Machine element or mechanism – Mechanical movements – Reciprocating or oscillating to or from alternating rotary
Reexamination Certificate
1999-09-15
2001-05-22
Herrmann, Allan D. (Department: 3682)
Machine element or mechanism
Mechanical movements
Reciprocating or oscillating to or from alternating rotary
C074S424830, C074S459500, C187S267000, C254S098000
Reexamination Certificate
active
06234034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a screw-driven linear actuator having an inner cylinder mounted for projection and retraction with respect to an outer cylinder so as to hoist or lower heavy things. More particularly, this invention relates to a linear actuator equipped with a mechanism for preventing an inner cylinder from dropping when a driven nut, which is threaded with a drive screw shaft for linear reciprocating movement of the inner cylinder, is damaged or otherwise broken.
2. Description of the Related Art
Conventionally, screw-driven linear actuators having an inner cylinder provided for projection and retraction with respect to an outer cylinder are widely used for hoisting or lowering heavy things. The outer cylinder is fixed to a housing, and the inner cylinder is telescopically connected to the outer cylinder so that the inner cylinder can move to project from and retract within the outer cylinder while a load, such as a heavy thing, is supported by the inner cylinder.
The housing is provided with a drive screw shaft driven for rotation in both forward and reverse directions at a fixed position by means of a rotating drive source such as a motor. The drive screw shaft is threaded with a driven nut fixed to the inner cylinder so that upon rotation of the drive screw shaft, the inner cylinder is moved to project from or retract within the outer cylinder.
The drive screw shaft is made of steel, and the driven nut threaded with the drive screw shaft is made of brass casing which is softer than steel. The conventional linear actuator of the foregoing construction may encounter a problem that the driven nut is broken due to fatigue caused by continuous use under loaded condition, or the driven nut becomes disengaged from the drive screw nut due to wearing-out of an internal screw thread of the driven nut.
Another problem is that when the inner cylinder is subjected to an overload which is in excess of an allowable maximum load of the linear actuator, the driven nut may be damaged or otherwise broken, allowing the inner cylinder to drop abruptly. If such a sudden dropping of the inner cylinder occurs during lifting or lowering operation of the heavy loads, it will result in a serious accident.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a linear actuator including a mechanism which is capable of preventing an inner cylinder from dropping under loaded condition when a driven nut threaded with a drive screw shaft is damaged or otherwise broken.
To accomplish the above-mentioned object, the present invention provides a linear actuator comprising: a housing; an outer cylinder firmly secured to the housing; an inner cylinder fitted in the outer cylinder such that the inner cylinder can project from and contract within the outer cylinder while it is prevented from rotating relative to the outer cylinder; a driven screw shaft rotatably supported by the housing and driven by a rotating drive source for rotation at a fixed position; and a driven nut secured to the inner cylinder and threaded with the drive screw shaft for causing the inner cylinder to project from or retract within the outer cylinder in response to rotation of the drive screw shaft. The linear actuator further includes a drop prevention nut threaded with the drive screw shaft in proximity to the driven nut, and a shear pin secured at one end to, and projecting in a radial direction from, one of the drop prevention nut and the inner cylinder or the driven nut. The other of the drop prevention nut and the inner cylinder or the driven nut has an axial shear pin guide groove in which a projecting free end portion of the shear pin is slidably received. The shear pin is designed to be sheared by engagement with an end wall of the shear pin guide groove when the inner cylinder and the driven nut are displaced relative to the drop prevention nut in a direction of a load acting on the inner cylinder. Upon shearing of the shear pin, the drop prevention nut supports thereon the inner cylinder or the driven nut to thereby keep the inner cylinder in position against drop.
In normal conditions, when the drive screw shaft is rotated in the forward or the reverse direction by the rotating drive source, the driven nut engaged with the drive screw shaft moves backwards or forwards along the drive screw shaft to thereby cause retraction projection of the inner cylinder with respect to the outer cylinder for achieving lifting of heavy loads or things.
In this instance, since the drop prevention nut is held non-rotatable by the shear pin slidably fitted in the shear pin guide groove, the drop prevention nut moves along the drive shaft in unison with the driven nut. During that time, the shear pin, which is slidable along the shear pin guide groove, is not subjected to thrust forces or loads. All the loads acting on the inner cylinder are born by the driven nut and, hence, the drop prevention nut is kept substantially free from wear.
When the driven nut is showing wear or damage sustained from continuous use, the driven nut is lowered relative to the drop prevention nut under the effect of thrust forces or loads acting on the inner. With this downward shift or displacement of the driven nut, the shear pin is displaced downwards along the shear pin guide groove.
When the internal screw thread of the driven nut is worn out, the driven nut is allowed to slip down along the drive screw shaft, causing sudden drop of the inner cylinder as the inner cylinder is subjected to the downward load. As a result of such sudden drop of the inner cylinder, the shear pin is broken or sheared when the projecting free end portion of the shear pin strikes against the lower end wall of the shear pin guide groove. With this shearing of the shear pin, the inner cylinder is allowed to further descending along the driven screw shaft, but the downward movement of the inner cylinder is terminated when the inner cylinder or the driven nut comes in abutment with the drop prevention nut.
The above and other objects, features and advantages of the present invention will becomes apparent to these versed in the art upon making reference to the following detailed description and accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the present invention are shown by way of illustrative examples.
REFERENCES:
patent: 2187390 (1940-01-01), Anderson et al.
patent: 3176963 (1965-04-01), Sturm
patent: 3309060 (1967-03-01), Villars
patent: 3468401 (1969-09-01), Letz
patent: 3687234 (1972-08-01), Gendreau
patent: 3833092 (1974-09-01), Flinchbaugh
patent: 4279329 (1981-07-01), Gehron
patent: 4644811 (1987-02-01), Tervo
patent: 4984657 (1991-01-01), Burns
patent: 5755310 (1998-05-01), Wourms
Dann Dorfman Herrell and Skillman, P.C.
Herrmann Allan D.
Skillman Henry H.
Tsubakimoto Chain Co.
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