Study of Pal®nut DIN 7967 bolt securing · PDF fileStudy of Pal®nut DIN 7967 bolt...

Study of Pal®nut DIN 7967 bolt securing

system

April 2013

2

1.1 Topic

In this report a bolt securing system comprised of stamped sheet metal counter nuts according

to the standard DIN 7967, most often referred to as Pal®nut, is studied. The study covers

aspects related to assembly, bolt securing against self-loosening, corrosion resistance and

quality assurance.

1.2 Project methodology and structure

The report will first describe and give a background of the Pal®nut bolt securing system.

Thereafter the Pal®nut will be investigated through testing. Tests which are considered relevant

are:

Assembly tests to study the time required to install the products and their dependence

on operator skill level.

Vibration tests to study the products capacity to secure bolted joints against self-

loosening and slackening.

Corrosion tests to study the lifetime of the product when exposed to a corrosive

environment.

Finally the quality assurance and certification level will be researched.

1.3 Summary

The study clearly shows that the Pal®nut bolt securing system has little or no functionality in

preventing self-loosening of bolted joints that are exposed to vibrations. It is also shows that the

DIN 7967 standard was declared invalid in 2003 without any replacement or reference to any

other standard.

1. Executive summary

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..................................................................................................................................... 2

1. Executive summary ................................................................................................. 2

1.1 Topic ................................................................................................................... 2

1.2 Project methodology and structure ..................................................................... 2

1.3 Summary ............................................................................................................. 2

2. Table of contents ...................................................................................................... 3

3. Introduction .............................................................................................................. 4

3.1 Technical background ......................................................................................... 4

3.2 Marketing / Sales background ............................................................................. 4

3.3 Purpose ............................................................................................................... 5

4. Technical study ........................................................................................................ 6

4.1 Testing ................................................................................................................ 6

4.2 Assembly test ...................................................................................................... 6

4.2.1 Summary assembly test ............................................................................... 7

4.3 Vibration test DIN 65151 ..................................................................................... 8

4.3.1 Vibration Test 1 ............................................................................................ 9

4.3.2 Vibration Test 2 .......................................................................................... 13

4.3.3 Vibration Test 3 .......................................................................................... 17

4.3.4 Vibration Test 4 .......................................................................................... 21

4.3.5 Summary vibration tests ............................................................................. 24

4.4 Corrosion test ISO 9227 .................................................................................... 25

Appendix 1 .......................................................................................................... 26

Appendix 2 .......................................................................................................... 27

2. Table of contents

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3.1 Technical background

The Pal®nut is a stamped sheet metal self-locking counter nut (see Picture 1). It is standardized according to DIN 7967 from 1963, however, this standard was declared as invalid in 2003 without any replacement or update. The locking principle is that Pal®nut deform when tightened and small protrusions jam into the bolt threads. Both assembly and disassembly procedures of a bolted joint secured by Pal®nuts are described in DIN 7967. After tightening the joint, the Pal®nut is placed on the bolt threads and turned by hand until contact is made with the preloaded nut. The Pal®nut is then further tightened between ¼ and ½ turn using a wrench. To disassemble the joint, the preloaded nut must be further tightened to relieve the Pal®nut. The Pal®nut can then be turned in the untightening direction and removed by hand. The preloaded fasteners are then free to be untightened. Because the Pal®nut is designed to lock a nut, the system is only implementable on bolted and stud joints.

Picture 1.

3.2 Marketing / Sales background

Pal®nut is a registered trademark of the company Trans Technology and is the most common name but is also known under the names Pahlnut, Pahlmutter, Buddy Lock-Nut and Speed Nut. Pal®nut has gained a high popularity in all types of industries and applications due to relatively low purchase price, high availability and the fact that it has been present on the market for approximately 50 years and has gained a reputation as an effective bolt securing system. The perceived added value of Pal®nut is the fact that only one side of a bolted joint must be secured as well as the combination of simple visual inspection and replacement. Pal®nut is most commonly sold through distribution and is available globally. Würth, Bossard, Fabory, Fastenal, Grainger, Emile Maurin are examples of distributors with strong regional and global networks that sell Pal®nuts. Pal®nuts are available for both Metric and Imperial threads in both coarse and fine thread pitch in the size range M4-M80. The available surface finishes are electro zinc plated, mechanically

3. Introduction

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zinc plated, phosphate, hot dip galvanized and stainless steel in corrosion classes A2/A4. Note that the carbon steel Pal®nuts are not made of spring steel. Because Pal®nut is not only a brand but also specified according to a DIN standard there are many manufacturers who subsequently supply numerous sales channels. Approximately 50 different manufacturers of Pal®nut can be identified. Since the competition to sell Pal®nut is high the price is often very low. A low purchase price is often the strongest sales argument for Pal®nuts and high volume applications are often targeted. For example, an offshore derrick may consist of well over 100 000 bolted joints that must be secured. Subsequently the bolt securing system may become a significant part of the total cost of such an application, which makes Pal®nuts due to low cost a preferred choice.

3.3 Purpose

Pal®nut has gained a high recognition based on best practice principles and is mentioned in some handbooks for safe securing of bolted joints. However, neither tests nor studies that verify the performance of Pal®nut are readily available. All that statements available regarding Pal®nuts seems to be based the DIN 7967 standard from 1963. However, this DIN standard was officially declared as invalid in 2003 without a replacement (see Appendix 1). Fasteners distributors also acknowledge the DIN 7967 standard as invalid without any replacement (see Appendix 2).

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4.1 Testing

In order to study the Pal®nuts tests in three 3 different aspects were performed. The purpose of the testing is to investigate:

1. the assembly procedure 2. the resilience against self-loosening 3. the corrosion resistance

All tests were performed on bolted joints comprised of ISO 4017 M16x2,0-60 and the corresponding M16x2,0 nuts. Bolts of property class 8.8, 10.9 and stainless steel A4-80 were investigated. The test included Pal®nuts of the surface finishes electro zinc plated, hot dip galvanized, phosphate and stainless steel of corrosion class A4.

4.2 Assembly test

Assembly tests were performed on joints secured by Pal®nut. The bolt was fixed in a vice and a simulated bolted joint was assembled according to DIN 7967. The time to assemble each joint was recorded. The joint assembly was repeated 5 times for the fastener property classes 8.8, 10.9, 10.9 hot dip galvanized, A4-80. Table 1. Assembly specification

Pal®nut 1. Fixation of the bolt 2. Start measuring time 3. Mount 1

st plain washer

4. Mount clamped parts 5. Mount 2

nd plain washer

6. Mount the nut 7. Tighten the nut with a torque wrench 8. Mount the Pal®nut 9. Hand tighten the Pal®nut 10. Further tighten the Pal®nut ¼ - ½ turn

using a wrench 11. Stop measuring time

Picture 2 Assembled joint

4. Technical study

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Table 1 Test matrix Tests

1.3

2.3

3.3

4.3

Bolt securing

Pal®nut

Pal®nut

Pal®nut

Pal®nut

Surface coating

Electro zinc plated

Phosphate

Hot dip galvanized

Stainless steel A4

Bolt [grade]

8.8

10.9

10.9

A4-80

Nut [grade]

8

10

10

A4-80

Plain washers

Steel

Steel

Steel

Stainless steel

Clamped parts

Steel Domex® electro zinc plated

Time [s]

Sample 1

53,0

32,0

35,0

37,0

Sample 2

44,0 34,0 37,0 30,0

Sample 3

42,0 43,0 59,0 26,0

Sample 4

38,0 30,0 34,0 29,0

Sample 5

46,0 31,0 48,0 33,0

Average

44,6 34,0 42,6 31,0

4.2.1 Summary assembly test

The zinc galvanized joints (Test 1.3 and 3.3) were the most difficult and time consuming to assembly because the Pal®nuts were difficult to hand tighten due to high thread friction. Two tightening tools were required for the tightening of Pal®nut. One torque wrench to tighten the hex nut and an open wrench to facilitate tightening of the Pal®nut. Additional parts or tools increase the risk of assembly mistakes and comprise a higher risk of dropped objects. Since the tightening instruction DIN 7967 stipulates further tightening of the Pal®nut between ¼ to ½ turn the tightening result is also operator dependent and not consistent, which may affect its locking performance.

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4.3 Vibration test DIN 65151

Vibration tests according to DIN 651 51 (Junker’s test see Picture 3.) were performed on bolted joints secured by Pal®nut. In a Junker test the joint is subjected to transverse movements while a load cell measures bolt tension continuously. The purpose of the vibration test is to compare the locking function of 4 different joint configurations secured by Pal®nut. All tests include an unsecured joint as reference.

Picture 3. Junker test principle according to DIN 651 51

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4.3.1 Vibration Test 1

Vibration conditions Standard: DIN 651 51 Duration: 30 seconds (900 load cycles) Amplitude: ± 0,9 mm Frequency: 30 Hz Bolted joint specifications: Bolt: ISO 4017 M16x2,0-60 grade 8.8 electro zinc plated Nut: ISO 4032 M16x2,0 grade 8 electro zinc plated Flat washers: Steel DIN 125 Contact surface: Steel 210HV1 electro zinc plated

Torque / Load diagram Green graph: Unsecured joint (reference test) Black graphs: Pal®nut (electro zinc plated) Five bolted joints secured by Pal®nuts were tightened according to DIN 7967 to a preload of 75 kN in dry condition in order not to obstruct the locking function. One reference joint was also tightened to a preload of 75 kN which corresponds to 75% of the bolt’s capacity in tension.

Torque [Nm]2302202102001901801701601501401301201101009080706050403020100

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration test diagram When exposing the bolted joints to the specified vibrations the following occurred. Green graph: The nut in the unsecured reference joint immediately started to rotate and the preload decreased from 74 to 16 kN. Black graphs: All nuts secured by Pal®nuts immediately started to rotate and the preload decreased from 74 kN to between 21 and 41 kN.

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[k

N]

80

70

60

50

40

30

20

10

0

Vibration test

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A sample of a bolted joint secured by Pal®nut before and after the vibration test. Markings were made to easily detect any rotation of the nut.

Picture 4 Joint secured by electro zinc plated Pal®nut before vibration test

Picture 5 Joint secured by electro zinc plated Pal®nut after vibration test

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Untightening diagram After vibration testing all joints were untightened and disassembled.

Torque [Nm]0-10-20-30-40-50-60-70-80-90-100-110-120-130-140-150-160-170-180-190-200-210-220-230

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration conditions Standard: DIN 651 51 Duration: 30 seconds (900 load cycles) Amplitude: ± 1,1 mm Frequency: 30 Hz Bolted joint specifications: Bolt: ISO 4017 M16x2,0-60 grade 10.9 phosphated Nut: ISO 4032 M16x2,0 grade 10 phosphated Flat washers: Steel DIN 125 Contact surface: Steel 210HV1 electro zinc plated

Torque / Load diagram Green graph: Unsecured joint (reference test) Black graphs: Pal®nut (phosphated) Five bolted joints secured by Pal®nuts were tightened according to DIN 7967 to a preload of 100 kN in dry condition in order not to obstruct the locking function. One reference joint was also tightened to a preload of 100 kN which corresponds to 71% of the bolt’s capacity in tension.

Torque [Nm]350300250200150100500

110

100

90

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration test diagram When exposing the bolted joints to the specified vibrations the following occurred. Green graph: The nut in the unsecured reference joint immediately started to rotate and the preload decreased from 98 to 34 kN. Black graphs: All nuts secured by Pal®nut immediately started to rotate and the preload decreased from 98 kN to between 26 and 73 kN.

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[k

N]

110

100

90

80

70

60

50

40

30

20

10

0

Vibration test

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Picture 6 Joint secured by phosphated Pal®nut before vibration test

Picture 7 Joint secured by phosphated Pal®nut after vibration test

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Torque [Nm]0-50-100-150-200-250-300-350

110

100

90

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration conditions Standard: DIN 651 51 Duration: 30 seconds (900 load cycles) Amplitude: ± 1,3 mm Frequency: 30 Hz Bolted joint specifications: Bolt: ISO 4017 M16x2,0-60 grade 10.9 hot dip galvanized Nut: ISO 4032 M16x2,0 grade hot dip galvanized Flat washers: Steel DIN 125 hot dip galvanized Contact surface: Steel 210HV1 electro zinc plated

Torque / Load diagram Green graph: Unsecured joint (reference test) Black graphs: Pal®nut (hot dip galvanized) Five bolted joints secured by Pal®nuts were tightened according to DIN 7967 to a preload of 100 kN in dry condition in order not to obstruct the locking function. One reference joint was also tightened to a preload of 100 kN which corresponds to 71% of the bolt’s capacity in tension.

Torque [Nm]450400350300250200150100500

110

100

90

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration test diagram When exposing the bolted joints to the specified vibrations the following occurred. Green graph: The nut in the unsecured reference joint immediately started to rotate and the preload decreased from 97 to 32 kN. Black graphs: All nuts secured by Pal®nut immediately started to rotate and the preload decreased from 97 kN to between 25 and 38 kN.

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[k

N]

110

100

90

80

70

60

50

40

30

20

10

0

Vibration test

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Picture 8 Joint secured by hot dip galvanized

Pal®nut before vibration testing

Picture 9 Joint secured by hot dip galvanized Pal®nut after vibration testing

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Torque [Nm]0-50-100-150-200-250

110

100

90

80

70

60

50

40

30

20

10

0

Clamp load [kN]

21


Vibration conditions Standard: DIN 651 51 Duration: 30 seconds (900 load cycles) Amplitude: ± 0,7 mm Frequency: 30 Hz Bolted joint specifications: Bolt: ISO 4017 M16x2,0-60 grade A4-80 Nut: ISO 4032 M16x2,0 grade A4-80 Flat washers: Stainless Steel DIN 125 Contact surface: Steel 210HV1 electro zinc plated

Torque / Load diagram Green graph: Unsecured joint (reference test) Black graphs: Pal®nut (Stainless steel A4) Five bolted joints secured by Pal®nuts were tightened according to DIN 7967 to a preload of 75 kN. In order to avoid galling of the threads and to be able to reach the desired preload the stainless steel joints were lubricated with Molykote 1000 (paste). One reference joint was also tightened to a preload of 75 kN which corresponds to 79% of the bolt’s capacity in tension.

Torque [Nm]200150100500

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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Vibration test diagram When exposing the bolted joints to the specified vibrations the following occurred. Green graph: The nut in the unsecured reference joint immediately started to rotate and the entire preload was lost. Black graphs: All nuts secured by Pal®nut immediately started to rotate and the preload decreased from 73 kN to between 0 and 40 kN.

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[k

N]

80

70

60

50

40

30

20

10

0

Vibration test

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Picture 10 Joint secured by stainless steel

Pal®nut before vibration testing

Picture 11 Joint secured by stainless steel Pal®nut after vibration testing

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4.3.5 Summary vibration tests

In the performed vibration tests according to DIN 651 51 all bolted joints secured by Pal®nut rotated. In several tests a joint secured by Pal®nut did not perform any better than the unsecured reference joints. This is a strong indication that Pal®nut have no or only very limited locking functionality even when assembled under strict control in a laboratory environment. Note: The vibration tests will be repeated by the independent research institute CETIM in France as validation. The test results will be added to this report as an appendix.

Torque [Nm]0-50-100-150-200-250

80

70

60

50

40

30

20

10

0

Clamp load [kN]

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4.4 Corrosion test ISO 9227

A corrosion test according to ISO 9227 will be performed for steel Pal®nut with the surface coatings: Electro zinc plating Phosphate Hot dip galvanizing Note: The results of this test will be added to the report as an appendix. The corrosion test will be performed by the research institute CETIM in France.

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Appendix 1

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Appendix 2

Study of Pal®nut DIN 7967 bolt securing · PDF fileStudy of Pal®nut DIN 7967 bolt...

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