Tool driving or impacting – Means to drive tool about an axis – Gear drive
Reexamination Certificate
1999-07-21
2001-06-05
Smith, Scott A. (Department: 3721)
Tool driving or impacting
Means to drive tool about an axis
Gear drive
C173S114000, C030S277400, C030S388000, C030S392000
Reexamination Certificate
active
06241027
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a hand-operated electric tool, especially a jigsaw, having a motor unit provided in the tool housing, which motor unit contains in a motor housing the stator and the rotor, wherein the rotor shaft projecting with one end from the motor housing is mounted in two bearings fixed in the motor housing, having a gear train which is arranged to be driven by the rotor shaft, which gear train is coupled to the rotor shaft by way of a countershaft that forms an extension of the rotor shaft and is rotatably mounted in a bearing, and having a tool chuck which is arranged to be driven by means of the gear train.
Electric tools, especially jigsaws, in which a pre-assembled motor unit is provided for the drive, are well known. The use of the pre-assembled motor unit, which is manufactured in large piece numbers and therefore economically, has the advantage that the very compact construction of the motor unit enables it to be mounted by fixing the motor housing in the housing of the electric tool, without the rotor and stator having to mounted in separate steps and without any need for the bearings of the rotor shaft to be set in specially constructed locating regions of the housing of the electric tool. With these motor units, however, for many applications, inter alia jigsaws, it is a disadvantage that the portion of the rotor shaft projecting from the motor housing is comparatively short, so that it cannot be coupled directly to the gear train arranged at distance from the motor housing in the housing of the electric tool. On the contrary, a countershaft has to be provided, which acts as an extension of the rotor shaft and has a toothed arrangement, preferably a pinion, which meshes with the input gearwheel of the gear train.
This countershaft has to be rotatably mounted in an additional bearing, and it is normally rigidly connected to the rotor shaft. This produces, however, a three-point bearing of the unit comprising rotor shaft and countershaft, namely, on the one hand in the two bearings of the rotor shaft and on the other hand in the additional bearing of the countershaft. The bearings of such a three-point bearing cannot be coaxially aligned with absolute accuracy, however, so that in operation the rotating unit comprising rotor shaft and countershaft is subject to bending stresses. These present no problems provided that the unit comprising rotor shaft and countershaft is of comparatively stable construction and the stresses acting on this unit in the region in which it engages with the gear train are not too great. As the stresses in the engagement region with the gear train increase, however, for instance in a relatively high-performance jigsaw, in which additional impact stress is transferred, via the gear, from the reciprocating saw blade engaged with the workpiece to the unit comprising rotor shaft and countershaft, it has been found that there is a risk that the three-point mounted unit will fracture.
The invention is based on the problem of eliminating the risk of fracture arising in the known construction.
BRIEF DESCRIPTION OF INVENTION
According the present invention, a portable power tool comprises a tool housing and a motor mounted in the tool housing. The motor has a motor housing and a rotor shaft rotatably mounted in and projecting from one end of the motor housing. A gear train is connected between a chuck for a power tool accessory and the motor. A countershaft is rotatably mounted in the tool housing via a bearing and connects the rotor shaft and the gear train. The connection between the rotor shaft and the countershaft is axially overlapping, is nonrotatable and permits radial play between the shafts. The connection comprises (1) mating projections and recesses on the shafts and (2) an annular projection on one of the shafts in line contact with a circumferential surface on the other of the shafts. The projections are on a first of the shafts. The recesses on the second of the shafts mate with the projections. The annular portion extends over a portion of the circumference of one of the shafts and is in substantially line contact with the circumferential surface on the other of the shafts.
With the construction in accordance with the invention, the rotor shaft mounted in the two bearings provided in the motor housing and the countershaft mounted in an additional bearing arc uncoupled from one another as a consequence of the engagement with radial play. Thus, the inevitable misalignment of the bearings does not lead to the disadvantageous bending stresses of a three-point bearing. On the contrary, because of the inevitable misalignment of their bearing arrangements, rotor shaft and countershaft are likewise not aligned exactly coaxially with respect to one another. Locking of the rotor shaft and countershaft to prevent rotation, and hence the transfer of the rotary movement from the rotor shaft to the countershaft and from there to the gear train, is achieved by the interlocking engagement of rotor shaft and countershaft by virtue of the radial projections and the recesses receiving them.
It has been found, however, that coupling of rotor shaft and countershaft with radial play can lead to the generation of considerable noise at high speed of the rotor shaft. Therefore, the radial play of the coupling is restricted in accordance with the invention by providing, on one of the shafts, an annular portion substantially in line contact with the adjacent circumferential surface of the other shaft. The line contact engagement between annular portion and circumferential surface does not hinder possible tilting of rotor shaft and countershaft relative to one another as a consequence of misalignments of their bearing arrangements and ensures a certain contact of rotor shaft and countershaft in the overlap region of the two shafts. This has unexpectedly led to a considerable reduction in noise during joint rotation of rotor shaft and countershaft at high speed.
In a preferred practical form of the invention, two projections lying diagonally opposite one another are present, and the annular portion extends on both sides between the projections, that is, forms two diagonally opposite component portions.
The rotor shaft can extend with a bushing affixed thereto into a coaxial opening in the countershaft, and the projections and the annular portion can then be formed on the bushing.
A further source of noise in the coupling of rotor shaft and countershaft provided is located in the region between the lateral surfaces of the projections and the lateral surfaces faces or axially running walls for the recesses receiving the projections. Depending on the direction of rotation a lateral wall of a projection comes into torque-transferring engagement with a lateral wall of the associated recess, this engagement varying in operation in dependence on the loading occurring. These variations in engagement can lead to noise.
To reduce this noise generation, the recesses can have resiliently deformable supporting portions abutting the projections on both sides. Although the variation in loading in the coupling region between rotor shaft and countershaft is not reduced thereby, the resilient construction of the supporting portions “damps” these variations, so that an effective noise reduction is achieved.
Preferably, a fan wheel of plastic material has a hub containing the recesses and is supported on the countershaft. The supporting portions are plastic and are formed integral with the fan wheel.
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Beck Reimund
Blickhan Stefan
Herting Rainer
Staas Ernst
Sutton David L.
Black & Decker Inc.
Leary Michael P.
Shapiro Bruce S.
Smith Scott A.
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