screw guide

Pocket guideto screwdriving

Pocket guide to screwdriving

Chapter Page1. The screw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

A versatile fastenerThe scope of screwdrivers

2. The screw joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6The tightening torqueQuality influence of the tightening tool

3. Torque control of screwdrivers . . . . . . . . . . . . . . . . . . 11Slip-clutchShut-off clutchDirect drive

4. Test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Static measurementDynamic measurementScrewdriver torque setting

5. Screwdriver selection . . . . . . . . . . . . . . . . . . . . . . . . . 17Power sourceAir motorUniversal electric motorLow voltage electric motorBattery powered motorCurrent controlled electric motor

6. Shape of tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Pistol grip screwdriversStraight screwdriversAngle head screwdrivers

7. Selection of performance data . . . . . . . . . . . . . . . . . . 23SpeedPower

8. Select the screwdriver to suit the joint . . . . . . . . . . . . 259. Screwdriver accessories . . . . . . . . . . . . . . . . . . . . . . . . 26

Screwdriver bitsSurvey of screwheadsBalancers and torque armsBatch counting – line controlESD approval

10. Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Air-powered screwdriversElectric screwdrivers

11. Screwdriver economics . . . . . . . . . . . . . . . . . . . . . . . 3212. Trouble shooting chart . . . . . . . . . . . . . . . . . . . . . . . 3313. Atlas Copco pocket guides . . . . . . . . . . . . . . . . . . . . . 34

3

4 P O C K E T G U I D E T O S C R E W D R I V I N G

The screw

A versatile fastenerEasy assembly and disassembly makes the threaded fastenerunmatched as a method for joining components. The screwjoint is by far the commonest way to assemble mechanicaldevices of different types. Motor vehicles, aircraft, railwaycarriages, washing machines, television sets, IKEA furnitureand mobile phones are all dependent on properly dimensio-ned and well-tightened screw joints to make assembly, hand-ling, maintenance and repair profitable for producers andaffordable for users. Screws for the purpose are available in avariety of types and sizes ranging from less than 1 millimetreup to several hundred millimetres. An ordinary motor car isput together with about 3,000 screws. If all these screws hadto be tightened by hand, cars would be so expensive thatvery few people could afford to buy them. To make produc-tion rational and economic and maintain consistent quality,power tools are extensively used to tighten screws in indus-trial production and in commercially run service and mainte-nance operations.

The scope of screwdriversBetween 50 and 60% of the screws in a motor car and over80% of the total number of all screws used in industrial pro-duction are up to and including 6 mm in diameter, i.e. sizessmall enough to be handled with a manual screwdriver.Beyond this size range the torque required to create sufficientprestress in screws becomes so great that some kind ofwrench is necessary. The same limitation applies to powertools used for tightening – not because of the tightening tor-que but because of the reaction torque that the operator willbe subjected to. Accordingly, power tools for screw sizes upto M6 can be direct-driven tools of straight or pistol grip typethat are called screwdrivers, whereas power tools for largerscrew sizes are usually described as wrenches or nutrunners.

P O C K E T G U I D E T O S C R E W D R I V I N G 5

This booklet only covers screwdrivers and the specific condi-tions that apply to the tightening of screw joints in the capacityrange where neither the driving force of the tool nor the reactionforce to the hand of the operator are normally critical issues, atleast not where just a single operation is concerned.


The screw joint

The purpose of a screw joint is to hold the members of thejoint together under the load conditions for which the joint isdesigned. Technically this means creating enough clampingforce in the joint to withstand the forces that the joint may besubjected to without being loosened, but not enough clam-ping force to break any member of the joint. In a screw jointthe clamping force is provided by tightening the screw to aprestress by converting rotary motion into linear elongationof the screw. The decisive factor in the balance of forces is the friction inthe joint that prevents the screw from loosening after setting.Essential as this friction is for the function of the joint, italso represents the major problem in tightening the joint.With ordinary machine screws, it takes as much as 90% ofthe total torque applied to the screw to overcome the frictionand turn the screw. This means that only 10% of the powerapplied remains for stressing the screw and building up theclamping force. It is obvious that even small variations infriction caused by differences in lubrication or thread qualitycan have a considerable influence on the quality of the joint.

Friction under head


Self drilling fasteners.These screws drill andtap their own holes,thus eliminating a cost-ly operation.

Self piercing screw.A screw to fasten alltypes of timber withoutthe need to pre-drill ahole.

Thread forming self-tapping screw, type ST.

Thread rolling screw. A coarse threaded tri-lobular screw designedfor thermoplastics.

Self tapping screw. Invery soft thermoplas-tics this screw canachieve greater pull-offstrength and can incre-ase the difference bet-ween driving and strip-ping torque.

Thread cutting screw.Suited particularly tobrittle plastics.

Thread clearing screw.Used in pretapped holesclogged with paint, weldspatter or other foreignmatter. Eliminates sepa-rate retapping opera-tions.

Thread rolling screw. A tri-lobular self tappingscrew with thread rol-ling action, suitable forductile materials.

Flange type heads. Increased locking with grooves in head.

Different screw tips.

Guide types Rotating guide Drill type

The problem of defining the correct torque is further compli-cated by the great variety of types of screws existing on themarket in addition to the plain machine screw. Some exam-ples are shown below.


The tightening torqueThere is at present no method for direct measurement of theclamping force in a screw joint in practical production. Thismeans in practice that, despite the uncertainty, the tighteningtorque is the only value usable for deciding the quality ofscrew joints. Screws are usually rated for a specific tighte-ning torque with regard to size and tensile strength (boltgrade). This torque has generally been calculated to give atension in the screw of approximately 60% of its tensilestrength. At first sight this seems to provide a good marginfor variations in tightening torque and yet result in properclamping. However, considering the distribution of forcesapplied to the screw, it is obvious that even moderate varia-tions in torque might lead to a joint that comes apart by itselfor an over-tightening situation with screw breakage if threadconditions are not fully under control.

Another decisive factor in the tightening process is the tor-que rate, which is the turning angle from snug level to therated tightening torque of the screw. A hard joint where thescrew is short and the pieces being fastened together arerigid and tight requires only a fraction of a turn to reach thefinal torque, whereas a soft joint where the screw is longand/or there are resilient components such as spring washersor gaskets may require several full turns to reach its finalprestress. It follows that the energy required to tighten a softjoint is greater than for a hard joint.


The torques correspond to approximatly 62% of tensile stress.

Bolt grade

Thread 3.6 4.6 4.8 5.8 8.8 10.9 12.9

M1.6 0.05 0,065 0,086 0.11 0.17 0.24 0.29

M2 0.10 0.13 0.17 0.22 0.35 0.49 0.58

M2.2 0.13 0.17 0.23 0.29 0.46 0.64 0.77

M2.5 0.20 0.26 0.35 0.44 0.70 0.98 1.20

M3 0.35 0.46 0.61 0.77 1.20 1.70 2.10

M3.5 0.55 0.73 0.97 1.20 1.90 2.70 3.30

M4 0.81 1.10 1.40 1.80 2.90 4.00 4.90

M5 0.60 2.20 2.95 3.60 5.70 8.10 9.70

M6 2.80 3.70 4.90 6.10 9.80 14.0 17.0

Recommended max tightening torque (Nm) for untreated lightly oiledscrews, (friction coefficient = 0.125) Metric coarse thread.

Torque

Thread Nm

M3 1.4

M4 3.2

M5 6.5

M6 11

M8 26

M10 52

M12 91

Torque

Thread Nm

ST 2.2 (B2) 0.2

ST 2.9 (B4) 1.0

ST 3.5 (B6) 1.8

ST 4.2 (B8) 2.9

ST 4.8 (B10) 4.2

ST 5.5 (B12) 6.7

ST 6.3 (B14) 9.1

Recommended tightening torque for thread rolling screws,Taptite®, Swageform® etc.

Case-hardened, threadformingscrews for metal. M thread.Recommended values apply tothe strength of the screw (thejoint may be weaker).

Recommended tightening torque for thread formingscrew – ST (sheet metal screw).

Recommended values apply tothe strength of the screw (thejoint may be weaker).

Note: If a screw locking element is included, increase torque by 10% forplastic insert and 20% for mechanical locking (prevailing nut).

Recommendations


Quality influence of the tightening tool

What influence can the selection of the tightening tool have onthe quality of the screw joint?

Different power screwdrivers have different qualities that arecrucial for the final tightening result. One important factor isthe speed of the tool and the dynamic influence it has on theapplied torque. The applied torque is always the sum of thetorque that the tool produces and the additional torque crea-ted by the stored energy of the mass of the rotating parts.This is valid whether the driving motion is ended by a clutchor by stalling. In a hard joint this additional dynamic torquecan have a considerable influence on the final torque and thehigher the tool speed the greater is this influence. Conversely,in a soft joint the dynamic energy is not sufficient to add sig-nificantly to the turning angle and torque. The difference inthe applied torque between a hard and a soft joint with thesame tool is called the mean shift. Depending on speed, thequality of the clutch of the tool and the mass of the rotatingparts, this mean shift varies from tool to tool. Consequently,the mean shift becomes another essential quality of the toolnecessary to consider to be able to rely on the joint, even ifthe torque rate varies.

M1, M2 = Final torque

1 = Motor torque

2 = Dynamic torque addition

3 = Mean shift

Area A1 = A2 =Dynamic energy

Dynamic energy influence on tightening torque


Torque control of screwdrivers

Screwdrivers (meaning power tools) are usually equippedwith an adjustable torque clutch. Air powered screwdriversare also available as direct drive tools where the maximumapplied torque is reached at stall of the motor. The commonadvantage of the tool with a clutch is that the dynamiceffects on the applied torque are limited by disconnecting thepower train from the drive shaft when the preset torque hasbeen reached, thereby reducing the mass influence of motorand gears.

Slip clutchThe most common type of clutch for general-purpose screwdriving is the friction clutch (slip clutch). The design is basi-cally a spring-loaded claw coupling with inclined claws thatslip when the torque exceeds the preset spring force. Theclaws continue to slam and slip until the operator releases thetrigger. The torque applied is not very accurate and dependson how long the operator lets the tool slip, as the slam of theclutch when re-engaging adds energy to the joint, resulting inincreased torque. However, this effect is sometimes anadvantage, e.g. when there is a variation in the torque requi-rement for achieving a tight joint, which may be the case forthread forming screws or wood screws. Disadvantages of theslip clutch are the limited torque accuracy, the noise andvibration from the slipping action and the torque reaction tothe operator. Apart from the ergonomic effects, the vibrationsmight also cause damage to screws and a high degree ofwear on drive bits.

Slip clutch permits a certaintorque addition after the pre-set level has been reached.


Shut-off clutchFor maximum torque accuracy with a minimum of operatorinfluence a shut-off clutch screwdriver is preferred. The shut-off function can be either a pure mechanical action of disen-gagement between the output shaft and the drive train, amotor shut-off or a combination of both. However, the basicdevice is usually an adjustable spring-loaded clutch that rele-ases torque transmission and stops motor rotation eitherdirectly by shutting off the air supply on an air poweredscrewdriver or indirectly by a signal to brake the electricmotor of an electric tool.

The fact that a power screwdriver has a shut-off clutch is noguarantee of high quality accuracy. The design of the clutchis of decisive importance for screw tightening quality. Thecritical factor is the clutch reaction time from reaching thepreset torque until full release resulting in a torque over-shoot. It follows that slow clutch reaction means a greaterdifference between hard and soft joint than a quick release.

Clutch response time influence.

The tool shuts off after presettorque.

The shut-off time is also crucial for the reaction torque. A high quality clutch with a short reaction time means thatmost of the reaction torque on hard joints will be absorbedby the mass of the tool, with hardly any jerk to the hand ofthe operator. The low noise level and little vibration of shut-off clutch tools result in better working ergonomics.

A smooth quick shut-off action with a minimum of jerkreduces the risk of recoil of the bit in its grip of the screwhead, which means less damage to the finish of the screwhead recess. Also, bit wear is reduced compared to slipclutch tools. 60-80% lower bit consumption is not unusual.

Moreover, power shut-off to the motor considerably reducesenergy consumption compared to slip clutch models.

Direct driveDirect drive air tools, in which the bit that drives the screwhas a direct connection to the output shaft of the motor viathe gears, have maximum torque at stall of the motor. Thismeans that the torque is decided by the motor stall torqueand the gear ratio. The applied torque can be adjusted byregulating the air pressure to the motor. However, directdrive tools are frequently used where final tightening isdetermined by visual control and the operator simply relea-sing the trigger when the joint is tight enough.


The screwdriver runs untilthe trigger is released orstalls when it reaches maximum torque.


Test methods

Static measurementThe quality of the completed screw joint is tested statically.This means using a hand torque wrench to measure the tor-que after tightening. The torque wrench can be either amechanical wrench with a torque scale or a wrench with a

torque transducer that converts the applied force to anelectrical signal to a torque read-out instrument.Measuring is done either by re-tightening thescrew and reading the torque immediately thescrew starts to turn or by measuring the peak tor-que when untightening. The loosening torque isusually approximately 75% of the tightening value.

The static torque measurement of a screw givesinformation on the torque rate and the general qua-lities of the joint. It is often also used for adjustingthe screwdriver clutch to the correct tightening tor-que level. However, it does not give much infor-mation about the quality of the screwdriver. Thereason is partly variation in the static friction inthe joint that has to be overcome before the screwwill turn and partly the relaxation in the joint thatreduces the clamping force after tightening anddecreases the turning torque.

Hand torque wrench withelectronic read-out for staticmeasurement.


Dynamic measurementAccurate evaluation of screwdriver qualities requires dyna-mic torque measurements. This means using an in-line torquetransducer to measure the torque during the actual tighteningprocess. This method gives an accurate value of the appliedtorque without the influence of static friction and relaxationin the joint. Dynamic torque measurement on the actual jointprovides the best value for clutch adjustment.

There are also in-line transducers with incorporated angleencoders that allow torque rate measurements to be donesimultaneously for screw quality testing. Some sophisticatedelectric screwdriver systems have built-in transducers andangle encoders for continuous monitoring and/or control ofthe tightening process.

In-line transducer with elec-tronic read-out for dynamicmeasurement.


Screwdriver torque settingWhatever the type of tool, the torque has to be measured beforefinal adjustment of the applied torque. It is practical to preset thetorque by means of a torque tester or an in-line transducer beforethe final adjustment on the actual joint for which it is intended.This can be made dynamically with the in-line equipment or bychecking the screw statically with a torque wrench. Full verifica-tion of joint qualities involves measuring both the dynamic andthe static torque.

Torque tester with built-intest joint.


Screwdriver selection

We have touched upon the general aspects of various screw-driver qualities with regard to the torque control techniquesavailable. Torque accuracy is not the only factor that dependson the type of clutch and quality of clutch selected. Torquereaction forces, noise, vibration, power bit wear, screw headdamage and power consumption are further factors influencedby the selection of screwdriver clutch. Considerations thatalso arise are the power source and such physical aspects asspeed and power, shape of tool, weight, dimensions and otherdesign features.

Power sourceScrewdrivers available are powered by electric universalmotors, by low voltage electric motors or by compressed airmotors.

Air motorAir motors are the most powerful motors used inportable power tools in terms of weight anddimensions ratio. They are also sturdy, wear resis-tant and insensitive to overload, which makesthem ideally suitable as power units in hand-heldpower screwdrivers. The tools can be made smalland easy to handle, they have a long service lifeand they can be operated and maintained withoutany risk of electric shock or burnout due to short-circuit.


Universal electric motorUniversal electric motors are not generally recommended forscrewdrivers for industrial purposes because of the unfavou-rable power to weight ratio and the safety aspects. Somemodels of small screwdrivers are available for direct mainsplug-in, which simplifies installation. However, it rarelycompensates for their heavier and bulkier appearance and theelectric safety risk.

Low voltage electric motorLow voltage DC motor powered screwdrivers are quite com-

mon in small apparatus assembly asin the electronic industry, where

low sound level and a cleanenvironment free from pollu-tion by exhaust air are essen-tial. The tools are poweredfrom the mains via a transfor-mer and a control box with arectifier at a voltage of 25-30VDC. The controller alsoshort-circuits the motor win-dings to bring the motor tostandstill immediately uponclutch release, to avoid slip-ping.

Low voltage screwdriverwith transformer and controller.

Electric screwdriver formains plug-in.


Battery powered motorBattery tools were originally developed for carpenters andcraftsmen whose prime requirement was mobility. However,air hoses and electric cables for power supply also present aproblem in some industrial applications because of the riskof scratching finished surfaces or jamming when workinginside closed compartments. This has led to changes in themode of operation in many industries and many screws arenow best tightened with a battery tool that allows freedom tomove along a running production line without the inconveni-ence and safety risk of carrying hoses or cables.

At the same time, the growing demand for battery tools in,among others, the motor vehicle industry has brought increa-sed demands on tightening control, reliability and servicelife. Consequently, battery screwdrivers today are availablewith torque shut-off clutches just as accurate as on industrialair tools and incorporating industrial quality battery motortechniques and sophisticated acoustic systems for communi-cation between the mechanical clutch and the electronicmotor control to meet the demand for accuracy and quality.

Pistol-grip batteryscrewdriver.

Battery angle screwdriver.


Current controlled electric motorThere are also electric screwdrivers with specially builtmotors providing continuous control of voltage, current andfrequency during the tightening process. These tools are usedin sophisticated systems for advanced process control incombination with computerised statistical quality control anddocumentation.

Electric screwdriver withpower control unit.


Shape of tool

Screwdrivers come in pistol grip models, straight configurationor with angle head. In most cases the selection is obvious –pistol models for horizontal operation, straight models for ver-tical operation and angle tools for access to awkwardly posi-tioned joints. However, some practical hints are usually worthconsidering prior to decision.

Pistol grip screwdriversPistol grip screwdrivers allow a firm grip of the tool withgood support for the reaction forces from the tightening pro-cess. Pistol screwdrivers are therefore always recommendedin the larger screw size range. For a torque above 8 Nm asupport handle should be considered to reduce the wrist loadon the operator. The pistol grip screwdriver is also the mostsuitable for self-drilling screws, sheet metal screws andwood screws that require quite a considerableamount of axial feeding force.

Pistol grip screwdrivers usually have atrigger for starting and stopping thetool. The trigger start is often comb-ined with a push start function toensure that the operation does not startuntil there is proper engagement between the power bit andthe screw recess.

The reversing valve control should allow operation with thehand that holds the tool without having to change grip andpreferably independent of whether the tool is operated withthe left or the right hand.

The exhaust air from air-powered tools and cooling fans ofelectric tools should not be allowed to blow up dust or coolthe hand of the operator. Many pistol screwdrivers offer pos-sibilities to lead away the exhaust air for operator con-venience.

Shape of tool is of outmostimportance for operatorcomfort.


Straight screwdriversStraight screwdrivers that have the motor, gearing, couplingand output shaft in line are convenient for use at fixed work-stations for vertical operations. The tools are often suspen-ded in balancers for easy access. Straight tools should not be used for torque above 4-5 Nm for male operators and 2-3 Nm for females because of the reaction torque, unlessthe screwdrivers are mounted in torque arms fixed to theworkbench.

Straight screwdrivers have lever or button start, push start orcombined lever and push start. Push start is the quickest typeif the screws are well oriented before the operation starts.Lever start on air tools allows smooth starting to secure pro-per entry of thread and bit engagement. Combined lever andpush start function is recommended where screws are pickedup with the screwdriver bit prior to placing in the screwhole.

On lever operated screwdrivers the reversing control is usu-ally of twist ring type that requires the use of the other hand.Push start tools are available with push button control forquick one-hand operation.

Low noise level and a clean working environment are oftencrucial issues on light assembly lines. Silencing and meansto lead away exhaust from air powered screwdrivers areoften decisive qualities.

Angle head screwdriversAngle screwdrivers are primarily used for accessibility rea-sons. They are basically built as straight tools with an anglehead attachment. The lever arm housing the motor, gear andclutch makes reaction forces negligible for the lower screw-driver torque range. The angle screwdriver can also be theideal solution for the higher torque range simply for the abil-ity to counteract reaction forces. However, it limits the possi-bility of exerting manual force for feeding, an important con-sideration in cases such as cross recess head screws.

For obvious reasons, angle screwdrivers are not equippedwith push start function.


Selection of performance data

As well as screw capacity and torque range, screwdrivers arealso selected with regard to other performance values such asspeed, power, energy consumption, noise and vibration emis-sion. Speed and power are interconnected values essential fortorque accuracy, productivity and operator convenience.

SpeedSpeed is naturally the prime factor in productivity but cannotbe increased beyond certain limits. Difficulties in controllingscrew entry and friction heat in threaded plastic are examplesof practical problems of high speed tightening, but the mostimportant reason to keep down rotary speed is the influencethat speed has on torque accuracy. An idling speed of 900-1200 rpm is usually the most practical solution. Higherspeeds are preferred for self-drilling screws and lower speedsshould be used for driving screws in soft materials such asplastics to avoid overheating the material.

Speed also influences operator exposure to reaction forces.On hard joints most of the reaction torque is absorbed by theinertia of the high-speed tool, whereas the tightening processwith a low-speed tool will seem softer but with a greater jerkon the hand of the operator. It follows that higher speeds areadvantageous from an ergonomic point of view but require aquick action clutch to avoid excessively increasing the torquemean shift.


PowerBeing a function of speed and torque, it follows that themotor size required is larger for a high speed tool than for alow speed model at the same torque capacity. From an ope-rator safety point of view it is better to select a tool with alarger motor size and a capacity well above the target torquethan a small tool at the limit of its capacity, as this mightlead to clutch failure, especially if the supply air pressuredrops below the nominal value.

A powerful tool is also recommended for tightening of softjoints, thread forming screws and wood screws, as the resis-tance in the threads reduces the speed and lengthens tighte-ning times.


Select the screwdriver to suitthe joint

To utilize fully the high performance of a high quality screwdri-ver, it is important to know the demands made on the screwdri-ver by different screw joints. Here are some examples that willhelp you select the kind of screwdriver you require.

Machine screw – Hard jointLow resistance to turning until thescrew head reaches its seating, afterwich resistance rapidly increases. For rapid operation with moderatetorque accuracy choose a high speedtool. For close torque tolerances orwhen the quality of thread is uneven,choose a lower speed tool. This is alsorecommended for brittle material.Most suitable clutch is shut-off type.Alternative choice slip type.

Machine screw – Soft jointLow resistance to turning until thescrew head reaches its seatings, afterwich resistance slowly increases. A lower speed than used for the rigidjoint utilizes the torque capacity of the tool. For more rapid tightening choose a larger size tool with a higherspeed. Most suitable clutches: Shut-off type. Slip type possible.

Self-drilling screwThe turning resistance graduallyincreases during drilling and threadcutting, but more slowly than withself-threading screws. Choose a toolwith speed over 1000 rpm. Slip typeclutch is generally suitable but for thinsheet choose shut-off.

Machine screw–locking element in threadRelatively high resistance to turningbefore screw head reaches its seating,after which resistance rapidly increa-ses. Choose a tool with high torquefrom the lower speed range. If rapiddriving is required choose a largertool with a higher speed. Suitableclutches: Slip type and shut-off.

Thread producing screwThe turning resistance graduallyincreases as the screw is producingthe thread and reaches a maximumjust before the entire hole is threaded.Choose a tool within speed range 800to 1300 rpm. Slip type clutches aregenerally suitable, but for thin sheetchoose shut-off type.

Wood screwThe turning resistance graduallybuilds up as the screw is being drivenand increases rapidly when the screwhead reaches its seating. The processvaries considerably with the degree ofpredrilling, different types of woodand sizes of screw. Choose a low-speed tool, 400 to 800 rpm. Suitableclutches: Slip type and direct drive.


Screwdriver accessories

Screwdriver accessories include a number of essential items aswell as optional equipment to extend and improve the field ofoperation. Some useful items are:

Screwdriver bitsGood quality bits are essential for satisfactory screwdriverperformance. Bit shape correctly fitting the screw headrecess and correct hardness are crucial for bit life and mini-mum damage to screws. Magnetic bit holders facilitate posi-tioning of screws but the insert bit must be kept clean fromiron chips to avoid misengagement with the screw head.

Extra-hard bits should beused for sheet metal screwsand other hardened screwtypes.

Finders are extra equip-ment and must be used forslotted screws unless thereare fixtures with bushingsto guide the bit.

Symbol Description Size Application CommentsSlotted Slot width Wood screw Inexpensive. Relatively

0.25-2.5 Sheet metal screw difficult to fit (the slot isMachine threaded screw ruined). Requires a finder

for guidance.

Phillips 00-3 Wood screw Inexpensive. Requires a(cross recess Sheet metal screw certain axial force whentype H) Board screw fitted. Note: Use the right

Machine threaded screw bits when mounting. Do notconfuse with Pozidriv bits.

Pozidriv® 1-3 P2D Wood screw Somewhat expensive. Easy(cross recess 1-3 S0V Sheet metal screw to fit. Only requires abouttype Z) 30% of the axial force

needed with Phillips screws.Note: Use the right bitswhen mounting. Do notconfuse with Phillips bits.


Hexagon 7-16 mm Machine threaded screw Suitable for higher torques.Thread forming screw Usually for screws M4 andSelf drilling screw upwards. Easy to fit.

Somewhat difficult to enter.

Internal Hex size Machine threaded screw Requires low axial force.hexagon 2-8 mm Difficult to enter. Allen head Available from M3 and

upwards.

Torx® T6-T40 Machine threaded screw Relatively expensive. EasySheet metal screw to fit. Saves bits. Easy to

enter. Low axial forcesneeded. Can transmit relati-vely high torques. Largerscrews also available withexternal grip.

A number of screws have been developed for special applications, for example:Torq-Set® TSO-O Special screw Expensive. Easy to

TSO-10 for the air industry. dismantle.

Survey of screwheads


Balancers and torque armsScrewdrivers are comparatively small and lightweight toolsbut when the small effort of lifting and lowering the screw-driver in the course of the tightening operation cycle is repe-ated maybe several hundred times per hour, even the lightweight of a screwdriver becomes a burden to the operator.The same applies tothe reaction torquefrom a screw tighte-ning operation.Balancers to reducethe load on the opera-tor and/or torquearms to limit thestrain from reactionforces should alwaysbe considered in repe-titive assembly work.

Batch counting – line controlIn this booklet we have merely touchedupon advanced systems for quality controlin the assembly of threaded fasteners. Oneway to zero-fault production, withoutcostly systems for current control or trans-ducerised tools, is to introduce batchcounting. Modern screwdrivers today haveconnections for conveying a signal thatthe clutch has released properly at the pre-set torque value. With an “RE Controller”this signal can be utilised to count thenumber of OK screws in an operationcycle, with alarm or line control to alertthe operator if the operation is incomplete.

Assembly Repair

Signal lightand alarm

Entrysignal

Clock Alert signal

OK signal

RE-Controller

RE-tool

Assembly station with batchcounting.


ESD approvalAll electronic components are more or less sensitiveto ESD (ElectroStatic Discharge). All materials andthe human body itself can accumulate an electrosta-tic charge that might discharge to any other objectof differing polarity, and if the object concernedincorporates for instance a semiconductor thedevice might have a functional breakdown.Screwdrivers are extensively used in the assemblyof electronic devices and are therefore criticalitems from an ESD point of view.

The risk that electrostatic charges will remain onthe surface of the tool and discharge to a criticalobject is eliminated by making plastic compo-nents such as insulating covers and triggers ontools out of a dissipative or conductivecompound and connecting the screwdriverbody electrically to ground.

When you assemble electronic devices, makesure that you use ESD approved screwdrivers.

When you assemble electronicdevices, make sure that you useESD approved screwdrivers.


Installation

The performance of the operator-tool combination is no moreefficient than whichever is the weaker member of this team.A good working position is necessary for the operator to beable to utilise the capacity of the tool. Proper tool installationis also essential to get as much productivity and service lifeout of the tool as are paid for.

Air-powered screwdriversAn air powered screwdriver consumes between 3 and 15litres of compressed air per second. Wrongly dimensioned orleaking airline accessories will not allow a sufficient air sup-ply to the air motor, which means power drop, lower produc-tion and increased production costs.

The airline installation at an assembly station with screwdri-vers should include a ball valve, a filter, a pressure regulator,an open flow hose coupling (claw or ErgoQuick), a PVChose of max. 5 m length or a spiral hose and a quick coup-ling (preferably ErgoQuick) to the tool inlet. Modern screw-drivers are lubrication free, which means that no in-linelubricator is necessary.

There is specially ESD approved PVC hose for air supply inESD sensitive environments.

Installation of air-poweredscrewdrivers.


Electric screwdriversThe installation of an assembly station with low-voltageelectric screwdrivers consists of a transformer with cable formains plug-in, a rectifier and controller built into a controlbox, a combined power and signal cable and the tool.Alternatively a combined transformer/controller can be usedto simplify installation but this is somewhat more expensive,especially if it breaks down. With a bigger transformer apower supply network can be built to supply several toolssimultaneously.

Electric installations also require proper dimensioning. A 3mcable between control box and tool is included as standardequipment with the tool. Optional extension cables can incre-ase the distance up to maximum 8m without causing excessi-ve power losses. A spiral cable is also available as an extraaccessory.

In ESD protected areas makesure that all items are ESDapproved.


Screwdriver economics

In deciding which power screwdriver to choose for an indus-trial assembly operation it is essential to consider not onlythe price of the tool but the cost of the whole process, inclu-ding rejects, remedial work due to bad quality tightening andloss of goodwill. And in this process the tool investment costis usually of minor importance. In most cases the labour costis the dominant part of the cost calculation.

With hand-held tools it is the combined performance of theoperator and the power tool that decides the productivity.How much a screwdriver can produce is limited by thepower output of the tool. But the tool also puts a limitationto how fast and accurately the operator can work. In additionto the capacity of the screwdriver, factors such as the qualityof the tool in terms of torque accuracy and how the handlingof the tool affects the operator by torque reaction, noise andvibration, weight, shape, physical load etc. are also decisivefor the economics of tightening.

Naturally, the quality of the screwdriver with regard to ser-vice life and maintenance cost must also be included in thetotal cost calculation.

RejectsInterruptions in prod.Physical exhaustionAversion to work

RejectsInterruptions in prod.Physical exhaustionAversion to work


Trouble shooting chart

The quality of a screw joint is the result of the combination of joint characteristics,screwdriver performance and the operator influence. When problems occur it is notalways obvious what causes the deviation in joint behaviour. The following table isintended to give a quick reference to possible causes and the relevant suggestion ofactions to cope with a number of typical problems in tightening of threaded fasteners.

Symptom

Insufficient clamping –screw works loose

Cross threading

Threads stripping

Tool does not back up thescrew after tightening

Cause

Too low tightening torqueadjustment

Poor toque accuracy

Supply air pressure drop

Insufficient screw lubrication

Joint relaxation

Short clamping length

Excess torque adjustment

Poor screw quality/ damagedthread

Too high screw enteringspeed

Poor tool balance

Excess speed

Varying screw friction

Excess hole diameter forthread forming screws

Insufficient thread engage-ment

Friction below screw head tolow

Too high tightening speed

Extremely soft joint with highresting friction

Solution

Check tightening specifications,adjust

Reconsider tool selection withregard to clutch type, speedand reliability.

Check pressure at tool idling

Adjust lubricating specifications

Try slip clutch or use screwswith flanged head or washer

Check possible change ofdesign or introduce spring was-her

Check tightening specifications,adjust

Make quality inspection, sortout failures

Select screwdriver with smoothstart (lever/button throttle)

Check if tool (handle type,weight, balance etc.) is right forthe job

Select lower speed model

Specify surface treatment

Adjust specifications

Increase screw length or threa-ded material thickness

Increase screw head diameteror introduce friction flange (forplastic material)

Select lower speed model orconsider slip clutch screwdriver

Consider slip clutch screwdriveror check with Atlas Copco foralternative


Title Ordering No.

Air line distribution 9833 1266 01

Air motors 9833 9067 01

Drilling with hand-held machines 9833 8554 01

Grinding 9833 8641 01

Percussive tools 9833 1003 01

Pulse tools 9833 1225 01

Riveting technique 9833 1124 01

Screwdriving 9833 1007 01

Statistical analysis technique 9833 8637 01

The art of ergonomics 9833 8587 01

Tightening technique 9833 8648 01

Vibrations in grinders 9833 9017 01

Atlas Copco Pocket Guides

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