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TORQUE WRENCHES: ONE CLICK IS ENOUGH

Like most drivers I often take my car into a franchised dealership or an independent garage for a change of viagra cost tyres.

Usually, the mechanic removes and then replaces the wheel with a torque wrench.  All generally goes well until the first click of the wrench which signifies that the nut has been tightened to its optimum torque.

However, all too often in my experience the mechanic continues to tighten the nut beyond its optimum torque.  In the past, I have counted two, three and even four clicks of the torque wrench before the operator stops tightening.

The assumption appears to be that a nut can never be “too tight”.  Unfortunately, this is untrue, as the impact of a nut being too tight is as serious as being too loose.

For this reason, all car manufacturers specify the correct torque for wheel nuts on their vehicles in the handbook.  For example, a wheel nut on a BMW 5 series is 120 N.m, whilst a Ford Focus Titanium requires only 95 N.m.

Why is this important?  Because, when the nuts are attached to the wheel bolts or studs, the bolts stretch as the nut is tightened.  By over-clicking you apply extra torque and apply extra load which further stretches the bolt above and beyond its optimum capacity.  This brings the bolt closer to its yield point which can ultimately lead to bolt failure.

Conversely, under-tightening can be equally as dangerous.  A wheel nut applied with inadequate torque can work itself loose through general use or vibration of the vehicle.

So how can we define best practise when it comes to tightening wheel nuts?  First off garage operators must check the torque required by different vehicles.  The variation may only be minimal, but it is still vital to check.

Secondly, it is crucial to use a torque wrench checker or a torque wrench calibration device to check the accuracy of the wrench.  Our recommendation is that a torque wrench is checked at least once a year or every 5,000 cycles, which ever event happens first.  Busy garages can get through thousands of calibration cycles, or clicks, every month and it is important therefore to align torque wrench calibration with work volumes, to ensure the unit remains within accuracy.

Of course, torque wrenches are not the only method of tightening wheel nuts.  I’m sure many of us have been to garages where pneumatic impact guns are used to tighten bolts.  Again, there is a basic rule of thumb that must be followed.  If an impact gun is being used it must apply less than the specified torque and the task completed with a torque wrench in order to ensure accuracy.

All of this may sound overly complex but garages must remember that, in the event of bolt failure, if it can be proven that a bolt has been over-tightened, liability may potentially rest with them.

Dave Waters,  Norbar Torque Tools Ltd


WIND POWER – A TURNING POINT IN TORQUE

Extracting power from the wind is, essentially, a simple process that has been understood and used for thousands of years.  Even modern wind turbines are often referred to by the general public as “wind mills” – a name that surely must make their designers cringe.  The truth is that modern wind turbines utilise a host of technologies to extract maximum power from the wind over a wide range of conditions and shut down safely when those conditions get too severe.

An area of similarity between Norbar products and the technology in a wind turbines can be found in the gearbox.  The challenge here for wind turbine designers is that for a 1 mW system the turbine blades will typically rotate at between 5 and 20 revolutions per minute (rpm) but the generator requires an input of about 1800 rpm – a 90:1 ratio in the case of the highest blade speed.  A gearbox is utilised for this purpose, increasing the speed to that required by the generator with a corresponding decrease in torque.

It would seem an attractive idea to increase the blade rotational speed and thereby reduce the number of stages in the gearbox making it lighter and cheaper, assuming that you are aiming for the same power output.  The disadvantage of this approach is that the resulting blade tip speed significantly increases noise which is a major issue for on-shore turbines.

The opposite approach would be to increase the blade diameter; doubling blade diameter gives about four times the energy output.  If the desired system output is still 1 MW, a generator with a larger blade diameter could potentially produce its full output at a much lower wind speed.  On the other hand, the gearbox would have to handle a lower input speed and higher input torque requiring more stages of gearing in order to feed the optimum speed to the generator, making it larger and more expensive.  The tower would also have to be taller making it more expensive and increasing its visual impact.

Visual impact was the major consideration when Norbar’s local government office, Cherwell District Council, rejected the planning application for wind turbines near the Oxfordshire villages of Fritwell and Ardley.  As in almost every instance, Cherwell supported the principles behind wind energy but “… not next to these villages”.

A large part of Norbar’s product range also relies on gearboxes but, in the exact opposite way to wind turbine gearboxes, we are increasing torque and decreasing speed.  The input to our gearboxes is either from a hand operated torque wrench in the case of a Handtorque™ multiplier or an air motor in the case of a Pneutorque® pneumatic torque wrench.  None the less, the challenges are similar: to produce robust gearboxes that are also light weight and compact; to use the best available materials but also consider cost and availability. Norbar have 50 years of experience in this field since Dr. H.E. Merritt designed our first epicyclic or planetary gearbox in the early 1960s.

Like Norbar torque multipliers, wind turbines use epicyclic gearboxes although the scale is quite different – much larger.  At some time the designers have probably referred to Dr. Merritt’s book “Gears” which has been the reference source on epicyclic gearbox design for over 40 years, no doubt, supplemented by modern design techniques such as finite element analysis (FEA).

For both wind turbine gearboxes and torque multipliers, a key element to increase reliability to reduce or eliminate non torsional forces.  On a turbine, this is primarily about supporting the weight of the blades and hub.  For Norbar, it is about good torque reaction – a subject explored in our “How to use a torque multiplier” video.

With billions of dollars worth of wind energy capacity coming out of warranty every year, reliability issues for turbines is a subject that is being closely monitored.  Due to the extreme engineering challenge faced by gearbox designers and manufacturers, this is an element of the system that has the potential to be costly to maintain as the turbine fleet ages.

As many Norbar gearboxes are used in the assembly, erection, maintenance and checking of wind turbines, the performance of our products plays a small but vital role in the reliability and ongoing cost of wind energy. For example, we very recently launched two new products in our Compact Series, the HandtorqueTM HT-92 and Handtorque HT-119, both of which are ideal for tightening windturbine nut bolts during assembly.

To talk to us about our torque solutions for the Wind Energy sector, email us at enquiry@norbar.com.

Philip Brodey, Sales and Marketing Director, Norbar Torque Tools


HOW TO TIGHTEN BOLTS WITH HIGH ACCURACY

Our HTM torque multiplier uses an integrated torque transducer to measure the actual output torque. Torque is displayed on a battery powered, handheld instrument rated as IP65 for its resistance to water and dust ingress.

 

Norbar Torque Tools

 

 


CARELESS TORQUE CAN COST LIVES

Health & safety requirements are having a worldwide impact even in countries that to date have had a poor history of H&S awareness, partly because multinational corporations now bring their own stronger safety cultures with them. Inaccurate torque is a frequent reason for accidents caused by wheel loss from vehicles. This can stem from over-tightening of wheel fastenings, which can cause them to break under stress, while if bolts are loose they can come free with vibration. To misuse the old wartime propaganda slogan a little, “Careless torque can cost lives.”

 

Of course, torque in health & safety is a particularly strong priority in certain sectors. For instance, the oil and gas industry has a strong interest in showing that high levels of accuracy in torque measurement and actuation are being achieved and recent oil spills have made the need to be seen to follow best practice especially important. Aerospace also has an essential need to accurately control threaded fasteners, since malfunction of components can be catastrophic and potentially life threatening.

 

Quality control is the major factor that has brought people into the worldwide torque learning curve. Calibration is of critical importance to all manufacturing companies and traceability is another key element in which torque has a significant part to play.  Establishing traceability involves a hierarchy of equipment, with transducers used to calibrate torque wrenches and transducers in turn being calibrated by calibration beams. The process starts in calibration laboratories, which are accredited under ISO/IEC 17025:2005 international testing and calibration standard.

 

There is mutual recognition by accreditation bodies worldwide. This means that Norbar’s UKAS accreditation for performing calibrations up to 108,500 N.m of torque under BS7882:2008 is also recognised by NVLAP (USA), NATA (Australia) and similar organisations in other countries. This level of agreement on competence is another element in the common language of torque. From huge scale operations on oil & gas industry pipelines to precision applications on delicate scientific instrumentation, torque is an ever present element requiring constant attention and input from an ever increasing range of stakeholders.

 

By Paul Carruthers, Applications Support Manager, Norbar Torque Tools

 


THE DIFFICULTIES OF MEASURING TORQUE OUTPUT FROM HYDRAULIC IMPULSE DRIVERS

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


TORQUE OPENS DOORS

Having worked at Norbar Torque Tools for over 40 years, I have witnessed a major upturn in the level of understanding and appreciation about controlling, measuring and accurately applying torque.

My international role as Applications Support Manager includes advising end users on their applications and supporting distributors with product training. This has offered firsthand experience of how almost every industry in every country now realises the importance of torque measurement and application.  As an engineer from a leading off-road vehicle manufacturer once said to me: “We realise that as around 30% of our assembly time is spent tightening bolts, we should at least make sure we do the job correctly!”

This is not to say that engineers and designers in the past overlooked the principles of torque control, joint behaviour, bolt tension, the effects of friction on threaded fasteners and the relevance of different surface finishes. A look at some of the famous and enduring bridges built in 18th century testifies to the skilful application of torque by engineers of the time. If you also consider aircraft engine manufacture from the 1930s onwards, together with shipbuilding, the motor industry, oil & gas and a number of other industries, a clear understanding existed about the implications of correctly torqued bolts.

What has changed is that individuals and companies across the world recognise the value of torque even if it does not directly impact on the day-to-day remit of their work. Torque is no longer a topic confined to engineers, engineering designers, technicians and mechanics but extends across all company operations. Today’s skilled workforces appreciate the vital part that accurately applied torque plays in the performance and longevity of equipment. Salespeople increasingly need to bring references to torque into customer facing discussions.

Torque is also a key that opens many doors to many unusual places, which makes working in this business particularly exciting. It has personally taken me into the locks at Faslane, Scotland to work on bolts for a ship’s lift within yards of nuclear submarine, a situation out of bounds to most people without high level security clearance. The journey has also included entering inside the caissons of the Severn Bridge, which links England and Wales, to ensure tension wires were bolted correctly and going literally underneath the Danube as it cascaded through a hydroelectric power station in Serbia, refurbishing a total of 12 turbines ranked on each side of the river. The project uses one of Norbar’s largest pneumatic tools, the 270,000 N.m Pneutorque PT 17, and work on each turbine takes about one year to complete. There is no room for shortcuts with this type of application!

Torque has become an international common language that is widely understood and creates links across national boundaries and between workpeople in different industries and professions. We’re all torqueing a common language.

By Paul Carruthers, Applications Support Manager, Norbar Torque Tools


HOW OFTEN SHOULD I RE-CALIBRATE MY TORQUE WRENCH?

This might well be the most frequently asked of all frequently asked questions and is rightly a subject of genuine concern to production and quality managers.

To answer the question, I am going to look to the standard BS EN ISO6789 – “Assembly tools for screws and nuts – Hand torque tools – Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure”.  Unsurprisingly, most of us refer to it as “the torque wrench standard”!

In 1992, ISO 6789 was very much a document covering the design and manufacture of torque tools and the requirement was that the tool should be tested at maximum capacity for 5000 cycles in each direction.  No guidance was given on recalibration intervals.

However, when the standard was revised to the 2003 edition, the scope was broadened to include “quality conformance testing and recalibration” and so became of relevance to people using torque wrenches rather than just those designing and manufacturing them.  This is the first time that the standard discussed the interval for recalibration.

For those looking for a simple answer to the question posed in the title, the default period of use between recalibrations is 5000 cycles or 12 months.  However, the standard recognises that many businesses will have their own procedures for the control of test devices and, as a torque wrench can be considered a test device, a company’s own procedures must take precedence over the default 5000 cycles/12 months.

The reason that there is really no simple answer to the recalibration interval question is that circumstances of use will vary widely and this will have a direct bearing on how long the torque wrench is likely to stay in calibration.  Factors such as the frequency of use, setting of the wrench as a percentage of full scale, general care taken in use and storage, ambient conditions in use and storage will all have their effect.  Another major consideration is the torque tolerance existing in every individual situation and degree of safety criticality of the bolted assembly.  For example, a helicopter assembly company that I have visited tests their wrenches before every single use.  A typical automotive garage might find this degree of control onerous and unnecessary.

The other important statement made by the standard is that if a torque wrench is subjected to an overload of 25% or more above the nominal maximum, it should then be recalibrated.  For many, this might be the ultimate decider on how often your wrenches should be recalibrated.  For some, it will be almost every time the wrench is used!

The draft version of the next release of ISO6789 is already in existence.  The good news is that in respect of the advice on recalibration, the standard has not changed.  When ISO6789 is published in early 2013 we will blog on the key changes that you should be aware of.

By Philip Brodey, Sales and Marketing Director


TWO NATIONS DIVIDED BY A COMMON 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


THE WHEELS ON THE BUS GO ROUND AND ROUND – THANKS TO EFFECTIVE TORQUE CONTROL!

The week of Coach & Bus Live at the National Exhibition Centre, Birmingham, England (you can visit us at stand K35) is an appropriate time to be addressing the issue of torque control in coach and bus applications.  Torque control of threaded fasteners can have a significant impact on the efficiency of a vehicle, but more importantly it can play a very significant role in terms of safety.

The road safety campaigners, BRAKE, call loose wheels from vehicles ‘bouncing bombs’ and while Barnes Wallis coined the phrase for an entirely different application, the end result can still be catastrophic.  It is estimated that ‘runaway’ wheels from vehicles kill 8 to 10 people and injure many more each year in the UK.

For example, a few years ago a van driver was killed on the M2 Motorway in Kent, England when a wheel detached itself from a truck and struck his vehicle.  This is not an isolated case and it highlights the fact that the correct wheel nut torque can have life or death implications.

The fact that technicians servicing and maintaining coaches and buses need to be skilled is a given.  Crucially, however, employers need to be able to prove through correct procedures and documentation that a job was carried out correctly.  In any kind of product liability claim it is not sufficient for the employer to claim that all their technicians are highly trained and are capable of doing the job.

One way of ensuring accurate torque in commercial vehicle workshops is to use pneumatic torque multipliers rather than a traditional impact wrench.  The reason for this is that impact wrenches, which are not torque controlled devices, require the torque to be checked using a manual torque wrench.  The torque wrench check will only correct under-tightened nuts and so frequently nuts can be left over-tightened.  A pneumatic torque multiplier like the Norbar TrukTorque™ undertakes accurate torque tightening in a single process.

A video on how to use a pneumatic torque multiplier can be found on the Norbar Youtube channel at http://www.youtube.com/user/TruTorque#p/u/7/Ib8dH4az1Dg

By Philip Brodey, Sales and Marketing Director


ALL ABOUT TORQUE

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