Actuating mechanism with a planetary roller screw mechanism

An actuating mechanism has a main rotational drive and a roller screw mechanism coupled to the main rotational drive. The roller screw mechanism has a screw, a nut, and a plurality of rollers. One of the screw and the nut is coupled to the main rotational drive, while the other is movable relative to the rollers both translationally and rotationally. The roller screw mechanism also has an annular guide for circumferential and axial retention of the rollers. The annular guide contains a cylindrical sleeve that extends axially outward from one of the annular heels beyond the nut. The screw extends into a bore of the sleeve, and the sleeve is coupled to a secondary rotational drive.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of European application EP 18210539, filed Dec. 5, 2018; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the field of actuating mechanisms containing a rotational drive and a roller screw mechanism that makes it possible to transform a rotational movement into a linear translational movement and vice versa, and more particularly to planetary roller screw mechanisms.

Such a roller screw mechanism is furnished with a screw that contains an outer threading, a nut disposed around the screw and containing an inner threading, and a plurality of longitudinal rollers that engage with the outer and inner threadings of the screw and nut, respectively. A roller screw mechanism has the principal advantage of having higher permissible load capacities, relative to a ball screw mechanism.

A first type of roller screw mechanism contains rollers with an outer threading that engages with the outer and inner threadings of the screw and nut, respectively. The roller threadings and the nut threading have helix angles that are identical to each other and different from the screw threading, so that when the screw rotates relative to the nut, the rollers rotate in place while rotating around the screw but do not move axially inside the nut. The rollers are guided in rotation parallel to the axis of the screw by teeth that are attached to the nut and engage with teeth furnished on the rollers. Such a mechanism is called a planetary roller screw.

A second type of roller screw mechanism with a similar operating principle but with a reverse arrangement is also known. The helix angles of the threadings of the rollers, screw, and nut are selected such that, when the screw rotates relative to the nut, the rollers will rotate in place around the screw and move axially in the nut.

The rollers are guided in rotation parallel to the axis of the screw by teeth that are arranged thereon and cooperate with teeth of the rollers. Such a mechanism is called an inverted planetary roller screw.

In some applications, it is desirable to have an element the output movement of which combines both linear translation and rotation. The current solution to such a problem is to provide an assembly that includes two associated screw mechanisms, one of which is dedicated to linear translation movement, while the other enables rotational movement. However, this has the drawback of high cost and large axial and/or radial dimensions.

Another structural limitation of planetary roller screw mechanisms is that they do not permit thread pitches that are sufficiently small to achieve high precision of linear displacement, particularly at low speed and under heavy load, and also do not allow thread pitches that are sufficiently large to achieve a high linear displacement speed, particularly at low load.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an actuating mechanism with a planetary roller screw mechanism that overcomes the above-mentioned disadvantages of the prior art devices of this general type, and aims to remedy the drawbacks of the prior art.

More particularly, the present invention aims to provide an actuating mechanism with a roller screw mechanism wherein, for a given speed and load, the linear displacement precision is optimized and, for a given load, the linear displacement speed is optimized, while also being compact and low cost, allowing an output movement that combines linear displacement and rotation.

The invention relates to an actuating mechanism containing a main rotational drive and a roller screw mechanism coupled to the drive.

The roller screw mechanism is furnished with a screw containing an outer threading, a nut disposed around and co-axial to the screw, the nut containing an inner threading, and a plurality of rollers interposed between the screw and the nut and each containing an outer threading that engages with the outer and inner threadings of the screw and the nut, respectively.

One of the screw and nut is coupled to the main rotational drive, and the other movable relative to the rollers both translationally and rotationally.

The roller screw mechanism also contains an annular guide for circumferential and axial retention of the rollers. The guide is mounted radially between the screw and the nut. The guide contains a first annular heel, a second annular heel, and a plurality of elongated portions that axially connect the first and second heels so as to define a plurality of recesses that separated from each other in the circumferential direction by the elongated portions; each of the recesses accommodates a roller.

According to the invention, the annular guide contains a cylindrical sleeve that extends axially outward from one of the annular heels to beyond the nut, the screw extending into a bore in the sleeve. The sleeve is coupled to a secondary rotational drive.

According to other advantageous but non-mandatory characteristics of the invention, taken separately or in combination:

a) The nut is coupled to the main rotational drive, and the roller screw mechanism is a planetary roller screw mechanism.

b) The screw is coupled to the main rotational drive, and the roller screw mechanism is an inverted planetary roller screw mechanism.

c) Each roller contains two cylindrical journals that extend axially outward from one axial end of the roller.

d) The first and second annular heels of the guide comprise a plurality of recessed areas formed at each axial end of the roller-receiving recesses, each recessed area receiving one journal of the rollers.

e) Each roller contains two sets of teeth on either side of the outer threading and at each axial end.

f) The mechanism contains two annular crown rings fixed in an unthreaded part of the bore of the nut, each internally containing teeth that engage with the corresponding teeth of the rollers.

g) The main drive comprises a first motor.

h) A secondary drive contains a second motor separate from the first motor.

i) The secondary drive contains the first motor and a gear train that rotatably couples the guide with one of the screw and the nut that is coupled to the main drive means.

j) The guide contains two symmetrical guide portions, each of which comprises a sleeve coupled to the secondary drive means.

k) The secondary drive contains two gears coupled by a shaft and driven rotationally by a motor, each of the gears being coupled with a sleeve of one of the guide portions.

Although the invention is illustrated and described herein as embodied in an actuating mechanism with a planetary roller screw mechanism, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first, particularly toFIGS. 1-3thereof, there is shown a roller screw mechanism, identified as a whole by reference numeral1. The roller screw mechanism1has a screw2, of axis X2, furnished with an outer threading3, a nut4mounted co-axially around the screw2and furnished with an inner threading5the inner diameter of which is greater than the outer diameter of the outer threading3, and a plurality of longitudinal rollers6that are arranged radially between the screw2and the nut4. The screw2extends longitudinally through a cylindrical bore of the nut4on which the inner threading5is formed.

The rollers6are identical to each other and are evenly distributed around the screw2. Each roller6extends along a longitudinal axis coaxial with the screw axis X2, and contains an outer threading7that engages with the outer threading3of the screw2and the inner threading5of the nut4. The threading7of each roller6is extended axially at each end by an external set of teeth8,9that is itself extended by a cylindrical journal10,11that extends outward.

The nut4further contains two annular crown rings12,13fixed in a non-threaded part of its bore, each of which internally contains a set of teeth that engages with corresponding teeth8,9of the rollers6in order to synchronize them. The crown rings12,13and rollers6are fully housed in the nut4; more precisely, they do not extend beyond the nut.

The roller screw mechanism1also contains an annular guide14for circumferential and axial retention of rollers6between the screw2and nut4. The guide14is mounted radially and co-axially between the screw2and the nut4.

As shown inFIG. 4, the guide14contains a first annular heel15and a second annular heel16, and the heels are axially opposed. The guide14also contains a plurality of elongated portions17that axially connect the first and second heels15,16so as to define a plurality of recesses18that are separated from each other in the circumferential direction by the elongated portions17.

The recesses18are advantageously distributed evenly in the circumferential direction. In the embodiment illustrated inFIG. 4, the guide14is made up of a single piece.

Each of the recesses18accommodates a roller6. The first and second heels15,16form axial stops for the rollers6. The outer diameter of the elongated portions17is strictly smaller than the inner diameter of the inner threading5of the nut4, so as to allow the threading5to engage with the outer threadings7of the rollers6. The outer diameter of the elongated portions17is strictly smaller than the inner diameter of the inner sets of teeth of the crown rings12,13so that the teeth of the crown rings12,13may couple with the outer teeth8,9of the rollers6. The inner diameter of the elongated portions17is strictly greater than the outer diameter of the outer threading3of the screw2, so as to allow the threading3to engage with the outer threading7of the rollers6.

The first and second annular heels15,16of the guide14respectively contain a plurality of recessed areas19,20, formed at each axial end of the recesses18that accommodate the rollers6. Each of the recessed areas19,20accommodates a corresponding journal10,11of the rollers6.

According to the invention, the annular guide14contains a cylindrical sleeve21that extends axially outward from the second annular heel16beyond the nut4. The screw2extends into a bore of the sleeve21. Advantageously, the outer surface of the cylindrical sleeve21is furnished with an outer set of teeth22.

Such a roller screw mechanism1according to the invention may advantageously be used in an actuating mechanism, as shown inFIG. 5according to a first embodiment of the invention.

An actuating mechanism, indicated as a whole by reference numeral30, contains the roller screw mechanism1, main rotational drive31, and secondary rotational drive32.

The nut4is rotatably coupled to the main rotational drive31. The main rotational drive31advantageously contains a rotating motor coupled by any suitable means to the nut4in order to transmit to the nut a rotational movement ω4about the axis X2.

The screw2is accordingly movable by linear displacement relative to the nut4. Thus, the roller screw mechanism1is a planetary roller screw mechanism.

The sleeve21of the guide14is rotatably coupled to the secondary rotational drive32. The secondary rotational drive32advantageously contains a rotating motor coupled by any suitable means to the sleeve21, so as to transmit a rotational movement ω14about the axis X2. For example, the secondary rotational drive32contains a gear (not shown) that is rotatably coupled to the motor and engages with the outer set of teeth22of the sleeve21.

When the nut4and guide14are rotated by the main31and secondary32rotational drives, respectively, the rollers6rotate in place and rotate around the screw2but do not move axially inside the nut4. The rollers6are guided in rotation parallel to the axis X2by the teeth of the crown rings12,13. The nut4and guide14are axially fixed and only the screw2has a linear displacement movement V2along the axis X2. In addition, when the guide14and the nut4are driven at different rotational speeds, a rotational movement ω2of the screw2is induced.

As a result of the invention, the output movement of the actuating mechanism30consists of a linear displacement movement V2combined with a rotational movement ω2of screw2such that:
ω2=ω14×(1+nut diameter/screw diameter)−ω4×(nut diameter/screw diameter);
V2=screw pitch×(ω4−ω14);
where the screw diameters2and nut diameters4are the pitch diameters.

In addition, it is possible to achieve higher speeds of linear displacement V2than is the case with a roller screw mechanism driven by a simple rotational drive. It is also possible to ensure very high precision of linear and rotational displacement by controlling the combined rotational speeds ω4and ω14. Particularly advantageously, the drive means may also be equipped with linear and/or angular position sensors.

Another advantage of the present invention is that it allows the speed V2of screw2to be modulated during linear displacement. For example, the linear displacement movement of the screw2may change from a very high speed V2to a speed that is lower but has more precision. This reduces the actuation time to position the screw at high speed, not requiring precise control, and then adjust the final position at a reduced speed.

FIG. 6illustrates a second embodiment of an actuation mechanism40according to the invention.

The actuating mechanism contains a roller screw mechanism43, a main rotational drive41, and a secondary rotational drive42.

The roller screw mechanism43is substantially similar to the above-described roller screw mechanism1, and differs only by a guide44for the rollers6, which contains two guide portions45,46.

The guide portions45,46are symmetrical, axially joined, and each respectively comprise a sleeve47,48coupled to the secondary drive42.

Preferably, the secondary rotational drive42contains two gears49,50coupled rotatably to a motor51; between them, each of the gears49,50engages with the outer teeth of a sleeve47,49of a corresponding guide portion45,46. The two guide portions45,46are thus coupled rotatably and rotate at the same speed ω44around the axis X2.

The main rotational drive means41is substantially similar to the means31described above, and the actuating mechanism40has an operating mode substantially similar to the above-described mechanism30.

FIG. 7illustrates a third embodiment of an actuation mechanism60according to the invention.

The actuating mechanism60contains the roller screw mechanism1, the main rotational drive61, and the secondary rotational drive62.

The nut4is rotatably coupled to the main rotational drive61. The main rotational drive61advantageously contains a rotating motor coupled by any suitable means to the nut4in order to impart a rotational movement ω4thereto. The screw2is accordingly movable by linear displacement relative to the nut4.

In this embodiment presented by way of example, the secondary rotational drive62contains a gear train63that rotatably couples the guide14with the nut4, which in turn is coupled to the main rotational drive61. Preferably, the outer surface of the nut4contains an outer set of teeth that cooperates with a first gear64, and the outer surface of sleeve21of the guide14contains an outer set of teeth that cooperates with a second gear65. The two gears64,65are coupled rotatably about an axis. The guide14is thus set in rotational movement ω14about the axis X2. By way of example, the gear train63may be of the epicyclic type.

In this case, a difference in speed between the nut4and the guide14may be created by dimensioning the gears64,65of the gear train63in a specific way. The actuating mechanism60thus has an operating mode that is substantially similar to the above-described mechanism30.

An actuating mechanism, identified as a whole by the reference numeral70, comprises a roller screw mechanism73, a main rotational drive71, and a secondary rotational drive72.

The roller screw mechanism73is substantially similar in structure to the roller screw mechanism1, with a screw82, a nut84, a plurality of rollers86and a guide88.

The screw82is rotatably coupled to the main rotational drive71. The main rotational drive71advantageously contains a rotating motor coupled by any suitable means to the screw82in order to impart thereto a rotational movement ω82about an axis X82. Accordingly, the nut84may be linearly displaced relative to the screw82.

Thus, the roller screw mechanism73is an inverted planetary roller screw mechanism.

The guide88contains a sleeve90that extends axially beyond the linear stroke of the nut84. The sleeve90is rotatably coupled to the secondary rotational drive72. The secondary rotational drive72advantageously contains a rotating motor coupled by any suitable means to the sleeve90in order to impart thereto a rotational movement ω88about the axis X82.

When the screw82and guide88are rotated by the main71and secondary72rotational drive, respectively, the rollers86rotate in place and rotate circumferentially in the bore of the nut84, but do not move axially on the screw82. The teeth of the crown rings guide the rollers86in rotation parallel to the axis X82. The screw82and guide88are axially fixed, and only the nut84has a linear displacement movement V84along the axis X82. Moreover, when the guide88and screw82are driven at different rotational speeds, a rotational movement ω84of the nut84is induced.

Moreover, the technical characteristics of the different embodiments may be combined with each other in whole or in part. Thus, the actuating mechanism may be adapted for purposes of cost, performance and ease of implementation.