Developed by engineers at Ford, the Variable Usage Screw’s benefits go well beyond automotive assembly.

Like mosquitos buzzing around their ears, hoop stress has consistently annoyed engineers who work on products with thermoplastic parts. Hoop stress occurs when a boss – the enlarged part of a shaft to which a wheel or gear is keyed – undergoes expansion and contraction. This can result in high stress fields and the cracking or failure of the joint. As a result, the quality and safety of the product is compromised.

In an effort to combat the effects of hoop stress, engineers at Ford Motor Co. have developed a new type of screw – the Variable Usage Screw (VUS) – that matches the interior draft angle of an injection molded boss. The threads have an optimized pitch and root that maximizes engagement for both a plastic boss, which Ford uses for a variety of its interior automotive parts, and a metal clip nut. The new screw accomplishes this feat because it contains both tapered and non-tapered threads in its design. The tapered threads on the lower part of the screw penetrate the plastic boss and the non-tapered upper threads ensure top performance in the metal clip nut.

This duality gives the VUS many advantages. For example, stresses are evenly distributed because the screw taper matches the inside draft angle of the boss. Even distribution of stresses results in fewer premature boss failures through cracking or splitting. The non-tapered portion of the screw provides the torsional strength necessary for use in metal nuts. The VUS brings together the best threading attributes of both plastic and metal tapping screws. As a result, it can replace several screw designs at once, reducing the complexity of a product’s design or even making it possible to use one torque specification for the whole job. In addition, the VUS’s tapered threads reduce injection-molded boss hoop stress by up to 25%. This means that engineers can now design these pieces with thinner wall-stock, achieving greater design and styling flexibility.

The VUS is a good replacement for small-diameter screws. The torsional strength of most small-diameter screws is pretty low, meaning that their heads may snap off when driven into a boss. The VUS has the largest minor diameter for a plastic-tapping screw and simultaneously provides the highest torsional strength and lowest driving torque. The VUS also provides a low radial force and an increased thread shear resistance that reduces both driving and thread forming torques, making the fastening of the screw an easier and more ergonomic operation.

One of the VUS’s biggest advantages is that it is capable of more repeat assemblies than any other type of screw – typically over 10 times. It is also ideal for plastic and light-alloy parts because it requires less torque for installation while providing high stripping torque. In addition to reducing internal hoop stress by 25%, the VUS provides 20% more overall strength and 20% higher strip torque.

Best of all, the VUS saves its users money on several accounts. First, it has a higher first run capability with more repeat assemblies for both polypropylene and magnesium materials than any other screw design. Also, due to prior field concerns, there is an anticipated warranty reduction because of the tapered root diameter that reduces boss hoop stress. Furthermore, unlike other screw designs, a uniform boss hole-size can be used with the VUS for several materials, a fact that reduces engineering time and complexity. VUS can bring savings by reducing tool maintenance, operator error and tool set-up time. And, since no special operator training or tooling is required to use the variable usage screw, it helps to increase assembly line efficiency. Users also save money because the VUS is less expensive than the screws that are currently recommended for plastics, magnesium and aluminum. Finally, the user company will realize savings simply because the Variable Use Screw will help prevent cracked and broken bosses that prove costly in the long run. The VUS truly is a screw of all trades.

Though Ford has both the U.S. and European patents for the VUS, they have licensed the technology to several fastener manufacturers. Since the VUS can be used in any product that requires screws to be driven into aluminum, magnesium, zinc or plastic, you can expect to see their use increase more and more frequently in the future. Moreover, the VUS will soon be designed in a variety of thread sizes for use in applications that currently require insert nuts, “U” nuts, “J” nuts, and boss attachments of any one of the aforementioned materials. VUS technology can be applied to any fastening joint where screws thread into materials or nuts. Also under development are VUS machine-screw thread forms and thread-forming screws.

Source: Fasteners: Old Problem, New Screw
Al Dapoz
Assembly Magazine, Dec. 1, 2001
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