Tightening method and device for producing screw connections and parts set for use in the tightening method

Abstract

Tightening method for producing a screw connection using at least one metallic screw (1) and a lock nut (2) matching the screw (1), characterized in that during the tightening process the lock nut (2) is moved back and forth in the free thread area of ​​the screw (1) with alternating, cyclical rotary movements to generate frictional heat and at the same time the screw temperature rising as a result of this cyclical friction movement is measured, wherein the lock nut (2) is tightened to the stop when a predetermined target value of the screw temperature is reached, and wherein the lock nut (2) is a self-locking solid metal nut.

Description

[0001] The invention relates to a tightening method and a tightening device for producing a screw connection, as well as a set of parts intended for use in the tightening method. The method and device are particularly suitable for tightening dynamically loaded, high-load screw connections, such as screw connections in steel construction and shipbuilding. Another area of ​​application is the tightening of screws in underwater installations.

[0002] In force-locked connections, which include bolted connections, the preload force—along with the friction between the connected elements—is the key parameter for the functionality of the connection, according to Coulomb's law of friction. Since friction can only be influenced to a limited extent, it is ultimately the preload force that must be defined as precisely as possible to ensure the force-locked connection.

[0003] However, with the most common torque-controlled tightening methods, the preload force can only be determined by indirect calculation, which requires knowledge of the friction conditions in the thread. In practice, thread friction is calculated using empirically based friction coefficients; in many cases, this is entirely sufficient to predetermine the preload force or to establish a tightening torque with which the required frictional connection of the screw connection can be ensured. However, since the conceivable material pairings are extremely diverse and, in addition, lubricants are sometimes used during tightening, the preload force when tightening to a specified tightening torque can rarely be determined precisely. Tightening devices for torque-controlled tightening of screw connections are described, among others, in DE 42 43 501 A1 or DE 10 2008 048 076 A1.

[0004] The also well-known method of angle-controlled tightening can generally be used to determine the preload more effectively, as the elongation or expansion of the screw - and thus also the preload - can be determined via the angle of rotation without having to take thread friction into account. However, there is usually the problem of determining the starting point for the angle of rotation measurement. The stop, which is relatively easy to determine under normal workshop conditions, i.e. the point at which the nut touches the component to be joined and consequently the tightening torque increases significantly during tightening, can only be used to a limited extent as a starting point for angle-controlled tightening, as in addition to the elongation of the screw, settlement phenomena occur in the thread. If the stop is selected as the starting point for an angle-controlled measurement, the measured tightening angle is split between the settlement phenomena and the elongation of the screw.These components cannot be separated from one another. The preload force determined in this way is therefore subject to significant measurement error. For this reason, a starting point for angle-controlled tightening is usually chosen that lies beyond the stop, i.e., well within the torque increase range, in order to anticipate the settling phenomena as far as possible. However, with regard to the starting point determined by torque-controlled tightening, the friction-related measurement errors described above again come into play. A angle-controlled tightening method and a corresponding screwing tool for implementing the method are disclosed, for example, in DE 10 2006 017 193 A1.

[0005] In addition, with both torque-controlled and angle-controlled tightening methods, the tightening force or torque increases continuously with the increase in preload. Particularly in screw connections that are intended to transmit very high forces, such as screw connections with large screw diameters or screw connections with high-strength screws, this can lead to the inability to apply the required tightening forces or torques using conventional tightening devices.

[0006] To avoid this problem, the screw can be heated before screwing on the nut, causing the bolt to elongate according to its thermal expansion coefficient. The nut is then screwed onto the screw thread while it is still heated, all the way to the stop or slightly beyond. During subsequent cooling, the bolt shrinks, thereby preloading the screw connection. The preload force can be calculated based on a temperature measurement using the known physical relationships between thermal expansion and elastic deformation. The particular advantage of this method, known as thermal tightening, is that only slight tightening forces or torques need to be applied, yet very high preload forces can be achieved. DE 10 2018 000 287 A1 describes such a tightening method and a corresponding tightening device.The disadvantage here, however, is that heating is only possible in conjunction with a screw that has a blind hole.

[0007] The object of the invention is to provide a tightening method and a tightening device that enable the creation of a screw connection with a defined preload force using conventional metal screws. Tightening should be possible with the lowest possible tightening forces and torques.

[0008] This object is achieved by a tightening method having the characterizing features of claim 1 and a tightening device according to claim 7; a set of parts intended for use in the tightening method according to the invention is described in claim 9. Expedient developments of the invention are set out in claims 2 to 6 and in claim 8.

[0009] The tightening process for creating a screw connection is carried out using at least one metal screw and a locknut that matches the screw. According to the invention, during the tightening process, the locknut is moved back and forth in the free thread area of ​​the screw with cyclical rotational movements, while the screw temperature is simultaneously measured. The locknut is tightened to the stop when a predetermined target screw temperature is reached.

[0010] The lock nut can only be moved by overcoming the low locking torque, i.e., only by overcoming the static or sliding friction in the thread. When the lock nut moves in the free thread, heat is generated due to friction, compared to a standard nut without a locking element. With unidirectional movement of the lock nut, the associated temperature increase is negligible. The alternating or cyclical movement according to the invention can increase the screw temperature so much due to thread friction that the screw elongates significantly.

[0011] The tightening method makes it possible to tighten the screw connection with low tightening forces or torques, since only the locking torque has to be overcome when tightening to the stop.

[0012] The good thermal conductivity of the metal screw ensures that the heat generated in the threaded area is quickly distributed throughout the entire bolt. After a short period of cyclic frictional movement, the screw exhibits a nearly uniform, elevated temperature, which leads to thermally induced elongation of the bolt. The screw temperature is measured either contact-free or contact-based; it serves as an indicator of sufficient elongation of the bolt. Once the specified target screw temperature is reached, the lock nut is tightened to the stop. As the bolted connection cools, the bolt shrinks, creating preload in the bolted connection.The preload force can be determined for the specific geometry of the bolted connection by calculating the thermal shrinkage based on the expansion coefficient and by subsequently calculating the elastic stress based on the Young's modulus and the thermal shrinkage strain.

[0013] The tightening method can be used with all conventional metal screws; special screws are not required. The tightening method is particularly suitable for tightening expansion bolts.

[0014] Furthermore, the tightening method according to the invention does not require additional heat sources to increase the temperature of the screw, such as gas burners and / or electric heating systems. The heat input forms part of the tightening process. Thus, slightly modified tightening devices based on known screwdriving tools, such as electric or pneumatic screwdrivers, but especially torque- and / or angle-controlled EC screwdrivers (electronically controlled), can be used.

[0015] For example, in bridge construction, especially in the rapid construction of temporary bridges, there is no need to resort to devices for inductive heating of the screws.

[0016] According to one embodiment of the tightening method, the torque of the locking nut's rotational movement is measured during the tightening process. Reaching the stop is defined as a predetermined increase in torque. This allows the stop point to be programmed, particularly when using automatic tightening devices.

[0017] For example, the screw temperature can be measured non-contact at the screw end, i.e., at the end of the screw opposite the screw head, using a pyrometer. The advantage of this tightening method is that the pyrometer can be integrated into the tightening device, which usually engages the nut.

[0018] Alternatively, a non-contact, but preferably contact-based, measurement of the screw temperature at the screw head is possible. Contact or tactile measurement of the screw temperature at the screw head can be performed, for example, using a thermocouple.

[0019] The method can also be carried out using a metal screw with a thermal color marking on the screw head. This thermal color marking serves as an indicator that indicates when the specified target screw temperature has been reached. When using screws marked in this way, the screw temperature is measured indirectly via the color change of the thermal color; additional measuring devices are therefore not required. This is particularly advantageous for underwater installations. A parts kit intended for use in the tightening method according to the invention contains, for example, the metal screw with the thermal color marking and the lock nut matching the screw.

[0020] According to the invention, the lock nut is a self-locking solid metal nut, for example, a Stover nut. Self-locking in solid metal nuts can be achieved, among other things, by partially deforming the nut (Stover nut) or by slightly altering the geometry of a thread.

[0021] It can further be provided that the rotational speed of the lock nut is increased and decreased during the rotational movement phases of the cyclical rotational movement between the dead centers in such a way that the rotational speed of the lock nut in the respective rotational movement phase is at least 50 rpm -1 preferably it is essentially in the range of 100 rpm -1 up to 200 rpm -1The temperature increase depends on the frictional power introduced into the thread; the frictional power, in turn, correlates with the friction speed and thus the rotational speed. The suggested rotational speeds lead to particularly rapid heating in the thread. At the preferred interval, local overheating on the friction surfaces is avoided.

[0022] According to the invention, the tightening device is configured to carry out the disclosed tightening method. For this purpose, the tightening device comprises, for example, a control unit with a stored program.

[0023] The tightening device for performing the tightening process can be an electronically controlled screwing tool with an integrated pyrometer for non-contact measurement of the screw temperature. The pyrometer is preferably arranged coaxially in the screw spindle of the screwing tool, which is connected to a wrench element for tightening the lock nut. The pyrometer's field of view is thus directed axially and centrally toward the screw end during tightening from the nut side.

[0024] The invention is explained in more detail below using exemplary embodiments and with reference to the schematic drawings, in which identical or similar features are provided with the same reference numerals. In the drawings: Fig. 1: three phases of the tightening process for producing a screw connection, each in longitudinal section, and Fig. 2: the tightening device in longitudinal section.

[0025] The screw connection according to the Fig. 1 comprises the two components 3 to be connected, the metallic screw 1 and the lock nut 2.

[0026] During the first phase of the tightening procedure according to Fig. 1a, the lock nut 2 is moved back and forth in cyclical rotational movements within the free thread area of ​​the screw 1. The arrows symbolize the cyclical rotational movements and the resulting, likewise cyclical axial movements of the lock nut 2 within the free thread area of ​​the screw 1.

[0027] The screw temperature can be measured in different ways, whereby Fig. 1a two different variants are given: According to the first variant, the screw temperature is measured non-contact using the pyrometer 4. The pyrometer 4 is directed toward the screw end 1.2 so that the infrared measuring radiation 4.1 emanating from the screw end 1.2 is detected by the pyrometer 4. The measuring radiation 4.1 is converted into electrical quantities in the pyrometer 4 and transmitted to a temperature display unit 6.

[0028] According to the second variant shown, the screw temperature is measured tactilely at the screw head 1.1 using the thermocouple 5. The screw temperature measured at the thermocouple 5 is also displayed on a temperature display unit 6.

[0029] As soon as the screw temperature has reached a specified setpoint, the cyclical rotations of the lock nut 2 are stopped and the lock nut 2 is moved into the second phase - see Fig. 1b - fully tightened. The arrow in Fig. 1b symbolizes the tightening to the stop, ie the contact of the lock nut 2 on the upper of the two components 3 to be connected.

[0030] In the third phase, Fig. 1c, the cooling of the screw 1 takes place. The locking nut 2, which is seated at the stop, presses the two components 3 against each other due to the thermal shrinkage of the screw 1. The arrows in Fig. 1c symbolizes the thermal shrinkage of screw 1.

[0031] The tightening device according to the Fig. 2 is an electronically controlled screwing tool 7 (EC screwdriver), with the wrench element 9 located on its screw spindle 8. The wrench element 9 can, for example, be an interchangeable socket wrench that can be attached to the screw spindle 8 in a known manner. The pyrometer 4 is located coaxially within the screw spindle 8 of the screwing tool 7.

[0032] To establish the screw connection between the two components 3 using the screw 1 and the lock nut 2, the key element 9 is placed on the lock nut 2. Tightening is achieved by first alternately moving the screw spindle 8 and, with it, the lock nut 2 by means of torque transmission using the key element 9. The further process for establishing the screw connection corresponds to the explanation of Fig. 1. List of reference symbols used 1 screw 1.1 Screw head 1.2 Screw end 2 lock nuts 3 components 4 pyrometers 4.1 Measuring radiation 5 thermocouple 6 Temperature display unit 7 screwing tools 8 screw spindle 9 Key element

Claims

[1] Tightening method for producing a screw connection using at least one metallic screw (1) and a lock nut (2) matching the screw (1), characterized by that during the tightening process the lock nut (2) in the free thread area of ​​the screw (1) is moved back and forth with alternating, cyclical rotary movements to generate frictional heat and at the same time the screw temperature increasing due to this cyclical friction movement is measured, wherein the lock nut (2) is tightened to the stop when a predetermined target value of the screw temperature is reached, and wherein the lock nut (2) is a self-locking solid metal nut. [2] Tightening method according to claim 1, characterized by that during the tightening process the torque of the rotational movement of the lock nut (2) is measured, wherein reaching the stop is defined as a predetermined increase in the torque. [3] Tightening method according to claim 1 or 2, wherein the screw temperature is measured contactlessly at the screw end (1.2) by means of a pyrometer (4). [4] Tightening method according to claim 1 or 2, wherein the screw temperature is measured tactilely on the screw head (1.1) by means of a thermocouple (5). [5] Tightening method according to one of claims 1 to 4, characterized by that the rotational speed of the lock nut (2) is increased and decreased during the rotational movement phases of the cyclical rotation between the dead centers in such a way that the speed of the lock nut (2) in the respective rotational movement phase is at least 50 rpm -1 reached. [6] Tightening method according to claim 5, characterized bythat the rotational speed of the lock nut (2) is increased and decreased during the rotational movement phases of the cyclical rotational movement between the dead centers in such a way that the speed of the lock nut (2) in the respective rotational movement phase is in the range of 100 rpm -1 up to 200 rpm -1 lies. [7] Tightening device, arranged to carry out the tightening method according to one of claims 1 to 6, wherein the tightening device is an electronically controlled screwing tool (7), characterized by that a pyrometer (4) for contactless measurement of the screw temperature is integrated in the screwing tool (7). [8] Tightening device according to claim 7, characterized by that the pyrometer (4) is arranged coaxially in a screw spindle (8) of the screwing tool (7) connected to a key element (9) for tightening the lock nut (2). [9] Kit of parts for use in a tightening method according to one of claims 1 to 6, comprising a metallic screw (1) and a locking nut (2) matching the screw (1), characterized by that the screw head (1.1) of the screw (1) has a thermal color marking.

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