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Linear Motion Control - 2012-04-16

Ultra-steady running, a low noise level, high precision and ever-increasing outputs: these are the key trends in linear technology that NSK is catering for by developing new series of ball screws and linear guides. Green engineering is another important element in our development work, a long side customised versions in response to clients’ increasingly specific demands. 
Machine manufacturers often use ball screws when linear movements have to be performed with a high degree of precision and quiet running. They are used in drives of all sizes. NSK – the world’s leading manufacturer of ball screws – produces everything from miniature linear drives with a spindle diameter of 4 mm for hand-held oral scanners to high-load drives for injection moulding machines which exert clamping forces of several hundred tonnes.

Customers want solutions, not components
As ever more specific requirements are being made of drive systems, the working relationship between manufacturers and users is changing in situations where high-end solutions are needed. Just a few years ago, clients would select systems from a catalogue. However, we are now seeing an increase in joint development projects involving ball screw producers and their clients, machine manufacturers.

This is partly due to growing demands concerning green engineering. Both manufacturers and users of machines want linear drive systems with the smallest possible environmental impact. In concrete terms, this means that the drives must be low-friction, i.e. energy-efficient. They should use a minimal amount of lubricant and release as little as possible into the environment. Special demands are also increasingly being made with regard to noise.

Roller guide for processing abrasive media
One such joint development project completed recently with a focus on durability and eco-friendliness was dedicated to the optimisation of roller guide seals. A German machine tool manufacturer needed a solution for machines that process abrasive media such as graphite, ceramic and sintered materials. NSK worked directly with the client to develop a new, highly effective seal concept for this area of application.

The sealing lip on the new V1 seal fits tightly around the roller guide profile without a gap and is made from an extremely abrasion-resistant material. Its design also enables a precise distinction to be made between repelling dirt and sealing the component. This means that it offers optimum protection from dirt, stopping it from penetrating into the inside of the carriage. That is good news for the environment too because the drives are very durable, despite the adverse operating conditions. This reduces the resources used – an environmental consideration that is often ignored.

High-load drive for injection moulding machines
Manufacturers of plastic injection moulding machines endeavour to use all-electric machines with drives that perform efficiently in the injection, clamping and ejector units which can also be precisely controlled. In recent years, NSK has helped to repeatedly raise maximum outputs in this field.

Lately, the company collaborated with a manufacturer of quality servo drives and a leading European producer of injection moulding machines to develop a drive system for the ejector unit that is capable of extremely high dynamic axial forces. This utilises heavy-duty spindles from the HTF series, which feature an optimum ratio between the ball’s diameter and the lead, and a patented ball guidance system that prevents direct ball-to-ball contact. Together with the material specifications for the nut, ball and spindle, these design features pave the way for very high speeds and peak-load capacities.

The newly developed A1 high-performance seals used in the HTF series offer further benefits in this regard. These provide a very effective, long-lasting grease retention seal, even when they are exposed to heavy loads and high speeds. They therefore guarantee cleaner processes, reduce grease consumption in high-performance linear drives and lessen the environmental impact related to lubrication.

System concept: ball screw with highly stressable bearings
NSK offers linear and rotary bearings as a system for many application scenarios, which also contributes towards the linear drives’ high performance capabilities. These systems feature the new angular-contact thrust ball bearings from the TAC series, which were specially developed for use in ball screws. Their extra large balls increase the working surface and are capable of absorbing very high axial loads. This gives them a service life that is approximately five times longer than that of conventional angular-contact ball bearings in this demanding scenario. At the same time, it makes for a high degree of effectiveness and energy efficiency thanks to a friction-optimised internal construction.

One of the differences between the latest range of angular-contact thrust ball bearings – the TAC-SHR series – and other ranges is the former’s smaller bore size. This enables ball screws with smaller shaft diameters to be used at the shaft end, without the need to increase the number of ball bearings per linear axle. This in turn makes it possible to realise more compact constructions.

The new BSBD series of bearing units was also developed specially for use in ball screws. The units feature two angular-contact ball bearings with a contact angle of 60°, arranged in an O-shape. This combination enables high axial loads to be absorbed in both directions. The shape and arrangement of the seal make for high efficiency twinned with low friction and heat generation. This helps to ensure energy efficiency.

Linear units for integration into bespoke drives
Many other applications – from the plastics machine and machine tool industries, for instance – could be added to the examples of customised linear drives. Bespoke systems have also been developed for handling technology and related areas. For example, NSK developed an ultra-slim ball screw for a German manufacturer of handling units, which can be integrated into a complex profile system. Furthermore, one of the world’s leading producers of electric drive systems uses an NSK ball screw at the heart of a highly sensitive servo-electric linear drive that replaces an existing hydraulic drive. Here too, the environmental footprint was a key criterion along with precision: the user now no longer needs a hydraulic axle, eliminating the risk of leakage, which is inevitably associated with hydraulics.

The examples show that users of ball screws and linear guides want more from their suppliers than just a broad portfolio consisting of high-quality, durable product ranges. They want engineering support when the catalogue range does not fit their specific needs. On top of this, they are increasingly looking at the environmental impact of drive systems or components and taking this aspect into account when machines and plants are developed.

A manufacturer of linear technology can meet these requirements by maintaining sufficient engineering capacity, fostering intensive dialogue with clients and operating a flexible manufacturing system that is capable of delivering top quality even for smaller series. NSK ticks all these boxes and supports users throughout linear systems’ life cycles by offering specialist services, some of which are provided in conjunction with local trade partners and distributors.

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