Note: Descriptions are shown in the official language in which they were submitted.
11365~9
The present invention relates to a circular saw blade
and more specifically it relates to circular saw blades of the
type which comprise a toothed annular peripheral portion, an
interior portion and strips connectino these to each other.
Gang saws, circular saws or band saws may be used
for sawing timber. In view of the fact that the operation of
a gang saw is cyclic, the continuously operating circular and
band saws have become more popular because of high yield re-
quirements. Both these types of saw have, however, some disad-
vantages. When a circular saw blade rotates, it is subjected
to centrifugal forces. A state of tension, which ~y be defined
in the form of radial and tangential stresses, arises in the
blade. Of these stress directions the radial stress is the more
detrimental because it makes the natural vibration frequency of
the blade approach the drive speed of the saw. When the vibration
frequency of the circular blade coincides with the drive speed,
it increases in amplitude, so that the blade diverges from the
sawing direction. To obviate this disadvantage,the vibration
frequency has been increased to avoid coincidence with the ro-
tational frequency by prestressing the blade. However, when
the outer periphery of the blade warms up and the teeth become
blunt, the vibration fre~uency often causes deformation of the
blade, because the sides of the blade touch the timber and
friction heats and deforms the blade.
In order to minimize this disadvantage, it has been
proposed to stretch the blade near the hub by ~ammering so that
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the radial stress due to rotation of the blade may be sub-
stantially diminished and excess vibration of the blade avoided.
For the same purpose it has also been proposed to tension the
blade hydraulically by means of an expanding hub. The decrease
of the radial stress by these means increases the tangential
stress of the blade especially at the outer periphery and this
permlts the blade to better withstand an increasa of temperature
of its outer periphery so that the sawing accuracy is improved.
This method gives satisfactory results with reasonable blade
thicknesses if the peripheral speed of the blade is not too high,
i.e. maximum 45-60 meters per second. However, very high peri-
pheral speeds make the hammering difficult and the results are
inaccurate so that they do not meet practical requirements.
It would be desirable, however, to achieve higher peripheral
speedsbecause they would allow higher feed speeds and higher
yields.
It has been suggested to counteract the effect of the
rise in temperature during sawing, that is the changes in radial
stresses and vibration frequency,by providing the circular saw
with a series of apertures in an annular arrangement along the
toothed periphery of the blade.
Curved perforations near the center of the blade have
been known, for instance they were suggested in U.S.P. 212,B13
which issued in 1879. In U.S.P. 1,083,645 which issued in 1914,
the perforations are located between the center and the toothed
edge and are so shaped that the portions between the perforations
form curved spokes. Some improvement has been achieved because
the slots facilitate the expansion at the periphery.
German patent 1, 959, 323 describes the use of eight
curved perforations similar to U.S.P. 1,083,645. German
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Offenlegungsschrift 2,654,625 describes the use of several
radial slot-type perforations which are arranged on several
diameters and the inner slots are so arranged in relation to the
outer slots that the angle between an outer and an inner slot is
less than one-half the angle between two outer slots. One dis-
ad~antage resulting from the blade of this patent is that the
strips run in the same direction as the rotation and are sub-
jected to compressive stresses by the drivin~ torque.
More recently U.S.P. 3,872,763 uses a series of
apertures in an annular configuration close to the edge and a
series of strips connecting the inner portion and the peripheral
edge portion, the strips being inclined substantially in the
same direction as the inclination of the teeth. The peripheral
portion, however is very narrow and the stiffness of the blade
decreases. In order to obviate this drawback, the perhipheral
portion must be made thick, a fact which causes sawing losses.
Further, frictional heat is still transmitted to the periphery
because the body of the blade is large with respect to the
peripheral portion. In conclusion, the improvements achieved
by all these proposals have not been substantial and still
hia,h feed rates cannot be achieved, deformation is likely to
occur with the result that the workpieceshave an irregular
surface.
An object of the present invention is to provide a
circular saw blade which is not subjected to substantial de-
formation during operation so that the surface of the workpiece
is smooth and regular. Still another object is to provide a
circular saw blade which does not reguire to be stiff~ Still
another object is to provide a circular saw blade which may be
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operated at high rate, with great accuracy and small material
losses.
The crux of the present invention resides in providing
a saw blade having a circular outer band which is not necessarily
stiff because the req~ired rigidity is achieved by stretching the
band in a manner similar to a gang saw or a band saw. The
stretching is accomplished as a result of the centrifugal force,
so that the peripheral speed is twice as high as the speed of
a conventional circular saw.
Characteristic features of the invention are that the
strips connecting the interior portion and the peripheral portion
do not essentially prevent the peripheral portion from expanding
and that the width of the peripheral portion is so great that,
due to radial acceleration, the weight of the peripheral portion
can give the required tension when the blade rotates. For this
purpose the width of the peripheral portion W must be 10.-30~,
preferably 15.-25~ of the diameter of the blade and the length
of the flexible portïon of the strips is at least 10% of the
diameter of the blade. Because of the small width of the
peripheral portion, preferably up to 25% the diameter of the
blade, the difference between the tangential stresses on the
inside and outside of the peripheral portion caused by the
centrifugal force, is small and the tendency to buckling is
eliminated.
When the blade rotates, the tangential stress of the
peripheral portion is approximately three times greater than
the stress in the same area in a conventional circular saw
blade. The maximum radial stress, on the other hand, is only
about 10% of the corresponding stress in a conventional circular
saw blade~ No vibration problems arise and the peripheral spee~
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can be increased according to the strength of the material.
The tension of the peripheral portion with a blade thic~ness of
a~out 3 mm,at a peripheral speed of about 100 meters/second
is the same as that in a band saw but the static stress is
only one th~rd of the,stress in a band saw. If one spot of the
peripheral,portion is heated by friction, the width of the peri-
pheral portion does not hinder the elastic expansion of this
spot in radial direction. Tangential expansion only causes an
increase of the diameter of the peripheral portion~ During
sawing,the peripheral portion of the blade must be guided
laterally by guides which determine the operation position of
the blade. The centrifugal force stretches the blade in the
tangential direction and keeps the tension constant. The blade
is made out of a circular saw blade by perforating its interior
portion so as to form strips which do not hinder expansion or
stretching of the peripheral portion but are capable of trans-
mitting to it the rotating force and withstanding the feeding
force. In the saw blade of this invention the aperture zone
cannot essentially guide the peripheral portion of the blade
laterally so that the potential vibrations of the interior portion
are not transmitted to the peripheral portion.
Another feature of the present invention is the great
flexibility of the blade. Essentially, the band is attached to
the shaft through flexible ribs so that the stiffness of the
ribs does not considerably prevent the stretching of the band.
The flexibility X is defined as follows:
L sin
K = B x
in which L = the length of the centre line of the flexible portion
of the ri~ and B = the greatest width of the rib of this length;
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= the angle between the radius running through the centre point
of the flexible portion and the tangent of the intersection point
of the radius and the centre line; N = the number of ribs. The
length L is chosen to be such that together with B, gives the
maximum value to the ratio L/B. In addition, the measurements
for L and B are taken for the stiffest rib, if there are differ-
ences between the ribs. It has been found that if the value of K
exceeds 1/3, the blade is considered to be within the scope of
the invention.
The strips may be inclined to the radius through the
center point or may be at right angles but must not run in the
direction of the rotation so as to be subjected to compressive
stresses by the driving torque.
The invention is described hereinbelow in more detail
with reference to the enclosed drawings in which
Fig. 1 shows a schematic side view of a sawing machine
provided with a circular saw blade according to the invention,
Fig. 2 shows an apparatus according to Fig. 1 in the
direction of the arrow A,
Fig. 3 shows an alternative embodiment of a circular
saw blade.
In Figs. 1 and 2 the numeral 1 refers to a timber to
be sawn which is fed by the feed roll pairs 2, 3 and 4,5 of the
sawing machine. The sawing machine has a circular saw blade 6
consisting of an annular toothed peripheral portion 7 and an
interior portion 8 which are connected to each other by strips 9
made of the same blade blank. The st~ips form an angle with the
radius of the blade so as not to hinder the expansion and stretch-
ing of the annular peripheral portion. The interior portion of
the saw blade is connected to the drive shaft 10 of the sawing
machine by known methods. Guiding elements 11 guide the blade
laterally.
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In the embodiment of the saw blade 6 shown in Fig. 3 the
annular toothed peripheral portion 7 and the inside portion 8 are
connected to each other mainly by tangential strips 9.
The stresses at different temperatures in a circular
saw blade according to Fig. 3 were determined mathematically.
The number of revolutions was 1800 4/min. Blade dimensions
outer diameter 1000 mm
thickness 2.8 mm
width a of peripheral portion 190 mm
length b of flexible
portion of the strips220 mm
width of the strips 50 mm.
It has been shown that due to stiffness of the strips,
the tension of the peripheral portion at an even temperature is
only 2.6~ lower and when the peripheral portion is 100C hotter
than the interior portion, 13.5% lower than the tension of a
freely rotating annular blade. The tensioning force of the
periphery is more than 2000 kg which gives it a greater stiff-
ness in the lateral direction than is possible to achieve in an
equally thick circular blade. The peripheral portion is kept
in the sawing direction by its own tension and by guides located
at its sides outside the sawing area. The interior portion in-
creases the lateral stiffness of the connection only to a minimal
extent.
By way of comparison, the slot structure presented in
Fig. 1 of German patent 1,959,323 gives the following values:
CC= 70, B = 4 mm, L = 7 mm and N = 8. Thus the value of the
flexibility is K = L 5i ~ = 7 x sin 70 = O,2056~ 1/3
A sawing machine provided with a circular saw blade
according to the invention has several advantages when compared
to a conventional circular saw, namely the peripheral speed
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may be increased, no hammering is required, the blade may be
thinner, the noise level of sawing is decreased due to elimination
of vibration, sawing accuracy is superior and the surface quality
is better than with other known annular saw blades.
Still another advantage of the saw blade of the
present invention is that the structural stiffness of the band
is almost insignificant, because it is only 5~ of the stiffness
caused by the stretching force. On the other hand structural
stiffness of the blade in the blades of known circular saws
is very substantial when compared to that caused by the stretch-
ing force.
When compared with a band saw, the present invention
offers the advantages that the peripheral speed and sawing
efficiency are higher, there are no dynamic bending stresses,
the blade can be made thicker, so that also the static strength
is better, sawing accuracy is superior, teeth made of hard
~etal may be used, due to hard metal teeth and sawing direction
the quality of the surface is better, the machine construction
is less expensiv~ The blades and maintenance of blades are also
less expensive.
