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Solving the problems of long-stroke motion

10 April, 2013

Achieving reliable, accurate linear motion over distances of more than 10m poses several problems. Anders Trygg, a product manager in Thomson’s linear components group, examines these problems and suggests how they can be solved.

Many industrial applications need linear movements, and this need is usually met by using belt or screw drive systems. Both have advantages and disadvantages and satisfy the demands of most applications, but issues start to arise when longer linear distances are needed.

In such cases, belt systems are an obvious choice. These relatively simple systems use pulley drives to create tension along the belt, and can be brought quickly up to high speeds. However, as they reach longer strokes, issues start to arise with the belts sagging because tension cannot be maintained across their whole length. There is also inherently a lot of give in the system from the rubber or plastic belts themselves.

This flexibility can cause vibration or springing which creates a whipping effect for the carriage. If a process cannot handle this, a screw system offers an alternative with a fixed mechanical element ensuring complete control at all times with precise stopping and positioning.

Safety may also be a consideration when choosing a belt drive, because of the possibility of a belt breaking. Such a fault would be uncontrolled and, in vertical applications, the load could fall and damage machinery or even harm personnel. If a vertical screw drive system fails, it stops the load from falling and ensures safety.

Historically, the issue with a screw drives has been the difficulty of achieving longer stroke lengths. These drives can commonly be provided in lengths of up to 6m using pairs of bearing blocks to support the screw and to prevent any whipping effect at higher speeds. Even at low speeds, longer screws need to be supported against bending as a result of their own weight. The bearing block support system traditionally consists of pairs of blocks, connected by a rod or wire, which move together along the linear motion system (above).

When a system needs a longer stroke, more bearing block pairs can be added to support the screw at regular intervals along its length. Having up to three – or, perhaps, four – pairs working together can be practical but, beyond this number, connecting the rods or wires between the blocks becomes difficult.

First challenge

The first challenge to achieving a longer stroke, therefore, is to create a system which can offer more support points for the longer screw. One possibility is to do away with a “connected” system for the blocks and, instead, use a system where the blocks can collapse into each other and separate out when needed (see diagram below). Once the blocks reach their position, they stay there to guide and support the screw. In such a system, 12 or even 13 support points can be realised with bearing block pairs.

This form of support for ball or lead screws can achieve long distances without bending or whipping, while maintaining rotation speeds.

To go beyond 6m long, the next challenge is to create a longer screw. However, screws are normally produced only up to 6m in length. So how can a stroke length of more than 10m be achieved? The answer lies in attaching two screws together and using some precision manufacturing techniques.

Lead and ball screws are manufactured on a rolling line and each part may be produced with a slightly different deviation. To join two parts together, therefore, differences in lead deviation need to be overcome. To join two screws successfully, the highest precision ballscrews with the smallest possible deviation must be used. These ballscrews need to be machined precisely. Heat must be prevented from entering the part and affecting its diameter and lead geometry, because a change of 0.01mm – or even 0.001mm – can create problems for the final system. Once the machining process delivers the accuracy needed, the screws are married together using a tap and hole with minimal deviation between the two leads. They are finally secured using a strong glue because any thermal or welding joins would again alter the geometry and create problems.

Avoiding whipping

Using the collapsible support block system and precision-manufactured long screw lengths allows strokes of 10.8m or longer to be achieved. A system with a stroke length of 2–3m would have a maximum speed of around 3–4,000 rpm. Normally, with a longer system, the rotational speed would have to be lowered considerably to avoid whipping, but Thomson has engineered a system using additional supports to maintain stroke lengths of more than 10m at 3–4,000 rpm. This offers the benefits of a stiff screw system, long stoke length and high transportation speeds.

Many existing applications work with screw systems up to 5m or 6m long where the screw is left completely open. There are two common issues with such systems:

•  the system cannot operate at the desired speed; and

•  is difficult to maintain because the open screw attracts dust and debris, requiring regular cleaning to avoid premature failure of the ball nut.

In such applications, the additional supports provided by a stacking bearing block configuration mean that the screw can be operated at much higher speeds. Cleaning and reliability issues can be resolved using a covered, sealed system which protects the screw and cuts maintenance. The enclosed screw (shown above) is protected against the ingress of dust and debris and can maintain optimum performance and reliability, without needing regular cleaning. In such a system, the carriage can be equipped with drilled channels and connected with a grease nipple. This enables lubrication from a single point without having to open the casing, thus allowing rapid and comprehensive maintenance. Because the system never has to be opened, only small amounts of dust or water can penetrate and the system is protected even in the dirtiest environments.

As with any process, specific application requirements dictate which technology is best. Linear motion systems using screws can handle larger loads than non-rigid, polymer belt systems. The screws are robust stainless-steel pieces and, if manufactured to the correct tolerance class, can reach stroke lengths well over 10m. The collapsible bearing block system supports the screw length to avoid whipping and bending while enabling it to operate at the higher speeds usually associated with shorter length systems. Long-stroke linear screw-drive systems offer precise positioning with fast, safe operation. When operating in the sealed enclosure, they are suitable for continued reliable use with minimal maintenance, even in harsh environments. 




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