Simple and safe machine actuation

24 April, 2007

The rodless pneumatic cylinder is perhaps less glamorous than the Space Shuttle, Eurofighter or Aston Martin’s latest; but it is at least as successful in its field.

Essentially, it is a simple product doing the simple job of creating linear movement. Its development began after the maturation of most pneumatic actuators, but it has joined the cannon of indispensable components for machine builders and automation systems designers.

The rodless cylinder was invented in the early 1970s by three Swedish consultant design engineers who, quickly realising their enormous potential, set up a manufacturing plant and renamed their company Origa. The design was based on a slotted aluminium profile where the slot was sealed both inside and out by stainless steel sealing bands. Inside the profile a piston ran and the force is transferred through the slot to an external carriage. At each end of the profile there were end covers to form pneumatic chambers.

Design engineers took the idea to their hearts almost immediately, although the adoption was steady rather than explosive due partly to a paradigm shift in the use of pneumatic cylinders (it became a load bearing structural design element and not simply a drive force) and also the significantly higher unit cost compared to a standard cylinder. Origa was soon selling rodless drives on the world stage, extending its range of sizes and working on plans for second and third generation designs.

Established pneumatic companies were also very interested in the rodless concept. They could see the obvious advantages of compactness, no overhanging or extending load, intrinsic guidance and non-rotation. Unfortunately, they were constrained by some fairly bullet-proof patent protection that Origa had had the foresight to take out.

But patents run out and Origa knew it had to protect its commercial flank. It did this by developing many variants on the basic rodless design – perhaps most notably with electric drive variants that did away with the need for a compressed air supply, and went on to develop a fully modularised range of rodless actuators and associated equipment. This move has freed machine builders from the effort of designing each new machine from the ground up; instead they can quickly realise their machine’s main infrastructure with modular actuators, and concentrate their efforts on developing the tool head.

ATEX explained

Origa found that by addressing the patents issue they were building an in-house expertise for constantly innovating with the basic rodless concept. This has lead to many timely developments over the years, such as new versions that comply with cleanroom specifications and most latterly the ATEX directive, the new legislative framework for equipment in explosive atmospheres.

ATEX uses the established Zone 0, 1 & 2 principle to define the severity of risk for gaseous environment and Zones 20, 21 & 22 for dust environments. Basically, Zone 0 and 20 are the more dangerous and refer to situations in which there is an ever-present possibility of explosion, possibly due to the continuous presence of a gas or vapour. Components used in these areas must be designed so that they are completely unable to create a spark (e.g. intrinsically safe electric elements that have such low power requirements that a spark is not possible).

Zone 1 and 21 are for when explosive gases may be present occasionally. In this case components with a small risk of sparking are acceptable, although there usually has to be some sort of secondary protection or containment mechanism, such as an explosion proof chamber or cabinet, that confines any explosion risk to a small controlled and unmanned area. Zone 2 and 22 covers the lowest level of risk, in which the frequency of an occurrence is both seldom and brief.

But ATEX is different in a very important principle from much recent industrial legislation. For ten years or more, many Regulations have been self-certifiable, so OEMs have created paper trails to prove compliance.

But with ATEX it is a different procedure: the onus is on the end user, they must take responsibility for owning and operating machinery that is safe.

This, however, does not let others off the hook, because every component in a machine must now be ATEX approved by an independent test house. This is usually the responsibility of the component manufacturers; and the OEM must be satisfied that valid approval is in place and that design and installation are safe.

ATEX tries to align legal responsibility with moral obligation, stopping the legal buck passing that has been evident in the past. ATEX’s first principle is that if machinery isn’t safe it should not be used, and that is primarily the machine owner’s responsibility. The owner can ensure his compliance by checking that both machine components and the way they are installed by machine builders are ATEX compliant.

With ATEX, machine safety certification is no longer a paper tiger. If there is an incident, it apportions blame appropriately and efficiently. It probably isn’t perfect, but it is definitely an advance in that it will be harder to dodge responsibility.