Internal-combustion engines – Poppet valve operating mechanism – Camshaft drive means
Reexamination Certificate
2000-06-09
2001-09-04
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
Camshaft drive means
C123S192100, C123S192200, C074S574300, C474S094000, C464S180000
Reexamination Certificate
active
06283076
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the reduction of vibrations in engine balance shaft drives. This invention has particular application to engine timing and balance drive systems driven by sprockets located on a rotating crankshaft. More particularly, the invention relates to a torsionally compliant sprocket for absorbing torsional loads and vibrations in a balance shaft drive system originating from a driving shaft.
Engine timing systems typically include an endless chain wrapped around a driving sprocket on an engine crankshaft and a driven sprocket on an engine camshaft. The rotation of the crankshaft causes the rotation of the camshaft through the endless chain system. A separate sprocket mounted upon the crankshaft may be used to drive a balance shaft system using a separate endless chain.
More complicated engine timing systems connect the crankshaft with two or more shafts by a pair of chains. The crankshaft includes two sprockets. Each chain is connected to one or more driven sprockets, including sprockets on each of the two overhead camshafts. Typically, the chain systems in more complicated engine timing systems will include tensioners on the slack side of each chain to maintain chain tension and snubbers on the tight side of each chain to control chain movement during operation.
Some engine timing systems have two (or dual) overhead camshafts for each bank of cylinders. The dual camshafts on a single bank can both be rotated by connection to the same chain. Alternatively, the second camshaft can be rotated by an additional camshaft-to-camshaft chain drive. The cam-to-cam drive chain can also include single or dual tensioners for chain control.
In some engine timing systems, especially those having a nonconventional firing order for the cylinders and those used in diesel engines, balance shafts are employed to counterbalance engine vibration. The balance shafts are driven by a chain connection from the crankshaft. Optionally, the balance shaft drive system may be utilized to operate an auxiliary drive such as a compressor or the like. The balance shafts are driven by the crankshaft and as a consequence, torsional vibrations and oscillations along the crankshaft may be transferred to the balance shafts and likewise throughout the timing system. Since balance shafts employ rotating weights, they are rotating systems having a high mass and inertia.
The rotating crankshaft may undergo resonance at certain frequencies. Since the balance shafts are coupled to the crankshaft by the balance shaft chain, the balance shafts are directly exposed to these extreme resonant torsional vibrations. The vibrations can cause high loads on the chain system for the balance shaft drive because of the high inertia of the balance shafts. Vibrations from the resonance of the crankshaft are often transferred throughout the system, including the balance shafts and associated engine elements and can significantly increase the load on the systems and components, increase the noise from the engine and increase wear and accelerate fatigue of the chains.
Some prior art timing systems use various damping devices to address the problem of vibrations. One example of such a prior art system uses a rubber damper piece which is placed against a sprocket and bolted to the shaft to absorb vibrations. However, the rubber damper piece may fracture from the extreme resonance vibrations. Other timing systems employ a weight that is positioned on the shaft and held against the sprocket by a Belleville washer. Frictional material is also placed at the area of contact between the sprocket and the weight. These systems, while effective at damping vibrations have drawbacks in terms of production, assembly and durability.
By contrast, the present invention focuses on absorbing the torsional vibrations of a crankshaft using a torsionally compliant sprocket. The torsionally compliant sprocket reduces the transfer of such vibrations and torque spikes to other parts of the engine system. The torsionally compliant sprocket interposes a resilient elastomeric layer between a hub portion and a sprocket rim portion of a driving sprocket mounted on the crankshaft to absorb vibrations and reduce transfer of the crankshaft vibrations.
SUMMARY OF THE INVENTION
On engines equipped with balance shafts, an endless chain connects a driving sprocket on the crankshaft to driven sprockets on balance shafts. The rotation of the driving sprocket advances the chain, which turns the driven sprocket and the shaft. Torsional vibrations occur during system operations and may be exceptionally severe at resonance conditions. To reduce these vibrations and to reduce the transfer of these vibrations from the crankshaft to other portions of the engine, the present invention provides a torsionally compliant arrangement of the balance shaft sprocket. By interposing an elastomeric element in the crank sprocket vibrations originating in the crankshaft are, to a significant extent, effectively prevented from being transmitted to the remainder of the balance shaft chain drive. This results in lower chain load, an increase in chain life and a system that can be optimized for cost rather than robustness.
In a first embodiment, the torsionally compliant sprocket comprises a hub portion securely mounted upon the crankshaft. The outer periphery of the hub portion has a plurality of extending splines formed thereon which project radially outwardly from the hub member. A sprocket rim portion having teeth formed on its outer periphery for engaging a chain is provided about the hub portion. Keyways on an inner surface of the sprocket rim portion are arranged to engage in a cooperative manner with the splines of the hub. Molded into the space between the splines of the hub and the keyways of the sprocket rim is an elastomeric material capable of absorbing torsional vibrations and providing beneficial damping.
Machined washers are provided on either side of the sprocket to maintain alignment of the hub and rim portions. The washers are either splined onto the crankshaft or may be freely rotatable about the crankshaft. One of the washers may be spring loaded to apply force to the sprocket but still allows the outer rim portion of the sprocket to move with respect to the inner hub portion. Preferably, the washers have a predetermined amount of clearance from the compliant sprocket. The clearance allows the elastomeric material in the sprocket to bulge out of the sprocket when compressed but still prevent the sprocket from becoming misaligned with the driven sprockets.
Stop blocks are optionally provided between the hub and the sprocket rim portions of the balance shaft drive sprocket to prevent damage to the elastomeric material. The stop blocks are formed on the side faces of the hub splines or formed on the inner faces of the sprocket rim keyways or on both. Engagement of the stop blocks prevent overrotation of the sprocket rim portion with respect to the hub portion when the drive sprocket rotates more than a predetermined amount.
When the system is excited at its natural frequency, large amplitudes of torsional oscillation may be generated, which tends to negate the effectiveness of the compliant sprocket. In this case, the resonant vibrations tend to have a broad frequency range. It is preferable that the natural frequency of the compliant material in the sprocket is less than the idle speed of the engine. Accordingly, it has been found that the compliant sprocket benefits by the addition of a damping mechanism.
Damping is provided to the torsionally compliant sprocket system. While many methods of adding damping to the system would be effective, the preferred method is to use the natural damping factor of the rubber elastomer itself When using a rubber elastomeric member as the compliant and damping aspect of the compliant sprocket, it is expected that no additional damping mechanism will be necessary. The rubber elastomeric insert is used to add sufficient damping to eliminate the issues associated with natural frequency oscillations, whil
Borg-Warner Inc.
Dziegielewski Greg
Lo Weilun
Sidley & Austin
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