Pocket Guide To Tightening Technique - Atlas Copco
POCKET GUIDETO TIGHTENINGTECHNIQUE
2POCKET GUIDE TO TIGHTENING TECHNIQUE
POCKET GUIDE TO TIGHTENING TECHNIQUEChapter.Page1. Why threaded fasteners?.42. The screw joint.43. Clamping force.64. Effect of lubrication.75. Screw quality classification.86. Joint types. 107. Torque and angle. 118. Measurement methods.129. The tightening process.1410. Mean shift.1511. Standards for measurement.1612. Certification.1613. Errors in tightening.1714. Damaged threads.1715. Missing joint components.1716. Relaxation .1717. Prevailing torque.1818. Tightening tools.18Summary.25POCKET GUIDE TO TIGHTENING TECHNIQUE3
POCKET GUIDE TO TIGHTENING TECHNIQUEThis booklet provides an introduction to the technique of using threaded fasteners for assembling components, the application of power tools for the assembly and the influence oftool selection on the quality of the joint.1. WHY THREADED FASTENERS?There are several ways of securing parts and components toeach other, e.g. gluing, riveting, welding and soldering.However, by far the most common method of joining components is to use a screw to clamp the joint members with anut or directly to a threaded hole in one of the components.The advantages of this method are the simplicity of designand assembly, easy disassembly, productivity and in the end– cost.2. THE SCREW JOINTA screw is exposed to tensile load, to torsion and sometimesalso to a shear load.The stress in the screw when the screw has been tightened tothe design extent is known as the pre-stress.The tensile load corresponds to the force that clamps the jointmembers together. External loads which are less than theclamping force will not change the tensile load in the screw.On the other hand, if the joint is exposed to higher externalloads than the pre-stress in the bolt the joint will come apartand the tensile load in the screw will naturally increase untilthe screw breaks.4POCKET GUIDE TO TIGHTENING TECHNIQUE
Tensile loadClamping forceShear loadShear loadShear load and tensile load.Tensile loadTorsion in the screw results from friction between the threadsin the screw and the nut.Some screws are also exposed to shear loads which occurwhen the external force slides the members of the joint inrelation to each other perpendicular to the clamping force. Ina properly designed joint the external shear force should beresisted by the friction between the components. A joint ofthis kind is called a friction joint. If the clamping force is notsufficient to create the friction needed, the screw will also beexposed to the shear load. Joints are frequently designed fora combination of tensile and shear loads.The screw is made up of the shank and the head. The shank isthreaded, either for part of its length or for the full length fromthe end to the head. Longer screws are usually only partlythreaded. There is no need to make a thread longer than isnecessary to tighten the joint as this will only make the screwmore expensive and reduce the tensile strength.The dimensions of threads, the shape of the thread and thepitch, i.e. the distance between successive threads, have beenstandardized. In practice there are only two different standards used today in industry; the Unified standard UN, originally used in the Anglo-Saxon countries, and the EuropeanMetric standard M.Basic screw design.POCKET GUIDE TO TIGHTENING TECHNIQUE5
Apart from the basic dimensional differences the UN and Mstandards have different angles and depths of thread. Bothstandards include separate specifications for fine t hreads. TheUN fine thread standard UNF is quite common parallel to thenormal UNC type.Pitch mm/rotation3. CLAMPING FORCEIn general it is desirable that the screw is the weakest memberof the joint. An over-dimensioned screw makes the productboth heavier and unnecessarily expensive. As a standard screwis usually comparatively inexpensive it is preferable that thescrew should be the first part to break.Furthermore, in most cases the dimensions of the screw arenot critical for the quality of the joint. What is decisive is theclamping force, i.e. whether it is sufficient to carry all the loadfor which the joint is designed, and whether the joint will remain tight enough to prevent loosening if exposed to pulseloads.The problem is that there is no practical way to measure theclamping force in normal production situations. Consequentlythe value of the clamping force is usually referred to as thetightening torque.As the clamping force is a linear function of both the turningangle of the screw and the pitch of the thread, there is a directrelation between the clamping force and the tightening torquewithin the elastic range of the screw elongation. However, onlyabout 10% of the torque applied is transferred into clampingforce. The remaining tightening force is consumed in friction inthe screw joint – 40% of the torque to overcome the friction inthe thread and 50% in friction under the screw head.6POCKET GUIDE TO TIGHTENING TECHNIQUE
4. EFFECT OF LUBRICATIONIf a screw is lubricated, the friction in the threads and underthe head is decreased and the relation between tighteningtorque and clamping force is changed. If the same torque isapplied as before lubrication, a lot more torque will be transformed into clamping force. At worst this might lead to thetension in the screw exceeding the tensile strength and breaking of the screw.On the other hand, if the screw is completely dry of lubricantthe clamping force might be too small to withstand the forcesfor which the joint is designed, with the risk that the screwbecomes loose.DryLightly oiledUntreatedBolt materialUntreatedNut material0.18-0.350.14-0.26Phosphorous coatedUntreated0.25-0.400.17-0.30Electro Zinc platedUntreated0.11-0.360.11-0.20Phosphorous coatedPhosphorous coated0.13-0.240.11-0.17Electro Zinc platedElectro Zinc plated0.18-0.420.13-0.22Table 1. Friction in threads ofdifferent material.POCKET GUIDE TO TIGHTENING TECHNIQUE7
5. SCREW QUALITY CLASSIFICATIONWhen a screw is tightened and the clamping force starts tobuild up, the material of the screw is stressed. After a shorttime when the thread settles the material will stretch in proportion to the force. In principle, this elongation will continueuntil the stress in the screw is equal to the tensile strengthat which the screw will break. However, as long as the elongation is proportional to the stress the screw will regain itsoriginal length when the load is removed. This is known asthe elastic area.StressYield pointFailureAngular displacementAt a certain stress, known as the yield point, plastic deformation of the material in the screw will occur. However, thescrew will not break immediately. Torque will continue toincrease but at a lower torque rate during the deformationabove the yield point. The plastic deformation will result in apermanent elongation of the screw if the joint is loosened.For very accurate clamping force requirements this area issometimes deliberately specified for the tightening process.Beyond the plastic area breakage occurs.8POCKET GUIDE TO TIGHTENING TECHNIQUE
M-Threaded screwboltsTightening torque Nm, according to ISO 898/1The material qualities of screws are standardized, i.e. theamount of tensile stress they can be exposed to before theyield point is reached and before breakage occurs. All screwsshould be marked according to their Bolt Grade – a classification standard in a two-digit system where the first digit refersto the minimum tensile strength in 100 N/mm2 and the seconddigit indicates the relation between the yield point and theminimum tensile strength. For example: Bolt Grade 8.8 designates a screw with 800 N/mm2 minimum tensile strength anda yield point of 0.8 x 800 640 N/mm2.Table 2. Table for different classes of screws.Bolt 0545076609190POCKET GUIDE TO TIGHTENING TECHNIQUE9
Example of screw d esignation.6. JOINT TYPESScrew joints vary not only in size but also in type, whichchanges the characteristics of the joints. From a tightening point of view the most important quality of a joint is the“hardness”. In figures this can be defined as the “torque rate”,which is the tightening angle necessary to achieve the recommended torque of the screw dimension and quality in question measured from the snug level – the point at which thecomponents and the screw head become tight.The torque rate can vary considerably for the same diameterof screw. A short screw clamping plain metal componentsreaches the rated torque in only a fraction of a turn of thescrew. This type of joint is defined as a “hard joint”. A jointwith a long screw that has to compress soft components suchas gaskets or spring washers requires a much wider angle,possibly even several turns of the screw or nut to reach therated torque. This type of joint is described as a soft joint.Obviously the two different types of joints behave differentlywhen it comes to the tightening process.10POCKET GUIDE TO TIGHTENING TECHNIQUE
7. TORQUE AND ANGLEAs mentioned above, the tightening torque is for practical reasons the criteria normally used to specify the pre-stressin the screw. The torque, or the moment of force, can bemeasured either dynamically, when the screw is tightened, orstatically, by checking the torque with a torque wrench aftertightening.Ta2rFTbrTorque specifications vary considerably depending on thequality demands of the joint. A safety critical joint in a motor car, such as the wheel suspension, cannot be allowed tofail and is consequently subject to very stringent tolerancerequirements. On the other hand a nut for securing the lengthof a workbench height adjustment screw is not regarded ascritical from a clamping force point of view and no torque requirement may be specified.A higher level of quality control is reached by adding thetightening angle to the measured parameters. In the elasticarea of the screw this can be used to verify that all the members of a joint are present, e.g. that a gasket or a washer is notmissing. Also, the screw quality can be verified by measuringthe tightening angle, prior to snug level as well as for finaltorque-up.30 In sophisticated tightening processes the anglecan also be used to define the yield point andallow tightening into the plastic area of the screw.60 POCKET GUIDE TO TIGHTENING TECHNIQUE11
8. MEASUREMENT METHODSKnowing the tightening specifications for a screw joint theobvious question is; how does one know that the joint hasbeen properly tightened?Torque measurements are made according to one of two principles – static measurement or dynamic measurement.Static measurement means that the tightening torque ischecked after the tightening process has been completed. Themeasurement is usually done by hand with a torque wrenchwhich has either a spring loaded torque scale or a straingauge transducer activated instrument.A very common method for checking the tightening torqueis to use a click wrench, which is a torque wrench equippedwith a clutch that can be pre-adjusted to a specific torque. Ifthe torque is greater than the preset torque value the clutchwill release with a click. If the torque is less, final torque-upis possible until the wrench clicks. Over-tightening cannot bedetected with the click wrench.To measure the static torque the torque value must be readinstantly as the screw starts to turn.An electronic torque wrench can be used for a more sophisticated static measurement of the joint. The tool has straingauge torque transducer which gives a high level of accuracy.Electronic torque wrench (staticmeasurement).12Dynamic measurement on the other hand means that thetorque is continuously measured during the complete tighten ing cycle. This is usually the preferred method in productionwhere power tools are used for tightening. The advantageover the static method is that dynamic measurement providesan indication of the tightening tool performance without theinfluence of relaxation in the joint and variations in restingfriction. It also eliminates the necessity for subsequent checking.POCKET GUIDE TO TIGHTENING TECHNIQUE
Dynamic measurement is done either directly by measuringwith a built-in or a separate in-line torque transducer, or indirectly by current measurement of some sophisticated electricpowered screwdrivers and nutrunners. In both cases torquemeasuring is only possible where the tools have direct torquetransmission, i.e. not a pulsating force as is the case with impact wrenches and pulse nutrunners.The in-line torque transducer is mountedbetween the driving shaft of the tool andthe screw drive socket or bit. It is basicallya drive rod with installed resistances, a socalled Wheatstone Bridge, which senses theelastic deformation of the body as a result ofthe torque applied and produces an electricsignal that can be processed in a measuring instrument.In-line transducers are also available witha built-in angle encoder for monitoring thetightening angle.As the housing with its connector for the signalcable has to be held to prevent it from rotating,the in-line transducer is not practical for use incontinuous monitoring in serial production. How ever, for tool installation and torque setting and for on-linequality checking, the in-line transducer is theinstrument commonly used for reading the applied torquevalues.In assembly line production where tightening requires 100%monitoring or if the tightening process itself is controlled bythe torque readings, the torque transducer is usually built intothe tightening tool. In geared tools there are several positionswhere the transducer can be installed, but for dimensionalreasons it is advantageous to place it as close to the motoras possible where the forces involved are the lowest. Insteadof putting the strain gauges on the shaft, as with the in-line model, the built-in torque transducer can utilize the reactionforces in the power train assembly of the tool.Dynamic measurement with a separatein-line torque transducer.POCKET GUIDE TO TIGHTENING TECHNIQUE13
Angle nutrunner with built-intransducer.Angle encoders can also be incorporated in the tool design forregistration of the joint characteristics during tightening or foradvanced tightening control.9. THE TIGHTENING PROCESSThe tightening process also has a major influence on thequality of the screw joint. A joint tightened by hand behavescompletely differently from one tightened using a power tool.Also, different types of tools have a decisive influence onthe result. Direct driven tools such as screwdrivers and nutrunners have a maximum capacity that is decided by thepower output of the motor and gear ratio. They can be of thestall type, where final torque is determined by the torqueproduced when the tool has no more capacity to overcomethe resistance to turn the screw. Nowadays they are usuallyequipped with a device which stops the tightening at a predetermined torque.There are also other types of tightening tools common inindustrial production today, i.e. impact wrenches and pulsenutrunners where the motor power is converted to torqueoutput by charging and discharging the energy intermittentlyduring the process. This means that very powerful tools canbe designed with a limited weight and size and with almostno reaction torque to the operator. However, from a torquemonitoring point of view these types do not lend themselvesto dynamic measurement and are, consequently, not discussed in this context.14POCKET GUIDE TO TIGHTENING TECHNIQUE
10. MEAN SHIFTThe fundamental reason for using a power tool for tighteninga screw joint is to shorten the process time within the ability of the operator and quality requirements. It follows that ahigh rotational speed of the tool is of prime interest.Most assembly tools are powered by a motor which gives ahigh speed during run-down of the screw, when the resistance is low, and slow down as the torquing up proceeds. Onhard joints there is an almost immediate brake from idlingspeed to stall or shut-off speed. However, due to the inertiaof all the rotating parts there is quite a lot of dynamic energystored in the tool, the socket or power bit and the screw itself.This energy has to be discharged somehow and most of it isdelivered to the joint in the form of added torque, so called“over-shoot”.TorqueOver-shootMean shiftTargetHardSoftDefinition of mean shift andSnuglevelTargetover shoot.Angle ofrotation ofscrewIf the joint looked the same all the time there would not bea problem, but if the same tool is run on a soft joint, requiring much more time and energy to build up the torque, thedynamic effects are only marginal. The result is a difference intorque between the hard and the soft joint which can be considerable. This difference is called the “mean shift”.In tools equipped with some kind of shut-off device the quality of the clutch also becomes decisive for the mean shift ofthe tool. As the tightening sequence is usually very short thetime necessary for the clutch to react on the torque impulsewill have an equally important influence on the final torque asthe dynamic effect, i.e. the shut-off delay gives a much highertorque over-shoot on the hard joint than on the soft.POCKET GUIDE TO TIGHTENING TECHNIQUE15
11. STANDARDS FOR MEASUREMENTThe variations in tightening torque which depend on jointhardness have made it necessary to establish common measurement standards in order to define the capability of a toolto meet certain quality specifications and to be able to compare different tool types with the specifications.The common standard used today is ISO 5393 – “Rotarytools for threaded fasteners – Performance and test method”.The standard and the principles for evaluation of the resultsare discussed in the “Pocket guide to statistical analysis oftighten ing results”.”12. CERTIFICATIONISO 5393 represents a common platform for the assemblytool manufacturers and users to evaluate the performance ofassembly tools. Based on this measurement standard, ma
standardized. In practice there are only two different stand-ards used today in industry; the Unified standard UN, origi-nally used in the Anglo-Saxon countries, and the European Metric standard M. Shear load and tensile load. Basic screw design. Clamping force Tensile load Tensile load Shear load Shear load
the service limit described in the body of the manual. Parts to be used 1. Cylinder head bolt 2. Bearing cap bolt 3. Connecting rod cap bolt Tightening method After tightening to the spec ified tightening torque, further tighten 90 and 90 , or 180 (90 90 ). Follow the tightening method described in the body of the manual because the tightening method differs from part to part .
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