Hoop stress has been around since the days when wagon wheels and wooden barrels were assembled by hand. It’s still a nagging problem facing engineers, especially in products that contain large amounts of thermoplastic parts, such as automotive interiors.
If undue hoop stress occurs on a boss undergoing expansion and contraction, it will lead to high stress fields, resulting in cracking and failure of the joint. When that condition occurs in a vehicle, it can cause annoying squeaks and rattles that often lead to higher warranty costs.
To minimize hoop stress, manufacturing engineers at Ford Motor Co. recently developed a tapered screw that matches the interior draft angle of an injection-molded thermoplastic boss. This new screw technology is currently used to assemble six different vehicle lines, including popular models such as the Explorer, Taurus and Windstar.
Injection-molded thermoplastic bosses are used on a variety of interior automotive components. Instrument panel substrates, interior trim, floor consoles, package trays, cowl vents, side seat shields and knee bolsters are just a few applications. To prevent damage, these components must be robustly designed to survive extreme environmental conditions.
Evaluation vehicles at Ford’s Arizona Proving Ground (Yucca, AZ) experience interior temperature variations ranging from 151 F for below-the-beltline trim to as high as 237 F for package trays. Instrument panels can experience surface temperatures ranging from 140 to 221 F. These large variations in temperature can change the physical properties of thermoplastics.
Temperature changes lead to expansions and contractions, depending on the materials’ coefficient of thermal expansion. With a subsequent reduction in the storage modulus, this condition ultimately results in loose fitting joints. In addition, if there are excessive hoop stresses on a boss undergoing thermal expansion and contraction, a high stress field will result. A crack will form to relieve the stress, and failure of the attachment is the end product.
Bold New DesignA team of Ford engineers developed a tapered screw design, dubbed the variable usage screw (VUS), which 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 and a metal clip nut.
The VUS contains both tapered and nontapered threads in one design. A tapered lower section provides even thread penetration inside a boss. A nontapered upper section provides optimum performance in a metal clip nut. It is the first screw that was purposely designed to be used in:
- A plastic boss of any modulus ranging from unfilled polypropylene to styrene maleic anhydride (SMA) with 20 percent glass.
- ST4.2 x 1.41 spring steel single-threaded nuts.
- Low modulus metals such as magnesium, aluminum or zinc.
By having tapered and nontapered threads, the VUS offers 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 nontapered portion of the VUS provides the torsional strength necessary for use in metal nuts.
The VUS offers optimum thread characteristics of both a plastic and metal tapping screw in one thread profile. It can replace several screw designs used in vehicle interiors, thereby reducing design complexity and commonizing assembly to one torque specification.
The tapered features of the fastener reduce injection-molded boss hoop stress by 25 percent. The reduced hoop stress allows engineers to design injection-molded bosses with thinner wall stock, preventing sink marks from appearing on class “A” component surfaces, leading to more design and styling flexibility.
BenefitsThe VUS is particularly beneficial with small-diameter screws—3 millimeters or smaller. The torsional strength of most small-diameter screws is low. That results in a high incidence of head shear when driving a screw into a boss. For instance, the screw head may snap off.
The VUS has the largest minor diameter ever used on a plastic-tapping screw. At the same time, it provides the highest torsional strength and the low-est driving torque. Re-search conducted by Ford’s engineering laboratory confirms this.
The thread design of the VUS provides several benefits:
- Low radial force—a force com-ponent that causes increased joint relaxation.
- Increased thread shear resistance allowing lower driving torques.
- Reduced thread forming torque, which also reduces the driving torque—an added ergonomic benefit.
Compared with competitive screws, the VUS has 20 percent more ultimate strength, 20 percent higher strip torque and 25 percent less internal hoop stress in a plastic molded boss.
ApplicationsThe VUS can be used in any product in which a screw is driven into aluminum, magnesium, zinc or plastic. It is currently being used at Ford to assemble steel steering column support brackets, plastic head lamp assemblies, die-cast transmission housings, magnesium and plastic air induction assemblies, and door trim panel assemblies.
The VUS thread form will eventually be designed in numerous thread sizes and released in applications where insert nuts, “U” and “J” nuts, and boss attachments of any material including plastic, magnesium, aluminum and zinc are used.
The technology can be applied to any fastening joint where screws thread into materials or nuts. The thread form is efficient across all materials and flexible enough for commonization of several fastener types into one VUS thread form. Other technologies under investigation include machine-screw thread forms and thread-forming screws.
Ford and Visteon assemblers install 25 million variable usage screws annually. The VUS is used to assemble components of several different vehicle lines, such as the instrument panel driver-side lower steering column cover of the F-Series pickup truck; the door trim panel of the Taurus and Sable sedan; the rear entertainment video screen of the Windstar minivan; the instrument panel of the Explorer and Mountaineer sport-utility vehicles; the center console of the Expedition and Navigator; and the polypropylene glove box door-to-bin attachment of the Excursion.
Cost SavingsThe variable usage screw has numerous cost savings. Nonfinancial benefits include reduced manufacturing complexity and material handling, improved customer satisfaction because of reduced squeaks and rattles, and increased dealer serviceability. Additional cost savings are attained because:
- The VUS fastener has demonstrated a higher first run capability with more repeat assemblies (serviceability) for both polypropylene and magnesium materials than any other screw design.
- Based upon previous field concerns, there is an anticipated warranty reduction because of the tapered root diameter that reduces boss hoop stress. Other warranty reduction opportunities are being identified.
- Unlike current screw designs, one common boss hole size can be used with the VUS for a variety of materials, including polypropylene, SMA, acrylonitrile-butadiene styrene (ABS), polycarbonate (PC), PC/ABS, magnesium and aluminum. That provides a reduction in both engineering time and complexity.
- Savings can be realized in assembly by reducing tool maintenance, operator error and tool set-up time.
- Assembly line efficiencies are increased, because no special operator training or tooling is required to use the variable usage screw.
The VUS fastener is less expensive than current screws that are recommended for plastics, magnesium and aluminum, while still offering many performance advantages. That translates into savings of $450,000 per year.
When the 2000 F-150 lower steering column finish panel material changed from PC/ABS to polypropylene, only the VUS screw met the assembly, molding and performance requirements of the boss material. That resulted in savings of $100,000 per year.
In North America, warranty cost for fasteners in interior systems is $4 per vehicle. In Europe, warranty cost is $0.80 per vehicle, based upon 1 year of warranty. Use of the VUS is expected to reduce Ford’s worldwide warranty expense by a minimum of 13 percent. Multiplied over 4 years and a worldwide automotive volume of 7.2 million vehicles per year, that translates into a savings of $18 million.
Another area for cost savings can be found in quality improvements based upon the following analysis: 3.5 million North American vehicles assembled per year x 450 interior fasteners per vehicle per year = 1.6 billion fasteners per year. With an average cost per fastener of $0.027, the total annual cost for interior fasteners in North America is $43.2 million, which translates into $88.9 million on a worldwide basis.
The VUS quality improvements, complexity reduction, improved serviceability and final cost will provide a minimum 7 percent cost reduction, yielding a savings of $6.2 million. These advantages will result in fewer cracked and broken bosses leading to increased high mileage durability, reliability, quality and customer satisfaction.
The variable usage screw is currently only used by Ford and Visteon. The fastener may look simple in design, but it was engineered to be a truly variable usage screw. One screw can be used in many different ways. The VUS replaces four different screw types that were used in the past.