TECHNICAL FIELD 5 The present invention concerns a screw and nut transmission intended e.g. to support a write head for a computer writer, comprising a screw and a nut assembly movable longitudinally along it, which is intended to be able to be stopped instantly from an axial movement along the screw at an exact position along the screw, with the nut assembly ¹⁰ featuring a first transmission nut which is connected by a compression spring or the like to a second transmission nut and with the two transmission nuts featuring an internal thread which is suited to the screw.

¹⁵ BACKGROUND TECHNOLOGY

Computer writers are conventionally fitted with a screw with large pitch. Along this screw is fed the write head, which is mounted on a nut that runs along the screw as this rotates, in both directions longitudinally to the screw. In this arrangement the nut is connected

20 to the fixed structure surrounding it in such a way that it does not rotate as a consequence of the screw's rotation. The nut is made e.g. of a plastic material with low weight and low friction so as to permit rapid reversals of the screw's direction of transport. With a single nut the friction is usually so low that the write head cannot be

25 brought to a standstill instantly in a desired position, but it has a tendency to backlash.

In order to eliminate this problem a well known method is to increase the friction in the nut among other ways by making it a nut assembly

30 in two sections, which sections are pressed against the respective

* flanks on the screw's threads in order to increase the friction in the

A

"* nut assembly in relation to the screw. Examples of such screw and nut transmissions are known among other instances from the U.S. patents

Nos 2493000, 2919596 and 3670583. 35

Further the U.S. patent No 4433590 shows a screw and nut transmission intended to eliminate backlash in a nut assembly movable along a screw. The screw is provided with an external thread which is suited " to the nut assembly which consists of a first nut with an internal thread that corresponds to the external thread on the screw and a second nut assembly with a thread which also corresponds to the exter¬ nal thread on the screw. The first and second nuts are linked with a' torsion element in the form of a spring which is then tensioned beyond a torsion moment on the nuts and thereby increases the friction between the thread on the nuts and the screw thread. A disadvantage with this screw and nut transmission is that the torsion moment between the first and second nuts is difficult to regulate. The torsion moment must be set in conjunction with the two nuts being fitted on the screw, and it cannot be changed without at least one of the nuts being unscrewed from the screw so that it can be rotated freely in relation to the other nut.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to produce a screw and nut trans- mission which does not show the disadvantages of the known arrange¬ ments. The aim has been achieved with a screw and nut transmission which comprises a screw and a nut assembly movable in a longitudinal direction to it, which is intended to be able to be stopped instantly from an axial movement along the screw at an exact situation along the screw, with the nut assembly featuring a first transmission nut which is linked by a compression spring or the like to a second transmission nut and with the two transmission nuts featuring an internal thread which is suited to the screw. The invention is characterised by the fact that the transmission nuts are also linked to each other by driver elements movable in an axial direction which prevent the nuts rotating in relation to each other under the influence of a pressure from the compression spring between the nuts. In order to achieve this the drivers exhibit a relatively great rigidity in a tangential direction and no springing or very little in this direction owing to the torsion moment which arises as a result of the pressure from the spring between the nuts.

Preferably the first transmission nut is fitted with drivers which extend axially out from one end of the first nut, and the second -f* transmission nut is fitted with drivers which extend axially along the casing surface of the second nut, with the drivers on the first nut 5 being able to be brought in to and moved axially in the driver slots on the second nut. The drivers on the first transmission nut can also be brought to spring elastically in a direction perpendicularly out from the screw when the first nut rotates in relation to the second transmission nut which is brought into engagement, as a result of

10 which the drivers are lifted up from the driver slots on to the adjacent ridges between the slots. Through this the nuts can be screwed in a direction towards or away from each other and thereby increase or reduce the pressure from the compression spring at the same time as the drivers on the first transmission nut go out of

15 engagement and into new engagements in the driver slots on the second transmission nut.

A guide bushing is preferably fitted so as to be axially movable along the sides of the drivers facing outwards in a direction from the screw

20 between an outer position, in which the drivers are prevented from springing out when the first transmission nut rotates in relation to the second transmission nut, and an inner position, in which the drivers spring out of the driver slots with such a rotation of the nuts relative to each other. One end of the guide bushing is main-

25 tained in a working position pressed against catches on one or more of the drivers with the aid of a spring fitted on the other side of the guide bushing and resting against a holder on the first transmission nut. The respective catches are preferably mounted projecting from the casing surface on the drivers and with the tip pointing to the free

30 ends of the drivers. A chage of pressure between the transmission nuts Jr occurs through the guide bushing being brought towards the holder on the first nut so far that the drivers on it are released so that they can spring elastically in a direction perpendicularly outwards from the screw. After that the two nuts are rotated towards or away from

35 each other, whereby the drivers spring out from their positions in the driver slots and take up new positions. Then the guide bushing is

freed and this again fixes the drivers to the driver slots and thereby also the nuts to each other.

Through the present invention a screw and nut transmission has thus been achieved in which the internal friction in the nut assembly can be set to the desired degree without the nut assembly being wholly or partially unscrewed from.the screw. By this means the maintenance is facilitated e.g. on a computer writer so that its screw and nut trans¬ mission can always be set in a simple manner for instant stop at the exact position along the screw.

DESCRIPTION OF DRAWINGS

The invention is described below in the form of an embodiment in connection with the appended drawings.

Fig. 1 shows a screw and nut transmission according to the invention seen from one side.

Fig. 2 shows the transmission as in Fig. 1 seen from the right.

Fig. 3 shows the transmission as ^' in Fig _* 1 seen from the left.

Fig. 4 shows a first transmission nut seen from one side.

Fig. 5 shows the first transmission nut as in Fig. 4 seen from the left.

Fig. 6 shows a second transmission nut seen from one side.

Fig. 7 shows the second transmission nut as in Fig. 6 seen from ^' the right.

Fig. 8 shows the second transmission nut as in Fig. 6 in longitudi¬ nal section.

Fig. 9 shows a guide bushing seen from one side.

Fig. 10 shows the guide bushing as in Fig. 9 seen from the left.

DISCRIPTION OF PREFERRED EMBODIMENT

The screw and nut transmission as in Fig. 1 comprises a transport screw 1, which is provided with a thread with five leads in order to allow a large pitch in each individual thread. On the screw 1 a first transmission nut 2 runs along, and this is provided internally with a thread complementary to the screw 1. The first transmission nut 2 features a cylindrical body 3, whose centre line coincides with the centre line of the screw and at one end of which an attachment plate 4 is fitted to receive e.g. a write head, not shown in the figures, for a computer writer. The attachment plate 4 is triangular and extends at right angles to the longitudinal direction of the screw and is also fitted with mounting holes 5 for the write head at each corner. In line with the casing surface of the cylindrical body 3 five drivers 6 extend from the other end of the cylindrical body, evenly spaced around its circumference, with each driver 6 exhibiting a continuous form and being fitted with a wedge shaped catch 7 at its free end, which projects from the casing surface and whose tip is pointing towards the free end of the driver.

On the screw 1 a second transmission nut 8 also runs along, and this is provided internally with a thread complementary to the screw. The second transmission nut 8 is formed from a cylindrical piece and provided with five similar driver slots 9, which run in its longitu¬ dinal direction in the casing surface and are arranged with an even distribution around its circumference. The outer diameter of the second transmission nut 8 is the same as the outer diameter of the cylindrical body 3 on the first transmission nut 2. The cross sectio- nal shape of the respective driver slots 9 in the second transmission nut 8 is the same as the respective drivers on the first transmission nut 2, and by this means the two nuts 2, 8 can be brought into engage¬ ment with each other by being rotated towards each other on the screw 1, whereby the drivers on the first nut 2 are brought to spring gradually out over the ridges that are formed by the remaining casing surface on the second nut 8 and take up a simultaneous position between them in the driver slots 9 in this.

The first and second transmission nuts 2, 8 are mutually affected by a cylindrical pressure adjustment spring 10, which is threaded on the screw 1 between the nuts 2, 8. The force from the pressure adjustment spring 10 is increased by rotating the nuts 2, 8 towards each other and reduced by rotating them away from each other. The outer diameter of the pressure adjustmentspring 10 is in this the same as the dia¬ meter of a circle inscribed inside the drivers 6, as a result of which this spring is completely contained between the screw 1 and the drivers 6 when the transmission nuts 2, 8 are in engagement with each other.

A guide buching 11 is pushed over the drivers G and the cylindrical body 3 on the first transmission nut 2 and features an internal diameter which is the same as the circumscribed outer diameter of the cylindrical body 3 and the drivers 6. In this push the guide bushing 11 slides against the catches 7 on the drivers 6, whereupon these spring in a direction inwards towards the screw. The guide bushing is kept pressed against the catches 7 on the drivers 6 with the aid of a guide bushing spring 12, through which these are prevented from springing out because of the force from the pressure adjustment spring 10 between the nuts 2, 8. If the transmission nuts 2, 8 are rotated inwards sufficiently far towards each other the guide bushing 11 likewise shoots out over the casing surface of the second nut 8.

A change of pressure between the transmission nuts 2, 8 is obtained by the guide bushing 11 being brought towards the attachment plate 4 on the first nut 2 so far that the drivers 6 on it are released so that they can spring"elastically in a direction at right angles out from the screw 1. The two nuts are then rotated towards or away from each other, whereupon the drivers 6 spring out from their positions in the driver slots 9 and take up the new positions. After that the guide bushing 11 is released, which again fixes the drivers 6 to the driver slots 9 and thereby also the nuts 2, 8 to each other.