WO2016097849A1

WO2016097849A1 - Nut for a linear actuator, actuator and manufacturing method

Info

Publication number: WO2016097849A1
Application number: PCT/IB2015/002379
Authority: WO
Inventors: Bertil Moritz
Original assignee: Hm Power Holding Ag
Priority date: 2014-12-19
Filing date: 2015-12-17
Publication date: 2016-06-23
Also published as: CN107208765A; SE542076C2; CN107208765B; SE1400604A1

Abstract

An operation nut (1) of a linear actuator comprises a threaded hole (2) for receiving a screw. The nut (1) comprises a plurality of elongate graphite elements (3) partly exposed in the threaded area whereby the graphite elements (3) become part of the thread.

Description

NUT FOR A LINEAR ACTUATOR, ACTUATOR AND MANUFACTURING METHOD

TECHNICAL FIELD

The present invention concerns a linear actuator. More precisely the invention concerns a linear actuator comprising a screw and a nut in cooperation to provide the linear motion. The invention concerns a method for providing lubrication of the actuator. Especially the

invention concerns an operating nut arranged to receive a screw in a maintenance-free operation.

A linear actuator is a mechanical device that creates a linear motion from a rotating motion. There are mainly two types of such linear actuators. A first type comprises a rotating screw and a linear moving nut. A second type comprises a rotating nut and a linear moving screw. The screw or nut may be operated by a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in jacks, in computer peripherals such as disk drives and printers, in valves and dampers, in high voltage switchgears and in many other areas where linear motion is required.

By the expression screw or spindle should be understood a threaded rod, pin, shaft or tube serving as an axis that revolves or on which something revolves in order to achieve a linear movement. The treads of the screw and nut are respectively in meshed engagement with each other. The spindle transfers the power generated by a motor into linear feed rate and feed force.

It is known a great number of linear actuators capable of transferring large forces. However these actuators involve grease or coating, such as Teflon, to lower the friction. These coatings are cumbersome in harsh environment where small particles like sand or metal chips may stick to the coating. If such particles would occupy the space between the screw and nut rapid wear or disruption would occur. Besides, grease and coatings need maintenance over time to keep the low friction properties. A maintenance-free operation and a low friction between the screw and nut are important when handling high pressure forces.

Teflon coated screws cannot handle large forces and specifically not high contact pressure. Plastic nuts commonly have high friction which results in high temperature. Consequences of high temperature may be severe wear or deterioration of material properties. Encapsulated screws comprising a flexible bellow for dirt protection has limited applications since the space requirements increases. Encapsulated systems are also difficult to inspect for function and wear.

Graphite is a good solid lubricant that is known to be added to bearings made of bronze or copper alloys. Solid plugs of graphite are inserted radially into holes in the base material. A manufacture of a screw or nut in such material is cumbersome. Since the screw threads would be processed axially the threads would be interrupted by the radially aligned graphite plugs. Thus parts of the thread would contain the base material but other parts of the thread would contain pure graphite. Pure graphite cannot handle great forces. From CN 202612517 an automatic lubrication device for a nut and screw driving mechanism is previously known. The object of the mechanism is to overcome the shortcomings of prior art mechanisms and to provide simple maintenance nut-screw drive mechanism with lubrication effect. The known mechanism consists of a graphite nut, a baffle, a driving nut and a lead screw. The graphite nut, the baffle and the driving nut are in rigid connection to enable the graphite nut to be clamped between the baffle and the driving nut. The graphite nut and the driving nut move synchronously.

The automatic lubrication device further comprises a torsion spring which is installed on the graphite nut. The device achieves lubrication actions by relying on self-movement of the nut-and-screw driving mechanism. The device is said to be free of maintenance and consists of a simple structure having few mechanism parts. The device is also said to be convenient to maintain, yet it is also said to be maintenance free. As best understood by the known device the driving nut is handling the forces and the graphite nut is handling the lubrication. Since they are attached to each other in an axial direction the full length of the assembled nuts may become severe. The assembled length of the nuts decreases the action length of the device. Also as understood by the figures the screw has two threads but the graphite nut is only

lubricating one of them. For lubrication of a multi-threaded screw-and- nut application the full length of the nuts increases since both the driving nut and the graphite nut must have a length equal to all the multiple threads of the screw. The use of a spring indicates that the graphite is pushed against the screw. The wear of the graphite nut is thus greater than the wear of the driving nut. This cannot result in a maintenance-free solution. When the graphite nut has been worn out it must be replaced regardless of the wear of the driving nut. Self lubricating and maintenance-free actuators require the lubrication to be an integral element of the original screw or nut material. No additional lubrication must be required at any point in the future, regardless of actuator lifetime. To be truly self lubricating the

lubrication cannot be an add-on. The actuator must not break down prior to its designed lifetime. Thus the lubrication must remain a part of the actuator for its entire life without the need for outside maintenance.

SUMMARY OF THE INVENTION

A primary object of the present invention is to seek ways to improve a linear actuator that is self-lubricating.

These objects are achieved according to the invention by an operation nut for a linear actuator characterized by the features in the

independent claim 1 or by a method characterized by the steps in the independent claim 8. Preferred embodiments are described in the dependent claims.

According to the invention a nut of a screw-and-nut actuator contains at least one elongated graphite element being partially exposed in the threaded area of the nut. The elongated graphite element is axially oriented with the threaded hole of the nut. In an embodiment the nut contains a plurality of elongated graphite elements.

In an embodiment of the invention the graphite element comprises solid graphite. In an embodiment the nut comprises a plurality of bores into which the solid graphite rods are inserted and fixed. In an

embodiment the bore is filled with a compound containing graphite. In an embodiment the positioning and fixing of the elongated graphite elements are made prior to the thread or threads being machined. This results in the exposure of the graphite in the threads such that part of the graphite elements will be part of the thread itself. By a continuous transfer of lubrication material to the meshing surfaces of the thread a self-lubrication actuator is provided. According to the invention this transfer process creates a film of graphite onto the threads. The graphite film provides lubrication of the threads and reduces friction over the length of the screw.

According to the invention the exposed parts of the graphite element in the threaded area of the nut will provide lubrication of the meshing screw by a thin coating of graphite dust. The surface of the thread wing of the screw will be in contact with the thread wing surface of the nut. While rotating the screw the surface of the screw thread will experience a contact path along the nut thread which partly contains graphite areas. By the rotation small particles of graphite will be peeled off from the exposed graphite areas and smeared on the thread. Thus the material of the nut and the graphite rod will be worn together during the lifetime of the actuator.

In an embodiment of the invention the bores containing graphite are made prior to the machining of the threaded hole. In an embodiment the plurality of elongated graphite elements are arranged in a circular pattern in which center the threaded hole is to be machined. By determining the size and exposure of the rods of graphite in the threaded area a great variation of lubrication areas may be

constructed. The material of the nut may be any kind of solid material such as plastics or metal. Preferably to achieve sufficient strength bronze compounds and stainless steel are used. Any kind of thread profile may be used such as trapeze and square profiles.

According to the invention the elongated graphite elements are positioned such that they become part of the screw thread to be machined. In an embodiment the cross section of the elongated graphite element is greater than the depth of the thread. In this embodiment a longitudinal part of the graphite element will extend through the whole nut as a backbone. The other part will be part of the threads. In an embodiment the elongated graphite elements are fixed by glue. However if a graphite element would come lose it would still be hold in place by the backbone. In an embodiment of the invention a diameter touching the innermost parts of the graphite element is greater than the innermost diameter of the thread. This results in the innermost part of the thread being uninterruptedly formed of the base material. The balance of lubrication properties and strength properties is therefore designed by choice of the position and the cross-section of the elongated graphite elements.

The nut according to the invention may be used in both types of linear actuators. Thus the inventive nut may be used in linear actuators with a rotating screw as well as in a linear actuator with a rotating nut.

In a first aspect of the invention the object is achieved by an operation nut for a linear actuator comprising a threaded hole for receiving a screw, wherein the nut comprises at least one elongated graphite element partly exposed in the threaded area and constitute part of the thread. In an embodiment the graphite element is aligned axially in a circle around the threaded hole. In an embodiment the graphite element comprises impregnating oil. In an embodiment the graphite element comprises a solid bar of graphite. In an embodiment the graphite element comprises a compound of graphite. In an embodiment the thread of the nut comprises a trapezoidal form.

In a second aspect of the invention the objects are achieved by a method of manufacturing an operating nut for a linear actuator, wherein a nut body of a solid material is provided, a plurality of bores in circle around a predetermined threaded hole in the nut body is arranged such that the bores will be partly exposed in the thread of the predetermined hole, the bores are filled with graphite, and the threaded hole is machined inside the circle. Further the method comprises impregnating the graphite with oil.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which:

Fig 1. is a three dimensional view of an operation nut according the invention,

Fig 2. is a body of an operating nut to be processed, and

Fig 3. is a linear actuator containing a nut according to the

invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

An operation nut 1 for receiving a not shown screw of a linear actuator according to the invention is shown in fig 1. In the embodiment shown the nut comprises nine elongated graphite elements in the form of rods 3 around a threaded hole 2. The nut also comprises a first actuator shoulder 4 and a second actuator shoulder 5 for transferring the movement. The graphite rods are arranged in parallel with the threaded hole and such that parts of the rods are being part of the thread of the nut. Consequently parts of the rod of graphite are exposed on both sides of the thread ridge.

In an embodiment of the invention a hole 7 is first processed. The hole has an inner diameter 2a which will later become the inner diameter of the thread to be machined. The thread to be machined will comprise an outer diameter 2b which in the embodiment shown is positioned half way into the graphite rods. The area between the inner diameter and the outer diameter will thus become the threaded area 9. When the thread is later machined part of the rod will therefore be part of the thread itself. By letting the graphite rods become part of the thread not only will a sufficient lubrication be achieved but also will the other part of the rod act as a backbone. Thus the processed rod would look like a toothed rack with the teeth being part of the thread. All the thread parts of the rod will be held together by the backbone.

According to the invention any position of the rods could be applicable as long as parts of the rods are included in the thread. In an

embodiment of the invention an imagined circle 8 touching the

innermost parts of the rods will be positioned between the inner thread diameter 2a and the outer thread diameter 2b.

The nut may have any number of threads. The axial length of the nut is determined by the strength of the nut material and the number of turns of the thread. Preferably the nut contains a single thread of for instance six turns. The nut may comprise any number of elongated graphite elements. The graphite elements may comprise rods or pellets which are glued to the body of the nut. The graphite elements may be two to three millimeter in diameter. Preferably the diameter of the rod is greater than the depth of the thread. After making the threaded hole the elongated graphite element becomes parts of the thread.

The bores for the graphite rods are arranged in a circle around the threaded hole to be machined. When the rods are firmly glued to the nut a hole with a trapezoidal thread may be machined inside the circle of graphite elements. The positions of the graphite elements are chosen such that parts of the elements are exposed in the thread. Both the body of the nut and the graphite rods are then processed by the same threading tool. Left around each threaded revolution is a plurality of exposed graphite pellets.

The graphite may be impregnated by oil to achieve a film which better stick to the surfaces and not being washed away easily. Thus when the sliding surfaces of the nut and the screw move in a relative rotation a thin coating of impregnated graphite will be left on the surfaces. If the nut is worn the surfaces of graphite will also be worn at the same rate thus providing and maintaining the graphite coating.

A linear actuator containing a nut 1 according to the invention is shown in fig 2. The nut comprises a common shoulder 4 and is rotatable arranged on an actuator screw 6. Although favorable the scope of the invention must not be limited by the embodiments presented but also contain embodiments obvious to a person skilled in the art. For instance a plastic nut may be used. The elongated graphite element may have any size and cross section. Any number of threads may be used in the nut. For faster operation double or triple threads may be used. Still the graphite rods will be exposed in every thread and constitute part of the thread. With equal effect the elongated graphite elements may be located in the screw. Also the machining of the thread may be made prior to the inserting of the graphite elements.

Claims

1. Operation nut (1) for a linear actuator, the nut consisting of a solid material and comprising a threaded hole (7) for receiving a screw (6), c h a r a c t e r i z e d i n that the nut comprises at least one elongated graphite element (3) being partly exposed in the threaded area (9) whereby the graphite element becomes part of the thread.

2. Operation nut according to claim 1, wherein the elongated graphite element is aligned axially with the threaded hole.

3. Operation nut according to any of the preceding claims, wherein the graphite element contains impregnating oil.

4. Operation nut according to any of the preceding claims, wherein the graphite element (3) comprises a solid bar of graphite.

5. Operation nut according to any of the claims 1 - 3, wherein the graphite element (3) comprises a compound containing graphite.

6. Operation nut according to any of the preceding claims, wherein the threads of the nut comprises a trapezoidal shape.

7. Linear actuator comprising a screw (6) and an operation nut (1) of solid material having a threaded hole for receiving the screw,

c h a r a c t e r i z e d i n that the nut comprises at least one elongated graphite element (3) being partly exposed in the threaded area (9) whereby the graphite element constitute part of the thread.

8. Method of manufacturing an operating nut (1) for a linear actuator, c h a r a c t e r i z e d b y providing a nut body of a solid material, machining a plurality of axially aligned bores (3) in a circle (8) around a predetermined threaded hole (7) in the nut body such that the bores will be partly exposed in the threaded area (9) of the predetermined threaded hole (7), inserting elongated graphite elements (3) into the bores, and machining the threaded hole whereby the graphite elements will become parts of the thread.

9. Method according to claim 8, wherein the exposed graphite is

impregnated with oil.

10. Use of an operating nut according to any of the claims 1 to 6 or a method according to any of the claims 8 to 9 to provide a linear actuator for the switchgear industry.