Abstract:
A variable-delivery external gear pump comprising 
     a casing, 
     at least one rotatively driven pair of gears comprising two intermeshing spur gears, which form with shell surface areas of the casing and axial sealing surface areas a delivery space comprising a low-pressure side connected to a pump inlet port and a high-pressure side connected to a pump outlet port, and 
     a piston, serving to rotatively mount one spur gear of the pair of gears, said piston being shifted relative to the other spur gear of the pair of gears so as to vary the delivery of the pump together with the one spur gear by charging the high-pressure side with fluid against the force of a return element, wherein 
     formed in at least one axial sealing surface area adjoining a sealing land of the sealing surface area is at least one pocket connecting a zone of deepest tooth engagement of the spur gears to either the pump outlet port or the pump inlet port.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a variable-delivery external gear pump comprising a casing, at least one rotatively driven pair of gears comprising two intermeshing spur gears which form with shell surface areas of the casing and axial sealing surface areas a delivery space comprising a low-pressure side connected to a pump inlet port and a high-pressure side connected to a pump outlet port, and a piston, serving to rotatively mount one spur gear of said pair of gears, the piston being shifted relative to the other spur gear of said pair of gears so as to vary the delivery of said pump together with the one spur gear by charging said high-pressure side with fluid against the force of a return element. More particularly, the invention concerns an external gear pump which automatically regulates itself with increasing speed. 
     2. Description of Related Art 
     One external gear pump in the field concerned is known from German Patent DE 41 21 074 A1. 
     SUMMARY OF THE INVENTION 
     It is on the basis of this prior art that an object of the invention is to provide a variable-delivery external gear pump which pumps a fluid to be delivered as consistently and pulsation-free as possible. 
     This object is achieved in that formed in at least one axial sealing surface area adjoining a sealing land of the sealing surface area is at least one pocket connecting a zone of deepest tooth engagement of the spur gears to either the pump outlet port or the pump inlet port. 
     The invention is based on an external gear pump comprising, in a casing, a rotatively driven pair of gears with two intermeshing spur gears. The pair of gears is enclosed by casing surface areas which together with axial sealing surface areas contacted by the faces of the spur gears define a delivery space comprising a low-pressure side connected to a pump inlet port and a high-pressure side connected to a pump outlet port. To define the volumetric displacement of the pump, one of the spur gears is rotatively mounted on a piston which, together with this spur gear, is mounted in the casing shiftable relative to the other spur gear. Preferably, the piston is linearly shiftable parallel to the axis of rotation of the spur gear mounted on the piston. By shifting the one spur gear relative to the other spur gear, the intermeshing length of the spur gears is varied for the preferred axial shift. 
     The intermeshing length is automatically reduced with increasing pumping pressure which increases/decreases depending on the pump speed. Varying the delivery or automatically adjusting the pump to increasing pumping pressure is achieved by charging the piston with fluid at the high-pressure side. The compressive force of the high-pressure fluid is counteracted by the force of a return element. The return element is formed preferably by a compression spring. The compressed fluid forces the piston in the direction of a reduction in the intermeshing length and the return element opposes this compressive force. 
     Preferably, the pump finds application as a lube pump, more particularly for engine oil delivery in reciprocating piston engines. 
     In accordance with the invention, a pocket or kidney-shaped recess is formed in at least one axial sealing surface area bordering a sealing land of this sealing surface area and connecting a zone of deepest tooth engagement of the gear teeth with either the pump outlet port or the pump inlet port. The axial sealing surface area directly adjoins a face of one of the spur gears. The sealing land of the sealing surface area covers the bottom of the tooth gap of the deepest tooth engagement of the two gears. The pocket connects the tooth gap in the zone of deepest tooth engagement to the next tooth gap at the high-pressure side or low-pressure side. 
     Opposite one sealing surface area without pocket, a lesser resistance discharge of the high-pressure fluid from the tooth gap of deepest tooth engagement is assured. The invention thus creates an external gear pump, in which the fluid of the high-pressure side is used to automatically limit the delivery while, however, eliminating or at least reducing the squeezed oil effects involved in such external gear pumps having the known single-plane axial sealing surface areas. 
     Preferably, the depth of the pocket is, at most, a 5th or, at least, a 20th of the maximum intermeshing length of the two gears, this also applying to any further pocket formed. When further pockets are formed, the depth of each may be the same. If a pocket is formed at both the high-pressure side and the low-pressure side of the sealing surface area, the pocket at the low-pressure side preferably has a depth less than that of the pocket at the high-pressure side. In one preferred embodiment, the otherwise single-plane axial sealing surface area is stepped down to the again single-plane pocket of constant depth. The maximum intermeshing length exists at least at zero delivery and in accordance with the reset characteristic of the return element in the lower speed range of the pump. 
     The pocket takes up a surface area of minimally 5% and maximally 20% of the surface area covered by the spur gear when in rotation, as measured at the upper edge of the pocket. With increasing pocket surface area, the friction between the sealing surface area and the sweeping face of the spur gear becomes advantageously less. 
     The pocket is preferably provided at the high-pressure side of the axial sealing surface area, since, in this case, the displacement of fluid in the delivery direction from the squeeze zone of the tightest tooth engagement is facilitated. However, the pocket can also be provided at the low-pressure side, since, in this case, the amount of fluid in the entrapment zone is reduced in any case. 
     Preferably, a pocket is incorporated into the axial sealing surface area at both the low-pressure side and high-pressure side adjoining the sealing land. Also preferred is that further axial sealing surface areas for the spur gears are each provided with a pocket at the high-pressure side, and even more preferably, each provided with a pocket at the low-pressure side of the respective sealing surface area. 
     The pump may also comprise several pairs of gears. In particular, a single spur gear, which is then preferably driven, may intermesh with one or several other spur gears and, in this way, form several pair of gears of the pump. Preferably, at least one pocket of the aforementioned kind is also formed in the same way in the zone of deepest tooth engagement of this additional pair of gears or in each additional pair of gears. In one preferred embodiment of the invention, one such additional spur gear is also shiftable relative to the common spur gear for regulating pump delivery by pressurization. In this arrangement, the delivery characteristics of the pair of gears may be the same as each other or differ from each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail by way of a preferred embodiment with reference to the drawings in which: 
     FIG. 1 is a longitudinal section through a variable-delivery external gear pump, 
     FIG. 2 is a plan view of a casing part of the pump, taken along the line  2 — 2  as shown in FIG.  1  and rotated 180°, 
     FIG. 3A is a plan view of a cover of the pump taken along the line  3 A— 3 A as shown in FIG. 1, 
     FIG. 3B is a plan view of the pockets taken along the line  3 B— 3 B as shown in FIG. 3A, 
     FIG. 4A is a plan view of sealing surface  16  viewed from the direction of line  3 A— 3 A as shown in FIG.  1 . 
     FIG. 4B is a cross-sectional view of cylindrical body  16  viewed from line  4 B— 4 B as shown in FIG.  4 A. 
     FIG. 4C is a plan view of cylindrical body  15 , taken along the line  4 C— 4 C, as shown in FIG.  1 . 
     FIG. 4D is a cross-sectional view of cylindrical body  15  viewed from the line  4 D— 4 D, as shown in FIG.  4 C. 
     FIG. 4E is a plan view of sealing surface  17  viewed in the direction of line  2 — 2 , as shown in FIG. 1, and 
     FIG. 5 is a view in perspective of the pump cover, as shown in FIG. 3, including a spur gear. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a longitudinal section through a variable-displacement external gear pump, whose location is marked as the line  1 — 1  in the FIGS. 2 and 3. 
     A casing part  1  and the cover  2 , bolted fluid-tight thereto, form a casing of the pump. The casing part  1  and the cover  2  are each individually depicted in FIG.  2  and FIG. 3 respectively, to which, for supplement information, reference is always made; reference also being made to the perspective view of the cover  2  with the mounted spur gear as shown in FIG.  5 . 
     The cover  2  comprises a cover plate and a cylindrical base  5  protruding therefrom. In the casing of the pump, two spur gears  6  and  11  are rotatably mounted around two axis of rotation distanced parallel from each other. The spur gears  6  and  11  intermesh by their toothings  7  and  12 . Porting into the meshing zone of the spur gears  6  and  11 , on one side of the spur pair of gears, is a fluid inlet port  3  indicated in FIG. 1 by a broken line and a pump outlet port  4  at the side of the spur pair of gears opposite the pump inlet port  3 . Rotary drive of the spur gears  6  and  7  delivers fluid from the pump inlet port  3  to the pump outlet port  4 . 
     Rotary drive is provided via a drive shaft  8  rotatably mounted in the base  5  of the cover  2 . The spur gear  6  is secured to the drive shaft  8  locked to prevent it twisting and shifting out of place. 
     The other spur gear  11  is rotatably mounted between two cylindrical bodies  14  and  15  on a journal  13  connecting the two cylindrical bodies  14  and  15  to each other and non-shiftingly relative to the two cylindrical bodies  14  and  15 . The two cylindrical bodies  14  and  15  form a piston being linearly shiftable back and forth in a bore of the casing part  1  along the line of the axis of rotation of the spur gear  11 . At an outer first face, this piston  14 ,  15  is charged by the fluid of the high-pressure side. The pump outlet port  4  ports directly into a high-pressure space  18  which is sealed off by the outer first face of the piston  14 ,  15  or of the cylindrical body  14 . At an outer second face of the cylindrical body  15 , opposite the outer first face, the piston  14 ,  15  is urged by an elastic returning force of a steel compression spring  20  extending through return space  19 . When the pressure in the high-pressure space  18  increases, the piston  14 ,  15  is displaced against the returning force of the compression spring  20  relative to the spur gear  6  axially parallel in such a direction that the intermeshing length of the two spur gears  6  and  11  is reduced, it being this reduction in the intermeshing length that diminishes the delivery of the pump. Since the pressure in the high-pressure space  18  increases with increasing pump speed of the spur gears  6  and  11  and this increase leads to a reduction of the intermeshing length of the two spur gears  6  and  7 , the pump is automatically down regulated infinitely variable due to pump outlet port  4  and high-pressure space  18  being short circuited. The maximum intermeshing length in the example embodiment corresponds to the length of the two spur gears equal in length, it being attained on zero delivery, more particularly on pump idle, the spur gears  6  and  11  thereby being located level opposite each other. 
     The fluid delivery from the pump inlet port  3  to the pump outlet port  4  is made through a pumping delivery space sealingly enclosed by the shell surface areas of the casing part  1  about the addendum circles of the spur gears  6  and  11  as well as by the axial sealing surface areas  9 ,  10 ,  16  and  17  and ports into that of the pump inlet port  3  and a communicating passage to the pump outlet port  4  on each side of the spur gears  6  and  11 . These axial sealing surface areas  9 ,  10 ,  16  and  17  are understood in the sense of the invention to be the surface areas of the casing  1 ,  2  or of the piston  14 ,  15  which each directly sealingly oppose each of the faces of the spur gears  6  and  11 . They seal off the pumping delivery space axially and separate its low-pressure side and high-pressure side from one another, i.e. prevent a short-circuit. 
     FIGS. 2 and 3 show, in a plan view, the axial sealing surface areas  9  and  10  for the rotatively driven spur gear  6 . The axial sealing surface area  9  is formed by the free face of the base  5  of the cover  2 . In FIG. 3, the base  5  is also shown in a view of the sealing land  21 . The axial sealing surface area  10  is formed opposite the casing part  1 . Plan views of the axial sealing surface areas  16  and  17  for the spur gear  11  are evident from FIGS. 4A and 4E. 
     The axial sealing surface area  9  is a circular ring surface area having an outer diameter corresponding to the diameter of the addendum circle of the spur gear  6 . It comprises a single-plane surface, in which on both sides of a sealing land  21 , a pocket  22  and  23  is incorporated. In the assembled condition of the pump, the sealing land  21  protrudes into the tooth gap of the deepest tooth engagement of the spur gears  6  and  11  and ensures separation of the low-pressure side from the high-pressure side at the intermesh. 
     The pockets  22  and  23  are open in the direction of the outer circumferential edge of the axial sealing surface area  9 . Each pocket is defined by two control edges  22   a ,  22   b ,  23   a , and  23   b , standing at an angle to each other, at which the axial sealing surface area  9  is sloped perpendicular or slightly inclined to the corresponding pocket  22  and  23 . The length of the sealing land  21 —as measured from the level of the intersection area of the control edges  22   a  and  22   b  of the pocket  22  and the intersection area of the control edges  23   a  and  23   b  of the pocket  23 , located at the same level, up to the outer circumferential edge of the sealing surface area  9 , which simultaneously forms the outer land edge—corresponds to roughly the tooth height of the spur gears. The depths T 1  and T 2  of the pockets  22  and  23 , or the height of their control edges, amounts to at least a 20th or at most a 5th of the maximum intermeshing length of the spur gears  6  and  11 . The bottoms of the pockets  22  and  23  are single-plane from their control edges up to their open edges. The depth T 2  of the pocket  23  at the low-pressure side is preferably less than the depth T 1  of the pocket  22  at the high-pressure side. In the example embodiment, T 1  is an 8th and T 2  is a 10th of the maximum intermeshing length of the spur gears  6  and  11 . 
     As measured at the surface area of the circular ring between the addendum circle of the spur gear  6  and the inner diameter of the spur gear  6 , the surface area of the pocket  22  at the high-pressure side is minimally 5% and maximally 20% of the surface area of the full, non-interrupted circular ring, the whole surface area between the upper edge of the defining control edges and the outer circumferential edge of the cited circular ring forms the basis of the surface area of the pocket  22 . In contrast, the pocket  23  at the low-pressure side is smaller, its surface area %—defined as before—of the whole surface area of the circular ring being between  5  and 15%. The recessed surface area of the pocket  22  is larger in relation to the recessed surface area of the pocket  23  for the same length of the control edge  22   a  defining the sealing land  21 , because the second control edge  22   b  oriented at an angle to the sealing land  21  in the case of pocket  22  is oriented at a greater obtuse angle to the control edge  22   a  at the sealing land  21  than the corresponding control edge  23   b  of the pocket  23 . The pocket  22  thus, from the outer circumferential edge of the axial sealing surface area  9 , covers a longer edge portion than the pocket  23 . 
     The control edges of the pockets  25  and  26  of the opposite sealing surface area  10  are similarly angled as pockets  22  and  23 , respectively (FIG.  2 ). In the assembled condition, the pockets  22  and  25  at the high-pressure side and pockets  23  and  26  at the low-pressure side of the pumping delivery space are located exactly opposite one another. The pairs of pockets are similarly shaped, except that the outer periphery of pockets  25  and  26  and sealing land  24  form a continuous, concave (or recessed) portion to accommodate spur gear  11 , in the same manner as sealing surface  16  (discussed below). 
     In FIGS. 4A,  4 B,  4 C, and  4 E, the cylindrical bodies  14  and  15  are shown individually in plan views of their axial sealing surface areas  16  and  17  and each in longitudinal section. The cylindrical body  15  is additionally depicted in a plan view of its outer face facing the compression spring  20 , as shown in FIG.  4 C. In FIG. 4E, sealing land  30  and pockets  31  and  32  are depicted. 
     The cylindrical body  14 , sealing off the high-pressure space  18 , is single-plane at its face facing the high-pressure space  18  (not shown). Recessed out at the outer shell surface area in the region of its sealing land  27  over the full length of the cylindrical body  14  is a cylindrical segment. In the assembled condition of the pump, this recess is penetrated by the teeth of the spur gears  6  and  11 . At the same time, the recess serves as an anti-rotation lock for the piston  14 ,  15 , in that the cylindrical body  14  is able to snugly slide along the base  5 . FIG. 4A shows the configuration of the axial sealing surface area  16  with the sealing land  27  adjoined by the pockets  28  and  29 . 
     The same applies to the axial sealing surface area  17  of the cylindrical body  15 , as evident from FIGS. 4C-4E. At its outer face facing away from the spur gear  11 , the cylindrical body  15  is formed as a cupped ring, including a circumferential annular passage  33  which serves as a guide for the compression spring  20 .