Designer's Corner 954

Tandem lift table

The required force to lift a conventional table with an inclined hydraulic actuator varies with the lifting height. This can result in jerky movements, particularly near the collapsed position. Using vertically oriented air-spring actuators between pairs of scissors with unequal long and short arms keeps the force on the actuators proportional to the load on the table regardless of tabletop height. The result is smooth tabletop motion throughout the stroke as the air spring inflates and deflates.

According to the lever principal, the relationship between lifting height and actuator stroke is directly proportional to the length ratio between long and short arms. In turn the relationship between lifting capacity and actuator force is inversely proportional to the length ratio.

Uneven scissors-arms also provides a more compact base relative to tabletop size, and compact air-springs offer a low-starting profile relative to lift height. Additionally, initial cost of an air spring is typically half the cost of the metal-walled cylinder of equivalent capacity.

Steve Wang, 800 E. Rockland Rd., Libertyville, IL 60048; Tel: (847) 247-1266; Fax: (847) 325-2959; E-mail: [email protected]

Encoder bearing

A new encoder bearing from Fafnir, a division of The Torrington Co., is a direct descendent of the company's advanced automotive wheel-speed sensing technology. Combining bearing and sensor functions saves space, simplifies design, and reduces cost and maintenance. A magnetic pulser ring with alternating north and south poles mounts inside the bearing seal. When the ring spins, a waterproof Hall sensor provides an output signal that measures speed or position of the rotating shaft.

Two different digital outputs are compatible with most tachometers, controllers, or PLCs. SB1 provides a single-channel output from 1 to 32 ppr. SB2 has a two-channel output (similar to a 32-line digital quadrature encoder) that gives double the resolution and the ability to detect rotational direction.

Available in pillow block and flanged units (two and four bolt) configurations with setscrew shaft retention, the sensor bearing provides corrosion resistance under washdown, steam cleaning and contaminated environments.

John Hanson, The Torrington Co., Box 1008, Torrington, CT; Tel: (860) 626-3251; Fax: (860) 496-3626; E-mail: [email protected]