Part four – meeting the needs of impulse wrench users
This is the fourth and last of this blog series on the difficulties of measuring torque output from hydraulic impulse wrenches. My first blog defined the tool type under discussion and how this differs from impact wrenches. The second explored the issues in testing these wrenches and the third examined which national and international standards applied to impulse wrench testing and what they had revealed.
In the early days of electronic torque measurement instruments, instruments had just three modes: track (for calibration and for zeroing the system), peak (to record the highest peak of torque seen) and first peak (to record the break point of “click” type torque wrenches).
Things were simple and not entirely bad. For impulse wrenches we would have recommended the peak mode using either the indirect measuring method with a static transducer and joint simulator or the direct method using a rotary transducer (see my previous blog). Provided that the customer could get consistent results, he could generate valid data for comparative evaluations of tools or performance monitoring of a given tool over time.
However, things start to get complicated when users compare the results from different torque measuring instruments and get completely different results (see my previous blog). Which instrument is right?
The user might then apply his digital torque wrench or dial torque wrenchto the tightened bolt to try to establish what the tightening torque was. There are several ways of doing this and I will describe one of them – the re-torque method. Put a fine mark on the head of the tightened bolt and the joint face. Back-off the bolt, a quarter turn should be sufficient, and then re-tighten it until your marks on the bolt head and joint face align, recording the peak torque to do this.
The person doing this test might reasonably conclude that the instrument giving him the closest match to the re-torque value is the best. All is well until he takes an impulse wrench from another manufacturer and then finds that the other instrument now gives better correlation with his re-torque test and he is back at square one!
When the Pro-Log became the basis for Norbar’s third generation of electronic torque measurement instruments in 1999, we adopted a different approach to the measurement of impulse tools and, for the first time, created an “impulse tool mode”. One of the features of this mode was (and still is on the Pro-Log derived TTT and TST instruments) that the user could change the frequency response of the system from the default 500 Hz. If the user applied his preferred method of evaluating the bolt torque using a torque wrench and did not get a correlation with the reading from the instrument, the frequency response could be changed to get more closely matching results.
The accusation levelled at us (and all of the other manufacturers offering a user settable frequency response) is that; “what is the point of this if you can tune the instrument to give whatever results you want”?
Our fourth generation of electronic measuring instruments, T-Box, was launched in 2009 and we adopted a different method again for calculating the torque from impulse wrenches. Rather than filtering the signal, the T-Box takes account of the energy in each pulse which is a function of both its duration and torque i.e. the area under the plotted torque/time line. This, combined with a lot of development and tuning of the algorithm, gives us an output which we believe and can demonstrate gives a good correlation between the recorded torque and the re-torque test in most situations.
We are not the only manufacturer to have applied logic similar to this and one might think that this generation of instruments using sophisticated algorithms to evaluate the torque would end the debate. Nothing could be further from the truth. In the absence of international standards that give a firm method for measuring the torque (as opposed to deriving the torque from the load created in a fastener), the debate rumbles on.
As a conclusion to this series of four blogs, my final point is that the ongoing debate about the torque output of impulse tools largely misses the point. In the final analysis, the only important fact is the clamping load that you have created in your fastener through the tightening process. We use torque as a control method because it is much more convenient than directly measuring load. However, in the case of impulse wrenches, measuring the torque output is fraught with problems, to the extent that VDI/VDE 2649 says that it is “not feasible”.
My suggestion is to establish, through research, the impulse wrench setting that gives the desired joint performance. Then, in devising a test for your impulse wrench, concentrate on getting consistent results against which you can make comparative measurements of tool performance. Results can be in torque or load measured from a load cell – that does not matter, consistency and repeatability does.
Perhaps, one day, a future standards committee will come up with a torque test, that we can all reproduce, that will finally end the debate. However, my instinct is that there is a lot more mileage in this subject yet!
By Philip Brodey, Sales and Marketing Director, Norbar Torque