Metal and Polymer Balls Unite

Contributing editor Jon Barrett looks at a new low noise, smooth running linear bearing developed to reduce rolling friction to minute levels

An innovative ball rail technology – developed to set a new benchmark in low noise, smooth running – is playing a key role in a new laser system. Thanks to the bearings, designed by Rexroth Star, the system can now deliver better edge, join and surface finish during operations such as cutting welding, drilling, marking and surface treatment. The bearings are so new that the laser system, designed and built in the UK by Mechtronic Industries, is the first machine in the world to use them.

In a conventional laboratory environment, a small laser with an X-Y table can be used to conduct prototyping. However, a new programme (see sidebar) has allowed Mechtronic Industries to develop a much larger system that closely mirrors the production systems that would ultimately be placed on the shopfloor.

The laser unit in the system is a 2kW, RF excited CO2 slab laser, representing the latest technology. When compared with a conventional 3kW CO2 laser, it provides similar performance, yet is smaller, lighter and more efficient, especially in gas usage. Wavelength is 10.6 micro m.

The system moves the workpiece under a fixed laser head. Maximum workpiece dimensions are in the region of 2440mm by 1220mm by 20mm mild steel, while the smallest parts could be tiny ceramic substrates just 0.1mm thick. Maximum traverse speed is 20m/min.

Jobs can vary from butt welding thick aluminium plates to drilling tiny holes in polymer circuit boards. Yet, the bearings on the X, Y and Z axes must be able to cope with these extremes while maintaining tight accuracy and repeatability tolerances.

Project manager, John Biffin, says: “When welding steel sheets, a noticeable pool of molten metal follows the laser beam as it moves along the joint line. In such applications, the bearings’ ability to safely handle the static and dynamic loads imposed by the sheer weight of the workpiece is probably more important than their ultimate smoothness. However, when we switch to trimming a complex profile from a thin ceramic substrate, any harshness in the bearings appears as a poorly finished edge. This problem is multiplied if the laser needs to rapidly reciprocate or follow constantly changing curves.”

Star’s Clive Straw was quick to recognise the limitions of conventional bearings used in such systems and to offer an alternative solution.

In a conventional ball rail system, a recirculating loop of balls provides the rolling contact between the raceways in the rail and carriage. As the balls roll, their touching surfaces will be moving in opposite directions. This simple fact is a key contributor to roughness, noisy running and heat generation. Also, reciprocating motion causes the balls to impact one way, then the other, adding to harshness.

One solution is to introduce a polymer cage that separates individual balls. This does help. Metal to metal contact is replaced by metal to polymer contact and two surfaces sliding in opposite directions are replaced by one moving surface sliding against a fixed surface. Although the load capacity per unit length is reduced because the number of balls falls, this type of bearing reduces noise and improves smooth running.

The secret to Rexroth Star’s new solution is to reduce rolling friction to minute levels, using a technique elegant in its simplicity. Instead of a string of steel balls, a loop of alternating hardened steel and polymer balls is used. However, these polymer balls are slightly smaller than their steel neighbours.

Under load, the steel balls provide the rolling point contact between the rail and carriage raceways. The polymer balls, due to their smaller size, are free to follow their preferred motion. As each steel ball rolls it causes its polymer neighbour to rotate in the opposite direction. Thus, a simple rolling motion `gear train’ is produced with each ball rotating in the opposite direction to the two balls either side. Rolling friction is now completely replaced with rolling motion, dramatically improving the running characteristics. Also, because of the steel/polymer mix, ball impact is dampened, improving the bearing’s ability to reciprocate smoothly at high speed. During tests carried out by Star, the new design was shown to halve noise output when compared with its nearest competitor.

Interestingly, customers keen to explore this innovation further will not need to worry about a heavy price penalty because the rails and carriage used in the new assembly are standard, off-the-shelf components. Only the balls are different.

Design engineers may argue that by halving the number of steel balls, load carrying capacity and working life would drop to unacceptable levels. However, Rexroth Star has a solution to this problem as well.

In linear bearing systems, load capacity and life are heavily dependent on the hardness and surface finish of the balls and rail/carriage tracks. On this point, it is the carriage that presents the most challenging production issues. Integral carriage tracks will obviously be of the same material as the carriage itself, while limited access reduces the range of groove cutting and surface engineering techniques that can be applied during manufacture.

To over come these problems, Rexroth Star chooses to manufacture hardened and ground bearing plates, in high quality bearing steel, that are subsequently inserted into the carriage. Using this technique the materials used for the carriage and raceways can be optimised without restriction. More importantly, because the tracks are produced independently of the carriage, advanced processes can be employed to super finish the surface. Rexroth Star says that this super finishing process increases the contact between ball and raceway surfaces, thus boosting the load carrying capacity. Also, the higher quality finish is beneficial to smooth running.

BALLSCREW DRIVES

Complementing the linear bearings, Rexroth Star also supplied the X and Y-axis ball screw drives. Both units feature zero backlash, flanged nuts and fixed angular contact end bearings. X-axis is 50mm dia by 20mm lead, while the Y-axis is 40mm dia by 20mm lead.

Rexroth Star claims that by precision rolling the screws, rather than grinding, the units are less expensive to produce and are available quicker. Straw said: “By picking the screw and nut components from stock, assemblies can be machined, assembled and delivered in days rather than weeks that some companies are used to.”

With the mechanical accuracy and repeatability of the machine assured, project team members can now quickly and confidently set up new jobs safe in the knowledge that the motion system’s accuracy and repeatability will be consistent, irrespective of widely varying application characteristics.

A number of organisations are supporting the programme and some £350,000 funding has been provided by companies including Rofin-Sinar Laser, Kuka Robot Systems, Mechtronic Industries, Total Process Cooling and V&S Scientific. The application development projects are customised to individual SMEs and specific manufacturing sectors, aiming to embed laser technology and stimulate improvements in their manufacturing processes and techniques.

Rexroth Star Tel: 01480 223200