Constant-Torque Hinge Adds Life To Computer Display

St. Paul, MN -When engineers recently needed a hinge to support a detachable computer display, they faced a vexing dilemma. The detachable display required them to design a hinge smaller than those used in most laptops, without a loss of consistency, durability, or torque capacity.

Their solution: A tiny, constant-torque, wrap-spring hinge that measures only half an inch long, but offers roughly 100 times more life than other designs. Displays on the Compaq Concerto and the LTE Elite computers employ the new hinge.

Key to the new hinge technology is an enclosed design that combines long life with small size. Developed by engineers at Reell Precision Manufacturing (RPM) Corp., the new enclosed design virtually eliminates two common modes of failure: failure due to excessive bending stresses and failure caused by loss of lubrication.

By virtue of its enclosed configuration, the new design solves both problems. During operation, an outer case surrounds the spring's lubricating grease, thus preventing it from drying out. The outer enclosure also captures the spring itself, which, in turn, dramatically reduces the bending stresses at or near the spring's toe. In conventional designs, the toe extends out from the spring and bears against a bracket to transmit torque from the hinge's shaft to the bracket.

In this new approach, however, engineers do not employ a conventional toe. Instead, the last half wrap of the spring functions as its "toe," then the design prevents this "toe" from lifting off the shaft.

How does this setup prevent high bending stresses? The answer is that forces enter axially as the tip of the spring pushes against a special surface on the inside of the housing. In contrast, forces enter a conventional wrap spring in a shearing fashion as the side of the toe pushes against the bracket. Using the new design virtually eliminates fatigue failure of the spring toe.

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Reduction of potential failure modes isn't the enclosed design's only advantage, however. The outer case plays a key role in reducing the hinge's overall length by eliminating the need for one extra bearing inside the unit. Unlike conventional hinges, which typically employ two bearings to support the shaft, RPM's new unit uses only a single bearing. "We only have one bearing on the end," explains Joseph Arnold, a project engineer at RPM. "Because we have such tight clearance between the spring and case, the spring itself acts as the other bearing."

Using the enclosed design, RPM engineers say they cut the hinge's length down to less than half an inch. With a conventional hinge configuration, they explain, they could achieve a minimum length of 0.75 inch, but no less. The unit also offers far longer life than other techniques examined by the Reell Precision engineering team. "We've tested it to 900,000 cycles," notes project engineer David A. Wahlstedt. "It offers approximately 100 times the life of anything but another wrap spring."

Additional details...Contact Marty Kottke, Reell Precision Manufacturing Corp., 1259 Wolters Blvd., St. Paul, MN 55110, (612) 484-2447.

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