Spring-loaded screw arrangement

Description

[0001] The invention relates to a spring head screw arrangement according to the preamble of claim 1.

[0002] Spring-head screws are known from the prior art, for example from DE 10 2016 100 446 A1, which have a conical skirt starting from the head in the manner of a disc spring, wherein the skirt widens towards the free end of the spring-head screw. The inner surface of the conical skirt merges with a radius into a threaded shaft. At its end facing the free screw end, the skirt has a contact area which, in particular, forms a support plane which is, in particular, orthogonal to the screw axis. The inner surface of the conical skirt encloses a contact angle α with the support plane.

[0003] Such screws are used to fasten clamping parts to substructures which are subject to a reduction in their component thickness over time, whereby the preload is maintained by the spring head.

[0004] In particular, under the influence of changing temperatures, some materials experience significant creep deformation under the applied preload.

[0005] According to the state of the art, the known designs have the disadvantage that unfavorable screwing cases can occur in which a washer gets caught on the screw, whereby in the case of prolonged exposure to temperatures of, for example, temperatures above 80°C to approximately 160°C in the screwed-in state, the preload force decreases over time to such an extent that it falls below a permissible minimum value.

[0006] The invention is based, among other things, on the finding that creep deformation occurs particularly in the area of ​​high surface pressure of the clamping part.

[0007] The object of the invention is to achieve a spring head screw arrangement with improved preload force retention under the influence of higher temperatures and an increase in assembly reliability.

[0008] The problem is solved by the characterizing features of claim 1 in conjunction with its preamble features.

[0009] The subclaims constitute advantageous further developments of the invention.

[0010] In a known manner, a spring-head screw assembly comprises a spring-head screw having a conical skirt extending from the head in the manner of a disc spring, the skirt widening toward the free end of the spring-head screw. The inner surface of the conical skirt transitions into a threaded shaft with a radius. At its end facing the free screw end, the skirt has a contact area which, in particular, forms a support plane which is located, in particular, orthogonal to the screw axis. The inner surface of the conical skirt forms a contact angle α with the support plane.

[0011] According to the invention, a washer is located between the threaded part of the shaft and the apron of the spring head screw, through which the shaft passes, so that the washer lies between the held component and the apron in the installed state.

[0012] The washer reduces the surface pressure exerted by the apron on the component, which allows the absolute amount of creep deformation of the clamping part to be reduced at the same preload force.

[0013] The washer is held, preferably captively, in an unthreaded portion of the shaft, wherein the unthreaded portion of the shaft lies between the head and a threaded portion of the shaft and has a bolt diameter. The inner diameter of the washer is preferably between the outer diameter of the screw thread and the bolt diameter.

[0014] The screw according to the invention is designed such that the shaft has a cylindrical area between the support plane and the radius in the shaft transition to the apron, the diameter of which corresponds to the bolt diameter.

[0015] The surface of the washer facing the head lies in the support plane when the washer just touches the apron.

[0016] The presence of the cylindrical area between the support plane and the shaft transition can reduce the probability that the radiused shaft transition between the shaft and the apron will come into contact with the washer when screwing in the spring head screw, even though the desired preload of the apron has not yet been achieved, which can then lead to a process-limiting increase in the screwing-in torque.

[0017] According to the invention, the cylindrical area between the support plane and the shaft transition allows the screw to be tightened over a larger screw-in angle without the shaft transition touching the washer. This allows the elastic spring travel of the skirt to be utilized more reliably.

[0018] For example, a washer's inner diameter that is only slightly larger than the shaft diameter, as is necessary for a rolled-on, captive washer, can be accommodated without negatively impacting the screw-in angle. Even in cases where the washer has an inner diameter that is generously tolerated compared to the shaft diameter, the likelihood of a blockage occurring between the shaft transition and the washer due to an eccentric position of the washer is significantly reduced.

[0019] By means of the measure according to the invention, the spring area of ​​the head of the spring head screw can then be increased, which then leads to an improved preload force retention under the influence of higher temperatures.

[0020] Preferably, the transition from the radius to the shaft and skirt can be tangential. This has the advantage of achieving a homogeneous stress distribution in the skirt and a reduced stress concentration for greater dynamic safety.

[0021] The skirt forms an annular surface in the support plane with an inner support diameter D KA . The unthreaded part of the shaft has a bolt diameter da.

[0022] According to a further preferred embodiment of the invention, the radius in the transition from the shaft to the inner surface of the skirt can be smaller than tan (a) * factor * (D KA -d 0 ) / 2, wherein the factor is smaller than 0.5 and preferably lies between 0.2 and 0.4.

[0023] The radius in the shaft transition therefore satisfies the formula: R U < tan α * Faktor * D KA − d 0 / 2 | Faktor < 0,5

[0024] Preferably, the inner diameter of the washer is less than 1.07 times the shaft diameter in the unthreaded part between the head and the threaded part of the shaft.

[0025] According to a further preferred embodiment, the contact angle α is between 20° and 40°, in particular between 28° and 32°. This provides the advantage that a sufficient angle of rotation is available for installation at the desired preload force.

[0026] According to a further advantageous embodiment, the thickness of the skirt is approximately 5% to 15% of the inner diameter of the support. The range of the preload force that can be generated can be adjusted by changing the thickness of the skirt.

[0027] According to a further aspect of the invention, a method for automatically tightening a spring-head screw according to the invention is provided. According to the method, the spring-head screw is tightened until a predefined torque is reached, and from the moment the torque is detected, the spring-head screw is further screwed in through a predefined angle of rotation.

[0028] The predefined angle is at least 180° and the predefined torque is preferably above a screw-in torque defined for the application and below a torque that must be applied to plastically deform the apron.

[0029] The range of plastic deformation can be easily determined for each application, since in the range of incipient plastic deformation the torque increase flattens noticeably and a plateau is formed in the screw-in angle-torque curve until the apron is completely plastically deformed and the torque increases steeply again with increasing screw-in angle.

[0030] The predefined torque is preferably in the middle between the torque required for plastic deformation and the screwing torque defined for the application.

[0031] According to the method for screwing a spring head screw described above, a clamping part to be fastened, which consists of a material that is subject to significant creep deformation under the influence of a corresponding temperature and the respective preload force applied, is clamped by the spring head screw to a substructure into which the spring head screw is screwed.

[0032] According to a further aspect of the invention, this relates to such a component connection comprising a spring head screw arrangement as described above comprising a spring head screw and a washer, a substructure into which the spring head screw is screwed, wherein between the washer and the substructure there is a clamping part which is held under prestress between the washer and the lower component layer.

[0033] Because the apron rests on the washer with its edge area, the local stress input through the apron to the clamping part is significantly reduced, which significantly reduces the creep of the clamping part material and thus the creep deformation of the clamping part.

[0034] Nevertheless, when the component connection is used in a temperature range permissible for the intended use of the component connection, creep deformation leads to a reduction in the strength of the clamping part by a maximum creepage distance under the preload applied by the spring head.

[0035] Accordingly, the spring head screw arrangement, in particular the spring head screw, is dimensioned such that the spring travel stored in the elastic deformation of the spring head and the elongation of the screw stored in the elongation of the shaft form a total travel that is greater than the maximum creepage distance in the permissible temperature range.

[0036] This leaves a stored spring travel, allowing a reliable preload to be applied to the clamping part even when it is subject to creep. Thus, the total travel exceeds the elongation of a conventional screw shaft with the same preload and screw diameter.

[0037] Preferably, the difference between the total travel and the maximum creepage distance over a specific time corresponds to a spring travel at which a minimum preload is applied to the component. This is achieved by dimensioning the washer and the skirt.

[0038] The minimum preload is application-specific and predefined with respect to the component connection.

[0039] The spring head screw and the washer are made of steel, for example, while the clamping part is preferably a copper part.

[0040] For example, the spring-head screw assembly is used to connect a clamping part that establishes a detachable electrical contact to a second component of the electrical contact. By maintaining the preload, reliable contact can be ensured even over a wide temperature range. The second component can be formed by the substructure or another clamping part.

[0041] The proposed method in combination with the spring head screw arrangement according to the invention allows the reliable production of a component connection according to the invention with very good preload force retention under the influence of temperature.

[0042] Further advantages, features and possible applications of the present invention will become apparent from the following description in conjunction with the embodiments shown in the drawings.

[0043] In the drawing means: Fig. 1 a sectional view of a spring head screw arrangement according to the invention, and Fig. 2 a component connection according to the invention comprising a spring head screw arrangement, a clamping part and a substructure

[0044] Fig. 1 shows a spring-head screw assembly 8 according to the invention, comprising a spring-head screw 10 with a head 12 having a conically extending skirt 14 in the manner of a disc spring. The skirt 14 forms a support plane EA, which faces the free end of the spring-head screw 10 and extends substantially orthogonally to the screw axis A.

[0045] The conical apron 14 forms an inner diameter in the support plane EA, which is referred to as the support inner diameter D KA.

[0046] The spring-head screw 10 also comprises a shaft 16, which has, at its end facing the head 12, a thread-free portion 18 with a diameter referred to as the bolt diameter d 0 . Toward the free end of the screw 10, which is opposite the head 12, the shaft 16 has a thread-bearing region 20 with an outer diameter d A that is larger than the bolt diameter d 0 , and with a core diameter d K that is generally smaller than the bolt diameter d 0 .

[0047] Furthermore, the spring-head screw assembly 8 according to the invention comprises a washer 30 that is rolled captively onto the shaft 16 of the spring-head screw 10. The washer 30 thus has an inner diameter DI that is smaller than the outer diameter d A of the thread-bearing part 20 of the shaft 16, but larger than the bolt diameter da.

[0048] According to the spring head screw 10 according to the invention, the thread-free part 18 of the shaft 16 merges into the apron 14 via a radius Ru, wherein between the support plane EA and the radius Ru there is another thread-free part 18 of the shaft 16 which has the bolt diameter d 0.

[0049] This has the advantage that, during the tightening process of the spring-head screw 10, in which the center of gravity of the head moves toward the washer 30 due to the resilient skirt 14, the washer 30 is prevented from contacting the widening shaft transition at the transition from the shaft 16 to the skirt 14 over a cylindrical area that is longer in the axial direction. In this way, the range in which the spring-head screw 10 with the washer 30 can be used in the elastic range can be increased.

[0050] In the present design, the contact angle α, which represents the angle between the inner surface 22 of the apron 14 and the support plane EA, is 30°.

[0051] The radius Ru for the underhead contour is given by (D KA -d 0 ) / 2 * tan (α =30°) * factor, whereby the factor in the present example is approximately 0.3.

[0052] The radius Ru merges tangentially into both the shaft 16 and the apron 14.

[0053] This ensures that an axial displacement of the center of gravity of the head 12 in the screwing direction does not prematurely lead to a contact of the transition from shaft to head with the washer 30, which would result in a process-limiting torque increase.

[0054] In this way, creep, especially of the clamping part, under preload can be better taken into account and the preload force retention can be improved.

[0055] Fig. 2shows a component connection 40, comprising a spring-head screw 10 according to the invention with a washer 30, and a substructure 44 into which the spring-head screw 10 is screwed, wherein a clamping part 42 is clamped between the washer 30 and the substructure 44, wherein the skirt 14 of the spring-head screw 10 exerts a prestressing force on the clamping part 42. In the present example, the clamping part 42 exhibits a stronger creep behavior under the influence of temperature than the substructure 44. Several clamping parts can also be clamped between the washer 30 and the substructure 44.

[0056] In the design of the component connection 40, the components are coordinated so that under the intended operating conditions—namely, preload, temperature gradient, and time—the spring-head screw compensates for creep of the clamping part to such an extent that a certain amount of spring travel is still maintained. This ensures that a minimum preload force is exerted by the spring-head screw on the clamping part 42.

[0057] For example, the clamping part 42 forms an electrical contact with the substructure 44.

[0058] For example, the spring head screw 10 and the washer 30 are made of steel and the clamping part 42 is made of copper.

[0059] The washer 30 is held captively on the spring head screw 10 so that it can be easily mounted.

Claims

1. Spring head screw assembly (8) comprising a spring head screw (10) with a head (12) having a conical skirt (14) that widens in the direction of the free end of a threaded shaft adjoining the head (12), characterized in that the spring head screw assembly (8) has a washer (30) that is arranged between the skirt (14) and the threaded part (20) of the shaft (16) in a non-threaded part (18) of the shaft (16) with a bolt diameter (d0), wherein the inner surface (22) of the skirt (14) transitions into the shaft (16) within a radius (RU), wherein the inner surface (22) forms a contact angle (a) with a bearing plane (EA), which bearing plane (EA) is determined by the end of the skirt (14) facing the free end of the screw and is orthogonal to the screw axis, wherein the shaft (16) has a cylindrical region with the bolt diameter (d0) between the bearing plane (EA) and the radius (RU) at the transition to the skirt (14).

2. Spring head screw assembly according to claim 1, characterized in that the washer (30) has a hole with an inside diameter (Di) which is smaller than the outside diameter (DA) of the adjoining threaded part (20) of the shaft (16).

3. Spring head screw assembly according to one of claims 1 or 2 above, characterized in that the radius (RU) transitions tangentially into the shaft (16).

4. Spring head screw assembly according to claim 3, characterized in that the radius (RU) transitions tangentially into the inner surface (22) of the skirt (14).

5. Spring head screw assembly according to any one of the preceding claims, characterized in that the skirt (14) defines an bearing inner diameter (DA) in the bearing plane (EA), wherein the radius (RU) is smaller than (DKA-d0) / 2 * tan (a) * factor, which factor is less than 0.5.

6. Spring head screw assembly according to any one of the preceding claims, characterized in that the factor is between 0.2 and 0.4.

7. Spring head screw assembly according to any one of the preceding claims, characterized in that the inside diameter (Di) of the washer (30) is smaller than 1.07 times the bolt diameter (d0).

8. Spring head screw assembly according to any one of the preceding claims, characterized in that the angle (α) is between 20° and 40°, in particular between 28° and 32°.

9. Spring head screw assembly according to any one of the preceding claims, characterized in that the thickness (s) of the skirt (14) is approximately between 5% and 15% of the bearing inner diameter (DA) of the head.

10. Spring head screw assembly according to any one of the preceding claims, characterized in that the washer (30) is held captive on the spring head screw (10).

11. Method for automatic screwing of a spring head screw assembly (8) according to any one of the preceding claims, characterized in that the spring head screw (10) is inserted through a clamping part (42) and screwed to a substructure (44) located under the clamping part (42), wherein the spring head screw (10) is tightened until a predefined torque is reached, whereupon, from the point at which the torque is detected, the spring head screw (10) is screwed in further over a predefined angle of rotation.

12. Method according to claim 11, characterized in that the predefined angle of rotation is at least 180°.

13. Component connection comprising a spring head screw assembly (8) according to any one of claims 1 to 10 above, a substructure (44) into which the spring head screw (8) is screwed, wherein a clamping part (42) is clamped in place between the spring head assembly (8) and the substructure (44), which clamping part (42), under the influence of a predefined temperature and predefined contact pressure, will undergo a reduction in thickness in the region of the bearing surface of the washer (30) by a creepage distance, with the spring head screw assembly (8) being designed such that, after the screw has been tightened, the sum of the elongation of the shaft (16) and the spring travel of the skirt (12) exceeds the length of the creepage distance.

14. Component connection according to claim 13, produced by the method of any one of claims 10 or 11 above.

15. Component connection according to any one of claims 13 or 14, characterized in that the spring head screw assembly is made of steel, and the clamping part is made of copper.