GB2187259A - Nut for a recirculating ballscrew arrangement - Google Patents

Abstract

The nut comprises a hollow cylindrical body (10) with an internal screw thread (14) defining a helical groove which, in use, receives a multiplicity of balls (16). An insert (11) is mounted in an aperture (12) in the body (10) and has a channel (15) which extends transversely between turns of the screw thread to form a continuous circulatory path for the balls. The insert is moulded in situ using a die (21) on which the nut is mounted, the die having screw thread portions (23) complementary to the helical groove. A male formation (22) on the die forms the channel (15) in the insert. <IMAGE>

Description

SPECIFICATION Nut for a recirculating ballscrew arrangement This invention relates to a nut for a recirculat- ing ballscrew arrangement in which the nut has a screw thread which receives balls which circulate as a screw is rotated within the nut.

Recirculation is permitted by a grooved insert located in a radial aperture in the nut and arranged so as to bridge adjacent turns of the screw thread.

The grooved insert must be accurately located in the thread in order to provide minimum resistance to the circulation of the balls.

This is usually achieved by precision engineering of the radial aperture and of the grooved insert.

In a known method of production, the nut first has the thread produced internally and the holes for the inserts are positioned accurately relative to the thread position by screwing the nuts onto a threaded mandrel against a stop and the insert holes are then machined. The position of each hole is determined either by locations on the milling fixture or by positioning the cutter splindle under computer numerical control. The radial position of the nut on the mandrel is determined by the torque applied by the operator whilst tightening the nut onto the mandrel, a higher torque producing a greater rotation of the nut when it contacts the stop. Due to variations of feel, different operators, therefore, can position the nut differently.

Any particles of machining swarf will also affect the accuracy. If a particle is trapped between the nut and the stop on the mandrel, a false positioning occurs. If a particle of swarf remains on the threaded portion of the mandrel, the torque required to traverse the nut along the mandrel is increased. If a similar torque to that for a clean mandrel is then applied, when the nut contacts the stop less of this torque is available for tightening and thus the radial movement due to this applied tightening toque is reduced. This, again, creates an incorrect positioning of the holes for the inserts. Any variation in the size of the thread in the nuts, within the accepted machining tolerances, will affect the tightness of each nut on the mandrel, with a similar affect upon radial positioning.If the nut is freely rotatable on the mandrel, the speed at which the nut contacts the stop will affect the final radial positioning. Any clearance in the thread will reduce positioning accuracy. An additional factor is the degree of precision of positioning of the cutter spindle and the amount of wandering of the cutter. The cutters must be changed frequently in order to maintain the size of the holes to give an acceptable fit for the inserts.

A nut normally has a multiplicity of insert holes, each of which is usually machined separately. It can be seen that there will be a variation from the true positioning of each hole relative to another in the same nut. Usually the thread in the nut is hardened. After hardening, if the thread in the nut is ground, it is now necessary to position the thread accurately relative to the insert positions. This is normally achieved by a locating peg in the grinding fixture entering one of the holes for the inserts and the machine is then set to grind the thread in the correct relationship.

Due to the deviation from the true positioning of the remaining holes, these will then be incorrectly positioned relative to the thread. The process of hardening can produce distortion which will increase any errors.

To reduce the adverse effect of cumulative machining tolerances and inaccurate positioning, high precision machine tools with commensurately highly skilled operators are necessary. In spite of these requirements the rejection rate is excessive, unless meticulous individual hand fitting and rectification is undertaken before final assembly of the nut to the screw.

Expert fitters are tied up on this time-consuming costly operation.

The present invention enables appreciable cost savings to be made. Less accurate machine tools may be used to produce the holes for the inserts, without the need for skilled personnel. The location of the nut on the mandrel is less exacting. The size of the hole does not need to be maintained to the same degree of accuracy, enabling cutters to be changed much less frequently. A greater cutter feed rate may be employed. A rough surface finish in the holes is acceptable and is, in fact, preferable for keying purposes. Within limits, the accuracy of positioning of the holes relative to each other is not important. Alteration in positions of the holes due to hardening distortion or cutter wandering can be ignored.

The effects of variations in the positioning and accuracy of the ball track in the insert relative to the outline profile of the insert, which create additional problems in the known method, are completely eliminated.

The present invention provides a nut for a recirculating ballscrew arrangement, wherein the screw thread of the nut is provided with a grooved insert bridging turns of the thread so as to permit recirculation of balls in the thread, the nut having a radial aperture in which the insert is secured, the insert being made of settable material and being formed in situ in the aperture.

The invention also resides in a method of making a nut as defined above comprising forming a radial aperture in the nut, providing a die having a formation complementary to the groove of the insert, locating the die in the nut with the formation aligned with the aperture, filling the aperture with settable material, allowing the material to set, and remov ing the die.

The method may include providing the die with a surface complementary to the interior threaded portion of the nut surrounding the aperture to facilitate location of the die.

Reference is made to the accompanying drawings, wherein: Figure 1 is a perspective view of a nut according to the invention, for a recirculating ballscrew arrangement, showing the circulation of balls; Figure 2 is a sectional view of the nut of Fig. 1, showing circulation of balls; Figure 3 is a sectional view of the nut prior to provision of apertures of inserts; Figure 4 is a plan view of part of a die for use in providing an insert in the nut; Figure 5 is a sectional view of the die; Figure 6 is a sectional view of the nut with an aperture formed therein; Figure 7 shows the nut mounted on the die; Figure 8 shows formation of the insert in the nut; and Figure 9 is a sectional view of the nut turned through 90 from the Fig. 2 position.

Referring to Figs. 1 and 2, the nut shown comprises a cylindrical body 10 having an internal screw thread 14. Bearing balls 16 are located in the grooves of the screw thread. A channel 15 extends between adjacent groove portions of the screw thread and also receives the balls 16, so as to form a bridge over the thread wall between the groove portions for the balls. A closed circuit is thereby defined for the balls, so that the balls can recirculate.

A pluraiity of channels 15 are provided between different turns of the screw thread to provide a plurality of closed circuits.

The channels 15 are defined by grooved inserts 11 mounted in apertures 12 in the cylindrical body. In manufacturing of the nut, after formation of a complete screw thread 14, the apertures 12 are formed in the cylindrical body at the locations where the inserts 11 are to be provided. In the prior art, the inserts were precision made units and the apertures were formed complementary to the inserts for locating the inserts accurately in the correct positions. The inserts were adhered or soldered in position.

In the method of the present invention, a die 21 is accurately provided with a raised, male formation 22 complementary to each channel 15. Fig. 4 shows a part of such a die with the formation 22. The die is formed with screw thread portions 23 complementary to the screw thread 14 of the nut to permit firm location of the die in the nut by engagement with the screw thread 14. The screw thread portions extend along a part-cylindrical surface of the die, only over a minor part of the die circumference. The formation extends transversely across the thread positions between male portions of the thread.

Each aperture 12 is roughly formed in the nut (Fig. 6) and is over-sized relative to the formation 22 on the die, to permit easy location of the formation within the aperture. Each aperture is sufficiently large to avoid the necessity for precision in formation of the aperture.

The die is carried at one end by a support member 30 (Fig. 7). The nut is located on the die to that its threads 14 engage with the thread portions 23 of the die 31 and with the formation 22 located in the corresponding aperture 12.

The aperture 12 is then filled with a settable material (Fig. 8), with the die in place, and the material is allowed to set, so as to form the insert 11 in situ. The settable material may comprise a plastics material which may include a metallic filler and/or may include metallic reinforcement. Any suitable material capable of setting in the aperture may be used. The aperture is preferably made with a roughened inner peripheral surface to key the insert. The nut is removed from the die after the insert has set (Fig. 9).

Although the die has to be precisely manufactured, this die can be used to produce the inserts for a multiplicity of nuts, each insert being easily made and automatically located without special skills being required.

Each support member 30 may mount a multiplicity of dies and each die has its formation 22 positioned relative to the shoulder stop 31 for central location within a specific one of the apertures 12 in each nut 10. The inserts may be produced in succession, therefore, by moving the nut from one die to the next along the support member 30. A multiplicity of dies will permit a multiplicity of nuts to be provided with inserts in a single operation.

After setting of the inserts, the nuts are moved along one die, with a new nut being added at the input end and a completed nut being removed from the output end.

Claims (7)

1. A nut for a recirculating ballscrew arrangement, wherein the screw thread of the nut is provided with a grooved insert bridging turns of the thread so as to permit recirculation of balls in the thread, the nut having a radial aperture in which the insert is secured, the insert being made of settable material and being formed in situ in the aperture.

2. A nut according to Claim 1 wherein the settable material comprises a synthetic resin.

3. A nut for a recirculating ballscrew ar rangement substantially as herein described with reference to the accompanying drawings.

4. A method of providing a channel bridg ing turns of a screw thread for producing a continuous path for balls in a nut for a recircu lating ballscrew arrangement, the method comprising forming a radial aperture through the body of the nut into the screw thread, mounting the nut on a die having complemen tary screw thread portions and a male formation complementary to the desired channel, locating the formation within the aperture, filling the aperture with settable material, allowing the material to set, and removing the nut from the die, whereby the set material forms an insert having said channel.

5. A method according to Claim 4, substantially as herein described with reference to the accompanying drawings.

6. A die for use in carrying out the method of Claim 4, the die comprising an elongate member having portions of a screw thread along a part-cylindrical surface thereof, and a male formation extending transversely of the screw thread portions between a pair of male portions of the thread.

7. A die constructed substantially as herein described with reference to the accompanying drawings.