Animation Fundamentals: Cranks
Here's a mechanism that ought to be familiar, since it's utilized in steam locomotives. A crank converts rotary motion into linear reciprocating motion. The underlying principle is that of two offset axles; rotating one causes the other to follow a broader circular motion. It's an extremely simple and versatile means of creating repeated action. Animations that need to self-index will usually have a windshield wiper control.
Note that index switches are optional. Location can be at the crank or at the object being animated. The object being animated may either be connected to the actuating rod, or directly to the connecting rod; in either case, some sort of guide is required. Also note that it's not an especially compact device. It can be reduced in size by folding the actuating rod back over the motor and crank, but there still needs to be sufficient space for the crank to rotate. If space is at a premium, an alternative method may be necessary.
Behavior
Cranks are ideal for most cyclical actions, except for those that require constant velocity. The movement over time of a crank resembles a sine wave, since the velocity changes: it starts out slow, accelerates, then slows down again before reversing. The behavior of some devices is very well-suited to the sine wave behavior. An example would be a rocking chair: it slows down gradually before stopping at the end of each cycle, rather than stopping and starting instantaneously. An example where this would not be acceptable would be something like rollup doors: they move at a constant rate in each direction, start to finish. In this case, a linear drive would be preferable. Described graphically, the crank drive would be the sine (curved) wave, below left; the linear drive would be the triangular wave, below right.
The choice of which to use is dependent on two factors: how the real-life device functions, and which is more practical to implement under the circumstances. Note, however, that a crank can simulate a linear drive if only a small portion of its full rotation is used, which would be constrained by limit switches; see the final mechanism for the crossing gates to see this implementation. (This is how Tortoise switch machines behave: they're an entirely rotary mechanism with an almost perfect linear motion output.)
Practical Application
Here's what an example of a crank can look like (this is the combine door):
Examples
- Combine Door
- Enginehouse Doors
- Fishermen
- Gas Station Car Lift
- Oil Column
- Oscillating Desk Fan
- Road Compactor
- Rocking Chair
- Rolling Lift Bridge
- Train Order Signals
- Water Tower