03_inspecting_e

Te ch ni ca lD at a Te ch ni ca lD at a 37Technical Data3-2/3-336 Technical Data 3-1/3-2 3. Inspecting Tightening Torque 3-1 Methods of Inspecting Tightening Torque 3-2 Retightening Torque Method ■ Loosening Torque ■ Retightening Torque ■ Marking ■ T point Tm：Measuring Torque Tt：Presumed Torque α：Coefficient Tm=α･Tt Tm is not alway equal to Tt. Advantages and disadvantage Loosening TorqueMethod Retightening Torque Marking 0.6～0.9 ※（0.8） 0.9～1.2 ※（1.05） 0.9～1.1 ※（1.0） α：Ratio between retightening torque and tightening torque ※：standard value Measured torque Tightened torque ＝α Measuring method Tm=α・Tt Tightening torque Loosening torque First, approximate how much torque was applied for the screw tightened and do an inspection of the tightening operation. Loosen or breakaway the bolts by torque wrench and read the torque when the bolt starts moving. Tighten a bolt further to determine an applied torque. Read the torque when the bolt starts moving again. Mark the position of the tightened bolt, loosen it and read the torque when re-tightened up to marked position. Relatively easy to measure. It is necessary to retighten the bolt. Accuracy is obtained if the point at which rotation begins is distinct. No further work is necessary after the inspection. Takes time and labor. After the inspection, the bolt is maintained at its original torque. T-Point 0.9～1.1 ※（1.0） When continuing to tighten a bolt that has already been tight- ened with a torque wrench until the bolt begins to turn again, this torque value can be calculated by using θ-T wave formula. It can be measured with the most accuracy when the test piece is secured. The bolt can be left "as is" after the inspection is complete and no individual variations will result. Table 3-1 Methods of inspecting tightening torque Figure 3-1 Typical variation of retightening torque Figure 3-2 θ T Tt T：Torque θ：Angle T θ A point C point B point Tt It is relatively easy to measure and is not affected so much by the personal interpretation or performance. But this method measures the torque value on statistic friction; the value will be higher than tightening torque (Tt) and relevance is not so clear. Sometimes max- imum value on statistic friction may not exist for some screws. ① A point method ② B point method It is easy to measure by checking maximum torque using a memory pointer. But the measurement value may change a lot depending on how the operator may feel and stop when the screw will start to turn, so personal interpretation and prefor- mance can be a large factor. Usually tightening torque vaguely means C point method. C point method sometimes shows A point method. ③ C point method 3-3 Proposal of New Retightening Method (Utilizing CTB series torque wrench)　 Figure 3-3 New retightening method Figure 3-4 Distribution of retightening torque T Tt Tt≒TT TT Unstable Stable Tightening process Retightening inspection process θ 　θ Tightening torque Retightening torque by CTB By bottom method (B point method) By peak method (A, C point method) The retightening method requires that additional torque is applied to a bolt or screw, which has already been tightened, and then measures the torque value when the bolt just begins to move. On the standard screw ・Torque to overcome against statistic friction of the screw (A point) ・Torque the screw starts to turn continuously (B point) ・Maximum torque for the inspection (C point) These laws are used properly by necessary accuracy. Typical torque change on retightening For retightening, at first it will start with the rotation of head, and then will gradually cause the rotation of the screw, then move to statistic friction to dynamic friction, finishing friction whip and then will become a stable straight line. (Figure 3-3) Basically this line is an extension of the torque-angle line of tightening. The torque (Tr) of the point of intersection on exten- sion line of measurement torque - twisting angle line and retightening line of stable screw can be calcula- ted by stable range torque, unstable angle ( θ), gra- dient of torque-angle (dT/αθ). The torque (TT) of T point is close value to tightening torque (Tt). The torque of T point is relatively stable and measure- ment of small measurement error (±5%) of tightening torque (Tt) can be achieved. This new retightening method will use data in a more stable range, so the personal interpretation and per- formance factor involved in the measurement is less compared with conventional A, B, C point method. Variation of tightening torque and break away torque Technical experience is necessary to measure this point. The retight- ening torque will reduce temporarily and the reading accuracy is not very high, but this value is the closest to the tighten- ing torque (Tt). Sometimes a clear minimum torque may not exist for some screws. Advantages of the New Retightening Method Anyone can measure the tightened torque No personal interpretation or performance variable involved in measuring the torque Requires less time to perform the measurement Dispersion of data is small. 022-105技術資料_E 06.10.19 5:14 PM ページ 36