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Tension rises as wrench set correctly

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Norbar Torque Tools was asked recently to carry out ultrasonic load testing of bolts and shell studs securing the Krupp SAG mill lifter bars on the shell feed and discharge ends at the Xstrata Ernest Henry copper mine near Cloncurry, Queensland.

This request was to see whether the correct clamping force had been applied using current tooling and methodology, and also to identify any limitations of the overall assembly.

This work was carried out by measuring the unloaded length of the bolts and shell studs with an ultrasonic stress meter (USM), then applying the torque to them using a Norbar pneutorque PT9 wrench and a prototype hydraulic version of the same unit owned by Pacific Relines.

Once loaded, subsequent monitoring with the USM indicated the elongation and calculated the applied tension.

The desired tension was concluded to be 65-70% of the reline bolt proof load. For the M52 bolts and shell studs, this equated to 527.4kN.

Tests on the bolts on the feed end lifting bars provided some confusing initial results. This raised questions about the maintenance history of the PT9 tools and the lubrication controls used for the job.

After investigation, it was found that one tool had not been serviced or calibrated since it had been put into service five or six years ago.

The other tool had been serviced and calibrated in mid 2001 after the tool had been damaged during use. This tool had been returned with a torque/pressure graph that was being used incorrectly to set up both tools.

After performing some changes to the hose from the lubrication unit, more tests were done to set the tools to the required residual bolt tension.

This resulted in major variations in the air pressure required to operate each of the PT9s compared with the previous single graph available.

There was apparently a continuing misunderstanding about the correct setting procedure for the tools.

The tool output must be set with the tool being run under no load condition. The method used by the local operators in the past of setting the tools at stall condition can produce low torque outputs often greater than 25% below the theoretical values.

When the verification was completed, more tests were performed on the feed end bolts that resulted in an even value of clamping force.

The conclusion was that because the torque process was so vastly upgraded from its previous status, the end results were likely to give an acceptable overall clamp of the lining.

It was evident there was no problem with the torque applied to the lifter bars on the shell section, as tests returned figures of 65-70% of proof load tension applied to the studs and converted back to a correct torque being applied.

There was no major variation shown on different studs on the same lifter bar as was found on the feed end, with the tension value of 520kN consistently achieved.

Tests were also performed on the discharge end bolts, the old bolts re-torqued and new bolts that had been fitted.

The re-torqued bolts were tested with the PT9 unit before and after adjusting the air pressure setting for the correct torque output.

As with the feed end, the tools were set low, mainly due to incorrect setting procedures. Even so, re-torque at this setting produced up to a quarter turn to take up bolt relaxation.

With the tools set to specification, bolt tensions increased to the required load region of 500kN when re-torqued.

The magnitude of rotation on these bolts indicated a significantly low torque up in the past.

Tests with the USM were performed on the new bolts to see the torque requirements for the fully unloaded assembly of the lifter bar bolts. Once again low tension values were obtained that were rectified by further adjustment of tool air pressure to give the required tension.

Testing at this final set pressure on the re-torque procedure produced results within the tension specifications in the region of 500-570kN.

According to Norbar, it was evident that these bolts, being the longest ones used on the mill, had more elasticity. As such, this resulted in a more consistent clamping force but, at the same time, they were susceptible to tension errors overall if the torque procedure was not maintained.

As a result of all these tests, Norbar concluded that the tools needed to be serviced and graphed.

To protect all the needs of Ernest Henry from damage, Norbar designed a carry cradle/trolley.

This included a cage or box for stowing the tool and provisions for rolling up the hose after use.

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