difficult to reach for the purpose of manually resetting conventional types of safety devices.

Inasmuch as the safety device and the ac coloration-responsive operator are completely mounted on the elevator car, the devices are eifective even though all ropes or cables extending between the elevator car and the hoistway are severed. Furthermore, the response of the braking units to increase in acceleration is so rapid that the elevator car may be retarded before the governor has had an opportunity to stop the governor cable. This feature is particularly desirable for the situation wherein the elevator car suddenly accelerates as it approaches its pit. The governor may fail to operate before the elevator car actually reaches the buffer commonly located in the pit and the buffer is not designed to absorb the energy resulting from such abnormal acceleration.

Although the invention has been described with reference to certain specific embodiments thereof numerous modifications are possible which fall Within the spirit and scope of the invention.

I claim as my invention:

1. In an elevator system, a movable body, means including a smooth continuous guide rail for guiding the movable body in a vertical path, a brake mounted on said body for movement in a path adjacent the guide rail, said brake normally being disengaged from the guide rail and being operable into braking engagement with the guide rail for retarding the movable body, and acceleration-responsive means completely mounted on the body for actuating the brake into said braking engagement in response to increase of the acceleration of the body above a predetermined value said acceleration-responsive means comprising a weight, means mounting the weight on said body for rotation relative to the body about a horizontal axis, spring means urging the weight about the horizontal axis against the influence of gravity, and means responsive to rotation of the weight about the horizontal axis under the influence of the spring means for actuating the brake.

2. A system as defined in claim 1, wherein the brake comprises a roller normally spaced from a surface of the guide rail with its axis parallel to the adjacent surface of the guide rail and perpendicular to the direction of travel of the body, said brake having a wedging surface mounted for movement relative to the body, and spring means urging the wedging surface towards said surface of the guide rail, the roller being positioned between said surface of the guide rail and said wedging surface, said wedging surface being shaped to force the roller against said surface of the guide rail when the roller is moved in an upward direction, relative to the wedging surface, said acceleration-responsive means comprising means for urging the roller in an upward direction in response to increase in the acceleration of the body above said predetermined value.

3. A system as defined in claim 2, wherein the brake includes a pair of pivoted levers defining jaws positioned to receive therebetween the guide rail, one of said jaws providing said wedging surface, said last-named spring means urging the jaws towards clamping relationship relative to the guide rail therebetween, and stop means stopping motion of the jaws short of the clamping relationship.-

4. A system as defined in claim 3 in combination with stop means establishing a predetermined position for said jaws clear of the guide rail, and resilient means urging the levers about their axis into engagement with the last named stop means.

5. In an elevator safety device, a pair of spaced rollers having parallel axes, a supporting structure having a pair of spaced surfaces engaging respectively said rollers, said surfaces being inclined with respect to each other for urging said rollers towards each other in response to movement of the rollers in a predetermined direction transverse to said axes relative to the supporting structure.

6. A device as claimed in claim 5 in combination with an operator engaging one of said rollers for moving the roller relative to the supporting structure in said predetermined direction.

7. A safety-jaw device for an elevator system, comprising a pair of pivoted levers defining jaws movable towards and away from each other, a pair of rollers, adjacent faces of said jaws having recesses for receiving respectively the rollers with the axes of the rollers substantially parallel to each other, said recesses having surfaces inclined relative to each other for urging the rollers towards each other in response to movement of the rollers in a predetermined direction substantially transverse to said axes.

8. A device as claimed in claim '7, in combination with spring means associated with said levers for urging said jaws towards each other and stop means preventing the jaws from closing completely.

9. In an elevator system, a guide rail having spaced, continuous guide surfaces, at supporting structure, means engaging the guide rail for guiding the supporting structure for movement parallel to the guide rail, and a safety brake mounted on the supporting structure for gripping the guide rail to retard movement of the supporting structure, said safety brake comprising a pair of pivoted levers defining pincers having jaws positioned to grip the rail, a pair of rollers, each of said jaws positioning a separate one of the rollers adjacent but spaced from a separate one of said guide surfaces, said rollers having their axes parallel to said guide surfaces and transverse to the direction of movement of the supporting structure, said rollers being free to move relative to the jaws in the direction of movement of the supporting structure, a first one of the jaws having a wedging surface inclined relative to the associated guide surface for urging a first one of the rollers into intimate engagement with the associated guide surface as the first one of the rollers is moved relative to the jaws in the direction of movement of the supporting structure, and operating means for moving the first one of the rollers relative to the jaws.

10. A system as defined in claim 9, wherein a second one of said jaws has a wedging surface inclined relative to the associated guide surface for urging a second one of the rollers towards the associated guide surface as the second roller moves in an upward direction relative to the jaws, said guide rail being vertically disposed.

11. A system as defined in claim 10 in combination with spring means biasing said jaws towards each other, stop means preventing full closure of the jaws on said guide rail, and stop means restricting upward travel and rotation of said rollers relative to the jaws, said rollers in travelling upwardly relative to the jaws acting to spread the jaws against the resistance of the spring means, whereby said spring means determines the force exerted by said rollers on the guide rail.

12. A system as defined in claim 11, wherein said second roller normally is displaced from the guide rail, said first roller in rising relative to the jaws acting to rotate the jaws to wedge the second roller into engagement with the associated guide surface.

13. The method of braking and releasing an elevator system which comprises gripping an elevator guide rail between a pair of rollers, forcing the rollers to roll and approach each other for one direction of travel of the rollers along the guide rail, and reversing the direction oi. travel of the rollers along the guide rail to release the rollers from the guide rail.

LENNIUS R. RISSLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 441,778 Stean, et a1. Dec. 2, 1890 583,125 Rossler May 25, 1897 751,330 Nameche Feb. 2, 1904 824,452 Taylor et a1. June 26, 1906 2,001,361 Hymans May 14, 1935

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