Tooth belt drive, especially for internal combustion engines

A tooth belt drive has tooth pulleys (5) with evenly distributed interruptions (22) in its teeth to temporarily interrupt the force excitation from the pulleys and to reduce the oscillation amplitude of the free parts of the belt between the pulleys.

The present invention relates to a tooth belt drive, comprising at least 
one driving and at least one driven pulley and a tooth belt running about 
the pulleys. 
When transmitting torque from a driving shaft to one or more driven shafts, 
belt drives of various types are often used. When relatively high torques 
are to be transmitted or when the relative angular positions of the 
driving shaft and the driven shaft must be kept constant, as in the case 
of the crankshaft and camshaft in internal combustion engines, tooth belt 
drives are often used running over driving and driven pulleys provided 
with notches which conform to a greater or lesser degree to the tooth 
profile of the belt. 
In recent years extensive development work has been devoted to optimizing 
the profile of the teeth for the purpose of reducing noise and extending 
the life of the tooth belt. 
Optimal tooth conformating can however only be achieved for a gear ratio of 
1:1; if there are varying pulley dimensions in the drive unit, the 
conformaty between the teeth and the notches can only be optimized for one 
pulley diameter, usually the smallest pulley diameter. Despite 
optimization, non-uniform belt movement does occurs between the pulleys, 
both longitudinally (i.e. in the primary direction of motion of the belt) 
and radially relative to the driving pulley. 
It is thus not possible by optimization of the tooth and notch profiles to 
eliminate the so-called polygon effect, which increases with decreasing 
pulley diameter and gives rise to cyclical variations in the belt velocity 
causing force variations in the belt and thus in the entire drive unit. 
The cyclical period is determined by the rotational frequency of one of 
the pulleys and the number of notches or teeth in this pulley. Between two 
pulleys the belt has a so-called free part, the dynamic properties of 
which depend on various factors such as the length of the free part, the 
materials used, the belt tension etc. In addition to the force excitation 
from the belt, there are also other force excitations in an internal 
combustion engine, due to non-uniform rotation of masses and non-uniform 
combustion pressure as a function of crank shaft angle. 
These types of excitations with varying frequencies produce different 
oscillations in the tooth belt which give rise to a burring in the belt, 
which can be heard both inside and outside the automobile. Belt burring in 
the passenger compartment due to sound transmission via the engine 
mounting is the most serious noise problem. Reduced belt tension can 
reduce the noise but it also increases the risk of misengagement. 
Increased belt tension means that the noise will be displaced towards 
higher rotational speeds. The noise level increases sharply, however, with 
higher belt tension. 
The general purpose of the present invention is to provide a tooth belt 
drive of the type described in the introduction, which makes it possible 
to reduce the oscillating amplitude of the sympathetic vibrations in free 
parts of the belt, thus reducing noise such as burring and increasing the 
useful life of the tooth belt. A particular purpose of the invention is to 
provide a tooth belt drive for driving twin cam shafts in an internal 
combustion engine, which drive unit, despite the appreciable length of the 
tooth belt, produces a low level of noise. 
This is achieved according to the invention by virtue of the fact that 
either at least one pulley or at least the belt has a series of teeth, 
which at at least one location has an interruption in its otherwise 
uniform normal pitch, at which location the pitch between two consecutive 
operative teeth is at least twice the normal pitch. 
By "removing a tooth" in accordance with the invention, i.e. by eliminating 
the effect of the tooth by removing it completely or partially, the force 
excitation is temporarily interrupted and the natural damping in the belt 
and the system produces a lower amplitude for the subsequent oscillations. 
By adjusting the number of interruptions, the noise generation can be 
affected so that the level can be reduced by up to 8 decibels.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, 1 designates a tooth belt running around a driving first pulley 
2 mounted on the crank shaft of an engine, a pair of driven cam shaft 
pulleys 3 and 4 and a driven pulley 5 on the engine oil pump. Three 
re-directing pulleys 6, 7 and 8 press against the flat outer surface of 
the belt 1, at least one of said pulleys serving as a tensioning pulley 
which is spring (not shown) biased towards the belt. The belt 1 has seven 
"free parts"10-16, i.e. parts between tooth pulleys and re-directing 
pulleys. 
If a free part between two tooth pulleys is excited by a single pulse, a 
maximum oscillating amplitude s is obtained at the natural point of 
resonance of the belt. This amplitude as a function of time t can be as 
represented in FIG. 2. The solid line in FIG. 2 shows schematically the 
maximum amplitude as a function of time t for an essentially un-damped 
tooth belt. For higher material damping (higher loss) a substantially 
different amplitude curve is obtained, as illustrated by the dashed line 
in FIG. 2. The oscillation amplitude of a damped belt thus decreases more 
sharply with time t. 
If there is essentially linear periodic excitation with the frequency of 
excitation corresponding to the natural frequency of the tooth belt, the 
maximum amplitude increases as a function of time to a value approaching 
infinity, if there is no damping whatsoever, which means that the belt 
will rupture. Normally, however, there is always some form of damping in 
the system. The maximum oscillating amplitude as a function of time is 
therefore relatively constant as long as the periodic excitation 
continues, as illustrated by the solid line curve in FIG. 3. 
If the excitation is temperarily interrupted by removing a tooth from one 
of the tooth pulleys in accordance with the invention (FIG. 4) or from the 
belt (FIG. 5) or by reducing the size of a tooth (FIG. 6) so that it is no 
longer operative, a lower amplitude is obtained, due to the natural 
damping of the belt and the system, for the immediately following 
oscillations, until the system returns to equilibrium with the oscillating 
amplitude again at its pre-interruption value. 
In FIG. 3, the dashed line illustrates the oscillation amplitude after an 
interruption caused by elimination of tooth No. 12 as an operative tooth 
in pulley 5. A complete return to maximum amplitude is achieved in the 
example shown at excitation by tooth No. 17. If, for example, every fourth 
tooth were eliminated as an operative tooth, a maximum oscillation 
amplitude is achieved which is lower than the amplitude illustrated by the 
solid line in FIG. 3. How much lower is determined by how powerful the 
excitation is from the crank shaft pulley 2. 
Tests performed with an engine and a drive unit of the type shown in FIG. 
1, have demonstrated that when every third tooth 20 on the oil pump tooth 
pulley 5 is eliminated as an operative tooth, there was a substantially 
lower oscillating amplitude in the free belt part 14 and a reduction of 
the noise level of up to 5 decibels. Correspondingly, there was a 
reduction in the noise level of up to 5 decibels by eliminating every 
seventh tooth on the crank shaft pulley 2. The measures together produced 
a noise reduction of up to 8 decibels. The tests also showed a substantial 
extension of the belt life. 
Interruptions in excitation can also be achieved by modifying the tooth 
belt 1 itself. For example, entire teeth 20 can be eliminated, as shown in 
FIG. 5, so that, as in the example in FIG. 4, spaces 22 are formed having 
a length equal to one tooth width 20 and two normal spaces 21. 
Alternatively, non-operative teeth 23 can be made shorter and with a 
thinner tooth profile, as shown in FIG. 6. 
In a cam shaft drive, it is, however, advantageous to first eliminate the 
operative teeth on a pulley which is not heavily loaded, such as the oil 
pump pulley 5 and to leave the belt 1, the crank shaft pulley 2 ad the cam 
shaft pulleys 3, 4 intact. 
The reduction in the burring noise level is determined by the number of 
operative teeth eliminated on each pulley and/or belt, the number of 
pulleys modified and the degree of damping in the system, and especially 
the natural damping of the tooth belt. Tests performed have shown that the 
number of teeth eliminated should not be fewer than 10%, preferably 15% or 
more, of the normal total number of teeth. Damping in the belt, measured 
as a loss factor, should be higher than 3%, preferably appreciably higher, 
for example 12%.