EP2486290A1

EP2486290A1 - Self locking nuts

Info

Publication number: EP2486290A1
Application number: EP10784852A
Authority: EP
Inventors: Marco Dezzani
Original assignee: Gally SpA
Current assignee: Gally SpA
Priority date: 2009-10-05
Filing date: 2010-10-05
Publication date: 2012-08-15
Also published as: ITTO20090757A1; WO2011042857A1; IT1396294B1; CN102597545A

Abstract

The present invention concerns self-locking nuts (1) comprising an upper threaded collar (3) provided with longitudinal slots (7), being V or U shaped, also with sharp edge, defining in the collar itself a plurality of circular arc segments (9) compressed by one or more turns of an external helical spring (5) made of a metal wire (5a), wherein the collar slots (7) have such a width as not to allow the mutual contact of the circular arc segments (9) compressed by the spring (5). The wire (5a) of the spring (5) is made of precipitation hardening stainless steel, and the nuts have values of unscrewing braking torque that differ from one another and are obtained, for nuts (1) having the same diameter, by modifying at least one dimensional parameter (Φd, Φj, 6) of the spring. The invention also concerns a method of manufacturing nuts according to the invention.

Description

SELF LOCKING NUTS

Technical field

The present invention generally relates to self- locking nuts in which one or more elements arranged to prevent nut loosening during use are provided.

More particularly, the present invention relates to self-locking nuts in which a cylindrical end is compressed by a helical spring, and reference will preferably be made herein to such kinds of nuts.

Prior art

Several kinds of self-locking nuts are known.

For instance, U.S. Patent n. 4,893,977 discloses a self-locking nut in which V- or U- shaped slots, arranged to be elastically compressed by a radially acting external helical spring, are provided in the cylindrical end.

Also, U.S. Patent n. 5,160,227 discloses a self-locking nut of the above kind in which, in order to improve its corrosion-resistance performance and to enable its use at temperatures above 250°C, it is envisaged that the helical spring is made of spring stainless steel, such as for instance AISI (American Iron and Steel Institute) 302 steel.

The Applicant has realised that, generally, due to the technology and market evolution, there is a growing need for nuts having, in particular, a loosening braking moment (unscrewing braking torque) that is constant for a high number of screwing/unscrewing cycles on screws, for instance 30 cycles, and is different depending on the kind of application.

Actually, the known nuts are sized so as to ensure, for instance according to technical specifications or manufacture standards, values of the loosening braking moment exceeding predetermined minimum values.

However, the known nuts are not sized so as to ensure performance constancy and values of the loosening braking moment differing in predetermined manner and such that they not only ensure minimum values above predetermined minimum values, but also maximum values that do not exceed predetermined maximum values.

A common problem in such a situation is that, in the presence of different needs, there is no guarantee for the nut buyer that the nuts have values of the loosening braking moment that do not exceed predetermined maximum values and, generally, that are within a predetermined range corresponding to the buyer's actual needs.

For instance, for applications in which screws or studs secured to a metal body are used and in which the stud braking torque must exceed that of the nut, the loosening braking moment of the nut must not exceed values possibly causing the unscrewing of the stud from the body.

Such a situation occurs for instance when fastening intake or exhaust manifolds on internal combustion engines.

The same kind of phenomenon could occur in devices with stationary or slowly moving parts, with low vibrations, where nuts with low values of the loosening braking moment are required. In such applications, nuts with high values of the loosening braking moment not only are useless, but they could entail damages to the screws being used or to the devices themselves because of an excessive rigidity.

On the contrary, for applications in devices with parts moving at high speed and strong vibrations, there is an equally strong need for nuts with values of the loosening braking moment exceeding predetermined minimum values since, otherwise, the risk of nut loosening would arise, with possible damages to the device and/or, possibly, injuries to the user of the device.

The need for nuts with particularly high values of the loosening braking moment also exists, for instance, when fastening parts with the interposition of electrically insulating material, which, of course, must not be damaged by too high a locking torque.

The Applicant has thus realised that, with the evolution of the applications of self- locking nuts, the need not only for performance constancy, but also for predetermined minimum and maximum limits for the performance, has become more and more apparent.

In particular, the Applicant has realised that, due to the technological and market demand evolution, ensuring values of the loosening braking moment above predetermined minimum values is no longer sufficient, but it is necessary to provide nuts having values of the loosening braking moment that are different from one another and are comprised between predetermined minimum and maximum values or within predetermined ranges. Description of the invention

It is an object of the present invention to provide self-locking nuts that, while maintaining the structure of the known nuts, allow obtaining unscrewing braking torques that differ from one another in controlled manner, whereby the following advantages can be achieved:

- the nuts can be selected depending on the actual application requirements;

- the nuts are free from risks that the values of the loosening braking moment are too high or too low; - the nuts ensure, within a predetermined range, a substantial constancy of the screwing/unscrewing braking torques for a high number of screwing/unscrewing cycles, for instance at least 30 cycles.

The object is achieved by the self- locking nuts and the relevant manufacturing method as claimed.

The claims are integral part of the technical teaching provided herein in respect of the invention.

According to a feature of a preferred embodiment of the present invention, the self- locking nuts comprise a threaded upper collar provided with suitably shaped longitudinal slots defining in the collar itself a plurality of segments shaped as arcs of circumference, and an external helical spring, arranged to compress the segments, which is made of precipitation hardening stainless steel and is so shaped that the nuts originate unscrewing braking torques that differ from one another and are obtained, for nuts having the same diameter, by suitably modifying at least one dimensional parameter of the spring.

According to another feature of the present invention, the spring is configured so that the nuts originate a plurality of partly overlapping unscrewing braking torques, arranged to cover respective different ranges of the unscrewing braking torque.

According to yet another feature of the present invention, the Applicant has identified three partly overlapping ranges of unscrewing braking torques, such as to cover an extremely wide range of unscrewing braking torque performance and hence, consequently, a correspondingly wide range of applications that would not be possible with the prior art nuts.

Brief Description of the Figures

The above and other features and advantages of the present invention will become apparent from the following description of preferred embodiments, given by way of non limiting example, with reference to the accompanying drawings, in which elements denoted by a same or similar numerical reference correspond to components having the same or similar function and construction, and in which:

- Fig. 1 A is a perspective view of an exemplary self-locking nut with helical spring;

- Fig. IB shows a spring that can be used on the self-locking nut shown in Fig. 1 A;

- Figs. 2 to 4 show the values of the braking moment, in Newton/metre (Nm), measured on samples of nut M10 as the number of screwing/unscrewing cycles increases, by changing dimensional parameters associated with the spring;

- Figs. 5 to 7 show the values of the braking moment, in Newton/metre (Nm), measured on nut samples as the number of screwing/unscrewing cycles increases, by changing dimensional parameters associated with the spring in a nut M20;

- Figs. 8 to 10 show, for respectively different nuts M10, Ml 2, M20, comparative graphs L, M, H of values of the braking moment, in Newton/metre (Nm), measured on nut samples, where each Figure refers to nuts with the same diameter and to springs with different dimensional parameters; and

- Figs. 11 and 12 show values of the braking moment, in Newton/metre (Nm), measured on nut samples as the number of screwing/unscrewing cycles increases, by using a spring made of spring stainless steel (AISI 302) in nuts M10 and M20, respectively.

Description of Preferred Embodiments

Referring to Fig. 1A, a nut 1 made in accordance with a preferred embodiment of the present invention is arranged to be screwed on a threaded rod, not shown in the Figure, located inside the nut, and is arranged for instance to fasten expendable parts of devices.

The nut has a conventional structure, including a collar 3 on which a helical spring 5 is wound, which exerts a radial compression on collar 3 and, through the collar, on the threaded rod onto which the nut is screwed.

The structure of nut 1 includes a plurality of slots 7, preferably U-shaped, provided in collar 3 itself.

Of course, in other embodiments, the slots may be V-shaped, or U-shaped with sharp corners.

Slots 7 define on collar 3 a plurality of segments 9 shaped as arcs of circumference and having an external upper rib 11.

In the preferred embodiment, a concave seat 13 is provided in the upper face of the structure of nut 1. Said seat houses a given number, even non-integer, of turns 6 (Figs. 1A and IB), of helical spring 5, so as to surround collar 3 and to radially compress segments 9, which, in turn, exert a given pressure or force against the threaded rod onto which the nut is screwed.

Spring 5 is preferably made of a wire 5a having a diameter < j that varies as the nut sizes vary, and it has an internal diameter < d that, in accordance with the preferred embodiment, is diversified for a same nut size or diameter, so as to allow diversifying in predetermined manner the minimum and maximum values of the loosening braking moment or torque, as it will be described in detail further on.

In use, the spring is preferably mounted in opposite position with respect to the fastened part, for instance an expendable part, so that it is generally subjected to temperatures lower than those of the fastened parts, as it will be readily apparent for a skilled in the art.

In the preferred embodiment, spring 5 is made of precipitation hardening stainless steel (PH steel), in which hardening compounds or elements, such as aluminium or an aluminium compound, soluble at high temperatures, are present.

More preferably, the spring is made of semi-austenitic PH steel, such as for instance 17-7 PH steel, also referred to as AISI 631 steel.

The Applicant has first of all realised, for instance by performing tests with springs of PH- 17-7 steel, that the nuts made by using precipitation hardening stainless steel have loosening braking moments with highly constant characteristics, even after repeated screwing/unscrewing cycles, for instance at least 30 cycles.

More particularly, the Applicant has realised that the constancy of the braking moment characteristics as the screwing/unscrewing cycles are repeated occurs in case of use both at ambient temperature, for instance at temperatures in the range -50°C to 100°C, and at high temperature, for instance at temperatures in the range 300°C to 400°C.

By summarising, the Applicant has noticed a high constancy of the braking moment characteristics, as the screwing/unscrewing cycles are repeated, at temperatures in the range -50°C to 400°C, so that the nuts according to the invention are particularly suitable in situations where a reliable repeated utilisation thereof is needed.

The Applicant has also realised that the constancy of characteristics is maintained also when modifying one or more dimensional parameters of spring 5 while keeping the nut diameter unchanged.

The detection of such a phenomenon has enabled performing experimental tests by preferably modifying a dimensional parameter of the spring while keeping the nut diameter unchanged, and obtaining nuts with unscrewing braking torques that differ from one another and are between predetermined minimum and maximum values.

The dimensional parameters that can be modified while keeping the nut diameter unchanged are:

inner diameter < d of spring 5;

diameter < j of wire 5 a;

the number (even non integer) of turns 6.

In the preferred embodiment, inner diameter < d of spring 5 has been selected as the dimensional parameter to be modified, since its modification has been deemed to less affect possible dimensional modifications (lengthening or shortening) of collar 3 of nut 1. Thus, hereinafter reference is preferably made to inner diameter < d of spring 5 as the dimensional parameter to be modified.

In performing the experimental tests on nuts of different diameters, a torque whose minimum value is the minimum value of the braking torque at the fifth unscrewing operation has been considered as the reference unscrewing braking torque, as provided for by standard ISO 2320.

Nuts having such a mean torque have been considered as nuts with standard characteristics (standard nuts or standard or medium torque nuts), and the dimensional parameters of spring 5 being used have been considered as the reference parameters (Figs. 3 and 6).

Having considered the standard nuts as a reference, diameter < d of spring 5 has been modified.

As a result of the experimental tests performed, the Applicant has realised that, by increasing inner diameter ("diameter") < d of spring 5 and by keeping the other dimensional parameters of the same spring unchanged, it is possible to obtain medium, minimum and maximum values of the unscrewing braking torque considerably lower than those of the standard nuts, while preserving the constancy of the characteristics for repeated screwing/unscrewing cycles, for instance 30 cycles (Figs. 2 and 5). In such case, nuts having characteristics of such kind are defined herein as low torque nuts.

The Applicant has also realised that, by reducing diameter < d of spring 5 and by keeping the other dimensional parameters of the same spring unchanged, it is possible to obtain medium, minimum and maximum values of the unscrewing braking torque considerably higher than those of standard nuts, while preserving the constancy of the characteristics over repeated screwing/unscrewing cycles, for instance 30 cycles (Figs. 4 and 7). In such case, nuts having characteristics of such kind are defined herein as high torque nuts.

By summarising, the Applicant has realised that, by suitably changing diameter < d for each kind of nut of predetermined size, a plurality of partially overlapping values, or ranges of values, of the braking torque can be obtained, where however the minimum, medium and maximum values are different, as it is shown hereinafter by table 1, which has been obtained by means of measurements on nuts in class 8 and by considering only three ranges of values. Table 1

In particular, in the example, the variations of the dimensional parameters of spring 5 were intended to obtain the following effect:

- the maximum value of the braking torque associated with low torque nuts is in any case higher than the minimum value of the braking torque of standard torque nuts;

- the maximum value of the braking torque associated with standard torque or medium torque nuts is in any case higher than the minimum value of the braking torque of high torque nuts; as it can be readily understood, this corresponds to the minimum value of the braking torque of high torque nuts being lower than the maximum value of the braking torque of standard torque nuts.

Thus, with each nut of predetermined size, the coverage has been obtained of an extremely wide range of unscrewing braking torque performance and hence, consequently, of an equally wide range of applications that would not have been possible by using the prior art nuts.

The result obtained moreover does not modify the performance constancy for repeated screwing/unscrewing cycles.

Preferably, as pointed out hereinafter in Table 2 and as shown in Figs. 8, 9 and 10 for class 8 nuts, the wide coverage range obtained by means of low, medium and high torque nuts also entails, for each nut of predetermined size, that:

- the average value of the unscrewing braking torque of low torque nuts is in the range of about 45 to 55% of the unscrewing braking torque of standard torque nuts;

- the average value of the unscrewing braking torque of standard torque nuts is in the range of about 75 to 85% of the unscrewing braking torque of high torque nuts. Table 2

In particular, the braking torque values reported in Tables 1 and 2 are preferably obtained:

- by using for instance, as inner diameter Od of low torque spring 5, a diameter increased by about 8 to 10% with respect to that of the standard torque spring for nuts with size lower than M20, and increased by about 2 to 4% for nuts with size greater than or equal to M20;

- by using for instance, as inner diameter Od of high torque spring 5, a diameter reduced by about 2 to 4% with respect to that of the standard torque spring

By summarising, the Applicant has realised that, in order to obtain high torque nuts, it is generally sufficient to reduce spring diameter Od with respect to that of the spring of standard torque nuts by substantially constant percentage values, and that a similar rule can be applied, in case of nuts with sufficiently large diameter, in order to obtain low torque nuts, yet by increasing spring diameter Od with respect to that of the spring of standard torque nuts.

Of course, in other embodiments, in particular when it is desired to obtain high torque nuts, it might be convenient also increasing diameter Oj of wire 5a besides reducing inner diameter Od of spring 5.

The Applicant has also surprisingly realised that, for the same dimensional parameters of the standard torque spring, substantially the same values of the unscrewing braking torque, with the same constancy of characteristics, as for the low torque nuts set forth in tables 1 and 2 are obtained by replacing the material the spring is made of, e.g. precipitation hardening stainless AISI 631 steel, with a spring stainless steel such as for instance AISI 302 steel.

By summarising, the Applicant has realised that, in order to obtain low torque nuts with high constancy of characteristics, it is sufficient to select a spring stainless steel, such as for instance AISI 302 steel, as an alternative for the material the spring is made of, while keeping substantially equivalent the dimensional parameters envisaged for the springs of standard torque nuts.

The term "substantially equivalent" is herein intended to denote dimensional parameters having the same values within the tolerance ranges usual in the technical field considered herein.

For instance and for the sake of clarity, the braking moment characteristics of samples of nuts M10, measured as diameter < d is changed as described, are shown in Figs. 2, 3 and 4, and a comparative graph of the average values of unscrewing braking moment is shown in Fig. 8.

For instance and for the sake of clarity, the braking moment characteristics of samples of nuts M20, measured as diameter < d is changed as described, are also shown in Figs. 5, 6 and 7, and a comparative graph of the average values of unscrewing braking moment is shown in Fig. 10.

A comparative graph of the average values of unscrewing braking moment for nuts M12 is shown in Fig. 9.

The experimental tests whose results are reported in Figs. 2 to 10 have been performed at ambient temperature on Class 8 nuts, by using springs made of precipitation hardening stainless steel, such as for instance AISI 631 steel.

Experimental tests have also been performed on Class 10 nuts and stainless steel nuts, with substantially equivalent results.

The results of such tests are not reported, since they do not exhibited significant differences with respect to those obtained for Class 8 nuts, which are more commonly used.

In all cases, the measures have been carried out in repeated screwing/unscrewing cycles (35 cycles).

Lastly, the braking moment characteristics of samples of class 8 nuts M10 and M20 determined at ambient temperature by using springs made of spring stainless steel, such as for instance AISI 302 steel, having substantially the same dimensional parameters as the springs used for standard torque nuts and made of AISI 631 steel, are shown in Figs. 11 and 12. Thanks to the invention, it is possible not only to use several times and without risks the nuts according to the invention on parts requiring a high number of replacements in their working life, but also to select, among the nuts of different diameters, the range of characteristics of the unscrewing braking torque that is the most suitable for the fastening requirements of the same parts.

Advantageously, the possibility of selecting nuts with different characteristics for the same shape allows the user of the nuts to manage them depending on the desired specific application and with the greatest flexibility.

The present description has taken the self-locking nuts shown in Fig. 1A as a reference but, as the skilled in the art can readily understand, it is also to be expected that wide ranges of unscrewing braking torques can be obtained with self-braking nuts having a metal insert with axial effect and frusto-conical nuts with calibrated deformation of the conical part on the thread, by suitably modifying the dimensional parameters of the metal insert or the calibrated deformation of the conical part.

Of course, obvious changes and/or modifications of the above description in respect of size, shape, components, as well as in respect of the details of the illustrated construction and the operating manner are possible without departing from the scope of the invention as defined in the following claims.

Claims

Patent claims

1. Self- locking nuts (1) comprising an upper threaded collar (3) provided with longitudinal slots (7), being V or U shaped, also with sharp edge, defining in the collar a plurality of circular arc segments (9) compressed by a certain number of turns (6) of an external helical spring (5) made of a metal wire (5 a), said slots having such a width as not to allow the mutual contact of said circular arc segments (9), characterised in that said wire (5a) of said spring (5) is made of precipitation hardening stainless steel; and in that said nuts have values of unscrewing braking torque that differ from one another and are obtained, for nuts (1) having the same diameter, by modifying at least one dimensional parameter (Od, Oj, 6) of said spring.

2. Self-locking nuts (1) according to claim 1, wherein said at least one dimensional parameter (Od, Oj, 6) is selected from a group comprising:

- inner diameter (Od) of the spring (5);

- diameter (Oj) of the wire (5a);

- number of turns (6) of the spring.

3. Self-locking nuts (1) according to claim 1 or 2, wherein said at least one dimensional parameter is the inner diameter (Od) of the spring (5).

4. Self-locking nuts (1) according to any one of claims 1 to 3, wherein said at least one dimensional parameter is selected so as to obtain:

- a first braking torque or standard or mean torque having, as minimum value, a predetermined minimum value of braking torque at a certain unscrewing and a certain maximum value of braking torque;

- a second braking torque in a range lower than that of the first braking torque, said second braking torque or low torque having, as maximum value, a maximum value of braking torque higher than the minimum value of the standard torque;

- a third braking torque in a range higher than that of the first braking torque, said third torque or high torque having, as minimum value, a minimum value of braking torque lower than the maximum value of braking torque of said standard torque;

whereby said dimensional parameter is selected so as to obtain partially superimposable values of braking torque adapted to cover different ranges of unscrewing braking torque, respectively.

5. Self- locking nuts according to claim 4, wherein said low torque is obtained by selecting, as a substitute for said spring made of precipitation hardening stainless steel, a spring made of stainless spring steel having dimensional parameters substantially equivalent to those selected for obtaining said standard torque by means of a spring made of said precipitation hardening stainless steel.

6. Self-locking nuts according to claim 5, wherein said substantially equivalent dimensional parameters are constituted by the inner diameter (Od) of the spring (5).

7. Method for realising self-locking nuts comprising the steps of

- realising a nut structure (1) comprising an upper threaded collar (3) provided with longitudinal slots (7), being V or U shaped, also with sharp edge, defining in the collar a plurality of circular arc segments (9) compressed by a certain number of turns (6) of an external helical spring (5) made of a metal wire (5a);

- making said wire (5 a) of precipitation hardening stainless steel;

- modifying, for nuts (1) having the same diameter, at least one dimensional parameter (Od, Oj, 6) of said spring so as to obtain nuts (1) having different values of unscrewing braking torque.

8. Method according to claim 7, wherein said step of modifying, for nuts (1) having the same diameter, at least one dimensional parameter (Od, Oj, 6) of said spring (5) comprises the step of modifying at least one dimensional parameter (Od, Oj, 6) selected from a group comprising:

- inner diameter (Od) of the spring (5);

- diameter (Oj) of wire (5 a);

- number of turns (6) of the spring.

9. Method according to claim 7 or 8 comprising the step of selecting said at least one dimensional parameter (Od, Oj, 6) so as to obtain:

- a third braking torque in a range higher than that of the first braking torque, said third torque or high torque having, as minimum value, a minimum value of braking torque lower than the maximum value of braking torque of said standard torque.

10. Method according to claim 9, wherein there is provided the step of selecting, as a substitute for said spring made of precipitation hardening stainless steel, a spring made of stainless spring steel having dimensional parameters substantially equivalent to those selected for obtaining said standard torque by means of a spring made of said precipitation hardening stainless steel.