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R-TECH Materials received a batch of five barrel nuts. The client was concerned that the machining tolerances were out of specification on three of the samples. The other two samples were considered to have correct machining tolerances. R-TECH Materials were requested to conduct mechanical tests to determine if the out-of-specification tolerances had affected the mechanical performance of the nuts.

The barrel nuts in question consisted of a short length of steel tubing with holes drilled into which two screws are located. The two countersunk screws attach the barrel nut into position.

A simple testing fixture was designed and manufactured in our workshop to enable the barrel nuts to be tested on our 300kN Avery tensile test machine. The screws from the barrel nut were screwed into a length of steel flat, which was itself attached to a rod which could be gripped in the bottom jaws of the Avery machine using Vee grips. The barrel nut itself was gripped in the upper jaws of the Avery using flat grips. The test arrangement is shown in Figures 1 and 2.

Figure 1: Test set up

Figure 2. Test set up on Avery 300kN tensile machine

The results of the tensile tests showed that the barrel nuts considered to have inadequate machining tolerances performed just as well as those with acceptable machining tolerances. In all cases, the failure was due to a fracture of one of the screws, and not to pull-out from the barrel of the fixing (see Figure 3).

Figure 3. Failed sample showing fracture of the screw


The out-of-tolerance machining on the barrel nuts had not produced a compromise of the mechanical performance of the fixing.

A thorough failure analysis investigation is a valuable and informative tool to reduce the likelihood of a repeat incident which improves safety and productivity and can enable evolution towards a better product. If you would like to know more about our Failure Analysis services? Contact us today!