Balance shaft for engine balancing systems

In a balance shaft for engine balancing systems for canceling an unbalance force of an engine, a journal portion of the balance shaft is provided with a recess on a side remote from the gravitational center of the counter weight, and a rib extends axially across this recess along a radial plane between full circular axial end portions of the journal portion to compensate for the reduction in the bending rigidity of the balance shaft due to the provision of the recess. The recess reduces the weight or mass of the balance shaft, and this recess does not diminish the performance of the bearing because the bearing load is essentially due to the unbalanced mass of the counter weight portion, and the recessed part of the bearing portion receives a significantly smaller part of the bearing load than the opposite side of the bearing portion. The recessing also contributes to increasing the eccentricity of the gravitational center of the associated part.

TECHNICAL FIELD
 The present invention relates to a balance shaft, and in particular to a
 balance shaft provided with a counter weight for canceling the unbalance
 force produced in reciprocating engines.
 BACKGROUND OF THE INVENTION
 Engine balancing devices are known. For instance, Japanese UM publication
 No. 5-39233 discloses a balancing device in which a pair of balance shafts
 each provided with a counter weight for canceling the unbalance force of
 the second order produced by pistons of an engine are disposed under the
 crankshaft in the oil pan, and the rotation of the crankshaft is
 transmitted to the balance shafts via a chain/sprocket mechanism or a gear
 mechanism. A similar balancing device is disclosed in U.S. Pat. Nos.
 4,703,724 issued Nov. 3, 1987 to C. Candea et al. and 4,703,725 issued
 Nov. 3, 1987 to W. L. Weertman.
 In such a balancing device, because the vibration control diminishes in
 effectiveness if the balance shafts deflect, the journal portion for
 rotatably supporting each balance shaft is desired to have as high a
 rigidity as possible. Therefore, the journal portion of a balance shaft
 generally consists of a solid structure having a fully circular cross
 section.
 However, it is desired to minimize the overall weight of the balance shaft
 because of the increasing demand for the weight reduction of engines.
 Also, the balance shaft is required to have a certain unbalance mass.
 Therefore, the mass of the balance shaft is desired to be allocated to the
 unbalance mass as much as possible, and the mass which does not contribute
 to the generation of unbalance for canceling that of the engine, such as
 the mass found in the journal portion is desired to be minimized.
 Such an effort to reduce the mass or weight of the various parts of a
 balance shaft should be implemented without compromising the required
 properties of the balance shaft such as the adequate bending rigidity and
 the load bearing capability of the journal.
 Also, because the balance shaft is required to be installed in a relatively
 limited recess within the confine of an engine, it is important that the
 assembly of the various components of the balancing device can be executed
 in a simple manner.
 BRIEF SUMMARY OF THE INVENTION
 In view of such problems of the prior art, a primary object of the present
 invention is to provide an improved balance shaft which can minimize the
 size and overall weight of the counter weight without reducing the bending
 rigidity of the balance shaft.
 A second object of the present invention is to provide a balance shaft
 which can minimize the weight of the balance shaft without sacrificing the
 load bearing capability of its journal portion.
 A third object of the present invention is to provide a balance shaft which
 can minimize the weight of the balance shaft without unduly increasing the
 rotational resistance of its journal portion.
 A fourth object of the present invention is to provide a balance shaft
 which is easy to assemble.
 A fifth object of the present invention is to provide a balance shaft which
 is suited for compact design.
 According to the present invention, such objects can be accomplished by
 providing a balance shaft for an engine balancing system for canceling an
 unbalance force of an engine, comprising: an counter weight portion having
 a gravitational center offset from a rotational center thereof; and a
 journal portion for rotatably supporting the balance shaft in a bearing
 bore defined in a fixed part of the engine; the journal portion being
 provided with a recess on a side remote from the gravitational center of
 the counter weight.
 The recess reduces the weight or mass of the balance shaft, and this recess
 does not diminish the performance of the bearing because the bearing load
 is essentially due to the unbalanced mass of the counter weight portion,
 and the recessed part of the bearing portion receives a significantly
 smaller part of the bearing load than the opposite side of the bearing
 portion. The recessing also contributes to increasing the eccentricity of
 the gravitational center of the associated part. Typically, the journal
 portion is provided with a full circular part having a full circular cross
 section, preferably on each axial end thereof, to ensure the load bearing
 capability of the journal portion under all circumstances.
 To compensate for any reduction in the bending rigidity of the balance
 shaft due to the recessing of a part of the bearing portion, the journal
 portion may be provided with a first rib extending across the recess
 axially along a radial plane between the full circular axial end portions
 of the journal portion.
 The counter weight portion typically comprises a radially offset lobe. The
 rigidity of the balance shaft against bending can be significantly and
 easily improved by providing a second rib which extends axially along a
 radial plane on a side of the balance shaft diagonally opposite the lobe.
 It is particularly preferable if the the first and second ribs extend
 along a common radial plane and merge at one of the full circular axial
 end portions.
 The first rib may be provided with an outer profile which is common to an
 outer profile of the full circular part of the journal portion. However,
 in view of reducing the resistance against rotation due to the deposition
 of lubricating oil in the recessed part of the journal portion may be
 provided with an outer profile which is recessed radially inward with
 respect to an outer profile of the full circular part of the journal
 portion. For the same reason, the outer profile of the first rib may be
 defined by a rounded edge. Additionally or alternatively, the first rib
 may be provided with an opening passed therethrough, preferably in a part
 of the rib adjacent to an axial center of the balance shaft.
 In view of maximizing the reinforcing effect of the second rib, the second
 rib may extend substantially over an entire length of the counter weight
 portion. To reduce the weight of the rib without detracting from its
 reinforcing performance, the second rib may be provided with a radial
 height which progressively diminishes away from the journal portion.
 Preferably, the journal portion has a substantially smaller rotational
 diameter than the counter weight portion. Thereby, the overall
 eccentricity of the balance shaft is maximized. In such a case, the
 housing for the balance shaft should consist of an upper and lower housing
 halves which jointly define a bearing bore for the journal portion so that
 the counter weight portion which is desired to have a relatively large
 rotational diameter is not required to be passed through the bearing bore
 for the balance shaft.
 According to a particularly preferred embodiment of the present invention,
 the counter weight portion is provided on each axial end of the journal
 portion. Thus, the bending rigidity of the balance shaft against the
 unbalance force of the unbalance mass portions of the engine can be
 maximized for a given amount of the material for the balance shaft. Often,
 a pair of balance shafts rotating in opposite directions are required, and
 such a synchronizing motion may be accomplished by a gear such as a spur
 gear or helical gear attached to each balance shaft. To support such
 balance shafts in a stable fashion, each balance shaft may be provided
 with a gear attached thereto at a part adjacent to an end of one of the
 counter weight portion remote from the journal portion, and a second
 journal portion on an axial side of the gear remote from the counter
 weight portion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows a reciprocating piston engine embodying the present invention.
 This engine E consists of an inline four-cylinder engine having a
 crankshaft 1 extending in the horizontal direction, and comprises a head
 cover 2, a cylinder head 3, a cylinder block 4, a lower block 5, a
 balancing device 6 and an oil pan 7. The crankshaft 1 is rotatably
 supported by bearings defined in the interface between the lower surface
 of the cylinder block 4 and the upper surface of the lower block 5.
 The balancing device 6 is designed to reduce the second-order vibration of
 the engine E resulting from the reciprocating motion of the pistons and
 other parts, and is attached by threaded bolts to the lower surface of the
 lower block 5 (under the crankshaft 1) fully enclosed in the oil pan 7.
 The rotation of the crankshaft 1 is transmitted to this balancing device 6
 via a large sprocket 8 fixedly attached to the front end of the crankshaft
 1 (the end adjacent to the crank pulley or the chain case is referred to
 as the front end in the following description), a small sprocket 9 fixedly
 attached to the front end of the balance shaft (which is described
 hereinafter) on the left hand side (the right and left is defined as such
 when the beholder faces the crank pulley or the chain case), and an
 endless link chain 10 passed around the large and small sprockets 8 and 9.
 The endless link chain 10 is prevented from oscillating by a chain guide 11
 fixedly attached to the front face of the lower block 5 at a point to the
 left of the center of the crankshaft 1, and is subjected to an appropriate
 tension at all times by a chain tensioner 12 which is fixedly attached to
 the front face of the balancing device 6 at a point immediately to the
 right of the small sprocket 9.
 As shown in FIGS. 2 to 4, the balancing device 6 comprises a pair of
 balance shafts 13L and 13R having a substantially identical shape, and an
 upper and lower halves 14U and 14L of a balancer housing 14 which are
 vertically separated from each other along a plane passing through the
 centers of the two balance shafts 13L and 13R.
 The two balance shafts 13L and 13R are mutually synchronized by helical
 gears 15L and 15R which are integrally connected to the corresponding
 balance shafts 13L and 13R and mesh with each other. As mentioned earlier,
 the rotation of the crankshaft 1 is transmitted to the left balance shaft
 13L via the large sprocket 8, the endless link chain 10 and the small
 sprocket 9. Therefore, the left balance shaft 13L is rotationally driven
 at twice the rotational speed of the crankshaft 1 and in the same
 direction as the crankshaft 1, and the right balance shaft 13R is
 rotationally driven in the opposite direction and at the same speed as the
 left balance shaft 13L by virtue of the meshing between the two helical
 gears 15L and 15R.
 As shown in FIGS. 2 and 3, the parts of the upper half 14U of the balancer
 housing 14 corresponding to the helical gears 15L and 15R define thrust
 bearing wall portions 16 which engage the axial end surfaces of the
 helical gears 15L and 15R integral with the balance shafts 13L and 13R.
 These parts are open in their upper parts so that the outer periphery of
 each of the helical gears 15L and 15R is always exposed to the interior of
 the oil pan 7, and is therefore adequately lubricated by a supply of
 lubricating oil into the meshing part of the two helical gears 15L and 15R
 and the thrust bearing wall portions 16, the supply of lubricating oil
 being derived from that dripping from above or circulating inside the
 interior of the oil pan 7 in the form of oil mist.
 Each of the balance shafts 13L and 13R is integrally provided with a first
 journal portion 18 having a relatively large diameter at a part near its
 rear end, and a second journal portion 17 having a relatively small
 diameter at its front end. Each of the balance shafts 13L and 13R is
 integrally provided with a pair of eccentric weights or counter weights 19
 which are separated into two parts ahead and behind the first journal
 portion 18. These counter weights 19 have gravitational centers which are
 offset from the rotational center in the radial direction, and the
 diameter of the rotational trajectory of each of the counter weights 19 is
 larger than the diameter of the first journal portion 18 (see FIG. 4).
 To achieve a required moment of inertia with counter weights of minimum
 size, each counter weight 19 is provided with a stem portion 20 which is
 reduced in diameter as compared to the remaining part of the balance
 shaft. To compensate for the reduction in rigidity due to the small
 diameter of the stem portion 20, tapering ribs 21 connecting to the
 corresponding axial ends of the first journal portions 18 are provided on
 the side of stem portions remote from the counter weight ahead and behind
 the first journal portion 18. These ribs 21 are each progressively reduced
 in height with the distance from the first journal portions.
 To reduce the weight of the first journal portion 18 and offset the
 gravitational center of the first journal portion 18 from its axial center
 toward the counter weight 19 as much as possible to minimize the size of
 the counter weight 19, the side of the first journal portion 18 remote
 from the counter weights 19 is recessed so as to reduce weight except for
 the parts thereof adjacent to the two axial ends of the first journal
 portion 18. To compensate for the reduction in bending rigidity due to
 this recessing, a rib 23 extends across this space or recess 22 along a
 plane passing through the central axial line of the first journal portion
 18 (see FIG. 5). The rib 21 formed in the stem portion 20 of the counter
 weight 19 and the rib 23 provided in the first journal portion 18 extend
 along a common plane.
 Thus, because the axial ends of the first journal portion 18 on the side
 radially remote from the counter weights are engaged by the inner
 circumferential surface of the metal bearing as described hereinafter,
 even though the surface area of the part of the first journal portion 18
 which is in contact with the bearing bore is somewhat reduced, there will
 be no break in the oil film, and the resistance to rotation can be
 adequately reduced.
 A hole 24 is passed through a part of the rib 23 adjacent to the axial
 center to allow the lubricating oil to flow freely in the recess 22 and to
 prevent any increase in rotational resistance due to excessive deposition
 of oil within the recess 22.
 In this embodiment, the recess 22 is defined by removing the material of
 the journal portion 18 approximately by one half or substantially along
 the diametric plane of the journal portion 18. However, the size and shape
 of the recess 22 can be freely selected depending on the condition of each
 application. For instance, the journal portion 18 may also be recessed by
 a recess which is substantially shallower and narrower. In any case, it is
 desirable for the journal portion 18 to have a full circular profile over
 at least 180 degrees so that the bearing capability against the unbalance
 force of the balance shaft may be ensured with a minimum outer diameter of
 the full circular profile of the journal portion.
 The second journal portion 17 of each of the balance shafts 13L and 13R is
 supported by a bearing bore 25a defined in a second bearing wall portion
 25 integrally provided in the front wall of the lower half 14L of the
 balancer housing 14. The first journal portion 18 of each of the balance
 shafts 13L and 13R is supported by a bearing bore 26a defined in a first
 bearing wall portion 26 consisting of two halves which are integrally
 formed with the upper and lower halves 14U and 14L of the balancer housing
 14, respectively.
 When installing the two balance shafts 13L and 13R within the balancer
 housing 14, the second journal portion 17 at the front end of each of the
 balance shafts 13L and 13R is first fitted into the bearing bore 25a
 defined in the second bearing wall portion 25 which is integral with the
 lower half 14L of the balancer housing 14, and the first journal portion
 18 of each of the balance shafts 13L and 13R is placed on the lower half
 of the bearing bore 26a defined in the lower half of the first bearing
 wall portion 26 which is integral with the lower half 14L of the balancer
 housing 14. During this process, the counter weight portions 31 are
 required to be turned away from the lower half 14L of the balancer housing
 14, in particular the lower half of the bearing bore 26a, so that the
 radially extending lobes of the counter weight portions 31 may not
 interfere with the lower half of the bearing bore 26a as the second
 bearing portion 17 is axially passed into the corresponding bearing bore
 25a.
 Thereafter, with the upper bearing half of the first bearing wall portion
 26 on the side of the upper half 14U of the balancer housing 14 aligned
 with the first journal portion 18 of the corresponding one of the balance
 shafts 13L and 13R, the upper and lower housings 14U and 14L are put
 together. As a result, the two balance shafts 14u and 14L are rotatably
 retained between the two halves 14U and 14L of the balancer housing 14.
 Thus, the counter weights 19 are not required to be passed through any of
 the bearing bores, and the diameters of the journal portions 18 and 19 can
 be reduced at will so long as the required mechanical strength is ensured.
 Therefore, the rotational resistance can be reduced, and the size and
 weight of the balancer housing 14 receiving the balance shafts 13L and 13R
 can be reduced to a level which has hitherto been impossible to achieve.
 The front end of the balancer housing 14 or the lower half 14L thereof is
 provided with a trochoid oil pump 27 for supplying lubricating oil to
 various parts of the engine as shown in FIG. 6 also. The trochoid oil pump
 27 comprises a pump housing 28 attached to the front surface of the
 balancer housing 14 by threaded bolts, an outer rotor 29 received in the
 pump housing 28, and an inner rotor attached to the front end of the right
 balance shaft 13R. The inner rotor 30 which integrally rotates with the
 right balance shaft 13R cooperates with the outer rotor 29, and supplies
 the lubricating oil drawn from the oil pan 7 via an oil strainer 31
 attached to the bottom wall of the lower half 14L of the balancer housing
 14 and a suction tube 32 integrally formed with the bottom wall of the
 lower housing half 14L to various parts of the engine via an output oil
 passage 33 defined by the pump housing 28 and communicating with oil
 passages (not shown in the drawings) formed inside the cylinder block 4
 and the lower block 5.
 Referring to FIG. 3, the bottom wall of the lower half 14L of the balancer
 housing 14 is integrally formed with a mounting boss 34 for supporting the
 oil strainer 31 which is connected to the first bearing wall portion 26.
 The bottom wall of the lower half 14L of the balancer housing 14 is also
 integrally formed with a suction tube 32 which extends from the mounting
 boss 34 to an open front end which is adjacent to the second bearing wall
 portion 25 and closed by a part of the pump housing 28. The mounting boss
 34 of the oil strainer 31 and the hollow suction tube 32 are integrally
 formed in the bottom wall of the housing lower half 14L in series and in
 continuation so that the bearing wall portions 25 and 26 of the housing
 lower half 14L supporting the front and rear ends of the balance shafts
 13L and 13R are joined by the mounting boss 34 of the oil strainer 31 and
 the suction tube 32, and this contributes to the increase in the rigidity
 of the bearing wall portions 25 and 26.
 A part of the suction tube 32 is located within the recess defined between
 the two balance shafts 13L and 13R (see FIG. 4) so that the downward
 protrusion of the suction tube 32 can be minimized. Also, because the oil
 strainer 31 is directly attached to the bottom wall of the housing lower
 half 14L, the size of the balancer housing 14 can be minimized, and this
 contributes to the compact design of the engine.
 The mounting boss 34 is internally provided with a pin-shaped projection 35
 extending from the housing lower half 14L to control the inward
 deformation of the oil strainer 31 which essentially consists of metal
 mesh. The projection 35 is also connected to the inner circumferential
 surface of the oil strainer mounting boss 34 with a rib 36. This rib 36
 increases the rigidity of the oil strainer mounting boss 34, in particular
 the bearing half of the bearing wall portion 26.
 The lateral side ends of the parting plane between the upper half 14U and
 the lower half 14L of the balancer housing 14 are each offset in the
 radial direction with respect to the corresponding balance shaft 13L or
 13R as shown in FIG. 4. In the illustrated embodiment, the lower half 14L
 extends sideways further the upper half U. This offset creates an upwardly
 opening gap 37 defined along a plane passing through the axial center of
 the corresponding balance shaft 13L or 13R on each side. The lubricating
 oil OL stored in the bottom of the balancer housing 14 is thrown upward by
 the counter weights 19 as the two balance shafts 13L and 13R rotate (in
 the direction indicated by arrows), and expelled out of the balancer
 housing 14 from these gaps 37.
 A ledge-like projection 38 axially extends along each lateral side of the
 housing upper half 14U. These ledge-like projections 38 oppose the open
 ends of the corresponding gaps 37, and prevent lubricating oil that may
 drip from above from entering the interior of the housing 14. These
 ledge-like projections 38 extend along either side of the housing upper
 half 14 over the entire length thereof as illustrated in FIGS. 7 and 8,
 and connect boss portions 39 for receiving threaded bolts B1 fastening the
 upper and lower halves 14U and 14L of the balancer housing 14 together,
 the first bearing wall portions 26, and the thrust bearing wall portions
 16 for abutting the helical gears 15L and 15R which are integrally
 attached to the corresponding balance shafts 13L and 13R. The ledge-like
 projections 38 thus contribute to increasing the rigidity of the balancer
 housing 14.
 The upper and lower halves 14U and 14L of the balancer housing 14 are
 additionally fastened together by three threaded bolts B2 arranged
 laterally along the first bearing wall portions 26 for supporting the
 first journal portion 18 so that the first bearing wall portion 26 may be
 kept free from any play even when subjected to the radial acceleration due
 to the rotation of the counter weights 19.
 The ledge-shaped projections 38 may be each extended to a desired length in
 lateral direction and provided with a desired cross sectional shape as
 illustrated in FIG. 9 so that they can be given with the function of
 baffle plates for preventing the disturbances in the surface of the oil in
 the oil pan.
 Referring to FIG. 10, the bearing bore 17a for supporting the second
 journal portion 17 may be formed in the parting plane between the upper
 and lower halves 14U and 14L of the balancer housing 14. According to this
 arrangement, because the common parting plane may be used for defining the
 bearings for both the first and second journal portions 18 and 19, the
 relative positional precision between the axial centers of the bearings
 can be improved.
 By providing additional boss portions 40 receiving threaded bolts B1 for
 fastening the upper and lower halves 14U and 14L of the balancer housing
 14 together in the area adjacent to the second bearing wall portion 17 and
 extending the ledge-shaped projections 38 up to the additional boss
 portions 40 as shown in FIGS. 11 and 12, the bearing wall portions in the
 front and rear ends can be connected to each other with the ledge-shaped
 projections 38, and the rigidity of the front and rear bearing wall
 portions can be increased even further.
 The balancing device 6 having the above described structure can be attached
 to the lower block 5 by threaded bolts B3 passed through the two halves of
 the balancer housing 14 from below as illustrated in FIG. 4.
 FIGS. 13 to 14 show a second embodiment of the balance shaft embodying the
 present invention. The parts corresponding to those of the previous
 embodiment are denoted with like numerals. This embodiment is different
 from the previous embodiment in the shape of the rib 23 which extends
 across the recess formed in the first journal portion 18 between the two
 axial ends thereof. In this embodiment, the outer edge of the rib 23 is
 somewhat recessed from the outer profile of the first journal portion 18,
 in particular the two axial ends thereof, and is rounded. The rib 23 is
 not provided with a through hole as opposed to the previous embodiment.
 Because the outer edge of the rib 23 is recessed and rounded, it receives
 less resistance from the lubricating oil as it rotates with the rest of
 the balance shaft. Also, recessing the outer edge of the rib 23 eliminates
 the need for a high precision because it is not required to conform to the
 outer profile of the remaining part of the first journal portion 18.
 Although the present invention has been described in terms of preferred
 embodiments thereof, it is obvious to a person skilled in the art that
 various alterations and modifications are possible without departing from
 the scope of the present invention which is set forth in the appended
 claims.