Linear motion off the shelf

Tom Shelley reports on some of the plethora of linear motion systems available from specialist suppliers.

With so many high-quality, competitively-priced linear actuators of different types, it makes no sense for an OEM engineer to try to design their own or manufacture them from bought-in parts. In addition to the cost of the parts, there is also cost of obtaining and assembling them, plus the cost of the time spent designing the actuator.

If the unit is to work as intended, it is necessary to calculate forces, loads and tolerance build up, plus, thanks to the requirements of BS EN 13849-1 and the Machinery Directive, the MTTFd - Mean Time To (dangerous) Failure for each component. Furthermore, if the completed machine is to perform in a medical, clean room, food industry or other critical environment, there will need to be a careful assessment of the suitability of all the materials used to make parts, lubricants and the methods of lubrication.

If, on the other hand, the engineer decides to purchase completed units, the vendor can be expected to specify dimensions, speed, accuracy, positioning repeatability, load, stroke and suitability for particular industries and environmental conditions. If it isn't in the catalogue, most vendors can make whatever a customer wants, calling up information from a database and configuring designs automatically or semi-automatically and downloading the relevant information electronically to their manufacturing system.

The automotive industries increasingly expect their Tier One suppliers to sell them complete packaged systems which they can assemble into vehicles in minimum times per vehicle. The same trend has spread into other industries such as makers of machine tools. German machine tool maker Deckel Maho Gildemeister (DMG) and the Italian BIGLIA Group have been closely working with Schaeffler on various developments involving the use of Schaeffler linear systems.

DMG is using the company's RUE-E recirculating roller bearing and guideway assemblies, while BIGLIA is using KUVE four row monorail guidance systems in its lathes and multi-spindle tools. The case to purchase packaged linear actuators is strongly made by Misumi, whose US arm has produced a White Paper on the subject, entitled 'Single-axis actuators: when to build, when to buy'. The company has recently expanded its LX-Series of ball screw units.

The LX20 now comes in installation lengths of 80, 250 and 300mm, while the LX26 is available to order in lengths of 100, 350, and 400mm and the LX30 version in a length of 125mm. The designs come in installation heights ranging from 20 to 45mm with spindle pitches between 1 and 20mm. All LX systems use clean room compatible lubricant, which includes LX30 and LX45 units with parallel mounted motors. Up to now, an installation length of 100 to 150mm had to be factored into the travelling distance: the new LX unit reduces this to 40mm.

The motor flange on the flanged nut housing is designed to accommodate almost any servo or stepper motor. All versions are also available as sealed units. Working with a supplier of packaged systems also means having access to the latest developments in a field in which the vendor has specialised. For example, igus, which has its own proprietary 'DryLin' plastic bearing technology, has recently announced 'Fast Forward' which allows quick positioning of a trapezoidal lead screw drive.

For those who want more bespoke, packaged systems, Gary Livingston of LG Motion points out that his company contributes "Innovative micro-positioning solutions" that typically combine precision mechanical assemblies, motors, control electronics and software as complete Mechatronics assemblies. Applications include positioning biomedical sensors to within a few thousandths of a millimetre for cancer drug research, helping film makers synchronise the movement of multiple cameras for 3D movies, and providing packaging machine manufacturers with cost reducing multi-axis motion controls to give their products a distinct market edge.

Offerings from HepcoMotion include an 'MCS' aluminium framework system onto which can be mounted a combination of its linear motor products. "Our resources both in terms of our design and manufacturing capability and applications knowledge allow us to provide this added-value service to our customers," explains sales director, Chris Rees. "Design cost is minimised for the customer as it is very likely that we will have worked on a similar configuration before. It can then be tailored to the new application with just minor changes."

All systems are thoroughly tested in the factory before being stripped down into modules for dispatch with a 3D drawing to the customer and all parts carry a 1-year warranty. Typical examples include an MCS frame with a 'DLS' driven unit in parallel with a slave system used in the handling of steel trolleys. Another involves having a standard HD ring fitted to the carriage of an 'HDLS' belt driven unit for the handling of heavy components that need to be rotated when placed.

Rockwell Automation argues that an alternative strategy, with many potential benefits, is to turn to linear motors rather than rotary motors producing linear motion through ballscrews, rack and pinion mechanisms and belt and chain drives. The motors themselves cost more, but do away with the need for other mechanical components, so that the overall cost can come out cheaper.

Furthermore, speeds and accelerations tend to be higher, because there is less mechanical inertia, and accuracy and precision is improved, because there is no possibility of backlash between mechanical components.

This can be a growing problem as the machinery becomes older, and meshing mechanical parts start to wear. The company also offers both 'MP'-series 'Integrated Linear Stages' as well as "Electric cylinders" which do convert rotary motor output to linear motion, but are designed to replace pneumatic and hydraulic cylinders and all their associated pipes, valves and other equipment.

John Smith of Olympus Technologies comments that linear motor drives are much more accurate but also "More expensive" than alternatives, and are hence, the solution of choice for linear gantries for robot welding or laser cutting where positional accuracy is, "Very important". On the other hand, Smith told us that, "A rack and pinion drive with a servo motor is the cheapest solution and suffices for most applications where you are simply manipulating things: palletising or loading or unloading machine tools, for example."

He adds that rack and pinion solutions allow the building of very large working areas, pointing out that, "Some production robots may be 30m or 40m long" whereas laser gantries tend to have smaller working areas, "e.g. 2.5m x 1m for example." Offering a completely modular system, Duncan Matthews, engineering manager with Machine Building Systems comments that they have quite a number of different ways of creating slides using the Item aluminium extrusion system.

"In general", Matthews observes, "It is possible to design slides to handle a wide range of loads using many of the same components." The slides are constructed by fitting hardened ground steel shafts to supporting aluminium profiles using a proprietary shaft clamp profile extrusion. The shafts can be 6mm, 10mm, 14mm or 25mm diameter and matching bearings are fitted to a carriage plate that fits the positions of the shafts.

The completed slides can be fitted with belts, ballscrews, chains or rack and pinion drive, standard items within the company's range, and can be fitted to, or made as integral parts of a machine frame. 'T' slots throughout any assembly allow the easy attachment of other components, such as sensors and guards.

Design Pointers
• Linear slides and motion systems are best bought from companies experienced in their design
• There is nothing useful to be gained by engineers trying to design their own
• Linear motors are the most accurate and fastest, but mechanical systems can often be cheaper, and can easily be made with very large traverses

Author
Tom Shelley

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