Description of the prior art An electric power-steering gear of the type described above is known from WO 98/12097. In this known power- steering gear, the stator of the electric motor is secured to the motor vehicle. As a result, this electric power- steering gear has the disadvantage that a releasable locking mechanism, such as a locking bolt or an electromagnetic clutch, is required in order to hold the stator of the harmonic drive in a fixed position.

The electric power-steering gear has the further disadvantage that its installation requires a large radial clearance around the steering shaft of the motor vehicle, on account of the design of the electric motor as a disk-type rotor motor and also of the harmonic drive as a disk-type gear mechanism.

Object of the invention The present invention seeks to provide an improved electric power-steering gear that requires less radial installation space and avoids the need for a releasable

locking mechanism for securing the stator of the harmonic drive.

Summary of the invention In accordance with the invention, there is provided an electric power-steering gear as described above in which the harmonic drive comprises a cylindrical externally toothed flexible sleeve, an internally toothed ring surrounding the flexible sleeve and a generator for resiliently deforming the flexible sleeve so as to urge the teeth of the flexible sleeve into migrating engagement with the internal teeth of the ring and in that the generator is driven relative to the flexible sleeve by a motor mounted within the flexible sleeve.

Because of the mounting of the motor within the flexible sleeve of the harmonic drive, it is possible to dispense with securing the casing of the electric motor in the vehicle, and with the releasable locking mechanism for securing the stator of the harmonic drive. As a result of this the installation of the electric power-steering gear is made significantly easier and requires less radial space.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a schematic section through of a first embodiment of an electric power-steering gear of the invention, coupled to a steering gear designed as a rack and pinion steering linkage; Fig. 2 shows a section along the line II-II in Fig. 1; Fig. 3 shows a schematic section similar to that of Fig. 1 of a second embodiment of the invention,

Fig. 4 shows a section along the line IV-IV in Fig. 3, and Fig. 5 a schematic section similar to that of Fig. 1 showing a still further embodiment of the invention, Description of the preferred embodiments In Figure 1, a steering shaft 1 connected to a steering wheel (not shown) is connected through an electric power- steering gear 2 to a steering linkage 13, shown in the present embodiment, by way of example only, as a rack and pinion linkage.

The electric power-steering gear 2 comprises a harmonic drive of which the input element is the steering shaft 1 and the output element is the pinion 3. The input and output elements of the harmonic drive are connected for rotation with an externally toothed elastically deformable sleeve 4 that serves as the rotor of the harmonic drive.

The harmonic drive 2 additionally comprises, as is known, an internally toothed ring 5 and an elliptical generator 6, which acts on the flexible sleeve 4 through a suitable ball-bearing arrangement 7. The internally toothed ring 5 in the present embodiment serves as the stator of the harmonic drive and is mounted in a fixed position in relation to the vehicle body by being bolted to the casing 14 of the rack and pinion linkage 13.

The generator 6 brings two regions of the external teeth 8 of the flexible sleeve 4 into engagement with the internal teeth 9 of the ring 5. These regions of engagement migrate with rotation of the generator 6 by an electric motor 10 so that the flexible sleeve 6 rotates relative to the internally toothed ring in the opposite direction to the generator 6 and a much slower rate.

The motor 10 is mounted concentrically within the flexible sleeve 10, its casing being fast in rotation with the flexible sleeve 4 and its output shaft being connected to the generator 6. Of course, the connections of the motor 10 to the flexible sleeve 4 and the generator 6 could alternatively be reversed. Electrical power is supplied to the motor 10 through slip rings 11 and 12 on the steering shaft 1.

The rack and pinion linkage 13 comprises the casing 14, in which the toothed rack 15 is axially displaceably supported in bearings 16 and 17. The casing 14 includes a connector 18 in which the end of the flexible sleeve 4 and the output element 3 of the harmonic drive are rotatably supported by means of a bearing 19. In the present embodiment, the output element 3 is constituted by the pinion 20 of the rack and pinion linkage 13.

The steering shaft includes a torque sensing transducer (not shown) that produces an output signal indicative of the applied steering torque. A torque sensing transducer may a conveniently comprise a torsionally resilient element, two angular position sensors arranged at the opposite ends of the torsionally resilient element and means for comparing the signals from the two angular position sensors to determine the angular deflection of the resilient element.

In an embodiment of the invention, it is possible to use part of the flexible sleeve of the harmonic drive as the torsionally resilient element of the torque sensing transducer. In particular, it is possible by incorporating recesses, folds of the like in the region above the electric motor 10, to design the flexible rotor so as to be torsionally flexible to a limited extent in order to be able to interact directly with angular position sensors for sensing the applied steering torque.

The signal from the torque sensing transducer, after suitable processing and amplification, is used to control the current supply to the electric motor 10 and powers the motor to rotate the generator 6 of the harmonic drive 2 in the appropriate direction to reduce the sensed torque.

As the steering shaft 1, the flexible sleeve 4 with the output element 3 are all directly connected for rotation with one another, the force that results from the rotation of the flexible sleeve 4 in the internally toothed ring 5 acts directly on the pinion 20, in order to displace the toothed rack 15, thereby assisting the driver and reducing the torque that he needs to apply to the steering wheel.

Should the electric power supply fail, the direct driving connection between the steering shaft 1 and the pinion 20 is maintained and the vehicle can be steered, though it will require more force. No special steps need to be taken to ensure that the vehicle remains drivable and in particular it is not necessary to release any form of clutch or locking mechanism.

The embodiments of Figures 3 and 4 and Figure 5 are generally similar to that of Figures 1 and 2 and to avoid unnecessary repetition of the description, like components have been designated by like reference numerals with the addition of a prime. The embodiments of Figures 3 and 4 and Figure 5 differ from that of Figure 2 essentially in that the connections to the harmonic drive are reversed. That is to say, the flexible sleeve 4'is held stationary relative to the casing of the rack and pinion linkage 13'and it is the internally toothed ring 5'that is connected to the pinion 20'and the steering shaft 1'and is free to rotate relative to the body of the vehicle. Because the motor 10' in these embodiments does not rotate in relation to the vehicle body, one can establish a directly wired electrical connection with the motor without the need for slip-rings.

Thus the embodiment shown in Figures 3 and 4, has a steering shaft 1', a harmonic drive 2'and a rack and pinion linkage 13'.

The harmonic drive 2'comprises an externally toothed flexible sleeve 4'which in this case serves as a stator, and which engages the internally toothed ring 5'. Migrating engagement between the teeth is brought about by an elliptical generator 6'acting by way of a roller bearing arrangement 7'.

The elastically deformable flexible stator 4'is secured to the casing 14'of the rack and pinion linkage 13' via a journal pin 21, while the steering shaft 1'is fast in rotation with the rotor ring 5'which is rotatably mounted on the casing 14'by means of a bearing arrangement 19'. The rotor ring 5'is provided on its external circumference with a pinion 20'that engages directly in the appropriate teeth of a the rack 15'.

This arrangement makes it possible to select a relatively large pinion diameter.

Fig. 5 shows a further embodiment in which the stationary support of the flexible 4'is provided again by means of a journal pin 21'which is secured to the casing 14'. The pinion 20''which is connected to the annular rotor 5'is support via a bushing 22 on the journal pin 21'. This embodiment provides a possibility of varying the transmission ratio of the gear over a relatively large range.

In both embodiments, the electric motor 10'for driving the elliptical generator 6'is arranged within the flexible sleeve 4'and has to be supplied with the necessary power lines in, a suitable way, it then being possible for this to be effected by means of simple line connections since the

stator 4'is stationarily mounted on the casing 14', it being possible to route the line connections via the stationary journals pins 21 and 21', for example.

The power-steering gear can not only be integrated into or built onto the rack and pinion linkage 13, as illustrated in the drawings, but it may be installed at any other desired point on the transmission path for steering torque.