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Solving the problems of long-stroke motion

10 April, 2013

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.

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