Chain tensioner for chain saw

A chain tensioner for a chain saw has a cam member in positive engagement with a flange of a plate attached to the guide bar. Rotation of the cam member forces the guide bar away from a drive sprocket of the chain saw to tension the chain. The cam member is clamped in position to maintain the guide bar in the adjusted position. Flats on the cam member in abutment with the flange resist rotation of the cam member when in the adjusted and clamped position. A slot is provided in the cam member to facilitate rotating the cam to force the guide bar away from the drive sprocket.

FIELD OF THE INVENTION 
This invention relates to a mechanism for mounting a guide bar and chain to 
a power head of a chain saw and more particularly it relates to that 
mechanism which achieves proper tensioning of the chain on the guide bar. 
BACKGROUND OF THE INVENTION 
A chain saw includes a power head having a drive shaft that drives a 
sprocket and a guide bar having a peripheral guide edge. The guide bar is 
mounted relative to the sprocket so that a loop of saw chain is guided in 
a path from the sprocket onto the guide edge of the bar (a groove in the 
periphery of the bar). The guide path continues around the nose of the bar 
in an oval-like pathway that leads back along the opposite side edge of 
the bar and back onto the sprocket. Because of the forces applied during 
cutting, the chain is required to tightly fit around the sprocket and 
guide bar to avoid having the chain jump free of the sprocket and/or bar 
edge. 
Typically the bar is mounted so as to have limited sliding movement toward 
and away from the sprocket. A screw and nut assembly on the power head 
controls the sliding movement and a clamping member clamps the relative 
position of the bar when the desired tension is obtained. 
When mounting a chain onto the bar and sprocket, the clamping member is 
loosened and the screw and nut assembly is manipulated first to allow 
mounting of the chain and then to tension the chain, at which position the 
clamping member clamps the bar in that position relative to the sprocket. 
Unfortunately that is not the end of the tensioning procedure. During 
operation of the chain saw, the chain develops slack either due to 
stretching and/or wearing of the components or perhaps due to some 
slipping of the bar relative to the clamping member. Repeatedly during 
operation of the chain saw, the operator will have to stop cutting, 
un-clamp the clamping member, manipulate the screw and nut assembly 
(typically located on the opposite side of the bar) and re-clamp the 
clamping member. 
As consumer saws have become more and more popular (as compared to 
professional saws used by loggers), the process of chain tensioning has 
become more of a problem for the operator. The operator often doesn't 
remember the process or will attempt manipulation of the screw and nut 
assembly without releasing the clamping member. This can result in 
stripping the threads or otherwise damaging the assembly. At the least, 
the process is seen to be time consuming and a source of frustration to 
the consumer/operator. It is accordingly an object of the present 
invention to simplify the chain tensioning procedure to make it safer and 
more convenient without adding significant cost to the manufacture of the 
chain saw. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention accomplishes the above-stated objective by combining 
the clamping and tensioning mechanisms (the tensioning mechanism of the 
invention replacing the above-described screw and nut assembly). Although 
not described above, the clamping mechanism on typical chain saws includes 
a pair of mounting studs that fit through elongated slots in the bar (thus 
allowing sliding adjustment). The studs are threaded and a clamping nut is 
threaded onto each stud and when the saw chain is appropriately tensioned, 
the clamping nuts are cinched down against the bar. 
In the preferred embodiment of the invention, one of the studs is used only 
for alignment, i.e., in cooperation with the second stud it allows linear 
sliding movement only of the bar. The second stud is provided with a cam 
member which may resemble a washer having an off center, stud receiving 
opening. With respect to a particular segment of the periphery of the 
washer, the distance from the stud gradually increases. A bearing surface 
or shoulder affixed to the bar engages the periphery of the cam member at 
the point on the periphery that is positioned outwardly from the sprocket. 
Turning the cam so that the shortest distance of that segment of the 
periphery is engaging the bearing surface, allows retraction of the bar 
and mounting of the saw chain. Turning of the cam to thereby increase the 
distance of the contact point from the stud forces outward sliding 
movement of the bar and thereby tensioning of the chain. Such turning is 
preferably facilitated by the provision of a tool such as a screwdriver, 
the cam member being provided with a slot for receiving the blade end of 
the screwdriver and whereby turning of the screwdriver induces turning of 
the cam member. 
In one embodiment, a separate plate is mounted between the clamping nut and 
the bar, the plate having a projecting finger projected into a hole 
provided in the bar. A typical guide bar is provided with oil holes that 
can serve this purpose. With the finger projected in the hole (e.g., an 
oil hole), the plate is fixed linearly relative to the bar and forced 
sliding movement of the plate results in similar forced sliding movement 
of the bar. The plate has a flange (a bearing surface) against which the 
cam member is engaged. Other embodiments are envisioned, e.g., with a 
flange provided integrally with the bar; with the bearing surface provided 
as an indentation or slot; and so on. 
Again for the preferred embodiment, the periphery of the cam member is 
formed by short intersecting flat edges or other configurations that 
initiate seating of the cam member against the flange at sequential 
positions on the periphery. 
A prior art saw chain tensioner is illustrated in U.S. Pat. No. 5,491,899. 
The mechanism of this patent is designed for finger (tool less) 
manipulation whereby an oversized camming member having a helical slot 
enables extensive rotative turning of the member, preferably 360 degrees 
to 450 degrees. The mechanism is more complex and adds undesired cost. The 
present invention excels in its simplicity and lower cost and will be more 
fully appreciated by reference to the following detailed description and 
the drawings referred to therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 illustrate a chain saw 10 incorporating a chain tensioner of 
the present invention. The saw 10 has a housing 12 which incorporates a 
conventional motor which drives a drive sprocket 14. The drive sprocket 14 
is arranged to propel an endless saw chain 16 around a guide bar 18. The 
guide bar 18 has a conventional guide groove 24 around the periphery in 
which the drive tangs of the saw chain 16 will travel as it is being 
propelled by the drive sprocket 14. The guide bar 18 is mounted to the 
housing 12 on studs 28 and 30 that extend from the housing 12. The guide 
bar 18 has a slot 20 (best seen in FIG. 1) that has a width that 
corresponds closely to the diameter of the studs 28, 30 with the slot 20 
permitting sliding adjustment of the guide bar 18 along the studs 28, 30 
as indicated by directional arrow 32. Typically the studs 28, 30 are 
threaded to receive clamping nuts 34. The clamping nuts 34 are utilized to 
clamp the guide bar 18 against the housing 12. The guide bar 18 has oil 
holes 22 to provide lubrication to the saw chain 16 and the guide groove 
24 of the guide bar in a conventional manner. As will be explained, these 
oil holes 22 also serve as adjusting holes. Use of the same holes as both 
oil holes and adjusting holes is a benefit of the preferred embodiment but 
the provision of separate holes for such use is contemplated. 
Initially the guide bar 18 is loosely mounted on the studs 28, 30 and is 
moved toward the drive sprocket 14 to permit mounting the saw chain 16 on 
the guide bar 18 and the drive sprocket 14. When the saw chain 16 is 
entrained around the drive sprocket 14 and the guide bar 18, the guide bar 
18 is moved away from the drive sprocket 14 (with nose 26 of the guide bar 
18 being lifted upwardly as indicated by arrow 27) to thus tension the saw 
chain 16 on the guide bar 18 and the drive sprocket 14. When the guide bar 
18 has been moved outwardly a sufficient distance to provide the proper 
tension for operation of the saw chain 16, the guide bar 18 is secured in 
the adjusted position. 
The chain tensioner of the present invention has a mechanism that will move 
the guide bar 18 outwardly from the drive sprocket 14 to the desired 
adjusted position and will maintain the guide bar 18 in that position. In 
this embodiment, the guide bar 18 is mounted on the studs 28, 30 as 
previously mentioned. The chain tensioner of the present invention has a 
plate 40 that is mounted on the studs 28, 30 adjacent to the guide bar 18. 
The plate 40 has projecting fingers 46 (studs) that engage the 
oil/adjusting holes 22 of the guide bar 18. The fingers 46 engaging the 
holes 22 couples the guide bar 18 and the plate 40 together so that the 
plate 40 and the guide bar 18 will move as a unit. 
A cam member 50 is rotatably mounted on the stud 30 adjacent to the plate 
40 and is arranged to engage a flange 42 of the plate 40. The nut 34 is 
loosely mounted on the stud 30 to hold the assembly on the studs 28, 30. 
The cam member 50 is initially installed such that the guide bar 18 may be 
moved toward the drive sprocket 14. With the guide bar moved toward the 
sprocket 14, the saw chain is entrained around the guide bar 18 and the 
drive sprocket 14. 
Rotation of the cam member 50 forces the peripheral edge 52 of the cam 
member 50 against the flange 42 of the plate 40 causing the plate 40 and 
the guide bar 18 to move away from the drive sprocket 14. The cam member 
50 is preferably mounted on the stud 30 such that rotation of the cam 
member in the direction indicated by arrow 53 in FIG. 2 will engage the 
flange 42 below the center line defined by studs 28, 30 and move the guide 
bar away from the drive sprocket 14. Whereas the cam action will provide 
the tensioning function in either clockwise or counter clockwise rotation 
of the cam member 50, the counter clockwise rotation (arrow 53 in FIG. 2) 
produces a lifting action against the nose end of the bar which is 
desirable. In a clockwise rotation of the cam member 50, the engagement 
with flange 42 will be above the center line defined by studs 28, 30 and a 
downward force vector will result. In this case it may be desirable to 
provide independent lifting of the nose end of the bar during tensioning. 
The cam member has multiple flats 58 on its peripheral edge 52 and one of 
the flats 58 will be in abutment with the flange 42. When the bar 18 is 
moved outwardly to the desired adjusted position (including moving the 
nose 26 of the bar 18 upwardly as indicated by arrow 27), the nut 34 is 
tensioned on stud 30. Tensioning the nut 34 clamps the cam member 50, the 
plate 40 and the guide bar 18 against the housing 12. In addition to the 
clamping force of the nut 30, the peripheral edge 52 (a flat 58) of the 
cam member 50 is in contact with the flange 42 of the plate 40 to further 
prevent the guide bar 18 from moving toward the drive sprocket 14. 
As illustrated, the stud 28 may also have a nut 34 threadably installed to 
enhance clamping but such is not deemed essential. Alternatively, stud 28 
is utilized merely as a guide member for the guide bar 18. It has been 
found that the flat 58 of the cam member 50 acting against the flange 42 
of the plate 40 and the cam member 50 being clamped in position on the 
stud 30 by the nut 34 is adequate to maintain the guide bar 18 in the 
adjusted position. 
The guide plate 40 is further illustrated in FIGS. 1 and 3. Note that FIG. 
3 illustrates plate 40 from the side hidden from view in FIG. 1. As shown, 
the guide plate 40 is a substantially flat member having an extending 
right angle leg or flange 42. The plate 40 has centrally positioned slots 
44 sufficiently long to be adjusted along the studs 28, 30 in the same 
manner as the guide bar 18. The plate 40 is mountable on the studs 28, 30 
with the studs 28, 30 being received in the slots 44. Extending fingers 46 
(studs) extend from the plate 40 (on the opposite side of the flange 42 as 
seen in FIG. 1). The studs 46 are positioned such that one stud 46 will 
fit into each of the oil holes 22 of the guide bar 18 when the plate 40 is 
mounted adjacent the guide bar 18. 
The cam member 50 is illustrated in FIGS. 1, 2, and 5 with FIG. 5 being an 
enlarged view of a portion of the periphery 52 of the cam member 50. The 
cam member 50 has a peripheral edge 52 that varies in distance from a 
center of a stud receiving hole 54. An example of a layout for the cam 
member 50 is illustrated in FIG. 4. A peripheral edge 52 extends at a 
distance from the center of the hole 54 in a spiral-like fashion such that 
the peripheral edge 52 varies in distance from the center of the hole 54. 
Preferably the peripheral edge 52 has generated flats 58 such as shown in 
the enlarged view of FIG. 5. Each flat 58 is at a different distance from 
the center of the hole 54 with the differential distance between adjoining 
flats being about 0.005 inch. 
A tool receiving slot 56 is provided in the surface of the cam member 50 as 
best seen in FIG. 2. A tool such as a tip of a flat bladed screw driver 
inserted in the slot 56 is utilized to rotate the cam member 50 when 
installed on the stud 30. The slot 56 is at a sufficient distance from the 
hole 54 so that the nut 34 when installed on the stud 30 does not cover or 
interfere with usage of the slot 56. 
The cam member may have different profiles than that illustrated for the 
cam member 50. An example of one variation is illustrated in FIG. 6. As 
shown a cam member 70 is a spiral-like wheel that has extending spokes 72 
on its periphery that are at different distances from the center of a stud 
receiving hole 74. The distances from the center of the hole 74 to 
adjacent spokes 72 is an incremental distance. The cam member has a slot 
76 to receive a tool such as a blade of a screw driver, the tool being 
utilized to rotate the cam member 70 when installed on the stud 30 of the 
chain saw 10. 
In use the cam member 70 is rotated such that two adjacent spokes 72 will 
be in contact with the flange 42 of the plate 40. The two spokes 72 in 
contact with the flange 42 will resist rotation of the cam member 70. 
It will be appreciated that the cam member utilized with the chain 
tensioner may have profiles and configurations other than those 
illustrated and described. 
The procedure for mounting and tensioning a saw chain will be described 
with reference to FIGS. 1 and 2 of the drawings. Initially the bar 18 is 
mounted on the studs 28, 30 and is moved toward the sprocket 14. The plate 
40 is mounted on the studs 28, 30 adjacent the bar 18 with the fingers 46 
fitting in the oil holes 22 of the bar (or such other holes as made 
available for the purpose). The cam member 50 is mounted on the stud 30 
with the cam member 50 positioned so that its peripheral edge will contact 
the flange 42 of the plate 40. 
A nut 34 is loosely fitted on the stud 30 to hold the assembly of the bar 
18, the plate 40 and the cam member 50 in position. The saw chain 16 is 
mounted to encircle the drive sprocket 14 and the guide bar 18 with the 
drive tangs of the saw chain 16 fitting in the guide groove 24. The cam 
member 50 is then rotated in the direction indicated by arrow 53 (FIG. 2) 
by utilizing the slot 56 to force the plate 40 and the guide bar 18 away 
from the drive sprocket 14. The cam member 50 is rotated until the guide 
bar 18 has been adjusted to a position to provide the desired tension of 
the saw chain, (e.g., the chain can be lifted away from the bar edge but 
without lifting the drive tangs out of the bar groove). As the cam member 
50 is rotated, one of the flats 58 will be in abutment with the flange 42 
of the plate 40. The nut 34 on the stud 30 is then tensioned to secure the 
guide bar 18 in the adjusted position. The guide bar 18 is maintained in 
the adjusted position by the clamping action of the nut 34 and the flat 58 
of the cam member 50 in abutment with the flange 42 of the plate 40. The 
cam member 50 is held in its adjusted position by the clamping action of 
the nut 34 and further the flat 58 that is in abutment with the flange 42 
of the plate 40 resists rotation of the cam member 50 and thus movement of 
the guide bar 18 toward the drive sprocket 14. 
Those skilled in the art will recognize that modifications and variations 
may be made without departing from the true spirit and scope of the 
invention. In one such modification, the cam member may be mounted on one 
stud and the clamping nut on the other stud. The manner of clamping may be 
other than with threaded inter-engagement of the nut and stud as well. The 
invention is therefore not to be limited to the embodiments described and 
illustrated but is to be determined from the appended claims.