The Basics
What is a Rope Start?
A “rope start,” or pull-start, is a mechanical part that is commonly used to start an engine’s combustion process, like in lawnmowers, chainsaws, or other types of generators. Instead of using an electric starter, which is more common in larger engines, a rope start is a manual way of starting an engine. It is also similar to that of a fishing pole. A rope start often composes a rope and a recoil spring. Pulling the rope feeds kinetic energy into the system that the rope start powers, while the recoil spring resets the rope back to its starting point once external load disappears.
In our project, we are using a rope start as the mechanical mechanism of harnessing wave energy, as the end goal would be to attach the “handle” piece of the pulley to the ocean floor, with the rest of the recoil mechanism attached to the buoy at the water’s surface. As the water moves, relative displacement between the buoy and the ocean floor causes the rope-start to extend, capturing the wave movement, converting it into rotation.
Shown below is one of our initial idea sketches of the buoy attached to a rope start.
Initial concept sketch of buoy and rope start (not drawn to scale)
The recoil spring within the rope start will be constantly pulling the buoy towards the attaching point at the sea floor until the rope length is reset to the starting position. As a result, the rope start will extend/compress according to the wave amplitude, and continuously harvest the constant motion introduced by ocean waves. We chose this approach for our system because it would allow for the device to attach to the seafloor, work well in underwater environments, and allow for easier prototyping.
What is Magnetic Coupling?
Magnetic coupling is a type of coupling that uses permanent magnets to connect two colinear shafts. There is no direct physical connection like a traditional coupler, instead the magnetic field can overcome the separation and allow transfer of kinetic energy [1].
Visual of how magnets are set up in a basic magnetic coupling set-up [1]
This type of technology is more optimal for devices that require isolation of subsystems as it allows separation between components. With our buoy device needing a coupling device to convert wave energy from the ocean to mechanical energy within the buoy without potential leakage, magnetic coupling becomes a great choice, allowing the internal components to be completely sealed off by the housing.
Additionally, magnetic couplings are great when it comes to regulating unpredictable external loads. The magnetic pull within the coupling is easier to overcome compared to traditional coupling, and often breaks when an abrupt load is introduced. Thus, in this project, if abnormal ocean activity introduces sudden overload, the coupling can serve as a failsafe to ensure internal components are not damaged.
Experimental Process with Magnets
Our first steps were to experiment with the number and magnetic strength of magnets we would need for our device of this size, as our gear size and magnet size were limited by our device and manufacturers, respectively.
We first looked at using 6 2.1lb magnets on both the drive and driven sides to see if this created magnetic field would work within the casing and wall between the buoy shell and water. As shown below, this magnetic field was strong enough to show feasibility of a magnetic coupling system for the size of our project. However, it was not strong enough to create the rotation needed in order to move the mechanical energy from the drive to driven component and therefore, we turned to looking for stronger magnets.
Shown below is a video demonstration of this magnetic coupling experiment. As seen, this magnetic field is stronger than the previous experiment, allowing for some rotation to occur on the shaft. However, when the gear is spun quicker, the torque is too great and the magnetic coupling disengages. The two sides are not able to stay aligned and therefore, a stronger magnetic force is needed in order for this method to be successful as the distance between these two components can not decrease anymore.
Shown below is a video demonstrating how the magnetic coupling is successful between the rope start and first gear within the gear train. When pulling on the rope start, the gear turns directly with the pulley and vice versa when it goes in the opposite direction. This showed that we could move on with this number of magnets and force onto the rest of the generator.
Future Experiments
There are many experiments that could be continued or expanded upon with magnetic coupling. One example is to measure and test the torque of different magnetic fields created by different magnetic configurations. This could be done by taking a magnetic coupling set-up, whether it be a pulley system and gear like ours or something else, attaching the driven shaft to a rotational load around the shaft axis, and measuring the maximum load under which the coupling is compromised.
A visual is shown below of what this experiment might look like:
Visual of how force (F), length (L), and torque (t) would be measured in this experiment
References
[1] Stanford Magnets, “Magnetic Coupling – An Introduction,” Stanford Magnets, https://www.stanfordmagnets.com/magnetic-coupling.html
