US3065708A

US3065708A - Fluid pressure transducer

Info

Publication number: US3065708A
Application number: US845909A
Authority: US
Inventors: Jay M Roth
Original assignee: MECHANISMS Co
Current assignee: MECHANISMS Co
Priority date: 1959-10-12
Filing date: 1959-10-12
Publication date: 1962-11-27
Anticipated expiration: 1979-11-27

Description

Nov. 27, 1962 J. M. ROTH 3,065,708

FLUID PRESSURE TRANSDUCER Filed Oct. 12, 1959 4 Sheets-snee?I 2 FIG.2

JNVENToR.

Doff JAY M. ROTH ATT YS;

Nov. 27, 1962 J. M. ROTH FLUID PRESSURE TRANSDUCER 4 Sheets-Sheet 3 Filed Oct. l2, 1959 INVENTOR. JAY M. ROTH ATTYS.

Nov- 27, 1962 J. M. ROTH 3,065,708

FLUID PRESSURE TRANSDUCER Filed Oct. l2, 1959 4 Sheets-Sheet 4- 0 200 400 @00 80o /000 J200 /400 M00 /oo 2000 .200 2400 .O/Sc/-l/LQGE eassl/QE 25J.

INVENTOR. JAY M. ROTH ATTYS.

United States Patent @dice 3,065,703 FLUED PRESSURE 'ERANSDUCER Jay M. Roth, Uhrichsvilie, hio, assigner to Mechaiisrns Company, Uhrichsvilie, (Bixio, a corporation of Filed Oct. 12, 1959, Ser. No. 845,969 6 Ciairns. (B2i. 163-136) This invention relates to iiuid pressure transducers for operations upon iiuid under relatively high pressures, and,

,more particularly, is concerned with improvements in slipper type transducers.

Heretofore in the hydraulic pump and motor art rotary vane type transducers have niet with very considerable commercial acceptance and use, particularly in the relatively low pressure held. However, as pressures are increased in vane type pumps the pressures wedge the vanes sideways in the slotted rotor radial guides for the vanes and so much friction and wear is encountered that con- Ventional vane type pumps and motors prove unsatisfactory for operation with high hydraulic pressures.

In endeavoring to overcome the objections to vane type transducers efforts have been made in the past to replace the varies with slippers utilizing the principle of the Kingsbury thrust bearing in which a slipper was free to pivot in one plane so that a wedge-shaped hydrodynamic oil iilm was `formed between the base of the slipper and the surface supported thereby. Professor Kingsbury of New Hampshire State College in 1900 invented his thrust bearing to give a coefficient of friction of .O02 and permitting high speed operation at pressures up to 9800 pounds per square inch of projected bearing area.

However, in adapting or attempting to adapt the Kingsbury slipper principle to a hydraulic transducer' many problems were encountered. One of the first efforts resulted in Wissler US. Patent No. 1,651,336, granted November 29, 1927. However, in this and most of the succeeding attempts to design a slipper pump the radial travel of the slippers was limited to only about 5 percent of the rotor diameter, and this means that the pump becomes quite bulky for any given capacity, and the load on the journal bearings is so large that it reduces the pump eiiiciency and places a severe limit on the maximum practical pressure. Moreover, in known slipper transducer designs true Kingsbury slipper action with its attendant low `friction has proven difficult to obtain and maintain particularly at high pressures.

It is the general object of the present invention to avoid and overcome the foregoing and other objections to prior art practices by providing a slipper pump or motor in which the radial travel of the slippers is in the neighborhood of percent of the rotor diameter, and in which the construction of the slippers is such that an oil fium thickness necessary to assure hydrodynamic lubrication is maintained between the slipper face and the inside of the transducer casing, sometimes called a earn ring, even at discharge or input pressures up to or in excess of 2400 pounds per square inch to maintain a true Kingsbury action between the slipper faces and the cam ring.

Another object of the invention is to provide a slipper transducer of the type described wherein springs are not required to hold the slippers against the cam ring, wherein liberal clearances can be provided between the rot-or and the casing or cam ring, and wherein end clearances between the rotor and casing are achieved by a pressure loaded end plate construction shown in one applieants earlier patent, namely No. 2,856,860, and wherein the slippers have the advanage that they need not fit the slots with clearances closer than .006 inch.

Another object of the invention is the provision of a Bbb Patented Nov. 27., 1962 slipper type transducer construction wherein all forces acting one the slipper body intersect at a single point in the body so located as to insure that a hydrodynamic oil film is formed and maintained on the bearing surface of the slipper body.

Another object of the invention is to provide a slipper having a truncated triangular shape in cross section with ilatly curved sides formed to a curve so that the effective force exerted on the slipper side by the radially outer corner of the rotor slot makes an angle of -rnore than about 14.5 degrees with a line drawn from the point of contact perpendicular to the curved sides of each slipper regardless of the radial position of the slipper in the slot or the direction of movement into or out of the slot by the slipper.

Another, and a more specific object of the invention, is to provide a slipper type transducer wherein each slipper has a slightly smaller radius on its face than the radius of the inside of the casing or cam ring with which it contacts, the radius of the slipper face being not more than about 2.0 percent less than the radius of the inside of the casing.

Another speciiic object of the invention is the provision of a slipper type transducer in which each slipper has oil grooves formed on its face which contacts with the inside of the casing or cam ring, the oil grooves extending up to at least about 25 percent of the width of the face from its leading edge.

The foregoing objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by the provision of a fluid pressure transducer inciuding a housing, a rotor of smaller diameter than the inside of the housing rotatably mounted in the housing to almost touch one side of the housing to provide a sealing arc and a diametrically opposed pumping arc, an inlet port in the housing between the sealing arc and the pumping arc, an outiet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumferentialiy spaced il-shaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape receive in each slot, each slipper having flatly curved sides adapted to engage with substantially a line contact with one or the other corner or side of the rotor slot, the width of each slipper being such that only a small clearance exists between one side of the slipper and its rotor slot during the movement of the slipper through the sealing arc and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the radially outer face of each slipper being formed to a radius up to about 2.0 percent smaller than the radius of the pumping and sealing are, and the outer lface of each slipper having oil grooves cut back from the leading edge at a distance of not more than about 25 percent to allow iuid pressure to get under the leading edge of the slipper, the forces acting on each slipper being substantially balanced to prevent it from rotating, turning over or wedging in its rotor slot, each slipper having a radial travel equal to at least about 10 percent of the rotor diameter.

For a better understanding of the invention reference should be had to the accompanying drawings wherein FIG. 1 is a cross sectional view through a transducer constructed in accordance with the invention;

FIG. 2 is a diagrammatic showing of the action of slippers of the inventive design in three positions in the pumping arc and under heavy pressure and indicating how the slippers cannot be maintained in the position shown of riding on their leading edges.

FiG. 3 is a view similar to FIG. 2 but showing the slippers with their faces seated on the cam ring;

FIG. 4 is a diagrammatic showing of a single slipper afa-earns provided with oil grooves on its face near `the leading edge and illustrating the diagram of forces on the slipper;

FIG. is a view similar to FIG. 4 but illustrating a diagram of forces on a single slipper in which the slipper face is not only formed with oil grooves, as in FIG. 4, but is provided with a radius up to about 2.0 percent less than the radius of the cam ring with which it engages; and

FIG. 6 is a chart showing the performance characteristies ofthe'improved transducer of the present invention.

in the drawings, the numeral 1t) indicates generally a transducer casing, sometimes more properly called a cam ring because the cam ring may be mounted in a surrounding housing or casing. The cam ring 10 is formed with a sealing arc 12 and a substantially diametrically opposed pumping arc 14, and with an intake port 16 being positioned between the sealing arc 12 and the pumping arc 14, and with an output or pressure port 13 being provided between the pumping arc 14 and the sealing arc 12. The inside surface of the cam ring y10 and marked with the numerals is coincident with the inside surfaces 22- of the sealing arc and the inside surface 24 of the pumping arc and is circular.

Rotatably mounted inside of the cam ring 10 is a circular rotor 26 of smaller diameter than the inside of the cam ring, and rotatably carried by a shaft 2S which is mounted in end plates, not shown, so as to be eccentric to the inside surface of the cam ring `10, the points of support of the shaft 28 being positioned substantially midway of the sealing arc 12, and with a clearance of liberal dimensions being provided between the rotor and the sealing arc at point 30 of approximately .010 inch. The rotor 26 is formed with a plurality of xl-shaped slots 32 extending transversely of the rotor and these slots receive slippers 34 of truncated triangular shape in cross section having radially outer faces engaging with the inner surface of the cam ring with an interposed hydrodynamic oil iilm (not shown because of its thinness), all in a manner more fully described hereinafter.

The dimension of the rotor 26 is such that the slippers 34 in the rotation of the rotor 25 and the shaft 28 in the direction of the arrow will have a movement in and out at least equal to 10 percent of the rotor diameter, and usually in the neighborhood of 10% of the rotor diameter. The cross sectional dimensions of each slipper 34 are such in relation to the extent of movement and the size of the slot l312 that a slipper cannot tilt over sideways or wedge in a slot 32 but would engage in the slot in the manner shown by dotted lines 36 at the point of greatest clearance between the slipper 34 and the slot 32 if the slipper attempted to turn over. On the other hand, up in the area of the sealing arc when the slippers 34 are at the most inward positions in the slots 32 clearances of at least .O06 inch exist between the sides of the slipper and the slot.

End plates for the cam ring 10, the rotor 26, and the slippers 34 are not illustrated or described in detail, it being suiiicient here to `say that these end plates are preferably spring-loaded in the manner shown and described in applicant I ay M. Roths earlier U.S. Patent No. 2,856,- 860. Thus, the pressure loaded end plates are provided with a fixed clearance so that movement backward is permitted to allow dirt particles to pass. This design can handle dirt particles in the oil since no fixed clearance as close as .001 inch are required.

Now looking at FIG. 2, a slipper 34 is shown as riding on its leading edge on the inner surface of the cam ring 10 and if this would occur the slippers leading edge would scrape through the oil iilm and cause metal-to metal-contact. A reasonable friction coefficient for metal to metal contact is .35 (tangent of 19.5 degrees). Thus the force EC in FIG. 2, between the slippers leading edge and the cam ring is shown at 19.5 degrees. CD is the resultant of the hydraulic force acting on line AE. AB is the perpendicular force that the rotor 26 would exert on the slipper without friction. Now since CD and EC intersect at C, it is known from the principle of equilibrium, sometimes called the Direction Condition of Equilibrium, that the third force AC which comes from the contact with the driving rotor must intersect at C also if the slipper is to remain in equilibrium riding on its leading edge. p

Now because of the slow relative motion and the presence of adequate lubrication at point A, we have assumed that the friction coeflicient is .25 so that the corresponding augle will be just under 14.5 degrees. And as long as angle BAC exceeds or equals 14.5 degrees it indicates that there is not enough friction at A to hold the trailing edge of the slipper away from the cam ring so that the slipper actually will seat properly instead of riding on the leading edge as shown in the drawing.

in a previous design of a slipper pump, angle BAC was only l() degrees minimum and while operation was satisfactory with steady ow, the pump was very sensitive to the slightest amount of air in the oil or to pulsations fed back from a large hydraulic system. A reduction of the angle BAC to 7 degrees minimum made the pump very noisy as the slippers rode up on their leading edges and were later forced back with their faces against the cam ring. However, with a minimum angle of 14.5 degrees at BAC no riding on the leading edge of the slipper was found to occur under any operating conditions even those involving relatively high pressure.

In other words, FIG. 2 emphasizes that the slippers 34 will never ride up on their leading edges as long as the angle BAC is greater than about 14.5 degrees, and if they did the slipper would tilt about its leading edge E, moving outwardly in respect to point A even though the slipper as a whole was moving inwardly in the rotor slot 32. This is why the lines AC are drawn at an angle equal to or greater than the friction angle of 14.5 degrees above, i.e. radially inside, lines AB in all three showings of the slipper in FIG. 2.

From the teaching of FiG. 2 it becomes evident that with proper shaping of the curved sides of the slipper to maintain the aforesaid angles of at least 14.5 degrees, the radially outer faces of the slippers 34 will always be seated on the inner face of the cam ring 10 with an interposed hydrodynamic oil lm. FIG. 3 shows the location and direction of forces acting on the seated slippers in the pumping arc. AC is the force exerted on the slipper by the rotor and its direction is changed by 14.5 degrees by friction away from the normal or perpendicular force AB normally exerted by the corner of the rotor slot against the slipper. The direction of change depends, of course, on whether the slipper is moving in or out. The slipper shown at the right hand side of FIG. 3 is still moving outwardly in the rotor slot whereas the two slippers shown to the left and center of the figure are moving into the rotor slot. In each case the rotor force AC and the hydraulic force CF intersect at point C. Then in accord with the principle of Direction Condition Equilibrium, the resultant bearing pressure on the outer face of the slipper must pass through C also. rhis resultant DC is drawn radially from point C inasmuch as it can be expected, because of the Kingsbury slipper action, to have a friction coeiiicient of about .002 between the slipper and the cam ring so that the friction angle would be negligible.

In the slipper at the right hand side of FIG. 3 the resultant DC is 24.4 percent ahead of the slipper contact area with the slipper moving out and only 5 to 6.4 percent ahead with the slipper moving in as shown at the center and left hand side of FIG. 3. In developing the theory of Kingsbury slipper action in Engineering Applications of Fluid Mechanics by Hunsacker and Rightmire, published 1947 by McGraw-Hill Book Company, it is shown that when the slipper is parallel to its mating surface the resultant bearing load is in the center 0f the slipper and if the slipper tilts up in front the resultant moves toward the rear. Thus, true Kingsbury slipper action was not obtained by the arrangement shown in FIG. 3 because of lack of bearing load at the center of the slipper.

However, in the hydraulic transducer high pressure oil is avail-able at the leading edge of the slipper and this provides an advantage over a normal Kingsbury type bearing in which high pressure oil is not available in the bearing system itself. In accord with the teaching of the present invention the high pressure oil available in the hydraulic transducer is deliberately employed to be passed underneath the leading edge of the slipper by providing a small oil groove 40 located about one-quarter the way back from the leading edge of the slipper, fed by two chamfers 42 at the ends of the slipper so that the high pressure oil will always get started under the leading edge of the slipper. This construction is diagrammatically shown in FIG. 4 together with an approximate pressure distribution curve which illustrates the resultant bearing pressure behind the center of the slipper. This curve is represented by the numeral 44. The other force 46 is the resultant of the rotor and hydraulic forces on the slipper. The several forces illustrated tend to tilt the slipper forward until its outer face is parallel with the cam ring. It has been found that in the construction shown in FIG. 4 that a true Kingsbury slipper action is still not obtained, and tests indicate that the adhesion of the oil provides adequate lubrication to a pump discharge pressure of about 1200 pounds per square inch.

Tests also indicate that

oil grooves

40 and 42 are unnecessary if the pump is supplying a very small hydraulic system. However, when the pump is supplying a large hydraulic system, particularly with rubber hoses present in the system, pulsations feed back from the system and tend to squeeze out the oil film underneath at least the leading edge of the slipper. With the oil grooves the pressure pulsations are carried part way under the slippers so adequate lubrication is still provided, so that the provision of the oil grooves at the leading edge of the slipper in the manner described does improve the action of the hydraulic transducer. Moving the oil groove 40 back towards the middle of the slipper face was found to require an increase in power which is believed to result from a shifting of the peak of the pressure distribution curve towards the rear or trailing edge of the slipper.

FlG. 5 shows a slipper 34 having a face radius slightly less than the cam ring radius. Usually the radius of the slipper is up to about 2.0 percent less than the radius of the cam ring inner face. Specifically, making the radius of the slipper 1/2 of l percent less than the radius of the cam ring inner face has been found to work effectively. With less radius on the slipper face than on the cam ring, as the slipper tilts forward, the point of minimum oil rilm thickness shifts forward until the hydrodynamic bearing pressure is in line with the forces acting downward on the slipper. In other words, the resultant 4o of the bearing pressure curve 48 comes into line with the resultant 50 of the forces acting downwardly on the slipper. This is -accomplished without losing the cohesive action of the oil film which helps hold the slippers -to the cam ring, as the variation in oil film thickness necessary to assure hydrodynamic lubrication is less than .0001 inches.

With this change of radius on the slipper face a true Kingsbury action was obtained up to a discharge pressure of 2400 pounds per square inch which was the limit of the test equipment. An analysis similar to FIG. 3 must be made for the slippers as they pass through the sealing arc l2 at the top of the cam ring l0 to assure hydrodynamic lubrication and stability through this arc. lf the slippers are permitted to leave the cam ring in the sealing arc, they will not be properly seated when they pass the suction port -and the discharge pressure will force them out and back against the rotor and cam ring with appreciable noise. However, in the pump illustrated and described the combination of centrifugal force on the slippers and cohesion of the oil iilrn between the slippers and the cam ring, together with the hydraulic pressure in the transducer has been found to keep the slippers against the cam ring during the entire operation of the improved transducer of the invention without the necessity for springs which bring various complicating factors into design.

Performance data for the transducer ot the invention, and operating as a pump is illustrated in FIG. 6. The particular pump tested employed a slipper radial travel of l0 percent of the rotor diameter. Polarine No. 101W oil at degrees Fahrenheit, and with the pump being driven by a U.S. Electric motor having the following characteristics: 5 H.P., 3 ph., 60 cyc. r.p.m. 1750, and rated amps 14. The graph shows the gallons per minute pumped, the motor revolutions per minute, the volumetric eiiciency, and the amperage requirements plotted against pressures from zero to 2400 pounds per square inch.

In the foregoing it will be recognized that the various objects of the invention have been achieved by the provision of a relatively simple slipper type pump in which a true Kingsbury slipper action has been found to be achieved up to discharge pressures of 2400 pounds per square inch and over. The only close tolerances required in the manufacture of the pump are on the length dimension which can be easily held by surface grinding. Wear on the loaded surfaces does not increase leakage even over various extended periods of use. The transducer is relatively quiet, has good volumetric ediciency and very considerable displacement even at relatively high pressures.

While in accord with the patent statutes certain best known embodiments of the invention have been illustrated and described in detail, it is to be particularly understood that the invention is not to be limited thereto or thereby, but that its scope is detined in the appended claims.

What is claimed is:

l. A fluid pressure transducer including a circular housing, a circular rotor of smaller diameter than the inside of the housing rotatably mounted in the housing so that the circle of the rotor is eccentric to the circle of the housing and so that the rotor will almost touch one side of the housing to provide a sealing arc and a diametrically opposed pumping arc, an inlet port in the housing between the sealing arc and the pumping arc, an outlet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumferentially spaced V-shaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape received in each slot, each slipper having flatly curved sides adapted to engage with substantially a line contact with one or the other corner of the rotor defining the radially outer corner of the rotor slot, the width of each slipper being such that only a small clearance exists between one side of the slipper and its rotor slot during the movement of the slipper through the sealing arc and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the radially outer face of each slipper being formed to a radius smaller than the radius of the pumping arc and sealing arc, said smaller radius being up to substantially 2.0 percent less than the radius of the pumping arc and the sealing arc, the outer face of each slipper having oil grooves cut back from the leading edge at a distance of up to substantially 25% of the circumferential length of the outer face of the slipper to allow iluid pressure to get under the leading edge of the slipper, the forces acting on each slipper being substantially balanced to prevent it from rotating, each slipper having a radial travel in the neighborhood of 10% of the rotor diameter.

2. A fluid pressure transducer including a housing having a circular interior, a rotor having a circular exterior of smaller diameter than the inside of the housing rotatably mounted eccentrically in the housing to almost touch one side of the housing to provide a sealing arc and a diametrically opposed pumping arc, an inlet port in the housing between the sealing are and the pumping arc, an outlet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumerentially spaced V-snaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape received in each slot, each slipper having flatly curved sides adapted to engage with substantially a line Contact with one or the other corner of the rotor defining the radially outer corner of the rotor slot, the width of each slipper bein such that only a small clearance exists between one side ofthe slipper and its rotor slot during the movement of the slipper through the sealing arc and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the outer face of each slipper having oil grooves cut back from the leading edge at a distance of substantially 25 percent of the circumferential length of the slipper face to allow lluid pressure to get under the leading edge of the slipper, the forces acting on each slipper being substantially balanced to prevent it from rotating, each slipper having a radial travel in the neighborhood of 10 percent of the rotor diameter.

3. A uid pressure transducer including a circular housing, a circular rotor of smaller diameter than the inside of the housing rotatably mounted eccentrically in the housing to almost touch one side of the housing to provide a sealing arc and a diametrically opposed pumping arc, an inlet port in the housing lbetween the sealing arc and the pumping arc, an outlet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumferentially spaced V-shaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape received in each slot, each slipper having flatly curved sides adapted to engage with substantially a line contact with one or the other corner of the rotor delining the radially outer corner of the rotor slot, the width of each slipper ybeing such that only a small clearance exists between one side of the slipper and its rotor slot during the movement of the slipper through the sealing arc and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the radially outer face of each slipper being formed to a radius up to about 2.0 percent and smaller than the radius of the pumping arc and sealing arc, the outer face of each slipper having oil grooves cut back from the leading edge at a distance of less than substantially 25 percent of the circumferential length of the slipper face to allow fluid pressure to get under the leading edge of the slipper.

4. A lluid pressure transducer including a circular housing having a circular bore, a circular rotor of smaller diameter than the inside of the housing rotatably mounted eccentrically in the housing bore to almost touch one side of the housing bore to provide a sealing arc and a diametrically opposed pumping arc, an inlet port in the housing between the sealing arc and the pumping arc, an outlet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumferentially spaced V-shaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape received in each slot, each slipper having flatly curved sides adapted to engage with substantially a line contact with one or the other corner of the rotor dening the radially outer corner of the rotor slot, the width of each slipper being such that only a small clearance exists between one side of the slipper and its rotor slot during the movement of the slipper through the sealing are and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the flatly curved sides of each slipper being formed to a curve so that the force from the radial outer corner of the rotor slot required on the slipper side to keep the slipper riding on its leading edge throughout the pumping arc makes an angle of more than substantially 14.5 degrees with a line drawn from the point of contact perpendicular to the curved side of each slipper regardless of the radial position of the slipper in the slot or the direction of movement into or out of the slot.

5. A slipper type hydraulic transducer including a circularly hollow casing, a slotted circular rotor eccentrically positioned in the casing, a slipper carried in each slot of the rotor and free for tilting movement therein, each slipper having oil grooves formed on its face which contacts with the inside of the casing, said oil grooves extending up to less than substantially 25 percent of the circumferential length of the face from its leading edge but leaving a land on the face of the slipper between the oil grooves and the leading edge of the slipper.

6. A uid pressure transducer including a housing having at least one pumping are and one sealing arc, a rotor rotatably mounted in the housing, an inlet port in the housing between the sealing arc and the pumping arc, an outlet port in the housing between the pumping arc and the sealing arc, said rotor having a plurality of circumferentially spaced V-shaped slots therein extending in axially parallel direction, a slipper of truncated triangular shape received in each slot, each slipper having flatly curved sides adapted to engage with substantially a line contact with one or the other corner of the rotor defining the radially outer corner of the rotor slot, the width of each slipper being such that only a small clearance exists between one side of the slipper and its rotor slot during the movement of the slipper through the sealing arc and the clearance of the parts is such that during movement of each slipper through the pumping arc the slipper cannot turn over in its rotor slot, the llatly curved sides of each slipper being formed to a curve so that the force from the radial outer corner of the rotor slot required on the slipper side to keep the slipper riding on its leading edge throughout the pumping arc makes an angle of more than substantially 14.5 degrees with a line drawn from the point of contact perpendicular to the curved side of each slipper regardless of the radial position of the slipper in the slot or the direction of movement into or out of the slot.

References Cited in the file of this patent UNlTED STATES PATENTS 1,917,054 Norling July 4, 1933 1,964,244 Benedek June 26, 1934 2,278,131 Livermore Mar. 31, 1942 2,830,543 Roth Apr. 15, 1958 2,983,226 Livermore May 9, 1961 2,984,186 Livermore et al May 16, 1961