Technical Field of the Invention
The present invention relates to a brush-brush holder assembly
for a direct current electrical machine.
Background of the Invention
The direct current (DC) machine is one of the first machines
devised to convert electrical power into mechanical power. Electric current flows
through a wire loop located in a magnetic field and presence of the two fields result
in a torque which tends to rotate the rotor. As already known, stationary parts
of the motor are referred to as the stator; moving parts, including the wire loop,
are referred to as the rotor or armature.
As the rotor turns, the current in the armature windings
is commutated to produce a continuous torque output. The electric current is fed
to the wire loop using carbon brushes that make contact with an extension of the
armature known as the commutator. Together, the brush and commutator act as a switch
that changes the direction of the electric current, causing the loop to continually
rotate in a single direction.
The commutator is made of conductive segments representing
the termination of individual coils of wire distributed around the armature. The
carbon brushes are usually held in place with a brush holder, securing the brushes
in the appropriate position relative to the commutator contact surface. The disadvantage
of commutator motors are that sparking tends to occur between commutator segments
and brushes, which eats away the metal of the segment. Even without sparks the rubbing
of brush on segments wears away both and more maintenance is required than in the
case of brushless machines.
There are quite a large number of patents/patent applications
directed at dc machine armature current switching systems. Among others, one such
system is disclosed in
EP 1 489 727
. Among the most prominent drawbacks thereof, one such lies in that the
brush basically consists of two separate parts, namely the holder portion and the
portion that actually provides electrical contact with the commutator; from among
those parts however, only the second part is used in conduction switching. The first
part only serves to the purpose of supporting the operational part in general terms
and transferring force to brush ends to keep electrical contact with the commutator.
The importance of the discussion of brush usability along
its entire length stems from the well-know fact that frequent maintenance requirement
is a major unpleasant factor for a DC machine, this machine although having convenient
operational characteristics such as for instance unsophisticated speed control.
Referring back to
EP 1 489 727
, the disclosure therein proposes three different embodiments, all of which
making use of a support shaft onto which brush portions are mounted through suitable
slots located on their bodies, eventually leading to a two-part non-monolithic design
utilizing a limited length portion of the brushes. More specifically, it is not
anymore possible to benefit from the carbon brush base portions after the brush
end portions are worn away. Over time, it is inevitable that the carbon brushes
wear away and require replacement; this naturally requiring periodic shutdown and
The operational design suggested by the present invention
ensures practical use of the carbon brushes along their entire length instead of
a limited portion thereof, as well as providing a reliable electrical contact with
the commutator. The brushes according to the present invention are directly connected
to support means, which are secured to the brush holder, to apply required contact
force on the brushes in order for holding them against the commutator. The contact
surface between the brush and the commutator is enhanced and a more reliable contact
The physical connection between said support means and
the brushes is arranged such that the force exerted on the non-linear brushes is
delivered to the brush ends and therefrom to the commutator lateral surface in the
manner that the intensity and the direction of the force applied consecutively from
one component to another component is constant for a given length of the non-linear
brush, ensuring both usability of a longer brush due to the operational and effective
design thereof and reliable contact force independent of how much of the brushes
have actually been worn away or left.
Objects of the Invention
The primary object of the present invention is to provide
a DC electric motor brush-brush holder assembly having a longer brush life and therefore
an improved practical usability for the motor.
Summary of the Invention
The present invention proposes an improved design allowing
use of longer brushes in order for extending brush life in which the intensity and
the direction of the contact force required for ensuring electrical conduction in
between the commutator and the brush end is devised in the manner to adapt a brush
body superimposed on an arc portion the center of which is used for pivoting support
means to apply required contact force on the brush body. The support means contacts
the brushes by a single point-connection provided on the longitudinal axis of the
non-linear brush body.
Brief Description of the Figures
Accompanying drawings are given solely for the purpose
of exemplifying a direct current motor brush card assembly whose advantages over
prior art were outlined above and will be explained in detail hereinafter:
Detailed Description of the Invention
- Fig. 1 demonstrates a side view of the brush card assembly taken along the cross-section
A-A depicted in Fig. 2.
- Fig. 2 demonstrates an upper view of the brush card assembly.
- Fig. 3 demonstrates a brush body according to the present invention.
- Fig. 4 demonstrates a brush holder assembly along with torsional spring element
and the brush.
- Fig. 5 demonstrates a brush holder assembly alone.
- Fig. 6 demonstrates a torsional spring element.
Referring now to the figures outlined above, under the
motor cover extend the brush card assembly along with the commutator (11), the brushes
(12) and the metal bars (13) connected to electrical elements (17) for filtering
the noise caused by the commutation of the brushes (12). Since the topology of the
filtering circuit is of no importance in regard to the scope of the present invmention,
and characteristics of a LC low pass filter, which can for instance be used to address
the noise issue, is known to those skilled in the art, no further details will be
given herein in regard to this filtering circuit.
The brush base (14) of Fig. 1 is made of a thermoplastic
material having a high heat deflection temperature. It is a single-piece monolithic
block manufactured along with the brush holders by a single moulding operation.
The thermoplastic material proves to remain solid up to 215° C under 8 MPa
pressure. It is a strong material in terms of mechanical properties (165 MPa tensile
strength and tensile modulus 14400 MPa). Said brush base (14) is fixedly mounted
in between the cover and the motor body by means of flaps (15).
The present invention enables use of longer brushes (12).
The length of a brush (12) is basically the key factor in determining brush life.
According to the present invention, the brushes (12) are conventionally held in
position and supported to push against the commutator by means of support means
in the form of torsional spring elements (16). Fig. 2 shows a 4-pole dc motor with
4 brushes (12). The non-linear brushes (12) contact the commutator (11) segments
in a manner to provide an optimized commutator-brush-brush holder relationship enabling
accommodation of longer brushes due to the way the contact force that is required
for switching the polarization is formed, delivered and applied as will be hereinafter
explained in detail.
The intensity and the direction of the contact force required
for ensuring electrical conduction between the commutator (11) and the brush (12)
end is devised in the manner to adapt a brush (12) body superimposed on an arc portion,
the center of which is used for pivoting said torsional spring element (16) to apply
required contact force on the brush body. The torsional spring element (16) pushes
the brush (12) by a single-point contact force which is tangential to the non-linear
longitudinal axis of the non-linear brush (12) body.
The single-point contact between the brush body (12) and
the torsional element (16) is provided by adapting said torsional element (16) to
have a hook-shape ending (18) along a point on which said brush (12) is pushed against
the lateral surface of the commutator (11). The hook-shape endings (18) of the torsional
spring elements (16) are suitable for engaging in an indentation (19) formed on
the brush (12) body. Said indentation (19) is suitable for receiving said hook-shaped
ending (18) in the manner that the largest diameter of the tubular body of said
hook-shaped ending (18) on the horizontal axis fits on the height of said indentation
(19). The indentation (19) is set parallel to the horizontal axis.
According to the present invention, the torsional elements
(16) are fixedly attached to locking slots (20) provided on the brush holder assembly.
Those locking slots (20) are cylindrical elevations along the periphery of the lateral
surfaces of which are encircled by a circular groove to receive the torsional spring
The brushes (12) according to the present invention are
designed to lie on arc portions on the center of which the torsional elements (16)
are present. The fact that the brush (12) bodies shaped in the form of a proper
arc of a circle whose center is occupied by the torsional elements (16) ensures
that the longitudinal body of the torsional element (16) substantially coincides
onto one of the outermost radiuses of the arc in question. Since the hook-shaped
ending (18) of the torsional element (16) is designed to exert the pushing force
in the direction of the surface normal of the contact surface, the torsional element
(16) precisely lying on the radius and exerting the pushing force in the direction
of the longitudinal axis of the brush (12) body results in a very effective design
n terms of the optimization and maximization of the contact force delivered to the
brush body (12).
As the brushes (12) wear away over time, they get shorter
and shorter and this will eventually lead to a significant change in the original
operation layout of the torsional element (16), brush (12) and commutator (11) trio.
On the other hand, since the original direction of the force exerted by the torsional
element (16) will remain intact due to the geometrical shape of the brush (12) body,
sound operation will go on independent of the brush (12) length. Since the originally
set operation conditions will remain unchanged until the brush (12) body is fully
consumed, the full length of the body will then be functional in terms of usability.
In sum, longer brush usability is ensured due to the operational design of the present
According to the present invention, the point onto which
the contact force is applied within the indentation (19) is exactly on the brush
(12) body longitudinal axis line, which is also joining the contact point to the
center of gravity of the actual brush (12) body. This is due to the fact that the
center of gravity of the brush body for any given length lies on the longitudinal
axis of the brush (12).
Referring now to Fig. 5, the brush holder assembly therein
comprises a set of special arrangements to support the operational design defined
above. The brush holder comprises a set of curvilinear projections (21) designed
to receive the brushes (12) from below. Those are given a corresponding arc form
adapted to the brushes (12) in order for keeping them on the trajectory as long
as they are sufficient in length for motor operation. The projections (21) contact
the brush (12) bottom surface from below by a contact connection extending along
a line. In other words, the points to which the projections (21) make contact with
the brush (12) form a linear contact line. The linear contact line extending longitudinally
along the peak points of the projections (21) basically eliminates unnecessary friction
caused by a two-dimensional contact area.
Again in Fig. 5, it can be seen delimiting elements 22
and 23, which surround the brush (12) of Fig. 4 in the manner to restrict its movement
from both sides. The delimiting element 22 is designed to occupy only a limited
space, in the form of a vertical support extension with a substantially narrow portion
that reaches over the brush (12) upper surface, allowing air flow through to enable
natural cooling by convection.
To evacuate heat occurring with friction and to enable
effective heat transfer, an evacuation window (24) so as to extend longitudinally
along a bottom line of the brush (12) lateral surface is arranged. Such an opening
(25) is also embedded in the middle of the brush (12) holder assembly to reinforce
The brush card assembly of Fig. 2 is designed for a 4-pole
dc motor. The operational design of the present invention so as to incorporate a
4-brush brush cart assembly comprising brush-brush holder assemblies as is explained
so far can accommodate longer brushes and therefore extend brush life in a 4-pole
According to the present invention, the brush (12) end
that makes contact with the commutator (11) is set eccentric to the commutator (11)
in the sense that the mediator of the line that passes from the two edge points
of the brush (12) arc that makes contact with the commutator (11) do not pass from
the geometrical center of said commutator (11). Therefore a more reliable contact
is established by increasing the contact surface between the brush (12) and the
commutator (11). It is apparent from Fig. 7 that the contact surface along a line
I2 is greater than that along I1.
Summarily, the present invention proposes a DC machine
brush cart assembly comprising a brush base (14), a commutator (11), an electrical
filtering circuit for filtering the noise caused by commutation, a plurality of
brush holders each of which with a non-linear brush (12) body and a support means
(15) for exerting required contact force on said brush (12) to push it against said
commutator (11) wherein said brush (12) body is superimposed on an arc portion,
in the center of which is pivoted said support means (16) so as to substantially
coincide onto one of the outermost radiuses of said arc portion and to exert contact
force on said brush (12) by a single-point contact force in the direction of the
surface normal of the contact surface, the contact force also being tangential to
the longitudinal axis of said brush (12) body.