DE10326898B4 - Method for reducing tolerances of the prestressing forces on the yield point of a workpiece - Google Patents

Abstract

method for reducing tolerances of the prestressing forces at the yield point (4, 5) in a thermally tempered Workpiece, in particular a screw or a thread forming part, wherein the thermally tempered workpiece to a defined stretching prestressing force (7, 8) above a yield strength (4, 5) is plasticized, characterized in that the stretching with a tensile force that consists of a tensile and torsional stress composite voltage state results.

Description

The The invention relates to a method for reducing the scattering of Montagevorspannkraft at a workpiece, in particular a screw, a thread molding or the like.

One well-known, relatively accurate and also economical assembly process for threaded parts is the over-elastic one Elongation and rotation angle controlled tightening, in which the screw until the yield strength of the material is reached or exceeded is. The yield strength is defined by the transition in the stress-strain curve from the linear elastic Hook'schen Area in the plastic deformation area.

These Preload force distribution can be achieved by exact temperature control the thermal pretreatment of the connecting elements can be reduced. But this is expensive, correspondingly expensive and can also not achieve a perfect elimination of these dispersions.

In the DE 101 15 403 A1 For example, in order to reduce the dispersion of the assembly biasing force, it is proposed to plasticize the workpiece, for example a screw, to a defined axial flow force (stretching biasing force) above the yield point before final assembly (final assembly). By newly generated in this way for all workpieces of a production lot equal yield strength, it should be possible to adjust by subsequent over-elastic tightening the assembly preload with far less tolerance than in fasteners that are either not annealed or heat treated alone, thermally tempered or from a pre-tempered material are made.

The The problem underlying the invention is that after the thermal Treatment of fasteners due to the dispersion of strength properties significantly reduce existing variations in the assembly preload.

The Task is solved with the features of claim 1. Thereafter is to achieve one approaching constant preload levels provided, the workpiece up over the Yield strength in the plastic area with a defined tensile force to stretch. Already when stretching while the screw is both a subjected to axial tensile stress as well as a torsional stress.

This embodiment is based on the consideration that in the off DE 101 15 403 A1 known processes in a disadvantageous manner, the stretching operation is carried out until the so-called Gleichmaßdehnungsbereich the screw before final assembly only in the axial direction. This creates a preload level in the plastic deformation range of the screw, which can not be achieved in the subsequent final assembly of the screw (screw mounting). In the subsequent screw mounting, in addition to the axial stress, torsional stresses are also introduced into the screw shaft. These superimposed torsional stresses lead to a reduction of the preload force, which uses plastic flow of the screw material.

The height of Torsion tension in turn depends in particular on the friction in the screw thread. As a result of the friction scattering also scatters the height of the torsional stress. This has the consequence that at the later screw mounting just not - despite previous stretching of all screws above the yield point beyond - one constant preload level can be achieved.

The here proposed solution starts from the knowledge that the dispersion of the assembly prestressing force in addition to the scattering of the yield strength and the tensile strength of the material significantly influenced by the thread friction of the connecting elements becomes. In order to minimize the dispersion of the assembly prestressing force, Therefore, the dispersion of the thread friction already at Vorrecken also detected. The stretching limited not on the stretching in the axial direction, in the only axial Tensions in the bolt arise. Rather, when stretching already applied torsional stresses.

The stretching process is therefore oriented to the actual installation conditions and thus carried out practically. Preferably, in this case, the screw is stretched with the same clamping length, with which it is used in the final assembly. Advantageously, the connecting elements to be stretched, in particular screws, are screwed by turning nut or screw head into a stretching device. The stretching device in this case comprises in particular a female thread part, in which the screw is screwed, and a spacer, which corresponds to the clamping length and on which the screw head is supported. Spacer and female thread part preferably form a one-piece unit. Alternatively, they are formed as separate components. During the stretching process, the screw is screwed into the female thread part until it comes to rest with its screw head on the spacer. By further tightening the screw, it is braced beyond the yield point to a uniform preload force level (stretching prestressing force) for all screws. Here, the preload force is measured exactly and In addition, the change in length of each individual workpiece (screw) is checked to ensure that in each case a plastic deformation has taken place during preheating.

All workpieces (Screws) of a production lot are at an identical preload level vorgereckt above the yield strength. In order to make this possible should the total scattering, in the scattering of the material strength and additionally the scatter of the thread friction numbers enter, if possible be small and in particular within the distance of the yield strength and the maximum traction lie to be stretched screws. The yield strength corresponds the applied voltage or force at the yield point. The maximum tensile strength corresponds to the maximum achievable voltage in the stress-strain diagram. The ratio of Yield strength to maximum tensile strength the workpieces (Components, screws) is therefore of particular importance. This ratio should not too big around all parts of a production lot above the yield point to be able to stretch one and the same prestressing force.

at the later following final assembly of the workpiece, for example, when connecting two components by means of the screw, this is preferably tightened to a tightening force, which at the Stretching exercised Reck-biasing force equivalent. Conveniently, this is the ratio of exerted Torque to the swept Angle of rotation monitored and the screw is tightened until the ratio changes abruptly. to Final assembly is therefore provided a suitable control device.

The relationship Corresponds to the slope in the torque angle of rotation diagram. Due to the previous stretching process into the plastic deformation range indicates the pre-extended screw at the yield point, ie at the transition from elastic in the plastic deformation area, a kink on. The slope therefore has a point of discontinuity at this point, which is easily detectable. So with this approach will be guaranteed that all screws are tightened to their yield strength become. Due to the previous stretching all screws have the same yield strength, so that all screws with the tightened substantially the same tightening force. Small scatters arise from the fact that the real mounting situation in particular with regard to the thread friction numbers of the situation can distinguish during the stretching process.

In The drawings are in the single figure in a schematic Stress-strain curve, in which the exercised Preload force is plotted against the angle of rotation, several curves of two Screws shown.

In the stress-strain diagram, the biasing force applied during stretching is plotted against the angle of rotation. And that's the curves 1 . 2 two thermally tempered screws of an identical material both in pure axial load (in the tensile test) as well as under tensile and torsional load (assembly trial) shown. For distinction, the reference numerals in the curves with pure axial load in white circles and in the curves with tensile and torsional load in black circles are shown. The curve 1 applies to a hard screw with a high maximum preload force or in the case of the curve with tensile and torsional stress for a hard screw and at the same time low thread friction. The curve 2 applies to a soft screw or a soft screw and at the same time high thread friction.

In linear elastic Hook's range 3 is the course of the two curves 1 and 2 because of the shape and material identity of the tested components also identical.

Even with the most careful temperature control during the heat treatment of the component (screw), variations in the material (component) yield strength can not be completely avoided. They lead to a different curve in the stress-strain diagram (curves 1 and 2 in a bright circle). This scattering is not identical with the dispersion of the assembly preload force with superimposed torsional stress. The yield strength defines the transition from the linear elastic Hook's range of deformation 3 in the adjoining plastic deformation range. Since this transition is often not exactly determined, the so-called 0.2% proof stress is usually used to determine a value for the yield strength.

The yield strength determined after thermal quenching is on the curves 1 with the reference number 5 and at the curves 2 with the reference number 4 characterized.

The difference caused by the strength tolerance during tempering and the dispersion of the thread friction 6 between the yield point forces 4 and 5 at the 0.2% proof strength of the components stretched in the assembly test, the difference which occurs during the later over-elastic tightening between the pretensions of the two screws is provided that the friction conditions remain constant after pre-stretching or change to the same extent.

These tolerances of the yield strength 4 . 5 , which always occur in the not yet advanced screw to make at the subsequent screw assembly completely ineffective or at least reduce a (thermally tempered) screw is pre-screwed before screwing, ie similar to the final assembly on train and additionally claimed torsion and thereby on the yield strength of the production process (obtained by thermal tempering) 4 . 5 stretched out. For this purpose, the screws are with the different curves 1 and 2 stretched to a previously determined in preliminary tests stretching prestressing force. The corresponding points in the two curves are with the points of equal tension 7 and 8th (in the dark circle). To by the stretching process to each of the points 7 and 8th Characterized to reach the same flow, the screw is turning 1 to the point 9 and the screw with the curve 2 to the point 10 plastically stretched. The newly established by the stretching process with superimposed torsional stress linear elastic range in the curves is with the distances 7 - 9 and 8th - 10 dash-dotted lines shown, wherein the linear elastic region ends at the same same stretching prestressing force and then merges into plastic flow. The size to be set in the stretching process is the same for all screws, previously defined axial tensile force, and not the elongation of the individual screws by a certain value (force control, not path control!).

If these screws are then stretch limit controlled during installation at their place of use up to the pre-set points (break points) 7 and 8th (Dark circle) biased, so sets for all pre-stretched by this method screws at the same or at least approximately the same biasing force plastic flow and all screws are tightened with the same or at least approximately the same tightening force.

Claims (5)

Method for reducing the tolerances of the prestressing forces at the yield point ( 4 . 5 ) in a thermally tempered workpiece, in particular a screw or a threaded molding, wherein the thermally annealed workpiece to a defined stretching biasing force ( 7 . 8th ) above a yield strength ( 4 . 5 ) is plasticized, characterized in that the stretching takes place with a tensile force resulting from a composite of tensile and torsional stress state. Method according to claim 1, characterized in that that this workpiece during the stretching process with the same clamping length, with which the workpiece in the Final assembly is used, with the tensile force is stretched. Method according to claim 2, characterized in that that the workpiece is screwed for stretching in a stretching device, the one Internal thread part and one of the clamping length corresponding spacer having. Method according to one of the preceding claims, characterized in that in the final assembly of the workpiece this is tightened up to a tightening force, the stretching force exerted during stretching stretching ( 7 . 8th ) corresponds. Method according to claim 4, characterized in that that during final assembly the stress-strain ratio of Monitored workpiece and the workpiece until a sudden change of the relationship is tense.