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Ever wondered why lego uses so many different axles ( and I’m not just talking about the different lengths ;))? Well so did I. Time for some explanations! As we could write an entire article about the different axles and when to use them, I will keep it at just one axle for today: the Axle with Center Stop (yellow/tan-coloured axle in left upper corner). The difference between this axle and other axles is not hard to guess: it has a center stop. But what is a center stop? And what is its use?
The center stop is more a less a ‘smooth area’ on the axle. This smooth area is initially designed to prevent (‘stop’) cross holed parts from sliding over the axle. The center stop has however another ace up its sleeve: since the smooth area is just a bit longer than one stud it provides a certain spacing around the center stop. Imagine a situation where you have both fixed and rotating parts on one axle. The fixed parts are just to hold the axle in place and should not cause any friction on the rotating part. Of course you could use a normal axle and make sure there is enough spacing on each side so everything runs smoothly. In practice, however, we often notice that bushings or gears are pressed too hard against the fixed part, causing quite some friction and disturbing the smooth working mechanism of modules. By using an axle with center stop you can make your mechanisms more reliable because the spacing is already defined on the axle. You can press the gears or bushing tight together and there will always be spacing, resulting in very little friction.
A nice example of the use of the axle with center stop can be found in the Shooter Module of the GBC 45 Stunt Circus. The Shooter Beam is lifted upwards by the Trigger Arm and when reaching the top it will fall down and hit a GBC Ball. To get the maximal energy/speed out of this freefall we want to have as little friction as possible. Again, you could use a normal axle and make sure there is enough spacing to have little friction. However this can have many user error during building, or over time, which makes the module unreliable. Therefore we used the axle with center stop (shown as pink axle). As explained the smooth area makes sure there is enough spacing, allowing the Shooter Beam to obey gravity and fall down freely. The Shooter is able to swing for- and backwards until it gets lifted upwards again by the Trigger Arm.
Another use for the center stop axle can be seen in the green axle. Since the dark grey beam is part of the framework, it will not rotate together with the axle. Therefore we used the axle with center stop, allowing the axle to rotate smoothly, while leaving the dark grey beam upright!