Roll on

As well as achieving highly accurate, fast movement, the latest linear technology maintains reduced rolling resistance, heightened performance and almost maintenance-free operation, says Martin Oakham.

As a lad the TV show Thunderbirds was a great influence on me, and I marvelled at the idea of being able to communicate with friends via my wristwatch — which of course carried a picture of the person I was talking to. Thirty years on and we are on the brink of commercially realising such technology.

Miniaturisation has a number of advantages. First, there is a saving on resources because products are lighter, more portable and cheaper to produce. Also, systems can travel faster and are more accurate as there is no requirement to accelerate/decelerate such masses.

Nanotechnology is a strong field of research, but for linear guidance systems anything under 15mm nominal rail width is deemed miniature.

Such systems minimise the friction generated by moving a load through a predefined or variable distance, thus reducing the power needed and heat produced through friction. Designs differ to accommodate different applications and the demands of the operating environment. This is especially true when the component in question is only millimetres in width or height.

For example, there is no need to use high load-bearing guideways in a light actuator, or environmentally-protected guideways designed for harsh working industrial conditions and process-orientated applications in a clean room environment.

Extreme temperatures, humidity and airborne contamination generally have only a limited effect on standard equipment, but as component size decreases, so the impact of environmental conditions becomes increasingly important. In particular, temperature changes can have a dramatic impact on miniaturised guideways. Even the smallest of thermal fluctuations can encourage expansion or contractions in the rail, resulting in a deterioration of production quality and reliability. Therefore, as the physical size of the linear guide decreases, so the environmental limitations on the system multiply.

Similarly, materials used to construct the systems must also be considered: clean room operations, for instance, can be hampered by emissions from grease or particles produced through wear and tear. To address this issue, it may be necessary to create a pressure differential within the system, or remove contaminants by vacuum operation.

INA, for example, produces two and four-row ball-type recirculating systems, using plain bearing and cage guided systems. These are available in stainless steel and other corrosion-resistant materials, such as INA’s Corrotect coating.

However, as a rule, plain type guideways can handle higher loads at reduced speeds, while recirculating systems can cope with speeds of anything up to three metres/sec with good load bearing capabilities — around 50kg for a 7mm carriage, although the length and carriage support must be considered.

Hepco systems produces a range of V-guide linear roller guideways that are capable of four metres/sec compared with recirculating ball systems which generally run up to two metres/sec.

According to Hepcos systems’ sales director Chris Rees these are designed to be more compliant than recirculating ball systems, which means that the alignment does not need to be as critical. They are also capable of working in harsher environments than most ball systems, even though many have seals to protect the recirculating tracks.

They are not as rigid as ball systems generally, and cannot take as high a load as a similar size system. But this is not an issue in many design applications providing that you are still within the guidelines and have taken mass into consideration and how it increases with velocity. In fact, Hepco has a low-cost unground version of its V-guide system for non-critical applications.

THK’s Caged Ball design enables smoother movements, less friction and less maintenance by relying on retainers to hold the ball bearings at an equal distance from each other. Used within a ballscrew-nut assembly, the system effectively eliminates excessive noise, friction, heat and wear that commonly plagues conventional ball-race configurations.

Popular applications include pick-and-place (high speed, high accuracy), PCB component placement, gluing and soldering machines, gauging equipment, measuring devices, medical applications, (pipette dosing and transport equipment) vending machines (coinage distribution and cup placement) ATM cash machines, dispensers and computer drives.

As you can see, environment and load-bearing capabilities are not the only factors regarding miniaturisation — control is also important. For example, the latest generations of miniaturised technologies enable the control of linear and reciprocating devices to reach sub-micron or nanometre levels. This degree of control is essential if component and chip sizes are to be reduced sufficiently to enable the next generation of miniaturised consumer products to be realised. Manufacturers of semiconductors, silicon wafers, electronics and laboratory test equipment, for example, incorporate precision guides to enhance accuracies and improve system control.

Small, compact and lightweight linear systems used for the positioning of computer drives, CD/DVD mechanisms, car seating controls, systems for step-and-scan medical cell analysis or the production of semi-conductors, increasingly require component parts that are compact, easy to assemble and cost efficient.

According to THK’s Bob Love, in response to this emerging market, the latest generation of linear motion technologies are being integrated with other component-enabling OEMs, machine builders and end users to purchase a single operating unit featuring a guide, controller, motor and interface.

Innovative miniature linear motion modules that combine linear motors, linear guides, encoders and cabling with a standard drive can reduce specification and build times, while improving levels of reliability and performance. More importantly, these systems can effortlessly transport significant loads, achieve high levels of accuracy and eliminate the problems associated with sourcing individual components.

A unique collaboration of THK, Parker, Renishaw and HMK, for instance, has launched a number of combination systems that are supplied pre-programmed and ready to install.

One of these is the RDM micro — with a stroke range from 9mm–409mm and only 49mm high x 60mm wide — which is is ideal where space is restricted, or where miniaturised equipment is required, such as in pick-and-place systems for electronics and telecommunications components, medical devices, test and measuring instruments and compact process lines.

Taking the modular concept a step further, Aerotech has taken the direct approach by replacing conventional drive systems with direct drive technology.

Unlike conventional systems, direct drive does not need motion converters such as couplings, gearboxes, belts, friction wheels or toothed wheels. This is a real advantage, as all motion converters lose energy and precision and suffer from wear. Furthermore, in terms of control technology they cause elasticity and hysteresis, which reduces overall performance.

According to Aerotech, the direct linking of drive, feedback system and load in motion increases rigidity in both mechanics and control, so that loads can be positioned or moved more dynamically — and therefore faster and more accurately.

But to use direct drive technology successfully, a number of issues, which are of minor importance in conventional drives, must be taken into account.

The direct connection between drive and load transfers power losses from the motor as thermal energy, and can cause unwanted expansion or distortion to the load or workpiece. This would obviously impair precision, so it is critical to take low-power loss into account when selecting a motor. As a result, cooling systems may need to be used more frequently than in conventional applications.

The difference between iron-cored and ironless motors is that the latter have no magnetic attraction between the coil and the magnet track. so in linear motors, the longer the path to be traversed, the less expensive iron core motors become compared to ironless versions as the corresponding magnet tracks are normally flat rather than ‘U’-shaped and can be produced more easily with a single row of magnets.

But the iron core design has a number of disadvantages, such as the increased mass or weight of the coils, which has a negative effect for lightweight payloads in motion applications. Their rotors have a strong magnetic attraction with the stator, thereby subjecting the bearing to greater loads. This not only makes assembly significantly more difficult when setting up and when replacing the motor during servicing, but also impairs synchronism due to the increased harmonics.

So with buoyant consumer electronics, automotive electronics and biotechnology equipment sectors, demand for miniaturisation across a wider range of products looks set to increase. As a result, smaller linear guides will undoubtedly become increasingly critical in a growing number of products, testing procedures and manufacturing processes.