Field of the Invention
[0001] The present invention relates to chain saws, and more particularly, to a chain braking
arrangement for chain saws.
Background
[0002] Chain saws are prone to exhibit a phenomenon called "kickback." Kickback typically
occurs when the tip of the saw contacts the wood being cut, or another solid object,
resulting in a violent upward motion which can endanger the operator. The risk of
injury can be minimized by stopping the saw chain whenever a kickback occurs or whenever
the operator is at risk of coming into contact with the saw chain.
[0003] Various chain braking arrangements have been discovered and are disclosed in the
prior art. Some devices use a safety handle which actuates the chain brake when the
handle is struck by the operator's hand or wrist. Other braking devices use an inertial
sensor to actuate the brake when a kickback or other violent motion of the saw occurs.
Both such devices are disclosed in Todero, U.S. Pat. No. 4,420,885.
[0004] Moore, U.S. Pat. Nos. 3,739,475 and 3,793,727 disclose a braking arrangement which
uses a brake drum and brake shoes. The brake drum is fixedly connected to a chain
sprocket which drives a saw chain, and the brake shoes make frictional contact with
the outer circumference of the brake drum to stop the saw chain.
[0005] Hirschkoff, U.S. Pat. No. 4,059,895 discloses a band brake for a chain saw comprising
a flexible metallic band which is wrapped around a brake drum. One end of the band
is fixed to the chain saw casing or chassis. The other end is attached to a mechanical
linkage via a biasing means, typically a spring. In a brake-off condition, the band
is loosely wound around the brake drum, and is not in contact therewith. In a brake-on
condition, the band is tightened around the brake drum by the biasing means such that
the friction between the band and the brake drum stops the saw chain substantially
instantaneously.
[0006] Wieland et al., U.S. Pat. No. 4,662,072 discloses a band brake for a chain saw wherein
a brake drum is located on a motor drive shaft. This patent alternatively discloses
that the brake drum can be located elsewhere in the drive train of the saw. Other
prior art designs use the outer casing of a centrifugal clutch as the brake drum.
See, e.g., Hirschkoff, U.S. Pat. No. 4,026,392.
[0007] Typically, chain saw brakes are only used in the event of a kickback or other violent
motion of the saw. In the future, it is anticipated that one or more regulatory agencies
will require that chain saw brakes must be activated whenever the throttle or trigger
is released. Wieland et al., U.S. Pat. No. 4,680,862 discloses a chain saw comprising
an electric coasting brake in addition to a mechanical safety brake. Such an arrangement
ensures that the operator will not come into contact with a moving saw chain as the
saw chain coasts to a stop after the throttle or trigger is released. Further, it
may be desirable to use the safety brake itself as a coasting brake. However, this
will require an extremely durable brake mechanism since the brake will be applied
at least once every time the saw is used.
[0008] Many saws, especially those with electric motors as their means for motive power,
use a reduction gear to ensure that the saw chain is driven at an optimal speed. In
order to produce a saw with a compact design, chain saws using a cup-shaped or ring-shaped
gear have been developed. Such a gear is generally cylindrical in shape and has teeth
disposed on the inner surface of a circumferential wall of the gear which are engaged
by a motor-driven pinion. A cup gear is disclosed in Haupt et al., U.S Pat. No. 3,857,179
and in the above-noted Wieland et al. '072 patent. A ring gear is disclosed in Irgens,
U.S Pat. No. 3,669,162.
[0009] Cup gears and ring gears have numerous advantages over conventional gears, such as
the conservation of space in the chain saw housing. Cup gears also protect the gear
and pinion mechanisms from contaminants, such as sawdust and dirt. As discussed in
the Irgens '162 patent, cup gears and ring gears can also reduce the torque on the
drive shaft by minimizing the moment arm acting on the drive shaft. In addition, cup
gears and ring gears have relatively large rotational moments of inertia, when compared
to conventional gears with external teeth. As discussed in the Haupt et al. '179 patent,
this results in enhanced performance of the chain saw, especially at low speeds.
Summary of the Invention
[0010] According to one aspect of the present invention, a cup gear is utilized as a braking
structure in a chain saw. More specifically, a pinion driven by a drive shaft of a
motive power means drives a cup gear which in turn drives a chain sprocket. The cup
gear carries a braking surface which is acted upon by a braking means to selectively
bring the saw chain to a halt.
[0011] The braking surface can be an outer circumferential surface of the cup gear. Alternatively,
the braking surface can be an outer circumferential surface of a brake drum secured
to the outside of the cup gear. An electric motor can be used as the motive power
means in a chain saw embodying the present invention, and a brake band can be used
as the braking means. The pinion can be directly driven by the drive shaft for rotation
therewith at all times during operation of the chain saw. Similarly, the chain sprocket
can be directly driven by the cup gear for rotation therewith at all times during
the operation of the chain saw. A mechanical linkage may be used to activate the braking
means.
[0012] In an embodiment of the present invention, a band brake is used in an electric chain
saw having a cup gear as a reduction gear. The band brake comprises a flexible metallic
band having two ends and a brake drum fixedly attached to the outer surface of the
cup gear. A mechanical linkage is used to actuate the band brake. The mechanical linkage
comprises a pivotally mounted safety brake lever which carries a pin engaging a cam
follower plate attached to the brake band. During a brake-on condition, a helical
compression spring biases the brake band in tension tightly around the brake drum.
[0013] The present invention allows for significant weight reduction in the chain saw braking
mechanism. The invention also provides for a compact braking configuration, thereby
reducing the width of the chain saw and the effective moment arm acting on the drive
shaft during braking. Furthermore, a chain saw constructed in accordance with one
embodiment of the present invention wherein the outer circumference of the cup gear
is used as a braking surface will have fewer parts, thereby reducing manufacturing
costs.
[0014] Other features and advantages are inherent in the apparatus claimed and disclosed
or will become apparent to those skilled in the art from the following detailed description
in conjunction with the accompanying drawings.
Brief Description of the Drawings
[0015]
Fig. 1 is a perspective view, taken from the rear and above, of a chain saw embodying
the present invention;
Fig. 2 is a fragmentary elevational view of the chain saw of Fig. 1 with the gear
housing removed and portions broken away illustrating a chain brake mechanism in the
brake-off position;
Fig. 3 is a view similar to Fig. 2 illustrating the chain brake mechanism in the brake-on
position;
Fig. 4 is a cross-sectional view taken generally along the lines 4-4 of Fig. 2 of
the chain brake mechanism and pinion together with a motor; and
Fig. 5 is a view similar to Fig. 4 of an alternative embodiment of the present invention.
Detailed Description of the Drawings
[0016] Referring to Fig. 1, a chain saw 10 includes a saw chain 12, a guide bar 13, a rear
handle 14, a front handle 16, a motor housing 18, a gear housing 19, and a safety
brake lever 20.
[0017] Figs. 2 and 4 show a braking and drive mechanism 21 of the present invention in a
first or brake-off position or configuration. A pinion 22 is fixedly attached to a
motor drive shaft 24 which is driven by a motor 25. A plurality of pinion teeth 26
disposed on the pinion 22 engage a plurality of cup gear teeth 28 disposed on an inner
surface 30 of a cylindrical wall 32 of a cup gear 34. A chain sprocket 36 is fixedly
attached to the cup gear 34. The chain sprocket 36 drives the saw chain 12 as it moves
along the guide bar 13. It should be noted that the bottom side of the saw chain 12
moves toward the chain sprocket 36. Thus the chain sprocket 36, cup gear 34, and pinion
22 all rotate in a clockwise direction as indicated in Fig. 2 by an arrow 37.
[0018] Although a chain saw powered by an electric motor 25 is depicted in the drawings,
the present invention is also applicable to chain saws powered by internal combustion
engines.
[0019] Referring to Fig. 4, the braking and drive mechanism 21 comprises a brake band 38
disposed around a brake drum 40. The brake drum 40 is rigidly attached to the cup
gear 34 by a plurality of stamped bosses 42 which extend into a plurality of corresponding
bores 44 in the cup gear 34. Each boss 42 includes an enlarged head 46 which, together
with an axial face 48 of the brake drum 40, capture the cup gear 34 therebetween.
Alternatively, the brake drum 40 could be secured to the cup gear 34 by another means
of attachment, such as bolts, screws, or rivets.
[0020] A motor housing partition 50 separates the motor 25 from the braking and drive mechanism
21. As seen specifically in Fig. 2, an angle bracket 51 having a vertical portion
52 and a horizontal portion 54, and a guide bar support 56 protrude from the motor
housing partition 50 and form a channel 58 therebetween. A fixed end 60 of the brake
band 38 is bent at approximately a right angle so that it fits in the channel 58 and
bears against the vertical portion 52 of the angle bracket 51. Other means of securing
the fixed end 60 of the brake band 38 may alternatively be used. A moveable end 62
of the brake band 38 is secured by any suitable means to a cam follower plate 63.
[0021] One suitable means of attachment comprises the following arrangement. The moveable
end 62 of the brake band 38 may be twisted 90 degrees so that it aligns with an outward
facing surface 64 of the cam follower plate 63. The moveable end 62 of the brake band
38 may be attached to the cam follower plate 63 by a post (not shown) extending from
the cam follower plate 63 which engages a hole (not shown) in the moveable end 62
of the brake band 38.
[0022] The cam follower plate 63 is pivotally mounted to the motor housing partition 50
by a cam follower plate mounting pin 65.
[0023] A fixed end 66 of a helical compression spring 68 is fixed within a retaining hole
70 in the motor housing partition 50. A moveable end 72 of the helical compression
spring 68 bears against a rearward side 73 of the cam follower plate 63, and may also
be attached thereto by any suitable means. A roller pin 74 carried by the safety brake
lever 20 engages a forward side 75 of the cam follower plate 63. The safety brake
lever 20 is pivotally mounted to the motor housing partition 50 by a safety brake
lever mounting pin 76.
[0024] In the brake-off configuration (as depicted in Fig. 2), the safety brake lever 20
is disposed in a substantially upward or vertical position and the roller pin 74 engages
an indentation 78 and a lobe 80 on the cam follower plate 63 such that the cam follower
plate 63 is prevented from rotation about the cam follower plate mounting pin 65.
Thus, the helical compression spring 68 is in a compressed state and the brake band
38 is loosely wrapped around the brake drum 40 such that the brake band 38 is not
in contact therewith.
[0025] As seen in Fig. 2, an arm 82 on the safety brake lever 20 engages a first arm 84
of a brake switch actuator 86 pivotally mounted on the motor housing partition 50
at a brake switch actuator mounting pin 88. A second arm 90 of the brake switch actuator
86 engages a spring-loaded plunger 92 of a microswitch 94. The microswitch 94 is configured
such that electrical power is supplied to the motor 25 (Fig. 3) when the plunger 92
is depressed. The plunger 92 is pushed outward from the microswitch 94 by a spring
(not shown) such that the plunger 92 is extended and the microswitch 94 is open in
the absence of pressure on the plunger 92.
[0026] A second or brake-on position or configuration is depicted in Fig. 3. When the safety
brake lever 20 is pushed forward (to the right in Figs. 2 and 3), for example by a
user's hand during a kickback condition, the roller pin 74 moves upwardly, releasing
the cam follower plate 63 for clockwise rotation under the influence of the force
applied by the helical compression spring 68. The brake band 38 thus is pulled tightly
around the brake drum 40 and the friction between the brake band 38 and a cylindrical
flange 96 (Fig. 4) of the brake drum 40 brings the saw chain 12 (not shown in Fig.
3) quickly to a halt.
[0027] In addition, in the brake-on position, the plunger 92 of the microswitch 94 is extended
and the microswitch 94 is open. Thus, electrical power is cut off from the motor 25
when the safety brake lever 20 is in the brake-on position.
[0028] As seen in Fig. 4, the brake drum 40 surrounds the cup gear 34 and the cylindrical
flange 96 of the brake drum 40 provides a braking surface 97 with which the brake
band 38 comes into frictional contact during braking. Although not necessary, the
cylindrical flange 96 of the brake drum 40 may be radially spaced outwardly from an
outer cylindrical surface 98 of the cup gear 34. This separation facilitates air flow
around the cylindrical flange 96 of the brake drum 40 for better cooling. Also, it
helps minimize heat transfer from the brake drum 40 to the cup gear 34. As a result,
the cup gear 34 can be made of a less durable, more inexpensive material, such as
plastic.
[0029] Fig. 5 shows an alternative embodiment of the present invention wherein elements
common to Figs. 2-4 are assigned like reference numerals. In this embodiment, an outer
surface 198 of a cup gear 134 is utilized as a braking surface. In such a design,
the cup gear 134 must be manufactured from a durable material, such as steel. Although
this design results in more weight in the cup gear 134, the absence of a separate
brake drum (40 in Fig. 4) simplifies the manufacture of the chain saw. Also, the additional
inertia of a heavier cup gear 134 improves performance of the saw at low speeds.
[0030] Numerous modifications and alternative embodiments of the invention will be apparent
to those skilled in the art in view of the foregoing description. Accordingly, this
description is to be construed as illustrative only and is for the purpose of teaching
those skilled in the art the best mode of carrying out the invention. The details
of the structure may be varied substantially without departing from the spirit of
the invention, and the exclusive use of all modifications which come within the scope
of the appended claims is reserved.
1. A chain saw, comprising:
means for developing motive power having a drive shaft;
a pinion connected to the drive shaft for rotation therewith and having pinion
teeth;
a rotatable cup gear having internal teeth engaged by the pinion teeth and carrying
a braking surface;
a chain sprocket connected to the cup gear for rotation therewith;
braking means surrounding the braking surface; and
means for moving the braking means between a first position wherein the cup gear
is free to rotate in response to rotation of the drive shaft and a second position
wherein rotation of the cup gear is arrested by the braking means.
2. The chain saw of claim 1, wherein the braking surface comprises an outer circumferential
surface of the cup gear.
3. The chain saw of claim 1, wherein the braking surface comprises an outer circumferential
surface of a brake drum secured to the cup gear.
4. The chain saw of claim 1, wherein the means for developing motive power is an electric
motor.
5. A chain saw, comprising:
a motor having a drive shaft;
a pinion connected to the drive shaft for rotation therewith and having pinion
teeth;
a rotatable cup gear having internal teeth engaged by the pinion teeth;
a chain sprocket connected to the cup gear for rotation therewith;
a brake drum surrounding the cup gear and rotatable therewith at all times during
operation of the chain saw;
braking means surrounding the circumference of the brake drum; and
means for moving the braking means between a first position wherein the brake drum
and cup gear are free to rotate in response to rotation of the drive shaft and a second
position wherein rotation of the brake drum and the cup gear is arrested by the braking
means.
6. The chain saw of claim 5, wherein the braking means comprises a brake band.
7. The chain saw of claim 5, wherein the pinion is directly driven by the drive shaft
for rotation therewith at all times during operation of the chain saw.
8. The chain saw of claim 5, wherein the chain sprocket is directly driven by the cup
gear for rotation therewith at all times during operation of the chain saw.
9. A chain saw, comprising:
a motor having a drive shaft;
a pinion connected to the drive shaft for rotation therewith and having pinion
teeth;
a rotatable cup gear having internal teeth engaged by the pinion teeth;
a chain sprocket connected to the cup gear for rotation therewith;
a brake drum surrounding the cup gear and rotatable therewith at all times during
operation of the chain saw;
a brake band surrounding the circumference of the brake drum;
a mechanical linkage for moving the brake band between a first position wherein
the brake drum and cup gear are free to rotate in response to rotation of the drive
shaft and a second position wherein rotation of the brake drum and the cup gear is
arrested by the brake band; and
means for applying a biasing force to the brake band in the second position.
10. The chain saw of claim 9, wherein the brake drum is rigidly attached to the outer
periphery of the cup gear for rotation therewith.
11. The chain saw of claim 9, wherein the mechanical linkage for moving the brake band
comprises a pivotally mounted lever which is coupled to the brake band.
12. The chain saw of claim 9, wherein the mechanical linkage for moving the brake band
comprises a pivotally mounted lever carrying a pin engaging a cam follower plate which
is attached to the brake band.
13. The chain saw of claim 9, wherein the pinion is directly driven by the drive shaft
for rotation therewith at all times during operation of the chain saw.
14. The chain saw of claim 9, wherein the chain sprocket is directly driven by the cup
gear for rotation therewith at all times during operation of the chain saw.
15. The chain saw of claim 9, wherein the applying means is a helical compression spring.