Tuesday, July 2, 2013

Limiting Strength with the Clutch Gear

LEGO Mindstorms NXT hobbyist, This topic is continue our topic about playing with gear: worm gear. Another special device LEGO Mindstorms NXT hobbyist should get familiar with is the thick 24t white gear, which has strange markings on its face (Figure 2.5). It is a clutch gear, and in the next part of this section we'll discover just what it does.

Figure 2.5 The Clutch Gear

Our experiment this time requires very little work; just put one end of an axle inside the clutch gear and the other end into a standard 24t to use as a knob. Keep the latter in place with one hand and slowly turn the clutch gear with the other hand. It offers some resistance, but it turns. This is its purpose in life: to offer some resistance, then give in!

This clutch gear is an invaluable help to limit the strength you can get from a geared system, and this helps to preserve your motors and your parts, and to resolve some difficult situations. The mysterious "2.5 "5 Ncm" writing stamped on it (as explained earlier, Ncm is a newton-centimeter, the unit of measurement for torque) indicates that this gear can transmit a maximum torque of about 2.5 to 5 Ncm. When exceeding this limit its internal clutch mechanism starts to slip.

What's this feature useful for? You have seen before that through some reduction stages you can multiply your torque by high factors, thus getting a system strong enough to actually damage itself if something goes wrong. This clutch gear helps you avoid this, limiting the final strength to a reasonable value.

There are other cases in which you don't gear down very much and the torque is notenough to ruin your LEGO monidstorms NXT parts, but if the mechanics jam, the motor stalls~this could be a very bad thing, because your motor draws a lot of current when stalled. The clutch gear prevents this, automatically disengaging the motor when the torque becomes too high.

In some situations, the clutch gear can even reduce the number of sensors needed in your robot. Suppose you build a motorized mechanism with a bounded range of action, meaning that you simply want your subsystem (arms, levers, actuators~anything) to be in one of two possible states: open or closed, right or left, engaged or disengaged, with no intermediate position.You need to turn on the motor for a short time to switch the mechanism from one state to the other, but unfortunately it's not easy to calculate the precise time a motor needs to be on to perform a specific action (even worse, when the load changes, the required time changes too). If the time is too short, the system will result in an intermediate state, and if it's too long, you might do damage to your motor.

You can use a sensor to detect when the desired state has been reached. If you are using LEGO mindstorms NXT servo motors, you could simply use the built-in motor encoders to determine when
you should start and stop the motor (we will discuss this more in next posting). However, you might choose to use a different LEGO motor to power your subsystem. In this case, you will not have a rotation sensor built-in to check. Without a secondary sensor, you will have to run a motor for a specific time. If you put a clutch gear somewhere in the geartrain, you can now run the motor for the approximate time needed to reach the limit in the worst load situation, because the clutch gear slips and prevents any harm to your robot and to your motor if the latter stays on for a time longer than required.

There's one last topic about the clutch gear we have to discuss: where to put it in our geartrain.You know that it is a 24t and can transmit a maximum torque of 5 Ncm, so you can apply here the same gear math you have learned so far. If you place it before a 40t gear, the ratio will be 24:40, which is about 1:1.67.The maximum torque driven to the axle of the 40t will be 1.67 multiplied by 5 Ncm, resulting in 8.35 Ncm. In a more complex geartrain such as that in Figure 2.6, the ratio is 3:5 and then 1:3, coming to a final 1:5; thus, the maximum resulting torque is 25 Ncm. A system with an output torque of 25 Ncm will be able to produce a force five times stronger than one of 5 Ncm. In other words, it will be able to lift a weight five times heavier.

The clutch gear isn't the only way to introduce slip into a system. Later in my posting we'll discuss pulleys and belts, another way to introduce slip into a system.

Figure 2.6 Placing the Clutch Gear in a Geartrain

From these examples, you can deduce that the maximum torque produced by a system that incorporates a clutch gear results from the maximum torque of the clutch gear multiplied by the ratio of the following stages. When you are gearing down, the more output torque you want, the closer you have to place your clutch gear to the source of power (the motor) in your geartrain. On the contrary, when you are reducing velocity, not to get torque but to get more accuracy in positioning, and you really want a soft touch, place the clutch gear as the very last component in your geartrain. This will minimize the final supplied torque. This might sound a bit complex, but we again suggest you learn by doing, rather than by simply reading. Prototyping is a very good practice. Set up some very simple assemblies to experiment with the clutch gear in different positions, and discover what happens in
each case.

See you again in next posting, about Placing and Fitting Gears!

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