PECM Issue 23 2016 - Page 99

be synchronised to work together, and that their screw operation can be used to amplify force. Essentially there are two types of screw jack: translating, in which the lifting spindle is driven axially through the gearbox, and rotating (or travelling nut), where the spindle is axially fixed and rotates to drive a travelling nut fixed to the load. The former is generally the more economic option, although with longer strokes the spindle may protrude through the bottom of the gearbox when retracted which can be a disadvantage. Unless fixed securely to the load, the spindle may rotate under friction when lifting, therefore an anti-rotation measure may be necessary. The latter, rotating type, ensures there is no protrusion below the gearbox - though the free end of the spindle will normally need securing by means of a flanged bearing. Depending on the duty cycle or lifting speed required, a trapezoidal spindle or ball screw spindle can be selected. Trapezoidal spindles have the great advantages of being simple, robust, reliable, low cost and inherently selflocking. However, the greater expense of a ball screw can be justified. Because frictional losses are reduced significantly, they can operate at greater speeds and are highly efficient, so will require smaller motor drives. Ball screw jacks tend to be specified when frequent actuation is expected while trapezoidal screw jacks are favoured for systems that will be used less often. However as ball screws are not normally self-locking, a brake motor must be used in all cases to prevent back driving. Selection of a screw jack for a given application may require some thought and Drive Lines will help identify the best choice. We will also help calculate the optimum size, typically a unit that is big enough to cope with likely shock loads but not so big that it requires excessive power. We will often sit down with the client and discuss the optimum solution – for example if travel speed is not important then a smaller motor may be specified, saving cost for the client. Screw jacks are usually driven by a standard industrial AC motor, to which it is coupled by a shaft, couplings and gearheads, as appropriate. However, other types of motors could also be used, such as: DC in remote locations where an alternating current supply is unavailable; compact servo in spacelimited applications; air or hydraulic motor in potentially explosive atmospheres. The configuration of the mechanical drive elements is often dictated by the physical layout of the system. Drive Lines always likes to carefully scrutinise this element of the design project; noting that simplicity is always better than complexity because it aids efficiency and reliability. It also looks for ways to design out possible failure modes: for instance an overhead system that pulls the load up is preferable to one that pushes from below because it is in tension rather than compression, therefore preventing buckling of the spindle if overloaded. Issue 23 PECM 99