The enemy of mechanic fasteners is vibration. Loosening can be infuriating and at the extreme, hazardous. In terms of the user such events are seen as poor design and many other reasons that could have contributed to the failure are also blamed. Whilst the variations of bolted joints are prolific, so are the types of assemblies!

Sheet metal fasteners have been specified in considerable applications for a number of years and have proven to be acceptable in numerous industries. The resistance to vibration can be variable by application and over time loosening may occur. Some industry statistics have shown that up to 37% of applications fail due to thread striping problems, thus requiring rework and refurbishment plus the inevitable variable costs of down time and re-assembly.

The High Torque Fastener System can overcome the problems of thread striping in several material types and is flexible for design engineers to use in a variety of applications when constructing sheet metal panels or enclosures.

The Principals of the High Torque Fastener System.

The High Torque Fastener system is designed to accommodate a large range of sheet material gauge thickness. Taking into account these various material combinations, the system has an advantage over conventional sheet metal screws, which are not specifically designed to accept multiple gauge material thickness’, as well as the relevant overall stack thickness within the joint.

An added premium with the High Torque system is the anti-vibration properties that are integral within the design. The formed sheet materials when clamped endeavour to return to their original position. This has the effect of reducing the possibility of the screw disengaging, unlike a conventional cut or formed thread which requires the possible addition of shake proof washers or applied patch technology to the fastener in order to resist loosening.

The High Torque Fastener System is designed to achieve the maximum engagement of the buttress screw thread into the pre-formed sheet material anchor threaded hole of the sheet metal panel. The thread-formed helix acts as a natural lead in for the screw, eliminating any cross-threading and allowing acute angles of pressure to be applied during assembly.

The Fastening System Attributes.

The intricate feature of the system is one of addressing the efficiency of the clamped joint. To achieve this, the natural pliability of the mating materials inherently secures the loading on the joint, endorsed by the buttress screw thread.

In this assembly example, the lower jointed material panel illustrates the functional aspects of the fastener nut. Note in the lower material panel the profile of the pre-pressed material forms a threaded hole, which is a reproduction of the mating fasteners thread geometry profile. The sheet material panel acts as the nut and once installed in application, the helix profile of the mating screw ensures engagement into the panel, which provides the rigidity within the system. At this juncture it is prudent to indicate that the buttress thread form of the screw is not to an international standard, but unique to the system.

The lower sheet material panel functions as the fastener nut.

Tooling is required to pre-form the sheet material panel being clamped. Various tooling configurations provide a flexible combination for the specifier with assistance from your suppliers Fastener Application Engineers who are available to advise on design criterion. The necessity to pre-form the buttress thread helix into the lower panel material and the corresponding deflection of material to create a thread pitch, ensures that the system is secure at the nut end.

Tooling for pre-forming the helical thread form into the panel.

The rigidity aspects of the system culminate in diminishing the likelihood of loosening due to the perfected thread engagement in combination with utilising the engineered strength within the joint itself and the materials being clamped.

Material thickness and thread lengths are important design ingredients that have a relationship to thread diameter and the depth of the joint. The inherent energy within the fastened joint provides a consolidated fixture, which can successfully resist most fatigue assaults.

The drive media and fastener head types conform to international standards and a suitable range will meet the requirements of the designer.

The Major Benefits

• In-place cost savings can be realised with no requirement for reworked components due to thread stripping as the matched material interfaces equate to the thread pitch of the material panel thickness.
• Rationalising multi-component fastener assembly by reducing or discarding proliferating of clips weld nuts, captive nuts, gage nuts, inserts and pre-applied processes.
• The tensile strength of mother material is enhanced through pliable work hardening, thus improving thread stripping strengths and resistance to loosening.
• Rationalisation of sheet material thickness may be applicable in certain applications.
• In assembly the contact between the helix nut end and the fastener is increased to more than 50%, compared conventional self-tapping screws and threadforming screws.
• Low clamp force compared with traditional sheet metal fasteners due to the clamping configuration in combination with the buttress screw thread and the anchorage at the nut end.
• Excellent electrical earthing performance where required.
• Due to lower insertion torque compared to that of conventional sheet metal fastenings, Repetitive Strain Injury is not an issue.

This Case Study emphasises the on-going innovation within the fastener community; there are always new ways to increase fastener and fixing efficiency and achieve cost savings.

Submitted by Christopher Payne – Materials Engineer High Torque Fastener Systems Ltd.

For further information e-mail: info@hightorque.co.uk