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MOTION CONTROL FEATURE


MOTION SOLUTIONS FOR achieving a longer stroke length


Belt or screw driven systems are beneficial where precise motion control is needed. But what are the options if your application demands a longer stroke length? Niklas Sjöström, product line manager systems group, Thomson Industries, comments


Figure 1: Bearing block support systems traditionally consist of pairs of blocks connected via rod or wire. The pairs move together along the linear system


blocks can collapse into each other and separate out when required, as shown in Figure 2, below. Once the blocks reach their position they stay there to guide and support the screw. In such a system 10, 12 or even 13 support points can be realised with bearing block pairs. This system of support for the ball or


A


conventional belt or screw drive system is often used for industrial


applications where linear motion is required. Although these meet most application demands, however, issues start to arise when longer linear distances are required. For where long linear movements are


required, belt systems are the obvious choice. These use pulley drives to create tension along the belt and can be brought quickly up to high speeds. But, as longer strokes are reached, tension cannot be maintained across the whole length of the system and belt sagging can occur. There is also inherently a lot of give in the system from the rubber or plastic belts themselves. The result can be vibration or springing which creates a whipping effect for the carriage. So, if a specific process cannot handle this, a screw system with a fixed mechanical element can ensure complete control with exact stopping and positioning. Another consideration is safety – as belts can break. This fault would be uncontrolled and, in vertical applications, the load could fall and damage machinery or hurt personnel. A screw drive system, however, would stop the load from falling. Screw drive systems have historically had difficulty in reaching longer stroke lengths. They are commonly provided in


lengths of up to 5.5 or 6m using pairs of bearing blocks to support the screw and stop any whipping effect at higher rotation speeds. Even at lower speeds, longer screws need support against bending caused by their own weight. This bearing block support system traditionally consists of pairs of blocks connected together with a rod or wire and the pair move together along the linear motion system, as shown in Figure 1, above. If a longer stroke is required, more


“Creating a solution with the


collapsible support block system and precision


manufactured long screw length means stroke lengths of 10.8m and over are achieveable”


bearing block pairs may be added to support the screw at regular divisions along its length. Up to three or maybe even four pairs working together can be practical, but connecting the rods or wires between the blocks starts to become a difficult configuration beyond this. 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 solution is to do away with the ‘connected’ system for the blocks and, instead, use a system where the


/ DESIGNSOLUTIONS


lead screws can enable long distances without bending or whipping, while maintaining rotation speed. To go beyond 6m in length the next challenge is to create a longer screw, but screws are only normally produced up to 6m in length. So, if a stroke length of over 10m is needed, the answer is to attach two screws together and employ some very precise manufacturing techniques. Lead and ball screws are manufactured on a rolling line and each piece part may be produced with a slightly different deviation – to join two parts together, differences in lead deviation need to be overcome. For two screws to be successfully joined the highest precision ball screws with the smallest possible deviation must be used. The ball screws need to be precisely machined, ensuring heat does not enter the part and cause changes to diameter and lead geometry – just one hundredth or even one thousandth of a millimetre change can create problems for the final system. Once the required accuracy is achieved, the screws are married together using a tap and hole with minimal deviation


between the two leads, and they are secured using high strength glue.


Figure 2:


Modular support system from Thomson with interlocking movable block pairs


>20 DESIGN SOLUTIONS | FEBRUARY 2015 19


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