Rexroth Tightening System 350 System Documentation - Rhino ...

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294/316 Bosch Rexroth AG | Tightening Technology 10 Working with the System10.4 Results storage and statisticsExample(1)(2)T [Nm]0-5 σ -3 σ M+3 σ +5 σT+7 σM UGTM set sollT –T +T– T –= 5σM - M UG = 5σMOG- M = 7σT +– T = 7σFigure 10–57: Normal distribution with a set torque tolerance and including the average value(1) Analysis C lowerM OG(2) Analysis C upperThe result according to Fig. 10–57:7σC upper= ------- = 2.333σ5σC lower= ------- = 1.673σCritical machine capability index C mk = 1.67BS350 automatically calculates the critical machine capability index C mk .Summary – C m represents the maximum reachable value for C mk .– If the actual value reached for C m is smaller than the requested value for C m and C mk , the tighteningresult is insufficient.– If the actual value reached for C m is larger than the requested value and only C mk smaller, the requestedC mk might still be reached by moving the average value.– A tightening system that provides very good indices for a specific tightening case is not automaticallyequally suitable for other tightening cases.10.4.3.2 Process capability testIn the process capability test the question is answered as to whether the quality of the tightening over thelong term and under realistic conditions within the tightening process can be kept within the set tolerances.In contrast to the machine capability test, in the process capability test disturbances such as user influence,material batches, thread tolerance etc. are taken into consideration by the selection of the randomsample. The process capability test also needs a larger random sample (at least 100 OK tightenings undernormal conditions). Otherwise the procedure is the same as that for the machine capability test.The process capability index C p and the critical process capability index C pk are calculated according tothe same formulas as the machine capability index. A sufficient machine capability is absolutely necessaryfor a high process capability.System 350 | 3 608 878 040/05.2008
10 Working with the System Tightening Technology | Bosch Rexroth AG 295/31610.4 Results storage and statistics10.4.3.3 Typical limit indicesIn tightening technology, the following typical limit indices have established themselves:Table 10–5: Limit indicesTightening is C m C mk C p C pkQuality capable > 2.00 > 1.67 > 1.67 > 1.33Conditionally quality capable > 1.67 > 1.33 > 1.33 > 1.00Not quality capable ≤ 1.67 ≤ 1.33 ≤ 1.33 ≤ 1.00Correlation between machine capability index C m and error ratesUnder the assumption that the random deviations of a tightening can be described with the help of thenormal distribution (Fig. 10–55), the size of the expected error rates can be calculated: Table 10–6 showsthe theoretical correlation between C m values and error rates. An unreserved statement on the expectedfaulty tightening cases is not possible, however, since proof of the exact validity of the normal distributionfor capability indices > 1.0 (edges of the bell curve) is not possible.10.4.4 Results storageTable 10–6: Theoretical correlation between C m values and error ratesMachine capability C m Rate Probable faulty tightening cases per1 million tightenings (rounded)1.00 0.25% approx. 25001.33 0.0063% approx. 631.67 0.00006% approx. 0.62.0 0.0000002% approx. 0.002To make tightening results available for a statistical evaluation they are saved in the tightening controllerresults storage. A separate, battery buffered RAM results storage is available for this purpose (factory installed).NoteThe storage of data in the tightening controller RAM results storage does not replace the long-term archivingof tightening result data on a server!The following will be described in the next sections:– Configuration of the tightening controller results storage (see page 295)– Calling up the storage capacity of the results storage (see page 298)– Changing the battery (see page 298)10.4.4.1 ConfigurationBefore using a statistical evaluation the tightening controller results storage must be configured. Here itis determined which result data will be stored within the results storage and subsequently used for statisticalevaluation.NoteThe configuration used in the BS350 menu item Display → Results storage and statistics determinesthe data output.To configure the results storage in offline mode:⇒ In the BS350 operating system, click on File → New → Results storage configuration.To load an already created configuration (*.sre):⇒ In the BS350 operating system, click on File → Open.System 350 | 3 608 878 040/05.2008
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