In this article I will talk about the magnetic gearbox developed by us. This gearbox is planned to be used in conjunction with a brushless motor. The article will describe the possible scope of this gearbox, its pros and cons compared to conventional gearboxes.
Introduction
Brushless electric motors (BLDC) have extremely high power with a minimum size. This is achieved thanks to powerful rare earth magnets. When creating a motor in a small form factor, such engines have no competition.
However, despite the high power, the small size of the motor imposes many restrictions on it, such as: small torque, high working revolutions, small cooling area. These restrictions do not allow to use the full potential of the motor. For example, a motor of form factor 540 (D = 36mm L = 54mm) is capable of delivering power over a kilowatt with good enough efficiency, but this is possible only at very high speeds, when it becomes quite difficult to remove this power from the motor. In the nominal mode, such a motor without forced blowing can produce 200-400 watts.
The main focus of our team MotoChrome is the design and creation of brushless motors. When designing a motor for one of the customers, we just ran into the problems described above. The customer needed a motor with a small diameter, but at the same time a rather high torque and efficiency. To meet the requirements, the motor turned out to be almost half a meter long with a diameter of 40 mm. The motor turned out to be quite expensive and very difficult to assemble. At the same time, to achieve the same power parameters, you can make the motor 3 times smaller, but using a gearbox. And in the end this option was chosen.
Using a gearbox seems like a good solution. But with him there are many other problems that do not allow to apply them fully in all engines.
- additional knot, having probability of breakage, and limited resource of work
- increase in overall dimensions;
- increased noise, which may be critical in the operation of underwater devices;
- Most mechanical gearboxes have limitations on the transmitted torque and shaft speed on the input shaft of the gearbox, which in turn imposes additional restrictions / requirements on the electric motor.
Picking up the gearbox, we rested in two main problems. 1) In order for the gearbox to realize the full potential of the brushless motor, it must be of a size comparable to the size of the motor. With him, the motor ceases to be compact. 2) Most of the mechanical gearboxes are not able to work at high turnover (> 15000 RPM), and we are interested in precisely high revolutions. Only 1 ... 2 companies agreed to take an order for the required gearbox and deliver it 3 months after receiving the payment. What will be its reliability and durability, we still have to find out, but the difficulties associated with finding the right gearbox, forced us to think about this problem further.
A good option, from our point of view, was a magnetic gearbox. They have not yet received widespread use and quite a bit of information about them, so we decided to look deeper into this issue and make the gearbox ourselves. This will allow us to work out an approach to its modeling and allow us to calculate its parameters in the future.
Magnetic gearbox
We decided to make a magnetic gearbox according to a common scheme, which is an analogue of a planetary gearbox. The magnets in the center are the "sun gear", the magnets on the outside are the "ring gear (epicyclic)", between them the teeth of the magnetic material act as the carrier.
This design has the maximum moment of retention in the minimum volume and it is well suited for our task. As a result, we designed and manufactured the following sample.
The diameter of this sample is 36mm. The input part of the gearbox is made according to the NEMA 17 standard. Instead of the output shaft, we made a universal faceplate, which allows attaching various loads to it in experimental work, tests and experiments, and, if necessary, installing the shaft, thanks to the base fitting hole in the center of the faceplate. By the way, we also applied the concept of universal mounts in our “Electron” motor, which we are preparing for mass production and sales in the territory of the Russian Federation (we will write a separate article about it).
The parameters of the resulting gearbox turned out to be quite close to the calculated ones. It has a decrease of 1:10 and is capable of holding a moment of 0.53 Nm at the output shaft under static load. This is a fairly good indicator for such dimensions. For comparison, the motor in the same dimensions will give such a moment with an efficiency below 50%, and a system with this gearbox will have an efficiency of 80%.
In addition, the gearbox turned out very smooth and quiet. It is absolutely not felt any jerks during rotation, the moment is transmitted very gently. This property has defined the name given to us by the gearbox - “Smoother”.
Another pair of rollers with gear
findings
After the initial testing, we were able to identify the following advantages of the magnetic gearbox compared to the mechanical one:
- Magnetic reducer silent
- Not afraid of high revs. The weak link in it is only the bearings, which also limit the motor.
- Durable. In it, as well as in the motor there is nothing to break except bearings.
- The gearbox is capable of smoothly feeding and holding the moment. For example, if you make a grip on such a gearbox and clamp a fragile object, then the motor jerk will not destroy it.
- This gearbox is protected from overload. If the shaft of the mechanical gearbox exceeds the permissible load, the gears in it will collapse and jam, which can also cause damage to the motor or controller. The magnetic gearbox will simply turn around and will not break, and the motor will not stop.
We also identified a number of shortcomings:
- The moment of the gearbox is limited by the volume and maximum force of the magnets. A mechanical gearbox is capable of transmitting a much larger moment in the same volume.
- The magnets of the gearbox are temperature sensitive and may demagnetize when overheated.
- Since rare-earth magnets are used in the gearbox, the creation of such type of gearboxes is difficult.
Based on the results obtained, we have formed application areas for gearboxes of this type:
- in biology, medicine, and other procedures for capturing and moving objects that require precise positioning on one side (which is achieved by the reducerlessness of the reducer) and there is a high risk of damage to objects with which the manipulator operates, for example, human tissues and organs (the softness of the output rotor and overload protection will limit the maximum impact force on the operated object).
- in industrial devices in which high-speed engines (electric or pneumatic) work in confined spaces, the torque of which needs to be increased and the speed reduced to the required permitted value.
- in underwater devices, where there are high requirements for low noise products
Currently testing gear continues. We carry out its dynamic test at various loads and speeds. If everything goes well, we will be able to implement this development in a real project.