Piston return mechanism
US 3303794 A
Description (OCR text may contain errors)
Feb. 14, 1967 D. w. HAGEMANN PISTON RETURN MECHANISM 2 Sheets-Sheet 1 Filed May 51, 1966 Feb. M, E967 D. w. HAGEMANN 3,303,794
PI STON RETURN MECHANI SM Filed May 51, 1966 2 Sheets-Sheet 2 United States Patent C) 3,303,794 PISTON RETURN IJIECHANISM Donald W. Hagemann, Rockford, lll., assignor to Sunil-strand Corporation, a corporation of illinois Filed May 31, 1966, Ser. No. 553,861 11 Claims. (Cl. 103-162) This application is a continuation-in-part of my prior copending application Ser. No. 466,589 filed June 24, 1965, and relates generally to hydraulic fluid energy translating devices and more particularly to axial piston pumps and motors.
In devices of the type described, `a rotatable cylinder block is provided with axially disposed cylinders having pistons reciprocable therein. An inclined cam plate or swashplate is connected to reciprocate the pistons as the block rotates against a porting member having inlet and outlet ports. The cylinders communicate serially with the inlet and outlet ports, and las the pistons reciprocate, uid is drawn into the cylinders and expelled therefrom through the ports. ln devices of this type, it is desirable that the cylinder block be resiliently biased toward the relatively stationary port member to `maintain the proper running engagement between the cylinder block and the port mem-ber and to prevent excessive fluid leakage from `between these members. In the absence of such a biasing means, the inherent forces placed on the cylinder block would tend to displace the cylinder `block from the port plate.
It is also desirable in this type of axial piston unit to maintain a force transmitting contact between the ends of the reciprocating pistons and the inclined cam plate. Conventionally, bearing slippers with spherical sockets are provided on the ends of the `pistons engaging the cam plate to transmit the necessary reciprocating force between the cam plate and the pistons. `It has been proposed in the past that spring means `be provided reacting between the cylinder 'block and the piston bearing slippers to maintain not only the contact of the piston bearing members with the cam plate, but also Contact between the cylinder block and the port plate. However, in some designs of fluid energy transl-ating devices, it has been found that the force desirable to `maintain the piston bearing members in contact with the cam plate is .different from and sometimes much greater than that necessary to maintain the cylinder block in proper contact with the port plate. 'ln the prior spring arrangements, biasing force on the cylinder block is at least as great as the piston hold-down force. Therefore, if a suitable holddown force is provided in these prior hydraulic units, the cylinder blocl: biasing force may be excessive. This leads to `unnecessary friction, wear and loss of eiciency.
It is, therefore, a general object of the present invention to provide a new and improved piston return mechanism for an axial piston pump or motor device which biases the pistons toward the cam plate without reacting on the rotating cylinder block.
Another object of the present invention is to provide a new and improved piston return mechanism of the type described immediately above and a separate and independent biasing means for urging the rotating cylinder block into engagement with the port mem-ber to assure the proper sliding enga-gement therewith.
A more specific object of the present invention is to provide an axial piston hydraulic pump or motor of the type having `bearing members for transmitting the reciprocating force between an inclined cam 4plate and the reciprocable pistons with a new and improved piston return mechanism including an axially extending coiled compression spring mounted `within the lmain drive shaft of the pump or motor and reacting against the interior of the shaft so that the biasing force Kwill be absorbed by the 3,33,794 Patented Feb. i4, 1957 ICC bearings which support the shaft rather than the cylinder block; the ho-llow shaft having a ybiasing piston slidable therein urged away Afrom the lport plate by the compression spring, the slidable piston having projections extending through the hollow shaft and engaging a spherical retaining member which urges the piston bearings against the inclined cam face, the hollow shaft also yhaving an abutment thereon which serves as a spring seat for another coiled compression spring surrounding the shaft and er1- gaging the cylinder block so that the block is urged into engagement with the port plate completely independent of the means for urging the piston lbearings into engagement with the cam plate.
A further object of the present invention is to provide an axial piston pump of the type described generally above, and according to a modified yform of the present invention, in which the spherical retaining member has a projection which extends rearwardly therefrom away from the cylinder block and around the end of the hollow shaft, with the `compression spring within the shaft biasing the projection to ur-ge the piston bearing means into engagement with the cam member.
Other objects and advantages of the present invention will 4become readily apparent from the Afollowing detailed description taken in connection with the accompanying 'drawing in which:
FIG. 1 is a longitudinal cross section of a hydraulic pump or motor embodying one form of the present invention;
FIG. 2 is a cross section of a hydraulic pump or motor of a somewhat different construction than that shown in FIG. 1, embodying another form of the present invention;
FIG. 3 is a fragmentary section taken generally along line 3 3 in FIG. 2;
FlG. 4 is a fragmentary section of a portion of the pump shown in FIG. 2 with the parts Irotated 90 degrees; and
FIG. 5 is a fragmentary section of a hydraulic pump or motor of the type illustrated in FIG. 2 embodying still another form of the present invention.
While illustrative embodiments of the invention are shown in the drawings and will be described in detail herein, the invention is susceptible of embodiment in different forms, and it should Abe understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended t0 limit the invention to the embodiments illustrated. The scope of the invention will be pointed out in the appended claims.
Referring now to the embodiment shown in FIG. l, an axial piston pump is generally designated `by the numeral it) and includes a suitable housing 1l. The housing consists of a generally bell-shaped housing member 12 and a port member 13 fixed on the open end of the housing member 12 by suitable threaded fasteners 15. The port member T13 has a surface 1.7 with arcuate inlet and outlet ports therein (not shown) for conveying fluid to and 'from the pump or motor device. Suitable plugs 18 a-re provided in the housing member 11 'for permitting the addition or withdrawal `of lubricating fluid from within the interior of the housing.
A hollow drive shaft 2@ is rotatably mounted within the housing 11 and is adapted to transmit torque to or from the hydraulic unit 16 depending upon whether the unit is operating as a pump or motor. The left end of hollow drive shaft 2G is rotatably mounted in bore 22 of housing member 12 by a suitable roller bearing 24. Shaft 20 has an annular flange 26 thereon engaging the inward side of the inner race of bearing 24 so that this bearing serves `as a thrust bearing, as well as a radial bearing, thereby preventing axial movement of the shaft 20 to the left, as shown in the drawing. The right end of hollow shaft is reduced or stepped as at 28 and mounted within roller bearing 29 in turn mounted within the port member 13. The stepped end 28 defines a shoulder 31 which abuts the inward face of the inner race of bearing 29 so that this bearing serves as a thrust bearing preventing any rightward movement of hollow shaft 20.
The drive shaft 20 has an extension shaft 35 splined thereto and extending from the housing member 12 where it may `be connected to a suitable prime mover when the hydraulic unit is acting as a pump, or a suitable load when the hydraulic unit is acting as a motor. A seal assembly 37 is provided for preventing the egress of fluid from within the housing 11 and also for preventing the entry of foreign matter into the hydraulic unit. As the details of the seal 37 form no part of the present invention, they are not described in detail. Further, a seal plug 38 is provided in the left end of hollow shaft 20 to prevent the egress of fluid from the unit through the center of the shaft.
A generally cylindrical cylinder block 40 is provided with an axial extension 41 having inwardly directed splines thereon which interengage splines 43 on the hollow drive shaft 20. In this manner, torque is transmitted from the shaft to the cylinder block or from the cylinder block to the shaft during operation of the hydraulic unit. Cylinder block 40 has a plurality of axially disposed cylinders 44 therein, in annular array about the axis of shaft 20. Cylinders 44 may have suitable bearing sleeves 45 bonded therein, as shown. Each of the cylinders 44 has a piston member 47 slidable t-herein. The pistons 47 have spherical ends 48 received in the sockets of bearing slippers 49.
A normally stationary cam or swashplate member is provided for reciprocating the pistons 47 within the cylinder block as the block rotates. Cam plate 55 is pivotally mounted within the housing 11 by suitable trunnions 56 (shown only in dotted lines in the drawing) for pivotal movement about axis 60 which lies in a plane containing the pivotal connections of the pistons with the slippers 49 and intersects the shaft axis. The cam plate 55 has a camming surface 63 which slidably engages the rear faces of the slippers 49 thereby transmitting reciprocating motion to the pistons 47.
The angular position of cam face 63 and the cam member 55 may be varied by a suitable control motor assembly 67 which includes a piston 69 slidably mounted Within a cylinder 70 formed in the valve member 13. Piston 69 is connected to rotate the cam plate 55 counterclockwise about axis 60 by a suitable link 71 mounted within the piston 69 at one end and within a socket in the cam plate 55 at its other end. A bracket 73 on cam plate 55 retains the link 71 within the socket in the cam plate.
A swashplate biasing assembly is provided for continuously biasing the swashplate 55 in a clockwise direction about pivot 60, as shown in the drawing. The assembly 75 includes a piston 76 slidably disposed in a bore 77 in valve member 13. The piston is continuously biased to the left by fluid in bore 77 from a suitable source. A link 80 interconnects piston 76 and the swashplate 55. A bracket assembly 82, similar to bracket assembly 73 is provided for maintaining the link 80 in engagement with its complementary spherical seat in the cam plate 55.
It will be seen that as fluid is ported to the control cylinder 70 from a suitable source of control iiuid that the cam plate will pivot counterclockwise from the position shown against the biasing force of uid in bore 77 to reduce the displacement of the pump. Subsequently, if it is desired to increase pump displacement, fluid may be ported from cylinder 70 and the bias assembly 75 will urge the `cam plate 55 in a clockwise stroke increasing direction. If desired, control fluid may as Awell be selectively ported to bore 77, as well as cylinder 70, so thata double acting control may be achieved.
A cylinder block spring assembly is provided for biasing the cylinder block 40 against the face 17 of valve plate 13 to maintain a iiuid tight relationship therebetween as the cylinder block rotates. Cylinder block 40 has an enlarged central recess 86 which accommodates the spring assembly 85. The latter includes a washer-like spring seat 88 which abuts a shoulder on the hollow shaft 20 such as the end of splines 43 preventing leftward movement of the seat S8. At the end of the central recess 86 adjacent the port plate, a second annular spring seat 89 is provided, xed with respect to the cylinder block 40. A suitable snap ring 91, seated within a complementary groove in the central recess 86, prevents the rightward movement of spring seat 89. A helical coiled compression spring 93 is seated at one end against the spring seat 88 and at its other end engages spring seat 89 on the cylinder block. In this manner, the spring assembly 85 continuously biases the cylinder block against the valve plate 13. The reaction forces of helical compression spring 93 are transferred to the drive shaft 20 and bearing 24 which serves as a thrust bearing for this purpose.
A piston return spring assembly 95 is provided for continuously urging the piston slippers 49 against the cam surface 63 on cam plate 55. The hollow shaft 20 has a stepped portion adjacent its right end forming a lspring seat 97 for one end of a relatively long compression spring 100. An axially slidable piston member 102 is provided within the hollow shaft 20 having a surface 103 defining the other seat for spring 100. Piston 102 has a spherical outer surface to permit limited pivotal movements thereof. Itis seen that the piston 102 is continuously biased away from the valve plate by spring 100. This biasing force is transmitted from the piston 103 to a spherical retaining ball 105 by radially extending pins 106 mounted in the piston 102. Pins 106 project through axially elongated slots 108 in the hollow shaft 20, so that the piston 103 is permitted to move axially with respect to the shaft. In the exemplary construction shown in the drawing, three pins 106 and slots 108 are provided, but it' should be understood that a diierent number may be employed, when desired.
The distal ends of pins 106 are received in complementary recesses 110 in the interior of the retainer ball 105, so that the ball is urged to the left, as shown in the drawing. The spherical retainer ball 105 is also driven in rotation by pins 106, so that the ball is driven in rotation with the shaft, but may move axially with respect thereto on extension 41.
A retainer plate is carried by the spherical ball 105 and has a complementary central opening permitting pivotal move-ment between the retainer plate 115 and the retainer ball 105 as the displacement of the hydraulic unit is varied by the control 67. Plate 115 has a plurality of openings therein for receiving the slipper members 49. One side of plate 115 engages the slippers and urges them against the cam surface 63. As the cyllinder block and drive shaft 20 rotate so do the pistons 103, spherical ball 105, and the retainer plate 115. It will be seen that the slippers 49 are continuously biased against the cam surface 63 by the compression spring 100 through the piston 103, pins 106, spherical retainer ball 105, and the retaining plate 115.
It should be noted that the piston return assembly 95 is completely independent of the cylinder hold-down spring assembly 85. The reaction force of spring 100 is transferred to the housing 11 by spring 93 through the spring seat 97 and `bearing 29 which serves in this regard as a thrust bearing. None of the piston hold-down force is transferred to the cylinder block 40, so that the cylinder block biasing force and the piston return force may be selected as desired, independently of one another.
A somewhat modified form of the present invention is illustrated in FIGS. 2, 3, and 4. As shown a piston return assembly 295 is shown incorporated into an axial piston pump or motor 210 of the general type disclosed in the copending application of C. J. Gantzer, Serial No. 421,483,
filed December 28, 1964, and assigned to the assignee of the present invention, to which reference may be made for details of construction and operation of a hydraulic unit of this type, although it is generally similar to the pump or motor disclosed with reference to FIG. 1.
As described in the above mentioned application, the hydraulic unit 210 comprises one-half of a pump and motor combination fixed to a valve plate 213 which has inlet and outlet ports 214 and 214a formed therein. A housing 211 shown diagram-matically in FIG. 2 surrounds the pump and motor combination along with associated gearing (not shown). A gear 216, which transfers torque to or from the hydraulic unit 210, depending on whether it is operated as a pump or as a motor, has an axial sleeve portion 217 formed integrally therewith. A bearing 224, fixed within the housing 211, receives the sleeve portion 217 and thereby rotatably supports the gear 216 in the housing. A hollow shaft 220 has one end thereof received within the gear sleeve 217, and is rotatably fixed thereto by a key 221 as shown in FIGS. 3 and 4. As shown more clearly in FIG. 4, the sleeve 217 has an inwardly extending flange 218 engaging the end of shaft 220. The other end of the shaft 220 is stepped as at 228 and received in a roller bearing 229 mounted within valve plate 213. Bearings 224 and 229 take the axial thrust force from shaft 220 in a manner similar to that in which `bearings 24 and 29 are loaded in the FIG. 1 e-mbodiment.
Shaft 220 is splined to cylinder block 240 as at 243. Cylinder `block 240 has pistons 247 axially slidable therein with bearing slippers 249 pivotally connected to the projecting ends thereof. Slippers 249 have bearing surfaces 250 slidably engaging a pivotally adjustably cam member 255. Cam 255 is pivotaly mounted in a partial housing 256 which is fixed to valve plate 213. The piston return mechanism 295 is generally similar in fuction to the return mechanism 95 shown in FIG. 1 although it is of somewhat simpler construction.
A retaining plate 315 is generally similar to the retaining plate 115 in FIG. 1 except that it has an additional spherical lip or flange portion 316 to provide more seating surface on a spherical retainer 305.
The spherical retainer 305 is piloted on a rearward extension 241 of the cylinder block 240 and has a generally U-shaped strap or projection 296 consisting of legs 297 fixed to the rearward end of the spherical member 305 and slidable on shaft 220 and a radial bight portion 298 extending over the end of shaft 220.
As shown more clearly in FIG. 3, the sleeve portion 217 of gear 216 has two axially extending diametrically opposed slots 299 slidably receiving the legs 297, and which serve to transfer torque from the gear 216 to the spherical ball 305.
The flanged portion 218 is slotted as shown at 219 in FIG. 4 to slidably receive the `bight portion 298 of the strap. A diametral slot 222, shown in FIG. 2, in the end of shaft 220 partially receives and permits axial movement of the bight portion 298 toward the cylinder block.
Within the lhollow shaft 320 is a coiled compression spring 300 housed in a bore 316 and reacting against a seat member 317 which in turn bears against a stepped portion of bore 316 near the valve plate 213. The other end of the spring biases a cylindrical plug 319 slidable in the other end of bore 316 and projecting somewhat therefrom. Spring 300 serves to bias the plug 319 into engagement with the bight portion 298 of the projection 296. This biases the spherical ball 305, and the retainer plate 315 as a unit away from the cylinder block urging the bearing slippers 249 into engagement with the cam member 355.
A cylinder block hold-down spring mechanism 285 is similar in operation `and construction to the assembly 85 shown and described with reference to FIG. l, so that a detailed description thereof is not believed necessary.
The embodiment shown in FIGS. 2, 3 and 4 permits a greater seating surface of the retainer plate 315 as shown at 316 on the spherical retainer 305. It further permits the use of a stiffer shaft than in the FIG. 1 embodiment, and permits the spring force to be directly applied to the spherical ball assembly 305. An additional advantage is that due t0 the relatively great length of the spring 300, it is not necessary to use shims on the spring 300 during assembly to assure a uniform hold-down force.
The embodiment shown in FIG. 5 is similar to that shown in FIG. 2, except that spherical ball 405 and its connection with spring 400 are somewhat different. Spherical ball 405 is piloted on cylinder blo-ck projection 441 and has a rearwardly extending sleeve portion 497 surrounding shaft 420 with an annular flange 498 positioned between gear 417 and cylinder block 440. Bores 499 in gear 417 slidably receive pins 501 fixed at one end to the flange 498 and at the other end to a transversely extending plate 502 across the open end of the shaft 420. Spring 400 biases plug 519 into engagement with the plate 502 thereby tending to slide the spherical ball assembly 405 rearwardly urging slippers 449 into engagement with the cam 455 in a manner similar to that in the construction of FIGS. 2, 3 and 4.
1. In an axial piston pump or motor device, housing means, a rotatable cylinder block in said housing means having a plurality of axial cylinders and a central recess, a drive shaft in said recess connected for rotation with the cylinder block, bearing means in said housing for constraining axial movement of said shaft, a plurality of pistons having ends reciprocable in said cylinders, a cam plate at one end of said cylinder block having a bearing face inclined to the end of the cylinder block, said drive shaft extending completely through said cylinder block and said cam plate, said bearing means supporting said drive shaft in said housing at both ends thereof, second bearing means on the outer ends of said pistons engaging said cam plate, ports communicating the cylinders with the opposite end of said cylinder block, a port member adjacent the opposite end of the cylinder block, and spring means wholly within said drive shaft for urging said second bearing means into engagement with said cam plate.
2. A pump or motor device as defined in claim 1, wherein said first spring means includes abutment means fixed on said shaft adjacent the central recess in the cylinder block, and a coiled compression spring surrounding said shaft within said central recess and having one end engaging the abutment means and an opposite end engaging the cylinder block, whereby the cylinder block is urged against the port member by the compression spring which reacts against the shaft.
3. A Ipump or motor device as defined in claim 1, wherein said shaft is hollow, said second spring means including a compression spring within said shaft having one end in reacting engagement therewith, projection means extending radially of said shaft and axially movable with respect thereto, the other end of said compression spring within the hollow shaft engaging said projecting means `and urging said projecting means away from said port member, a portion of said projecting means extending radially from said shaft and connected to urge said second bearing means into engagement with said cam plate.
4. A hydraulic pump or motor device, comprising: `a housing member having a port member adjacent one end thereof with inlet and outlet ports therein adapted to convey fluid to and from the device, a hollow shaft rotatable within said housing, bearing means in said housing for supporting said shaft and for preventing axial movement thereof, a rotatable cylinder block surrounding a portion of said shaft and having a port face in sliding engagement with said port member, a plurality of axial cylinders in annular array in said cylinder block, a plurality of ports in said cylinder block providing communication between the cylinders and said port face whereby the cylinders serially communicate with said inlet and outlet ports during rotation of the cylinder block, pistons slidable within said cylinders, a pivotal cam plate mounted within said housing adjacent one end of said cylinder block for reciprocating said pistons, pivotal bearing means on the end of each piston engaging said cam plate, a retaining plate engaging said pivotal bearing means for urging the bearing means and the pistons toward said cam plate, a generally spherical retaining member surrounding said shaft and rotatable therewith, said spherical retaining member having an outer surface engaging said retaining plate permitting pivotal movement therebetween, said cylinder block having a central recess therein, first spring means in said recess for urging the cylinder block against the port plate, and second spring means within said hollow shaft for biasing said retainer ball against said retainer plate, said retainer plate against said piston bearing members, and said piston bearing members against said cam plate.
5. A pump or motor device as defined in claim 4, wherein said first spring means includes an abutment fixed on said shaft adjacent said central recess in the cylinder block, a coiled compression spring having one end engaging said abutment, shoulder Imeans on said cylinder block defining one end of said central recess, the other end of said coiled spring engaging said cylinder block shoulder whereby the cylinder block is urged into engagement with the port plate, said second spring means including an axially disposed coiled compression spring mounted within said hollow shaft, said hollow shaft having axially extending recess means `therein adjacent said spherical retaining member, an axially slidable member within said hollow shaft, said axially slidable member having radial projections `extending through said shaft recess means and engaging said spherical retaining member, one end of said coiled spring Within the Ihollow shaft engaging said axially slidable member and urging it away from the port member whereby the piston bearings are urged into engagement with the `cam plate.
6. A pump or motor device -as defined in claim 5, wherein said shaft is hollow throughout a substantial portion of the length of the cylinder block, vthe inside of said hollow shaft being stepped adjacent the port member to define a spring seat therein, said second coiled compression spring extending throughout a substantial portion of the length of the cylinder block, said axially slidable member having a curved outer surface to permit limited pivotal movement thereof, said elongated recess means in the hollow shaft including a plurality of circumferentially spaced axially elongated slots, said radial projections including a plurality of pins mounted in said axially slidable member and extending through said slots, said spherical retaining member having grooves for receiving said pins.
7. In an axial piston pump or motor device, housing means, a rotatable cylinder block in said housing means having a plurality of axial cylinders and a central recess, a 'hollow drive shaft disposed in said recess and connected for rotation with the cylinder block, bearing means in said housing means supporting the shaft for rotation and restraining it against axial movement, pistons reciprocable in said cylinders, respectively, a swashplate at one end of said cylinder block having a bearing face inclined to the end of the cylinder block, bearing means on the outer ends of said pistons engaging said swashplate, a hold-down member engaging the last recited bearing means to hold the latter on the swashplate, ports communicating the cylinders with the opposite end of said cylinder block, a port member adjacent said opposite end of the cylinder block, first spring means surrounding said shaft and second spring means in said drive shaft, both spring means grounded on said drive shaft, means transmitting the force of one of said spring means to urge the cylinder block toward the port member, and means transmitting the force of the other of said spring means to urge the hold-down member toward the swashplate.
8. A pump or motor device as defined in claim 1, wherein said shaft has a bore therein opening to the end of the shaft adjacent the cam plate, said second spring means including a compression spring lwithin said shaft having one end in reacting engagement therewith, a retainer member surrounding said shaft for urging :the second rneans into engagement with the bearing face, and projection means extending axially from said retainer member to said end of the shaft and having a radially .inwardly extending portion adjacent said shaft bore, said spring being connected to bias said radially inwardly extending portion of :the projection means axially away from the cylinder block.
9. A pump `or motor as defined in claim 8, and further including a generally fiat retainer plate engaging said second bearing means, said retainer member being a spherical member, a gear surrounding and rotatably fixed to said shaft between said one end of the shaft and the spherical retainer, said axial projection extending through said gear.
10. A :pump or motor device as defined in claim 9, wherein said bearing means includes a bearing in said housing supporting said gear, said gear having two axial slots therethrough along said shaft, said projection means including a generally U-shaped member having legs extending through said slots and fixed to said spherical retainer, and a cylindrical plug slidable in said shaft bore and engaging said U-shaped member, said spring biasing said plug into engagement with said U-shaped member which in turn urges the second bearing means into engagement with the bearing face on the cam.
11. A pump or motor as defined in claim 9, wherein said bearing means in said housing includes a bearing supporting said gear, said gear having at least one axial opening therethrough, said spherical ball having a rearward extension, an annular fiange extending radially from said rearward extension, said projection means including at least one pin fixed to said flange at one end and extending through said opening and projecting from said gear, a plate fixed to the other end of said pin and extending radially over the open end of said bore, and a cylindrical plug slidable in said bore and engaging said radially extending plate, said spring biasing said plug into engagement with said radially extending plate to urge said second bearing ymeans into engagement with said cam face.
References Cited by the Examiner UNITED STATES PATENTS 2,776,628 l/1957 Keel 103-162 3,191,543 6/1965 Hann et al 103-162 3,232,056 2/1966 Heinrich et al 103-162 FOREIGN PATENTS 1,038,588 5/1953 France.
441,952 3/1927 Germany.
628,472 4/1936 Germany.
855,582 12/1960 Great Britain.
DONLEY I. STOCKING, Primary Examinez'.
R. M. VARGO, Assistant Examiner'.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,303,794 February 14, 1967 Donald W. Hagemann rs in the above numbered pattifed that error appea Letters Patent should read as It is hereby cer tion and that the said ent requiring correo corrected below.
means includes"; Iine 53,
ealed this 6th day of May 1969.
line 44, strike out "wherein said first spring strike out "second".
Signed and s (SEAL) t W f' MM Attest:
Edward M. Fletcher, Jr.
Commissioner of Patents Attestng Officer