US2898709A

US2898709A - Electric motor driven knife sharpener

Info

Publication number: US2898709A
Application number: US649744A
Authority: US
Inventors: Thaddeus E Bednarski
Original assignee: John Oster Manufacturing Co
Current assignee: John Oster Manufacturing Co
Priority date: 1957-04-01
Filing date: 1957-04-01
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11

Description

Aug. 11, 1959 T. E. BEDNARSKI 2,893,709

' ELECTRIC MOTOR DRIVEN KNIFE SHARPENER Filed April 1, 1957 2 Sheets-Sheet 1 5 1 ZIQIII M) III/III g- 1959 T. E. BEDNARSKI 2,898,709

ELECTRIC MOTOR-DRIVEN KNIFE SHARPENER FiledApril 1, 1957 2 Sheets-Sheet 2 2,898,709 Patented Aug, 11, 1959 2,898,709 ELECTRIC MOTOR DRIVEN KNIFE SHARPENER Thaddeus E. Bednarski, Milwaukee, Wis., assignor to John Oster Manufacturing Co., Milwaukee, Wis., a corporation of Wisconsin Application April 1, 1957, Serial No. 649,744 4 Claims. (Cl. 51-102) This invention relates to knife Sharpeners and refers more particularly to electric motor driven knife sharpeners of the type illustrated in the McMaster et a1. Patout No. 2,775,075. Elecric knife sharpeners of this type have a single abrasive Wheel, and the grinding or sharpening is done on the opposite faces of the wheel as distinguished from the periphery thereof, as in the sharpener of the Fillweber Patent No. 2,707,852.

One of the difficulties encountered in designing a commercially practicable motor driven knife sharpener, as discussed at length in the aforementioned McMaster et al. Patent No. 2,775,075 resides in providing assurance against damage to the knife blade resulting from abnormal pressure of the blade on the wheel. In the Me- Master et a1. patent, this problem is met by a particular relationship between the diameter of the wheel, the speed of the motor and the power delivered by the motor, to the end that the motor stalls before the knife can be damaged. i

The present invention has as its purpose, a simpler solution to this problem of assuring against damage due to excessive pressure of the knife blade against the abrasive Wheel. The invention achieves this objective through a novel impositive torque transmitting connection between the motor shaft and the abrasive Wheel by which the wheel'is driven only so long as the pressure of the knife blade against the wheel is less than a safe maximum. More specifically, this torque transmitting connection between the motor shaft and the abrasive wheel comprises two oppositely acting spring loaded friction slip-clutches, one at each side or face of the wheel. These clutches constitute the sole means of transmitting torque to the wheel which 'is otherwise free on the shaft, both axially and rotatably.

The clutches are so designed and constructed that together they are capable of overcoming the braking effect of the knife blade against the abrasive wheel under normal use of the sharpener, but individually they cannot drive the wheel against such braking efiect.

Hence, in the event of abnormal pressure against either face of the wheel by the knife being sharpened, and the resulting axial displacement of the wheel, the slip-clutch at the engaged face of the wheel is disengaged or disabled, and although the spring loading of the other clutch is increased by such axial displacement of the wheel, the torque transmitted thereby is incapable of overcoming the braking effect. Consequently, the wheel stops before the knife blade is damaged.

Although the sharpener of this invention is primarily intended for the sharpening of knives, it is also adapted to the sharpening of scissors. Such sharpening is done on the periphery of the abrasive wheel, and to guard against damaging scissors blades by a too rapid grinding action, the invention has as another of its objects to provide a predetermined relationship between the shape of the wheel periphery and the rest which supports and guides the scissors blades during the sharpening thereof, by which relationship abnormal pressure of the blade against the wheel periphery results in axial displacement of the wheel and consequent disengagement of one of the clutches.

Another object of this invention is to protect the two spring loaded friction slip-clutches from being rendered inoperative by the abrasive particles resulting from the sharpening action. Specifically, this object of the in-, vention is achieved by enclosing the slip-clutches in pockets formed in the opposite faces of the abrasive wheel.

With the above and other objects in view which will appear as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate one complete example of the physical embodiment of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a view partially in side elevation and partially in longitudinal section through a knife sharpener embodying this invention;

Figure 2 is an exploded perspective view of the motor. shaft, the abrasive wheel, and the parts which together form the two slip-clutches, one at each side or face of the Wheel, the wheel being shown in cross section;

Figure 3 is a cross sectional view through the abrasive, Wheel showing the same properly mounted upon and drivingly connected with the motor shaft;

Figure 4 is a top plan view of the sharpener, illustrating how it is used in sharpening scissors blades; and

Figure 5 is a cross sectional view through Figure 4 on the plane of the line 5--5.

Referring now more particularly to the accompanying drawings, the numeral 5 designates the housing or body of the entire knife sharpener, and which preferably consists of two mating upper and lower sections 6 and 7. The upper housing section 6 has a transversely extending depression or recess 8 in its top wall defined by two downwardly sloping and forwardly converging side walls 9 and a bottom wall 10 joining the adjacent lower edges of these sloping side walls. The bottom wall 10 has a hole 11 through which an abrasive wheel 12 projects upwardly into the space between the walls 9. The walls 9 thus provide guide surfaces to support and properly position a knife blade to be sharpened by engagement with first one and then the other face of the abrasive wheel. The opposite faces of the abrasive wheel are bevelled, as shown, so that forwardly of the center of the wheel its opposite faces are tangent to planes substantially parallel to the sloping walls 9. The periphery of the wheel is frusto-conical to adapt the sharpener for use in sharpening scissors, as will be more fully explained hereinafter.

A guard plate 13 is preferably fixed across the top of the depression 8; and, obviously, of course, the side edges of this plate must be spaced a short distance from the sloping walls 9, as shown, to permit knife blades to be presented to the wheel. The guard plate 13 rests upon a pedestal 14 forming part of the upper housing section, and is suitably secured to the back of the housing as at 13'.

The abrasive wheel 12 is mounted on the shaft 15 of an electric motor 16, located within the housing and suitably fixed to a mount 17 formed as part of the bottom wall of the housing. The motor need not be of any particular'type, as long as it is adequate to drive the abrasive wheel at the desired speed.

The manner in which the abrasive wheel is mounted upon and drivingly connected with the motor shaft constitutes the chief feature of this invention. As already briefly explained, two similar spring loaded friction slipclutches' 18, one confined between each side or face of the abrasive wheel and an adjacent abutment on the shaft, provide the sole torque transmitting connection between the shaft and the wheel, which is otherwise free of the shaft, both torsionally and axially. One of the shaft abutments is a shoulder 19 provided by reducing the diameter of the shaft, and the other is provided by a friction type lock nut 20 threaded onto the reduced diameter shaft portion 21. The wheel is thus located between the two shaft abutments and, since the springs of the two clutches act in opposition to one another, they hold the wheel substantially centered with respect to the abutments. Hence, if the motor is so positioned that the

shaft abutments

19 and 20 are centered with respect to the knife guides 9, the clutches will yieldingly hold the wheel in proper relation to the knife guides. Since the

abutments

19 and 20 are directly on the motor shaft, it follows that to assure proper centering thereof with respect to the knife guides, the motor must be accurately positioned and to assure such proper positioning, the holes in the motor mount 17 through which the motor fastening screws 16' pass are elongated in a direction parallel to the axis of the motor.

Although the specific construction of the slip-clutches is subject to modification, good results have been obtained through the use of a bowed spring Washer 22 confined between an outer washer 23 which bears against the adjacent shaft abutment, and a pair of contacting

fiat washers

24 and 25 which bear against the side of the wheel. The fiat washers 24 are preferably thinner than the

washers

23 and 25. By virtue of the tension imposed upon the spring washers 22 by means of the nut 20, the frictional engagement between the contacting surfaces of the several washers and between the endmost washers and the shaft abutments and the sides of the wheel yieldingly resists relative rotation between the shaft and the wheel.

The magnitude of these friction forces is, of course, dependent upon the setting of the nut 20, which should be so adjusted that the tension upon the clutch springs 22 issuch that together the two clutches are capable of transmitting sufficient torque from the shaft to the wheel to overcome the braking effect upon the wheel during normal use of the sharpener so that the wheel turns with the motor shaft. However, if a knife blade is pressed against the wheel with a force great enough to cause damage to the blade, the resulting side thrust upon the wheel will shift the wheel axially out of its normal positionand thereby decrease the spring tension of the clutch at the engaged face of the wheel to the point where that clutch no longer transmits torque. In other words, under such abnormal pressure, one of the clutches is disabled or disengaged, and although the tension upon thespring in the other clutch is increased by such axial displacement of the wheel, that clutch alone is incapable of overcoming the braking effect of the knife upon the wheel, so that the wheel stops.

As will readily appear, since the friction clutches are duplicated at opposite sides or faces of the abrasive wheel, it makes no difference at which side of the wheel the excessive pressure is exerted. In either event, the rotation of the wheel is stopped before damage to the knife blade can result. Of course, stoppage of the wheel in this way does not stop the motor, which continues to run and turn the shaft.

To prevent the slip function of the clutches from being destroyed by an accumulation therein of abrasive particles resulting from the grinding or sharpening action, the clutches are nested in counterbores 26 at the opposite ends of the hole 27 which extends Centrally through the 4 wheel and by which the wheel is loosely fitted on the shaft. These counterbores are substantially closed by the washers 23 which bear against the shaft abutments. The washers 23 thus not only constitute part of the slip-clutches but also cooperate with adjacent surfaces of the Wheel to from enclosures for the clutches.

In the sharpening of scissors blades, the edge of the blade is held against the frusto-conical periphery of the wheel as indicated in Figure 4. To assure proper presentation of the scissors blade to the periphery of the wheel, the upper housing section 6 has its front shaped to expose the abrasive wheel at the front thereof and to provide a ledge 28 which extends across the front of the wheel and lies in a downwardly and rearwardly sloping plane which intersects the wheel and is tangent to a circle concentric to the wheel axis and slightly larger than the shaft diameter so that a scissors blade resting flatly on the ledge is in position to have its edge properly ground as the blade is moved longitudinally across the periphery of the wheel.

It is important to observe that the described relationship between the frusto-conical wheel periphery and the disposition of the ledge 28 is such that as a blade resting on the ledge is pressed against the wheel periphery a side thrust is imposed upon the wheel tending to shift the wheel axially toward the motor. Thus, in the event of abnormal pressure of a scissors blade against the wheel, one of the two clutches, specifically the one adjacent to the nut 20, becomes disengaged and the wheel stops before damage can be done to the scissors blade.

From the foregoing description, taken in connection with the accompanying drawings, it will be readily apparent to those skilled in this art, that this invention provides an extremely simple and reliable way of assuring against damage to a knife or scissors blade during sharpening thereof by an electric motor driven abrasive wheel.

What is claimed as my invention:

1. An electric motor driven knife sharpener of the type wherein an abrasive wheel is carried directly upon the shaft of a drive motor and means are provided to guide the application of a knife blade to be sharpened against the opposite faces of the wheel, characterized by: the fact that the abrasive wheel has a bore at its center by which the wheel is loosely mounted on the shaft, and a counterbore at each end of said bore; the provision of opposing abutments on the motor shaft, one adjacent to each face of the wheel; the provision of a washer bearing against each abutment and substantially closing the mouth of the adjacent counterbore, said abutments locating the washers in spaced relation to the bottoms of the counterbores; and the provision of means for impositively transmitting torque from the shaft to the wheel comprising spring means confined under tension between the bottom of each counterbore and the adjacent washer and cooperating with the latter to provide two fricton slipclutches between the motor shaft and the abrasive wheel, the spring means of the two clutches acting in opposition to one another and yieldingly holding the wheel in a position substantially centered with respect to said shaft abutments and from which position the wheel is axially shiftable by thrust applied against either face thereof, and the friction forces of said clutches being such that together the clutches are capable of transmitting sufficient torque from the shaft to the abrasive wheel to overcome the braking effect on the wheel resulting from normal use of the sharpener, whereas individually said clutches are incapable of overcoming said braking effect, so that axial shifting of the wheel resulting from abnormal side thrust against either face thereof disables the clutch at the engaged face of the wheel and thus disrupts the torque transmitting connection between the shaft and the wheel.

2. The knife sharpener of claim 1, further characterized by the fact that one of said abutments on the motor shaft is provided by a nut threaded onto an end portion of the motor shaft and by axial adjustment of which nut the tension of said spring means may be adjusted.

3. In an electric motor driven knife sharpener, the

combination of: an electric motor; a shaft driven by the motor; an abrasive wheel loosely fitted on the shaft; blade supporting and guiding means adjacent to the wheel to support and guide a blade to be sharpened into proper engagement with the wheel, said means being so disposed that a blade supported and guided thereby, upon being pressed against the wheel, imposes side thrust on the wheel to thereby tend to shift the wheel axially on its shaft; and common means for impositively transmitting torque from the shaft to the abrasive wheel and for yieldingly holding the abrasive wheel in a predetermined operative position with respect to the blade supporting and guiding means, said common means comprising a pair of axially spaced abutments on the shaft, one adjacent to each face of the abrasive wheel, and a spring loaded friction slip-clutch connected between each of said abutments and the adjacent face of the abrasive wheel, the springs of said clutches acting in opposition to one another and the friction forces of the clutches being such that together the two clutches deliver sufiicient torque from the shaft to the wheel to overcome the braking eifect on the wheel resulting from normal use of the sharpener, whereas said clutches are individually incapable of overcoming said braking effect so that axial shifting of the wheel resulting from abnormal side thrust on the wheel brought about by excessive pressure of the blade against the wheel, disables one of the clutches and thus disrupts the torque transmitting connection between the shaft and the wheel.

4. The electric motor driven sharpener of claim 3 further characterized by the fact that the periphery of the abrasive wheel is frusto-conical, and by the fact that the blade supporting and guiding means comprises a ledge extending crosswise of the periphery of the wheel and lying in a plane which intersects the wheel and is tangent to a circle concentric to and spaced from the wheel axis but considerably smaller than the wheel diameter so that a blade resting on the ledge and pressed against the periphery of the wheel imposes side thrust on the wheel.

References Cited in the file of this patent UNITED STATES PATENTS 1,370,964 Hansen Mar. 8, 1921 1,445,676 Furber Feb. 20, 1923 1,535,611 Le Roux Apr. 28, 1925 2,114,106 Geveke Apr. 12, 1938 2,638,720 Hock May 19, 1953 V FOREIGN PATENTS 1,091,563 France Oct. 27, 1954