ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

Подождите немного. Документ загружается.

Fig. 21 Response of components in a bolted joint to applied load

The torque-tension testing procedure specifies a generic method for evaluating the effect of different surface

finishes, coatings, or manufacturing processes on the torque versus tension characteristics of threaded fasteners.

Test specifications published by standards organizations and engineering societies are identified in the section

“Selected References.”

Test Equipment

Test Bolts, Nuts, and Washers. Test bolts should be of sufficient length so as to be easily fixtured into the

adapters that are used with the test equipment. Test bolts should have a uniform application of finish, coatings,

or both.

Test nuts should be plated, coated, and lubricated according to the test plan or experimental design.

Test washers should be square torque-tension test washers corresponding to the bolt size being tested. They

should be appropriately hardened, finished, plated, or coated according to the test plan. The test washer

provides the reference surface for contact with the underhead of the nut or bolt element to which torque is

applied. The square shape provides means to prevent the washer from slipping on the test plate when the

friction coefficient between the washer and plate is less than that between the bolt or nut and washer.

Test Equipment. A fixtured direct-current (dc) electric nutrunner or other drive system capable of maintaining

the required constant test speed is used to tighten and remove the test bolt or nut. The nutrunner should have the

capability of producing a torque output greater than the torque values specified for the size and grade of bolts to

be tested and should be capable of reaching the continuous speed required by the test specification. Test

specifications generally require 25 to 100 rpm for most fasteners. For tests that require torque application above

250 N · m, the continuous speed may be reduced to the range of 5 to 25 rpm. Speed should be held constant

within 10% of the specified test rpm. For valid comparison to be made between results obtained by two or more

laboratories, the testing speeds must be specified and controlled.

Torque Transducer. A strain-gage torque sensor is used to measure the torque required for the installation of the

bolt being tested. The torque sensor should have an accuracy of ±2% at the point of measurement (more

information can be found in the section “Measurement Accuracy”). A torque sensor equipped with an angle

encoder is recommended in the event that additional analysis is desired.

Tension Load Cell. A tension load cell is used to hold the test bolt, nut, and washer in position and to measure

the tension generated in the bolt as the test nut is tightened. The recommended accuracy of the tension load cell

is ±2% at the point of measurement. A tension load cell with thread torque measurement capability is

recommended in the event that additional analysis, such as frictional characteristics, is desired.

Data Acquisition and Control Equipment. A multiple channel recorder capable of recording the torque, tension,

and angle and thread torque (if required) data during the entire test cycle should be used. Equipment that can

control the dc electric nutrunner, calculate statistics, and provide graphic plots of the test is highly

recommended.

Procedure

The following procedure can be used as a reference for conducting torque-tension testing. This procedure

assumes that the bolt is secured and the nut is tightened. Some test evaluations of bolt underhead characteristics

require that the nut be secured and the bolt rotated. In that case, the position of the nut and bolt should simply

be reversed:

1. Determine the torque and tension loads that are appropriate for the size of the fastener to be tested.

These are typically specified by the required test standard or special requirements of the customer.

2. Select an appropriately sized nut and test washer. A new nut and test washer should be used with each

bolt that will be tightened.

3. Place the bolt being tested through the test fixture (load cell and adapters) and test washer so that the

bolt extends beyond the test nut approximately six threads after bolt rundown. Make sure that the test

washer is secured so that it does not rotate during testing (Fig. 17).

4. Assemble the nut onto the bolt approximately two threads.

5. Place the nutrunner tool on the bolt and tighten to the specified torque or tension value. Simultaneously

record the torque that is applied and the tension that is achieved.

6. Loosen the nut, and return to the original starting position.

7. Repeat steps 5 and 6 for the specified number of bolts to obtain data that can be statistically analyzed to

produce torque at tension to 3 sigma limits. A new nut and washer is to be used for each test bolt.

Note: In addition to obtaining torque-tension data for previously unused bolts and nuts as described, it is often

desirable to tighten and loosen the same bolt-nut-washer combination multiple times to evaluate the effect of

repeated tightening and loosening on the torque-tension relationship.

Test Report

The test report for torque-tension testing generally requires a multiple graphic plot of torque versus tension for

the prescribed number of rundowns, as shown in Fig. 22. A statistical graphic plot of torque at tension also is

desirable. This testing can be further documented by a report of the numerical data to which statistics can be

added.

Fig. 22 Multiple rundown torque-tension plot

Mechanical Testing of Threaded Fasteners and Bolted Joints

Ralph S. Shoberg, RS Technologies, Ltd.

Locknut Testing

A locknut is a device that provides extra resistance to vibration loosening beyond that produced by proper

preload, either by providing some form of prevailing torque or, in free-spinning locknuts, by deforming,

crimping, or biting into mating parts when fully tightened. A locknut counters the back-off torque created by

the inclined planes of the thread. Locknuts and locking mechanisms take many forms. Such forms include

thread interference locknuts, also known as prevailing torque locknuts, that use several different forms, such as

nuts with out-of-round holes, a nylon locking collar in the nut, a nylon patch, or mechanically deformed

dimples or crimps. Another form of locking devices are free-spinning locknuts, such as springhead nuts, beam-

type nuts, and serrated nut bearing surfaces.

Because of the particular characteristics of prevailing torque locknuts, a unique testing process is required for

them. In the past, there has been a wide range of equipment available for prevailing torque locknut testing from

basic hand tool and indicator systems to various semiautomated systems. Test results for locknut prevailing

torque are particularly sensitive to control of testing speed or a lack thereof. Automated testing systems have

been developed with the capability to conduct multiple tightening cycles on production lots or research and

development samples of locknuts, with tests preprogrammed according to Industrial Fasteners Institute (IFI)

100/107 for inch, ISO 2320 and ANSI B818.61.1 for metric, and other common test specifications. Precisely

controlling test speeds, tension shut-off points, and cycle timing is necessary to ensure repeatability of test

results.

The locknut testing procedure specifies a generic method for evaluating the effectiveness of prevailing torque-

locking devices. Test specifications published by standards organizations and others that may apply to these

types of fasteners are listed in the section “Selected References.”

Principle

There are several published standards for testing and evaluating prevailing torque locknut devices. The

standards all generally attempt to determine how well the prevailing torque element works the first time it is

used and how well it holds up when reused a specified number of times. The Industrial Fasteners Institute (IFI)

publishes well-known and widely accepted test specifications. The IFI test is often referred to as a “first-on,

fifth-off” test. In this test, the locknut is torqued on and off the bolt five times. Torque is measured and recorded

the first time on and the fifth time off. On the first cycle, the locknut is tightened to a specified clamp load. The

second through fifth cycles involve installation and removal without achieving clamp load.

Multiple locknut and bolt samples are tested to develop a summary report. On completion of the test batch,

statistical data are generated for the breakthrough torque, where the locking device is first penetrated, the first-

on maximum prevailing torque, the torque to reach the specified clamp load, and the fifth-off minimum

prevailing torque.

Figure 23 shows a graph of a typical locknut test tightening cycle. The clamp-force-versus-angle curve is

superimposed on the torque curve. The graph indicates the breakthrough torque, prevailing on-torque,

prevailing off-torque, and the clamp force attained.

Fig. 23 Prevailing torque locknut testing cycle

Variations of the IFI test have been developed by other organizations and include the following differences

(users may also specify their own variations):

• Tighten, but do not attain clamp-load cycle test, up to 30 cycles

• Tighten to clamp load each of three or five times on

• Standard torque versus tension cycles, from one time up to 30 cycles

When multiple tightening cycles are required, the user can specify dwell times after each tightening-and-

loosening cycle to permit cooling of the test specimen. This procedure helps to ensure uniform testing

conditions, which are particularly important when testing certain prevailing torque locknuts.

Test Equipment

Test Bolts. Test bolts should be specified to meet the strengths and finish characteristics of the fasteners

intended to be used with the locking device being tested. Bolt length must be sufficient so that at least two

threads are projecting past the nut after tightening. Test bolts should have a uniform application of finish,

coatings, or both. If an applied adhesive is the locking device, its application must be uniform and consistent

throughout the test sample. When production conditions are to be simulated, the maximum thread projection

found in the production assembly must be replicated in the test assembly.

Test Nuts. The test nuts used in this type of testing generally contain the locking device described previously.

However, if the locking device is an adhesive applied to the bolt, the bolt should have the same surface finish

and be of the same size and grade as the bolt normally used in production.

Test Washers. Test washers should be hardened, finish-plated, or coated square torque-tension test washers

corresponding to the surface conditions related to the application for which the locking device is being tested.

Nutrunner. A fixtured nutrunner is used to tighten and remove the test bolt or nut. The nutrunner should have a

capability for producing a torque output greater than the torque values specified for the size and grade of nuts

and bolts to be tested and should be capable of reaching and maintaining the continuous speed required by the

test specification. Test specifications for prevailing torque testing are typically slower than those for torque-

tension testing, around 25 rpm, ±5. However, a number of standards require tests to be run at 100 rpm.

Torque Transducer. A strain gage torque sensor is used to measure the torque required for the installation of the

locknut being tested. The recommended accuracy of the torque sensor is ±2% at the point-of-measurement. A

torque sensor equipped with an angle encoder is required so that the position of the locknut can be used to

establish certain windows of measurement to obtain prevailing on- and off-torque values, as well as the

breakthrough torque value, if required.

Tension Load Cell. A tension load cell is used to hold the test bolt, nut, and washer in position and to measure

the tension generated in the bolt as the test nut is tightened. The recommended accuracy of the tension load cell

is ±2% at the point of measurement.

A tension load cell with thread-torque measurement capability is recommended in the event that additional

analysis, such as thread and underhead frictional characteristics, is desired. The use of a torque-tension research

head is mandatory if the separation of underhead friction torque from thread friction torque is to be achieved for

control of manufacturing tolerances for thread-loading friction.

Data Acquisition and Control Equipment. A multiple-channel recorder capable of recording the torque, tension,

and angle and thread torque (if required) data during the entire test cycle should be used. Equipment that can

control the dc electric nutrunner as well as calculate statistics and provide graphic plots of the test is highly

recommended. Since achieving a specified clamping load is normally a part of the test, the test nutrunner drive

should be capable of being stopped with no more than 2% overshoot once the target tension is achieved.

Excessive overshoot of assembly torque can be a major source of variation in test results.

Procedure

The following procedure can be used as a reference for conducting prevailing torque locknut testing. This

procedure assumes that the test bolt is secured, and the test nut is tightened:

1. Determine the torque and tension loads that are appropriate for the size of the fastener to be tested.

These loads are typically specified by the required test standard or special requirements of the customer.

2. Select an appropriately sized bolt and test washer. A new bolt and test washer should be used with each

locknut that will be tested.

3. Place the bolt being tested through the test fixture (load cell and adapters) and test washer so that the

bolt will extend beyond the test nut approximately six threads after bolt rundown. Be sure the test

washer is secured so that it does not rotate during testing (Fig. 17).

4. Assemble the nut onto the bolt approximately two threads so that the locking device will be ready to

engage the threads.

5. Place the nutrunner tool on the bolt and tighten to the specified torque or tension value. Simultaneously

record the torque that is applied and the tension that is achieved.

6. Loosen the nut and return it to the original position. Simultaneously record the removal torque that is

required and the tension such that once the clamp load has been released, the prevailing off-torque can

be measured in the specified window as the nut is loosened to the starting position.

7. Repeat steps 5 and 6 the indicated number of times to obtain the specified data.

Evaluation

The locknuts are inspected to determine conformance to the specific standard. Inspection procedures may be

designated by the purchaser or may be agreed upon between the purchaser and the supplier prior to the

acceptance of the order.

Test Report

As an example, the illustration in Fig. 24 shows a representative printed locknut analysis according to IFI

specifications. The upper chart indicates the following:

• Measurements for five samples of breakthrough torque for the first cycle (initial angle window

specified)

• Maximum on-torque for the first cycle (generally after the breakthrough is achieved and recorded in a

360° window before any clamp force is generated)

• Clamp load for the first cycle at a verified tension shut-off point

• Maximum off-torque for the first cycle (measured in a 360° window of turn after the clamp force has

been released)

• Minimum off-torque for the first cycle (measured the same as maximum off-torque)

• Maximum off-torque for the fifth cycle (same window as first cycle)

• Minimum off-torque for the fifth cycle (same window as first cycle)

The lower chart provides statistics for the above values including high, mean, low, range, standard deviation,

±3 sigma, and the process capability index, C

Fig. 24 Representative printed locknut analysis according to IFI specifications

Other test specifications may require that the on- and off-torque be reported for cycles other than those required

by IFI. A common variation of the IFI 101/107 locknut test is to require that the specified clamp force be

achieved on each of three or five loading cycles. The maximum and minimum off-torques are specified and

checked on the last off cycle.

Mechanical Testing of Threaded Fasteners and Bolted Joints

Ralph S. Shoberg, RS Technologies, Ltd.

Angular Ductility and Rotational Capacity Tests

The ability of a fastener to provide additional clamping load after it has been taken into yield has been used to

advantage in certain assemblies. The critical challenge has been to determine the point at which the fastener

begins to yield. By measuring and recording torque and clamp load versus angle of turn as a fastener is taken to

failure, the resulting signature can be analyzed to determine the angular ductility or rotational capacity of the

fastener, which is an additional measurement indicative of the overall energy-absorbing capacity of the fastener.

Angular ductility measurements are the torque-tension test equivalents to the reduction-in-area or elongation

measurements made in stress-strain tensile testing.

This section describes a generic method for determining the clamp load at the torque-tension yield point and the

angular ductility of threaded fasteners. Test specifications published by standards organizations and engineering

societies are identified in the section “Selected References.”

Principle

This testing quantifies the ductility of the fastener as it relates to rotational capacity. It locates the point of yield

and determines the fastener rotation from yield to failure. This measurement can then be used to establish the

relative toughness of the fastener.

The yield point may be located by offsetting the slope of the elastic tightening curve by an amount established

by the following equation:

(Eq 18)

where L

is the effective grip length of the fastener, and P is the pitch of the thread. The constant, 0.002, relates

the angular yield point to the 0.2% offset often used in tensile testing to establish the yield point. For example,

to calculate the offset angle for a –20 × 1 English fastener, the effective grip length is 1 in., and the pitch of the

fastener is or 0.050. Thus, the calculation would appear as follows:

(Eq 19)

Test Equipment

Test Bolts, Nuts, and Washers. Test bolts should be selected from a representative production lot with all

dimensions, finishes, and coatings certified according to the appropriate manufacturing specification.

Test nuts or drilled and threaded test bars should be plated, coated, and lubricated according to the testing plan

or experimental design.

Test washers should be square torque-tension test washers or suitably prepared test bars with holes prepared

corresponding to the bolt size being tested. Test washers should be appropriately hardened, finished, plated, or

coated according to the testing plan.

Nutrunner. A fixtured dc electric nutrunner or other drive system capable of predictable control of test speed

and torque is used to tighten the test bolt or nut. The nutrunner should have a capability for producing a torque

output sufficient to bring the test bolts to failure. Test specifications generally require 25 to 100 rpm, ±10%, for

most fasteners. For tests that require torque application above 250 N · m, the continuous speed may be reduced

to the range of 10 to 25 rpm, ±10%.

Torque Transducer. A strain-gage torque sensor is used to measure the torque required to fail the bolt being

tested. The torque sensor should have an accuracy of ±2% at the point of measurement (more information can

be found in the section “Measurement Accuracy”). The measurement system must be equipped with an angle

encoder with sufficient resolution for determining the yield point and plotting the torque and tension versus

angle of turn.

Tension Load Cell. A tension load cell is used to hold the test bolt, nut, and washer in position and to measure

the tension generated in the bolt as the test nut is tightened. The recommended accuracy of the tension load cell

is ±2% at the point of measurement.

Data Acquisition and Control Equipment. A multiple-channel recorder capable of recording the torque, tension,

and angle data during the test should be used. Equipment that can control the dc electric nutrunner and

construct the required offset for yield determination in graphic form is highly recommended.

Procedure

The following procedure can be used as a reference for conducting angular ductility testing. This procedure

assumes that the nut is secured and the bolt is tightened. Some test evaluations require that the bolt be secured

and the nut rotated. In that case, the position of the nut and bolt are simply reversed:

1. Determine the torque and tension loads that are appropriate for the size of the fastener to be tested.

These loads are typically specified by the required test standard or special requirements of the customer.

2. Select an appropriately sized nut and test washer.

3. Place the bolt being tested through the test fixture (load cell and adapters) and test washer so that the

bolt extends beyond the test nut approximately six threads after bolt rundown. Be sure the test washer is

secured so that it does not rotate during testing (Fig. 17).

4. Assemble the nut onto the bolt approximately two threads.

5. Place the dc nutrunner tool on the bolt and tighten to failure. Simultaneously record the torque that is

applied, the angle of turn, and the tension that is achieved.

6. Repeat steps 3 to 5 for the statistically agreed-upon sample of fasteners.

7. Construct a plot of the data as torque or force versus angle of turn.

8. Using the test-reporting software or by hand, draw a straight-line tangent to the elastic tightening slope

of the curve back to zero torque or force.

9. Draw a line parallel to the line drawn in step 8 at the offset calculated using Eq 18, extending the line

upward so that it intersects the plotted curve. This intersection locates the point of yield in the fastener.

10. Calculate the angular ductility by determining the angle of turn from the yield point to the point of

fracture (Fig. 25).

Fig. 25 Angular ductility calculation

Test Report

The test report for angular ductility generally requires a multiple graphic plot of torque or force versus angle of

turn for the prescribed number of rundowns, as shown in Fig. 26. This testing can be further documented by a

report of the numerical data to which statistics can be added.

Fig. 26 Angular ductility report

Mechanical Testing of Threaded Fasteners and Bolted Joints

Ralph S. Shoberg, RS Technologies, Ltd.

Torque-Angle Signature Analysis

The factors that must be considered when establishing a threaded-fastener, bolted-joint analysis program are

described in this section. Included are methods for modeling the joint, experimental testing of components and

assemblies, and procedures conducting postassembly audits.

The basic torque-angle signature is used as a starting point for all analysis. As a first example, it can be used to

illustrate the influence of underhead and thread friction on the tightening process. An increase in friction, in

either the thread or underhead regions, results in a proportional increase in the slope of the torque-angle

signature. The study of the slope of the elastic tightening zone is an important element in analyzing the

performance of threaded fasteners in bolted joints.

To apply torque-angle signature analysis, a torque-angle transient recorder is used for measurement and curve

plotting. The transient recorder can provide curves onscreen for analysis as well as print them out for detailed

study. Tightening, audit, and release-angle signatures for a given fastener can be simultaneously displayed and

printed.

Classical Design Concepts: Modeling the Tightening Process

When developing a testing program to correlate the design of a bolted joint and the actual assembly, it is

necessary to document the relationship between torque and turn in the development of tension. Before control

of a tightening process can be gained, it is necessary to become familiar with what actually happens when the

fastener is tightened. The process of tightening a fastener involves turning, advancing the lead screw, and

torque, or the turning moment, so that preload, or tension, is produced in the fastener. The desired result is a

clamping force that holds the components together. A torque-versus-angle signature correlated to the clamp

force-versus-angle plot offers the best model that can be used to explain this process. The most general model

of the torque-turn signature for the fastener tightening process has four distinct zones, as illustrated in Fig. 27.

Fig. 27 Four zones of torque-angle tightening

Zone 1: Rundown. The first zone is the rundown, or prevailing torque, zone that occurs before the fastener or

nut contacts the bearing surface. Prevailing torque, due to thread-locking features, such as nylon inserts or

deformed threads, will show up in the rundown zone. Frictional drag on the shank or threads due to

misalignment of parts, chips, or foreign material in the threads, as well as unintended interference due to out-of-

tolerance threads, are additional causes of prevailing torque in the rundown zone.

Zone 2: Alignment. The second zone is the alignment, or snugging, zone wherein the fastener and joint mating

surfaces are drawn into alignment, or a stable, clamped condition. The nonlinear alignment zone is a complex

function of the process of drawing together the mating parts and bending of the fastener as a result of

nonparallelism of the bearing surface to the fastener underhead surface. In addition to the macroeffects related

to the alignment of parts, there are microeffects within the alignment zone. The microeffects include contact

stress-induced deformations of plating and coatings as well as local surface roughness and thread deformations.

These macroeffects and microeffects are illustrated in Fig. 28.