Can the speed of a falling object be regulated using gears?

Question

I am not an engineering student, but can understand if explained, so excuse me if this is too basic.

For a personal DIY project I envisioned, I need to make an object, attached to a set of gears, fall at a certain speed - 6ft / 24 hours. This object will rotate the first driving gear and the resultant driven gear at 3000-5000 RPM. I estimate the ratio to be 1:100000.

So basically my question is whether the speed at which the object falls (from gravity) be regulated? The weight of the object doesn't matter, since I can set it to whatever is required for my rate of descent.

The setup will be similar to: http://s.hswstatic.com/gif/clock-weight-train.gif

Answer 1

A gearbox on its own won't really regulate speed, apart from the fact that it will have some internal friction but this is difficult to regulate. For example just having a 1000:1 gear ration won't make the object fall 1000 times slower unless there is some external torque applied.

A simple solution is to use viscous drag for example having the gearbox drive a fan or paddle wheel. The works because viscous drag increases with the square of speed so you will get to a point where the drag force equals the weight of the object at some reasonably constant speed.

There are also various types of mechanical governor which can be used to adjust the force on a mechanical brake in response to speed. A step further is to use an electronic system with speed sensors and actuators on a brake, this potentially allows a lot of flexibility and the ability to self adjust on the fly.

Another option is to use an escapement of they type used in clocks. This doesn't give true constant speed as the motion becomes a series of discrete steps but for the very low speeds you are talking about this may not be a problem.

Having said that the gear ratios you are talking about are huge, to put it into perspective if you did this with a single pair of gears and the driving gear was 5cm in diameter then the driven gear would need to be 5km in diameter

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Edit in response to comment:

If you want to use this arrangement to generate power then you need to bear in mind that gravitation potential energy has pretty poor energy density compared to other small to medium scale energy storage.

Specifically the total gravitational potential energy is 9.8J per kg per metre of drop. 24 hours is 86400 seconds so for a 2m drop this translates as 1.1e-4 Watts ie you would need 5000kg to provide 1W over 24 hours before considering losses.

Answer 2

In theory this is completely possible. In practice you may have problems. Remember that if you have a 100000:1 ratio of speed, you also have a 1:100000 ratio of torque. At that ratio, a tiny torque at the high speed shafts becomes huge at the low speed shaft. E.g let's say the bearing on your high speed shaft has a drag torque of 0.05 in-lbf. That becomes 5000 in-lbf at the low speed shaft. If your low speed shaft is 1" radius, then your object needs to be 5000 pounds just to overcome the friction needed to turn the output shaft. With very high precision bearings with very low drag torque, might be possible. But I suspect those would be expensive.

Answer 3

The 100k to 1 overall gear ratio you want makes the realization difficult, as Danial Kiracofe has already pointed out.

However, to answer your question, yes it is possible to regulate the speed. There are quite a few different ways. A conceptually simple one is a centrifugal brake. You have a vertical rotating shaft with weights hinged from it so that the weights move outward due to centrifugal force as the shaft speed goes up. When the weights move a certain distance from the shaft, they touch something that causes friction. This slows down the rotation speed of the shaft. The net effect is that the frictional drag on the shaft is little up to some set speed, then increases rapidly for higher speeds.