Spiral Spring Units For Pressing Brushes

Abstract

A spiral spring unit for pressing brushes, which consists of a plurality of spiral spring elements bonded together at the opposite ends thereof and which is capable of pressing the brush always with a constant pressure.

Description

This invention relates to improvements in the spiral spring unit for pressing brushes, which is usually used at the current collecting portions of rotary electric motors, etc., and more particularly to improvements in the spiral spring proper thereof.

The brush pressing spiral spring unit of the present invention can be used at the current collecting portions of any and all rotary electric machines, but herein it will be described as used at the current collecting portion of an electric motor for vehicles, to facilitate best understanding of the invention.

At the current collecting portion of a vehicle motor or the like are generally provided conductor wires, a brush and a brush holder, by which current is conducted between a stationary portion and a rotating portion. However, since this type of motor is usually arranged at a place, such as below the floor of a vehicle, where the space is not substantially large, the motor is required to be small in size per. se. and accordingly not a large space is available around the current collecting portion of the motor.

For this reason, the brush and the brush holder are also required to be small in size as a matter of course.

Should the frequency of changing the brush not be considered, simple reduction of the brush and brush holder to meet such demand would be possible. However, in the light of the fact that this type of motor is usually arranged in a limited space, such as beneath the vehicle floor, along with other parts as stated above, it is usual that much time is required for the inspection of the motor and frequent performance of such operation is quite cumbersome and detrimental to the effort of saving labor. Thus, it becomes necessary to use a brush which is low in rate of wear or has a large height, so as to minimize the necessity of changing the brush.

In view of the above, it will be understood that the most ideal collector of this type is the one which is small in size and free of maintenance, or in other words, the one which is operable for an extended period of time without the necessity for changing the brush used therein.

Furthermore, in this type of motor slipping of the brush or jumping and oscillation of the brush on a rotary collector ring, such as commutator, tends to occur due to the high rotational speed of the motor and the vibration of the vehicle, impairing smooth conduction of current. Therefore, means for holding the brush or a brush holder is desired, which is capable of holding the brush against jumping and vibration even when the motor is subjected to intense vibrations.

Under such circumstances, a brush holder including a spiral spring is presently being employed as the most desirable brush holding means, which satisfies substantially all of the requirements set forth above.

The brush holder having a spiral spring has many advantages, such as that since the spring does not have a rotating or sliding portion, there is no fear of the relative rotation or sliding of the brush holder being impaired by the brush powder or dusts in air attaching to the contacting portion; that it is relatively small in size and can be used with a brush having a large height; and that it can absorb the vibrations of the brush because the spring is small in inertia per. se.

Because of such advantages, the brush holder having a spiral spring has been widely used as the most effective one, at least to the present time.

However, with the demand for increasing the speeds of vehicles more and more, which is expected to become greater in the future, and with the demand for rendering vehicles maintenance-free for a longer period of time, the conventional brush holder including a simple spiral spring very recently has become unsatisfactory to meet such demands.

The reason why the brush holder, having heretofore being considered as ideal, has reached the limitation of its use will be explained hereunder with reference to an example thereof shown in FIG. 5 of the drawings.

In FIG. 5 there is shown a spiral spring 10 having a relatively large number of turns. The spring is subjected to a displacement as indicated by the chain line when set in a brush pressing position. Each turn of the coiled portion of the spring is also subjected to a slight displacement in this case but such displacement is not shown.

In the Figure, the displacement of the spring from the position Q to the position P, namely the angle of torsion of the spring, is indicated by symbol .alpha. and it will be obvious that the larger the angle of torsion .alpha. is, the more advantageous, when it is desired to use a brush of larger height.

The angle of torsion .alpha. and the stress .sigma. occurring in the spring are generally expressed by the following formulae respectively:

.alpha. = 687.sup.. P.sup.. a.sup.. l/E.sup.. b.sup.. t.sup.3

.sigma. = 6.sup.. P.sup.. a/b.sup.. t.sup.2

wherein:

P = the pressure

l = the effective length

E = the Young's modulus

t = the thickness

b = the width of the spring plate.

It will be understood from the above formulae that in order to make the pressure change of the spring smaller with respect to the margin of wear of the brush, it becomes necessary to reduce the spring constant by making the effective length l of the spring large and increasing the number of turns of the same. However, this will result in such disadvantages that the stress in the spring increases and the turns of the spring get so close to each other as to interfere with each other, and that the diameter of the spring becomes large.

In general, the brush portion of a rotary electric machine rises in temperature during operation of said machine. Therefore, an increase of stress in a brush holding spring tends to result in a lowering of the spring pressure due to the creeping phenomenon and the interturn interference of the coiled portion of the spring causes an abnormal vibration of the spring, all of which provide causes of unsatisfactory commutation.

Furthermore, a motor for vehicles is normally used in a wide range of speed from zero to 500 r.p.m. but a spiral spring consisting of a single spring plate has its resonance point in the neighbourhood of 1,000 - 6,000 C.P.m., so that such a spring during operation tends to become defective in both the pressure characteristic and vibration characteristic, due to occurrence of resonance of the turns of the spring.

The present invention has been achieved under the circumstances described above. It is, therefore, the object of the invention to provide a spring unit of the type described, which is small in size, capable of pressing a brush with a minimum pressure change and with a minimum stress, and has a large resonance suppressing and damping effect.

According to one aspect of the invention, there is provided a spring unit of a small size, a low stress and a small pressure change, which is composed of a first spiral spring having one end secured to a stationary body for pressing a brush at the other end thereof and a second spiral spring coiled into a configuration substantially concentric with and disposed within the spiral space of said first spiral spring, one end of said second spiral spring being secured to the same stationary body to which the first spiral spring is secured, and the other end thereof being bonded to the other end portion of said first spiral spring.

According to another aspect of the invention, there is provided a spring unit of the character described, in which said first and second spiral springs have different spring constants thereby to enhance the resonance suppressing and damping effect of the spring unit, and further said second spiral spring is smaller in width than said first spiral spring and arranged in close contact with the side of said first spiral spring closer to the axis thereof thereby to facilitate the fabrication of the spring unit.

FIG. 1 is a perspective view of a brush holder comprising the spring unit of the present invention;

FIG. 2 is a side elevational view of the spring unit only of the brush holder shown in FIG. 1;

FIGS. 3 and 4 are side elevational views respectively showing other embodiments of the spring unit of the invention; and

FIG. 5 is a side elevational view of a conventional spring unit.

The present invention will be described in detail with reference to the embodiments thereof shown in the drawings.

In FIG. 1 there is shown in perspective a brush holder 3 comprising a brush 1 and a spring unit 2.

The brush holder 3 has at one end thereof a brush receiving box 4 in which the brush 1 is slidably received.

The brush holder 3 also has a fixed pin 5 which has the inner end 6 of the spring unit 2 secured thereto and thus supports the entire spring unit 2.

The outer end 7 of the spring unit 2 is placed on the top of the brush 1, and the brush is normally urged into pressure contact with a collector ring (not shown) as indicated by the arrow, under the biasing force of the spring unit. The brush holder 3 comprising the spring unit 2 and the brush 1 is fixed to a predetermined stationary portion (not shown) through an insulation rod 8.

The spring unit 2, as shown in FIG. 2, is composed of a first spiral spring 2a secured to the fixed pin 5 of the brush holder at one or inner end and having a brush pressing portion at the other or outer end thereof, and a second spiral spring 2b disposed within the spiral space of said first spiral spring. The second spiral spring 2b, similar to the first spiral spring 2a, is secured to the fixed pin 5 at the inner end thereof, and the outer end thereof is bonded integrally with the outer end of the first spiral spring 2a.

In this case, while the outer ends of both springs are bonded together, it is not essential that the outer ends are always aligned with each other, it being only necessary that the outer end of the second spiral spring is bonded to the first spiral spring near the outer end extremity thereof.

Alternatively, the spring unit 2 may be fabricated in the form shown in FIG. 3.

In this form of the spring unit, the thickness t of a first spiral spring 2c is differentiated from the thickness T of a second spiral spring 2d.

Differentiating the thicknesses means to differentiate the natural vibrations of the springs and has the effect of suppressing the vibration of the spring unit as a whole, though will be described in greater detail later, and thus makes it possible to use the spring unit with machines and equipments which are subjected to intense vibrations. Besides differentiating the thicknesses, many other methods may of course be considered to differentiate the natural vibrations of both springs, such as differentiating the materials or widths of the springs.

In FIG. 4 is shown a spring unit in which the first and second spiral springs of different thicknesses mentioned above are bonded together over substantially the entire lengths thereof and especially the second spiral spring 2d having a relatively small thickness is bonded to that side of the first spiral spring 2c having a relatively large thickness which is closer to the axis of the springs. The function and effect of this spring unit will not be described here as they will be described in detail later.

Now, the functions of the various forms of spring unit described hereinbefore will be explained hereunder: As may be apparent from the formulae given previously, a stress .sigma. occurring in a spiral spring is in inverse proportion to the square of the plate thickness and the angle of torsion .alpha. thereof is in inverse proportion to the cube of the plate thickness, for the same output. Therefore, by forming the spiral spring unit by laminating the first and second spiral springs of smaller thicknesses (than the conventional one) as described herein, it becomes possible, for the same output, to reduce the effective length of the spring unit and hence to reduce the number of turns and to make the angle of torsion large, or in other words, to minimize a change in pressure of the spring occurring incident to wear of the brush and to reduce the stress.

Further, where the first and second springs are respectively made of materials which have different spring constants, they have different natural frequencies, so that the energy loss occurring at the time of vibration due to mutual inteference of the laminated spring plates, i.e., the vibration damping effect, becomes very large and thus a spiral spring unit can be obtained which is excellent in its effect of suppressing resonance or abnormal vibration.

The production of a prototype spiral spring unit of the invention has revealed that it is extremely difficult to assemble two spiral springs into a unitary spring unit, with the circular configurations of their innermost turns matching with each other, and the assembly work is quite time-consuming. However, when spiral springs of different thicknesses are used and especially the spiral spring of smaller thickness is to be disposed inwardly of the other spring with respect to the axis of the springs, the smaller thickness spring leans backward slightly more than the other spring in the case when both springs are formed concurrently by a cold working, with one ends thereof fixed. Therefore, the coiled portion of the inner spring can be easily bonded to the coiled portion of the outer spring without requiring any additional effort.

The spring unit of the invention, as described herein, is composed of the first and second spiral springs bonded with each other at the opposite ends thereof and, therefore, is small in size, low in stress and small in pressure change, owing to the inherent properties of the springs. Further, the springs are made of material having different spring constants and the plate thicknesses of the springs are differentiated, and further the spring of smaller thickness is disposed inside of the spring of larger thickness with respect to the axis of the springs, so that the spring unit has a large effect for suppressing or damping resonances and can be easily produced, in addition to the advantageous features set forth above.

Although the present invention has been described herein with reference to spring units having first and second springs, it is to be understood that the spring units of the invention may be composed of three or more springs .

Claims

What is claimed is:

1. A spiral spring unit for pressing brushes, comprising: a stationary body, a first spiral spring having one end secured to said stationary body, the other end of said first spiral spring pressing a brush in a predetermined direction, a second spiral spring having a spiral configuration substantially concentric with said first spiral spring, said second spiral spring being disposed within the spiral space formed by said first spiral spring with a space being provided between said first and second spiral springs, one end of said second spiral spring being secured to the same stationary body to which said first spiral spring is secured, the other end of said second spiral spring being bonded to said other end portion of said first spiral spring, said other end portions being bonded together to cause said space between said first and second spiral springs to decrease when said first and second springs are wound up.

2. A spiral spring unit for pressing brushes, as defined in claim 1, wherein said first and second spiral springs have different spring constants.

3. A spiral spring unit for pressing brushes, as defined in claim 1, wherein the spring constant of said second spiral spring is smaller than that of said first spiral spring.

4. A spiral spring unit for pressing brushes, as defined in claim 1, wherein said second spiral spring has a plate thickness smaller than that of said first spiral spring.