GB1592275A

GB1592275A - Self arresting threaded elements

Info

Publication number: GB1592275A
Application number: GB3791977A
Authority: GB
Original assignee: Bauer & Co Carl GmbH
Current assignee: Bauer & Co Carl GmbH
Priority date: 1977-09-01
Filing date: 1977-09-12
Publication date: 1981-07-01
Also published as: FR2426825A2; DE2822928B2; FR2402099A1; DE2822928C3; DE2822928A1

Description

(54) SELF ARRESTING THREADED ELEMENTS (71) We, CARL BAUER GmbH & BR< Co., a Germany Kommanditgesellschaft of 5600 Wuppertal-Cronenberg, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to self arresting threaded elements.

Self arresting threaded elements, for example a cap screw or a nut, can have, on an annular portion to bear against a component to be secured thereto or thereby, a ring of saw-tooth-shaped projections extending generally radially, which projections, on tightening the threaded element, are pressed into the surface of a component as a function of the axial tension produced in the screw or a screw on which the nut is engaged. The extent of penetration of the projections also depends upon the area of the active contact surfaces and the difference in strength between the material of the threaded element and the material of the component. The contact surfaces which become progressively larger as tightening proceeds are eventually able to take up the load without further deformation.Faces of the saw-tooth-shaped projections which lead in the direction of rotation for tightening form axially active contact surfaces of the threaded element and are inclined only slightly away from the component, whilst faces of the saw-toothshaped projections which trail in the direction of rotation for tightening are directed substantially perpendicularly to the annular shoulder and form, with the parts thereof which penetrate into the component to be braced, stop surfaces for absorbing loosening torque.

It has been found that in previously proposed threaded elements of this kind, the rear faces of the saw-tooth-shaped' projections acting as stop surfaces against.

the loosening torque have been made unnecessarily large. Where the contact diameter is fixed according to a set standard or by practical experience, the size of the effective annular portion of the screw member is predetermined. Therefore, the stop surface sizes to be realized thereon by saw-tooth-shaped projections are thereafter determined by the number of teeth and the tooth depth. If there are too many teeth or the tooth depth is too great, this leads to unnecessarily steep pitch angles for the teeth, i.e. to a pointed tooth shape. Exterior forces such as vibration can act in addition to the intitial stressing force, after tightening, to modify the previously regulated equilibrium between the initial stressing force and the pressure of the braced parts to increase the bearing surface due to so-called settlement.

The amount of settlement, presupposing a constant difference between the initial stressing and the screw force, is determined by the increase in contact area attainable per settlement unit, i.e. a larger settlement in the case of a pointed tooth shape and a smaller settlement in the case of a flat tooth shape, and the clamping force reduces by an amount corresponding to the amount of settlement. The relatively large amount of settlement in known self arresting threaded elements must be looked upon as a possible source of undesired relative movement between the threaded element and the component in the case of an oscillating load, so that fatigue fractures may occur in a relatively short time.

Therefore, to obviate this, additional smooth contact surfaces have been provided on the annular shoulders of self arresting threaded elements and these engage against the component surface after the saw-tooth-shaped projections have penetrated into the component. However, in the case of such threaded elements the saw-tooth-shaped projections must be adapted to the difference in strength between the material of the threaded element and that of the component, because otherwise in the case of too great a difference in strength between the threaded element and the component that is, the former having the greater strength, the portions of the component engaging against the trailing faces of the arresting projections are no longer large enough to absorb without deformation a vibration induced or other undesired loosening torque, whilst when the difference in strength between the threaded element and the component is too small, there is no engagement of the smooth contact surface with the component.

According to the invention, there is provided a combination of a self arresting threaded element with a component to be secured thereto or thereby, the element comprising a threaded portion having an axis, a support portion with an annular support face juxtaposed with the component in a relatively tightened condition of the element and the component achieved by rotating the element in a tightening direction, and, for arresting retrograde rotation of the threaded element in a tightened position thereof, a plurality of arresting projections on the annular support face and extending therefrom into the component, each of the arresting projections having a leading face and a trailing face as considered in the rotary direction of tightening, the arresting projections having deformed the material of the component during said tightening to an extent that is determined by the specific axial force acting on the threaded portion in the tightened position of the threaded element, the trailing faces having a combined effective area, Asp in contact with the deformed material of the component in the tightened condition of the threaded element and the component sufficient to counteract any loosening torque acting on the threaded element opposite to the direction of tightening and arising from an axial vibration which gives rise to an axial force on the element up to the value of the specific axial force, and wherein the combined effective area, Asps of the trailing faces of the teeth is at least equal to

where As is the tension cross-sectional area of the threaded portion of the threaded element; D and Dl are the outer and inner diameters of the annular support face respectively d2 is the pitch diameter of the threaded portion; and is the lead angle of the thread of the threaded portion.

Thus the specific axial force and the loosening torque can be transmitted solely by the saw-tooth-shaped projections, whereby the amount of settlement can be reduced and the loosening torque acting on the threaded element is absorbed without deformation by those portions of the component which cooperate with the trailing faces of the saw-tooth-shaped prqiections.

With a given configuration of the threaded element, the same axial forces acting on the threaded element result in the same loosening torque which is proportional to the axial forces and, hence, can be calculated. It has also been recognised that it is desirable to accommodate the magnitude of the contact zones to the retaining capability of the cooperating parts, that is to say primarily to the limiting- surface pressure with which these parts act on one another just short of plastic deformation of the material of the component.Threaded elements used in a combination according to the invention have, according to a preferred embodiment, an area of contact 2.3 times the tension cross-sectional area of the threaded portion of the threaded element, that is to say the predetermined load-bearing cross-sectional area of the screw forming the threaded element or of the screw on which a nut forming the screw threaded element is engaged, and a correspondingly large stop surface suited to the loosening torque. The minimum strength of the component can therefore be up to 1:2.3 times lower than the strength of the threaded element, without the interface pressure being exceeded.In the case of differences in strength which fall within these limits, the saw-tooth-shaped prqjections penetrate by varying amounts into the component, whereby the contact area and stop surface size corresponding to the interface pressure is automatically set.

Such threaded elements require for the same restraining force a lower tightening torque and have a smaller amount of settlement compared with those proposed by the prior art, so that the initial stressing force can be lower and the service life can be increased. Furthermore, a more stable tooth shape is obtained, because the sawtooth-shaped prqjection has a flatter leading face and it is possible to use a smaller threaded element which nevertheless can be more effective than hitherto proposed constructions.

The area size of the support portion formed by the ring with the saw-tooth-shaped projections and consequently the contact surface size of the component for the axial tension of the threaded element is at least 1.4 times and preferably 2.3 times the ,tension cross-sectional area of the threaded portion of the 'threaded element. The threaded elements should be made from a material, whose surface strength i.e.

hardness is at least 30% greater than the surface strength of the component, so that the saw-tooth-shaped prqjections can penetrate into the component and are not flattened by the component. As a result, the area size of the ring with the saw-toothshaped projections and consequently the contact surface size of the component for the axial tensions of the threadedelement must be at least 1.4 times the tension crosssection of the threaded element. However, this would mean that for each component special threaded elements would be necessary corresponding to the 30V strength difference.Therefore, to adapt to conventional thread and screw standards, it is advantageous to make the area size of the ring with. the saw-tooth-shaped projections and consequently the contact surface size of the component for the axial tension of the threaded element,'2.3 times greater than the tension cross-sectional area of the threaded portion of the threaded element. The threaded element strength can therefore be more than 30% greater than the component strength.

The support portion formed by the ring with the saw-tooth-shaped projections has an internal diameter suited to the maximum standard hole diameter and the difference between such hole diameter and the nominal diameter of the thread. This ensures in simple manner that the ring with the saw-tooth-shaped projections even when arranged in a non-central manner in the hole of the greatest permitted and expected diameter, still comes to rest by means of its entire surface against the component and does not project into the hole.

The necessary stop surfaces are relatively small compared with the necessary contact surfaces. In order to reduce the settlement occurring when using thin metal sheets and soft materials, in the case of pointed teeth, without modifying the overall action, it is advantageous to flatten the pointed tooth in accordance with the teaching of our Patent No 1 444 675, so that the flank of the'tooth engages directly with a flat surface.

The invention is diagrammatically illustrated by way of example with reference to the accompanying drawings, in which: Figure 1 shows a self arresting threaded element of a combination according to the invention in side view, partly broken away and on an enlarged scale: Figure 2 is a view from underneath the head of the elements of Figure l; Figure 3 shows the profile of the sawtooth-shaped projections at an outer marginal portion of a flange of the element of Figures 1 and 2; and Figure 4 shows an alternative profile of the saw-tooth-shaped projections at an outer marginal portion of the flange of the element of Figures I and 2.

Referring to the drawings, a self arresting threaded element in the form of a threaded bolt 10 has a nominal diameter of d=10 mm.

The threaded bolt 10 has a hexagonal head 11 at one end with the underside of the head 11 increased in dimension by a flange 12. At the underside of the head 11, on an annular shoulder 13, a ring 14 of saw-toothshaped projections 15 directed generally radially is provided directed towards a component (not shown) to be secured thereto or thereby. On tightening the threaded bolt 10, the saw-toothshaped projections 15 penetrate into the surface of the component as a function of 1) the axial applied screw force, 2) the effective contact surfaces and 3) the difference in strength between the material of the threaded bolt and the material of the component, until the contact surfaces which become progressively larger upon tightening of the bolt are able to absorb the load without further deformations.Faces 16 of the saw-tooth-shaped projections 15 which lead in the direction of tightening are inclined slightly with respect to the general plane of the annular shoulder 13 so that they rise in the tightening rotation direction and form the contact surfaces of the threaded element effective to provide a clamping force. Trailing faces 17 of the saw-toothshaped projections 15 are directed substantially perpendicularly to the general plane of the-annular shoulder 13 and, when portions thereof are pressed into the surface of the component, form stop surfaces for absorbing a loosening torque.

As can be gathered in particular from Figure 2, in the represented embodiment, each saw-tooth-shaped projection 15 is slightly curved and extends at an angle of 25 to a radius line. The saw-tooth-shaped projections can however be linear and precisely radial if desired. In the represented embodiment, the tooth flank angle 70 over virtually the entire length of each saw-tooth-shaped projection 15, so that the tooth depth is greater at the outer edge than at the internal diameter of the ring 14.

The saw-tooth-shaped projections 15 of the self arresting threaded element should be approximately 30% harder than the component to be braced. This ensures that on tightening the threaded element, the saw-tooth-shaped prqjections 15 press into the component and are not flattened by the component. The area size of the ring 14 with the saw-tooth-shaped prqjections 15 and consequently the size of the contact surface of the component for the axial tension of the threaded elements is at least 1.4 times the predetermined cross-sectional area of the threaded element. In the case of nuts and cap screws with a smooth bearing surface, the contact surface is 1.65 times the crosssection area As of the threaded element.

This is necessary so that the threaded elements can cover a larger range of strength differences between the threaded element and the component. So that the self arresting threaded elements can be used in roughly the same range, the factor of 1.65 is also used. The area of the ring 14 with sawtooth-shaped projection 15 and consequently the contact surface size of the component for the axial tension of the threaded element is preferably 1 .4x 1.65 i.e.

2.3 times the cross-sectional area of the threaded element. In the case of the represented bolt 10, the cross-section area As is 58 mm2, corresponding to standard specifications. Multiplying by the factor 2.3, the area, Ak of the ring 14 with the sawtooth-shaped projections 15 is required to be 132 mm2.

In order that the ring 14 with the sawtooth-shaped projections 15 always engage completely against the surface of the component it has an internal diameter Dl suited to the maximum standard hole diameter and the difference between such maximum standard hole diameter and the nominal hole diameter for the threaded element. In the case of the bolt 10 shown, the diameter of the maximum standard through hole is 11 mm, so that the ring internal diameter Dl is 12 mm. The external diameter of the ring 14 of 19 mm is obtained from the required surface area of the ring 14 and the internal diameter thereof.

The area of the ring 14 with the sawtooth-shaped projections 15 is such that the latter can be completely submerged in the component, provided that the hardness of the threaded element is 2.3 times greater than the hardness of the component. If the difference in hardness is smaller, the sawtooth-shaped projections 15 penetrate less deeply into the component, corresponding to the given screw force, so that only a part of the leading faces 16 of the saw-toothshaped projections 15 are brought into engagement. A threaded element with a hardness greater than 2.3 times that of the component should not be used i.e. a lower quality threaded element must be selected.

When the saw-tooth-shaped projections 15 penetrate into different component surfaces, different areas of the leading faces 16 of the projections 15 act as contact surfaces. Penetration is dependent on the axial screw force, the contact area and the difference in hardness between the screw number and the component. The forcing-in-' process continues until the increasing contact areas are able to absorb the load without further deformation. The larger the areas of the faces 16 of the saw-toothshaped projections 15, which are contact areas, the larger the parts of the trailing faces 17 of the saw-tooth-shaped pro,jections 15 which are available for absorbing a loosening torque generated for example by axial vibration.The trailing faces 17 of the saw-tooth-shaped projections 15 have according to, the principle of the present invention only such a combined area that those portions of the trailing faces 17 of the saw-tooth-shaped projections 15 which become effective as stopping surfaces acting against a loosening torque when the projections 15 have penetrated into the component, exert such a contact pressure on the corresponding counter-surfaces of the component as can be absorbed without deformation of the counter-surfaces, utilizing as far as possible the strength of the component.

In the paper by Dr Ing D G Paland entitled "Reliability of screwnut connections in the case of dynamic axial stressing" published in the German language publication "Konstruktion" vol. 9 of 1967 and particularly at page 463 thereof, a formula is given for the maximum loosening torque ML resulting from axial vibration.

The minimum required retention area Asps that is to say the combined effective area of the trailing faces 17 is proportionate to the retention force related to the length of the lever arm at which the retention force acts, that is, the central radius of the annulus 14 amounting to 1/4 (D+Dl). Thus

Once the minimum area necessary for the stop surface has been calculated and since the length of the saw-tooth-shaped projections 15 is fixed by the internal and external diameters of the ring 14, the combined height of the teeth can be calculated for all the saw-tooth-shaped projections 15 The number of saw-toothshaped projections 15 can now be selected at random and divided into the combined height to obtain the individual height for each tooth.However, the number of teeth should be as large as possible, because an increase in the number of teeth reduces the initial pehetration. This is best kept as small as possible in order to increase the fatigue life of the component.

In case of the bolt 10 shown, the number of saw-tooth-shaped projections chosen was 52. A typical individual tooth height is 0.1 mm. . Such teeth can be economically manufactu'red by conventional manufacturing methods. A limited inclination of the tooth flank ensures the necessary strength for the tip of the sawtooth-shaped projections 15. By reducing the tooth flank inclination a reduced tightening torque is required for obtaining the given screw force of the screw member, because the tightening torque rises as the angle of the toothed -flank inclination increases.

The necessary stopping surfaces are relatively small compared with the necessary bearing surfaces. In order to be able to reduce the large amount of settlement which occurs with thin metal sheets and soft materials when pointed teeth are used, without modifying the overall action, the upper part of the pointed tooth 15 can be flattened, as shown in Figure 4, so that the face 16 is contiguous with a flat surface 18.

WHAT WE CLAIM IS:- 1. A combination of a self arresting threaded element with a component to be secured thereto or thereby, the element comprising a threaded portion having an axis, a support portion with an annular support face juxtaposed with the component in a relatively tightened condition of the element and the component achieved by rotating the element in a tightening direction, and, for arresting retrograde rotation of the threaded element in a tightened position thereof, a plurality of arresting projections on the annular support face and extending therefrom into the component, each of the arresting projections having a leading face and a trailing face as considered in the rotary direction of tightening, the arresting projections having deformed the material of the component during said tightening to an extent that is determined by the specific axial force acting on the threaded portion in the tightening position of the threaded element, the trailing faces having a combined effective area, Asp, in contact with the deformed material of the component in the tightened condition of the threaded element and the component sufficient to counteract any loosening torque acting on the threaded element opposite to the direction of the tightening and arising from an axial vibration which gives rise to an axial force on the element up to the value of the specific axial force, and wherein the combined effective area, Asps of the trailing faces of the teeth is at least equal to

where As is the tension cross-sectional area of the threaded portion of the threaded element; D and D1 are the outer and inner diameters of the annular support face respectively; d2 is the pitch diameter of the threaded portion; and i is the lead angle of the thread of the threaded portion.

2. A combination according to claim 1, wherein the arresting projections are elongate in a generally radial direction.

3. A combination according to claim 1, wherein the leading faces of the arresting projection slope away from the support face so that they rise in the tightening direction so that the leading faces constitute effective contact zones with the component, the contact zones gradually increasing in area during the tightening of the threaded element until the axial forces at the surface exerted on the contact zones by the deformed material of the component are in equilibium with the specific axial force in the tightened position.

4. A combination according to any one of claims I to 3, wherein the arresting projections are substantially identically shaped and are arranged in an annulus about the threaded portion.

5. A combination according to claim 4, wherein the component has a hole through which the threaded portion passes in the tightened condition, the hole having a diameter exceeding that of the threaded portion by a given distance; and wherein the inner diameter D, at least equals the sum of said diameter and said distance.

6. A combination according to any one of claims 1 to 5, wherein the arresting projections have substantially saw-tooth-' shaped cross-sections.

7. A combination according to any one of claims I to 6, wherein each of the arresting projections has a flat face intermediate the

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Claims

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projections 15 The number of saw-toothshaped projections 15 can now be selected at random and divided into the combined height to obtain the individual height for each tooth. However, the number of teeth should be as large as possible, because an increase in the number of teeth reduces the initial pehetration. This is best kept as small as possible in order to increase the fatigue life of the component.

In case of the bolt 10 shown, the number of saw-tooth-shaped projections chosen was 52. A typical individual tooth height is 0.1 mm. . Such teeth can be economically manufactu'red by conventional manufacturing methods. A limited inclination of the tooth flank ensures the necessary strength for the tip of the sawtooth-shaped projections 15. By reducing the tooth flank inclination a reduced tightening torque is required for obtaining the given screw force of the screw member, because the tightening torque rises as the angle of the toothed -flank inclination increases.

The necessary stopping surfaces are relatively small compared with the necessary bearing surfaces. In order to be able to reduce the large amount of settlement which occurs with thin metal sheets and soft materials when pointed teeth are used, without modifying the overall action, the upper part of the pointed tooth

15 can be flattened, as shown in Figure 4, so that the face 16 is contiguous with a flat surface 18.

WHAT WE CLAIM IS:- 1. A combination of a self arresting threaded element with a component to be secured thereto or thereby, the element comprising a threaded portion having an axis, a support portion with an annular support face juxtaposed with the component in a relatively tightened condition of the element and the component achieved by rotating the element in a tightening direction, and, for arresting retrograde rotation of the threaded element in a tightened position thereof, a plurality of arresting projections on the annular support face and extending therefrom into the component, each of the arresting projections having a leading face and a trailing face as considered in the rotary direction of tightening, the arresting projections having deformed the material of the component during said tightening to an extent that is determined by the specific axial force acting on the threaded portion in the tightening position of the threaded element, the trailing faces having a combined effective area, Asp, in contact with the deformed material of the component in the tightened condition of the threaded element and the component sufficient to counteract any loosening torque acting on the threaded element opposite to the direction of the tightening and arising from an axial vibration which gives rise to an axial force on the element up to the value of the specific axial force, and wherein the combined effective area, Asps of the trailing faces of the teeth is at least equal to

where As is the tension cross-sectional area of the threaded portion of the threaded element; D and D1 are the outer and inner diameters of the annular support face respectively; d2 is the pitch diameter of the threaded portion; and i is the lead angle of the thread of the threaded portion.
2. A combination according to claim 1, wherein the arresting projections are elongate in a generally radial direction.
3. A combination according to claim 1, wherein the leading faces of the arresting projection slope away from the support face so that they rise in the tightening direction so that the leading faces constitute effective contact zones with the component, the contact zones gradually increasing in area during the tightening of the threaded element until the axial forces at the surface exerted on the contact zones by the deformed material of the component are in equilibium with the specific axial force in the tightened position.
4. A combination according to any one of claims I to 3, wherein the arresting projections are substantially identically shaped and are arranged in an annulus about the threaded portion.
5. A combination according to claim 4, wherein the component has a hole through which the threaded portion passes in the tightened condition, the hole having a diameter exceeding that of the threaded portion by a given distance; and wherein the inner diameter D, at least equals the sum of said diameter and said distance.
6. A combination according to any one of claims 1 to 5, wherein the arresting projections have substantially saw-tooth-' shaped cross-sections.
7. A combination according to any one of claims I to 6, wherein each of the arresting projections has a flat face intermediate the

leading and trailing faces thereof, located in a plane substantially normal to said axis.
8. A combination substantially as hereinbefore described and illustrated with reference to Figures 1, 2 and 3 or Figures 1, 2 and 4 of the accompanying drawings.
9. A self arresting threaded element specifically designed for use in a combination according to any one of claims 1 to 8.