Power Transmission
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.
Les vis de calage sont parfois négligées et confondues avec un moyen de créer un mouvement linéaire contrôlable. Jon Severn propose un aperçu de ces vérins mécaniques utiles et observe quelques uns des récents développements dans ce domaine.
Schraubengewinde werden manchmal nicht verstanden und als Methode zur Erzeugung eines linearen Bewegungsablaufs übersehen. Jon Severn bietet einen Überblick dieser nützlichen mechanischen Aktoren und betrachtet einige neue Entwicklungen in diesem Feld.
Screw jacks deliver benefits for diverse applications
S
crew 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
Fig. 1. Unlike ‘Classic’ designs of screw jack, ‘cubic’ types such as the Power Jacks C-Series offer a choice of mounting faces.
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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
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