Saturday, June 29, 2013

Worming Your Way: The Worm Gear

LEGO Mindstorms NXT hobbiyst, Do you remember about the geartrain in the past my posting?
This posting, I dont review it.  We will learn about the worm gear. Do you know the worm gear? In your NXT box, you've probably found another strange gear, a black one that resembles a sort of cylinder with a spiral wound around it. It is called worm gear. Is this thing really a gear? Yes, it is, but it is so peculiar we have to give it special mention.

In Figure 2.4, you can see a worm gear engaged with more familiar gears. The assembly on the right uses a special LEGO part. It is called a worm gear block. With this single piece you can connect the worm gear to a 24t gear. In just building these simple assemblies, you will discover many properties. Try to turn the axles by hand. Notice that although you can easily turn the axle connected to the worm gear, you can't turn the one attached to the other gears. We have discovered the first important property: The worm gear leads to an asymmetrical system; that is, you can use it to turn other gears, but it can't be turned by other gears.The reason for this asymmetry is, once again, friction. Is this a bad thing? Not necessarily. It can be used for other purposes.

Figure 2.4 Worm Gears Engaged with Other Gears

Another fact you have likely observed is that the two axles are perpendicular to each other. This change of orientation is unavoidable when using worm gears.You may also have noticed when building these assemblies that the worm gear slides easily along an axle. Sometimes this is useful, but most of the time you will have to fix it in place along the axle with bushings.

Turning to gear ratios, LEGO Mindstorms NXT hobbyist are now an expert at doing the math, but you're probably wondering how to determine how many teeth this worm gear has! To figure this out,
instead of discussing the theory behind it, we proceed with our experiment. Taking the middle assembly used in Figure 2.4, we turn the worm gear axle slowly by exactly one turn, at the same time watching the 24t gear. For every turn you make, the 24t rotates by exactly one tooth. This is the answer you were looking for: The worm gear is a It gear! So, in this assembly, we get a 1:24 ratio with a single stage. In fact, we could go up to 1:40 using a 40t instead of a 24t.

The asymmetry we talked about before makes the worm gear applicable only in reducing speed and increasing torque, because, as we explained, the friction of this particular device is too high to get it rotated by another gear. The same high friction also makes this solution very inefficient, as a lot of torque gets wasted in the process. As we mentioned earlier, this outcome is not always a bad thing. There are common situations where this asymmetry is exactly what we want. One example would be when designing a robotic arm of LEGO Mindstorms NXT to lift a small load. Suppose we use a 1:25 ratio made with standard gears: What happens when we stop the motor with the arm loaded? The symmetry of the system transforms the weight of the load (potential energy) into torque, and the torque into velocity, and the motor spins back, making the arm go down. In this case, and in many
others, the worm gear is the proper solution, its friction making it impossible for the arm to turn the motor back.

We can summarize all this by saying that in situations when you desire precise and stable positioning under load, the worm gear is the right choice. It's also the right choice when you need a high reduction ratio in a small space, because it allows very compact assembly solutions.

OK. lego mindstorms NXT, see you again with next topic about Limiting Strength With The Clucth Gear.

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