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(b)(4).The investigation including the following contents has been conducted: 1.Analysis of production: (3331, 4109): the dhr of the reported lot 3000190786 has been reviewed.No non-conformity indicating the reported failure was observed.All the products had been performed 100% mechanical function test during production.They all passed the function test, which demonstrate the knob can be tighten and also can tighten the link arm.2.Trend analysis: (4110): in the last 12 months, 25-may 2021 to 25-may 2022, the occurrence rate for ¿knob difficult/ unable to tighten- arm does not lock¿ is approx.(b)(4) which is under anticipated rate.3.Testing of actual/suspected device & testing of raw/starting materials: (10 & 4105) returned product evaluation: the device was received and the following evaluations have been conducted: 1) physical inspection: no visual defects were observed on the product; 2) functional test: assemble the device securely onto a reference retractor blade by sliding the positioner base onto the rail and locking the lever.Evaluate the knob for its ability to tighten the flexlink arm while the locking lever in both the locked and unlocked positions.The knob was rotating normally, the knob can be tightened and flexlink arm can be tightened.3) disassemble the device and check the assembly status according to the applicable manufacturing process instructions knob assembly mp-bh-0004.It is found that the lead-in thread on acme nut was broken.4) verify 2 to 4 threads of the acme screw are exposed past the housing mount.It shows about 3 threads out of the housing mount, the returned product is conforming to this requirement.5) check the pilot crimping and its position, the pilot crimping is conforming, without defects on it, and its position is there without moving.6) the dimension of the relevant components are measured and within drawing specification, no deficiency indicating the knob tighten and flexlink arm tighten issue was observed.There's no non-conformity observed indicating the acme nut thread broken or defect from raw material inspection.All the products had been performed 100% mechanical function test during production.They all passed the function test which demonstrate the knob can be tighten and also can tighten the link arm.7) to try to understand how was the acme nut broken, the further simulation test was performed.When rotating the knob with extreme strength pulling the flexlink arm forward from mount, the knob rotating was found stuck or idling, and knob cannot be tightened and flexlink arm cannot be tightened.Release the extension of the flexlink arm, and rotating the knob, the knob can be tightened and flexlink arm can be tightened.Replace the broken acme nut with a new one and perform the test again, it¿s found knob rotating stuck by pulling the flexlink arm forward with extreme strength, keep rotating rapidly with high strength, the knob rotating found to be idling.Dismount the knob, we can see the acme nut lead in thread broken.Pulling the flexlink arm forward would potentially make the acme nut and acme screw misaligned, if keep rotating with high strength, the acme nut lead in thread could be broken.Reviewing the mechanism of the knob tightening, it¿s indicated the knob rotating idling or stuck was caused because of the misalignment of acme nut and acme screw lead in thread, when rotating the knob by pulling the arm forward with extreme strength, or rotating anti-clockwise rapidly for several circles and then rotating the knob clockwise rapidly, or rotating the knob leaner with too much strength, the acme screw could not be well engaged or even not engaged with acme nut, then rotating become stuck or idling, which resulted in the knob could not be tightened, and link arm could not be tightened.Based upon the above evaluation, there¿s no deficiency identified from production relevant to the mechanical issue- knob difficult/ unable to tighten-arm does not lock prior to this complaint.It is not indicated that it is the product problem, and also not indicates a manufacturing defect caused or contributed to the reported failure based on the investigation.The most probable root cause of the knob difficult/ unable to tighten-arm could be the misalignment of acme nut and acme screw when using with extreme strength pulling flexlink arm forward or rotating the knob rapidly or leaner with too much strength.
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