The present invention relates to a linear actuator for a machine, particularly a bending press.

More precisely, the invention relates to a linear actuator comprising:

- first and second bodies mounted for sliding relative to each other and interconnected by means of prismatic guides extending parallel to a working line,

- a male-and-female screw mechanism in which the male screw and the female screw are supported for rotation by the first and second bodies respectively, and

- first and second motors for alternately rotating the male screw and the female screw respectively.

An actuator of the type defined above is known from U.S. patent No. 4,693,131 which describes a device for effecting a linear advance , the device having a fixed base on which a table is mounted for sliding.' A male screw is carried for free rotation by the base and is rotatable by a first motor. The male screw engages a female screw which is carried for free rotation by the movable table and is rotatable by a second motor. The two motors can be rotated in the same sense at slightly different speeds to achieve a slow approach stroke or in opposite senses to achieve a fast stroke.

Other linear actuators with male-and-female screw mechanisms, in which the male and female screws are associated with respective motors are described in U.S. patents Nos. 4,614,128 and 4,703,666.

In many machines, there is a need to cause a tool (for example, the die of a bending press) to perform a linear movement constituted by two clearly distinct stages: an approach stroke, during which the tool must be moved at a high speed of translation with a low thrust (sufficient to overcome the friction, the inertia and possibly the weights of the movable bodies) , and a working stroke with exactly the opposite characteristics, that is, a high thrust (in bending presses this may even be of the order of hundreds of tonnes) and a low speed of translation.

The known actuators mentioned above have been found unsuitable for use in a machine of this type. In fact, if the motor for effecting the approach stroke were dimensioned to supply only the low thrust necessary during this stroke, its torque would be insufficient to counteract the high reaction torque acting on the male-and-female screw mechanism during the operation of the motor which operates the working stroke. If the known actuators mentioned above were used in applications for which markedly different thrusts are required, the motor effecting the approach stroke would have to be oversized.

The object of the present invention is to provide an actuator of the type indicated at the beginning which can achieve a slow working stroke with a high thrust and a high-speed approach stroke with a low thrust, the powers of the motors being kept as low as possible.

According to the present invention, this object is achieved by the provision of a linear actuator of the type defined above, characterised in that the motors are associated with respective speed-reduction devices

having substantially different reduction ratios, and in that it includes a braking device associated with the member of the male-and-female screw mechanism which is connected to the speed-reduction device with the lower reduction ratio.

By virtue of these characteristics, two strokes characterised by markedly different thrusts can be effected with the use of two fairly low-powered motors which are therefore compact and inexpensive.

Further characteristics and advantages of the present invention will become clear in the course of the detailed description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which:

Figure 1 is a partially-sectioned plan view of an actuator according to the present invention,

Figures 2 and 3 are sections taken on the lines II-II and III-III of Figure 1,

Figure 4 is a view of the part indicated by the arrow IV in Figure 2, on an enlarged scale,

Figure 5 is a partially-sectioned schematic side view of a bending press with an actuator according to the present invention, and

Figure 6 is a view taken on the arrow VI of Figure 5.

With reference to the drawings, a linear actuator, indicated 1, includes first and second bodies, indicated 2 and 4 respectively, mounted for sliding

relative to each other along a working line A. As can be seen in Figures 1 and 2, the first body 2 has a pair of dia etrally-opposed guides 6 slidable in respective channels 8 formed in guide elements 10 fixed to the second body 4. The body 4 is intended to be fixed to the fixed structure of a machine, such as, for example, a bending press, by means of a conventional fixing system (not shown) . The first body 2, however, is intended to be connected to the movable part of the machine, for example, to the punch or the die of the bending press. In Figures 2 and 3, a male-and-female screw mechanism is indicated 14, the male screw 16 extending parallel to the working line A and being supported for free rotation by the first body 2 by means of a radial bearing 18 and a thrust bearing 20.

A bevel gear 22 is fixed to the screw 16 and is driven, by means of a pinion 26 (Figure 2) , by an electric motor 24 carried by the first body 2.

The male screw 16 engages a female screw 27 carried by a sleeve 28 which in turn is supported for free rotation about the axis A by the second body 4 by means of a thrust bearing 30 and two radial bearings 32. The sleeve 28 is connected to the output member 34 of a mechanical, geared speed-reduction device 36, for example, of the epicyclic type. The reduction device 36 is driven by a second electric motor 38 by means of a toothed belt 40 (Figure 3) .

The respective transmission ratios of the two speed-reduction devices 22, 26 and 36 are substantially different; in particular, the reduction ratio of the device 22, 26 will be low (for example 5 : 1) and that of the device 36 will be high (for example 100 : 1) .

The male-and-female screw mechanism 14 may be of any known type; because of the large forces involved, it is preferable to use a mechanism with a female screw 27 of the (known) type which has satellite screws and can withstand very large loads whilst maintaining optimal performance.

An auxiliary braking device is associated with the screw 16 and, in the embodiment shown in Figure 2, is constituted by a disc brake comprising a disc 42 fixed to the gear 22 and a hydraulically-operable caliper 44 carried by the first body 2 (see in particular Figure 4) .

As will be explained further below, the actuator according to the present invention can perform a working stroke characterised by high thrust and a low speed of translation, driven by the second electric motor 38, and an approach stroke with a high speed of translation and low thrust, driven by the first electric motor 24. In order to achieve very precise positioning during the working stroke, the second electric motor 38 is controlled numerically by a conventional electronic control unit (not shown) . The first electric motor 24 may also be controlled numerically in order to control the speed and position of the tool during the approach stroke.

The numerical control units which control the two motors 24, 38 use a single transducer which detects the relative displacement of the bodies 2 and 4. The transducer may be constituted by a visible scale 25 fixed to the body 2 and an optical reader 29 carried by the body 4 (Figure 3) .

In order to effect the working stroke, the male screw

16 is made fast with the first body 2 by the application of the disc brake 42, 44 and the female screw 28 is rotated by the second electric motor 38.

The working stroke is actually characterised by very high thrust, even of tens of tonnes, with a corresponding reaction torque which is not negligible.

The brake 42, 44 must be able to counteract this torque and prevent the male screw from rotating. The ratio of the reduction brought about by the toothed belt 40, the speed-reduction device 36 and the pitch of the screw 16 is such that the first body 2 moves along the working line A at a low speed but with a high thrust, even with the use of a motor 38 of quite low power.

In order to effect the approach stroke, the disc brake 42, 44 is released and the screw 16 is rotated by means of the first electric motor 24. The friction in the speed-reduction device 36 is sufficient to have a restraining effect which prevents the female screw 27 from rotating relative to the second body 4 since, in this case, the thrust is low. Alternatively, this restraining effect may be achieved by the second electric motor 38 which is kept in a stopped condition by its associated control unit. The reduction ratio between the first electric motor 24 and the screw 16 is quite low, and the first body 2 therefore moves at a high speed of translation but with a thrust which can overcome only the small reactions which the tool encounters during its unloaded movements.

With reference to Figures 5 and 6, a bending press with a linear actuator according to the present invention is indicated 50.

The press 50 includes, in known manner, a support structure 52 carrying an upper, fixed table 54 to which a punch 56 is fixed.

The support structure 52 carries a pair of fixed guide tables 58 between which a movable table 60 carrying a die 62 is slidable.

The movable table is fixed to the first body 2 of the actuator 1 whilst the second body 4 is supported in a hole 64 in the guide table 58.