Rexroth Tightening System 350 System Documentation - Rhino ...

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292/316 Bosch Rexroth AG | Tightening Technology 10 Working with the System10.4 Results storage and statistics10.4.3.1 Machine capability testThe machine capability test offers information about whether the nutrunner is able to do a defined tighteningunder nearly constant conditions for a specific tightening case. Disturbances such as user influenceor faulty components must be eliminated as much as possible. Since the tests for machine capability takeplace within a relatively short amount of time (approx. 50 OK tightenings), no judgement on the long-termabilities of a nutrunner (e.g. lifespan, process capability) can be made. The machine capability indicatorC m (machine capability) and C mk (critical machine capability) are relevant for machine acceptance.Sequence for themachine capabilitytest1. Determination and documentation before beginning measurements:a. Tightening program with– Set torque value– Upper and lower torque limits as monitoring function– Speed, i.e. cycle timeb. Quality of the bolts/nutsc. Tolerance range for the dispersion (e.g. 7%).d. MCE information:– ErgoSpin: The values (MCE factor, efficiency) are stored on the hand-held nutrunner and are automaticallytaken over during commissioning.2. Torque measurement on the tightening case being tested:At least 50 OK tightenings under ideal conditions must be conducted (one tightening program, oneemployee, advantageous ambient conditions, no changes, e.g. adjustment or changes to the tighteningprogram).NoteThe measurements must be done at realistic rpms, since too-high nutrunner speeds (especially in hardtightening cases * ) lead to a larger dispersion in the actual torque value and thus automatically to poorercapability indices.* See the definition of “Hard tightening case” on page 42The torque measurement can take place in different ways:– With the measurement transducer on the hand-held nutrunnerAdvantage: simple realizationDisadvantage: friction torque in the spindle dependent upon speed are incorporated into thetorque measurement– With the attachable measurement transducerAdvantage: no distortion of the torque through the friction torque in the hand-held nutrunnerDisadvantage: additional measurement technology3. Evaluation of the measurement data:To calculate the machine capability index C m the following are taken into consideration:– Torque dispersion of the results– Upper and lower torque limits as monitoring function.To calculate the critical machine capability index C mk the average for the measurement data is alsotaken into consideration.4. If there is an MCE, in which the C mk value was not reached in the tolerance class, the individual components(e.g. measurement transducer, output drives) must be checked, efficiency or correction factorsadapted, and, if necessary, the defective components repaired or exchanged.Machine capabilityindex C mThe machine capability index C m represents the ratio of the torque tolerance 1) given by the user and thedispersion for the torque measurement results (formula (1)). For data with nearly a normal distribution, asthey are in tightening technology, a value of 6σ (= ± 3σ) is usually required for the dispersion of a randomsample (formula (1)). The average torque value is not taken into consideration.1) The torque tolerance corresponds to the T + and T – torque monitoring limits in the tightening program.System 350 | 3 608 878 040/05.2008
10 Working with the System Tightening Technology | Bosch Rexroth AG 293/31610.4 Results storage and statisticsC m is calculated in the BS350 with the following formula:T +– T –C m= ----------------------6σ(1)T + , T – = set monitoring limits.ExampleIf a value of 2.00 for C m is required, the set torque tolerance must observe at least 12 times the standarddeviation of the random sample (Fig. 10–56).T +– T –C m= ---------------------- = 2 ,006σT +– T –= 2 • 6σ = 12σ0 TM [Nm]TMTM – UGTM set sollT M + OGMT + OG–- TM - = UG12σFigure 10–56: Normal distribution with a set torque tolerance and excluding the average valueBS350 calculates the machine capability index C m automatically based on the measurement values of thespindle.NoteC m does not offer information on how exact the average was reached (see “Critical machine capabilityindex Cmk” in the following section)!Critical machinecapability index C mkTo calculate the critical machine capability index C mk the dispersion of the measurement torque results,the torque tolerance set by the user, as well as the average torque value of the random sample are used.C mk therefore provides information on how “exact” the average was “reached”.Two values are used to determine C mk : C upper (formula 3) and C lower (formula 4):T +– TC upper= ------------------3σ(3)T–T –C lower= -----------------3σ(4)The smaller of the two values C upper and C lower is designated C mk .System 350 | 3 608 878 040/05.2008
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