Abstract:
A balance shaft unit for internal combustion engines consists of at least one balance shaft ( 6, 7 ) supported in a balance casing ( 5 ) and counterweights ( 8 ) connected onto it. In order to achieve maximum precision, reliable fastening and minimum external dimensions at minimum manufacturing costs, the counterweight ( 8 ) is essentially a cylindrical ring with two end surfaces ( 61 ) normal to the axis and with a cutout ( 55 ) in the longitudinally central region, so that the counterweight consists of two ring parts ( 60, 61 ) abutting the two end surfaces ( 61 ) and an intermediate segmental part ( 62 ). One of the end surfaces ( 61 ) of the counterweight ( 8 ) forms, jointly with a machined surface ( 33 ) of the balance casing ( 5 ), a thrust bearing ( 32 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a balance shaft unit for reciprocating machines, consisting of a balance shaft supported in a balance casing and at least one counterweight fastened to the balance shaft. Balance shafts have the objective of balancing the inertia forces and moments occurring in reciprocating machines. They are preferentially employed in light, high-speed internal combustion engines, in particular in pairs in engines with four in-line cylinders, for balancing the second order inertia forces. In the latter application, they are usually driven from the crankshaft at double rotational speed, i.e. at up to more than 10,000 rpm. 
     This means extreme requirements with respect to precision and bearing arrangements. At the same time, they should be as light as possible, cheap to manufacture and assemble and demand as little installation space as possible in the crank case. Two different designs are fundamentally possible — the balance shaft is either produced in one piece with its counterweights or it is “built up”, the counterweights being fastened onto the finished shaft. 
     The first possibility is for instance described in DE 37 05 346 A and the second in U.S. Pat. No. 4,425,821. The one-piece design is very expensive, demands maximum accuracy and leads, in the case of shafts with more than two bearings, to large bearing diameters for which, given the high rotational speeds, the lubrication technology cannot be successfully mastered. Precisely these reasons also prevent rolling contact bearings. On the other hand, built-up balance shafts have the main advantage of permitting smaller bearing diameters, it being however necessary to ensure sufficient shaft stiffness. In the case of more than two bearings, furthermore, single-piece casings with bearing bushes closed all round can also be used. It is, however, difficult to achieve maximum precision, reliable fastening and sufficient imbalance in the case of limited external dimensions. Thus in the case of the clamping connection of U.S. Pat. No. 4,425,821, for example, precision, minimum external diameter and connection strength is dubious. 
     The object of the invention is, therefore, to design a built-up balance shaft in such a way that, at minimum manufacturing cost, it meets the requirements of maximum precision, reliable fastening and minimum external dimensions. 
     SUMMARY OF THE INVENTION 
     The foregoing object is achieved according to the invention, wherein the counterweight is essentially a cylindrical ring with two end surfaces normal to the axis and with a cutout in the longitudinally central region, so that the counterweight consists of two ring parts abutting the two end surfaces and an intermediate segmental part. Trouser suspenders give the idea. The rings closed at the edge accept the tensile force, offer a firm connection with an accurate fit and load the shaft in the vicinity of its bearing arrangement so that no bending load occurs. They can be shrunk on in the most simple manner. The cutout in the longitudinally central region—it extends practically over half the periphery—permits a large mass eccentricity at a small external diameter. The segmental part surrounds the other half of the periphery. It is located at the tensile zone of the shaft loaded by bending and, by this means, increases its stiffness substantially. Preventing deformation in this way by increasing the resistance moment is of benefit to the shaft (reduced bending stresses) and the bearings (better support profile due to less sag). In consequence, the shafts can be produced with smaller diameter. 
     If the mass eccentricity which can be achieved in this way is not sufficient, the counterweight can—with only a small increase in the external diameter—have a thickening increasing the eccentric mass on the side facing away from the cutout. 
     In an advantageous embodiment, at least one of the end surfaces of the counterweight forms, jointly with a machined surface of the balance casing, a thrust bearing. By this means, a thrust bearing is also created without additional parts and which ensures precise axial positioning. 
     In the case of a balance shaft supported in a singlepart balance casing in light metal, the scope of the invention also includes the bearing surfaces of the friction bearing being formed in the balance casing itself. In this way, particularly accurate support is achieved with minimum complexity. The bearing surface is an uninterrupted cylindrical surface which is machined, with maximum accuracy, directly in the light metal casing. It has been found that purely hydrodynamic lubrication remains because of the small diameter of the shaft in the region. 
     In an advantageous embodiment, the counterweight is connected to the balance shaft by a beam welded seam in the cutout, which beam welded seam is produced on both sides at the intersection of the cutout plane with the balance shaft. This produces a particularly rapid and reliable connection, which is also completely distortion-free in the case of oppositely located welded seams, preferably manufactured by a laser. 
     In another advantageous embodiment, the balance shaft has at least a first transverse hole which is aligned with at least one second transverse hole in the counterweight, which two holes accept an essentially cylindrical connecting element, which can be a bolt, a dowel pin, a clamping sleeve or a rivet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described and explained below using illustrations. In these: 
         FIG. 1  shows an arrangement diagram of a unit according to the invention, 
         FIG. 2  shows, in plan view, an axonometric view of a balance casing as part of the unit according to the invention, 
         FIG. 3  shows a longitudinal section through a unit according to the invention in a first and a second embodiment, 
         FIG. 4  shows a cross section along III—III and IV—IV in  FIG. 3 , 
         FIG. 5  shows a longitudinal section through a unit according to the invention in a third and a fourth embodiment, 
         FIG. 6  shows a cross section along V—V and VI—VI in  FIG. 5 , 
         FIG. 7  shows a longitudinal section through a unit according to the invention in a fifth and a sixth embodiment, 
         FIG. 8  shows a cross section along VII—VII and VIII—VIII in  FIG. 7 , 
         FIG. 9  shows an axonometric view of a counterweight according to the invention, and 
         FIG. 10  shows an enlarged cross section along X—X in FIG.  2 . 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , the reciprocating machine  1  is symbolized only by its crankshaft  2  and its main bearings  3 . The main bearings  3  represent the complete engine block, which can be produced both in tunnel design and also with free bearing brackets. The mass balance appliance fastened to the engine block below the crankshaft  2  is generally designated by  4 . It consists of a balance casing  5  and two balance shafts  6 ,  7 , with counterweights  8 , counter-rotating within the balance casing  5 . The normal planes  9  through the main bearings  3  are indicated by dashed lines; the bearings of the mass balance appliance  4  are also located in these normal planes. The balance shafts  6 ,  7  are driven, via a drive gearwheel  11 , by a gearwheel  10  torsionally connected to the crankshaft  2 ; the synchronizing wheels  12 ,  13  ensure equal counter-rotating rotational speed. 
       FIG. 2  shows the balance casing  5  which, in the embodiment example represented, is a part cast in light metal. It consists of a bottom shell  15  with oil drain holes  16  and a number of bearing brackets  17 ,  18 ,  19 . Each bearing bracket has, on both its sides, a cast-on part  20  with a vertical hole  21  for, in each case, a threaded bolt  22  (which is only indicated) by means of which the balance casing  5  is bolted to the engine block. For this purpose, mounting surfaces  23  are formed in the surroundings of the holes  21 . The assembled balance casing is connected by means of these mounting surfaces to the corresponding positions in the engine block, which is located in a common normal plane  9  with the bearing brackets  17 ,  18 ,  19 . The connection, which is not actually represented, takes place either with the engine block transverse ribs formed by the bearing pedestal of the main bearing or with the bearing brackets of the main bearing or, in the case of a tunnel construction, on the latter. 
       FIG. 3  is a vertical longitudinal section through the first balance shaft  6  which, because of the special features of the design described, can be a simple, purely cylindrical shaft of constant diameter. The three bearing brackets  17 ,  18 ,  19  form friction bearings  30  for supporting the balance shaft  6 . They exhibit the special feature that bearing surfaces  31  for the radial support are machined in the basic material of the balance casing  5  or the bearing brackets, without a bearing bush of its own being necessary. Thrust bearings  32  are configured on the two outer bearing brackets  17 ,  19 , for which purpose fine-machined bearing surfaces  33  are likewise provided on the basic material. 
     The counterweights  8  can be fastened onto the balance shafts  6 ,  7  in various ways. For this purpose, two different types of fastening are represented in each of the longitudinal sections of  FIGS. 3 ,  5 ,  7  and the two balance shafts  6 ,  7  are correspondingly associated, in each case, to one embodiment and the other embodiment in  FIGS. 4 ,  6  and  8 . It is, however, obvious that the same type of fastening will normally be selected for both balance shafts and all the counterweights. 
     In  FIG. 3 , the counterweight  8  is simply shrunk ( 34 ) onto the balance shaft  6  on the right hand side; on the left-hand side, it is connected by means of two laser weld seams  35 ,  35 ′, which are arranged diametrically opposite to one another and are longitudinally directed. With this arrangement of the weld seams and given the use of a closely focused high-energy beam, the balance shaft  6  remains distortion-free. 
       FIGS. 5 and 6  show two further types of connection between balance shaft  6  and counterweight  8 . On the left-hand side, a threaded hole  38  is provided in the balance shaft  6  and a fitting hole with counterbore  39 , through which the one or two fitting bolts  40  are screwed in from the side of the counterweight, is provided in the counterweight  8 . On the right-hand side, the position is reversed—fitting bolts  43  are screwed in through a hole  41  in the balance shaft  6  and a threaded hole  42  in the counterweight  8 . 
     In  FIGS. 7 and 8 , the connection takes place by means of a fitting hole  45  in the balance shaft  6  and a preferably stepped fitting hole  46  in the counterweight  8 , into which during assembly at least one dowel pin  47  (in this case there are two) is driven in. On the right-hand side, two fitting holes  48 ,  49  of the same diameter are provided into which, during assembly, two clamping bushes  50  are pushed; in an alternate embodiment rivets  51  are used. 
       FIG. 9  shows the counterweight  8  in detail. Its basic shape is that of a cylindrical ring or of a hollow cylinder with a thick wall, which is indicated by a dashed line. It can be manufactured in various ways, possibly forged or as a lost-wax casting. Independently of this, the explanation of the shape refers to a lantern-type cutout  55 , which takes place over a part of the length and approximately over half the periphery. The cutout  55  is bounded by two cutout planes  56 ,  57  and by lateral cutout surfaces  58 . The cutout planes  56 ,  57  are essential where the connection to the shaft takes place by means of laser welding; in this case, the weld seam is located at the intersection line of the cutout planes  56 ,  57  with the cylinder of the balance shaft  6 . In the longitudinal direction, and abutting on both sides of the cutout  55 , two ring parts  59 ,  60  remain which are closed rings and accept the centrifugal forces and, in the case of a shrunk connection, also the peripheral forces generating the shrinkage stress. In each case, the ring parts  59 ,  60  have, on the outside, an end surface  61  which, in association with the bearing surface  33  of the bearing bracket  17 , forms a thrust bearing  32 . The cutout  55  extends approximately over a semicircle and, on the residual semicircle, there is a segmental part  62  which forms the eccentric mass. Because absolutely no material is located between the ring parts  59 ,  60  on the side of the cutout  55 , it is possible to achieve a high degree of eccentricity with only small thickness of the segmental part  62 . If this is not sufficient, it is additionally possible to form a thickening  63 . 
       FIG. 10  mainly shows the oil supply. The main bearing  3 , and therefore the complete engine block, are only indicated. The main bearing itself is located above the illustration and can no longer be seen. The balance casing  5  is also only partially represented and the threaded bolts  22 , by means of which it is bolted onto the engine block or main bearings  3 , are only indicated by a chain-dotted line. The section is taken in the bearing bracket  19  (FIG.  2 ). A first vertical lubricating duct  70  is located in the balance casing  5 ; it is parallel to the bolt holes  21  ( FIG. 2 ) and can be drilled, with the latter, in one clamping operation. A second vertical lubricating duct  71  is provided in the main bearing  3  or engine block; this is aligned with the first lubricating duct but ends at a small distance away  72 . By this means, any displacements caused by thermal expansion are accommodated in such a way that they cannot lead to any clamping of the balance casing  5 . The two lubricating ducts  70 ,  71  are connected by an inserted sleeve  73 . The first vertical lubricating duct  70  can be produced as a blind hole but leads to under the plane in which are the two balance shafts  6 ,  7 . The connection to the bearings  30  is produced by plunge drillings  74 ,  75 , which again lead upward. They intersect the lubricating duct  70  and can be drilled from underneath into the balance casing  5 . They are closed towards the outside by pressed-in balls  76  and open into an oil distribution nut  70  of the friction bearing  30 .