Patent Publication Number: US-8525625-B2

Title: Starter solenoid with spool for retaining coils

Description:
FIELD 
     This application relates to the field of vehicle starters, and more particularly, to solenoids for starter motor assemblies. 
     BACKGROUND 
     Starter motor assemblies that assist in starting engines, such as engines in vehicles, are well known. A conventional starter motor assembly is shown in  FIG. 15 . The starter motor assembly  200  of  FIG. 15  includes a solenoid  210 , an electric motor  202 , and a drive mechanism  204 . The solenoid  210  includes a coil  212  that is energized by a battery upon the closing of an ignition switch. When the solenoid coil  212  is energized, a plunger  216  moves in a linear direction, causing a shift lever  205  to pivot, and forcing a pinion gear  206  into engagement with a ring gear of a vehicle engine (not shown). When the plunger  216  reaches a plunger stop, electrical contacts are closed connecting the electric motor  202  to the battery. The energized electric motor  202  then rotates and provides an output torque to the drive mechanism  204 . The drive mechanism  204  transmits the torque of the electric motor through various drive components to the pinion gear  206  which is engaged with the ring gear of the vehicle engine. Accordingly, rotation of the electric motor  202  and pinion  206  results in cranking of the engine until the engine starts. 
     Many starter motor assemblies, such as the starter motor assembly  200  of  FIG. 15  are configured with a “soft-start” starter motor engagement system. The intent of a soft start starter motor engagement system is to mesh the pinion gear of the starter into the engine ring gear before full electrical power is applied to the starter motor. If the pinion ring gear abuts into the ring gear during this engagement, the motor provides a small torque to turn the pinion gear and allow it to properly mesh into the ring gear before high current is applied. The configuration of the solenoid, shift yoke, electrical contacts, and motor drive are such that high current is not applied to the motor before the gears are properly meshed. Accordingly, milling of the pinion gear and the ring gear is prevented in a starter motor with a soft-start engagement system. 
     Starters with a soft start engagement system, such as that of  FIG. 15 , typically include a solenoid with two distinct coils. The first coil is a pull-in coil  212  and the second coil is a hold in coil  214 . As shown in  FIG. 15 , the pull-in coil  212  is wound first on the spool  220 . On top of this winding the hold-in coil  214  is wound. Sometimes this order is reversed such that the hold-in coil  214  is wound first on the spool  220  followed by the pull-in coil  212 . 
     During operation of the starter, the closing of the ignition switch (typically upon the operator turning a key) energizes both the pull-in coil  212  and the hold-in coil  214 . Current flowing through the pull-in coil  212  at this time also reaches the electric motor  202 , applying some limited power to the electric motor, and resulting in some low torque turning of the pinion. Energization of the pull-in coil  212  and hold-in coil  214  moves a solenoid shaft (also referred to herein as the “plunger”) in an axial direction. The axial movement of the solenoid plunger moves the shift lever  205  and biases the pinion gear  206  toward engagement with the engine ring gear. Once the solenoid plunger reaches the plunger stop, a set of electrical contacts is closed, thereby delivering full power to the electrical motor. Closing of the electrical contacts effectively short circuits the pull-in coil  212 , eliminating unwanted heat generated by the pull-in coil. However, with the pull-in coil is shorted, the hold-in coil  214  provides sufficient electromagnetic force to hold the plunger in place and maintain the electrical contacts in a closed position, thus allowing the delivery of full power to continue to the electric motor  202 . The fully powered electric motor  202  drives the pinion gear  206 , resulting in rotation of the engine ring gear, and thereby cranking the vehicle engine. 
     After the engine fires (i.e., vehicle start), the operator of the vehicle opens the ignition switch. The electrical circuit of the starter motor assembly is configured such that opening of the ignition switch causes current to flow through the hold-in coil and the pull-in coil in opposite directions. The pull-in coil  212  and the hold-in coil  214  are configured such that the electromagnetic forces of the two coils  212 ,  214  cancel each other upon opening of the ignition switch, and a return spring forces the plunger  216  back to its original un-energized position. As a result, the electrical contacts that connected the electric motor  202  to the source of electrical power are opened, and the electric motor is de-energized. 
     In order to produce a high performing vehicle starter with a soft start motor engagement system, such as that described above, designers are faced with numerous design challenges. First, the pull-in coil must be properly designed to avoid various issues that may arise during operation of the starter. As described above, when the pull-in coil of a soft-start starter motor engagement system is energized (i.e., when the ignition switch contacts close due to operator turning engine switch key on), the pull-in coil provides electromagnetic force to pull the plunger toward the plunger stop and to the closed position. However, the pull-in coil is connected electrically in series with the starter motor, and should only have a low resistance. With low resistance through the pull-in coil, sufficient current flows through the pull-in coil and to the electric motor such that the electric motor can deliver a sufficient output torque to rotate the pinion gear and avoid abutment with the ring gear, as described previously. This required torque is typically 8-12 N-m. For a 12V motor, the resistance may be on the order of 0.030 ohms so that several hundred amps flow through the motor, and also the series connected pull-in coil, during soft start. However, this low of resistance of the pull-in coil creates other design challenges. First, if the soft start period is prolonged, or repetitive starts are performed, a high amount of ohmic heat is generated in the pull-in coil because of the large amount of current flowing through the pull-in coil. For a 12V system this can be on the order of 3-4 kW, and this can lead to thermal failure of the insulation system of the wiring that forms the coils. Second, the large current through the pull-in coil creates a much stronger electromagnetic force on the plunger during closure than is needed. This may become a problem when an abutment between the pinion gear and ring gear occurs, and the impact force of the pinion gear on the ring gear can exceed 4500N. As a result, the ring gear could fracture or chip. Over time and thousands of starts, the surface of the ring gear may deteriorate and require replacement for proper starting. 
     Design challenges related to the pull-in coil, such as those discussed in the preceding paragraph result in additional design challenges with respect to other components of the starter, such as the hold-in coil. For example, as discussed in the previous paragraph, the pull-in coil has specific design limitations related to the current flowing through the pull-in coil. Since the electromagnetic excitation is the product of coil turns times current, and since current is fixed, this generally leaves the number of turns of the pull-in coil as the primary design variable for the pull-in coil. While the number of turns of the pull-in coil can be reduced to reduce the impact abutment force issue described previously, this presents a problem with the hold-in coil. In particular, the number of turns in the hold-in coil should match the pull-in coil so that during disengagement of the pinion gear and the ring gear following vehicle start, the electromagnetic forces of the two coils will cancel each other and allow the pinion gear to pull cleanly out of the ring gear. However, before vehicle start, the hold-in coil stays energized for a much longer period of time than the pull-in coil. Therefore, the hold-in coil should not be of low resistance or it will thermally fail. Thus, the resistance of the hold-in coil generally is an order of magnitude higher than that of the pull-in coil. The high resistance of the hold-in coil means that current flow through the hold-coil before start is relatively low, resulting in a relatively low amp-turn product. If the number of turns of the hold-in coil is too low, then the hold-in coil will deliver an insufficient magnetic force to hold the plunger closed and the starter motor will disengage before vehicle start. 
     As explained in the previous paragraphs, designers of vehicle starters with soft start motor engagement systems are faced with opposing design challenges for two coils that should produce equivalent electromagnetic forces. On the one hand designers strive to limit the turns of the pull-in coil in order to reduce the impact force during engagement of the pinion gear and the ring gear. On the other hand designers strive to increase the turns of the hold-in coil such that the hold-in coil delivers sufficient electromagnetic force to maintain the plunger in a closed position during engine cranking. Accordingly, it would be desirable to provide a solenoid for a vehicle starter with a pull-in coil that limits the impact force during engagement of the pinion gear and the ring gear. It would also be desirable to provide a hold-in coil for the solenoid that delivers sufficient electromagnetic force to maintain the plunger in a closed position during engine cranking. Additionally, it would be desirable if such a solenoid were relatively simple in design and inexpensive to implement. 
     SUMMARY 
     In accordance with one embodiment of the disclosure, there is provided a solenoid for a vehicle starter. The solenoid comprises a spool including a first coil bay, a second coil bay, and an interior passage defining an axial direction. A first coil is positioned in the first coil bay of the spool, and a second coil positioned in the second coil bay of the spool. A plunger is positioned within the interior passage of the spool and configured to move in the axial direction when the first coil is energized. In at least one embodiment of the solenoid, the first coil bay is positioned adjacent to the second coil bay in the axial direction. 
     In at least one alternative embodiment, the spool of the solenoid includes a middle flange separating the first coil bay from the second coil bay. The spool may further include two end flanges, and wherein the middle flange is not centered on the spool between the two end flanges such that the first bay and the second bay are of different lengths. Additionally the center flange of the spool may be thicker than each of the two end flanges. One or more of the flanges may include a plurality of coil mounting features positioned along the outer perimeter of the flange. 
     In at least one alternative embodiment, the solenoid is provided as part of a vehicle starter including an electric motor configured to be energized by a source of electric power when the solenoid is energized. 
     The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. While it would be desirable to provide a solenoid that provides one or more of these or other advantageous features, the teachings disclosed herein extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above-mentioned advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a vehicle starter including a motor and solenoid; 
         FIG. 2  shows a perspective view of a spool, pull-in coil, and hold-in coil of the solenoid of  FIG. 1 ; 
         FIG. 3  shows a diagram illustrating lines of magnetic flux through the solenoid when the pull-in coil and hold-in coil of  FIG. 2  are energized and the plunger is removed from a plunger stop; 
         FIG. 4  shows a diagram illustrating lines of magnetic flux through the solenoid when the pull-in coil and hold-in coil of  FIG. 2  are energized and the plunger is in transition toward the plunger stop; 
         FIG. 5  shows a diagram illustrating lines of magnetic flux through the solenoid when only the hold-in coil of  FIG. 2  is energized and the plunger is engaged with the plunger stop; 
         FIG. 6  shows a cross-sectional view of the spool of  FIG. 2  taken along a centerline of the spool; 
         FIG. 6A  shows a cross-sectional view of the spool along line A-A of  FIG. 6 , illustrating one side of a middle flange of the spool; 
         FIG. 6B  shows a cross-sectional view of the spool along line B-B of  FIG. 6 , illustrating another side of the middle flange of the spool; 
         FIG. 6C  shows an side view of the spool along line C-C of  FIG. 6 , illustrating an end flange of the spool; 
         FIG. 7  shows a perspective view of an alternative embodiment of the spool of  FIG. 2 ; 
         FIG. 8  shows the spool of  FIG. 7  with the hold-in coil being wound in one direction on a second coil bay of the spool; 
         FIG. 9  shows the spool of  FIG. 8  with the hold-in coil being wound in an opposite direction on the second coil bay of the spool; 
         FIG. 10  shows the spool of  FIG. 9  with the hold-in coil completely wound on the second coil bay of the spool; 
         FIG. 11  shows the spool of  FIG. 10  with the pull-in coil being wound on a first coil bay of the spool; 
         FIG. 12  shows the spool of  FIG. 11  with the pull-in coil completely wound on the first coil bay of the spool; 
         FIG. 13  shows a cross-sectional view of the spool along line D-D of  FIG. 12 , including the hold-in coil and pull-in coil positioned on the spool; 
         FIG. 14  shows a cross-sectional view of an alternative embodiment of the spool, hold-in coil and pull-in coil of  FIG. 13 ; and 
         FIG. 15  shows a cutaway view of a conventional starter motor with a soft start starter motor engagement system 
     
    
    
     DESCRIPTION 
     General Starter Arrangement 
     With reference to  FIG. 1 , in at least one embodiment a starter  100  for a vehicle comprises an electric motor  102  and a solenoid  110 . Although not shown in the  FIG. 1 , the starter  100  also includes a drive mechanism and pinion gear, similar to the conventional starter assembly  200  described above with reference to  FIG. 15 . The electric motor  102  in the embodiment of  FIG. 1  is positioned in a motor circuit  104  that is configured to connect the motor to the vehicle battery (not shown) via the B+ terminal. The solenoid  110  is positioned in the motor circuit  104  to facilitate connection of the motor to the vehicle battery. The solenoid includes a pull-in coil  112 , a hold-in coil  114 , a plunger  116 , and an ignition switch  118 . 
     The motor circuit  104  of  FIG. 1  includes a first current path  106  and a second current path  108  configured to provide electrical power to the electric motor  102 . The first current path  106  begins at the B+ terminal, travels across the contacts  119  of the ignition switch  118 , continues to node  115 , travels through the pull-in coil, and ends at the input terminal  103  of the electric motor  102 . Accordingly, this first current path  106  is only a closed path when the contacts  119  of the ignition switch  118  are closed. 
     The second current path  108  begins at the B+ terminal, travels across the motor contacts  117  associated with the plunger  116  and ends at the input terminal  103  of the electric motor  102 . Accordingly, this second current path  108  is only a closed path when the plunger  116  has closed the motor contacts  117 . Moreover, when the second current path  108  is closed, the first current path  106  is shorted by the second current path  108 , and no current flows through the pull-in coil  112 . Upon closing of the ignition switch  118 , the solenoid  110  and motor  102  cooperate to provide a soft start motor engagement system for a vehicle. 
     Axially Adjacent Coils 
       FIG. 2  shows the pull-in coil  112  and the hold-in coil  114  of the solenoid  110  positioned on a spool  120  of the solenoid  110 . In the embodiment of  FIG. 2 , the pull-in coil  112  and the hold-in coil  114  are adjacent to one another in an axial direction of the spool  120 . The axial direction is represented in  FIG. 2  by axis  132 . 
     The pull-in coil  112  is comprised of a first length of wire wound around a first portion of the spool  120  to form a first plurality of conductor windings (i.e., turns). The wire for the pull-in coil  112  has a relatively large cross-sectional area such that the resistance of the conductor windings is relatively low. Similarly, the hold-in coil  114  is comprised of a second length of wire wound around a second portion of the spool to form a second plurality of conductor windings (i.e., turns). The wire for the hold-in coil  114  is has a relatively small cross-sectional area such that the resistance of the conductor windings is relatively high. 
     The pull-in coil  112  and the hold-in coil  114  are retained in a side-by-side arrangement on the spool  120 . In the embodiment of  FIG. 2 , the spool  120  is a single component comprised of a glass-filled nylon material. However, it will be recognized that the spool may alternatively be comprised of different materials. The spool  120  may be manufactured using any of various known processes, such as a straight pull mold or other molding process. 
     The spool  120  includes a first end flange  122 , a middle flange  124 , a second end flange  126 , and a hub  128 . The hub  128  of the spool  120  is generally cylindrical in shape and provides a coil retaining surface for the pull-in coil  112  and the hold-in coil  114 . Although a right circular cylinder is shown in the embodiment of  FIG. 1 , it will be recognized that the hub  128  make take on other forms, including cylindrical and non-cylindrical forms. Furthermore, the term “spool” as used herein refers to any appropriate solenoid coil holder, regardless of whether the hub is provided as a cylinder or if flanges are included on the ends of the hub. 
     The hub  128  in the embodiment of  FIG. 2  extends from the first end flange  122  to the second end flange  126 . The hub  128  defines a cylindrical interior passage  130  that extends through the spool  120  from the first end flange  122  to the second end flange  126 . The cylindrical hub  128  also defines a spool axis  132  that extends through the interior passage  130 . The spool axis  132  defines a centerline for the spool  120  and an axial direction along the spool. 
     The first end flange  122  provides an end wall for the spool  120  that is configured to retain coil windings on the spool. The first end flange  122  is generally disc shaped and includes a circular center hole at the interior passage  130  of the spool. This end wall may be solid with a central hole for the plunger passage  130 , as shown in  FIG. 2 , or may include a plurality of openings. Moreover, although the flange  122  is shown as a relatively thin circular disc in the embodiment of  FIG. 2 , it will be recognized that the end flange  122  may be provided in various different forms and shapes. 
     The middle flange  124  also provides a wall that is configured to retain coil windings on the spool. The middle flange  124  is positioned on the hub  128  between the first end flange  122  and the second end flange  126 , but not necessarily centered between the first end flange  122  and the second end flange  126 . Indeed, in the embodiment of  FIG. 2 , the middle flange  124  is positioned closer to the second end flange  126  than to the first end flange  122 . The space between the first end flange  122  and the middle flange  124  provides a first coil bay  142  on the spool  120  where the pull-in coil  112  is wound around the hub  128 . 
     Similar to the first end flange  122 , the middle flange  124  in the embodiment of  FIG. 2  is also disc shaped. The middle flange  124  is generally thicker than the first end flange and includes coil mounting features  134  such as slots  136  along the outer perimeter of the flange  124 . These slots  136  provide a passage for wire leads on the pull-in coil  112 . It will be recognized that additional coil mounting features  134  are also possible, and examples of such coil mounting features will be discussed in further detail below with reference to  FIGS. 6-12 . Although the center flange is shown in  FIG. 2  as having a circular perimeter, it will be recognized that the middle flange  124  may be provided in various different forms and shapes. For example, although the middle flange  124  is shown as being solid with a single central opening, the middle flange may also include a plurality of openings. 
     The second end flange  126  provides another end wall for the spool  120  that is configured to retain coil windings on the spool. The space between the second end flange  126  and the middle flange  124  provides a second coil bay  144  on the spool that is adjacent to the first coil bay  142  in the axial direction. The hold-in coil  112  is wound around the hub  128  at the second coil bay  144 . Similar to the first end flange  122 , the second end flange  126  is also generally disc shaped and includes a circular center hole at the interior passage  130  of the spool. The second end flange  126  is generally the same thickness as the first end flange  122 . Similar to the middle flange  124 , includes mounting features  134  such as slots  138  along the outer perimeter of the flange  126 . These slots  138  provide a passage for wire leads on the pull-in coil  112  and the hold-in coil  114 . The second end flange  126  may be solid, as shown in  FIG. 2 , or may include a plurality of openings. Moreover, although the second end flange  126  is shown as a relatively thin circular disc in the embodiment of  FIG. 2 , it will be recognized that the flange  126  may be provided in various different forms and shapes. 
     As described above with reference to  FIG. 2 , the spool  120  of the solenoid  110  is configured such that the pull-in coil  112  is positioned adjacent to the hold-in coil  114  of the solenoid in the axial direction. As a result of this adjacent coil arrangement, greatly increased flux leakage can occur around the pull-in coil, as described below with reference to  FIGS. 3-5 . The increased flux leakage reduces the magnetic force experienced by the plunger as a result of the pull-in coil  112 , thus allowing the resistance of the pull-in coil  112  to be low while still minimizing the abutment force issues previously described. At the same time, the adjacent coil arrangement provides for minimal flux leakage with the hold-in coil  114  when the plunger gap is zero and the contacts are closed, thus allowing the number of coil turns in the hold-in coil to be low but maximizing its hold-in force. 
       FIGS. 3-5  are diagrams illustrating lines of magnetic flux through the solenoid when the pull-in coil  112  and the hold-in coil  114  are in various energized and non-energized states. In each of  FIGS. 3-5 , the pull-in coil  112 , hold-in coil  114 , plunger  116 , solenoid case  150  and plunger stop  152  are illustrated as a cross-sectional view of the solenoid taken radially outward from the solenoid centerline  132 . The solenoid spool  120  of  FIG. 2  is not illustrated in  FIGS. 3-5  for clarity, allowing the lines of magnetic flux  170  passing through the solenoid  110  to be more clearly displayed. However, it will be recognized that the spool  120  is present in the illustrations of  FIGS. 3-5  with the pull-in coil  112  and hold-in coil  114  wound around the spool, and the plunger  116  inserted in the interior passage  130  of the spool  120 . 
     With particular reference to  FIG. 3 , the solenoid  110  is housed by the solenoid case  150 . The plunger stop  152  is a generally disc shaped member that is fixed to the solenoid case  150  and extends radially inward from the solenoid case. The plunger stop  152  includes a cylindrical protrusion  154  that fits within an end of the interior passage  132  of the spool  120  (not shown in  FIG. 3 ). This cylindrical protrusion  152  provides a stop surface  154  configured to engage the plunger  116  when the plunger is moved in the axial direction by the pull-in coil  112 . 
     The plunger  116  is a solid component with a cylindrical shape. The cylindrical shape of the plunger  116  is provided with a first larger diameter portion  160  and a second smaller diameter portion  162 . A shoulder  164  is formed between the larger diameter portion  160  and the smaller diameter portion  162 . The plunger  116  is slideably positioned within the solenoid case  150 . In particular, the plunger  116  is configured to slide in the axial direction along the centerline  132  to close an air gap  168  (which may also referred to herein as a “plunger gap”) between the plunger shoulder  164  and the stop surface  154  of the plunger stop  152 . Each of the plunger  116 , the solenoid case  150 , and the plunger stop  152  are comprised of a metallic material having relatively low magnetic reluctance, such that magnetic flux lines may easily pass through the solenoid case and the plunger. 
     With continued reference to  FIG. 3 , the pull-in coil  112  of the solenoid  110  is positioned within the solenoid case  150  and encircles the larger diameter portion  160  of the plunger  116 . The pull-in coil  112  is removed from the plunger stop by a distance d in an axial direction. An axial end of the pull-in coil is aligned with the shoulder  164  of the plunger  116  when the plunger is in the leftmost position of  FIG. 3 . As discussed previously, the pull-in coil  112  is comprised of a length of conductor including a plurality of windings that wrap around the spool  120  (not shown in  FIG. 3 ). When the pull-in coil  112  is initially energized, the plunger  116  is urged in the axial direction to the right, as indicated by arrow  166 . 
     The hold-in coil  114  is positioned adjacent to the pull-in coil  112  in the axial direction within the solenoid case  150 . The hold-in coil  114  encircles the protrusion  154  of the plunger stop  152  and the associated stop surface  156 . Accordingly, the hold-in coil  114  also encircles the smaller diameter portion  162  of the plunger that extends through the plunger stop  152 . Furthermore, the pull-in coil encircles the air gap  168  when the plunger is in the leftmost position of  FIG. 3 . As discussed previously, the hold-in coil  114  is comprised of a length of conductor including a plurality of windings that wrap around the spool  120  (not shown in  FIG. 3 ). When the hold-in coil  114  is initially energized, the plunger  116  is urged in the axial direction to the right, as indicated by arrow  166 . 
     Coil Position within the Solenoid Results in Leakage Flux 
     As represented by flux lines  170  in  FIGS. 3 and 4 , when the pull-in coil  112  and the hold-in coil  114  are energized, magnetic flux is created within the solenoid. Leakage flux is any flux that does not contribute to the axial force acting on the plunger  116 . The axial force acting to pull the plunger  116  toward the plunger stop  152  and close the plunger gap  168  is dependent upon the total flux linkage between the pull-in coil  112  and the plunger  116  and between the hold-in coil  114  and the plunger  116 . When flux leakage occurs, the flux linkage is reduced and so is the resulting force on the plunger  116 . 
     By placing the pull-in coil  112  away from the plunger gap  168  and plunger stop surface  156 , as shown in  FIGS. 3 and 4 , the flux leakage of the pull-in coil  112  is intentionally greatly increased in order to reduce the resulting force on the plunger  116 . As shown in  FIGS. 3 and 4 , rather than traverse directly from the plunger  116  to the plunger stop  152 , an increased amount of flux by-passes the plunger  116  and couples directly from one side of the case  150  to the stop  152  or even back to the case  152  outside wall  151 . Examples of this leakage flux is are indicated in  FIGS. 3 and 4  by lines  171 . The leakage flux  171  effectively lowers the magnetic force on the plunger  116  for a given amp-turn excitation of the pull-in coil  112 . Since the magnetic force on the plunger  116  is reduced, and because the pinion gear is mechanically connected to the plunger via the pivoting shift lever, the impact and steady-state abutment force of the pinion gear on the ring gear is also reduced. Therefore with the embodiment of  FIGS. 1-5 , the resistance of the pull-in coil  112  can be made low to increase soft start current to the electric motor  102 . Accordingly, the torque of the electric motor  102  is increased during soft start, without having excessive abutment force between the pinion gear and the ring gear which traditionally results from the high amp-turn excitation of the pull-in coil  112 . 
     While coil arrangement in the embodiment of  FIGS. 1-5  is configured to increase the leakage flux for the pull-in coil  112 , the arrangement is configured to do the opposite for the hold-in coil  114 . In particular, the hold-in coil  114  in  FIGS. 1-5  is configured to minimize flux leakage with the plunger  116  in order to maximize the electromagnetic hold-in force on the plunger  116  for a given number of turns of the hold-in coil  114 . This is accomplished by centering the hold-in coil  114  at the plunger stop surface  156  interface. In this fashion leakage flux  171  is minimized with the hold-in coil  114 , and the electromagnetic force on the plunger is maximized. Accordingly, by the geometrical layout of the windings of the pull-in coil  112  and the hold-in coil  114 , it is possible to reshape the force-travel curves of the plunger  116  to values more desirable for a starter with a soft start system. 
     In addition to the benefits related to flux leakage, the side-by-side arrangement for the pull-in coil  112  and the hold-in coil  114  can also have thermal benefits. In particular, with the conventional coil over coil winding such as that shown in  FIG. 15 , the hold-in coil  214  suffers in strength if the abutment time between the pinion gear  206  and the ring gear is prolonged. During a prolonged abutment, the pull-in coil  212  will rapidly heat and then increase the temperature of the hold-in coil  214 . When the temperature of the hold-in coil  214  increases, the electrical resistance increases and the current decreases. This decreases the resulting hold-in force provided by the hold-in coil and thus the risk of the plunger contacts opening and plunger disengagement is increased. However, with the side-by-side coil arrangement shown in the starter embodiment of  FIGS. 1-5 , the thermal influence of the pull-in coil  112  on the hold-in coil  114  during starting is minimal, as the thermal conductive path resistance is much higher with the two coils separated from one another in the axial direction. 
     Spool with Additional Mounting Features 
     With reference now to  FIGS. 6-7 , an alternative embodiment of the spool  120  of  FIG. 2  is shown. Similar to the spool of  FIG. 2 , the alternative embodiment of the spool also generally includes a first end flange  122 , a middle flange  124 , a second end flange  126 , and a hub  128 . The hub  128  is generally cylindrical about an axial centerline  132 , and an interior passage  130  extends through the hub from one end of the spool  120  to the other. However, as explained in further detail below, in the embodiment of  FIGS. 6-7 , the middle flange  124  and the second end flange  126  include a number of additional mounting features  134 . 
       FIGS. 6A and 7  show views of the side of the middle flange  124  that faces the first coil bay  142 . The middle flange  124  includes various mounting features including a first winding post  172  positioned between a lead-in slot  174  and a lead-out slot  176 . The first winding post  172  extends radially outward from the centerline of the spool  120  and is configured to engage the wire from the hold-in coil. Sufficient space is provided around the first winding post  172  to allow the hold-in coil  114  to be wrapped around the winding post. Moreover, the first winding post  172  is sufficiently long to allowing wire from the hold-in coil  114  to be wrapped around the first winding post  172  several times. Accordingly, as explained in further detail below, the first winding post  172  provides a mounting feature  134  that allows the hold-in coil to be securely anchored to the spool  120  and also provides a feature for reversing the direction of the turns of the hold-in coil  114  on the spool. A reverse turn post may be advantageous in solenoids for starters with soft start systems, as described in U.S. patent application Ser. No. 12/767,710, filed Apr. 26, 2010, the content of which is incorporated herein by reference in its entirety. 
     With continued reference to  FIGS. 6A and 7 , the lead-in slot  174  provides an axial groove in the outer circumference of the middle flange  124  which is designed and dimensioned to receive the wire used to form the pull-in coil  112 . Additionally, in the embodiment of  FIGS. 6A and 7 , the lead-in slot  174  includes an entry ramp  175  for the start lead of the pull-in coil  112 . This entry ramp  175  extends in a substantially radial direction to the hub  128  of the spool  120 . The entry ramp  175  is configured such that the depth of the slot  174  into the middle flange  124  is slightly tapered moving toward the hub  128 . Accordingly, the lead-in slot  174  with entry ramp  175  allows the start lead of the pull-in coil  112  to be guided on the spool  120  from the perimeter of the middle flange  124  toward the hub  128  without consuming space in the first coil bay  142  before the start lead reaches the hub  128 . Once the start lead does reach the hub  128 , the first layer of turns for the pull-in coil  112  begin. While the lead-in slot  174  has been disclosed as including the entry ramp  175 , it will be recognized that in at least one alternative embodiment, the lead-in slot extends directly to the hub without the entry ramp  175  positioned in the slot  174 . 
     Similar to the lead-in slot  174 , the lead-out slot  176  provides another axial groove in the outer circumference of the middle flange  124  which is designed and dimensioned to receive the wire used to form the pull-in coil  112 . However, unlike the lead-in slot  174  in the embodiment of  FIGS. 6A-7 , the lead-out slot  176  does not include a ramp portion that extends in the radial direction to the hub  128  of the spool. Instead, the lead-out slot  174  is simply provided on the perimeter of the middle flange  124  and extends radially approximately the thickness of the wire for the pull-in coil in order to allow the finish lead of the pull-in coil to cut across the middle flange  124  once the pull-in coil is completely wound in the first coil bay  142 . 
     With reference now to  FIG. 6B , the opposite face of the middle flange  124  is shown. The face of the middle flange  124  shown in  FIG. 6B  is the face presented to the second coil bay  144  of the spool  120 . The first winding post  172 , the lead-in slot  174 , and the lead-out slot  176  are all visible on this side of the middle flange  124 . In addition, this side of the middle flange  124  includes an entry ramp  182  for the start lead of the hold-in coil  114 . This entry ramp  182  is similar to the entry ramp  175  for the pull-in coil, extending in a generally radial direction toward the hub  128  and gradually tapering as the ramp extends toward the hub  128 . Furthermore, the side of the middle flange  124  shown in  FIG. 6B  includes a second winding post  178  that is only accessible on this side of the middle flange  124 . Accordingly, an indentation  180  is formed in this face of the middle flange  124 , and the second winding post  178  is situated in this indentation  180 . As explained in further detail below, this second winding post  178  provides a mounting feature for the hold-in coil  114  that may be used as an anchor or a reversing turn feature. 
     With reference now to  FIG. 6C  the second end flange  126  includes additional mounting features, including a dual start lead slot  184 , a first finish lead slot  186 , and a second finish lead slot  188 . The dual start lead slot  184  is designed and dimensioned to allow the start leads for both the pull-in coil  112  and the hold-in coil  114  to pass through the perimeter of the second end flange  126 . When both start leads are positioned in the slot  184 , the start lead for the hold-in coil  114  is positioned radially inward from the start lead for the pull-in coil  112 . The first finish lead slot  186  is configured to allow the finish lead for the pull-in coil  112  to pass through the perimeter of the second end flange  126 . Similarly, the second finish lead slot  188  is configured to allow the finish lead for the hold-in coil  114  to pass through the perimeter of the second end flange  126 . 
     It will be recognized that the middle flange  124  is thicker in the axial direction than the two end flanges  122  and  126 . This increased thickness naturally follows because of the desired separation of the pull-in coil  112  and the hold-in coil  114  in the axial direction such that the coils are properly positioned on the spool  120 . However, the increased thickness also provides increased space for the various coil mounting features  134  included on the middle flange  124 . Without this middle flange design, the end flanges  122 ,  126  would need to be the thickness of the center flange to provide the same features, and this would decrease the available space for the coil bays  142 ,  144 . 
     The winding of the pull-in coil  112  and the hold-in coil  114  on the spool  120  is now described with reference to  FIGS. 8-12  in order to provide a better understanding of the design of the foregoing mounting features  134  of the spool  120  and arrangement of the coils  112  and  114  on the spool. 
     The process of winding the spool  120  begins with the hold-in coil  114 .  FIG. 8  shows the hold-in coil  114  being wound in the second coil bay  144  of the spool. To begin the winding process, a start lead  190  of the hold-in coil  144  is wrapped around the first winding post  172  in order to anchor the wire for the hold-in coil to the spool  120 . The start lead  190  is then channeled down the entry ramp  182  (not shown in  FIG. 8 ) on the middle flange  124  toward the hub  128 . After the start lead  190  reaches the hub  128 , the spool  120  is rotated in the direction of arrow  191 , causing a length of wire from a reel (not shown) to be wound around the hub, and create winding turns for the hold-in coil  114 . These winding turns are wound in a first turn direction in the second coil bay  144  of the spool  120 . 
     As shown in  FIG. 9 , after a predetermined number of turns in the first direction are created in the second coil bay  144 , the length of wire for the hold-in coil  114  is again wrapped around the first winding post  172 , and the spool  120  is rotated in the opposite direction as indicated by arrow  192 . Rotation of the spool in the direction of arrow  192  results in reverse winding turns being created in a second direction in the second coil bay  144  of the on the spool  120 . Such reverse winding turns may be advantageous on the hold-in coil in a vehicle starter, as described in U.S. patent application Ser. No. 12/767,710, filed Apr. 26, 2010, the content of which is incorporated herein by reference in its entirety. 
     With reference now to  FIG. 10 , after the reverse winding turns are created, the wire for the hold-in coil is wrapped around the second winding post  178  (see  FIG. 6B ) on the middle flange  124  to securely anchor the hold-in coil  114  in the second coil bay  144 . The finish lead  194  of the hold-in coil is then directed through the second finish lead slot  188  on the second end flange  126 . The start lead  190  is also directed through the dual start lead slot  184  on the second end flange  126 , and this completes the hold-in coil  114  on the spool  120 . 
       FIG. 11  shows the pull-in coil  112  being wound in the first coil bay  142  of the spool  120  after the hold-in coil  114  is wound in the second coil bay  144 . To begin winding the pull-in coil, a start lead  196  of the pull-in coil  144  is routed through the dual start lead slot  184  on the second end flange  126  and through the lead-in slot  174  on the middle flange  124 . The start lead  196  is then directed down the entry ramp  175  on the middle flange  124  toward the hub  128 . After the start lead  196  reaches the hub  128 , the spool  120  is rotated in the direction of arrow  197 , causing a length of wire from a reel (not shown) to be wound around the hub, and create winding turns for the pull-in coil  112  in the first coil bay  142  of the spool  120 . 
     With reference now to  FIG. 12 , after the turns of the pull-in coil  112  are completely wound in the first coil bay  142 , the finish lead  198  is routed through the lead out slot  176  on the middle flange  124 . The finish lead  198  is then directed across the turns of the hold-in coil  114  and through the first finish lead slot  186  on the second end flange  126 . This completes the winding of the pull-in coil  112  on the spool  120 . 
     Coil Comprised of Rectangular Wire 
       FIG. 13  shows a cross-sectional view of the spool  120  along line D-D of  FIG. 12 . In this embodiment of the solenoid  110 , the pull-in coil  112  is comprised of rectangular wire  146  (i.e. wire having a substantially rectangular cross-section), and the hold-in coil  114  is comprised of traditional round wire  147 . In particular, the rectangular wire  146  used for the pull-in coil  112  is square wire in the embodiments of  FIGS. 12 and 13 . The rectangular wire  146  is jacketed with a layer of insulation on the outer perimeter. The wire  146  also includes slightly radiused corners  148  that are provided for manufacturing concerns and to avoid any sharp edges on the wire which might cut into the insulation layer on neighboring wires. As explained below, the rectangular wire  146  is advantageous for use in the pull-in coil  112 , as it provides an increased stacking factor for the coil while also providing thermal benefits for the coil. 
     The stacking factor for a coil is the ratio of the total volume consumed by conductors only (i.e., not including air voids between conductors) to the total volume consumed by the complete coil (i.e., including all conductors and air gaps between conductors). Traditional round wire has an effective stacking factor of about 78%. In contrast, the square wire disclosed herein has an effective stacking factor of 90% or more. In particular, the square wire  146  used in the embodiment of  FIGS. 12 and 13  has a stacking factor of 92%. As a result, when comparing square wire and round wire, square wire will require less space to provide the same electromagnetic force (i.e., less space to provide the same amp-turns). This space savings is particularly useful for vehicle starters where the starter is often situated in a crowded engine compartment. 
     Another benefit of the rectangular wire  146  of  FIGS. 12 and 13  is that it provides a better thermal conduction path than round wire for transporting the ohmic heat of the coil  112  to the edges of the coil, where the heat may be removed by conduction or convection. With a round wire coil, there is only point contact between adjacent windings, as the conductors layers are wound on top of each other (i.e., two adjacent circles will only touch in a single point). In contrast, as shown in  FIG. 13 , with square wire  146  the interface between conductors on adjacent windings is much larger since there is contact between adjacent conductors along the entire flat portion of the sides of the conductors. Therefore, the heat being transmitted from coil wire to coil wire is transported via the copper wire rather than the air between the wires, and this copper-to-copper conduction provides a significant thermal advantage. For example, the improved conduction reduces the delta temperature difference between the outside edges of the coil and the typical center hot spot of the coil. 
     With reference now to  FIG. 14 , yet another alternative embodiment of the solenoid spool  120  and coils  112 ,  114  is shown. In this embodiment, the pull-in coil  112  is comprised of rectangular wire  146 , and the hold-in coil  114  is also comprised of rectangular wire  149 . The rectangular wire  146  of the pull-in coil  112  is essentially the same as the rectangular wire  149  of the hold-in coil, but the width of the pull-in coil wire  146  is greater than the width of the hold-in coil wire  149 . Accordingly, the hold-in coil wire is square wire with radiused corners. Additionally, the rectangular wire  149  is jacketed with a layer of insulation on the outer perimeter. The rectangular wire  149  of the hold-in coil  114  also provides similar advantages to those described above for the pull-in coil  112 . For example, the rectangular wire  149  provides an increased stacking factor for the hold-in coil  114  while also providing thermal benefits for the coil. 
     The foregoing detailed description of one or more embodiments of the starter solenoid with spool for retaining coils been presented herein by way of example only and not limitation. It will be recognized that there are advantages to certain individual features and functions described herein that may be obtained without incorporating other features and functions described herein. Moreover, it will be recognized that various alternatives, modifications, variations, or improvements of the above-disclosed embodiments and other features and functions, or alternatives thereof, may be desirably combined into many other different embodiments, systems or applications. Presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the appended claims. Therefore, the spirit and scope of any appended claims should not be limited to the description of the embodiments contained herein.