Specifying linear motion systems

It doesn't help that the terminology can be confusing. Linear systems use a number of methods to achieve linear motion, but the main methods are belt-driven and ballscrew-driven linear actuators. So what do you need to consider when starting out? "System configuration, including the number of axes of motion, is often the first factor that needs careful thought," says Mike Hughes, an applications engineer at Schaeffler UK. "The most common are two-axis (X-Y) configurations, but single-axis and three-axis configurations are also possible. "System orientation and mounting are also key factors. In a single-axis linear system this is fairly straightforward, but in a multiple-axis system it becomes more complex. Factors to consider include: the direction of travel in each axis; whether the load need to be moved simultaneously in multiple axes or if each axis moves individually; and does the system require a standard moving carriage or a moving rai? Also, consider if the axes are vertical, horizontal or inclined and if the mounting positions of each actuator is 0, 90 or 180° to the horizontal." The mass and geometry of the object to be moved and the position of its centre of gravity as it moves relative to a co-ordinate or datum point on each axis must also be calculated. Clearly, as a mass is accelerated or decelerated along multiple axes of travel, the position of its centre of gravity relative to each axis will change. This needs careful consideration so that the moment loads at multiple points in the system can be established. Calculating the best and worst-case scenarios using specialist design calculation software – such as Schaeffler's Linear EasySolution – is sufficient for most applications. "The effective and total stroke length for each axis is also critical," says Hughes. "With ballscrew-driven linear actuators, for example, the stroke length is limited to the length of the ballscrew itself. Therefore, maximum stroke lengths tend to be around 3m. But with belt-driven systems, there are no such restrictions and stroke lengths can be as much as 20m if required. If linear motors are specified, in theory, stroke length is unlimited. However, in reality lengths above 10m are rare." Accuracy and repeatability will differ greatly depending on the application. As a rule of thumb, typical off-the-shelf accuracy of a ballscrew-driven linear actuator is 0.16mm per metre with repeatability of +/- 0.01mm, and for belt-driven actuators typical accuracy is around 0.5mm per metre, with repeatability of +/- 0.10mm. The true limitation for traverse speeds and times is essentially contained in the bearings of the ballscrews. Typically with ballscrew-driven actuators, maximum speed is around 3m/s. For belt-driven actuators with track roller guidance systems the maximum speed can be as much as 8-9m/s. "Acceleration itself is not normally a defining issue in multi-axis positioning systems," says Hughes. "It is the load due to acceleration that is critical. The highest acceleration of any linear actuator to date is around 40m/s2 although typically accelerations are likely to be less than this, often between 0.5m/s2 and 5m/s2. Deceleration is also important, particularly if emergency stops are required." Additionally environmental factors such as temperature, humidity and contamination – i.e. dust, oil, water, washdowns, chemicals and coolants – will also affect the choice of linear systems. A dusty working environment may require external bellows or dust extraction devices. Linear actuators can be protected from the environment by incorporating special seals, corrosion-resistant materials and coatings, special greases or by using plastic parts where necessary. Additionally, for some applications the overall noise of a system may be a factor that needs addressing. A lower-speed linear actuator may be a solution here, but if high speeds need to be maintained, special components, materials or coatings can be specified in order to keep noise levels to a minimum. As with Schaeffer, green engineering is an important element of the development work at NSK. It has increasingly had customers demanding low-noise for linear systems as well as a number of other key attributes such as ultra-steady running, high precision and ever-increasing output. Machine manufacturers often use ballscrews when linear movements have to be performed with a high degree of precision and quiet running. NSK produces miniature linear drives with a spindle diameter of just 4mm for hand-held medical applications to larger high-load drives for injection moulding machines with clamping forces of several hundred tonnes. As ever more specific requirements are being made of drive systems, the working relationship between manufacturers and users is primarily changing in situations where high-end solutions are needed. Like Schaeffler, NSK is seeing an increase in joint development projects with machine manufacturers. The company collaborated with a manufacturer of servo drives and a producer of injection moulding machines to develop a drive system for an ejector unit that is capable of extremely high dynamic axial forces. This utilises heavy-duty spindles from the HTF series, which feature an optimum ratio between the balls diameter and a patented ball guidance system that prevents direct ball-to-ball contact. Together with the material specifications for the nut, ball and spindle, these design features pave the way for very high speeds and peak-load capacities. Bespoke systems have also been developed for handling technology and related areas. A producer of an electric drive system used an NSK ballscrew at the heart of its highly stressable servo-electric linear drive to replace an existing hydraulic drive. Here, the environmental footprint was a key criterion along with precision. Users of ball screws and linear guides want more from their suppliers than just a broad portfolio of high-quality, durable products. They want engineering support when the catalogue range does not fit their specific needs. They are also increasingly looking at the environmental impact of drive systems and taking this aspect into account when machines and plants are developed.