16-09-2016, 03:57 PM
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Abstract: Contactless mechanical components are mechanical sets for conversion of
torque/speed, whose gears and moving parts do not touch each other, but rather they
provide movement with magnets and magnetic materials that exert force from a certain
distance. Magneto-mechanical transmission devices have several advantages over
conventional mechanisms: no friction between rotatory elements (no power losses or heat
generation by friction so increase of efficiency), no lubrication is needed (oil-free
mechanisms and no lubrication auxiliary systems), reduced maintenance (no lubricant so
no need of oil replacements), wider operational temperature ranges (no lubricant
evaporation or freezing), overload protection (if overload occurs magnet simply slides but
no teeth brake), through-wall connection (decoupling of thermal and electrical paths and environmental isolation), larger operative speeds (more efficient operative conditions),
ultralow noise and vibrations (no contact no noise generation). All these advantages permit
us to foresee in the long term several common industrial applications in which including
contactless technology would mean a significant breakthrough for their performance. In
this work, we present three configurations of contactless mechanical passive components:
magnetic gears, magnetic torque limiters and superconducting magnetic bearings. We
summarize the main characteristic and range of applications for each type; we show
experimental results of the most recent developments showing their performance.
Introduction
Contactless mechanical components are mechanical sets for the conversion of torque/speed, whose
gears and moving parts do not touch each other, but rather they provide movement with magnets and
magnetic materials that exert force from a certain distance without any kind of contact between
moving parts. Magneto-mechanical transmission devices have several advantages over conventional
mechanisms: lack of wear, silent operation, reduced vibration, no need of lubrication, overload
protection, reduced maintenance and improved reliability.
Furthermore, they are able to work in cryogenic environments, requirement which is more and more
demanded for the new space telescopes and astronomical instruments because the lower the
temperature, the better the sensitivity of some new sensors. At very low temperatures, conventional
mechanisms present tribological problems in bearings and joints like backlash, cold spots, fatigue and
wear [1,2]. Only solid lubricants such as PFTE or MoS2 can be a solution at low temperatures [3].
However, for long life-time operation solid lubricants turn out not to be a very reliable solution.
Contactless mechanical components may represent the optimal solution to aerospace, space and
mechanical engineering industry where lifetime and reliability are a key factor. In this way,
governments and companies are investing in the research and development of such a kind of
mechanical devices. The specific development of magnetic gears for aerospace has been the objective
of several projects carried out in MAG SOAR facilities; some of them funded by the European
Community’s SPACE and CLEAN SKY Programs.
In this work, we present three types of contactless mechanical components: magnetic gears,
magnetic torque limiters and superconducting magnetic bearings. All of these components are
completely passive; therefore, external mechanical power supply is always needed. In this work, we
state the main characteristic and range of applications for each type; we show experimental results of
the most recent developments using both models and prototypes comparing their performance to
commercial conventional mechanical systems.
2. Magnetic Gears
Magnetic gears were proposed almost a hundred years ago. The absence of contact and wear
between teeth seemed a worthy feature to prompt their development, but low magnetic product,
difficulties with manufacturing techniques and cost were strong handicaps. At the beginning of this
century, attention was paid to their development, using new magnetic materials with higher
magnetization or permeability, new precise manufacturing techniques and advanced magneto-mechanical
modeling tools. The number of papers devoted to magnetic gears has increased exponentially in the
last two years and the technology has overcome many of the first difficulties [4,5].
The specific development of magnetic gears for aerospace has been the objective of several projects
carried out in MAG SOAR facilities; some of them funded by the European Community’s SPACE and
CLEAN SKY Programs ([FP7/2007-2013]) [6–8].
The main advantages of magnetic gears are a consequence of the absence of contact between teeth.
There is no wear. No lubricant is needed. They can be operated at a broad range of temperature ranging
from −270 °C up to 350 °C depending on the kind of bearings they mount. They present intrinsic
antijamming properties and there is a clutching effect if the applied torque overpasses a limit therefore
protecting the output from overloads. This effect is completely reversible. No damage or wear is
produced while operating. The motion direction is also reversible with highly reduced backlash. Input
and output axles can also be exchanged so that the same device can be used as a reducer or a
multiplier. An additional advantage is that they are suitable for through-wall transmission requiring no
joints or sealing. Magnetic gears are also compatible with the presence of dust, sand or non-magnetic
particles. As there is a gap between the moving parts, sand can flow not producing significant
scratches, wear or stalling.
Magnetic gears can be designed in configurations similar to conventional gears: mainly spur gears,
planetary gears and “harmonic drives”.
2.1. Spur Gears and Planetary Gears
Direct spur gears consist of a pinion and wheel set with permanent magnets alternating their poles
able to magnetically engage. These poles are equivalent to the teeth in conventional gears. The
characteristics of this sort of gears are greatly dependent on size, shape, materials and geometry. As in
conventional spur gears only moderated ratios can be achieved. Although useful for some applications,
spur gears are worth to be combined in planetary configurations to achieve high ratios with relatively
low mass and volume. Figure 1 shows two examples of spur gear and planetary gear configuration.
Table 1 shows the performance of a magnetic planetary gear. The data shown for the planetary gear
correspond to a general configuration not yet optimized. An optimization of parameters with a defined
objective can typically improve greatly any of the characteristics.
The torque density of the magnetic planetary gear is smaller than the purely mechanical one
(planetary gear from HD company Size 14 has 65 kNm/m3
[9]) despite the fact that the maximum
torque, reduction ratio and maximum speeds are similar. Efficiency is expected to be greater due to the
lack of losses in the teeth contact. Temperature operational ranges are larger because there is no need
to lubricate the teeth contact; however, it will always depend on the bearing selection
The case is quite different in conventional gears (blue line in Figure 3). A backlash is present due to
the clearances needed for the movement. Therefore, if the movement is reversed there is a small but
not negligible backlash around the origin. Once the gears are engaged, the input axle presents a rigid
behavior with stiffness of the order of magnitude of the flexural stiffness of the teeth. Once the
maximum admitted torque is reached a plastic deformation and fracture appears with permanent
damage of the conventional gear.
The radically different behavior of magnetic and mechanical gears makes the meaning of
“maximum admissible torque” to be quite distinct. For mechanical gears this means: “if you overpass
that value you break the device”. For magnetic gears this means: “it will transmit the movement up to
that value of the torque, but if you try to overpass it will slide and nothing breaks down”. The zero
backlash properties are very desirable for the accurate control of the position in mechanism. Since
these devices are torsionally more compliant, some specific considerations must be taken when
controlling the position [10,11].
3. Magnetic Torque Limiters
Other magnetomechanical devices that can be used for power transmission are magnetic torque
limiters, Figure 4. They behave as 1:1 transmissions whenever the torque is under a limit. If the applied torque overpasses this limit they present an antijamming clutching effect that protects the
output structure. Specific and optimized designs make these devices quite compact and competitive.
Abstract: Contactless mechanical components are mechanical sets for conversion of
torque/speed, whose gears and moving parts do not touch each other, but rather they
provide movement with magnets and magnetic materials that exert force from a certain
distance. Magneto-mechanical transmission devices have several advantages over
conventional mechanisms: no friction between rotatory elements (no power losses or heat
generation by friction so increase of efficiency), no lubrication is needed (oil-free
mechanisms and no lubrication auxiliary systems), reduced maintenance (no lubricant so
no need of oil replacements), wider operational temperature ranges (no lubricant
evaporation or freezing), overload protection (if overload occurs magnet simply slides but
no teeth brake), through-wall connection (decoupling of thermal and electrical paths and environmental isolation), larger operative speeds (more efficient operative conditions),
ultralow noise and vibrations (no contact no noise generation). All these advantages permit
us to foresee in the long term several common industrial applications in which including
contactless technology would mean a significant breakthrough for their performance. In
this work, we present three configurations of contactless mechanical passive components:
magnetic gears, magnetic torque limiters and superconducting magnetic bearings. We
summarize the main characteristic and range of applications for each type; we show
experimental results of the most recent developments showing their performance.
Introduction
Contactless mechanical components are mechanical sets for the conversion of torque/speed, whose
gears and moving parts do not touch each other, but rather they provide movement with magnets and
magnetic materials that exert force from a certain distance without any kind of contact between
moving parts. Magneto-mechanical transmission devices have several advantages over conventional
mechanisms: lack of wear, silent operation, reduced vibration, no need of lubrication, overload
protection, reduced maintenance and improved reliability.
Furthermore, they are able to work in cryogenic environments, requirement which is more and more
demanded for the new space telescopes and astronomical instruments because the lower the
temperature, the better the sensitivity of some new sensors. At very low temperatures, conventional
mechanisms present tribological problems in bearings and joints like backlash, cold spots, fatigue and
wear [1,2]. Only solid lubricants such as PFTE or MoS2 can be a solution at low temperatures [3].
However, for long life-time operation solid lubricants turn out not to be a very reliable solution.
Contactless mechanical components may represent the optimal solution to aerospace, space and
mechanical engineering industry where lifetime and reliability are a key factor. In this way,
governments and companies are investing in the research and development of such a kind of
mechanical devices. The specific development of magnetic gears for aerospace has been the objective
of several projects carried out in MAG SOAR facilities; some of them funded by the European
Community’s SPACE and CLEAN SKY Programs.
In this work, we present three types of contactless mechanical components: magnetic gears,
magnetic torque limiters and superconducting magnetic bearings. All of these components are
completely passive; therefore, external mechanical power supply is always needed. In this work, we
state the main characteristic and range of applications for each type; we show experimental results of
the most recent developments using both models and prototypes comparing their performance to
commercial conventional mechanical systems.
2. Magnetic Gears
Magnetic gears were proposed almost a hundred years ago. The absence of contact and wear
between teeth seemed a worthy feature to prompt their development, but low magnetic product,
difficulties with manufacturing techniques and cost were strong handicaps. At the beginning of this
century, attention was paid to their development, using new magnetic materials with higher
magnetization or permeability, new precise manufacturing techniques and advanced magneto-mechanical
modeling tools. The number of papers devoted to magnetic gears has increased exponentially in the
last two years and the technology has overcome many of the first difficulties [4,5].
The specific development of magnetic gears for aerospace has been the objective of several projects
carried out in MAG SOAR facilities; some of them funded by the European Community’s SPACE and
CLEAN SKY Programs ([FP7/2007-2013]) [6–8].
The main advantages of magnetic gears are a consequence of the absence of contact between teeth.
There is no wear. No lubricant is needed. They can be operated at a broad range of temperature ranging
from −270 °C up to 350 °C depending on the kind of bearings they mount. They present intrinsic
antijamming properties and there is a clutching effect if the applied torque overpasses a limit therefore
protecting the output from overloads. This effect is completely reversible. No damage or wear is
produced while operating. The motion direction is also reversible with highly reduced backlash. Input
and output axles can also be exchanged so that the same device can be used as a reducer or a
multiplier. An additional advantage is that they are suitable for through-wall transmission requiring no
joints or sealing. Magnetic gears are also compatible with the presence of dust, sand or non-magnetic
particles. As there is a gap between the moving parts, sand can flow not producing significant
scratches, wear or stalling.
Magnetic gears can be designed in configurations similar to conventional gears: mainly spur gears,
planetary gears and “harmonic drives”.
2.1. Spur Gears and Planetary Gears
Direct spur gears consist of a pinion and wheel set with permanent magnets alternating their poles
able to magnetically engage. These poles are equivalent to the teeth in conventional gears. The
characteristics of this sort of gears are greatly dependent on size, shape, materials and geometry. As in
conventional spur gears only moderated ratios can be achieved. Although useful for some applications,
spur gears are worth to be combined in planetary configurations to achieve high ratios with relatively
low mass and volume. Figure 1 shows two examples of spur gear and planetary gear configuration.
Table 1 shows the performance of a magnetic planetary gear. The data shown for the planetary gear
correspond to a general configuration not yet optimized. An optimization of parameters with a defined
objective can typically improve greatly any of the characteristics.
The torque density of the magnetic planetary gear is smaller than the purely mechanical one
(planetary gear from HD company Size 14 has 65 kNm/m3
[9]) despite the fact that the maximum
torque, reduction ratio and maximum speeds are similar. Efficiency is expected to be greater due to the
lack of losses in the teeth contact. Temperature operational ranges are larger because there is no need
to lubricate the teeth contact; however, it will always depend on the bearing selection
The case is quite different in conventional gears (blue line in Figure 3). A backlash is present due to
the clearances needed for the movement. Therefore, if the movement is reversed there is a small but
not negligible backlash around the origin. Once the gears are engaged, the input axle presents a rigid
behavior with stiffness of the order of magnitude of the flexural stiffness of the teeth. Once the
maximum admitted torque is reached a plastic deformation and fracture appears with permanent
damage of the conventional gear.
The radically different behavior of magnetic and mechanical gears makes the meaning of
“maximum admissible torque” to be quite distinct. For mechanical gears this means: “if you overpass
that value you break the device”. For magnetic gears this means: “it will transmit the movement up to
that value of the torque, but if you try to overpass it will slide and nothing breaks down”. The zero
backlash properties are very desirable for the accurate control of the position in mechanism. Since
these devices are torsionally more compliant, some specific considerations must be taken when
controlling the position [10,11].
3. Magnetic Torque Limiters
Other magnetomechanical devices that can be used for power transmission are magnetic torque
limiters, Figure 4. They behave as 1:1 transmissions whenever the torque is under a limit. If the applied torque overpasses this limit they present an antijamming clutching effect that protects the
output structure. Specific and optimized designs make these devices quite compact and competitive.