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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).

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