Torsionally compliant sprocket system for balance shaft drive

The present invention relates to a torsionally compliant sprocket assembly which reduces the transfer of the crankshaft torsional oscillations to other components in the engine such as the balance shaft drives. The assembly comprises a first sprocket, second sprocket and a planar torsion spring located between the two sprockets and the shaft.

FIELD OF INVENTION 
This invention relates to the isolating of vibrations in rotating devices. 
More particularly, this invention has particular application to engines 
timing systems with two sprockets located side by side in close proximity 
on a rotating shaft. This invention provides a torsionally compliant 
sprocket system that absorbs torsional loads and vibrations from the 
shaft. 
BACKGROUND OF THE INVENTION 
Engine timing systems typically include at least one driving sprocket 
located on the engine crankshaft and at least one driven sprocket on an 
engine camshaft. The rotation of the crankshaft causes the rotation of the 
camshaft through an endless power chain transmission. 
More complicated engine timing systems connect a 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. 
Some engine systems, such as three cylinder engines, due to the number of 
cylinders and arrangement of the cylinders are inherently unbalanced. In 
these engines, balance shafts are employed to balance the inherent 
inbalance of the engine. Since the balance shafts are driven by the 
crankshaft, torsional vibrations and oscillations along the crankshaft may 
be transferred to the balance shafts and through the chain drive and 
create unnecessary high chain tensions throughout the engine timing system 
and accessory drive. 
The rotating crankshaft may undergo resonance at certain frequencies. Since 
the balance shafts are coupled to the crankshaft by one or more balance 
shaft chains, the balance shafts are directly exposed to these extreme 
resonant torsional oscillations. Vibrations from the resonance of the 
crankshaft are often transferred throughout the system, including the 
balance shafts and can significantly increase the load on the systems and 
components, increase the noise from the engine, and increase wear and 
fatigue loading of the timing chains and components. 
Conventional approaches to this problem have focused on reducing rotational 
perturbations of the crankshaft by means of internal devices such as 
Lanchaster dampers and harmonic balancers. External devices such as fluid 
engine mounts and engine mounts having adjustable damping characteristics 
have also been used. By contrast, the present invention focuses on 
absorbing the torsional vibrations of a crankshaft by using a torsionally 
compliant sprocket system on the crankshaft to absorb the crankshaft 
torsional vibrations and prevent their transfer to other parts of the 
engine system. 
Some prior art timing systems use a rubber damper piece 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 to 
absorb vibrations. These systems, while effective at damping have 
drawbacks in terms of production, assembly and durability. 
An example of the above-described prior vibration damping techniques is 
found in Wojcikowski, U.S. Pat. No. 4,317,388, which issued on Mar. 2, 
1982. That patent discloses a gear with split damping rings of diameter 
slightly smaller than the gear bolted to each side of the gear with a 
tapered bolt and nut assembly. Tightening of the bolt cams the damping 
ring outward, producing pressure circumferentially against the rim of the 
gear and causing tensile stress on the gear. Additionally, tightening of 
the bolts presses the elastomeric washers associated with the bolt and nut 
assembly firmly against the web of the gear which damps the stress wave 
passing from the rim through the web and into the shaft. In contrast to 
this prior art structure, the present invention utilizes a novel 
arrangement of sprockets to produce a torsionally compliant sprocket 
assembly to reduce the transfer of vibrations of the crankshaft to other 
parts of the engine system. 
Another example of the above-described prior art damping techniques is 
Funashashi, U.S. Pat. No. 5,308,289, which issued on May 3, 1994. The 
damper pulley disclosed therein consists of a pulley joined to a damper 
mass member with a resilient rubber member. The pulley and the damper-mass 
member each have at least two projections, and the projections of the 
pulley contact the sides of the projections of the damper mass member. A 
second resilient rubber member is placed between the contacting 
projections. Bending vibrations from the crankshaft cause the pulley to 
vibrate in the radial direction and the first resilient rubber member 
deforms, causing the dynamic damper to resonate with the pulley and 
restrain the bending vibrations. Torsional vibrations cause the pulley to 
vibrate in the circumferential direction. The second resilient rubber 
member undergoes compression deformation, decreasing the spring force and 
raising the resonance frequency against the torsional vibrations. The 
present invention avoids the use of rubber which has wear problems in use. 
Another example of a prior damping technique is Kirschner, U.S. Pat. No. 
4,254,985, which issued on Mar. 10, 1981. That patent discloses a damping 
ring for rotating wheels that includes a viscoelastic damping material 
disposed within an annular groove in the surface of the wheel. A metal 
ring is positioned in the groove at the top of the damping material. In 
operation, the damping material undergoes shear deformation. 
SUMMARY OF THE INVENTION 
In an engine timing system, an endless chain connects a driving sprocket on 
the crankshaft to a driven sprocket on a camshaft. The rotation of the 
driving sprocket advances the chain, which turns the driven sprocket and 
the cam shaft. Torsional vibrations arise in the system and are 
transmitted along the crankshaft which is spinning at a nonconstant speed. 
These vibrations may be excessively large at resonance conditions. To 
absorb these vibrations, and minimize the transfer of these vibrations to 
other portions of the engine, the present invention utilizes a torsionally 
compliant sprocket assembly positioned along the crankshaft. The 
torsionally compliant sprocket includes a first driving sprocket, a 
serpentine planar torsion spring, and a second sprocket positioned on the 
crankshaft, used in conjunction with the engine balancer drive system. The 
planar torsion spring is held abuttingly in contact with the driving 
sprocket on its one side and the second sprocket on its other side. 
However, the planar torsion spring moves independently of the two 
sprockets. Movement of the planar torsion spring acts to absorb the 
crankshaft torsional vibrations and prevent their transfer to the tension 
on the balance shaft chain and balance shaft drive. The planar torsional 
spring serves to isolate the driving sprocket from the balance shaft 
without compromising the integrity of the drive.

DETAILED DESCRIPTION 
An example of a multiple axis timing chain system, in which the present 
invention is utilized, is schematically illustrated in FIG. 1. The 
sprockets can be either single units or sprocket pairs, with the pairs 
having aligned teeth or having their teeth phased, or offset, by a portion 
of one tooth. Phased sprockets and chain assemblies are described in U.S. 
Pat. No. 5,427,580, which is incorporated herein by reference. 
Crankshaft 100 provides power output through sprocket 102, and sprocket 
pair 104. Crankshaft sprocket pair 104, which includes sprockets 103 and 
105, carries load or transmits power to chain assemblies (or chain pairs) 
106. The chain assemblies 106 provide the primary drive of the two 
overhead camshafts 108 and 110. Camshaft 108 includes a pair of phased 
sprockets 112, and camshaft 110 also includes a pair of phased sprockets 
114. The chain assemblies 106 also drive idler sprockets 116, which may or 
may not be phased. 
The second crankshaft sprocket 102 is a balance shaft drive sprocket that 
provides power transmission through chain 120 to a pair of balance shafts 
122, 124, and idler shaft 126 and to an accessory drive 128, such as an 
oil pump drive. Chain 120 therefore transmits power from the crankshaft 
balance shaft sprocket 102 to a first sprocket 130 on balance shaft 124 
and to a second sprocket 132 on balance shaft 122. The chain also drives 
idler sprocket 134 on idler shaft 126 and accessory drive sprocket 136 on 
accessory drive 128. The balance shaft drive system can include a pair of 
chains in place of single chain 120, and a pair of sprockets in place of 
single sprocket 102. In such a system, one chain of the pair of chains 
would preferably drive the idler sprocket while the other chain of the 
pair of chains would drive the accessory drive. 
The chain assemblies of the multi-axis chain drive system shown in FIG. 1 
utilize conventional snubbers and tensioning devices to maintain tension 
and lateral control in various portions of the chain drive. Such devices 
are known to those skilled in the chain art. 
In this system, the crankshaft itself and the driving sprocket mounted on 
the crankshaft is subject to torsional loads and vibrations and may 
undergo resonance at certain frequencies. Vibrations from the resonance 
condition are transferred through the system due to the interconnection of 
various components of the system. Since the balance shafts are coupled to 
the crankshaft by a chain, the balance shafts are directly exposed to 
crankshaft torsional oscillations and vibrations. 
FIGS. 2, 3 and 4 illustrate a torsionally compliant sprocket system, with 
crankshaft drive sprockets 3 and 5, of sprocket pair 4. Sprocket pair 4 
corresponds to sprocket pair 104 illustrated in FIG. 1. Crankshaft drive 
sprocket 2 drives the balance shafts. Sprocket 2 corresponds to sprocket 
102 in FIG. 1. The sprockets are shown schematically in FIG. 2, without 
the individual sprocket teeth. 
The crankshaft sprockets 3, 5 may be conventional involute tooth sprockets. 
The driving crankshaft sprockets 3,5 are mounted on a hub 20. The 
sprockets 3,5 are mounted on the hub 20 by any suitable fastening means 
such as weldment, splines or keyways. A pair of chains are disposed on 
teeth 21, 23 of the sprockets 3, 5 which connects the crankshaft to the 
camshafts. The size and dimensions of the driving sprocket are dependent 
upon the engine and configuration of the system. 
The torsionally compliant sprocket assembly further includes a balance 
shaft drive sprocket 2 which is also mounted on the hub 20. The balance 
shaft drive sprocket 2 can also be any conventional sprocket with 
dimensions dependent upon the system configuration and power requirements. 
This sprocket is also mounted on the hub 20 and includes a central opening 
7 to permit connection of the hub and sprocket assembly to the crankshaft. 
A chain is disposed about the teeth 11 of the sprocket 2 and drivingly 
connects the sprockets on the balance shafts. 
The planar torsion spring 15, shown in FIG. 2, is sized to fit between the 
balance shaft sprocket 2 and hub 20. The planar torsion spring is adapted 
to move independently of the two sprockets. The planar torsional spring 
serves to absorb the vibrations and the torsional forces from the 
crankshaft. 
The planar torsion spring 15 can be mounted around the crankshaft in any 
manner provided that it is located between the sprocket and the hub. 
Preferably, the planar torsion spring is wrapped in a circular fashion 
around the crankshaft in a spiral shape. A first end 17 of the planar 
torsion spring 11 is attached to the balance shaft sprocket 2 by any known 
means, but is preferably fitted into a slot 19 formed in the side of the 
sprocket. The second end 21 of the planar torsion spring 11, is preferably 
fitted into a slot 23 formed in the hub 20, as shown in FIG. 2. 
In operation of the torsionally compliant sprocket assembly, the planar 
torsion spring absorbs torsional oscillations from the crankshaft during 
rotation of the crankshaft. Without the spring 11, vibrations in the 
crankshaft will be transmitted from the crankshaft through the hub and 
sprockets and then to the balance shafts and their components. The 
torsional spring will deform and permit some relative rotation between the 
balance shaft sprocket 2 and the hub 20. Thus, the sprocket 2 must be 
mounted on the hub to permit some relative rotation. 
The torsionally compliant sprocket assembly can be used on any rotating 
shaft with two sprockets located thereon in close proximity of each other. 
In particular, the torsionally compliant sprocket assembly can serve as a 
compliance mechanism and be used on any rotating shaft in which the 
dissipation of torsional forces and vibrations is desired. 
Although specific embodiments and examples have been disclosed herein, it 
should be borne in mind that these have been provided by way of 
explanation and illustration and the present invention is not limited 
thereby. Certainly modifications which are within the ordinary skill in 
the art are considered to lie within the scope of this invention as 
defined by the following claims.