Patent Publication Number: US-7908775-B2

Title: Two-stage snow plow

Description:
This application is a divisional of U.S. patent application Ser. No. 11/438,442, filed May 23, 2006, now U.S. Pat. No. 7,730,643. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to plows and more particularly relates to plowing arrangements for clearing snow from pavement such as a road, a highway or a runway as well as to methods of clearing snow from pavement. 
     BACKGROUND OF THE INVENTION 
     An accumulation of snow is usually removed from pavement by a truck that is provided with a snowplow having a moldboard mounted on the front end of the truck. Typically, the plowing operation leaves some amount of snow or ice or slush on the pavement being cleared. When the snow or ice is packed down on the pavement surface, the ability of the moldboard to remove all or substantially all of the snow and ice is significantly reduced. 
     During a plowing operation, it is conventional to raise and lower the moldboard of the snow plow as desired and to change the angle that the moldboard of the snow plow makes with the longitudinal center axis of the truck, and therefore with respect to the longitudinal axis of the lane of pavement being cleared. 
     The moldboard of the snow plow may be selectively raised and lowered so that the plow truck may be driven with the lowermost edge of the moldboard either in contact (for conducting a plowing operation) or out of contact with the road, such as when the truck is being driven over pavement which has already been cleared of snow. Also, the snow plow is typically arranged to enable the angle of the plow with respect to the truck to be changed so that the snow plow can be used to divert snow to the left or to the right of the truck or used to push snow directly in front of the truck such as when clearing a driveway or parking lot. 
     A wing plow or another attachment may be provided to effectively extend the width of the lane that can be plowed by a single truck in a single pass. Such wing plows are typically mounted at one side of the truck. 
     Snow plow vehicles at airfields may sometimes have a front plow blade and a broom which is towed by the vehicle. 
     The need remains for a snowplow arrangement in which some or essentially all of the snow, ice and slush which has been left by the moldboard may be removed from the pavement being plowed in a single pass of a snowplow vehicle. 
     SUMMARY OF THE INVENTION 
     These and other needs are met by the invention. In one embodiment, a snow plow for mounting to a vehicle includes a first frame, a second frame coupled to the first frame for rotation about the first frame, a main plow having a scraping edge and mounted to the second frame, a secondary plow, and a member that couples the secondary plow to the second frame and configured to allow translation of the secondary plow relative to the second frame and in a direction that is parallel to the main plow scraping edge. One example of the member for coupling is sleeves provided on the second frame. These sleeves have bearing surfaces upon which the secondary plow slides as it translates in the parallel direction. A linkage may also be provided which, when combined with the member, allows the secondary plow to translate. The linkage is connected at one end to the secondary plow and at the other end to the first frame. In another embodiment, the member may be formed by a gear train where portions of the gear train are located on the drive frame and the secondary plow. 
     In another embodiment, a snow plow for mounting to a vehicle includes a drive frame, a main plow coupled to the drive frame, and a secondary plow coupled to the drive frame and positioned behind the main plow, wherein the secondary plow includes a plurality of fingers, each having a straight portion and a curved portion wherein the curved portion is adapted for collecting snow. Each of the tines may be a one piece tine, or a two piece tine. For a two piece tine, the straight portion may be formed by spring steel while a scraping tip may be formed of carbide. 
     In another embodiment, a snow plow for mounting to a vehicle includes a drive frame, a main plow coupled to the drive frame, a secondary plow coupled to the drive frame and positioned behind the main plow, and a remotely controlled actuator, mounted to the drive frame and configured for selectively placing the secondary plow into a plowing position. The actuator may be a hydraulic cylinder. 
     In another embodiment, a method for deploying a snow plow mounted to the front of a vehicle includes the steps of lowering a main plow so as to bring it into a plowing position, and lowering a secondary plow, located between the vehicle and the main plow, so as to bring the secondary plow into the plowing position. In this method, the secondary plow may be placed in a plowing position after the main plow has begun plowing. The plows may be raised/lowered by hydraulic cylinders. Further, both plows may have separate hydraulic cylinders and the pressure applied to the secondary plow by its hydraulic cylinder may be remotely controlled by an operator-enabled valve so that as tines of the secondary plow blade begin to erode, the operator can increase the pressure applied to the tines. 
     In another embodiment, a method for positioning a snow plow at the commencement of snow plowing includes the steps of rotating a main plow relative to a vehicle carrying the main plow and translating a secondary plow, positioned between the main plow and the vehicle, in a direction parallel to a scraping edge of the main plow. In this embodiment, the secondary plow may be translated by allowing it to freely slide along bearing surfaces which may be formed on a drive frame. Additionally, the steps may include lowering the main and secondary plows after the rotating and translating steps. 
     In another embodiment, a method for snow plowing using a vehicle having a snow plow attached at a front end of the vehicle includes the steps of providing a first plow in a stowed position, providing a second plow that is located between the first plow and the vehicle, and lowering the second plow so as to place it into a plowing position while maintaining the first plow in the stowed position. 
     In another embodiment, a method for adding a secondary plow to an existing plowing apparatus, the plowing apparatus having a frame, a main plow supported by the frame, and a bracket for securing the frame to a front end of a vehicle, includes the steps of providing a secondary plow blade, an actuator having a first end and a second end, and an actuator mount, securing the actuator mount to the frame, coupling the secondary plow to the frame for pivotal motion relative to the frame and attaching the actuator first end to the actuator mount and a second end to the secondary plow blade. In this method, the conventional frame for the main plow may provide adequate clearance for operating the secondary plow, or it may require a modification to the frame. 
     Additional features and advantages of the invention will be set forth or be apparent from the description that follows. The features and advantages of the invention will be realized and attained by the structures and methods particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation without limiting the scope of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Several preferred embodiments of the invention are illustrated in the enclosed figures in which: 
         FIG. 1  is a side view of a snow plow according to the prior art with the plow in contact with the pavement; 
         FIG. 2  is a side view of a snow plow according to the prior art with the plow raised out of contact with the pavement; 
         FIG. 3  is a top schematic view of a snow plow that is angled with respect to a center line of a truck carrying the snow plow; 
         FIG. 4  is a side view of a first embodiment of a snow plow mounted to the front of a vehicle; 
         FIG. 5  is a side view of a portion of the snow plow of  FIG. 4 ; 
         FIG. 6  is a top partial schematic view of a portion of the snow plow and vehicle of  FIG. 4 ; 
         FIG. 7  is a side view of a portion of the snow plow of  FIG. 5 ; 
         FIG. 8  is a side view of the snow plow of  FIG. 5  showing three angular positions of a secondary plow; 
         FIG. 9  is a partial exploded side view of the snow plow of  FIG. 5  illustrating an assembly of a trip mechanism; 
         FIG. 10  is a partial schematic top view of the vehicle and snow plow of  FIG. 4 , but with the snow plow rotated about an axis A so as to divert snow to the right of the vehicle and without the secondary plow positioned completely behind the plowing path of a moldboard; 
         FIG. 11  is a partial schematic top view of the vehicle and snow plow of  FIG. 4 , but with the snow plow rotated about an axis A so as to divert snow to the right of the vehicle and the secondary plow positioned completely behind the plowing path of the moldboard; 
         FIG. 12  is a side view of a second embodiment of a snow plow; 
         FIGS. 13 and 14  are respective side and front views of a portion of a secondary plow of  FIG. 12 ; 
         FIGS. 15 and 16  are respective side and front views of a portion of the secondary plow according to the first embodiment; 
         FIG. 17  is a side view of an alternative embodiment of a portion of the secondary plow according to the first embodiment; 
         FIG. 18  is a side view of a conventional plow illustrating a modification thereto in connection with a third embodiment of a snow plow; 
         FIG. 19  is a side view of a third embodiment of a snow plow; and 
         FIG. 20  is a schematic of a hydraulic circuit of the first embodiment of a snow plow. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a snow plow according to the prior art is shown with a moldboard  100  of conventional design which is carried by a truss  102 . The truss  102  is arranged to be removably mounted on the front of a truck or other suitable vehicle (not shown) through a bracket  104  in a suitable and conventional manner well known in the art. A back brace  106  is provided to support an upper portion of the moldboard  100 . 
     An arrangement  108  including a plow shoe  108  and vertical member  112  is provided behind the moldboard  100 . The vertical member  112  has a plurality of holes  114  which correspond to holes in a bracket  116  so that the plow shoe may be adjusted vertically to provide a support for the moldboard on the pavement. 
     The truss  102  has a mounting member  118  which is formed from steel square tubing and which comprises a box beam, i.e., a member having a square cross-section, which is hollow along the length of the box beam. The moldboard  100  is pivotally attached to the mounting member  118  by a bracket  120 . Typically, the back brace  106  is formed by a pair of hydraulic cylinders which are provided to selectively orient the moldboard  100  with respect to the truss  102 . In this way, the angle that the moldboard makes with respect to the pavement may be varied as desired. In addition, the truss  102  includes an arrangement (not shown) such as one or more hydraulic cylinders to lift the moldboard  100  when desired. 
     If desired, the scraping edge  122  of the moldboard  100  may be made of a flexible or resilient material in order to minimize damage to the moldboard in the event that the cutting edge  122  should strike an obstruction during plowing. The cutting edge  122  may also be provided with a trip mechanism such as is described in U.S. Pat. No. 5,079,866, which is incorporated herein by reference. 
     With reference to  FIG. 2 , a trip mechanism  124  is provided for the moldboard  100  with the trip mechanism having a compression spring  125  which urges the lower portion of the moldboard against the pavement being plowed. The trip mechanism  124  includes a slot  126  through which a member  128  may slide to allow the moldboard to lift above the pavement upon striking an object. With reference to  FIG. 2 , the movement of the member  128  along the slot  126  moves a member  130  to compress the spring  125 . After the moldboard has passed over the object, the moldboard is urged back against the pavement by the spring  125  which urges the member  128  downwardly along the slot  126 . 
     With reference now to  FIG. 3 , vehicle which carries the moldboard is illustrated schematically by the tires  130 . The moldboard may be angled with respect to a center line  132  of the vehicle in order to divert snow and ice to one side of the vehicle. If desired, the moldboard (not shown) may be oriented perpendicular to the center line  132  or angled to the left or to the right of the centerline  132  of the vehicle. If desired, a wing plow  134  may be provided on one side of the vehicle to extend the width of the pavement being cleared by the vehicle in a single pass over the pavement. 
     With reference to  FIG. 4 , a two stage snow plow  200  of a preferred embodiment of the invention has a primary moldboard  202  and a secondary plow  204  which is mounted to a drive frame  212  for pushing the moldboard  202 . Plow  200  is shown attached to the front of a vehicle  130 , secured to a bracket  266 . A lifting mechanism  10  is mounted to the vehicle front end to lift and lower drive frame  212  and moldboard  202 . Mechanism  10  includes a pivot arm  13  and lifting arm  12  that are controlled by a double action hydraulic cylinder  11 . Moldboard  202  is of suitable conventional form and may be rigid or flexible, made from metal or from a non-metal material such as plastic, and it may also have a predetermined cross section or a cross section that can be changed to alter the ability of the moldboard  202  to divert snow and ice to one side of the vehicle. Moldboard  202  is coupled to drive frame  212  at a lower end to a trip mechanism  218  and at an upper end to a brace  206 . Two vertically adjustable shoes  15  (only one shown in  FIG. 4 ) are located on opposed sides of drive frame  212 . Shoes  15  are connected to a bracket  16  that is mounted (by way of removable bolts) to an L-frame  17  which is connected at ends of a stiff, square tube  220  ( FIG. 5 ) of drive frame  212 . Shoes  15  are used to support drive  212  when moldboard  202  is raised off the pavement by the trip mechanism  218 . Shoes  15  also protect secondary plow  204  so that the much heavier moldboard  202  does not cause damage to secondary plow  204  as moldboard  202  skips over obstacles encountered on a road surface. 
       FIG. 5  illustrates plow  200  shown in  FIG. 4 , but with lifting mechanism  10  and shoes  15  removed for purposes of illustration, and  FIG. 6  illustrates a top partial schematic view of plow  200 . Referring to  FIGS. 5-6 , secondary plow  204  is mounted to drive frame  212 , located directly behind moldboard  202 , and operated independently of moldboard  202 . Plow  204  is pivotally mounted to each of two supporting plates  236  of the plow&#39;s two trip mechanisms  218  and pivotally controllable by a pair of hydraulic cylinders  224  which pivot secondary plow  204  about an axis corresponding to the polar axis of a pivot tube  230  in  FIG. 5 , or axis B in  FIG. 6 . Plow  204  may also slide lengthwise over pivot tube  230 , for purposes of repositioning plow  204  relative to moldboard  202  (as discussed in greater detail, below). A plurality of individual tines  208  are secured to and removable from a U-channel  210  of secondary plow  204  using, e.g., removable bolts (an angle iron, an I-beam, or round tube or other similarly suited supporting structure may be used in place of U-channel  210 ), extend along the length of U-channel member  210 , and have substantially the same length as moldboard  202 . Tines  208  form a scraping edge of the secondary plow  204  for purposes of removing residual snow left behind by moldboard  202 . Tines  208  are discussed in greater detail, below. 
     Referring to  FIG. 6 , secondary plow  204  has four hinge points (shown schematically in  FIG. 6  as  214   a ,  214   b ,  214   c  and  214   d ) that are located on the side of the U-channel member  210  that faces moldboard  202 . These hinge points pivotally mount secondary plow  204  to drive frame  212 . As indicated earlier, the rotation axis for secondary plow  204  is illustrated schematically in  FIG. 6  by broken line B which corresponds to the polar axis of pivot tube  230  in  FIG. 5 , a tube which has a longitudinal extent approximately equal to the length of square tube  220 . Two of the secondary plow  204  hinge mounts, namely,  214   a  and  214   d  are formed by sleeved holes in support plates  236  of trip mechanisms  218 , one of which is illustrated in  FIG. 7  as hole  236   a , while the other hinge mounts  214   b ,  214   c  are formed on U-channel  210 . 
     To position the outermost ends of secondary plow  204  within the path of moldboard  202  during snow plowing, pivot tube  230 , which is mounted and secured with two lock nuts and bolts to U-channel  210 , is arranged to slide either to the right or to the left over bearing surfaces provided by the holes formed in support plates  236  as moldboard  202  is angled to discharge snow to the right or to the left, respectively, of the plowing vehicle. Separate bearing sleeves are preferably installed in holes  236   a  to facilitate sliding motion of secondary plow  204 . The sliding feature of secondary plow  204  is described in greater detail, below and illustrated in  FIG. 11 . 
     Drive frame  212  is coupled to bracket  266  by an A-frame  268 . A pair of hydraulic cylinders  270 ,  272  (not shown in  FIG. 5 ) are used to rotate drive frame  212  about a rotation axis A so that moldboard  202  may be angled to divert snow left or right of the vehicle path, as illustrated in  FIG. 11 . Drive frame  212  includes an arcuate member  260  and truss members  261   a ,  261   b , which provide structural support for drive frame  212  and mount flanges  263   a ,  263   b , respectively, which are used with braces  206  to pivotally support moldboard  202  at an upper end thereof. Hydraulic cylinders may also be used in place of braces  206 . A-frame  268  is pivotally coupled to drive frame  212  at hinge  264  located centrally on square tube  220 , while actuating ends of hydraulic cylinders  270 ,  272  respectively are pivotally connected at locations  262   a ,  262   b  respectively. Synchronous actuation of hydraulic cylinders  270 ,  272  effect rotation of drive frame and thus plow  204  and moldboard  202  about rotation axis A. 
       FIG. 7  illustrates structure mounted on drive frame  212  and associated with moldboard  202  and secondary plow  204 . This structure is also shown in  FIG. 5  with secondary plow  204  and moldboard  202 . Square tube  220  supports a pair of trip mechanism support plates  236  having lower slots  252  that receive a pins (not shown) coupling moldboard  202  to drive frame  212 , as discussed below, and flanges  263  for braces  206 . A hole  236   a  is formed in plate  236  to allow tube  230  to pass through and be supported by support plate  236 . As indicated earlier, tube  230  may also slide within hole  236   a . Drive frame  212  also includes a pair of flanges  226  for mounting hydraulic cylinders  224  for secondary plow  204 , as discussed in greater detail below. 
     With reference to  FIG. 9 , which shows a partially exploded view of plow  200 , the attachment of moldboard  202  to drive frame  212 , by way of trip mechanism  218 , will now be described. As mentioned earlier, drive frame  212  mounts a trip mechanism support plate  236  at square tube  220  (recess  238  is mated with tube  220 ). A rod  242  is fixed at its lowermost end  242   a  to an upper portion  236   b  of support plate  236 . A sleeve  259  is placed over rod  242 . A flange  259  of sleeve abuts a washer  256  and rod  242  is secured to nut  256 . A trip helper plate  248  is provided with lower and upper pins  250   a ,  250   b  which are arranged to slide within corresponding slots  252  of support plate  236 . Lower pin  250   a  is also pivotally connected to a lower portion of moldboard  202  by pin  250   a  being simultaneously received in a hole  202   a  of moldboard  202  and slot  252  of support plate  236 . The upper end of helper plate  248  has a flange  258  that engages the lower end of spring  240 , so that spring  240  is compressed between flanges  258  and  259   a  when helper plate  248  is pushed upwards by moldboard  202 . Accordingly, the trip support plate  236 , fixed to drive frame  212 , is arranged to allow upwards and rearwards movement of support plate  248  along slots  252  against the force of compression spring  240 , which lifts the scraping edge of moldboard  202  off of the ground. When the moldboard  202  encounters an obstacle, helper plate  248  and therefore moldboard  202  are urged upward and over the obstacle. After the moldboard  202  has passed over the obstacle, the compression spring urges the trip helper plate  248  and therefore the moldboard  202  downwardly until the moldboard again encounters the pavement. Shoes  215  ( FIG. 4 ) are used to stabilize plow  200  and protect secondary plow  204  when moldboard  202  is pushed off the ground by an obstacle. 
     With reference again to  FIG. 6 , rotation of secondary plow  204  about drive frame  212  (axis B in  FIG. 6 , bar  230  in  FIG. 5 ) is controlled by a pair of hydraulic cylinders  224 . Each hydraulic cylinder  224  is identical to that illustrated in  FIG. 5 . The line of action for both these hydraulic cylinders is illustrated schematically in  FIG. 6  by C. Hydraulic cylinders  224  are coupled to U-channel  210  by way of a box  215  formed by two opposed plates ( FIG. 5  shows one of these plates) that mount a rod attaching actuating end  224   a  of hydraulic cylinder  224  to secondary plow  204 , specifically, U-channel  210 . Flange  226  mounts the housing end  224   b  of hydraulic cylinder  224 . The box  215  for actuating end  224   a  of hydraulic cylinder  224  has two half-round grooves that receive two rods  210   a  which are welded to the inner surfaces of U-channel  210 . When secondary plow  204  translates as illustrated in  FIG. 11 , the box  215  slides along rods  210   a . At the same time, secondary plow  204  is supported by way of the coupling between box  215  and U-channel  210  when secondary plow  204  is raised and lowered by hydraulic cylinder  224 . 
     Referring to  FIGS. 6 and 8 , hydraulic cylinder  224  raises and lowers secondary plow  204  by applying a force at position C, which causes secondary plow  204  to rotate about bar  230  (rotation axis B). Hydraulic cylinder  224  may be used to raise tines  208  from (position I) or lower tines  208  to (position II) the plowing surface. Additionally, hydraulic cylinder  224  may be operated to rotate tines  208  to a position III, which would be needed to bring tines  208  into contact with the ground after the lower portion of tines  208  (indicated by D) has eroded. Thus, even when the tines  208  have undergone a significant amount of erosion, tines  208  may still be used by further extending hydraulic cylinder  224  so that an appropriate pressure may be applied to tines  204 . 
     Hydraulic cylinders  224  provide an appropriate and steady pressure for tines  208  to scrape the residual snow/ice from the road. Further, hydraulic cylinders  224  provide the steady pressure regardless of and compensating for, the wear that takes place at a scraping edge of the tines  208  or fingers while they are plowing. The appropriate pressure provided by the hydraulic cylinders to urge the tines  208  against the pavement is dependent on the condition of snow (i.e., lightly packed to highly packed snow) on the road. The pressure can be set as well as monitored accurately at a gauge installed in a cab of the vehicle, through-out the plowing operation. See  FIG. 20  and related discussion, infra. 
     If the pressure urging the tines  208  downwardly is unnecessarily high, the tines  208  may be subjected to undue wear at a scraping edge. Unnecessarily high pressure may also cause damage to the pavement. However, inadequate pressure at the tine tips may be ineffective for removing packed snow and ice from the pavement. Because the drive frame  212  is supported by plow shoes  15 ,  FIG. 4 , the amount of downward pressure provided by the two hydraulic cylinders is independent of the weight of moldboard  202 . 
     In the preferred embodiment, tines  208  are urged downward by hydraulic cylinders. Springs may be used, however, it is preferred to use controllable hydraulic cylinders because it may be difficult for one or more springs to provide a relatively constant amount of downward pressure on the tines  208 , especially by one or more coil springs. Further, the coil springs may not deliver a relatively constant pressure at the tips of the tines or fingers because of the shortening of the tines at the ends or tips as the tines start to wear during a plowing operation. 
     If the tines  208  were urged downwardly by coil springs, the downwardly directed pressure exerted by the coil springs may not be easily compensated for as the fingers  208  wear. Therefore, the downwardly directed pressure exerted by the coil spring will tend to decrease as the tines erode and get shorter and shorter. In order to scrape the snow and ice from the road efficiently, in the preferred embodiments an appropriate and steady downwardly directed pressure is applied by the tips of the fingers or tines against the pavement during the entire plowing operation. Of course, an arrangement, not shown, could readily be provided for adjusting (either automatically or manually) the downward force applied by one or more coil springs to the tips of the tines against the pavement. 
     Sliding Feature 
       FIGS. 10-11  are top view illustrations of plow  200  without tines  208  of secondary plow  204  or moldboard  202  shown. Instead, secondary plow  204  and moldboard  202  are represented by their respective scrapping edges T S  and M S , i.e., edges that come into contact with the ground. Plow  200  is located at the front of a vehicle (the vehicle orientation is indicated by the tire silhouettes  10 ) with the vehicle path being left to right and the drive frame  212  rotated approximately 30 degrees so as to divert snow right of the vehicle.  FIG. 10  illustrates the position of tine edge T S  relative to moldboard edge M S  if secondary plow  204  were fixed relative to drive frame  212 . As shown, an upper portion of edge T S  encounters a section of snow S S  that is not first met by a corresponding upper portion of edge M S  while a lower portion of edge T S  does not cover a section of snow S 2  that is encountered by a corresponding lower portion of edge M S .  FIG. 11  illustrates the positioning of edge T S  relative to edge M S  if secondary plow  204  is free to slide relative to drive frame  212  and coupled to linkages  267   a ,  267   b . By comparison with  FIG. 9 , it is seen that edge T S  covers the same path of snow first encountered by edge M S . Thus, by repositioning secondary plow  204  behind moldboard  202  along its entire edge, all areas of the roadway encountered by edge M S  are also covered by edge T S . Edge T S  is repositioned relative to edge M S  in  FIG. 11  by a force F applied to secondary plow  204  by linkage  267   a , which is under tension. The force applied by linkage  267   a  pulls upon secondary plow  204 , causing it to slide along its track, toward the lower end of square tube  220 . If drive frame  212  were rotated counterclockwise, then linkage  267   b  would come under tension and pull secondary plow  204  towards the upper end of square tube  220 . This behavior is evident when considering that linkages  267   a ,  267   b  have a fixed length and thus, when drive frame  212  rotates, one end of drive frame  212  is brought closer to the location where both linkages  267  are fixed, while the other end is further away, causing the connecting linkage to go under tension (thereby pulling secondary plow towards it). Accordingly, in an embodiment of the invention, when the plow is positioned to plow snow, the moldboard  202  is rotated and the secondary plow  204  is rotated and translated along a direction parallel to a scraping edge of the moldboard  202 . Additionally, as discussed above and below, secondary plow  204  may also be operated independently of moldboard  202 . 
     A variety of mechanisms may be employed as alternatives to the first embodiment for translating secondary plow  204  when drive frame  212  rotates into a plowing position. For a example, three interlocking gears (two rotary and one linear) may be used. One rotary gear would mount to the drive frame  212 , e.g., at axis A, and would engage a second rotary gear, which could be mounted to the A-frame  268 . This pair of gears would have a greater than 1:1 gear ratio. The second gear would then engage with a linear gear on secondary plow  204 , e.g., a rear surface of U-channel  210  would have gear teeth adapted for engaging the second rotary gear. When drive frame is rotated, the second gear rotation would cause the U-channel  210 , and therefore tines  208 , to translate parallel to the moldboard  202  scrapping edge. 
     In preferred embodiments, two sections of steel chain are used to pull secondary plow  204  over the bearing surfaces defined by holes  236   a  and towards one or the other end of square tube  220  of drive frame  212 . One end of each chain is attached to two lugs, each of which is welded to two ends of a rear flange of U-channel  220  (not shown in  FIG. 6  but illustrated schematically in  FIG. 11 ). The rear flange faces rearwardly toward the truck. The other end of each chain is attached to a common sleeve  266   a  with lugs that swivel within a vertical short shaft that is welded to a bottom face of a stiffener plate of the swivel hitch. 
     Tines  208   
     As mentioned earlier, tines  208  form a scraping edge of the secondary plow  204  for purposes of removing residual snow left behind by the primary moldboard  202  while it is plowing. Referring to  FIGS. 15-16 , each have a first portion that is curved, preferably substantially semi-circular in shape, and a second portion that is generally straight. They are made from flat spring steel, preferably one-piece, and are readily available for replacement as suitable tines are often used for agricultural applications. When plow  204  is mounted to drive frame  212 , tines  208  will extend in a concave manner towards the main moldboard  202 . Tines  208  may be one-piece. However, upon conducting a series of road tests, it was found that tines  208 , when formed from spring steel, can erode at an undesirable rate. A two piece tine was therefore employed. In this design, tines  208  have a first part made of spring steel and a tip made of carbide that is bolted to the first part. The carbide tip may form a portion of the curved section of tine  208  illustrated in  FIGS. 15-16  or a straight part secured at the distal end of a tine.  FIG. 17  illustrates one example of a secondary plow with tines that include a carbide tip. Tine  209   a  has an end in which a carbide scraping tip  209   b  is secured thereto by a releasable fastener  209   c.    
     In the preferred embodiments, the spacing between adjacent tines is preferably about 0.016 inch. In the preferred embodiments, the tines do not overlap one another because overlapped tines or fingers are unduly rigid because each tine or finger effectively becomes an integral part of effectively a single blade extending along the length of the U-channel member. Accordingly, overlapped tines or fingers are effectively prevented from individually following the contour of the road or pavement and the scraping ability of the tines is relatively poor and inefficient. On the contrary, when the fingers or tines are not overlapped, the fingers are flexible and able to oscillate especially when they are made of spring steel. 
     Oscillating fingers are considered to be especially desirable for scraping bonded snow and ice because the oscillating fingers provides an impact force against the packed snow and ice when they oscillate (move back and then forth) during the plowing operation. 
     With reference to  FIGS. 15-16 , in one configuration for the individual tines  208 , the concave portion of the tines or fingers are substantially semi-circular with two substantially tangential straight top and bottom end portions. The bottom straight end portion of the tines together essentially functions as the blade of a plow even though the individual tines are spaced apart from one another. The top straight portion, made of spring steel, is fastened to U-channel  210 . The bottom straight portion, i.e., the portion in contact with pavement, is made from carbide or another sufficiently hard material. The inside surface of the arrangement of tines or fingers formed by the lower relatively flat lower portion and the curved semi-circular portion may be made relatively smooth to essentially provide a continuous surface for facilitating efficient snow and ice flow along the plurality of tines or fingers. 
     The inside surface and contour of the arrangement of tines or fingers corresponds closely to the inside surface and contour of a conventional plow or moldboard. In this way, secondary plow  204  may be used as a small-scale reversible plow. The tines or fingers  208  face toward the front of the vehicle (i.e., in the plowing direction) as does the moldboard  202 . In the preferred embodiment, a space adequate to accommodate at least about 80% of the residual snow left behind by the main plow, is provided between the rear of the moldboard  202  and the front of the secondary plow  204  beneath the drive frame  212  so that the snow and ice left by the moldboard  202  and scraped by the secondary plow  204  can flow without interruption along the inside curvature of the secondary plow  204  and be discharged from one end of the secondary plow  204 , substantially as in a curved moldboard of a typical snow plow. 
     An obstacle or shield provided in front of secondary plow  204  would narrow down the space needed between plow  204  and moldboard  202  and tend to prevent scraped snow and ice from flowing. As a result, secondary plow  204  may clog. In a preferred embodiment, secondary plow  204 , the uppermost portion of the individual tines or fingers are not inclined with respect to the plowing direction because such an incline would tend to pack the snow and thereby clog the flow of snow and ice along the inside surface of plow  204 . 
     Because secondary plow  204  operates independently of moldboard  202 , it is not necessary that plow  204  be used every time moldboard  202  is used for snow removal. Instead, the operator may decide based on conditions. For example, plow  204  may not be needed if the snow is not packed to the ground and plowing with moldboard  202  is deemed sufficient to keep the road open and safe. Additionally, it may not be necessary to use secondary plow  204  when residual unpacked snow is left behind by moldboard  202  if the road that has been treated with anti-icing treatment before a snowstorm and warm weather is expected. In this situation, most or all of the residual snow will be melted by the anti-icing treatment and the warming weather. By selective use of secondary plow  204 , the life of the individual tines or fingers can be extended. 
     When cold weather is forecasted to continue or worsen after plowing, when another snowstorm is expected, or when anti-icing treatment would need to be reapplied, it is desirable to remove most or all of the residual snow (whether packed, unpacked or slushy). Removing this residual snow and ice prevents an excessive dilution of the anti-icing chemicals which makes the chemicals ineffective to prevent the packed snow or ice from developing a bond with the pavement. In a situation such as this, use of secondary plow  204 , either with tines or a resilient blade (discussed infra) would be helpful. 
     Anti-icing chemicals are applied to pavement, typically before a winter storm to prevent bonding between snow or ice and the pavement. The anti-icing chemicals depress the freezing point of water. If the snow or ice is not bonded to the pavement, plowing of the un-bonded snow and ice is relatively effortless. Accordingly, the use of anti-icing chemicals is well suited to roads that have a relatively high level of traffic and is considered to be relatively cost effective. 
     De-icing of pavement is considered to be a highway snow and ice control operation. The typical, traditional procedure of snow and ice control practice is to wait until an inch or more of snow accumulates on the pavement before beginning to plow and to treat (de-ice) the highway with chemical abrasives and then plow away the slushy snow. The amount of residual packed or unpacked snow and ice that typically remains on the road (after the application of the conventional anti-icing chemicals) is generally considered to be high. Therefore to keep the road open and safe, the amount of de-icing material needed to penetrate the pavement is relatively high and considered to be expensive. 
     Secondary plow  204 , when used in conjunction with moldboard  202 , reduces the amount of residual snow and ice left on the pavement after plowing. Therefore, the amount of de-icing chemicals can be reduced and the time taken for chemicals to reach the pavement (by melting through the ice and snow) is reduced. A reduction in the use of anti-icing chemicals is usually considered beneficial to the environment. 
     Secondary plow  204  facilitates the reduction of anti-icing chemicals, such as sodium chloride, calcium chloride, magnesium chloride and salt etc., required by anti-icing and de-icing treatments of roads in order to keep them open and safe in the winter storm. Tines  208  remove a layer of snow from the pavement that is left behind by the moldboard  202 . In addition, tines  208  help break apart frozen snow on the pavement being plowed into tiny pieces so that chemicals may more quickly penetrate through the snow. In this way, the amount of time needed to melt any remaining snow on the pavement is shortened and the amount of chemicals that are needed to treat the road is reduced. 
     With reference to FIGS.  8  and  15 - 16 , preferably, the angle that tines  208  make with the pavement, almost vertical (about 75 to 90 degrees), is desirable with 85 to 90 degrees preferred, and close to 90 degrees is most preferred. Close to vertical is more effective and good for scraping. Tines  208  are preferably inclined more than 45 or 50 degrees (like a plow blade) because a more shallow angle typically cannot take hard pack off. Instead, the tines tend to slide over hard packed snow. Tines  208  may be formed from one-piece metal with slots up to four inches from the securing bolts but it is preferred to have industrial tines which better follow the contour of the road. The tips of tines may be square (commercially available ones have a notch) and of the type used for cultivators, as are available from John Deere (a support spring from a tooth cultivator). Preferably, tines are two-piece, with tips made of carbide. 
     When the secondary plow uses tines  208 , about 70% or more of the residual snow and ice left on the pavement by the primary moldboard  202  is reduced and therefore the amount of the chemical needed to clean the road from a snowstorm is reduced. Additionally, secondary plow  204  reduces the time required for chemicals to penetrate through to the pavement and melt the remaining snow left by secondary plow  204  (typically less than about 30% of the residual snow of the primary moldboard). Thus, delays caused by snowstorms are significantly reduced. 
     In one embodiment of secondary plow  204 , ninety-six tines are arranged vertically, with a 1/16 inch gap provided between adjacent tines. Tines are composed of a flat spring bar which is 1 inch× 5/16 inch thick and shaped to an overall height of 13 inches with a depth of 16 inches. The top horizontal arm is 7 inches long and the bottom vertical arm is 6 inches long with the curved section having a radius of 6 inches. In another embodiment, the bottom vertical arm may be made of carbide or another relatively hard material. 
     In one embodiment, tines were set at an angle of 37 degrees with respect to the forward direction of the vehicle and the hydraulic cylinders  224  provided a downward force of about 3500 lbs. to about 4000 lbs. on the tines  208  to scrape the packed snow from the road. This arrangement produced satisfactory results. 
     Secondary Plow  304   
     A second embodiment of a secondary plow, plow  304 , is illustrated in  FIGS. 12-14  and described below. Drive frame, moldboard and other structure associated with the use of plow  304 , was sufficiently described in connection with the first embodiment above in order to fully appreciate much of the attributes and construction of a plow incorporating plow  304 . Reference will therefore be limited in discussing secondary plow  304 . Plow  300  engages the ground with a resilient blade  308 , preferably rubber that is impregnated with vertical steel cable. The mounting arrangement is essentially the same as in secondary plow  204 . Plow  304  may be employed with or without the primary moldboard  202  as it is especially suitable for snow removal in large cities where snow usually does not accumulate excessively or develop a bond to the pavement. 
     Resilient blade  308  may be used when the pavement has been treated with solid chemicals and/or with liquid chemicals (typically after one inch or more of snow has accumulated on the pavement). Plow  300  is used to plow away the slushy snow and reduce or minimize the ability of the slushy snow from re-freezing into ice. Plow  300  is also beneficial, especially in relatively congested areas and heavily traveled streets and roads such as in the center of cities, where snow typically does not bond to the pavement road but instead remains slushy due to dense traffic. Plow  300  is also especially useful to prevent an excessive dilution of anti-icing chemicals by residual slushy snow remaining on the pavement before an anti-icing treatment of the road is to be provided (such as before a snowstorm is expected). 
     As illustrated in the drawings, blade  308  is mounted to a series of supporting plates  310  which are connected to drive train  212 . Hydraulic cylinders  224  may be used to raise or lower blade  308  and may selectively apply pressure to blade  308  when it engages the road surface. Plow  300  may be mounted with or without secondary plow  204 . Additionally, as both of these secondary plows may have a common mounting device, either may be interchangeably mounted with moldboard  202 . In other embodiments, conventional plows may be modified to mount a secondary plow controlled by a hydraulic cylinders, where the secondary plow may use one or both of tines and a resilient blade. As in the previously described embodiments, the blade types may be used separately or together, and the plow may be configured to readily to switch one for the other as needed. 
     Secondary Plow  404   
     To mount the secondary plow  204  on an existing, conventional plow, the frame between the swivel plate at the back and the square tube, e.g., tube  120  in  FIG. 1 , for mounting the plow at the front may require modification to accommodate a secondary plow  204  behind moldboard  202 . The space that is available under the drive frame to accommodate secondary plow  204  may be limited and may not permit the discharge end of secondary plow  204  to be extended without having tines interfere with a front post of a wing plow (at the rear) and without having the mounting channel of the tines interfere with the forward trip springs of the primary moldboard  202 . To have the snow discharged beyond the trail of the tire of the vehicle and beyond the intake end of the wing plow, the secondary plow  204  is preferably installed relatively close to the primary moldboard  202 . Thus, the discharged snow from secondary plow  202  can be removed from the road and thrown away into the ditch by a wing plow for safer driving in winter months. Secondary plow  204  may be more effectively integrated into an existing plow (e.g., by bringing it closer to the primary moldboard by about 10 inches) without jeopardizing the efficient operation of both the primary moldboard and the secondary plow  204 , as well as the tripping device of the primary moldboard  202 , by modifying the drive frame. These modifications may include relocating the primary moldboard trip mechanism including replacing the inclined plow lift trip and compression spring with a modified inclined plow lift trip and compression spring arrangement. In addition, a parallel lift for the push frame may be provided and the push frame may be replaced with a push frame having a high bow configuration. 
       FIGS. 18-19  illustrate an example of a secondary plow  404  fitted to a conventional snow plow, such as the plow illustrated in  FIGS. 1-3  (shoes  110  and the associated shoe mounting bracket are not shown). Referring to  FIG. 18 , the fitting of secondary plow  404  begins with removing a section  402  from truss  102  and replacing it with a modified support structure including support member  406  having end plates  408 ,  410  and a square tube section bridging plates  408  and  410 . Support member  406  effectively provides a raised area in the truss  102  extending over the length of the moldboard&#39;s scrapping edge so that the secondary plow  404  may be raised and lowered without interference from truss  102  and independently of moldboard  100 . Support member  406  may be welded to truss  102  at plates  408  and  410 . Referring to  FIG. 19 , the mounting and operation of secondary plow  404  is similar to that described for secondary plows  204  and  304 . Secondary plow  404 , which includes a pair of hydraulic cylinders  424 , is configured to rotate tines  408  (or a resilient blade  308 ) about a rotation bar  430  by operation of hydraulic cylinders  424 . Hinge mounts, which retain bar  430  (not shown), may be located on plate  408  and a channel holding tines or a resilient blade. Hydraulic cylinders  424  are attached at a housing end to flanges  426 , which are mounted to a square tube  120 , and at an actuating end to mounts  415 . Flanges may, of course, be mounted to any suitable hard point on the truss. These mounts hold pins for receiving the actuating ends. For reversible plows, e.g.,  FIG. 9 , secondary plow  404  may be configured to slide relative to the truss and may also be fitted with linkages in a similar manner as described above to reposition secondary plow  404  behind moldboard  100 . Of course a one-way plow, e.g.,  FIG. 1  may already have sufficient clearance for operation of secondary plow and thus only relatively minor modification may be needed. In these embodiments, secondary plow  204  may be mounted to a square tube and positioned at an effective distance behind moldboard  202 , and fitted with hydraulic cylinders to raise and lower secondary plow. 
     Hydraulic Circuit for a Plow 
     A hydraulic circuit for a snowplow configured to operate in the manner previously described for the first embodiment, plow  200 , will now be described with reference to the  FIG. 20 . The circuit is used to extend and retract a first double action hydraulic cylinder  11  (see  FIG. 4 ) and a pair of second double action hydraulic cylinders  224  ( FIG. 5 ) which are used to raise and lower moldboard  202  and secondary plow  204 , respectively. 
     When moldboard  202  and secondary plow  204  are in their fully retracted positions, i.e., raised off the ground, the arm of cylinder  224  is fully retracted whereas the aim of cylinder  11  is fully extended. To lower the plows, an operator opens a four-way, three positional directional control valve  508 , permitting liquid, e.g., oil, to flow from a reservoir or tank  501  via pump  502 , through fluid line  520  and towards cylinders  11  and  224 . A pressure relief valve  502   a  is used to limit the fluid pressure generated by pump  502 . 
     As the moldboard  202  is more massive than secondary plow  204 , it is preferred to lower moldboard  202  first, followed by secondary plow  204 , to avoid damaging secondary plow  204  when moldboard  202  is lowered. This may be accomplished by incorporating a reversible valve  516  which prevents flow towards cylinders  224  until cylinder  11  is filled with fluid, i.e., moldboard  202  is on or near the ground. As cylinder  11  fills with fluid, the pressure above valve  516  increases to a level that causes valve  516  to open. When valve  516  opens, fluid begins to flow into cylinders  224 . This delay in the fluid flow into cylinders  224  results in moldboard  202  being lowered first, followed by secondary plow  204 . Moldboard  202  and secondary plow  204  may be raised in any order or simultaneously. Thus, the fluid may be emptied from both cylinders  11  and  224  at the same time, pass through a common node  532  and drain into tank. A check valve  510  is opened to allow fluid flow back to the tank (“T” in  FIG. 20 ) from cylinders  11  and  224 . 
     The circuit allows an operator to vary the pressure applied to tines  208 . It also gives the operator the option of deploying both moldboard  202  and secondary plow  202 , or only moldboard  202  for snow plowing, by monitoring the fluid pressure at input line  522  using a pressure gauge  512 . If only moldboard  202  is used for snow plowing, valve  508  is placed into a neutral position when fluid begins to pass through input line  522 . If both moldboard  202  and secondary plow  204  are used, the operator allows fluid to enter cylinders  224 , thereby deploying secondary plow  204 , until an acceptable pressure level is reached that is not too great as to cause damage to the tines  204  and/or roadway but sufficient to lower the tines  204  and adjust the applied pressure as needed. 
     In another embodiment of a hydraulic circuit, an operator may also have the option of lowering only the secondary plow  204  (e.g., as when a resilient blade is used to remove slush). For example, an additional, one-way valve may be placed in parallel with and upstream of valve  516  with only one of these two valves being in fluid communication with the tank at a given time. If an operator wants to use both the moldboard and secondary plow  204  (or only moldboard  202 ), valve assembly  516  is used. If an operator only wants to use secondary plow  204 , then the additional one-way valve is opened and valve  516  is closed. In the later case, the additional one-valve is both opened and fluid is prevented from entering cylinder  11 , thereby causing only secondary plow  204  to lower when valve  508  is opened. 
     The principles, preferred embodiments and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. The embodiments are therefore to be regarded as illustrative rather than as restrictive. Variations and changes may be made without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such equivalents, variations and changes which fall within the spirit and scope of the present invention as defined in the claims be embraced thereby.