Electric motor with an air duct

An electric motor includes a stator (3) secured in the motor housing (2), a rotatable rotor (4), a fan impeller (5) arranged on the rotor (4), and an annular air duct (6) arranged between the fan impeller (5) and the rotor (4) and having a shroud (7) spaced from the impeller (5) by a slot having an axial width (X), a dimensionally stable, with respect to the shroud (7), housing stop (8), an axially, resiliently displaceable fixing element (9) forming a stator stop (10) facing in a same direction as the housing stop (8), and a locking element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an air-cooled electric motor, in particular, for a hand-held power tool, and having an air duct.

2. Description of the Prior Art

The stator of an electric motor is formed of stacked sheet metal lamellas in order to prevent any eddy current losses in an a.c. field. The necessary discrete number of lamellas results, upon assembly, to a discrete lengths, with a length tolerance of about 0.5 mm with a minimum one lamella.

For removal of heat, which is generated during operation of the electric motor, in compact electrical motors which are preferably used in hand-held power tools, a cooling air stream is directed through a gap between the stator and the rotor. The air stream is generated by a fan impeller mounted on the rotor shaft. In order to increase the efficiency, the fan impeller is movable in a suitably shaped, air duct with a shroud, whereby a critical, axially narrow gap for the fan impeller is maintained. Even smaller changes of +/−0.2 mm with a slot having a width of about 1 mm cause strong fluctuations of +/−50% of the generated air flow, which leads to different cooling characteristics within a tool series.

According to German Publication DE 42 20 078 A1, in front of the fan impeller, an annular air duct, which forms a foreground grid, is secured to the tool housing of a hand-held tool. The securing of a stator with the air duct cannot be carried out.

In U.S. Pat. No. 5,343,101, behind the fan impeller, an annular or ring-shaped air duct, which is formed of a plastic material, is secured to a rotor end shield with screws. The rotor end shield is secured in the tool housing and is provided with bushes each having an inner thread for a locking bolt for securing the stator. U.S. Pat. No. 5,343,101 does not disclose a freely projecting shroud having a predetermined position relative to the impeller and arranged on one side of the impeller.

According to German Publication DE 26 17 860, a naturally stiff receiving ring, which functions as an annular air duct, is arranged in front of the impeller, with the receiving ring being secured to the tool housing and provided with bushes having an inner thread for looking bolts for securing the stator. Upon securing the stator using the rigid receiving ring, the stator axial tolerances result in the change of the critical axial width of the gap in front of the fan impeller.

Accordingly, an object of the present invention is an electric motor with an air duct which is arranged between the rotor and the fan impeller and which, in addition to functioning as means for securing the stator, provides for a gap in front of the impeller that does not depend from stator tolerances.

SUMMARY OF THE INVENTION

This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing an electric motor including a housing, a stator secured in the housing a rotor rotatable about an axis, a fan impeller arranged on the rotor, an annular air duct arranged between the fan impeller and the rotor and having a shroud spaced from the impeller by a slot having an axial width, a dimensionally stable, with respect to the shroud housing stop, an axially resiliently displaceable fixing means forming a stator stop facing in a same direction as the housing stop, and locking means.

By separating fixing means with the stator stop, which serves for securing the stator in the housing, and the locking means from the housing stop, which serves for exact axial positioning of the air duct, the slot width becomes independent of the stator tolerances.

Advantageously, the shroud is formed rotationally symmetrical so that width remains constant during rotation of the impeller.

Advantageously, the resiliently displaceable fixing means is connected with the shroud by a leaf spring securable with one of its ends to a radially inwardly located, connection point. Thereby, with the air duct formed as a one-piece part, the radially outwardly located, axially resiliently displaceable, fixing means can be formed technologically easy.

Advantageously, the locking means is formed as an inner thread which extends parallel to the axis about which the rotor rotates and in which a locking bolt is screwed in for securing the stator from a side remote from the fan impeller.

Advantageously, there are provided two locking means which are, preferably, arranged diametrically opposite with respect to the rotor rotational axis. Thereby, the bending torque generated upon tightening of locking bolts for securing the stator in the housing is reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An air-cooled motor according to the present invention, which is shown in the drawing, includes a housing2, a stator3secured in the housing2, and a rotatable, about an axis A, rotor4on which a fan impeller5is arranged. Between the rotor4and the impeller5, an annular air duct6with a rotationally symmetrical shroud7is arranged. The shroud7is spaced from the impeller5by a narrow slot having a width X. The one-piece air duct also forms a dimensionally stable housing stop8and an axially resiliently displaceable fixing means9that includes a stator stop10facing in the same direction as the housing stop8. The fixing means9further has locking means in form of two inner threads11(of which only one thread is shown) for locking bolts14and which extend parallel to the axis A about which the rotor3rotates. A leaf spring12, which is formed by the air duct6, connects the fixing means9with the shroud7at a located radially inwardly, connection point13. Two diametrically opposite locking bolts14(only one is being shown), which partially extend through the housing2, are screwed into respective inner threads11from a side remote from the impeller5, whereby the stator3is secured between the stator stop10and a stator counter-stop15free of bending stresses, as shown with dash-dot lines. The housing stop8, which is provided on the air duct6and which abuts a duct stop16provided on the housing2, provides for setting of a predetermined slot width X between the shroud7and the impeller5independently of an axial displacement V of the fixing means9.