Torque-limited chain tensioning for power tools

A chainsaw (100) has a chainsaw body (101), a cutting chain (103) driven by a motor, and a guide bar mounting stud (111) extending from the chainsaw body (101). The chainsaw (100) also has an adjustably connected guide bar (102) having a track for supporting the cutting chain (103) around a periphery of the guide bar (102) and an adjustment pin (114) extending from a side face thereof. The guide bar mounting stud (111) is configured to extend through an elongated slot (112) in the guide bar (102). The chainsaw (100) also has a cam (116) received on the guide bar mounting stud (111) and having an eccentric groove facing the guide bar (102). The adjustment pin (114) is received in the eccentric groove such that rotation of the cam (116) causes the guide bar (102) to move with respect to the chainsaw body (101), thereby tensioning the cutting chain (103). The chainsaw (100) also has a clutch (118) connected to the cam (116) for allowing no more than a predetermined load to be applied to the guide bar (102) for tensioning the cutting chain (103).

TECHNICAL FIELD

The present disclosure generally relates to the field of chain tensioning for power tools, and more particularly to a torque-limiting chain tensioning device for a chainsaw.

SUMMARY OF THE INVENTION

The present disclosure is directed to a device for reducing the range of chain tension created when a user replaces or re-tensions a chain. Providing load limited tension brings the chain up to a specified tension and then maintains that tension during the clamping of the bar. This configuration may also provide a solid surface that relies more on solid material interaction rather than coefficients of friction and normal force generation.

In one embodiment, the invention is directed to a chainsaw powered by a motor housed by a chainsaw body. The chainsaw includes a cutting chain connected so as to be driven by the motor, and a guide bar mounting stud extending from the chainsaw body. The chainsaw also has an adjustably connected guide bar. The guide bar has a track for supporting the cutting chain around a periphery of the guide bar, an adjustment pin extending from a side face thereof, and an elongated slot formed therein, wherein the guide bar mounting stud is configured to extend through said slot in the guide bar. The chainsaw also has a cam received on the guide bar mounting stud and having an eccentric groove facing the guide bar. The adjustment pin is received in the eccentric groove such that rotation of the cam causes the guide bar to be positioned along its longitudinal dimension with respect to the chainsaw body, thereby tensioning the cutting chain. The chainsaw also has a clutch connected to the cam for allowing no more than a predetermined load to be applied to the guide bar for tensioning the cutting chain.

In one embodiment, the clutch includes a wrap spring between the knob and the cam to limit the amount of tension which may be supplied to the cutting chain. In another embodiment, the clutch includes a first series of magnets, and the cam includes a generally opposed second series of magnets, wherein the first and second series of magnets hold the cam in relation to the clutch until sufficient force is created such that the magnetic forces between the first and second series of magnets are no longer great enough to prevent a slip between the clutch and the cam, thereby limiting the torque that the cam can apply. In yet another embodiment, the clutch includes a ring gear connected to one of the clutch and the cam, and one or more flexible fingers comprising a center gear connected to the other of the clutch or the cam.

Corresponding reference characters indicate corresponding parts throughout the views of the drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring generally toFIGS. 1 through 6, a chainsaw100is illustrated in accordance with one exemplary embodiment of the present disclosure. The chainsaw100has a body101which incorporates a conventional motor and a guide bar102for supporting a cutting chain103. The guide bar102comprises a longitudinal frame having two flat sides, one of the flat sides facing towards the body101and the other one of the flat sides facing away from the body. The guide bar102has rails104for keeping the cutting chain103centered on a track106extending around the periphery of the guide bar102. The cutting chain103is supported by the guide bar102at a first end108of the guide bar102, and is connected to and driven by a chain-drive output proximal to a second end110of the guide bar102, such as an engine's drive sprocket inside the body101of the chainsaw100. As best seen inFIGS. 5 and 6, the guide bar102is mounted to the body101on stud111that extends from the body101. The guide bar102has a slot112that has a width that corresponds closely to the diameter of the stud111. While the positions of the stud111and the drive sprocket are fixed with relation to the body101of the chainsaw100, the guide bar102may be adjusted along its longitudinal dimension as shown by the arrow A inFIG. 5with relation to the body101of the chainsaw100.

As best seen inFIG. 6, the guide bar102includes an adjustment pin114fixedly connected to the guide bar102for tensioning the cutting chain103. The adjustment pin114is connected to a cam116, and the cam116is connected to a clutch118. In one embodiment, the cam116is received on and rotates about the axis of the stud111. The cam116has an eccentric groove119that receives the adjustment pin114. The clutch118is configured to allow a predetermined load to be applied to the guide bar102via the adjustment pin114. The predetermined load is set by the design of the clutch118. For example, the clutch118may include a knob120mounted on the stud111, driven by a user. While there is not a sufficient reaction force from the tension of the cutting chain103, the cam116will turn, and the sliding of the adjustment pin114in the eccentric groove119drives the guide bar102forward (i.e., in a direction away from the body101of the chainsaw100proximal to the second end110of the guide bar102).

Once sufficient tension exists in the cutting chain103, the clutch118will begin slipping and prevent the cutting chain103from being tensioned further. Thus, components of the clutch118can be selected to prevent the chain103from being over tensioned. The knob120will continue to turn while the clutch118slips to clamp the guide bar102down against the mounting surfaces of the power head of the chainsaw100. Further, the configuration of the chainsaw100, including the adjustment pin114and the cam116, may hold the guide bar102and prevent it from rotating about guide bar mounts.

In one specific embodiment, illustrated inFIGS. 1 through 6, the clutch118includes a wrap spring122between the knob120and the cam116to limit the amount of tension which may be supplied to the cutting chain103. In another specific embodiment, illustrated inFIGS. 7 and 8, the clutch118includes a first series of magnets124, and the cam116includes a generally opposed second series of magnets126. The first and second series of magnets124and126hold the driven parts in relation to the driving parts until sufficient force is created such that the magnetic forces between the first and second series of magnets are no longer great enough to prevent a slip between the parts limiting the torque that the driven parts can apply. In a further embodiment, two friction surfaces may be utilized such that the surfaces will slip relative to one another when the resistance torque of the driven parts is greater than the torque of the friction surfaces.

In a still further embodiment, illustrated inFIG. 9, the clutch118includes a ring gear128rotated by the knob120and one or more flexible fingers130comprising a center gear132connected to the cam116. However, one skilled in the art will realize that these components may be reversed such that the gear is connected to the cam, and the flexible fingers are connected to the handle without departing from the scope of the invention. In a specific embodiment, the fingers130may be constructed from a plastic material, such as Acrylonitrile Butadiene Styrene (ABS) or another type of plastic. The ring gear128may also be constructed from a plastic material. The ring gear128may include directioned teeth134, providing slip torque between the ring gear128and the fingers130that is much higher in one direction than in an opposing direction. Thus, when a finger130bends past kick-over, the clutch118slips, stopping tensioning or tightening of the ring gear128. Thus, the clutch118may comprise a torque-limited transmission.

While the present disclosure describes the wrap spring122, the first series of magnets124and the generally opposed second series of magnets126, the two friction surfaces, and the ring gear128and the fingers130with some specificity, it will be appreciated that these embodiments are not meant to be limiting of the present disclosure, and that other clutch mechanisms may be utilized with the torque-limiting chain tensioning device of the present disclosure.

While this invention has been described in conjunction with the specific embodiments described above, it is evident that many alternatives, combinations, modifications and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative only, and not in a limiting sense. Various changes can be made without departing from the spirit and scope of this invention.