Field of the invention
[0001] The invention relates to a rotary conductor comprising a first circular body having
a first metal circular contact surface, a first electrical terminal attached to the
first contact surface, a second circular body having a second metal circular contact
surface attached to a second electrical terminal, wherein the first and second metal
contact surfaces engage in rolling contact, the metals of the contact surfaces having
a predetermined hardness and a corresponding yield pressure.
Background of the invention
[0002] When electrical current needs to be transferred between parts that show relative
rotation, such as machine parts, wind turbines or offshore high voltage swivels, many
different solutions are known, some of which allow a limited angle of rotation while
others allow unlimited rotational angles.
[0003] In known slide contacts, such as available from Schleifring or Cavotec, a stack of
rings or discs is contacted by one or more sliding contacts or carbon brushes per
ring to provide electrical contacts. The slide contacts have several disadvantages
such as wear of the contact surfaces. Wear is counteracted by the use of expensive
metal alloys and reduced contact pressures between the slide contacts and the rings.
Many carbon or composite brushes also contain oil providing lubrication and reducing
wear. Typical carbon brushes are used for power transfer, whereas gold or silver brushes
are used for transfer of electrical signals.
[0004] Furthermore, the known slide contacts are sensitive to vibrations, due to the low
contact pressures between the sliding contact members and the rings. Too low contact
pressures may lead to spark forming. Also, in corrosive environments such as in wind
turbines and cranes that are used in maritime environments, the conductivity between
the sliding contact members and the rings may decrease due to corrosion. Finally,
the known conductors are less suitable for successive smaller rotations or oscillations
and changes in the direction of rotation.
[0005] Another category of current transfer devices is formed by electricity chains connected
to machines or robots, which are suitable for limited rotational angles. Despite limited
angles of rotation, fatigue loading of the copper conductors by repeated bending may
result in a reduced operational life cycle.
[0006] Other solutions for transferring electric current from a stationary body to a rotating
member utilize liquid metal (e.g. mercury), which however is highly toxic and can
only be used for transfer of limited power.
[0007] A rotary conductor is known having coaxial rings, the gap between which is bridged
by circular rings that are deformed into a slightly oval shape by the pressures applied.
This system is relatively costly due to the expensive gold/silver surface and is subject
to fatigue weakening of the deforming rings. Furthermore, the whole conductor needs
to be protected against oxidation by preventing oxygen from entering the internals,
which would lead to corrosion of the contact surfaces.
[0008] From
US 5,501,604 a rotary conductor according to the pre-amble of claim 1 is known. In this reference
a planetary power or signal transmission band-gear system is described in which the
flexible metal bands of planet gear assemblies are preloaded against the flexible
metal bands of the sun and ring wheels. In so doing, the bands on the planet gear
assemblies deform elastically to provide greater area contact, band deformations ranging
between 50 µm and 250 µm. The degree of preloading is dependent on the particular
application, a device designed for the transmission of power requiring a higher preloading
force than that designed for signals only.
The power that can be transmitted with the known device is limited in view of the
relatively high electrical resistance across the rotating conductors. The lay-out
of the known transfer device is relatively complex in view of the combination of the
conducting bands with intermeshing gears that drive the rotary motion.
[0009] It is, in view of the above, an object of the present invention to provide a rotary
conductor with which high currents can be transferred in a stable and continuous manner
between parts that rotate relative to one another. It is a further object of the invention
to provide a rotary conductor which is suitable for high speed signal transfer between
rotating parts.
[0010] It is again an object of the present invention to provide a rotary conductor that
is of a simple construction and can be easily manufactured, installed and serviced
or repaired. It is finally an object of the invention to provide a rotary conductor
that has reduced sensitivity to vibrations, corrosion and that can operate reliably
when subject to small reciprocating movements.
Summary of the invention
[0011] Hereto a rotary conductor according to the invention is characterized in that the
metals of the contact surfaces having a predetermined hardness and a corresponding
yield pressure, a contact pressure between the contact surfaces being greater or equal
to at least 10%, preferably at least 15% of the yield pressure of the contact surface
metal having the lowest hardness such that the contact surfaces are smoothed by plastic
deformation.
[0012] It was found that by placing the contact surfaces into rolling contact at high contact
pressures, a very stable and efficient transfer of electric current can be obtained.
It is assumed that due to the high pressures, uneven surface textures in the metal
on metal contact surfaces are evened out and the contact surfaces are brought in a
closely mating relationship such that a highly conductive current path is established,
while wear is prevented by the rolling metal on metal contact. Furthermore, the high
pressures ensure that sufficient frictional engagement between the contact surfaces
is present to avoid slipping and to ensure a pure rolling motion without requiring
the presence of intermeshing teeth.
[0013] During manufacture of the conductor, the contact surfaces are smoothened by any suitable
means such as machining, forging, rolling or other methods. However, at microscopic
level still significant valleys and peaks remain. To further smoothen the surface
of the conductors, the invention teaches to apply prior to and/or during use, the
rolling configuration of the conductors at high contact pressures such that the microscopic
peaks on the contact surfaces are smoothened by plastic deformation and bright smooth
contact surface is obtained.
[0014] By the high rolling contact pressures it was found for copper alloy rings that the
Ra value, which is the arithmetic average of the absolute values of profile height
variations from the mean line, recorded over the evaluation length, ranged between
1.6 µm and 6.3µm prior to plastic deformation, while the Ra value after rolling at
high contact pressures was found to lie between 0.1 µm and 0.8 µm. The continuous
rolling contact at high pressures according to the invention was found to have a fine
mechanical "cold forming" effect (plastic deformation), causing a smoothening of the
surfaces while also the surface hardness was found to increase (work-hardening) by
a factor of 2-2.5 for the investigated copper alloy.
[0015] With the term "yield pressure" as used herein, the pressure is intended at which
the deformations in the metal change from being elastic to being plastic. In absence
of data for the yield pressure of a particular metal, as an approximation the yield
stress can be taken. For instance for a copper alloy of which a yield stress of 120N/mm
2 is indicated, a contact pressure on this basis is set at least 12 N/mm
2, preferably at least 18N/mm
2.
[0016] With the term "rolling contact" as used herein, is meant a movement of one contact
surface along the other substantially without any slip between the contact surfaces,
one of which rotates around a central axis.
[0017] The "hardness" as defined herein can be measured by the Brinell Hardness (BH), wherein
the contact pressure during manufacturing of the conductors by pre-rolling of the
contact surfaces, or during use, is about at least 50% of the Brinell Hardness (HB).
Hereby plastic deformation of the contact surfaces is achieved. For a copper alloy,
the contact pressure on the basis of a Brinell Hardness of between 40 and 45 is set
at at least 20-22.5 N/mm
2
[0018] The rotary conductor according to the invention may be produced by applying an initial
rolling contact of the first and second contact surfaces at an initial relatively
high value, the contact pressure during use of the rotary conductor being reduced
to 33%- 50% of the initial value. The rotary conductor may comprise a first ring with
an internal contact surface, and a second wheel-shaped or ring-shaped conductor of
smaller diameter rolling on the internal contact surface. Alternatively, the first
rotary conductor may be ring-shaped or wheel -shaped with an external contact surface,
one or more second ring- or wheel -shaped conductors rolling along the external contact
surface.
[0019] The metals used in the rotary conductor comprise highly conductive metals such as
silver, gold, copper and aluminium or an alloy thereof.
[0020] Preferably, the contact pressure between the first and second contact surfaces is
at least 20N/mm
2 for contact surfaces comprising copper and at least about 40 N/mm
2 for contact surfaces comprising phosphor bronze
[0021] Although it is preferred that a flat metal on metal contact is established, the rotary
conductor according to the invention may have first and second contact surfaces that
are provided with meshing teeth in order to counteract any slipping movement.
[0022] The first body may comprise a ring-shaped outer body having an inner contact surface
with a first centre line and a first radius, the second body comprising a second circular
body having an outer contact surface with a second centre line and a second radius,
the second center line being at a distance from the first center line, which distance
is smaller than the second radius.
[0023] The outer body may be stationary and the second body may be rotatable around the
first centre line, the second terminal comprising a universal joint conductor that
is with one end connected to the rotatable inner body and with its other end connected
to a rotary support that is situated on the first centre line. The transfer of current
via the universal joint provides a stable and reliable solution which does not suffer
from vibrations, which allows rotation of the bodies at high speed, which accommodates
high-frequency signal transfer and/or transmission of high currents without the risk
of spark formation at little loss.
[0024] In an embodiment of a rotary conductor according to the invention , the first conductor
is ring-shaped, the second conductor being wheel-shaped, the second conductor having
a smaller diameter than the first conductor and having at least a 20% higher yield
value than the first conductor. The wheels material is harder than the ring material.
The wheels are pre-rolled before assembly of the rotary conductor, and the rings are
machined before assembly. After assembly, the rings are rolled by a temporarily higher
contact pressure that is sufficient to roll the weaker ring material.
[0025] A further embodiment of a rotary conductor according to the invention comprises at
least two outer bodies that are supported in a spaced-apart relationship, each connected
to a respective terminal, the inner bodies comprising corresponding spaced-apart ring-shaped
members rotatably mounted on a rotary support that is rotatable around the first centre
line, about an axis situated at a radial distance from the central first center line.
The outer bodies form a stack of ring-shaped conductors, one for each phase of current
to be transferred. The inner ring-shaped bodies rotate jointly and roll along the
inner tracks of the outer bodies to provide parallel current paths. The current is
divided evenly over the various rotating conductors. Hereby it is ensured that even
if one conductor would lose proper contact, no sparking and consequent material damage
will occur as the other conductors can temporarily accommodate the higher current.
[0026] A further rotary conductor has a first body that comprises a ring-shaped angled contact
surface with a central axis, the second body comprising at least one radial angled
ring-shaped contact surface rotatably mounted around a radial axis, which axis is
rotatable around the central axis of the first body. The conical second bodies that
rotate about the radial axis provide for stable and even load distribution on the
first ring shaped conductors, allowing high contact pressures while not being subject
to wear.
[0027] Preferably the first body is provided with opposed and spaced-apart angled contact
surfaces that are each contacted by a respective body having a radial angled ring-shaped
contact surface rotatably mounted around a radial axis, which axis is rotatable around
the central axis of the first body. In this even load distribution the axial pressures
exerted on the first body compensate each other so that high contact pressures are
possible.
[0028] The second body may comprises a spring element that contacts the at least one radial
contact surface for biasing the radial contact surface in the direction of the central
axis. The spring biasing elements provide an adjusting force for equalising the contact
forces and for removing any play in the radial direction.
[0029] In a preferred embodiment, the rotary conductor comprises conducting oil between
the first and second contact surfaces. Surprisingly it was found that the voltage
loss between the conductors is strongly reduced by use of oil film between the rotating
bodies. In combination with the high pressure, a reduction in resistance of over 20%
could be achieved. The oil used may be insulation oil, such as transformer oil. However,
the best results were found when using an oil that is a non-conducting penetrating
oil that comprises a suspension of conducting lubricating particles, preferably graphite
particles.
[0030] The rotary conductor according to the invention is suitable for conducting currents
from the first electrical terminal to the second electrical terminal of at least 10
A, preferably at least 25 A, more preferably at least 100 A. For copper electrodes
having a contact surface area of about 2mm
2, currents of up to 60A/mm
2 were measured at a contact pressure of 100-150 N/mm
2 and of up to 4-5A/mm
2 at pressures of 30-50N/mm
2. For copper electrodes, a minimum pressure of 20N/mm
2 is applied. For electrodes comprising phosphor bronze, currents of up to 40A/mm2
were achieved at pressures of 40-600N/mm
2.
[0031] The rotary conductor according to the invention can be used in wind turbines, offshore
installations such as Floating Production Storage and Offloading vessels (FPSO's),
or in machine parts. The rotary conductors can also be used for transmitting electrical
signals from one contact surface to the other at data rates of up to 1 Gb/s and higher.
Brief description of the drawings
[0032] Some embodiments of a rotary conductor will by way of non-limiting example be described
in detail with reference to the accompanying drawings. In the drawings:
Fig.1 shows a schematic representation of a stationary ring-shaped outer conductor
and an eccentric rotating inner conductor,
Figs. 2a and 2b-2c show a schematic cross-sectional view through the contact interface
of the conductors prior to, and after rolling contact at high contact pressures, respectively,
Fig. 3 shows a detail of a universal joint conductor connecting to the rotating conductor,
Fig. 4 shows a perspective view of a rotary conductor in a stacked configuration,
having a universal joint conductor,
Fig. 5 shows a schematic view of an embodiment with spring-biased conical conductors,
and
Fig. 6 shows a side view of the embodiment of fig. 4.
Detailed description of the invention
[0033] Fig. 1 shows a rotary conductor 1 for the transfer of current from a rotating terminal
4 to a stationary terminal 5. In figure 1, conductor 3, in the form of a stationary
outer ring, has a first centreline C1, and an internal radius R1 and forms a raceway
for second conductor 2, being formed by an inner ring or cylinder having a second
centreline C2 at a distance s from first centreline C1 and a radius R2, wherein s
= R1-R2. The centreline C2 will move along the circular path with radius s about the
first center line C1. The pattern of movement of the terminal 4 connected to the circumference
of conductor 2 is formed by the combined rotation about the second center line C2
and the rotation of the center line C2 about C1. In this embodiment, R2 preferably
is about the size of R1 so that the curvature of inner and outer rings only slightly
differ and a large contact surface for current transfer is available.
[0034] For the following metals, the minimum contact pressures apply:
Metal |
HB value |
Min pressure N/mm2 |
Pure Aluminium |
15 |
7.5 |
Gold |
20 |
10 |
Silver |
25 |
12.5 |
Pure Copper |
40 |
20 |
Electrical Copper |
45 |
22.5 |
Phosphor Bronze |
90 |
45 |
Mild Steel |
110 |
55 |
[0035] Fig. 2a shows a schematic cross-sectional view through the contact surfaces of conductors
2,3 prior to engaging the surfaces at high contact pressures. The Ra values are relatively
high and the contact interface is limited.
[0036] Figs. 2b and 2c show the conductors 2,3 in a contacting and in a separated state,
respectively after having been in rolling contact at high contact pressures over a
time period of a significant number of cycles, such as during several hours, preferably
days. The Ra value has decreased and the number of contact surfaces 2' has increased
due to smoothening caused by the plastic deformation.
[0037] Fig. 3 shows a perspective view of the rotary conductor 2, that is supported in a
bearing 6 that rotates around the first center line C1. The terminal 4 is formed by
a universal joint conductor 7 having a first set of perpendicular hinge axes 8,9 and
a second set of perpendicular hinge axes 10,11 connecting to a drive axis 12 along
the first center line C1. In this way, the combined translational and rotational movement
of the inner conductor 2 are transferred to the rotation of the drive axis 12 about
first center line C1. Current from the rotating drive axis 12 can hence be transferred
via the universal joint conductor 7 and rotary conductor 2 to the stationary conductor
3.
[0038] Fig. 4 shows an embodiment of a stacked rotary conductor 20 comprising a base plate
21 and two spaced-apart stationary conductors 22, 23 supported by axial supporting
rods 24,25 that interconnect the base plate 21 with a top plate 26. A support guiding
plate 27 is attached to the base plate 21 so that it can rotate via a bearing construction
(that is not shown in the drawing) around the center line C1. A stack of rotating
conductors 30, 31 is placed onto the support guiding plate 27, the surfaces of which
roll along circular contact surfaces 32,33 of the stacked stationary conductors 22,23
that are mounted on the supporting rods 24,25. Support guiding plates 27, 28 are provided
that interact with rotating bearing elements 39,40 which are in line with the conductors
30,31, for providing a stable rolling motion of the conductors 30,31 along the circular
tracks of the stationary conductors 22,23. The contact surfaces 30,31 may for instance
be provided with teeth that mesh with corresponding teeth of the internal gear plate
37.
[0039] At the top end of the stacked rotary conductor 20, a universal joint conductor 41
connects the conductors 30,31 to the drive member 42, that is rotating around the
axis C1.
[0040] Fig. 5 shows an embodiment wherein a first conductor comprises a conical member 50
rotatably supported on a radial axis 51. The radial axis 51 rotates around center
line L. The conical member 50 contacts with angled contact surfaces 52, 53 corresponding
angled contact surfaces of upper rotary electrode 54 and lower stationary electrode
55. A biasing spring member 56 provides an axially compressive force to maintain a
predefined contact pressure between the angled surfaces of the conical member 50 and
the upper rotary electrode 54 and the lower stationary electrode 55.
[0041] In fig. 6 it can be seen that rotary electrode 62 comprises upper and lower angled
contact surfaces 63,64 that are encased between upper and lower conical electrodes
60,61, such that forces on the electrode 62 balance out and effective current transfer
at high contact pressures and high rotational speeds can be obtained. The high contact
pressure results in a smooth rolled surface 65.
[0042] With the embodiment according to fig. 6, multiple contact points between the upper
and lower electrodes 63, 64 and a number of conical electrodes 60,61 can be constructed
such that the current transferred between the electrodes 63,64 and the electrodes
60,61 can be strongly increased.
1. Rotary conductor (1,20) comprising a first circular body (3,22,23) having a first
metal circular contact surface, a first center line (C1) and a first electrical terminal
(5) attached to the first contact surface, a second circular body (2,30,31) having
a second metal circular contact surface attached to a second electrical terminal (4)
and rotatable about a second center line (C2), wherein the second center line (2)
is rotatable about the first center liner (C1) wherein the first and second metal
contact surfaces engage in rolling contact, the metals of the contact surfaces having
a predetermined hardness and a corresponding yield pressure, Characterised in that the second circular body (2,30,31) is wheel-shaped and that upon rotation of the
second center line (C2) about the first center line (C1), a contact pressure between
the contact surfaces is greater or equal to at least 10%, preferably at least 15%
of the yield pressure of the contact surface metal having the lowest hardness such
that the contact surfaces are smoothed by plastic deformation.
2. Rotary conductor according to claim 1 or 2, wherein prior to use and/or or during
use, the contact pressure is about at least 50% of the Brinell Hardness (HB) of the
contact surfaces.
3. Rotary conductor according to claim 1, wherein at least one of the metal contact surfaces
comprises copper, the contact pressure being at least 20N/mm2.
4. Rotary conductor according to claim 1 or 2, wherein at least one of the metal contact
surfaces comprises phosphor bronze, the contact pressure being at least 40N/mm2.
5. Rotary conductor (1,20) according to claim 1 or 2, wherein the contact pressure between
the first and second contact surfaces is at least 20N/mm2.
6. Rotary conductor (1,20) according to any of the preceding claims, wherein the first
body comprises a ring-shaped outer body (3) having an inner contact surface with a
first centre line (C1) and a first radius (R1), the second circular body (2) having
an outer contact surface with a second centre line (C2) and a second radius (R2),
the second center (C2) line being at a distance (S) from the first center line (C1),
which distance (S) is smaller than the second radius (R2).
7. Rotary conductor (1,20) according to claim 6, wherein outer body (3) is stationary
and the second body (2) is rotatable around the first centre line (C1), the second
terminal (4) comprising a universal joint conductor (7) that is with one end connected
to the rotatable inner body (2) and with its other end connected to a rotary support
(12) that is situated on the first centre line (C1).
8. Rotary conductor (20) according to claim 6 or 7, wherein at least two outer bodies
(22,23) are supported in a spaced-apart relationship, each connected to a respective
terminal, the inner bodies comprising two spaced-apart cylindrical members (30,31)
rotatably mounted on a rotary support (27) that is placed at a radial distance from
the first center line (C1) and that is rotatable around the first centre line.
9. Rotary conductor according to any of claims 1-6, wherein the first body (54,55) comprises
a ring-shaped angled contact surface, with a central axis (L), the second body (50)
comprising at least one radial angled ring-shaped contact surface (52,53) rotatably
mounted around a radial axis (51), which axis is rotatable around the central axis
(L) of the first body (54,55).
10. Rotary conductor according to any of the preceding claims, the conductor being suitable
for leading currents from the first electrical terminal to the second electrical terminal
of at least 10 A, preferably at least 25 A, more preferably at least 50 A.
11. Rotary conductor according to any of the preceding claims , the first conductor being
ring-shaped, the second conductor being wheel-shaped, the second conductor having
a smaller diameter than the first conductor and having at least a 20% higher yield
value than the first conductor.
12. Method of transferring electricity between a first and second circular body in a rotary
conductor according to any of the preceding claims, wherein after initial rolling
contact of the first and second contact surfaces at an initial relatively high value,
the contact pressure is reduced to 33%- 50% of the initial value.
13. Method of transferring electricity between a first and a second circular body in a
rotary conductor according to any of the preceding claims, wherein data signals are
transferred between the first and second bodies.