Field of the Invention
The invention relates to a belt drive with a
compression span, and more particularly, to a toothed
belt drive having a drive length which is less than a
toothed belt length such that the toothed belt forms a
free-standing arcuate span between the first sprocket and
the second sprocket on a toothed belt compression span.
Background of the Invention
The initial installation tension is the most
critical factor to a successful synchronous belt drive. A
synchronous belt drive is used to drive machines where a
timed or synchronous condition is required. A
synchronous belt uses a toothed belt to achieve the
desired timing effect.
The conventional synchronous belt drive requires the
tension ratio, which is defined as the ratio between the
tight side tension and slack side tension, to be about
eight (8) for good results. Using a tension ratio of
about 8 proves to be a good approach for the constant
load drive, for example, the fan drive, conveyer belt
drive, and so on.
With high quality synchronous belt development,
automotive industries explored the possibility of using
synchronous belts to replace the metal chain for the
transfer case applications. The traditional transfer
case uses a metal chain to split the power from the
engine to drive the front wheel and rear wheels. This
arrangement requires a chain guide, sealed case, and oil
lubrication. The advantage of the belt drive is the
complete elimination of the oil, for a so-called dry
transfer case.
The challenge of a dry transfer case is the belt
initial installation tension. Because the large
variation of the torque passing through the transfer
case, if a tension ratio of 8 is used the required
initial tension is significantly high. The high initial
tension results in a poor drive efficiency and belt
noise. On the other hand, if the initial tension is too
low, the belt may jump a tooth when high torque is
encountered .
The concept of using belt backward buckling to
achieve the zero tension drive has been explored in the
prior art. The idea is to use a belt pitch length longer
than the drive length, and let the belt buckle backward
on the slack side span. In the prior art device two
bearings are placed at the exit and entrance of the slack
span and serve as the guide to lead the belt into the
desired backward buckling. When the forward torque is
applied, the extra belt length is taken up by the
backward buckling that forms an arc shape. The belt back
bending stiffness is relevant to allowing the belt
buckling in the backward direction.
In comparison to a metal chain drive, there is no
bending stiffness due to the flexible chain link
connections, therefore making it impossible to achieve
the backward buckling.
Representative of the art is US patent no. 8,308,589
which discloses a belt and chain drive for vehicles or
for use in drive technology with an input shaft and an
output shaft supported on a frame, the input shaft and
the output shaft project out of the frame, with the
following features: a ) gear transmissions with gear
wheels, which are embodied as belt and chain drives, are
located between the input shaft and the output shaft, b )
all of the gear wheels are constantly in rotation during
operation, c ) the gear transmissions located between the
input shaft and the output shaft are embodied as belt and
chain drives with toothed belts as traction mechanisms
and with pulleys as gear wheels, d ) the traction
mechanism are reinforced with aramid, Kevlar, carbon
fibers or other fibrous materials, is characterized in
that e ) the traction mechanism is pressed into a kidneylike
shape onto the pulleys by at least one component
during no-load rotation, and that under the effect of
load this component does not touch the traction
mechanism, and f ) the kidney-like shape of the traction
mechanism during no-load rotation is formed by a convex
curvature of the driving side and by a concave curvature
of the slack side, and g ) the kidney-like shape of the
traction mechanisms under load is formed by a straight
shape of the driving side and by an intensified concave
curvature of the slack side.
What is needed is a toothed belt drive having a
drive length which is less than a toothed belt length
such that the toothed belt forms a free-standing arcuate
span between the first sprocket and the second sprocket
on a toothed belt compression span. The present
invention meets this need.
Summary of the Invention
An aspect of the invention is to provide a toothed
belt drive having a drive length which is less than a
toothed belt length such that the toothed belt forms a
free-standing arcuate span between the first sprocket and
the second sprocket on a toothed belt compression span.
Other aspects of the invention will be pointed out
or made obvious by the following description of the
invention and the accompanying drawings.
The invention comprises a toothed belt drive with
compression span comprising a first sprocket, a second
sprocket, a toothed belt having a toothed belt length and
trained between the first sprocket and the second
sprocket, a first linear guide member in cooperative
relation to and disposed a predetermined distance (B)
from the toothed belt, a second linear guide member in
cooperative relation to and disposed a predetermined
distance (B) from the toothed belt, and the toothed belt
length greater than a drive length such that the toothed
belt forms a free-standing arcuate span between the first
sprocket and the second sprocket on a toothed belt
compression span.
Brief Description of the Drawings
The accompanying drawings, which are incorporated in
and form a part of the specification, illustrate
preferred embodiments of the present invention, and
together with a description, serve to explain the
principles of the invention.
Figure 1 is a schematic view of a system.
Figure 2 is a chart of the inducing torque versus
push out force.
Figure 3 is a chart of the inducing torque versus
push out force.
Detailed Description of the Preferred Embodiment
Figure 1 is a schematic view of a system. The system
100 comprises a first sprocket 10 and a second sprocket
20. Toothed belt 30 is trained between the sprockets. A
first linear guide 40 is in cooperative relation to the
toothed belt between the first sprocket and the second
sprocket. A second linear guide 50 is in cooperative
relation to the toothed belt between the first sprocket
and the second sprocket.
Arcuate section 31 of belt 30 is formed in the
compression span between the first and second sprocket.
The concave arcuate span 31 is with reference to linear
guide 50 in the Figure. Concave span 31 forms in the
described position when sprocket 2 0 is the driving
sprocket. Span 32 will become concave when sprocket 10
is the driving sprocket. The arcuate concave span is
free-standing and only forms on the slack side of the
belt. The concave, slack side of the belt is under an
axial compression.
In Figure 1 span 32 is under tension and span 31 is
under compression. Under reverse torque span 32 will
become concave and span 31 will then be under tension.
In the illustrated example system variables are:
Belt length overall 784mm
Belt pitch length 14mm
Sprocket center distance (A) 219mm
Difference belt length to drive length 10mm
Initial guide gap clearance (B) 2mm
Each sprocket 24 teeth
The belt length is determined by belt pitch length
multiplied by the number of teeth on the belt. For
equivalent sprocket diameters, the drive length is two
times the sprocket center distance (A) , plus the ½ the
number of teeth in the first sprocket (10) plus ½ the
number of teeth in the second sprocket (20) multiplied by
the belt pitch length.
In equation form:
Drive length = 2x (A) + (1/2 number of sprocket 10
teeth) x belt pitch length + (1/2 number of sprocket
2 0 teeth) x belt pitch length
Using two stationary linear guides of nylon or other
suitable low friction material and placing the linear
guides parallel to the belt span can achieve the stable
belt backward buckling, i.e. the concave arc portion,
under both forward and reverse load on the belt slack
side.
The gap (B) between the stationary linear guide 40
and 50 and the belt can be adjustable. A suitable gap
(B) is between 1 mm and 2 mm. Assuming a 10 mm
difference between the belt length and drive length gives
a locked center distance (A) of 219 mm. All numeric
variable values are examples only and are not intended to
limit the invention.
The inventive drive comprises the combination of a
tension span on one side and a compression span on the
other. The compression side of the system can be modeled
as a straight beam subject to an axial compression force.
When the load reaches the critical value, i.e. buckling
load, the beam, or in this case the belt, will buckle.
The buckling shape will depend on the boundary condition,
i.e. clamping support and simple hinged support. This is
called instability of the beam buckling. Whether the
beam will buckle to one side or the other is not
predictable, each having a 50/50 chance as the system has
two potential solutions. By placing a guide on one side
the beam will buckle away from the guide, thereby
becoming a single solution system. That is, the guide
forms a boundary condition. Once formed the concave span
is self -perpetuating, that is, no further mechanical
contact with the concave portion of the belt is required
by any other apparatus such as the linear guide in order
for the concave span to maintain its concave shape during
operation .
A prior art drive, i.e. chain or belt, uses an
idler/tensioner to push the span into the concave arc.
In the prior art system the idler or slack side span is
also under some tension. A chain drive cannot operate
under compression because the chain will collapse due to
the non-rigid link connections.
In the inventive system the proper difference
between the belt length and the drive length must be
selected to assure a stable concave arc portion is
formed. The greater the difference between the belt
length and the drive length, the more pronounced the
concave form of the concave arc portion. If the belt is
too long, the concave arc portion becomes too steep and
belt life is compromised due to the sharp bending of the
belt cord as it passes through the concave arc. If the
belt length is too short, the concave arc portion is very
shallow and un-stable and can buckle forward putting it
in contact with the linear guide. This in turn can
generate noise as the belt slaps against the guide due to
engine torsional vibrations.
Drive length divided by the belt pitch length
determines the number of teeth in the belt. The
expression f can be used where N" is an integer and "f"
is a fractional value. If f = 0 , or the belt length
matches the drive length an "N+l" belt length is
selected, that is, the belt length is one belt pitch
(belt tooth) longer than the drive length. If 0 < f < ½ ,
the "N+l" tooth belt length is selected. If f > , an
"N+2" tooth belt length is selected, that is, the belt
length is greater than one and half pitch of the drive
length and thereby two teeth or twice the belt pitch
length. Using the strategy outlined above, the
difference between belt length and drive length is
between ½ of the belt pitch length and one and half of
the belt pitch length. For example, for a 14mm pitch
belt the range would be (0.5) x 14mm = 7mm and 1.5 x 14mm
= 21mm.
The difference in the overall length of the belt
compared to the drive length can also be described as
follows. Under a tensile load the belt span length
between tangent point 11 and tangent point 2 1 is equal to
the center distance (A) . Formation of a stable concave
portion requires the length of this belt segment 31
between tangent point 11 and 2 1 to be greater than the
center distance (A) . The additional belt length required
is in the range of ½ times the belt pitch length to 1.5
times the belt pitch length. In this example the belt
pitch length is 14mm and so the range of extra belt
length in excess of the drive length is approximately 7mm
to 21mm.
The inventive drive is advantageous for use in lock
center drives. In a lock center drive no provision is
made to change the center distance (A) of the sprockets
because each sprocket mounting position is fixed. As
described, the belt length is slightly longer than the
sprocket center distance (A) which would otherwise cause
both belt spans 31, 32 to be linear and not buckled or
arcuate. The extra belt length is taken by the slack
side concave arc portion, 31, 32.
The inventive drive also has a significant impact on
the cost of a system. The need of the fraction pitch to
match the drive length in a typical system is completely
eliminated due to the longer belt length and the ability
to take up the extra length by the slack side concave
arc .
In operation, the guide 40 or 50 does not contact
the belt. The guide only provides a boundary condition
to assure the concave portion forms in the proper
direction, that is between the sprockets. Otherwise, the
guide does not continually contact the belt once the
concave portion is formed. Nor does the guide contact
the belt in order to maintain the concave shape. The
concave portion is free-standing and requires no
mechanical intervention to maintain the arcuate form.
The guides 40, 50 momentarily guide the belt when the
drive switches the torque, or driver becomes the driven,
and driven becomes the driver. The concave span becomes
the straight span under the torque reversal, but the belt
still does not continually contact the guide due to the
gap (B) .
Figure 2 is a chart of the inducing torque versus
push out force. The belt in this example is 50 mm in
width and has a 14 mm pitch. The push out force is the
force required to push the concave portion away from the
driving sprocket. The push out force increases as the
difference between the belt length and drive length
increases .
The inducing torque is the torque required to induce
the slack side span into the concave arc 31. At start
up, a concave arc portion can exist but the belt will
contact a guide 40 or 50 at the sprocket tangent point
due to the bulging effect. When the inducing torque is
applied, the bulging portion straightens and the gap (B)
develops between the guide and the belt. The lower the
inducing torque the more easily the belt forms the
concave arc portion.
The inducing torque decreases as the difference
between the belt length and the drive length increases.
The difference between the belt length and the drive
length determines the "depth" of the concave or buckled
portion of the belt.
Figure 3 is a chart of the inducing torque versus push
out force. The belt in this example is half the width of
the belt in Figure 2 , that is, 25mm in width.
Although a form of the invention has been described
herein, it will be obvious to those skilled in the art
that variations may be made in the construction and
relation of parts without departing from the spirit and
scope of the invention described herein.

Claims
I claim:
1 . A toothed belt drive with a compression span
comprising :
a first sprocket;
a second sprocket;
a toothed belt having a toothed belt length and
trained between the first sprocket and the second
sprocket ;
a first linear guide member in cooperative relation
to and disposed a predetermined distance (B) from the
toothed belt;
a second linear guide member in cooperative relation
to and disposed a predetermined distance (B) from the
toothed belt; and
the toothed belt length greater than a drive length
such that the toothed belt forms a free-standing arcuate
span between the first sprocket and the second sprocket
on a toothed belt compression span.
2 . The belt drive as in claim 1 wherein the difference
between the drive length and the toothed belt length is
at least ½ times a toothed belt pitch length.
3 . The belt drive as in claim 1 wherein the arcuate span
is concave with respect to the first linear guide member.
. The belt drive as in claim 1 wherein the arcuate span
is concave with respect to the second linear guide
member.
5 . A toothed belt drive having a compression span
comprising :
a first sprocket;
a second sprocket;
a toothed belt having a toothed belt length, the
toothed belt trained between the first sprocket and the
second sprocket;
a first linear guide member in cooperative relation
to a toothed belt first span;
a second linear guide member in cooperative relation
to a toothed belt second span; and
the toothed belt first span having an arcuate
concave form when in compression.
6 . A toothed belt drive having a compression span
comprising :
a first sprocket;
a second sprocket;
a toothed belt having a toothed belt length and
trained between the first sprocket and the second
sprocket ;
a first linear guide member in cooperative relation
to the toothed belt and having a predetermined clearance
distance (B) from the toothed belt;
a second linear guide member in cooperative relation
to the toothed belt and having a predetermined clearance
distance (B) from the toothed belt; and
the toothed belt length greater than a drive length
by at least one belt pitch length such that the toothed
belt forms an arcuate span between the first sprocket and
the second sprocket.