HEICO LOCK? Wedge Lock Washers

The HEICO-LOCK? Wedge Locking System delivers high quality anti-vibration security for the most demanding of bolted joint applications.

In an era of optimised construction with less volumes and material inputs, in accordance with the principles of lightweight design, reliable securing methods for fasteners have become ever more critical. With space limitations resulting in smaller clamping lengths, even high strength bolted joints are susceptible to self-loosening. With system operations causing high dynamic shear loads, specifically high vibrations, bolted joints can lose preload by self-rotating, which can lead to system failure or significant danger to life and limb.

HEICO-LOCK? Wedge Lock Washers prevent bolted joints from self-loosening by rotation even under the most extreme vibrations. HEICO-LOCK? Wedge Lock Systems are used in many fields of applications, because vibrations and dynamic loads occur in nearly every industry.  Whether it be oil and gas, railway, agriculture and forestry, automotive or renewable energy sector, safety and reliability in case of bolted connections are always important.

Since 1900 HEICO has been providing high class fastening solutions. Starting with the production and distribution of upholstery and decorative nails, mainly for the furniture industry, the range of products has extended steadily over the intervening years. In the early 1990s HEICO began manufacturing design-specific cold formed parts and moldings, mainly for the automotive sector. After passing on the management of the family owned company to the fourth generation, the product range was extended significantly by the introduction of HEICO-LOCK? Wedge Lock Washers.

Today the company has approximately 400 employees across two production plants in Germany and eleven worldwide distribution subsidiaries.

There are a wide variety of bolt locking devices available on the market but unfortunately not all of them meet the basic requirement of reliability preventing bolted joints from self-loosening. The locking performance of fasteners under vibration, specifically locking devices, can be evaluated on a vibration test rig according to DIN 65151 known as the ‘Junker’ test after it’s inventor, Gerhard H. Junker.

The renowned German Institute for Standardisation (DIN) conducted a program of these ‘Junker’ tests and as a result in 2003 withdrew several standards referring to locking devices. A number of devices were verified as not applicable in preventing high strength bolts from self-loosening, including spring washers (according to DIN 127), toothed lock washers (according to DIN 6797) and locking plates (DIN 93). Locking devices would now be catagorised according to their locking performance as either reliable ‘bolt locking’ or ‘loss preventing’ device.

While reliable ‘bolt locking’ devices prevent self-rotation in the bolted joint during vibration, as well as obtaining high preloads, ‘loss prevention’ devices are only preventing the nut or bolt from falling off after the preload is mostly lost.

Schematic preload curves of bolt locking devices during the vibration test (Junker test)

An example of a ‘loss prevention’ device is a prevailing torque type nut (DIN 980 and DIN 985). The HEICO-LOCK? Wedge Lock Washer is then categorised as a ‘bolt locking’ device. A detailed overview was included in DIN 25201 part 4 in table A13.

In response to these changes a number of surveys were undertaken by independent research institutes to test the ‘bolt locking’ effectiveness of the HEICO-LOCK? Wedge Lock Washers. A vibration test rig was used, according to the specifications of DIN 65151 (Junker test), and the tests were undertaken according to the specifications of E DIN 25201 part 4 appendix B4.

HEICO-LOCK? Wedge Lock Washers met the E DIN 25201 requirements of maintaining a residual preload >80% after an alternating transverse load, representing the effects of vibration, of over 2,000 load cycles.

The outstanding locking performance of HEICO-LOCK? Wedge Lock Washers is due to their functional principle:

Functional principle of HEICO-LOCK? Wedge Lock Washers

The remarkable property of HEICO-LOCK? Wedge Lock Washers is that they positively lock fasteners using tension instead of friction. The HEICO-LOCK? system consists of a pre-assembled pair of glued washers that have inclined cams on the inside and radial teeth/ridges on the opposite sides (the use of the glue is simply to ensure the correct position of the single washers in relation to each other before the first assembly and is not part of the actual functional principle). Tightening of the bolt and/or nut leads to a gripping function of the teeth thus seating the mating surfaces (the surface hardness of the washer is greater than that of structural grade and high-tensile bolts).

Tightening lock the HEICO-LOCK? Wedge Lock Washer in place (Image A). Due to the low friction between the single washers movement is only possible across the faces of the cams on the inner side (Image B). As the wedge angle of these cams (o) is greater than the angle of the thread pitch (?) any movement of the bolt increases the thickness of the washers as the cams move across each other (Image C). When the bolt rotates in a loosening direction this cam movement increases the preload force and blocks the self-loosening action (Image D).

Bibliography:

? DIN 65151: Aerospace series - Dynamic testing of the locking characteristics of fasteners under transverse loading conditions (vibration test). August 2002

? JUNKER, G: New criteria for self-loosening of fasteners under vibration, transactions of the Society of Automotive Engineers, Vol.78, 1969. pp. 314-335, New York

? DIN 25201-4: Design guide for railway vehicles and their components - Bolted joints - Part 4: Securing of bolted joints. June 2004

? E DIN 25201-4 Appendix B: Design guide for railway vehicles and their components - Bolted joints - Part 4: Securing of bolted joints - Appendix B. Draft version. March 2010

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