Ian Carr explains the key considerations that should be made when specifying screw jacks
Screw jacks are often the perfect solution for precise linear motion and positioning applications, they can even be supplied as a bespoke system complete with their own dedicated control system for particular applications. However, without proper consideration during the specification stage, it’s possible to miss some of the design advantages that the latest technology offers.
Unlike the cheap and imprecise car jacks that most of us have used when changing a flat tyre, engineering screw jacks are based on a direct screw drive mechanism. They are precision motion systems capable of lifting large loads and holding position to a high degree of accuracy. With this in mind they can’t be considered a ‘one-size-fits-all’ solution.
Typically available with cast aluminium or iron bodies and a choice of either trapezoidal or ballscrew spindles, the dimension, speed, load capacity and precision capabilities are so varied that no two screw jacks are alike. Jacks are mainly motor driven (though manual operation is often also allowed for) with the type and power of the motor matched to specific application needs. It’s also possible to specify unique control components which can be integrated into automated control systems.
As with many power transmission components, there are also multiple accessories available for screw jacks which enable them to perform specialised functions for different applications.
While screw jacks are relatively simple technology, with such variation it’s clearly important that engineers understand the different options available to them in order to secure optimal performance once installed. Working with an experienced distributor will help to ensure that all considerations are made so the right product is specified first time.
First and foremost, it’s important to decide whether the mechanism should be rotating or translating. With a rotating unit the spindle stays in a fixed position and rotates, causing the load-carrying nut to move up or down. A translating unit is the opposite of this as the nut stays still and the spindle moves through it. Often the nature of the application dictates which mechanism is required; though when designing-in a translating unit, space must be allowed to accommodate the spindle in both its extreme positions.
The decision between a trapezoidal or ball screw and a worm gear or bevel gear is not quite so ‘open and shut’. Much of the decision comes down to individual requirements in terms of load and speed requirements, as well as the expected duty of the application once installed. A good supplier should be able to offer guidance and explain how the different combinations may complement the requirement of your application in different ways, helping you to source the most efficient solution.
As with any equipment involved with the movement of potentially heavy loads, safety must be considered from the outset. Buckling is a very real hazard if a screw jack has not been properly specified; the mechanics of buckling are related to the material, length and diameter of the spindle, as well as the load it’s carrying. For guidance, manufacturers use a system called the Euler Number which helps engineers to work out minimum safety figures.
A further safety consideration is that some screw jacks are self-sustaining - meaning they can hold position without a brake – while others are not. It’s important to understand this when specifying a jack. Trapezoidal spindles are normally self-sustaining but vibration and shock loads may compromise this ability. Ballscrew spindles are not self-sustaining and must be braked if used vertically. As with all safety issues, it is probably best to seek expert advice.
A final comment needs to be made on sizing and selecting a drive motor for a screw jack. Each project is unique and deserves individual attention to be paid to the motor selection. Usually a standard induction motor is used, but there may be occasions when a servo or other motor type is a better option. The formulae for motor selection are generally simple, and these should be calculated using the dynamic load rather than the static load. The screw jack supplier should be happy to help – and they should always recommend a slight oversizing of the motor to account for unexpected friction, misalignments, increased load, shock loads, and adverse operating conditions.
When specified correctly screw jacks should be a simple and reliable solution for precise linear motion which can be used across many industries. However, like all power transmission components there are a lot of unique variables for almost every application, all of which must be considered to come to a safe and reliable solutions. This article has only touched on some of the more important aspects of screw jack specification – for more advice speak to your supplier.
Ian Carr is Managing Director, Drive Lines Technologies Ltd.