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

Part 1 – defining the tool types

The issue that perhaps causes the most debate and even argument between customers, distributors and torque measurement equipment manufacturers is that of how to measure the torque output of hydraulic impulse drivers (impulse or “pulse” wrenches).  In a series of blogs, I am going to explore some of the issues why this is so problematic and what, if anything, can be done about it.

First of all, let’s define what type of tools I am referring to.  Tools with non-continuous rotation fall broadly into two types:  impact wrenches and hydraulic impulse or “pulse” wrenches.

Impact wrenches use a rotating hammer driven by a pneumatic motor (sometimes an electric motor) to directly strike an anvil that is attached to the output square drive of the tool.  Noise and vibration levels are high, as is the torque scatter.  Because of the aggressive nature of the tool operation, most torque measurement equipment manufacturers recommend that impact wrenches are not used on electronic torque transducers.  A better method is to use robust, mechanical devices that utilise hydraulic load cells such as those made by Skidmore-Wilhelm in the USA or Flexaulic in the UK.  Of course, you get an output measured in load, not torque, but this is perfectly fine as a comparative method.

Impulse wrenches are similar in operation to impact wrenches except for the inclusion of a hydraulic ‘cushion’ between the hammer and anvil.  Noise and vibration are reduced compared with impact wrenches and impulse wrench manufacturers claim a high degree of joint consistency when their tools are used.

These articles are going to be about impulse wrenches and there will be no further reference to impact wrenches beyond this paragraph.  If you are really concerned about the torque output of your impact wrench, you are using the wrong tool!  Impact wrenches are generally cheap and they are fast and for those reasons will continue to earn an important place in the world of bolt fastening and, perhaps even more relevant, unfastening.  Check out the UK Health and Safety Executive (HSE) website that gives excellent guidance on maximum exposure limits to noise and vibration.  You will find that for larger impact wrenches, maximum permissible exposure time can be down to a few minutes due to high vibration levels.

In part two, which we will release in two weeks, I will examine what the issues are in relation to determining the torque output of impulse wrenches.

By Philip Brodey, Sales and Marketing Director, Norbar Torque

We’ll shortly be attending Farnborough International Air Show 2012 so it seems fitting that prior to this we consider torque in aerospace engineering.

Now I know that Norbar’s specific offerings to this sector have been outlined already in a previous blog but it is also worth thinking about the industry as a whole and how torque application and measurement impacts aerospace manufacturing, especially with recent pressures to decrease fuel consumption for cost and environmental reasons.

Wider industry concerns like this naturally impact on aerospace engineering which requires absolute precision and rigorous attention to health and safety details. In short, torque calibration and measurement play a vital role in maintaining the performance, reliability and longevity of vital equipment. Aerospace has a critical need to accurately control threaded fasteners.

The accurate measurement and application of torque on joints and fastenings is critical for aircraft manufacturing and maintenance where close tolerance levels are required.

Torque calibration equipment is used by air forces, airlines and OEMs worldwide and accurate testers are sited in production facilities, calibration laboratories and  hangers for regular checks on the wide variety of tools being used.

One of the key issues is access to fasteners, because if the tool operators’ ability to apply the correct torque to a fastener is impeded by space restrictions, then a malfunction is likely to occur.

Constraining factors in aircraft design and construction mean that torque tools used for manufacture and maintenance frequently have to be engineered to order to overcome these challenges.

We’ll be in Hall 1 on stand C9 at Farnborough International Air Show 2012 if you want to talk more torque.

Philip Brodey, Sales and Marketing Director, Norbar Torque Tools

There are a good number of calibration laboratories across the globe of varying sizes, offering various scientific services. Now I won’t go into the complete history of the Norbar Torque Calibration Laboratory, this has been more than adequately touched upon in previous blogs. What I would like to do however, is give you an idea of the scale of the lab and how this compares to others.

At Norbar we offer quite an extraordinary range of independently accredited capabilities that very few laboratories in the world can match. These capabilities extend from over 100,000 N·m for projects such as measuring torque for fasteners on massive oil and gas pipelines down to 0.005 N·m for delicate scientific calibrations.

Back in 2010, our calibration laboratory in Banbury, Oxfordshire achieved an extended level of endorsement from the government recognised United Kingdom Accreditation Service, being awarded UKAS accreditation for performing calibrations to 108,500 N·m of torque under BS7882: 2008.

This sort of measurement requires an enormous calibration test rig – a good analogy for the torque would be to think of 7 Ford Focus’s hanging from a 1 metre length (based on 1500kg per car, so perhaps the LX, not the Ghia!). Only the national laboratories of France (LNE) and Germany (PTB) work to a higher capacity in Europe – not bad seeing as both these labs are government funded and Norbar’s isn’t!

The main point of difference between our laboratory and many others is that it is solely focused on torque calibrations, having been an accredited laboratory, since 1989. This reflects a dedication to consistent quality and accuracy in torque, meaning that it is possible for us to calibrate with uncertainties of 0.02% (class 0.1 to BS 7882: 2008). Users across all sectors from aerospace to mining can have total confidence that our products are fully fit for purpose as calibrated.

Chris Young, European Sales Engineer Norbar Torque

Like me, you may shudder at Jeremy Clarkson’s determination to dumb-down BBC’s Top Gear by referring to an engine’s output in “torques” which is as meaningless as saying that the car’s maximum velocity is “150 speeds”.  However, I will let him off the hook slightly by conceding that torque units are quite confusing.  In Europe we are most commonly using the SI (Système International d’unités) unit of Nּm, in Asia the metric units of kgfּm or kgfּcm are often preferred whilst in the United States the Imperial units of ftּlbf or inּlbf are almost universally used (with the final ‘f’ standing for ‘force’ commonly omitted).  Just to add to the confusion, in the UK we usually call the Imperial units lbfּft or lbfּin, reversing the order of the force and the distance.

So, from a British perspective, which is correct, ftּlbf or lbfּft?  We do sometimes hear British people referring to “foot pounds”, and this certainly trips off the tongue easier, but is it technically correct?

My main reference for this argument is BS 350:2004.  The British Standard refers to a convention under which units of torque and units of energy (both dimensionally force times length) are distinguished by reversing the order of the units.  Under this convention, the foot pound-force (ftּlbf) is a unit of energy while the pound-force foot (lbfּft) is a unit of torque.  So, on this side of the Atlantic, you will be correct in always using lbfּft if you are referring to a torque unit.

You may have noticed that I have used a centre dot between the force and the units rather than a dot on the base line i.e. Nּm rather than N.m.  This is because in SI units the centre dot indicates a multiplication so Nּm is equivalent to N x m.  The ‘x’ symbol is not used to indicate multiplication as it could be confused with the letter ‘x’.  This convention is not unique to SI units.  In mathematics in general the centre dot is interchangeable with the ‘x’ symbol to indicate a multiplication whereas a dot or comma on the baseline is usually the indicator of a decimal.

So, my plea to Top Gear and others that have followed suite is to pick a torque unit and stick with it.  Please stop confusing us by talking of “torques” and allow us to make meaningful comparisons between the vehicles you are discussing.

It’s also worth noting that we have a Torque Convertor on the Norbar website and as a smartphone app with both apple and android!

Philip Brodey, Sales and Marketing Director

Aerospace - Space Shuttle Mission STS-123 Measuring preload on critical bolts on the SLP carrying Space Robot Dextre

At the start of November we exhibited at Advanced Engineering UK and within this wider group of events sat Aero Engineering 2012 which received a record attendance this year. We came away with some really valuable knowledge from the event and it prompted me to blog about this mammoth industry and how torque technology innovations have had such an impact.

Given the sometimes conflicting demands of aerospace for unparalleled safety, whilst keeping weight to a minimum, no other industry has a greater need for control when using threaded fasteners.

Norbar has been at the forefront of supplying torque equipment for the aerospace sector since WWII, when we supplied wrenches for the Rolls Royce Merlin engines that powered most of Britain’s fighters and bombers.

Norbar produced its first torque wrenches specifically to help Rolls Royce engineers deal with the problem of uneven tightening of the cylinder head which leads to distorted cylinder bores.  In fact, several companies were licensed to build the Merlin engine to mitigate the risk of one or more of the factories being bombed.  This would have increased the need to standardise methods of making the engines and eliminate areas of potential quality problems.

Precision remains a key factor in aerospace and is at the heart of everything we do at Norbar.  The starting point is our UKAS accredited torque calibration laboratory which has an accredited range from 0.005 N.m to 108,500 N.m.  Every calibrated product that we make is traceable back to the laboratory and the laboratory equipment itself is traceable to international standards for length and mass (the components of torque).  Prior to 2004 the Federal Aviation Authority demanded NIST (National Institute of Standards and Technology) traceable certificates for torque equipment.  On 2^nd March 2004 NIST confirmed to the FAA that certificates from laboratories accredited by a member of the Mutual Recognition Agreement (MRA) are equivalent to NIST.  As Norbar’s laboratories in the UK, Australia, USA and Singapore are accredited by bodies in the MRA, customers in the aviation industry will accept our calibration certificates.  As a result, our torque calibration equipment has almost become the standard for European aircraft manufactures and is also used by air forces around the world.

Many torque applications in the aerospace industry can not be handled by our standard range of products.  This is mainly because of the very poor bolt access in many cases, particularly but not uniquely on small aircraft.  We have recently done a couple of jobs for BAE Systems to provide tools for hard to access bolts on the Panavia Tornado.  Another example of an ‘Engineer to Order’ application was our recent design to help disassemble the turbine stage of the GE CF6-80 engine.

Nowadays, we manufacture a variety of torque equipment for the aerospace industry from the humble (but critical) ‘clicker’ torque wrenches to torque multipliers often used in helicopter motor blade bolting, through to torque calibration equipment and ultrasonic bolt load measurement (USM-3).  Our product coverage is quite literally from private light aircraft to the space programme.  Our USM instrument was selected after extensive evaluation by NASA’s Stennis Space Center and was used for numerous applications on the Space Shuttle and expendable launch vehicles and satellites. There is even rumoured to be a Norbar gearbox driving the emergency hatch on the International Space Station!

I often wonder what my grandfather who started Norbar or those first Rolls Royce engineers would make of the different torque tools their sector uses nowadays!

By Philip Brodey, Sales and Marketing Director

Torque, what it is and why you need it!

Although many methods exist to join two or more parts together, the ease of assembly and disassembly provided by threaded fasteners make them the ideal choice for many applications.  The object of a threaded fastener, such as a bolt, is to clamp parts together with a tension greater than the external forces trying to separate them. The bolt then remains under constant stress and is immune from fatigue under normal circumstances.

Why then, I hear you ask don’t we just tighten our bolts as much as we possibly can?  No chance of anything coming apart then.

Unfortunately, it isn’t that simple.  If the initial tension is too high, the tightening process may cause bolt failure.  If that bolt is holding a wheel onto a Formula 1 car the consequences are obvious.  Equally, if the tension is too low, varying loads act on the bolt and it will also quickly fail.

There is therefore an optimum tension and the most reliable way of ensuring this is by specifying and controlling the tightening torque.

Torque is any force or system of forces that tends to cause rotation about an axis.  Imagine someone tightening a bolt using a socket attached to a meter long bar.  If they apply 10 kg of force (kgf) perpendicular to the bar they will produce a torque of 10 kgf·m at the axis or the centre of the bolt.

Under the S.I. system of measurement, force is expressed in Newtons (N) rather than kgf. The conversion between kgf and N is x 9.807 so the person is applying 98.07 Newton metres (N·m) of torque.

However, it must also be remembered that any surface treatment of a bolt can have an impact on torque measurement.  Untreated bolts or nuts require different calculations than bolts or nuts treated with zinc, cadmium or phosphate. It should also be noted that when a threaded fastener is tightened, the induced tension results in friction under the

head of the bolt and in the threads. As much as 50% of the applied torque is expended in overcoming friction between the bolt head and the abutting surface and another 30% to 40% is lost to friction in the thread.

Further information on torque measurement and calculations can be found at www.norbar.com

Philip Brodey, Sales and Marketing Director