Conical Wedge Lock Washer Testing

Introduction

Our Quality Management System is tightly integrated into our Product Design process and we are bound by our QMS that complies with 8.2 to 8.5 of the ISO9001:2015 standard.

The Junker test is the established method used for analysing the spontaneous self-loosening behaviour of secured threaded fasteners which make up a bolted connection under transverse loading conditions caused by vibrations. The engineer, Gerhard Junker, pioneered research into the behaviour of bolted connections under transverse loads, and his original methodology in this paper has been adopted into the international fastener standard, DIN 65151 - Dynamic testing of the locking characteristics of fasteners under transverse loading conditions. The DIN 25201-4 B, published in 2010, supersedes DIN 65151 and is significantly more rigorous.

The purpose of the tests is to determine and facilitate a comparative assessment of the self-loosening behaviour of bolted joints subject to dynamic transverse stress under defined test conditions. The comparative assessment is made possible by comparing the results of:

1. reference tests using an unsecured bolted joint.
2. verification tests using a bolted joint secured with the aid of a securing element

Reference Test

To start the reference test, the unsecured bolted joint (i.e., the fastener without any fastener locking element) must be placed in the test bench. Starting from zero, the displacement is gradually increased until the point at which the fastener completely self-loosens after 300 load cycles, ±100 load cycles. Once this efficient displacement has been discovered, three subsequent control tests with fresh components are required to ensure that the initial reference test results are consistent (see graph below).

Verification Test

The next stage is to conduct the verification test. The exact same test conditions are set, including the same effective displacement. Then, the locking product is introduced, and the secured bolted joint is tested to determine whether and at what point the secured bolted joint starts to loosen. Several verification tests must be made in order to ensure the test reproducibility (See graph below)

Junker Test Bench

The testing of our Conical Wedge Lock Washers proved to be much more challenging than we had initially assumed it would be. We thought that either, our existing aerospace and mining clients or the independent industry testing laboratories, would have well established testing facilities and methodologies that we could leverage. For a variety of reasons, not least of which was the large bolt size which we needed to test (up to M110!), we had to develop our own testing facilities and methodologies ourselves. This took us well over a year to design, develop and implement. The testing of our M10 to M27 CWLWP (Conical Wedge Lock Washer Pairs), has been done on the apparatus shown below.

Testing Parameters

Junker Test Parameters
M10 thru to M36

Ref	Bolt Size	Class	Preload	Lubrication	k-factor	Torque	Clamp Length	Clamp Ratio	Disp	Freq
1	M10	8.8	27 kN	dry	0.258	69 Nm	40 mm	4	3mm	6Hz
2	M16	12.9	122 kN	greased	0.206	401 Nm	70 mm	4.375	3mm	5Hz
3	M24	10.9	234 kN	oiled	0.153	1158 Nm	100 mm	4.7	4mm	3Hz

Video Evidence