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Journal of

Materials Processing Technology

ELSEVIER Journal of Materials Processing Technology 56 (1996) 321-332

EVALUATION OF ANTI-LOOSENING NUTS FOR SCREW FASTENERS

N. Sase, S. Koga, K. Nishioka and H. Fujii

Dept. of Mech. Eng., Faculty of Eng., Gifu University, Japan

A B S T R A C T

There are various types of screw fasteners and/or parts with additional features designed to prevent loosening. Their actual ability is questioned and concretely examined. Seven types of those so-called anti- loosening nuts which are readily available in the market are compared with conventional nuts and evaluated their ability, including their manoeuvrability. Two types of loosening devices are developed for this purpose. One is to loosen fasteners by forcing a fastened material to displace and the other by applying high acceleration to a whole structure. The conclusions obtained are as follows: (1) Nuts with edged spring at the bottom, nylon inserted nuts, metal inserted nuts and rings with sharp inner edge to cover nuts are all unable to stop loosening. (2) A nut with serrated flange at the bottom has the ability to suppress the initiation of loosening to some extent. It makes the fastener resistant to higher levels of acceleration than fine screws. (3) A pair of double nuts and a combination of a nut with an eccentric external taper and a nut with an internal taper can slow the loosening process considerably. The latter may be an answer at present, but it is effective only under limited conditions.

1. I N T R O D U C T I O N

The screw fastener was first invented, it seems by a Spanish artisan of steel armour in the 15th Century [1]. Since then, fasteners have been used almost everywhere as the most convenient way to connect two parts. Modem screw fasteners can provide a high clamping force using a very simple tool and can be made with high productivity. However, they have an inherent and inevitable fault; they all loosen eventually.

Aiming to prevent screw fasteners from loosening, many methods have been proposed and some of them are even marketed. They are supposed to be effective in stopping or, at least, slowing the loosening tendency. Never-the-less, a considerable portion of the troubles on production lines and in industrial products are still found to be caused by the loosening of screw fasteners. It should be questioned, therefore, whether these anti- loosening screw fasteners are really effective or not.

In this paper, the authors chose some of those so-called anti-loosening nuts, which are readily available in the market, and examined their ability to resist loosening. Two types of devices to loosen the fasteners were developed for this purpose and used all through this test. Manoeuvrability of these anti-loosening nuts was also discussed. The overall performance of these nuts was finally evaluated to find out whether they are useful or not.

2. T H E T E S T E D A N T I - L O O S E N I N G NUTS

Seven types of so-called anti-loosening nuts were examined. The shape, method of use and principle of anti-loosening characteristics of those nuts are listed in T a b l e 1. These nuts were chosen based on the following thought:

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322 N. Sase et al. /Journal of Materials Processing Technology 56 (1996) 321-332

Table 1 So-called anti-loosening nuts /methods tested

)ne- fiece method

Shape and code name

EDGED SPR

Nut with an edged spring washer welded. SR TD

e Nut with a frange with rugged surface on it. NYLON INS

it Nylon inserted nut. METAL INS

\ /

Nut with two metal plates with sharp edges.

Method of use

Fasten as an ordinary nut, until [the washer is )ressed hard

'against fastened ~material.

Fasten as an ordinary nut.

Fasten as an ordinary nut. Unable to turn by hand. Wrenching is always required.

Fas t en as an

ordinary nut. Unable to turn by hand. Wrenching is always required.

Unloosening mechanism

expected or claimed

The edge of the spring pressed into the fastened material will result m giving resistance against loosening rotation of the nut.

Ratcheted shaped ]serrated teeth on [flange will provide an extra friction on fastened material !t2].

]Nylon inserted at the top part of nut will resist !loosening and give extra pressure on screw.

Two metal plates mounted at the top of nut will cut into the screw of bolt and will resist [loosening.

Clasifica- tion

Extra resistance at the surface of fastened material

Extra resistance at the screw thread

continued next page

N. Sase et al. / Journal of Materials Processing Technology 56 (1996) 321-332 323

Fwo-

piece method

COVER RNG

Sharp edged ring to cover a nut. DOUBLE NUTS

Two conventional nuts pressed against each other.

Fasten on top of ordinary nut by finger first and tighten it harder by 120 ° with wrench.

ECCNTRC NUT I

double nuts with eccentric tapers, i

Inner brim of the ring cuts into the bottom of screw groove and will ~rovide extra ~ressure.to it.

Fasten with two The groove Extra ordinary nuts first surfaces of both pressure and return the nut nuts press against on the of fastened each other and will screw material side in mcrease the thread the opposite pressure on them surface direction. [2].

Fasten the nut with eccentric taper first and fasten the female nut on top. Tighten them leaving no clearance.

By fastening until the limit, the eccentric male nut is pressed at a right angle to the centre line and will increase the pressure on the groove surface.

The advantages of screw fasteners are that they can offer a high clamping force by using a very simple tool and that they can be taken off at any time and used repeatedly. The nuts chosen and examined in this experiment are limited in those which do not need to sacrifice any one of the above conditions. Because of this reason, those methods like putting a pin in a hole drilled beside/on a nut or like deforming a nut with a crusher were not tested here.

The principles of anti-loosening characteristics given in the fourth column in the table are just what the companies which are selling those nuts claim they are. The claims have neither been objectively proved, nor even thoroughly tested yet.

324 N. Sase et aL /Journal of Materials Processhzg Technology 56 (1996) 321-332

The aims of the products seem to be classifiable into three types as follows; (1) The ones which are supposed to provide additional resistance at the bottom of the nut: an edged spring nut (EDGED SPR) and a serrated flange nut (SRRTD FLG.) It is claimed that the friction force between the nut and the fastened material beneath will be increased because of the extra resistance. (2) The ones which are supposed to provide additional resistance at the screw thread: a nylon inserted nut (NYLON INS) and a metal inserted nut (METAL INS.) It is claimed that the inserted nylon will fill the gap between the external thread and internal thread, and that the inner edge of a metal ring will cut into the threaded groove. All these are expected to provide extra resistance against loosening of the nuts. (3) The ones which are supposed to provide an increase in friction at the threaded groove: a pair of nuts (DOUBLE NUTS) and a combination of a nut with an eccentric external taper and a nut with an internal taper (ECCNTRC NUT.)

These are supposed to press the screw thread of the bolt and to lead to an increase in friction force. It is also claimed that the nuts are self-standing regardless of the movement of fastened material. In addition to the nuts above, a conventional nut (CONV NUT) with a standard coarse screw and a nut

with a fine screw (FINE SCR) were used as the controls. Both are produced in Japan in accordance with the Japanese Industrial Standard, which follows I.S.O. standard [3].

The size of bolt is fixed at 8mm in diameter. The initial tightening force fo is fixed at 18kN, which is 77% of the yield strength of the bolt. The dimensions, material and other specifications are shown in Table 2.

3. E X P E R I M E N T A L SETUP

Two types of loosening devices were produced for this test. Both were designed to apply a force at a right angle to the center line of the bolt. The purpose of the present paper is neither to evaluate the performance of these devices nor to discuss the mechanism of loosening. These will be presented in some other occasion. However, before the structure and the mechanism of movement are explained, it is essential to discuss the necessary conditions for a proper testing device and to review the works which have been done in this field.

3.1 A brief review of loosening testers and a background of the authors' development Generally speaking, a proper loosening device to test the loosening characteristics of a fastener should be

the one which can fulfill the following conditions: (1) Its structure and the method to use are not too complicated. (2) It is constructed easily.

Table 2 Mechanical characteristics of the bolts used for test

S~aecification

Nominal diameter

Coarse screw

Nominal length

mm 8.0 Pichi diameter mm 7.2 7.5

mm 50 40 mm Thread length

Pitch 35

Fine screw

8.0

20 '0.75 mm 1.25

Material % 0.15-0:25 0.25-0.45 Carbon steel Carbon steel

Vicker's Surface Hv 150 610 hardness Inside Hv 368 250

MPa Yield strength Tensile stlen[gh Others

M~a

640 or higher 800 or hisher

480 or hi~gher 600 or higher

Bolt:Hexagon headed Standard:metric


(3) It simulates what is actually happening on a fastener. (4) It loosens a fastener in a reasonable period of time. (5) The data obtained are the direct indication of the performance of a tested fastener. (6) The anti-loosening characteristics should be evaluated from two points of view. One is an ability to resist the initiation of loosening, and the other to resist the progress of loosening. A loosening test device must be what is able to do both of these. As you can imagine from these conditions, it is not easy to make a proper loosening tester. Several

testers have been proposed and examined by many investigators, including by Junker [4], Goodier [5], Yamamoto [6] and Koga [7]. However, none of these have been widely accepted as a standard device. That is probably because the testers require a complicated adjustment or produce the data difficult to evaluate. At least, not all the conditions above have been fulfilled yet by them.

In order to find out appropriate ways to loosen the fasteners, the authors also made many loosening devices and tested them. Two steel blocks were connected by a conventional bolt-nut fastener, and external forces were applied from the direction of the center line of the bolt, at a right angle to the center line, or along the circumference of the center line. Impacts were also applied from various directions. The results obtained by all these preliminary survey showed that the most dangerous external force is vibration applied at a right angle to the center line. This is practically the same condition as most researchers have concluded. The problem is what kind of tester is the best to give this force and to induce loosening. The authors' proposal is shown in the following sections. The results will be seen in Section 4.

3.2 Displacement based loosening device The first type is shown in Fig. 1. An eccentric c amO mounted on a motor spindle drives a

pendulum(~ which moves a rocking plate(3)reciprocally. The rocking plate is fastened by the bolt and the n u t , which is actually being tested. The bolt is bent because of the forced reciprocal movement of the rocking plate. This movement results in slipping of the nut at the contact surface with the fastened material.

Crossection at B-B' (enlarged)

Fig.1 Displacement-based loosening device

326 N. Sase et al. / Journal of Materials Processing Technology 56 (1996) 321-332

This device is similar to the Junker tester [4] from the viewpoint that the clamped material is forced to move. But this is contrasted to the Junker tester from the viewpoint that the amplitude is kept constant during a test.

The amplitude of the rocking plate can be adjusted arbitrarily by changing the shape of the cam and its location. The setup is designed to be as rigid as possible, so that the amplitude will stay constant throughout an experiment. The actual amplitude was 0.74mm when the tested bolt and nut were fastened, and the amplitude when they were loosened was 0.8mm. The frequency is fixed at 1Hz. In order to reduce the testing time and to minimize the scattering of data, the bottom surface of each nut was ground by sand paper and was lubricated by Molibdenium di-Sulfide together with the screw thread.

Changes in the clamping force during the loosening process were measured by a load cell(~)shown in Fig. 1. The relative rotation of the nut against the bolt was measured by a video camera.

The advantages of this setup are considered to be as follows:

• It is possible to move a fastened material with an approximately constant amplitude.

• Adjustments of the amplitude and frequency are easy.

• Difference in loosening processes according to the fasteners are clearly defined.

• A test for a fastener is completed in 3 to l0 min.

3.3 Acceleration based loosening device Another loosening device produced for this experiment is shown in Fig. 2. A we igh t~ the mass of

which is 5kg, is fastened by the tested bolt and nut(~onto a rigid supporting plate(~ The plate is mounted on the base of an accelerator~)which vibrates plate(~)at a right angle to the bolt.

While increasing the acceleration of the base, the level of acceleration at which the initial tightening force starts dropping suddenly was recorded. Changes in the clamping force after the above event were also recorded.

Lr~

L'q

I

Fig.2 Acceleration-based loosening device


The relative rotation angle of the nut against the bolt was measured by a video camera. Both the threaded groove and contact surface were lubricated in the same way as in the former method.

The advantages of this setup are considered to be as follows:

• The structure is extremely simple and very easy to use.

• Adjustments of the acceleration amplitude and frequency are also easy.

• The conditions under which loosening starts (the initiation of loosening) can be defined by physical parameters; the acceleration, amplitude, frequency and accumulated number of oscillations.

• The initiation of loosening can be also judged basing upon the difference between the acceleration applied and the acceleration induced on fastened materials as well.

• This setup simulates the actual situations where fasteners are used.

• It takes only 3 to 5 min to test a fastener,

Because of the above reasons, the authors would like to propose this testing method for objective evaluation of fasteners.

4. E X P E R I M E N T A L RESULTS

4.1 Results of a displacement based test Using the displacement based tester, changes in the clamping force of a bolt according to the number of

oscillations were examined. Figure 3 shows the results obtained when the conventional coarse screws and the fine screws were tested. The initial tightening force fo, 18kN, is plotted at the first oscillation. It is clear from the figure that the conventional nut starts loosening right after the oscillation begins, even if the nut was tightened at the optimum force. It is also clear that the remained clamping force f becomes nil after 100 oscillations. The rate of decrease in the clamping force f becomes slower as the number of oscillations increases. The loosening process of the fine screws seems to be the same, except that the loosening rate is a little slower than that seen in the coarse screw. This is probably caused by a smaller pitch of fine screws, as it is widely believed [6].

The relative rotation of nuts observed by a video camera decreases in the same manner as observed above. It was also found by repeating the same experiment on the same bolt and nut combination that the loosening caused by wear did not occur in 10 repetitive usages.

As shown above, the loosening process differs according to the individual sets of bolts and nuts. In order to compare the anti-loosening ability of each type of anti-loosening nut, the most typical ones should be chosen after testing several nuts. The typical loosening processes thus chosen are shown in Fig. 4 for comparison. As is seen in the figure, no anti-loosening nuts are worse than the coarse screw, but most of them,

2 0 ~ - ....... , . . . . . . .

' *"15-

0 O i , i , , , o , I J , , , , , , , 1 i i | | i | l

o 101 102 Number of osci lat ions n

Fig.3 Loosening characteristics of standard nuts (by displacement-based test)

0 3

328 N. Sase et aL / Journal of Materials Processing Technology 56 (1996) 321-332

except the double nuts (DOUBLE NUTS) and eccentric nut (ECCNTRC NUT), do not seem to be very effective at all. Only the nylon inserted nut (NYLON INS) showed a little resistance against loosening after it lost half of the initial tightening force. This nut, however, loses its resistance when the same nut is used repeatedly. Upon the third use, inserted nylon loses all of its efficacy and becomes the same as the other nuts.

Figure 5 shows the additional results obtained with the double nuts and eccentric nuts which were found to have considerable anti-loosening characteristics in Fig. 4. The results here are all obtained using new nuts. It is clearly recognized that these two types certainly have some anti-loosening ability. But the effect scatters a lot. Especially for the double nuts, some pairs do not loosen regardless of the initial tightening force, some loosen gradually and some loosen rapidly. Their anti-loosening characteristics are not stable.

The above mentioned tendency of the double nuts is probably caused by the fact that the tightening procedure is difficult to control. In this experiment, firstly the bottom nut was tightened to a tentative target value, secondly the top nut was tightened by the same torque, and thirdly the bottom nut was returned backward

~ 2 0 . . . . . . . . t . . . . . . . . w . . . . . . . .

'~'~ 1 5 ~ '

10 o ECCNTRC NUT '~"~ A NYLON iNS "k'~%, t~ [] FINE SCR ~. '~ '~IL~.

O SRR'I'D FLG ke-~'~l ~ • F_.I~ED SPR ~I, ~ _

• , , . . . . _ . 7 " i j , , , J , I

0 101 10 2 10 3 Number of oscilatiens n

Fig.4 Comparison of the performance of all the nuts tested (by displasement-based test)

. . . . . . . . , . . . . . . . . , . . . . . . . .

""15

~ I , i i D l l t l I . , , , . , , I 0 101 10 2 10 3 Number of oscilations n

Fig.5 Loosening characteristics of two-piece nuts (by displacement-based test)


slowly until the torque starts increasing suddenly. The tightening was approved and used for testing, only when the right clamping force was obtained after the procedure. Although the tightening was carded out controlling both the tightening force and torque as explained above, the loosening process scattered widely as shown in Fig. 5. Double nuts, therefore, are not always considered to be an effective way to stop or slow loosening.

The eccentric nut is inferior to double nuts which perform well, but is superior as a whole because their data do not scatter as much. The scattering did not change even when one nut was tested repeatedly. Therefore, as far as general usage is concerned, the eccentric nut is a practical answer.

4.2 Resul ts of a n a c c e l e r a t i o n based test In the former test, the clamped material was forced to move. In this test, both the clamped material and

the fastened bolt-nut are oscillated together. The principle of this test is explained schematically in Fig. 6. Suppose that a screw fastener and a clamped material are vibrated at a fixed acceleration level, the construction will loosen and start rattling noisily after some number of oscillations. If we repeat this at different levels of acceleration, a line like the one shown by the solid line in the figure will be obtained. Empirically, the line thus obtained is almost horizontal in most part in the range shown in the figure. Therefore, when a clamped construction is vibrated increasing the acceleration level gradually starting from the zero level, the construction loosens at some point marked a e in the figure. That is because the friction force which can support the inertia force is exceeded at this a c point. The acceleration level at this point is practically constant regardless a change in the total number of oscillations and in the amplitude, and is proportional to the initial tightening force fo of the fastener and the mass of the weight.

Figure 7 shows an example of the results obtained by fastening a conventional nut with various initial tightening forces fo. Although the results were scattered in the width shown by the standard deviation in the figure, the critical acceleration a c remains fundamentally proportional to fo.

As mentioned in Section 3.3, this type of test takes a very short time to perform. The longest test, which is represented by the point at the top right corner in the figure, took only 304 seconds until it completely loosened. This is an additional advantage of this testing method.

The average critical acceleration a c found for every nut is shown in Fig. 8. The initial tightening force fo is fixed at the optimum force:18kN. The amplitude was fixed at 1.524 mm and the frequency was gradually increased. The blank parts of each bar shows the range of the standard deviation of the data. It is observed from the figure that, although most nuts can endure a higher acceleration than the conventional nut can stand, the difference is very little. Only the serrated flange nut seems to have the ability to suppress the initiation of loosening.

/ ,,, ~1 with larger/amplitude / /

5O /

I / I /

i / o ac / /

/ / / /

/ / / / /

/ /

/ t / / ~ ' - . . . , with smaller amplitude / /

?0 ~ , , . . . . . I , , • , , . , , I , , . . . . . . I , , . i i i i I

o 10 2 10 4

N u m b e r of osci lat ions n Fig.6 Loosening characteristics obtainable using the accelerating device and the points a c defined as the critical acceleration

330 N. Sase et al. / Journal of Materials Processing Technology 56 (1996) 321-332

After the initiation of loosening, the clamping force of fasteners starts decreasing. It is important to understand this loosening process in order to evaluate the ability of a nut to resist loosening. Among the recorded data of the loosening processes, the most typical examples for each type of nut was chosen and are shown in Fig. 9. It is clear from the figure that no improvement is achieved, except by the double nuts and the eccentric nut. This result is quite similar to the one seen in Fig. 4 obtained by the displacement based test.

A nut starts turning left, right at the initiation of loosening. The turning speed of the nut is so high that you cannot see it clearly by the naked eye. An analysis of the images taken by a video camera indicated that a similar process as that observed in the former test is going on here as well.

70 ' ' I ' ' i , I • ~ " '

/ / / ¢,.I / / f / /

l'~ 60 i i i I ~ i i i -

l i i l ~ l l l i l I B i i I I , i 50 ," / b'-

i / . I I I I / . I I 11 •

,q: ..." / ..." ! ,," ,,,/ , . ' , , i ,

10 15 20 Initial tightening force fo kN

Fig.7 Change in the critical acceleration ac of conventional nuts (CONV NUT) for various initial tightening forces fo (by acceleration-based test)

EDGED SPR

SRRTD FLG

NYLON INS

METAL INS

COVER RNG

DOUBLE NUTS

ECCNI"RC NIJ I

CONV NUT

FINE SCR

3O

i i i

I • I

o I

El , I ~ I ,

40 50 Acceleration

A I

I • I

• I

@ I

? I

I

1 I

60 ac G

70

Fig.8 The critical acceleration a¢ at which the fasteners started loosening (by acceleration-based test, the initial tightening force fO =18kN)


4.3 Overall evaluation of anti-loosening nuts Neither discussion ~f the mechanism of loosening nor proposal of the authors' method for preventing

loosening are the purposes of the present paper. But nobody would disagree that an ideal anti-loosening screw fastener can be made if, by -:--qy chance, one or more of the following conditions happens to be realized:

1. the making of a ~-rew thread which has no lead (if we can still call it a screw.) 2. the making of a ~rew thread which has no flank angle. 3. the prevention of the relative slip between an external thread flank and an internal thread flank. 4. the prevention of the relative slip between the bottom surface of a nut and the fastened material. It would seem imp~sible to realize the above conditions without giving up all the other advantages that

screw fasteners have. Probably, all we can do is to make a screw fastener with characteristics which come as close to the above conditions as possible. Considering the test results of these nuts from the above point of view, the performance of each nut may be evaluated as follows:

The edged spring nut (EDGED SPR) fails to provide any improvement with regard to condition 4. It has no appreciable efficacy.

The serrated flange nut (SRRTD FLG) is somewhat effective. The ratchet shaped serrated teeth on the flange are imbedded into the fastened material. The teeth seem to be able to provide a grip which makes the fastener resistant to higher levels of acceleration. But after it starts loosening or when it is tested by the displacement based testing device, no efficacy is observed. Proper wrenching with a well controlled torque is necessary.

The nylon inserteA nut (NYLON INS), the metal inserted nut (METAL INS) and the cover ring (COVER RNG) fail to attain condition 3. These nuts did not perform any better than conventional nuts in both either test.

The double nuts (DOUBLE NUTS) and the eccentric nut (ECCNTRC NUT) are useful. Neither of them suppress the initiation of loosening, but they slow the loosening process considerably. Condition 3 is fulfilled to some extent, However, the double nuts require a very careful wrenching to achieve the right clamping force. Less care is needed in the L~ase of the eccentric nut, but it remains effective only before experiencing a limited number of oscillations.

C , , , , , , i , I i , i , , i l l I i , ~ i , i l l

10 o ECCNTRC NUT \ ~ ~3 5 [] FINESCR ~ \ ~

. . . • • METAL INS "~ , • EDGED SPR \ ~

SRRTO FLG • c o . v . u T . . . . . . .

, , . . , q l K I , , i l a a |

o 101 10 2 10 3

Number of oscilations n Fig.9 Cor~parison of the loosening processes of all the nuts tested (by acceleration-based test)

332 N. Sase et al. /Journal of Materials Processing Technology 56 (1996) 321-332

5. C O N C L U S I O N S

Seven types of so-called anti-loosening nuts were evaluated from the viewpoint of their ability to prevent loosening. Two types of devices that are able to loosen screw fasteners efficiently were developed for this purpose. The following conclusions were obtained.

(1). Edged spring nuts, nylon inserted nuts, metal inserted nuts, and cover rings are not able to stop loosening.

(2). A serrated flange nut has the ability to suppress the initiation of loosening to some extent. It makes the fastener resistant to higher levels of acceleration than fine screws.

(3). A pair of double nuts and an eccentric nut can slow the loosening process considerably. The eccentric nut is an answer so far, but it is effective only under limited conditions.

ACKNOWLEDGMENT

The authors would like to thank Mr. Mark Adams for the genuine interest he showed in this research and for spending so much of his valuable time to check this manuscript.

REFERENCES

[ 1 ] H. Fujii, The History of Screw Fasteners, Nitto Technical Report, No.43, 1991, pp. 2-3 [2] D.B. Dallas (Ed.), Tool and Manufacturing Engineers Handbook, 3rd ed., McGraw-Hill Company, 1975,

p. 27-27 [3] Japanese Standard Association (Ed.), JIS Handbook: Screw, Japanese Standard Association, 1989, p. 84 [4] G.H. Junker, New Criteria for Self-Loosening of Fasteners under Vibration, SAE Transaction, 1969,

pp. 314-335 [5] J.N. Goodier and R.J. Sweeney, Loosening by Vibration of Threaded Fastenings, Mechanical

Engineering, 1945-12, p. 798 [6] A. Yamamoto and S. Kasei, Investigations on the Self-Loosening of Threaded Fasteners under Transverse

Vibration, JSPE, 1976, p. 507 [7] K. Koga, A Discussion on Screw-Fasteners Loosening Caused by Impacts, JSME, 35-273, 1969,

pp. 1104-1111

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