Power Transmission
GmbH recently introduced the Type KH1 - KH3 screw jack bevel gearboxes and ball screw spindles that can achieve linear speeds in excess of 3 m/min provided the loads are not too high. Typical applications will be in automation and handling machinery, where duty cycles can be as high as 50 per cent. The bevel gear set features hardened and ground spiral-
toothed bevel gears, lubricated with grease as standard or with oil for more demanding applications. Both moving- spindle and moving-nut variants are offered with transmission ratios of 1:1 or 2:1 (3:1 is possible in special cases). Note that brake motors or separate brakes are required with the KH gearboxes, since the screw jacks are not self-locking. Another company that offers bevel-geared screw jacks is
Duff-Norton, which points out that these units offer speeds that are four-times higher than worm-driven screw jacks and efficiencies that are higher too.
Novel alternatives
Fig. 2. Power Jacks has supplied 50 kN ball screw jacks with a stroke of 1050 mm for positioning radio telescope antenna dishes at the Cavendish Laboratory, Cambridge.
nut and a second nut adjacent to it that carries the load in the event of the main nut failing. A small gap between the two nuts also enables wear on the main nut to be measured. Another safety-related option is ‘emergency stop nuts’ or ‘full power stop nuts’ that contact the jack body in the event of overtravel - though it is generally recommended that external stops should be used, rather than relying on devices that could damage the screw jack body in the event that they have to stop the motion under full power. Screw jacks are high-precision engineered mechanisms,
yet standard designs feature an exposed screw. This can be protected in a number of different ways, including fixed protective tubes, elastomeric bellows and spiral springs, depending on the nature of the likely contaminants and the physical constraints of the equipment design. For applications in the food, beverage, pharmaceutical or nuclear industries, screw jacks can be manufactured in stainless steel. Other options and accessories typically offered for screw jacks include adjustable anti-backlash nuts, limit switches, wear indicator switches, shaft encoders and handwheels. Often the manufacturer can also supply drive shafts, couplings and other components that enable the movement from multiple screw jacks to be synchronised when only a single motor is employed (Fig. 3). An interesting option from Marzorati is multi-start ball screw spindles that offer higher efficiency and, importantly, slower screw rotation to produce the same linear speed.
Higher speeds
Most screw jacks feature a worm drive that, in combination with the small pitch on the screw, results in a relatively low linear speed. To address the need for higher speeds, Grob
Fig. 3. These four Zimm Z 2 screw jacks are all controlled by a single motor via a series of drive shafts and couplings.
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Some light-duty applications might benefit from the main advantages of screw jacks but the price of conventional products is prohibitive. For such situations, Unimec offers the Aleph series of trapezoidal screw jacks that have an attractive price/performance ratio because some of the internal components are moulded from polyaryl amide instead of being machined from metal, and the casing is also moulded from polymer. These self-locking units operate without lubrication and are available with load ratings from 7 to 18kN.
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