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POCKET GUIDE

TO TIGHTENING

TECHNIQUE

POCKET GUIDE TO TIGHTENING

TECHNIQUE

This booklet provides an introduction to the technique of us- ing threaded fasteners for assembling components, the ap- plication of power tools for the assembly and the influence of tool selection on the quality of the joint.

1. WHY THREADED FASTENERS?

There are several ways of securing parts and components to each other, e.g. gluing, riveting, welding and soldering. However, by far the most common method of joining compo- nents is to use a screw to clamp the joint members with a nut or directly to a threaded hole in one of the components. The advantages of this method are the simplicity of design and assembly, easy disassembly, productivity and in the end

• cost.

2. THE SCREW JOINT

A screw is exposed to tensile load, to torsion and sometimes also to a shear load.

The stress in the screw when the screw has been tightened to the design extent is known as the pre-stress.

The tensile load corresponds to the force that clamps the joint members together. External loads which are less than the clamping force will not change the tensile load in the screw. On the other hand, if the joint is exposed to higher external loads than the pre-stress in the bolt the joint will come apart and the tensile load in the screw will naturally increase until the screw breaks.

Torsion in the screw results from friction between the threads in the screw and the nut.

Some screws are also exposed to shear loads which occur when the external force slides the members of the joint in relation to each other perpendicular to the clamping force. In a properly designed joint the external shear force should be resisted by the friction between the components. A joint of this kind is called a friction joint. If the clamping force is not sufficient to create the friction needed, the screw will also be exposed to the shear load. Joints are frequently designed for a combination of tensile and shear loads.

The screw is made up of the shank and the head. The shank is threaded, either for part of its length or for the full length from the end to the head. Longer screws are usually only partly threaded. There is no need to make a thread longer than is necessary to tighten the joint as this will only make the screw more expensive and reduce the tensile strength.

The dimensions of threads, the shape of the thread and the pitch, i.e. the distance between successive threads, have been 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

4. EFFECT OF LUBRICATION

If a screw is lubricated, the friction in the threads and under the head is decreased and the relation between tightening torque and clamping force is changed. If the same torque is applied as before lubrication, a lot more torque will be trans- formed into clamping force. At worst this might lead to the tension in the screw exceeding the tensile strength and break- ing of the screw.

On the other hand, if the screw is completely dry of lubricant the clamping force might be too small to withstand the forces for which the joint is designed, with the risk that the screw becomes loose.

Table 1. Friction in threads of different material.

Bolt material Nut material Dry Lightly oiled Untreated Untreated 0.18-0.35 0.14-0. Phosphorous coated Untreated 0.25-0.40 0.17-0. Electro Zinc plated Untreated 0.11-0.36 0.11-0. Phosphorous coated Phosphorous coated 0.13-0.24 0.11-0. Electro Zinc plated Electro Zinc plated 0.18-0.42 0.13-0.

5. SCREW QUALITY CLASSIFICATION

When a screw is tightened and the clamping force starts to build up, the material of the screw is stressed. After a short time when the thread settles the material will stretch in pro- portion to the force. In principle, this elongation will continue until the stress in the screw is equal to the tensile strength at which the screw will break. However, as long as the elon- gation is proportional to the stress the screw will regain its original length when the load is removed. This is known as the elastic area.

At a certain stress, known as the yield point, plastic defor- mation of the material in the screw will occur. However, the screw will not break immediately. Torque will continue to increase but at a lower torque rate during the deformation above the yield point. The plastic deformation will result in a permanent elongation of the screw if the joint is loosened. For very accurate clamping force requirements this area is sometimes deliberately specified for the tightening process. Beyond the plastic area breakage occurs.

Yield point

Stress

Angular displacement

Failure

6. JOINT TYPES

Screw joints vary not only in size but also in type, which changes the characteristics of the joints. From a tighten- ing 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 recom- mended torque of the screw dimension and quality in ques- tion measured from the snug level – the point at which the components and the screw head become tight.

The torque rate can vary considerably for the same diameter of screw. A short screw clamping plain metal components reaches the rated torque in only a fraction of a turn of the screw. This type of joint is defined as a “hard joint”. A joint with a long screw that has to compress soft components such as gaskets or spring washers requires a much wider angle, possibly even several turns of the screw or nut to reach the rated torque. This type of joint is described as a soft joint. Obviously the two different types of joints behave differently when it comes to the tightening process.

Example of screw designation.

7. TORQUE AND ANGLE

As mentioned above, the tightening torque is for practical reasons the criteria normally used to specify the pre-stress in the screw. The torque, or the moment of force, can be measured either dynamically, when the screw is tightened, or statically, by checking the torque with a torque wrench after tightening.

Torque specifications vary considerably depending on the quality demands of the joint. A safety critical joint in a mo- tor car, such as the wheel suspension, cannot be allowed to fail and is consequently subject to very stringent tolerance requirements. On the other hand a nut for securing the length of a workbench height adjustment screw is not regarded as critical from a clamping force point of view and no torque re- quirement may be specified.

A higher level of quality control is reached by adding the tightening angle to the measured parameters. In the elastic area of the screw this can be used to verify that all the mem- bers of a joint are present, e.g. that a gasket or a washer is not missing. Also, the screw quality can be verified by measuring the tightening angle, prior to snug level as well as for final torque-up.

In sophisticated tightening processes the angle can also be used to define the yield point and allow tightening into the plastic area of the screw.

Ta

T (^) b

2r F

r

60°

30°

Dynamic measurement is done either directly by measuring with a built-in or a separate in-line torque transducer, or indi- rectly by current measurement of some sophisticated electric powered screwdrivers and nutrunners. In both cases torque measuring is only possible where the tools have direct torque transmission, i.e. not a pulsating force as is the case with im- pact wrenches and pulse nutrunners.

The in-line torque transducer is mounted between the driving shaft of the tool and the screw drive socket or bit. It is basically a drive rod with installed resistances, a so- called Wheatstone Bridge, which senses the elastic deformation of the body as a result of the torque applied and produces an electric signal that can be processed in a measur- ing instrument.

In-line transducers are also available with a built-in angle encoder for monitoring the tightening angle.

As the housing with its connector for the signal cable has to be held to prevent it from rotating, the in-line transducer is not practical for use in continuous monitoring in serial production. How- ever, for tool installation and torque setting and for on-line quality checking, the in-line transducer is the instrument commonly used for reading the applied torque values.

In assembly line production where tightening requires 100% monitoring or if the tightening process itself is controlled by the torque readings, the torque transducer is usually built into the tightening tool. In geared tools there are several positions where the transducer can be installed, but for dimensional reasons it is advantageous to place it as close to the motor as possible where the forces involved are the lowest. Instead of putting the strain gauges on the shaft, as with the in-line model, the built-in torque transducer can utilize the reaction forces in the power train assembly of the tool.

Dynamic measure- ment with a separate in-line torque trans- ducer.

9. THE TIGHTENING PROCESS

The tightening process also has a major influence on the quality of the screw joint. A joint tightened by hand behaves completely differently from one tightened using a power tool.

Also, different types of tools have a decisive influence on the result. Direct driven tools such as screwdrivers and nut- runners have a maximum capacity that is decided by the power output of the motor and gear ratio. They can be of the stall type, where final torque is determined by the torque produced when the tool has no more capacity to overcome the resistance to turn the screw. Nowadays they are usually equipped with a device which stops the tightening at a prede- termined torque.

There are also other types of tightening tools common in industrial production today, i.e. impact wrenches and pulse nutrunners where the motor power is converted to torque output by charging and discharging the energy intermittently during the process. This means that very powerful tools can be designed with a limited weight and size and with almost no reaction torque to the operator. However, from a torque monitoring point of view these types do not lend themselves to dynamic measurement and are, consequently, not dis- cussed in this context.

Angle nutrunner with built-in transducer.

Angle encoders can also be incorporated in the tool design for registration of the joint characteristics during tightening or for advanced tightening control.

11. STANDARDS FOR MEASUREMENT

The variations in tightening torque which depend on joint hardness have made it necessary to establish common meas- urement standards in order to define the capability of a tool to meet certain quality specifications and to be able to com- pare different tool types with the specifications.

The common standard used today is ISO 5393 – “Rotary tools for threaded fasteners – Performance and test method”. The standard and the principles for evaluation of the results are discussed in the ”“Pocket guide to statistical analysis of tightening results”.

12. CERTIFICATION

ISO 5393 represents a common platform for the assembly tool manufacturers and users to evaluate the performance of assembly tools. Based on this measurement standard, many car manufacturers have their own quality programs. These programs involve the categorisation as well as quality clas- sification of tools available on the market. Usually, the perfor- mance of a tool, when new as well as after a certain time of operation, must be verified before the tool becomes accepted for use by the manufacturer’s assembly plants worldwide.

The most extensive certification program is the one from the Ford Motor Co. In principle, it is based on a classification of all the joints in a car in the relevant tool classes with regards to torque requirement. Tools are tested according to those re- quirements, from maximum to minimum torque in each class according to the ISO 5393 testing procedure. For approval each tool has to meet the accuracy requirements both when new and after 250 000 cycles, and for preferred certification after 500 000 cycles without major repairs and within the same tolerance specification.

Other car manufacturers have similar programs. Most of them use ISO 5393 as the test method, but demands might vary.

The tests for tool performance are primarily developed by the car manufacturer but the tool manufacturer may be author- ized by the user to perform the practical testing.

13. ERRORS IN TIGHTENING

The purpose of monitoring the tightening torque is to ensure that the proper clamping force has been reached. However, tightening torque alone is not a 100% guarantee that the clamping force is sufficient for the load for which the joint has been designed. There are a number of errors that might occur and result in inadequate pre-stress in the screw despite the correct tightening torque.

14. DAMAGED THREADS

Damage to the thread or insufficiently cut threads will result in increased resistance to turning the screw and hence the pre- determined torque will be reached before the correct clamp- ing force is achieved.

Damaged threads can be detected by monitoring the tighten- ing angle.

15. MISSING JOINT COMPONENTS

A common problem in industrial production is that the opera- tor forgets a washer or packing in the assembly of a joint. Apart from having other purposes for the design, missing components will change the torque rate of the joint and con- sequently also the clamping force.

16. RELAXATION

All joints set after tightening. This means that after a short time, less than 30 milliseconds, the clamping force in the joint is less than it was when the tightening stopped. For joints which include elastic components such as gaskets this relaxa- tion can be considerable and a subsequent torque test may show that the torque is just a fraction of the intended speci- fication. Relaxation is usually overcome by tightening in two stages. A pulse tool or impact wrench might also be a practi- cal solution as the pulsating drive allows relaxation of the joint between the pulses or impacts.

Damaged thread.

Missing washer.

Relaxation.

The disadvantages are the comparatively high noise level of the impact wrench and the difficulty in measuring the ap- plied torque and consequently also the limited possibility to achieve accurate torque control.

Consequently, the impact wrench is the ideal tool for loosen- ing rusty and stuck bolts in maintenance work in chemical plants, refineries and other heavy industries. Impact wrenches are also suitable for a variety of applications that do not re- quire the highest degree of accuracy.

Pulse tools

The hydraulic pulse tool has all the advantages of the impact wrench, i.e. high speed and power from a lightweight and handy tool without reaction forces, but none of the disadvan- tages except the difficulty of dynamic monitoring of the ap- plied torque.

In pulse tools the torque is built up, not by metal to metal blows, but via a hydraulic cushion. This results in low noise, a minimum of vibration and, above all, good accuracy in tightening. This is achieved by controlling the hydraulic pressure in the pulse mechanism that limits the torque ap- plied once the pre-set value is reached.

The handiness, the speed, the low noise and vibration levels and the torque accuracy has made the pulse tool very popular in the manufacturing industries, including the motor vehicle industry. The limitation is for applications which require docu- mentation of the applied torque values.

Torque

Time

Principle of pulse tools.

Principle of how torque is built up with pulse tools.

Pneumatic screwdrivers and nutrunners

Direct driven air powered tools for tightening screws range from the smallest screwdrivers for up to M6 (1/4’’) screw size to high torque nutrunners for several thousand Nm tighten- ing torque. The torque is built-up by transforming the power of a high-speed air motor into low-speed, high-torque output of the driving shaft by gearing. Planetary gears are normally used.

Screwdrivers

The term screwdrivers defines those tools used for the smallest screws where the tightening torque required is low enough to allow the reaction torque built up during tighten- ing to be taken up by the operator just by gripping the tool. In practice this limits the range to between 4 and 12 Nm (M5- M6) capacity, depending on the type of tool, the type of joint and the working position.

The simplest form of screwdriver is the stall type tool in which the applied torque is determined by how much the motor after gearing is capable of tightening before it stops. Adjustment of torque is made by regulating the air pressure that powers the tool. Often this type of tool is used for ap- plications with varying torque demand such as sheet metal screws, where the operator stops the tightening process by visual control.

However, screwdrivers are usually equipped with a torque ac- tuated mechanical clutch. The clutch can be either of slip type or have a shut-off function. With the slip clutch a spring-load- ed coupling disengages when the pre-set torque is reached but re-engages as long as the trigger is activated. This is a comparatively inexpensive solution and gives some possibil- ity to add torque or compensate for relaxation but the action is rather noisy and torque control is poor. Shut-off screwdriv- ers have good accuracy.

Pneumatic screwdriver.