Method for effecting a screwed connection

A screwed connection has a threaded bolt; a nut; a washer arranged between the nut and a surface of a component to be clamped; a screwing device with a housing, a holding tool connected to the housing, and a tightening tool that engages positively the nut; a measuring element arranged in the washer for measuring a compressive force generated by tightening the screwed connection. The washer has an anti-rotation device oriented toward the surface of the component. The outer periphery of the washer is positive-lockingly engaged by the holding tool. The washer has no positive or frictional connection to the threaded bolt, and the holding tool has no torque supporting action on the threaded bolt or on any other part so that a reaction moment, generated when tightening the nut, is introduced from the housing of the screwing device through the washer into the component to be clamped.

BACKGROUND OF THE INVENTION

The invention concerns a screwed connection comprised of a threaded bolt, a nut, and a washer positioned between the nut and a surface of a component to be clamped.

Such screwed connections wherein the washer is engaged by a holding tool and the nut by a tightening tool are disclosed in EP 0 988 935 A2, EP 1 080 847 A2, and EP 1 435 279 A1.

According to EP 0 988 935 A2 a positive locking connection between the threaded bolt and the washer exists while according to EP 1 435 279 A1 a friction connection between the threaded bolt and the washer is provided. Additionally, according to EP 0 988 935 A2 a further holding element connected to the first holding element can positively engage the threaded bolt. In contrast to this, in the screwed connection according to EP 1 080 847 A2 there is no connection between the threaded bolt and the washer but holding tools that are connected to one another engage the washer as well as the threaded bolt in a positive locking way.

These prior art solutions have in common that the holding tool is supported on the washer as well as directly and/or indirectly on the threaded bolt to be tightened that therefore absorbs when tightening the nut the reaction forces of the screwing tool which forces are directed in a direction opposite to the tightening torque.

The frictional anti-rotation action of the washer relative to the threaded bolt according to EP 1 435 279 A1 can cause damage on the thread of the threaded bolt and thus lead to fatigue occurring too early. Moreover, the washer is designed in a special way so that its manufacture is complex.

The positive connection between the threaded bolt and the washer according to EP 0 988 935 A2 requires a matching configuration of the threaded bolt and, below the threaded part, it must be provided with one or several longitudinal grooves that are engaged by matching elements of the washer. In this way, the manufacture of the threaded bolt and of the washer is more complex and the fatigue strength of the threaded bolt may be impaired.

When in accordance with EP 0 988 935 A2 and EP 1 080 847 A2 a positive-locking engagement on the threaded bolt by means of a holding tool that, in turn, engages a holding tool engaging the washer is provided additionally, the arrangement is made more complicated and more complex.

The screw clamping force is determined in these known screwed connections by measuring the torque applied to the nut but this measurement is extremely imprecise because the frictional conditions in the screwed connection can fluctuate within a wide range as is well known in the art.

SUMMARY OF THE INVENTION

The invention has the object to provide a screwed connection that is of a simple configuration, requires only a screwing tool of a simple design, and enables a precise measurement of the screw clamping force.

Based on this object, a screwed connection comprised of a threaded bolt, a nut and a washer arranged between the nut and a surface of a component to be clamped is proposed wherein, in accordance with the present invention, the outer periphery of the washer is designed to be positively engaged by a holding tool and has a measuring element arranged therein for measuring the compressive force that is caused by tightening the screwed connection.

The invention is based on the concept that the screw clamping force acting on the threaded bolt and the force acting on the surface of the component to be clamped and transmitted by the washer are directed opposite and equal but that they act by a different radius so that the friction torque acting between the washer and the surface of the component to be clamped is greater than the friction torque acting between the thread of the threaded bolt and the nut, while the friction torque that is acting via the nut on the washer and this proportion of the reaction torque acting on the washer will cancel each other. Therefore it is sufficient to cause a holding tool that is connected to a housing of a screwing device to engage the washer in a positive locking way and to tighten the nut by means of a tightening tool of the screwing device that has been brought into positive engagement with the nut. The housing of the screwing device then remains stationary together with the washer because the reaction torque that is caused by tightening the nut is diverted from the housing through the washer into the component to be clamped without a support of the housing on the threaded bolt or on other parts being required. By means of the measuring element that is arranged in the washer the screw clamping force can be measured directly because the compressive force generated by the nut and acting on the washer is directly proportional to the screw clamping force without any friction effects occurring or having to be taken into account in this connection.

The outer periphery of the washer is of a polygon shape and has, in particular, a hexagon shape, the measuring element can be arranged so as to be recessed within one of the flat surfaces so that in this way it is protected from being damaged by the holding tool.

The measuring signals generated by the measuring element can be transmitted by a cable connection or without a cable connection inductively or by means of a radio connection to an evaluation and display device.

Preferably, the measuring element can be embodied a measuring bridge with a strain gauge or as an inductance part or as a capacitance part as a measuring part.

In order to achieve a sufficient friction torque of the washer relative to the component to be clamped from the beginning when tightening the screw, the washer can have an anti-rotation device that is acting on the surface of the component to be clamped.

This anti-rotation device can be, for example, comprised of at least one pin that is guided in a bore of the washer, wherein a pressure spring acts on one end thereof and wherein the other end is sharply pointed.

Moreover, the anti-rotation device can also be comprised of several annular segments that are arranged in grooves in the washer and are loaded by spring force.

The spring force acting on the pin or the annular segments can advantageously be increased when tightening the nut by means of at least one moveable pressure element that interacts with the nut and projects past the surface of the washer that is facing the nut.

A further simple anti-rotation device can be comprised of at least one pointed projection that upon tightening the screwed connection is forced into the surface and that is preferably comprised of a planar toothing.

The screwed connection according to the invention can be used preferably in connection with a conventional simple nut that is to be screwed onto the threaded bolt when the torsional moment is acceptable that is caused when tightening the nut on the threaded bolt and that is transmitted by thread friction. Of course, it is however also possible to use the screwed connection according to the invention for a torsion-relieved tightening of the screwed connection, for example, when according to EP 0 988 935 A2 a holding tool is brought into positive-locking connection with the threaded end of the threaded bolt or when, according to EP 1 080 847 A2, a double nut is employed wherein the inner nut is in positive-locking connection with the washer and secured by the holding tool while the outer nut is tightened by the screwing tool.

DESCRIPTION OF PREFERRED EMBODIMENTS

The component1to be clamped is schematically shown only a bore11for receiving a threaded bolt9and by a surface2against which the washer10is resting.

Onto the threaded end of the threaded bolt9a nut3is screwed that, in the illustrated embodiment, has a cylindrical area4and an area5with a configuration for positive-locking engagement of a nut-tightening tool25. The nut3is screwed onto the threaded end6of the threaded bolt9and has on the end that is facing the washer10a conical recess7as well as a recess8that leaves off at least one turn of the thread, which recesses serve for distributing the force transmission onto several thread turns.

The washer10is also provided with configurations for positive locking engagement of a holding tool24; in the illustrated embodiment flat surfaces12of an external hexagon are provided. Recessed in the area of a flat surface12there is a measuring element14by means of which the compressive force caused in the washer10when tightening the nut3can be measured. This compressive force corresponds to the screw clamping force applied onto the threaded bolt9that is not affected by any friction effects. The measuring signals generated by the measuring element14can be transmitted by an interface15onto an evaluation device and display device that is not illustrated. The interface15can be a plug connection fora cable connection to the evaluation and display device. It is likewise possible to configure the interface for inductive transmission or a radio transmission of the measured values. The interface15is recessed like the measuring element14in the flat surface12of the washer10and is connected to the measuring elements14in an appropriate way.

Since the average diameter of the washer10, and thus the friction torque of the washer10relative to the surface2of the component1to be clam ped which friction torque is caused by the compressive force acting on the washer10, is greater than the diameter of the thread6, and thus greater than the friction torque that is generated in the thread6when tightening the nut3, the washer10when tightening the nut3by means of a suitable screwing device remains stationary so that it is possible to attach a holding tool24non-rotatingly to the housing of the screwing device which holding tool24is brought into engagement with the outer hexagon of the washer10while a tightening tool25of the screwing device is brought into positive engagement on the nut3. When the tightening process is triggered by means of the screwing device and the nut3force the washer10against the surface2, the housing of the screwing device is secured against rotation relative to the washer10because the washer10absorbs the reaction moment of the housing of the screwing device that is opposite to the tightening torque and diverts it into the component to be clamped without this requiring an additional torque support on the component1to be clamped or on the threaded bolt9.

In order to already absorb the reaction moment when the nut3is tightened only minimally against the washer10, the washer10can have an anti-rotation device that is comprised, for example, according toFIG. 1, of a pointed pin17. This pointed pin17is guided on the one hand in a plug20provided with a throughbore and on the other hand in a bore16in the washer10by means of a pressure plate18connected thereto and is loaded by a pressure spring19. When positioning the washer10on the threaded bolt9in contact with the surface2of the component1to be clamped, the tip of the pin17will dig into the surface2and will effect immediately a rotational securing action that, when tightening the nut3, is enhanced by the friction effect between the washer10and the surface2in such a way that the reaction moment of the screwing device is absorbed completely by the washer10and guided into the component1to be clamped.

The embodiment illustrated on the right side ofFIG. 2corresponds with regard to the pointed pin and its guide to the embodiment illustrated inFIG. 1but the pressure spring19does not act directly onto a part of the washer10but by means of a pressure plate22onto a further plug20by means of which a pressure element21in the form of a pin is guided through the surface13facing the nut3. Upon tightening the nut3it first meets the pin21, forces it into the bore16in the washer10and reinforces thus the pressure generated by the spring19onto the pointed pin17that in this way is forced with increased spring force into the surface2of the component1to be clamped.

These two illustrated embodiments show only one pointed pin17as an anti-rotation device but, of course, it is possible to arrange in the washer several such spring-loaded pins17about its bore.

A further simple anti-rotation device is illustrated on the left side ofFIG. 2and is comprised of a planar toothing23on the bottom side of the washer10. This planar toothing23is forced when tightening the nut3into the surface2of the component1to be clamped and provides a quasi positive-locking connection between the washer10and the component1to be clamped after the nut3has generated the required screw clamping force.