Encapsulated armature and shaft assembly

An armature and shaft assembly useful in gauges and stepper motors is encapsulated for ease in manufacture and diversity of application, the capsule being suitable for sealing in dampening fluids and for allowing magnetization of the armature after assembly of the capsule, providing protection against contamination by dust, etc.

This invention relates to electric gauges and stepper motors and more 
particularly to providing an improved encapsulated armature and shaft 
assembly for electric gauges and motors. 
BACKGROUND OF THE INVENTION 
It is commonplace in automotive vehicle instrumentation to use electric 
gages driving pointers mounted on the ends of gauge shafts to indicate 
measure of various vehicle parameters such as vehicle speed or engine 
RPMs. A common type of gauge has a plastic bobbin supporting an outer 
winding and a permanent magnet armature on a spindle within a cavity 
defined by the bobbin. Often a dampening fluid is placed within the cavity 
to prevent erratic pointer movement caused by fluctuations in the signal 
driving the gauge. 
A characteristic of these previous types of gauges is the difficulty of 
creating a leak resistant seal for the cavity within which the dampening 
fluid and the armature are placed. A leak problem can cause a high scrap 
rate of gauges. Also, because the coils are crosswound on the bobbins, the 
flexibility of coil designs for the gauges is limited. 
Another characteristic of the previous gauges is that when the armature is 
manufactured, often it is at first not a magnet, but a material such as a 
polymer bonded ferrite or any equivalent material which can be magnetized 
through application of a strong magnetic field to become a permanent 
magnet. In these cases, the magnetizing process must take place some time 
during the construction of the gauge. Because the magnetizing fixture 
cannot get close enough to the armature once the armature is inserted in 
the bobbin, it is difficult to magnetize the armature after insertion into 
the bobbin. Consequently, magnetization of the armature is usually done 
before insertion into the bobbin. The difficulty with this method of 
manufacture is that the armature is subject to attraction of dust and dirt 
during and after the magnetization process. These impurities are an 
additional contributing factor to the scrap rate of gauges. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is an improved encapsulated armature assembly useful 
in gauges and stepper motors which overcomes many of the limitations of 
previous gauges. The capsule design provides a more leak resistant cavity 
for gauge dampening fluid and is characterized by a small magnetic air gap 
between the armature and the outside of the capsule. This small air gap 
allows the armature to be assembled within the capsule before it is 
magnetized and to be later magnetized once it is within the capsule or 
even within the final gauge assembly. 
This invention is suitable for implementation into various types of gauges 
For example the capsule may be subassembled and then placed within a 
crosswound bobbin and operated similar to previous gauges mentioned above. 
Or the capsule may be placed in proximity to various windings on separate 
coil forms and operated in response to magnetic fields created by those 
windings. 
Structurally the improved encapsulated armature and shaft assembly 
comprises a shaft upon which is mounted an armature portion made out of 
material capable of being magnetically charged to form a permanent magnet. 
A first bearing is placed in the capsule shell which defines a cavity with 
a closed end and an open end. The first bearing is placed in the closed 
end. One end of the shaft is placed in the first bearing and a second 
bearing within which the shaft rotates is mounted in the open end of the 
capsule shell, forming a leak resistant seal with the shell. The armature 
portion of the shaft is within the capsule between the two bearings. The 
capsule shell is magnetically permeable and the magnetic air gap between 
the outer surface of the capsule shell (which is considered part of the 
magnetic air gap) and the armature is small such that the armature portion 
of the shaft may be magnetized by a charging magnetic field imposed on the 
capsule from outside the capsule. 
Various other improvements and modifications to the present invention are 
set forth in the detailed description below.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, reference numeral 14 generally designates the 
preferred encapsulated armature and shaft assembly which includes the 
outer shell 26, the shaft 16 containing armature 24, and first and second 
bearings 30 and 22. The invention can be implemented into a wide variety 
of gauges ranging in sizes but is particularly well suited for miniature 
gauge applications as the width of the encapsulated armature and shaft 
assembly 14 may be in the range of 0.25 inches or smaller. 
The outer shell 26 is manufactured from copper, aluminum or any other 
suitable magnetic permeable material and may be extruded from a 
cylindrical blank through a proper form or manufactured through any other 
suitable technique. As can be seen the outer shell 26 is closed at the end 
32 near bearing 30 and opened at the end near bearing 22. Because of the 
magnetic permeability of the outer shell 26, the outer shell 26 is 
considered part of the air gap between armature 24 and any magnetic field 
generator (such as coils 40 in FIG. 2) outside of shell 26. 
In the manufacture of the encapsulated armature and shaft assembly 14, the 
bearing 30 is placed in the bottom of the shell 26. The bearing 30 may be 
plastic and may be press fit into the outer shell 26. In the example 
shown, a separate spacer 12 is placed between the bearing 30 and the 
armature 24. In actual practice, this spacer is not necessary or may be 
molded out of bearing 30 if desired. Next the shaft 16 with the armature 
24 is inserted in the bearing 30. The closed end 32 of the outer shell 26 
acts as a stop to keep the shaft 16 in the proper position in the 
direction of the bearing 22. 
The armature 24 is a preferably a material which may be charged into a 
permanent magnet, such as a polymer bonded ferrite, glass ferrite, 
Alnico.TM., any other equivalent material, and forms a cylinder around the 
shaft 16. In order to prevent contamination from different forms of dust 
and dirt, it is preferable that the armature 24 not be magnetized before 
insertion into the encapsulated armature and shaft assembly 14. Another 
spacer 13 is placed above the armature 24 as shown and bearing 22 is 
placed in the open end of the outer shell 26 which is widened as shown. 
The ends of the outer shell are crimped around the bearing 22 as shown to 
seal the capsule. In many cases it is desired to have dampening fluid 25 
within the capsule to prevent erratic pointer movements with slight 
variations in signal to the gauge in which the encapsulated armature and 
shaft assembly 14 might be used. In these cases, a small amount of 
dampening fluid is placed in the cavity 28 before the second bearing 22 is 
sealed into the assembly. The viscosity of the dampening fluid may vary 
with the particular gauge application. A small amount of fluid is usually 
required to dampen the gauge so the majority of cavity 28 is usually 
filled with air. 
The crimp 10 of the outer shell around the outer bearing 22 provides a leak 
resistant seal to keep the dampening fluid within the cavity 28. The gap 
18 between the shaft 16 and the bearing 22 is preferably on the order of 
two thousandths (0.002) inches and is also leak resistant. To discourage 
the dampening fluid from leaking out of the gap 18 a channel 20 is 
preferably placed in the bearing 22. This channel tends to trap any 
dampening fluid and prevent leakage in event that the encapsulated 
armature and shaft assembly 14 is oriented so that the dampening fluid 
tends to flow toward the gap 18. 
Once the second bearing 22 is sealed into place, the capsule is a complete 
unit ready for use in a variety of applications. It is at this time that 
it may be desirable to magnetize the armature 24. This is done by simply 
applying a strong magnetic field across the encapsulated armature and 
shaft assembly 14 and armature 24 perpendicular to the axis of shaft 16 
and maintaining the field for several seconds. This process of 
magnetization of polymer bonded ferrites and equivalent materials is well 
known to those skilled in the art. 
FIG. 2 is one example of a wide variety of implementations in which the 
present invention is well suited. Reference numeral 42 generally 
designates four coil forms which may be plastic or metal and are mounted 
so as to be stationary. Each coil form is wound with electrical wiring to 
form coils 40 which, when flowing with electrical current, create magnetic 
fields which flow through the outer shell of the encapsulated armature and 
shaft assembly 14. The shell 26 (FIG. 1) of the capsule 14 is 
non-rotatably mounted as by tabs 33 (FIG. 1) attached to the outer shell 
26 and a support for the encapsulated armature and shaft assembly 14 
(support not shown). The magnetic fields created by the coils 40 act on 
the armature 24 (FIG. 1) to rotate the shaft 16. 
As an example of the diversity of uses of the present invention, the 
apparatus shown in FIG. 2 can be operated either as an air core gauge or a 
stepper motor. To operate the apparatus as an air core gauge, each coil 40 
is connected in series with the coil 40 opposite the encapsulated armature 
and shaft assembly 14 and, when current is applied generates a magnetic 
field in the same direction as the coil 40 opposite the capsule. The 
circuit can then be driven in the manner of conventional air core gauges 
with two coils. In this instance a pointer would be attached to the end of 
shaft 16 and the unit could be used in a variety of gauge applications. 
Alternatively the coils may be driven in phases for operating the apparatus 
as a stepper motor. One skilled in the art would easily be able to 
implement the structure shown in FIG. 2 as a stepper motor or as a gauge 
suitable for automotive instrumentation. 
As is obvious to those skilled in the art, this invention is not limited to 
the above described examples but includes a wide variety of 
implementations of the capsule assembly. For example the bearing 30 may be 
molded to include a stop for the shaft 16 so that the shaft doesn't stop 
against the closed end 32 of the outer shell 26 but against the stop 
molded into the bearing 30. Additionally a wide variety of coil 
combinations using one or more coils may be used to rotate the armature 24 
and shaft 16, including conventional crosswindings. If the invention is 
used in a gauge, return springs or weighted pointers may be used to return 
the gauge to zero. Any of these above implementations and many more may be 
easily implemented by those skilled in the art. Many of the modifications 
and improvements to the present invention which may occur to those skilled 
in the art may fall within the scope of the invention as set forth below.