Plain bearings make toughest slides

Tom Shelley reports on some useful innovations in linear systems for use in difficult environments

Flexibility and durability are key attributes of new dry, polymer bearing linear systems. One system copes with overhanging loads beyond the capabilities of traditional monorails, while another is specifically designed for harsh environments and goes down to small sizes. The power of the Drylin-W system from Igus is its use of circular cross section rails, either formed on each side of extruded, hard anodised channel sections, or mounted separately. The option of separate mounting not only allows the rails to be any distance apart, but to be placed at 90 deg to each other on adjacent sides of a rectangular construction. The carriages are zinc castings lined with the company's proprietary Iglidur J polymer based compound material. Matthew Aldridge of Igus told Eureka, "Furthermore, with round rails, dirt falls off instead of being trapped in a traditional rail design." Applications are found in automation, including pneumatic powered systems and robotics, but a first sale has been made for a use in construction equipment. Profile height is 18mm, dynamic loads are up to 2,400N and static loads are up to 4,800N. Maximum speed is 15 m/s. Life calculations and drawings are downloadable from the Igus web site the system now has its own web site at Hepco is also offering a new range of linear guides with polymer bearings. The Pacific Mini Rail uses extruded aluminium rails with Pacific Bearing made PTFE based bearings in its carriages. Its particular speciality is its going from 20mm down to 7mm sizes, in lengths up to 3000mm. UK sales manager John Palmer told Eureka that, "Miniature ballrails suffer from contamination and start running rough. As the ball size inside is tiny, it only takes a few specks of dirt to make the runner block run rough. Once the dirt gets into the block it can't get out and debris builds up inside until it fails." Miniature slides with solid state bearings on the other hand do not suffer from such problems. The perceived downside of designs based on plain bearings is possibly lower speeds and reciprocating rates. The limiting factor is heat generation, which in Aldridge's opinion is, "The big enemy of polymer bearings." However, provided excessive heat is not generated by fast reciprocating movements and friction, temperature endurance for slides with solid state bearings can actually be higher than for those using balls. Maximum working temperature of the Drylin-W products is 250 deg C, and the Pacific bushes and blocks, we are told "Will run at temperatures from -240 to +260 deg C." The Mini Rail is available in three versions: Precision, Compensated Precision and Commercial. The MRC Compensated Precision Series provides additional clearance on the carriage to tolerate misalignment. Intended Mini Rail applications include the manufacture of semiconductors, medical equipment, printers, packaging and textile machines, material handling equipment and pick and place modules. Pacific Bearing is based in Rockford Illinois. Igus Hepco Slide Systems Pointers * Slides fitted with plain bearings have high load capacities, resist dirt, corrosion and shock, do not need lubricating, are quiet and able to dampen vibration * The downside is that they cannot move or reciprocate as fast as those based on reciprocating balls and cannot be allowed to run too hot, although they can work at temperatures as high as 300 to 350 deg C with the right bearing material * New designs include split rails whose component halves can be mounted far apart, and not necessarily in line with each other as well as new mini products