A BALANCE SHAFT ASSEMBLY

Abstract: In an internal combustion engine (E) of a vehicle there is provided a balance shaft assembly (11) connected to the crankshaft support member (2). There are aligned fixing apertures in the crankshaft support member (2) and the cylinder block by which the member and block may be connected together. The balance shaft assembly (11) comprises a pair of rotary balance shafts (14) supported in a housing that has fixing apertures aligned with those in the crankshaft support member and the cylinder block. The longitudinal axis defined by certain aligned apertures (25, 26) intersects the swept volume of rotation described in rotational operation by at least one of the balance shafts (14) and so at least one balance shaft is provided with a recess so that in a predetermined orientation it allows access to the aligned apertures so as to permit insertion of a fixing member into the apertures with the balance shafts (14) in situ. The design allows a balance shaft assembly to be fitted with only minor modification to the engine block and without providing an increase in noise.

Description: A BALANCE SHAFT ASSEMBLY The present invention relates to an internal combustion engine and in particular to a balance shaft assembly for such an engine. It is well known for internal combustion engines such as those used in automobiles to include balance shafts to counteract second order vibrations in the form of vertical shaking forces generated by the reciprocal movement of the pistons in their cylinders. Balance shafts are typically mounted in a support housing that is disposed beneath the engine crankshaft. The housing has a plurality of mounting points designed to connect to a part of the engine block. Above the crankshaft is what is often referred to as the cylinder block and this in combination with a bearing support which may be in the form of a bearing beam or a plurality of bearing caps, provides annular bearing portions in which the crankshaft journal portions rotate and are lubricated. In a typical arrangement two oppositely but synchronously rotating balance shafts are driven by the crankshaft via a chain at a rotation rate of twice the speed of the crankshaft. The shafts have intermeshing timing gears to ensure synchronous and opposite rotation. Eccentric portions of the shafts provide the balancing forces and shell bearings are typically provided for each of these portions. One of the bearings generally supports the load of the eccentric portions and the other serves as a guide for the other end of the balance shaft. In engine design significant effort is directed to limiting engine noise as much as possible. One significant source of noise is intermeshing gear mechanisms such as those of the balance shafts and the driving connection between the crankshaft and the balance shafts. Generally components of an engine block are manufactured from different materials that each have different coefficients of thermal expansion and, in the case of gear mechanisms, there is a tendency for the intermeshing wheels to separate as the temperature increases thereby causing an increase in the noise levels of the running gears. This is a particular problem in gear drive mechanisms between the balance shafts and the crankshaft. Various alternative fixing arrangements have been proposed to secure the balance shaft assembly to the engine block without involving significant cost in having to modify parts of the engine block and without adding to the noise levels of the engine. However, there has been limited success and invariably significant design changes to the design of the bearing support or other parts of the engine block are required. This is undesirable since it is well known for vehicle manufacturers to use the same engine on all vehicles within a particular range but to provide a balance shaft assembly only to certain vehicles within that range (such as higher specification vehicles). One example of such a design is described in US patent No. 5743230 in which a balance shaft assembly is connected to the engine block by means of elongate bolts that pass through aligned bores in the balance shaft support housing, the bearing block and into the cylinder block. The bores are disposed outboard of the balance shafts and the respective housing and blocks are designed to be correspondingly wide to accommodate them. These fixing bolts and bores are provided in addition to those that connect the bearing block to the cylinder block and thus there is added manufacturing and installation costs associated with this design. It is an object of the present invention to obviate or mitigate the aforesaid disadvantages. According to a first aspect of the present invention there is provided an engine having: a cylinder block in which pistons are reciprocally received; a crankshaft to which the pistons are connected a crankshaft support member connected to the cylinder block and defining a bearing for the crankshaft; a balance shaft assembly connected to the crankshaft support member; aligned fixing apertures in the crankshaft support member and the cylinder block by which the member and block may be connected together; the balance shaft assembly comprising at least one pair of rotary balance shafts supported in a frame, the frame having at least one fixing aperture aligned with those in the crankshaft support member and the cylinder block; characterised in that the longitudinal axis defined by the aligned apertures intersects the swept volume of rotation described in rotational operation by at least one of the balance shafts and in that said at least one balance shaft is provided with a recess so that in a predetermined orientation it allows access to the aligned apertures so as to permit insertion of a fixing member into the apertures with the balance shafts in situ. The assembly ensures that little modification is required to an engine in order to add the balance shaft assembly and that the number of parts is reduced from that of a conventional design. This is achieved by being able to use existing aligned fixing apertures of an engine block. Accordingly, the assembly is cheaper to produce and install. Moreover, since the aligned apertures used to fix the assembly to the rest of the engine are in a central area of the support member and the cylinder block where noise and vibration is eliminated or reduced by engine design and therefore there is reduced potential for noise and/or vibration to be transmitted from the assembly. Preferably a single fixing member is fixed into each group of aligned apertures thereby adding stiffness to whole engine structure. In being connected to each of the aligned aperture groups the balance shaft assembly further stiffens the engine structure and assists in the limitation of noise. The fixing member is generally a bolt that has a coefficient of thermal expansion that is less than that of other components of the engine block and therefore it serves to prevent excessive thermally-generated separation of the gear mechanism of the balance shaft assembly from the driving mechanism of the crankshaft. The balance shaft assembly frame may be conveniently in the form of a housing having openings to provide access to the aligned aperture for the fixing member. The housing may be constructed in two parts with one part having attachment members in which the fixing aperture is defined. The balance shaft assembly is preferably disposed in a chamber defined between the crankshaft support and a lubricant sump. Preferably shell bearings flank an eccentric portion of at least one balance shaft. According to a second aspect of the invention there is provided a balance shaft assembly comprising at least one pair of rotary balance shafts supported in a frame, the frame having at least one fixing aperture, characterised in that the longitudinal axis defined by the aperture intersects the swept volume of rotation described in rotational operation by at least one of the balance shafts and in that said at least one balance shaft is provided with a recess so that in a predetermined orientation it allows access to the aperture so as to permit insertion of a fixing member into the aperture with the balance shafts in situ. A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an underneath perspective view of an internal combustion engine with its cylinder block removed and the oil sump cut away for clarity; Figure 2 is an end perspective view of the engine of figure 1 with a balance shaft assembly housing removed for clarity; Figure 3 is an underneath plan view of the engine of figure 2 with the oil sump removed for clarity; Figure 4 is a perspective view from above of the balance shaft assembly; Figure 5 is a perspective view from below of the assembly of figure 4; and Figure 6 is a perspective view from below the balance shaft assembly with a lower part of the housing removed for clarity. Referring now to the drawings, the engine E is an in-line four cylinder internal combustion engine that is commonly used in automobiles and has a cylinder block (not shown) of conventional design with cylinders and pistons. The pistons are connected, as is conventional, to a crankshaft 1 via connecting rods (not shown). The crankshaft 1 is rotatably supported between bearings defined at spaced intervals between the cylinder block and a lower support block 2 connected thereto. The lower support block 2 is generally rectangular in outline having parallel side walls 3 and two upstanding end walls 4. Intermediate the end walls 4 there are three spaced cross-members 5 that are similarly upstanding from the side walls 3. Each of the end walls 4 and the cross members 5 have through bores 6, 7 that are intended to receive fixings that connect the lower support block 2 to the cylinder block. An oil sump 10 is connected to the underside of the engine E as shown in figure 1 and 2. The engine E has a balance shaft assembly (generally indicated by reference numeral 1 1) disposed in a chamber 12 defined between the oil sump 10 and the underside of the lower support block 2. The assembly 11, shown in detail in figures 4 to 6, comprises a two-part housing 13a, 13b and two balance shafts 14a, 14b. An upper portion 13a of the housing has three spaced attachment blocks 15, 16, 17 each having two fixing bores 18, 19 that are in alignment with the bores 6, 7 in the cross members 5 of the lower support block 2. Inside the housing the two parallel balance shafts 14a, 14b have integral eccentric portions 14c and are seated in complementary recesses provided in the housing 13a, 13b. Both shafts 14a, 14b have meshed gear wheels 21, 22 (also recessed into the housing body) at one end so that, in use, the shafts 14a, 14b rotate synchronously in opposed directions. At the opposite end to the meshed gear wheels 21, 22 there is a drive wheel 23 supported on one of the shafts 14a. The periphery of the drive wheel 23 projects through an opening 24 in the upper portion 13a of the housing and in use is engaged by a driving gear wheel, chain or belt (not shown) that is in turn driven by the crankshaft as is well-known. Shell bearings (not shown) are arranged to flank the eccentric portion on each balance shaft. This configuration of bearings allows both shells to share the load so each can be designed to be of a smaller size and lower specification (and therefore lower cost) than conventional bearings It will be seen especially from figures 3, 4 and 6 that two of the attachment blocks 15, 17 lie outboard of the region occupied by the balance shafts 14a, 14b and a lower portion 13b of the housing so that there is clear access to their fixing bores 18, 19 whereas the third is 16 positioned intermediate the others and is disposed above the shafts 14a, 14b. In order to provide access to the fixing bores 18, 19 of the intermediate attachment block 16 cut-out apertures 25, 26 are provided in the upper and lower portions 13a, 13b of the housing and the balance shafts 14a, 14b have arcuate slots 27, 28 at an intermediate point along their length. Each balance shaft 14a, 14b is fitted with a shell bearing that is not shown in the appended figures. When the balance shaft slots 27. 28 are in the orientations shown in figures 3 and 6 it is possible to pass elongate fixing bolts (not shown) through the cut-out apertures 25, 26 in the housing portions 13a, 13b via the arcuate slots 27, 28 and into the fixing bores 18, 19 of the intermediate attachment block 16. The bolts pass into the aligned bores 6, 7 of the respective cross member of the lower support block 2 and into aligned bores in the cylinder block. Whilst the extended longitudinal axis of the bores 18, 19 intersects the swept volume defined by revolution of the eccentric portions 14c of the balance shafts 14a, 14b, once the bolts are fully inserted their heads are clear of the path of the shafts 14a, 14b and therefore do not interfere with rotation of the shafts. The balance shaft assembly 11 is attached to the rest of the engine E by means of four elongate bolts (not shown) that pass through the bores 18, 19 in the outer attachment blocks 15, 17 and, as described above, into aligned bores in the lower support block and the cylinder block. The principal advantage of the balance shaft assembly described above is that it can be installed on a vehicle with little or no modification to the engine block. The assembly is connected by means of the existing fixing bores in the engine block that are used to interconnect the lower support block and the cylinder block and therefore there is no requirement to machine additional fixing points although it may be necessary in practice to make a minor modification to the lower support block by machining the area around the bores so as to ensure the connection is in tolerance. This means that within a range of vehicles having identical engines the manufacturer can selectively fit certain vehicles (e.g. those with a higher specification) with a balance shaft assembly with relative ease. Since there are also fewer parts to the assembly the manufacturing and installation cost is reduced. Moreover, if necessary, customers can have an assembly fitted to an existing vehicle at relatively low installation cost. In the above described design the fixing points of the balance shaft assembly to the remainder of the engine block are kept towards the central area of the engine. This reduces the potential for the transmission of noise or vibration from the balance shaft assembly as the engine has generally already been designed to reduce significantly the vibration in the central structure. The use of elongate fixing bolts passing through as much of the engine block as is possible increases the overall stiffness of the engine as well as the balance shaft assembly. Moreover, such bolts are generally constructed of metal that has a lower coefficient of thermal expansion than that of the rest of the engine block, the tendency of components of the engine to separate under thermal expansion is reduced. In particular, the driving gear mechanism of the crankshaft is held in position relative to the driven mechanism of the balance shaft assembly so as to reduce the noise created by inadequate intermeshing of the gears. It is to be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. In particular, the configuration of the engine described is only one of a number of possibilities and it is to be understood that the lower crankshaft support block may be replace by another design of bearing beam or bearing caps. Indeed there may be one or more crankshaft support members. Moreover, the particular drive mechanism between the crankshaft and the balance shafts may be of a different configuration to that described above, the alternative drive possibilities being well known to those skilled in the art.

Claims: CLAIMS 1. An engine having: a cylinder block in which pistons are reciprocally received; a crankshaft to which the pistons are connected a crankshaft support member connected to the cylinder block and defining a bearing for the crankshaft; a balance shaft assembly connected to the crankshaft support member; aligned fixing apertures in the crankshaft support member and the cylinder block by which the member and block may be connected together; the balance shaft assembly comprising at least one pair of rotary balance shafts supported in a frame, the frame having at least one fixing aperture aligned with those in the crankshaft support member and the cylinder block; characterised in that the longitudinal axis defined by the aligned apertures intersects the swept volume of rotation described in rotational operation by at least one of the balance shafts and in that said at least one balance shaft is provided with a recess so that in a predetermined orientation it allows access to the aligned apertures so as to permit insertion of a fixing member into the apertures with the balance shafts in situ. 2. An engine according to claim 1, wherein there is a single fixing member is fixed into each group of aligned apertures. 3. An engine according to claim 1 or 2, wherein the balance shaft assembly frame is in the form of a housing having an opening to provide access to the aligned aperture for the fixing member. 4. An engine according to claim 3, wherein the housing is constructed in two parts with one part having attachment members in which the fixing aperture is defined. 5. An engine according to any preceding claim, wherein the balance shaft assembly is disposed in a chamber defined between the crankshaft support and a lubricant sump. 6. An engine according to any preceding claim, wherein the assembly further comprises shell bearings that flank an eccentric portion of at least one of the balance shafts. 7. A balance shaft assembly comprising at least one pair of rotary balance shafts supported in a frame, the frame having at least one fixing aperture, characterised in that the longitudinal axis defined by the aperture intersects the swept volume of rotation described in rotational operation by at least one of the balance shafts and in that said at least one balance shaft is provided with a recess so that in a predetermined orientation it allows access to the aperture so as to permit insertion of a fixing member into the aperture with the balance shafts in situ. 8. An engine substantially as hereinbefore described with reference to the accompanying drawings. 9. A balance shaft assembly substantially as hereinbefore described with reference to the accompanying drawings

Inventors: COLQUHOUN CALLUM MAXWELL, COLQUHOUN, CALLUM, MAXWELL
Applicants: COLQUHOUN CALLUM MAXWELL, SIMPSON INTERNAT UK LTD, COLQUHOUN, CALLUM, MAXWELL, SIMPSON INTERNATIONAL (UK) LIMITED
Application year: 2000
Main classification: H02J9
IPC codes: F16F15