Assembly of a Screw Connection

Abstract

A method and a system for assembling a screw connection for a predefined operating load. The system includes a detector configured to detect values of at least one influence factor on the screw connection, a calculator configured to determine, based on the operating load, a required pretensioning force of the screw connection, and, based on the determined pretensioning force, to determine an expression of at least one assembly parameter, and an assembler configured to assemble the screw connection in accordance with the determined expression of the assembly parameter.

Claims

1. A method for assembling a screw connection provided for a predefined operating load, comprising the acts of: determining a required pretensioning force of the screw connection in dependence of the predefined operating load; determining a nominal expression of an assembly parameter in dependence of the required pretensioning force; recording a value of an influence factor impacting the screw connection; determining a set expression of the assembly parameter on a basis of the nominal expression and in dependence of the recorded value of the influence factor; and tightening the screw connection until the set expression of the assembly parameter has been reached.

2. The method according to claim 1, wherein an adjustment factor is assigned to the recorded value of the influence factor with regard to the assembly parameter, in dependence of which the set expression is determined on the basis of the nominal expression.

3. The method according to claim 2, wherein, in order to determine the adjustment factor, the recorded value of the influence factor is compared to a stored value of the influence factor which has an adjustment contribution assigned to it with regard to the assembly parameter, and wherein the recorded value has the adjustment contribution of the stored value assigned to it.

4. The method according to claim 1, wherein one value each is recorded for several influence factors and wherein the set expression of the assembly parameter is determined in dependence of a respective value of the several influence factors.

5. The method according to claim 1, wherein set expressions for several assembly parameters are determined and wherein the several assembly parameters include a tightening torque and a tightening angle.

6. The method according to claim 1, wherein the value of the influence factor is recorded by a recording means.

7. The method according to claim 1, wherein the influence factor impacting the screw connection is one of: a geometry; a surface roughness; a coating; a wear; a raw material; a raw material pairing; a friction coefficient of the raw material pairing; a thread geometry, and a degree of soiling.

8. The method according to claim 1, wherein an empirical data record is created and stored, in which the determined required pretensioning force, the value of the influence factor, the nominal expression, and the set expression are contained and/or are set into relation with each other.

9. The method according to claim 8, wherein the set expression of the assembly parameter is determined in dependence of the empirical data record.

10. A system for assembling a screw connection provided for a predefined operating load, comprising: a recording means which is configured to record a value of an influence factor impacting the screw connection; a calculation means which is configured to: determine a required pretensioning force of the screw connection in dependence of the predefined operating load; and determine an expression of an assembly parameter in dependence of the determined required pretensioning force; and an assembly means which is configured to assemble the screw connection in accordance with the determined expression of the assembly parameter; wherein the calculation means is additionally configured to adjust the determined expression of the assembly parameter prior to assembly in dependence of a recorded value of the influence factor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 shows a system as per an exemplary embodiment of the invention with an exemplary screw connection, in two lateral sectional views along the same cutting line;

[0042] FIG. 2 shows a diagram with a screwing process curve, in which a green window of the screwing process curve is plotted in terms of a nominal expression of two assembly parameters;

[0043] FIG. 3 shows the diagram of FIG. 2, wherein additionally adjusted green windows of the screwing process curves have been plotted in terms of two exemplary set expressions of the assembly parameters;

[0044] FIG. 4 shows a method as per an exemplary embodiment of the invention in a schematic view of the method steps; and

[0045] FIG. 5 shows a method as per a further exemplary embodiment of the invention, wherein additional steps are shown in relation to the method as per FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

[0046] For better clarity of the depiction FIG. 1 has been divided into FIG. 1a and FIG. 1b. In FIG. 1a the screw connection 1 is depicted, in particular showing the active forces, torques and assembly parameters, in FIG. 1b the screw connection is depicted, in particular showing the system 50 and its components.

[0047] In all, an exemplary screw connection 1 is depicted in FIG. 1. A first metal sheet 2 or second metal sheet 3 are screwed together by means of the screw 4 and the counternut 5. The operating load in the example is an axial force F A X engaging eccentrically to the screw axis and a transverse force F Q. In order to raise the required clamping force (plus settling force and other additional forces), the screw 4 is screwed together with the nut 5 using a tightening torque M. To this end the screw 4 and the nut 5 are twisted about a tightening angle .

[0048] When the desired tightening torque is reached using the desired tightening angle, the desired pretensioning torque FV is present at the screw connection 1. With an exemplary embodiment of the method it is provided that the screw 4 and the nut 5 are twisted about a tightening angle .sub.E,SOLL with a final tightening torque M.sub.A,SOLL. The manner, in which these expressions are determined, is explained in more detail by way of the description of FIGS. 2 to 5.

[0049] FIG. 1b depicts a system 50 for assembling a screw connection provided for a predefined operating load according to an exemplary embodiment of the invention.

[0050] The system 50 comprises a recording means 6, which is configured as a surface sensor with image capture for recording values 310 (see FIG. 4) of a surface roughness R2 of the metal sheet 2 and a surface roughness R3 of the metal sheet 3. In addition the system 50 comprises, as further recording means not shown, at least one input means such as a keypad and the option of database queries. The input means may be used to record, for example, values 320 (see FIG. 4) relating to raw material pairing of the metal sheets; a database query may be used to record, for example, values 330 (see FIG. 4) relating to the surface condition R4, R5 of the respective head bottom of the screw 4/the nut 5.

[0051] In addition the system 50 comprises a calculation means 7, which is for example configured as a desktop computer or as an industrial control with a screen interface. The calculation means 7 is configured to determine a required pretensioning force FV in dependence of the operating load and to determine, in dependence of the determined pretensioning force, a nominal expression M.sub.A, NENN, .sub.E, NENN of the assembly parameters of tightening torque M and tightening angle respectively. Moreover the calculation means 7 is configured, in dependence of recorded values 310, 320, 330 of different influence factors R.sub.2-5, to determine a set expression M.sub.A,SOLL, .sub.E,SOLL of the assembly parameters. Exemplary functions of the calculation means 7 are described in more detail with reference to FIGS. 4 and 5.

[0052] The system 50 further comprises an assembly means 8 (shown only schematically by a dotted line), which in the exemplary embodiment is configured as an industrial robot with a tightening-torque-controlled and/or tightening-angle-controlled screw head. The assembly means 8 may however also be a hand-controlled screw head connected to the calculation means 7. Using the assembly means 8 the screw connection can thus be assembled, i.e., tightened according to the requirements in the system 50.

[0053] FIG. 2 shows a diagram with a conventional screwing process curve, in which a green window 20 of the screwing process curve is plotted in terms of a nominal expression M.sub.A, .sub.N, NENN of the two assembly parameters M, . The green window frames the range between the upper threshold values M.sub.A, MAX, .sub.E, MAX and the lower threshold values M.sub.A, MIN, .sub.E, MIN. Within this range, which can be monitored without problems for its adherence by means of internal sensors of the assembly means, the screwing process must end, so that the nominal expression is achieved. With the exemplary curve 15 shown, a successively rising tightening torque M is applied during the screwing process as from a threshold torque MS for a certain angle difference Delta PHI, until in the range between angles .sub.E,MIN and .sub.E,MAXgiven the case that screwing is performed according to requirementsa tightening torque between M.sub.A,MIN and M.sub.A,MAX is achieved and the screwing process is finished (see circle with x in FIG. 2).

[0054] The green window 20 of FIG. 2 has for example been determined by means of an experience-based estimate of an experienced screw-fitter after several iterations with further tightening torque measurements. Alternatively it may also have been determined as per process step 200 in a process according to an exemplary embodiment of the invention with a simplifying calculation method.

[0055] FIG. 3 on the other hand shows the diagram of FIG. 2, wherein additionally adjusted green windows 21 and 22 of the screwing process curves are plotted in terms of two exemplary set expressions M.sub.A, SOLL, .sub.E, SOLL of the assembly parameters. The corresponding threshold values of the green windows 21 and 22 are marked with */** in order to differentiate between the set expressions and the nominal expressions of the green window 20.

[0056] The depiction of FIG. 3 shows in summary, how in this exemplary embodiment starting from the nominal expression (see green window 20), different set expressions (see green windows 21 and 22) can deviate. For example the green window 21 has been moved upwards as regards the set expression for the tightening torque M.sub.A* and comprises a larger tolerance range towards the maximum value as regards the set expression .sub.E*. For example the green window 22 has been moved upwards as regards both set expressions M.sub.A** and .sub.E** for the minimum values and for the maximum values. The depicted moves are in particular the result of executing the method according to an exemplary embodiment of the invention, as will now be described with reference to FIG. 4.

[0057] FIG. 4 shows a process according to an exemplary embodiment of the invention in a schematic depiction of the process steps. The exemplary embodiment shown is based on a screw connection, the design parameter of which is the pretensioning force and the control variables of which, during assembly, are the tightening torque and the tightening angle. To aid understanding, only the influence factors listed for FIG. 1 from a plurality of relevant influence factors have been used for explaining the individual process steps. The operating load to which the screw connection to be assembled shall be exposed, comprises the combination of operating loads as per FIG. 1 and is assumed to be predefined.

[0058] In process step 100 a pretensioning force FV required as a minimum is determined in dependence of the operating load F A X, FQ by means of the calculation means 7. The calculation model required for this is picked, with operation-specific adjustments, from a collection of formulae for screw connections (depicted as f(x) in 100).

[0059] In process step 200 nominal expressions are determined from the ascertained pretensioning force FV for the tightening torque M.sub.A,NENN and the tightening angle .sub.E,NENN, wherein additionally maximum values and minimum values are determined in terms of a derivation of a green window 20. To this end an experience-based computing model is used, in which suitable expressions are derived via the creation of regression functions from empirical values of expert screw-fitters.

[0060] Taking account of different influence factors begins with process step 300: in each case a value for the surface roughnesses R2 and R3 is recorded with the aid of sensor 6. The user of the system (normally the designer for the screw connection 1, but he/she may also be the screw-fitter) inputs the raw materials of metal sheets 2 and 3 via a keypad, from which, in the exemplary embodiment, the calculation unit 7 ascertains the friction coefficient id. When the designer designs the screw connection 1, a database query is also automatically activated by the calculation unit 7, which as a result returns among others the surface roughness R4 and R5 of the head bottom of both the screw 4 and the nut 5. Thus a value is recorded for each of the influence factors R2, R3, R4, R5 and .

[0061] In process step 450 each of these values is compared in comparisons 410, 420 and 430 to a table associated with the value. This includes in each case determining an adjustment contribution 411, 421, 431 for the recorded value.

[0062] Depending on the programming logic on which the calculation means 7 is based, individual adjustment contributions 411, 421, 431 are merged in a suitable manner to form one overall adjustment factor 490.

[0063] In process 500 the determined nominal expressions of the assembly parameters are then used as the basis andagain depending on the programming logic of the calculation means 7adjusted in a suitable manner according to the adjustment factor 490. The result of the adjustment are the set expressions of the assembly parameters, from which then adjusted green windows (see e.g., depiction of the green windows 21 and 22 in FIG. 3) can be derived. The screw connection 1 is then assembled in process step 600 in accordance with the ascertained adjusted green window.

[0064] In FIG. 5 a process is described according to a further exemplary embodiment of the invention, wherein the process as per FIG. 5 is different from the process of FIG. 4 essentially in that it contains additional steps following on from process step 500, which allow the system 50 to be learnt with regard to the characteristics of the presently observed screw connection 1.

[0065] In process step 700, during or after carrying out the calculations for screw connection 1, the nominal expressions from process step 200, the set expressions from process step 500, the pretension and, if required, also the operating loads from process step 100 as well as the recorded values 310, 320, 330 of influence factors R.sub.2, R.sub.3, R.sub.4, R.sub.5, and from process step 300 are merged (see merger 710) and stored in a storage means of the calculation means 7.

[0066] From the merged data an empirical data record 720 is created in the storage means, which, among others and in particular permits, for future process curves analogue FIG. 4, to assign an improved adjustment contribution to the then recorded values of the corresponding influence factors. To this end the calculation unit 7, when processing the new empirical data record 720 from the data stored in there, determines via suitable multivariate calculation methods a suitable adjustment contribution in each case for each value of an influence factor in the data record 720.

[0067] If required, an already previously stored adjustment contribution must be adjusted for a certain value of a certain influence factor. A suitable prioritization of the old and the new adjustment contribution may be effected automatically by means of the calculation unit 7 or manually by means of a request to the user.

[0068] The empirical data record 720 is stored in this form in the calculation unit 7 such that it is available for future comparisons with recorded values.

LIST OF REFERENCE CHARACTERS

[0069] 1 screw connection [0070] 2 first metal sheet [0071] 3 second metal sheet [0072] 4 screw [0073] 5 nut [0074] 6 recording mans (surface sensor) [0075] 7 calculation means [0076] 8 assembly means (screwing tool) [0077] 10 screwing process curves [0078] 15 tightening-torque/tightening-angle curve [0079] 20 exemplary nominal window [0080] 21 first exemplary set window [0081] 22 second exemplary set window [0082] 50 system [0083] F.sub.AX axial operating load [0084] F.sub.Q transverse operating load [0085] F.sub.V pretensioning force [0086] M applied tightening torque for rotary angle PHI [0087] M.sub.S trigger torque [0088] M.sub.A tightening torque at end of screwing process [0089] already tightened rotary angle [0090] .sub.S rotary angle at reaching trigger torque [0091] .sub.E rotary angle at end of screwing process [0092] R surface roughness [0093] friction coefficient between two surfaces [0094] 100 process step for determining pretensioning force [0095] 200 process step for determining a nominal expression of at least one assembly parameter [0096] 300 process step for recording a value of at least one influence factor [0097] 310 measured values of surface roughness of metal sheets [0098] 320 input of a raw material pairing of the metal sheets [0099] 330 database query regarding surface condition of the screw head bottom [0100] 400 process step for assigning an adjustment factor [0101] 410, 420, 430 comparison of recorded values to stored values [0102] 450 process step for comparing recorded and stored values of an influence factor [0103] 411, 421, 431 adjustment contribution for each recorded value [0104] 490 adjustment factor for all recorded values [0105] 500 process step for determining a set expression of the assembly parameter [0106] 510 merging the nominal expressions with the adjustment factor [0107] 600 process step for tightening the screw connection [0108] 700 process step for creating and storing empirical data records [0109] 710 create [0110] 720 empirical data record

[0111] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.