Patent Publication Number: US-6218922-B1

Title: Bearings for proportional solenoid

Description:
TECHNICAL FIELD 
     Manufacture of proportional solenoids. 
     BACKGROUND OF THE INVENTION 
     Proportional solenoids position an armature axially within a housing as a function of applied current or voltage. Ideally, the armature can move to infinitely variable positions corresponding accurately to infinite variations in the applied voltage or current. To accomplish this in practice, though, requires that the armature move with very low friction within the housing, and achieving this has made proportional solenoids expensive. 
     The housing, the armature, and sometimes bearings arranged between the armature and the housing all have to be machined accurately for the sliding axial fit of the armature within the housing to have minimal friction. Diameters and concentricity of bearing surfaces must be highly accurate to minimize friction, and location and shape of bearing surfaces must be considered to minimize effects of side or off-axis loading. These needs have required that all contacting surfaces be accurately machined, and errors that inevitably occur in attempting to accomplish this adversely affect solenoid performance. 
     The machining accuracy that is required to keep friction low in proportional solenoids increases their price sufficiently so that some users who could benefit from proportional solenoids avoid them in favor of simpler and lower cost non-proportional solenoids, even though performance is less than optimum. Our invention aims at reducing the cost of making proportional solenoids accurate enough to minimize friction. 
     SUMMARY OF THE INVENTION 
     Our invention recognizes a way that precision bearing surfaces can be molded on a solenoid armature so that precision machining can be limited to a housing for the armature to simplify and reduce the cost of making proportional solenoids. Precision molding of resin bearings formed in imprecise grooves in an armature ensures precise and accurate diameters and concentricity for a pair of armature bearings. These can then slide precisely within one or two machined surfaces of a housing to minimize friction. In effect, precision invested in a bearing mold for an armature eliminates any precision machining of the armature so that relatively simple precision machining of one or two inside diameters of a housing is all that is needed for an accurate fit between the housing and the molded armature bearings to minimize sliding friction of an armature of a proportional solenoid. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a partially cross-sectioned and partially schematic view of a prior art proportional solenoid having bearings between an armature and a housing. 
     FIGS. 2 and 3 show alternative preferred embodiments of the inventive molded bearings formed in grooves in a partially cutaway armature to achieve a low friction sliding axial fit in a solenoid housing shown in cross section. 
     FIG. 4 is a schematic diagram of steps involved in making a proportional solenoid having the inventive bearings applied to the preferred embodiments of FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION 
     A typical prior art way of arranging bearings in a proportional solenoid  10  is shown schematically in FIG.  1 . This requires precision machining of several surfaces. Housing  11  of solenoid  10  has precision machined internal surfaces  12  and  13  to engage bearings  15  and  16 . These are typically press fitted onto precision machined surfaces  17  and  18  of armature  20 . Also, bearings  15  and  16  often have radially outer surfaces precision machined for an accurate sliding fit within the interior of housing  11 . Several variations on the illustrated prior art arrangement are also possible, but they all involve expensive precision machining of multiple surfaces that must achieve an accurate sliding fit. 
     FIG. 2 illustrates a preferred embodiment of the inventive way of providing precision molded bearings  35  and  36  for an armature  40  axially slideable within a housing  31  of proportional solenoid  30 . Bearings  35  and  36  are spaced apart axially of armature  40  to minimize side loading or off-axis forces. The configuration of solenoid  30  allows bearings  35  to be concentric and have equal diameters to fit accurately within a single precision machined interior surface  32  of housing  31 , to minimize precision machining. 
     Each of the annular bearings  35  and  36  is formed in a respective annular groove  37  and  38  machined or otherwise formed in armature  40 . Grooves  37  and  38  are imprecise and do not have to be accurately machined. 
     Bearings  35  and  36  are molded to be retained in grooves  37  and  38  and to extend radially beyond a peripheral radial surface of armature  40  as illustrated. Since bearings  35  and  36  are precision molded, their radially outer or peripheral surfaces  33  and  34  are made accurately concentric and accurately equal in diameter for a precise and low friction sliding fit within housing surface  32 . 
     Another preferred embodiment of proportional solenoid  50  is illustrated in FIG.  3 . Its housing  51  has a pair of precision machined interior surfaces  52  and  53  having different diameters and being accurately concentric. Armature  60  has one molded bearing  61  formed in an imprecise groove  63  in an armature body and another bearing  64  formed in another imprecise groove  66  machined in a push rod  65  forming a portion of armature  60 . This results in bearing  64  having a radially outer surface  68  with a smaller diameter fitting housing surface  53 , and bearing  61  having a larger diameter outer surface  62  fitting housing interior surface  52 . Radially peripheral surfaces  62  and  68  of annular bearings  61  and  64  are also molded to be accurately concentric for a low friction sliding fit within precision machined surfaces  52  and  53 . 
     Many solenoid armatures have push rods, which rods can be grooved to receive one of the armature bearings. The attachment of push rods to armature bearings raises a possibility of concentricity error; but the inventive way of molding armature bearings automatically cancels out any concentricity error between the body portion and the push rod portion of an armature. This results from a single accurate mold forming both armature bearings, whether located on the body portion or the push rod portion of the armature. 
     Many other arrangements of bearings molded on solenoid armatures to fit within solenoid housings are possible beyond the preferred embodiments illustrated in FIGS. 2 and 3. All of these arrangements have in common that a pair of bearings are axially spaced on the solenoid armature and are precision molded for an accurate fit within one or more precision machined interior surfaces of a solenoid housing. They also have in common the fact that the pair of bearings formed on a solenoid armature are molded in a single precision mold having a pair of cavities that ensure accurate concentricity as well as accurate outer diameters of the molded bearings. 
     Outer surfaces of molded armature bearings can vary in configuration and in area of contact with a solenoid interior. Some experimentation is needed to minimize friction by configuring the peripheral surfaces of molded bearings, and such configurations can vary with the bearing material selected. 
     A resin chosen for molding armature bearings is preferably designed for bearing purposes and preferably has high dimensional stability (between 0 to 0.0005 inches) and a low coefficient of friction (ranging from 0 to 0.2). Using the smallest practical amount of resin for each of the armature bearings helps minimize shrinking after molding; and we prefer that armature bearings be formed of 0.01 to 0.02 cubic inches per bearing, for typical bearing diameters. This means that grooves formed in solenoid armatures to receive molded bearings can be shallow and that bearings need extend radially only a small distance beyond the radial periphery of an armature. By using a small volume of resin for each bearing, dimensional changes in the bearings after molding can be held to a range of 0 to 0.0005 inches. These measures, along with precision configuration of a bearing mold, can form bearings having radially outer surfaces held to a high degree of accuracy. 
     FIG. 4 illustrates a preferred method of forming low friction solenoid bearings after armatures and housings are roughly formed. A first step is machining or otherwise forming imprecise grooves  37  and  38  in armature  40  or grooves  66  and  63  in armature  60 . Then, bearings are molded precisely in the grooves of armatures  40  or  60  to form precision bearings  35  and  36  having outer surfaces  33  and  34  or precision bearings  61  and  64  having outer surfaces  62  and  68 . In each case, the radially outer surfaces of the pair of bearings on each armature  40  and  60  are accurately concentric and have accurately predetermined diameters. A single mold having a pair of bearing cavities registering with the grooves of each armature ensures this precision. 
     In preparation for solenoid assembly, an interior surface  32  of solenoid housing  31  is accurately machined to a predetermined diameter; or for an alternative embodiment, interior surfaces  52  and  53  of solenoid housing  51  are accurately machined to concentric but different diameters. Armatures  40  and  60  with their respective molded bearings  35 ,  36  and  61 ,  64  are then assembled into respective solenoid housings  31  and  51 . This gives bearings  35  and  36  a low friction sliding fit within interior housing surface  32  and correspondingly gives bearings  61  and  64  a low friction sliding fit within respective housing surfaces  52  and  53 . 
     Solenoids  30  and  50 , formed and assembled by the inventive method, have optimally low friction movement of armatures  40  and  60  and are also made at significantly less expense than is required for precision machining of multiple surfaces of prior art proportional solenoids. The invention can thus make low friction proportional solenoids available at a lower cost, allowing the advantages of proportional solenoids to be used in previously unaffordable circumstances.