RELATEDAPPLICATIONS

This application claims priority and benefit from Swedish patent application No. 0500992- 3, filed May 1, 2005, the entire teachings of which are incorporated herein by reference. TECHNICAL FIELD

The present invention is generally concerned with electric motors.

BACKGROUND

In some electric motors or mountings/installations of electric motors the energy stored in the rotor inertia can cause damage when the load suddenly jams. One example of this is machine tools where work pieces may drop into the active area of a moving part thus forcing it to stop in a very short distance. Another example is an actuator in an aircraft or in a special vehicle where the power from a motor is transferred over long shafts. If a shaft is driven at one end of the shaft and jams in the other end, the high torque required to stop the motor rotor may cause permanent deformation of the transfer shafts. Torque limiting devices have been used together with electric motors to solve various problems. Many of these applications are designed to eliminate problems originating in the properties of earlier motor technology and can presently be handled by current controlled brushless motors.

One such purpose is to be able to regulate the output torque during normal operation. EP 0 164 846 A2 (Wickham) discloses a system where an electric motor 1 is connected to a load over a torque limiter 6-8-4 (single friction disc, two friction surface places) external to the motor and a cog gear 10-11 to roller nut. The nut operates a screw 3 that acts as the final output element. The force over the torque limiter can be varied by a combination of springs and hydraulic piston, thus permitting the torque when slip occurs to be controlled by a variation of the pressure in the hydraulic fluid. The invention is from 1985, and the same purpose can with present technology be obtained by a current controlled brushless motor acting without any torque limiter.

DE 03012637 Al for Schulz discloses an electric hand tool having a motor rotor 2 with an output shaft that is shaped as a cog wheel 3 that drives a cogwheel 4 in which a torque limiter 13 (single friction surface place) is integrated. The speed of both sides of the torque limiter can be sensed by sensors 14 and 15 thus permitting the control system to detect a slip and reduce the motor torque accordingly. I this way the heat dissipation of both motor and torque limiter and the torque to the hand tool output can be limited.

Another purpose is to protect the motor.

U.S. patent 3,555,317 for Allan discloses an electric motor in the housing of which a torque limiter and a cooling fan are located. The application was filed in 1969 and the motor designs common at that time, brush DC motors and induction motors, have high heat dissipation if operating at a speed substantially lower than the no load speed for the supply voltage. Allan uses a torque limiter 25 (single friction surface place) on the front side of the rotor and another 37 (single friction surface place) at the rear side of the rotor. During start, the motor torque is assumed to be large enough to surpass the torque of the torque limiter, thus permitting the motor rotor to run a high speed. Thereby the power loss in the motor will be acceptable and the cooling fan will operate. The same purpose can with present technology be obtained by a current controlled brushless motor acting without any torque limiter.

U.S. patent 4,474,428 for Wunsch discloses a rear mirror for a vehicle including an integrated combined electric motor and gear box 1. The application was filed in 1981 and the brush DC motors common for small motors at mat time have high heat dissipation if operating at a speed substantially lower than the no load speed for the supply voltage. The output shaft 2 from the gear box end in a torque limiter 3 (single friction surface place). When the mirror has reached an end stop, the torque limiter will permit the motor to run, thus reducing its current and heat generation. The same purpose can with present technology be obtained by a current controlled brushless motor acting without any torque limiter. U.S. patent 4,499,409 for Bauer discloses a method to permit a series wound brush DC motor to be reversed by simply shifting the polarity of the supply voltage. If this is done while the motor runs at high speed, the new supply voltage will add to the emf of the motor, thus creating a very high current and torque. Bauer solves this by a switches activated by a torque limiter (single friction surface place) connected to a rear motor shaft. U.S. patent 6,268,669 Bl for Wakao and others discloses an electrically assisted steering for a vehicle. There is an electric motor body 21 on which a board case 22 and a worm case 90 are assembled, hi the worm case a torque limiter is inserted with a motor side torque limiter disc 82 and an output side torque limiter disc 83 (single friction surface place, splines at inner shaft, disc spring). The speed of both can be sensed by sensors 102 and 112 thus permitting the control system to detect a slip and reduce the motor torque accordingly. In this way the heat dissipation of both motor and torque limiter can be limited.

Yet another purpose is to limit the power required to run a cooling fan.

EP 0 826 266 for Lienert discloses a cooling fan assembled on the rear shaft of a motor.

Between the motor shaft and the fan there is a permanent magnet torque limiter. The slip torque of the torque limiter is selected to permit the fan to rotate at the maximum speed required for the cooling purpose of the fan. If the motor runs faster, the torque required to run the fan would increase and the torque limiter would start to slip. DE 19935733 Al for Kershaw and others shows another fan application where an electric motor and a clutch is used. Li this case the clutch is not intended to operate in the slip mode. The fan can be driven by the DC motor at a low speed, by the DC motor at a high speed or by the engine over a belt wheel connected by the clutch; in this case the DC motor is disconnected.

SUMMARY A purpose of the invention is to provide a compact and light electric motor where the rotor will release from the output shaft if the torque surpasses a certain level.

The purposes above are achieved by the invention, the characteristics of which appear from the appended claims.

An electric motor includes an external shaft, an electromagnetically active rotor and a stator having a winding. It further includes a torque limiter that is coupled to transfer forces and/or torque between the external shaft and the rotor. The torque limiter or at least the major part thereof is placed inside the motor.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

- Fig. 1 is a sectional view of a motor with an integrated torque limiter in the rotor, - Fig. 2 is a front view of an inner friction disc corresponding to item 102 of fig. 1,

- Fig. 3 is a front view of an outer friction disc corresponding to item 104 of fig. 1,

- Fig. 4a is a sectional view of a motor with an integrated torque limiter in the rotor, and

- Fig. 4b is a detail view of the torque limiter components of the rotor of Fig. 4a.

DETAILED DESCRIPTION

Fig. 1 is a sectional view of a motor provided with an integrated torque limiter located in the rotor. The rotor external shaft 101 is torsionally rigidly connected to four inner friction discs like 102. These are torsionally kept in position by splines like 103. Interleaved between the inner friction discs 102 are five outer friction discs 104. The discs are pressed against each other by a rear pressure hub 105 and a front pressure hub 106. The rear pressure hub 105 is axially locked by the external rotor shaft 101 on one side and the inner ring of internal bearing 107 and a lock nut 108 on the other side. The front pressure hub can move axially along the external shaft 101 and is pressed by a disc spring 109 against the adjustment hub 110. This has a tapped hole acting against a tapped surface of the external rotor 101. This permits an adjustment of the pretension of disc spring 109.

The outer friction discs have splines that forces them to keep a fix torsional position relative to the rotor tube 111 that carries the rotor magnets 112. The rotor tube 111 is radially kept in position in the front end by a PTFE bearing 113 between the rotor tube 112 and the adjustment hub 110. In the rear end, it is kept radially by the internal shaft 114 and the centre bearing 107. A torque limiter is thus placed inside the motor and even inside the rotor, taken in a radial direction, this e.g. giving a small total length of the motor. Generally, at least a part of the torque limiter may be placed inside the motor to give a reduced length compared to conventional motors. As long as the torque between the rotor tube and the external shaft 101 is below a limit torque, the two shafts will behave as a conventional rotor, being suspended between a rear bearing 116 and a front bearing 115 that is preloaded by a spring 120. The internal bearing 107 is static during such operations. The preload from spring 120 will pass the internal bearing 107, thus reducing its radial play. The PTFE bearing 113 can be made with a rather tight for applications where the torque limiter is assumed to be active very seldom if ever.

If the external shaft suddenly becomes almost locked by some jamming in the load connected to the external shaft during high motor speeds, the friction force between the five discs 104 attached to the rotor tube 111 and the four discs 102 attached to the external shaft will no longer be sufficient to avoid slip between the two disc sets. The commutation and position feedback is attached to the internal rotor shaft 114 that is angularily fixed to the rotor tube 111. Li the embodiment shown, a resolver having a stator 117 and a rotor 118 is provided that is kept in place against the rear bearing 116 inner ring by a screw 119. It is important that the angular position of the rotor magnets 112 is fixed relative to the

feedback rotor like 118, since otherwise the commutation information taken from the feedback would be useless.

Fig. 2 is a front view of an inner friction disc corresponding to item 102 of Fig. 1. The channel that fits to a spline 103 of Fig. 1 is shown at 103'. Fig. 3 is a front view of an outer friction disc corresponding to item 104 of Fig. 1.

Fig. 4a is a sectional view of another electric motor having an integrated torque limiter located inside the rotor 402. The difference from the motor shown in Fig. 1 concerns the bearing structure between the motor external shaft 401 and the rotor 402. hi the embodiment of Fig. 4, the two parts are radially aligned by two ball bearings 403 and 404 with pretension to reduce the play.

The pretension is arranged by a set 405 of disc springs. To the left these springs press against the pressure transfer ring 406 that presses against the inner race of the ball bearing 403. The outer race is pressed towards the right by a hollow screw 407 that is screwed into the rotor 402. To the right the springs 405 press against a pressure distribution washer 408 that compresses a set of three inner torque limiter disks 409 and two outer torque limiter disks 410. After yet a pressure distribution washer 411 the force ends in a flange 412 of the shaft 401. Further to the left, the force will enter the inner race of bearing 404 pressing it towards the right. The outer race of the same bearing will be pressed towards the left by the rotor 402 which is engaged with the hollow screw 407, thus closing the force loop. The same set of springs 405 can in this manner supply both the pretension for bearings 403 - 404 and the torque limiter axial force. The friction discs 409 are connected to the output shaft 401 by splines in the same manner as shown for Figs. 1 - 3. The friction discs 410 are also connected to the rotor tube 407 by splines. Magnets 419 are assembled on the rotor tube and are acting together with the stator winding 420 to produce the motor torque.

The two bearings 403 and 404 will rotate only if a jam occur in the system driven by the motor when the motor speed is high. In the machine tool and aircraft examples given above, this will happen very seldom if at all during the life of the system. The bearings 413 and 414 will rotate in the normal manner for a motor. The play in these bearings is cancelled by pretension from a set 415 of disc springs. It is important to note that the pretension force from the springs 415 will pass bearing 404 in the same direction as the force from springs 405. If in another design they would counteract each other, the net pretension of bearing 404 could be too small or nil.

Felt rings 416 and 417 are added to absorb grease that could eventually leak from the bearings 403 - 404. The purpose is to stop eventual grease leaks to contaminate the friction surfaces of the torque limiter discs 409 - 410.

The arrangement of having a common spring set to handle both the pretension of the rotor internal bearings and the axial force in the torque limiter package is not always economical but reduces component count. The required force for the system can be adjusted by turning the hollow screw 407. In the case where a common spring set cannot be used, two separate sets according to normal motor bearing pretension and torque limiter design can be used.

As is obvious for those skilled in the art, the principle of having a torque limiter inside the motor rotor can be implemented in many different ways. The pretension of the torque limiting elements can be independent of the internal bearing pretension as shown in Fig. 1 or common as in Fig. 4a. The torque limiting elements can be discs with an axial pretension as shown but can alternatively be made as a drum brake with a radial pretension force. The bearing pretension can be common for three bearings as shown in Fig 1. The bearing pretension can be made common for all four bearings using a separate pretension arranged for the torque limiter. Finally the bearing pretension can be made common for all four bearings and for the torque limiter, thus reducing component count compared with the embodiment shown in Fig. 4a.

The implementations shown have the complete torque limiter inside the magnetically active part of the rotor, that is inside the cylinder radially limited by the magnets such as 419 of Fig. 4a and axially by the magnet length shown as 421 in Fig. 4a. This can alternatively be described as the cylinder defined by the stator-rotor air gap. Alternatively, the torque limiting device can be located inside the larger cylinder defined radially by the stator winding 410 and axially by its length 418.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous other embodiments may be envisaged and that numerous additional advantages, modifications and changes will readily occur to those skilled in the art without departing from the spirit and scope of the invention. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention. Numerous other embodiments may be envisaged without departing from the spirit and scope of

the invention.