The past, present and future of linear motion technology

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Denis Eckstein of Hoerbiger-Origa discusses the development of engineered linear motion, from Victorian hydraulics to rodless cylinders and belt-driven actuators.

Today you could stock a supermarket with the huge choice of linear motion products that are available from the 250 or so suppliers operating in the UK. But it wasn't always like this. It is sometimes difficult to appreciate that most of these systems have been developed within the career span of many engineers working today. And it is perhaps even more difficult to imagine what life would be like without them.

In the early 1970s man was regularly walking on the moon and Concorde was promising supersonic civil aviation; computers had become cheap enough for major blue chip companies to be able to buy one of their own, and television transmissions were in colour. Technology was on the march in every sector. Yet if you wanted to create linear motion, you had to engineer the system yourself! This was very expensive and required that considerable expertise be available, but simpler ways to create linear motion were needed more and more.

The roots of linear actuators can be traced back to the great Victorian civil engineers, who used mechanisms to open bridges, locks and boat lifts. These were hydraulic, using water, usually the natural flow of a river or tide as the motive force. By the standards of the day, they were incredibly accurate and fast, and they proved engineering principles that are still useful today.

Moving on several decades the first recognisably modern linear actuators appeared as oil hydraulic expandable struts. An early use of these was in mechanical diggers and farm equipment, helping to 'automate' the boom industries as Britain rebuilt and remodelled itself after the Second World War. They offered tremendous power from a compact size and redrew the definition of positional accuracy.

A generation after the war, the manufacturing industries needed to improve productivity and started automating various processes. Many of these required precision and/or high speed linear movements and hydraulics provided a ready answer.

Exploring alternatives

However, hydraulics was expensive, heavy, complicated and required considerable maintenance. Many potential applications could not justify the use of hydraulics and alternatives began to be explored.

An obvious alternative to hydraulics is pneumatics, compressed air driving pistons in cylinders. Originally pneumatics were used only in a 'bang-bang' mode, that is two set positions, fully retracted and fully extended, with travel between the two being relatively high speed and uncontrolled. It was a fairly obvious thought to develop a valve that let the air into and out of the cylinder slowly, but this did not give precision speed control or intermediate positioning.

It was not until the 1990s that such a performance was perfected, and by that time other products were available. Therefore pneumatic positioning has never achieved much more than niche status: it is useful only for applications with light loads and requiring moderate accuracy of positioning, and preferably when there is already a pneumatic system in place.

One of the great leaps forward was the development of the rodless pneumatic cylinder in the early 1980s. This was a reconfiguration of the basic cylinder, using a radial yoke instead of a longitudinal rod to connect the piston to the load.

So that the yoke could move back and forth a slot was formed in the wall of the cylinder and sealed using a patented sliding seal. The main advantages of the rodless cylinder were its compactness and ease of installation, as it was not necessary to allow room for the full extension of the rod, Also, its ability to directly carry a load without the need for additional bearings and structures. It was the rodless cylinder that spurred companies to develop positional controllers for air powered devices.

Braithwaite's work on electric linear motors, although diminishing, did lay the foundations for the development of many different types of linear motor now used throughout industry. (Braithwaite's legacy is not entirely lost; there are now several Maglev train systems around the world and British politicians are once again showing interest in developing a high speed national network.) Given that a linear motor is basically a reconfiguration of a conventional motor, it is not surprising to find that they can be divided into induction, stepper and servo variants.

Servos are perhaps the most commonly used linear motors in Europe; with their high load capacity and high speed operation they are generally used as precision axes in manufacturing and assembly cells. In Asia, linear steppers are commonly used for component placement in electronics assembly operations.

Linear induction motors have not found a great deal of favour in industry, partly because of cost and partly due to innate conservatism for proven induction technologies. There is another form of basic linear motor that owes much to solenoid technology; this has found some favour in light, simple, highly repetitive duties.

Main thrust

But for all their advantages, these technologies were still wanting in many respects. Simpler solutions were needed at lower cost, so designers looked to machine tools which were using ball screws, lead screws and similar mechanisms to traverse headstocks etc. At this time, machine tool engineers were busy redeveloping traditional lathes, milling machines etc into multi-functional machining centres that could produce finished pieces in a single operation. But design engineers realised that the screw mechanisms were purely mechanical, simple and cheap and therefore attractive for general linear motion duties.

However, a bare ball screw still needed skilled designers and engineers to install them and maintenance would be a problem. There was also considerable expertise required to select the best screw for a particular application. There are several variations of the basic screw mechanism, each with its own strengths and shortcomings, while the bearing surface of the mechanism has to be matched to the load characteristics.

Even with these issues, ball screws were increasingly vital in the ever-more automated world of the 1980s and 1990s. However, the need to simplify their specification and installation became critically important as engineering budgets were squeezed due to global competition.

By this time pneumatics suppliers had long since standardised their actuators' mounting arrangements so that they offered users plug-and-play installation, and it was decided to apply the same logic to mechanical equivalents.

Although this sounds straightforward, considerable effort had to be put into designing the internal linear drive mechanisms: each type of drive had to be configured to fit within the standardised housings and be easy to assemble so that users could specify exactly what they required. The Millennium had turned before most linear actuator manufacturers could claim to have a comprehensive and rational range of actuators.

It was also realised that belt-type actuators would be required as well if a range was to be truly comprehensive. Belt drives are cheaper and faster than screw drives (though not as accurate or powerful), so were attractive for many potential applications.

Manufacturers are now turning to the development of a second generation of actuators, with new formats and features to meet the emerging needs of a twenty-first century industry.

For example, Hoerbiger-Origa's new generation BHD series includes: a low-profile belt actuator for use in restricted space applications; units for high-accuracy, extreme-load, high-speed and very-long-stroke applications; position options for the drive motor; and units for demanding environments such as clean rooms, sterile areas, explosive atmospheres and marine use.

Indeed, there are still many more avenues for the developers of linear actuators at Hoerbiger-Origa to explore.



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