Screw jacks are sometimes misunderstood and overlooked as a method of creating controllable linear motion. Jon Severn provides an overview of these useful mechanical actuators and takes a look at some recent developments in this field.
Screw jacks are an ancient concept but just as relevant today as ever before. For the right type of application a screw jack - or a set of screw jacks - can provide smooth, controllable linear motion by operating a motor, which is usually electrical though it can also be hydraulic or pneumatic. Normally a screw jack is self-locking and cannot be back-driven by the load, although this is not always the case, especially if ball screws are used. The self-locking feature is often seen as an attractive feature when replacing hydraulic and pneumatic actuators and it is necessary to prevent the load from moving once it is in position. Typically screw jacks are available with ratings of a few kilograms up to 100 kN, depending on the manufacturer and model.
'Classic' designs of screw jack have a single mounting face, but most manufacturers today offer designs that are 'cubic' and can be mounted on their other faces (Fig.1). Likewise, manufacturers generally offer two classes of screw jacks: those in which the spindle (screw) moves, and those in which the spindle rotates and a travelling nut moves. Manufacturers of screw jacks are also tending to develop modular designs that enable products to be assembled quickly from standard components so that customers' needs can be met without incurring the expense associated with fully customised units (see panel page 50).
Applications for screw jacks are extremely diverse, but the following is an indicative list: moving platforms on theatre stages; settings on wood working machinery; alignment of radio telescope dishes (Fig.2); plastics processing machinery; height-adjustable platforms in manufacturing industries; handling systems in the nuclear industry; and adjustable manufacturing equipment that enables different product lines to be processed. In most cases screw jacks are only used periodically, so care needs to be taken if high duty cycles are anticipated. For example, this is what Servomech states for four of its models of screw jack:
MA Series: high-efficiency screw jacks with acme screw, oil-lubricated gearbox, duty cycle of 40 per cent over a 10 minute period or 30 per cent over 1 hour at 25° C.
SJ Series: low-maintenance screw jacks with acme screw, grease-lubricated gearbox, duty cycle of 30 per cent over a 10 minute period or 20 per cent over 1 hour at 25° C.
MA BS Series: MA type with ball screw, oil-lubricated gearbox, high efficiency and up to 100 per cent duty cycle at 25° C.
SJ BS Series: SJ type with ball screw, grease-lubricated gearbox, and up to 60 per cent duty cycle at 25° C.
Options and accessories
To increase further the versatility of screw jacks, manufacturers offer an array of options and accessories. Often screw jacks are used for safety- or mission-critical applications in which it is essential that the load does not drop when power is removed form the driving motor. Although screw jacks are usually inherently self-locking and cannot be back-driven, there is a risk that excessive wear or other factors could cause catastrophic failure of the nut. For such applications, manufacturers offer 'safety nuts' that comprise the main load-bearing 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.
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 GmbH recently introduced the Type KH1 - KH3 screw jack bevel gearboxes and ball screw spindles that can achieve linear speeds in excess of 3m/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.
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.
Most screw jacks are connected either directly or indirectly to a motor, but Inkoma has developed an innovative model that eliminates both the separate motor and the internal gearing. Its DSH screw jacks feature a special torque motor that drives the ball screw spindle directly, resulting in smooth, backlash-free motion with virtually no mechanical losses and high energy efficiency.
Specially selected bearings ensure the spindle can withstand both tensile and compressive loads, and the unit is suitable for highly dynamic applications; seven cycles per second and a maximum linear speed of 32m/min are quoted, with 100 per cent duty cycles possible. Repeatable positioning accuracy is also claimed to be very high.
Multiple DSH units can be synchronised, with the torque motors used to control the speed and position of the ball screw spindles, and integrated brakes are available for increased safety. Inkoma offers both rotating-spindle and rotating-nut designs in five sizes, with a maximum lifting force of 1000kN. Compared with conventional screw jacks, the DSH units are claimed to offer simplified machine construction, easier installation with mounting in any position, and lower maintenance.
Each application has to be considered on its own merits, but screw jacks - and the Inkoma DSH alternative - are undoubtedly versatile. Provided they are specified correctly, screw jacks can be robust, reliable and cost-effective.
140 million standard configurations
Towards the end of 2010 Power Jacks launched the compact cubic C-Series screw jack that is available in around 140million standard configurations (Fig.4). The C-Series complements the company's existing high-performance S-Series cubic screw jack and is said to deliver new application versatility.
Bruce Hamper, Power Jacks' customer service and marketing director, says: "The C-Series is our response to an identified market need for a screw jack possessing its very specific qualities and characteristics.
"It represents the latest stage in the evolution of our product range, based as it is upon the principles of our classic E-Series screw jack that is long established and proven in the market.
"We have adapted those principles into this new C-Series cubic design, which we believe offers exciting new market opportunities.
"The C-Series is durable and adaptable and boasts a modern, sharp design. One of its key assets is the versatility it offers in terms of mounting methods, reflecting our commitment to offering customers the widest possible range of options. It is ideally placed to serve a host of industrial applications, operating individually or as part of a multi-unit jacking system."
Special features of the C-Series jacks include two screw lead options and two gear ratio options for each screw jack size, an aluminium-bronze worm gear, and lubrication via two integrated systems.
The gearbox housing is made of either highly durable spheroidal graphite (SG) iron or aluminium, providing a robust unit that firmly and accurately holds the gear set in a reservoir of lubricant.
The worm gear set is of a proven design commonly used across Power Jacks' portfolio of screw jacks, and there is forced grease lubrication of the lead screw. Standard C-Series units are complemented by anti-backlash, anti-rotation and safety nut versions, and they are available in four standard capacities from 10kN to 100kN.