This invention relates to a rim sprocket for driving a saw chain of a chainsaw, the saw chain comprising, e.g. center drive links connected to paired side links, certain ones of which are side cutting links, and more particularly to a structure for such rim sprocket that uses less material and is thus lighter in weight and equal or greater in strength. More particularly and/or additionally it relates to the process for producing the sprocket while reducing scrap rate.
Rim sprockets can be described as having a star shaped center section (i.e., having radially extended teeth) positioned between disc shaped side walls. The spacings between the teeth circumferentially, and between the side walls laterally, define gullets that receive the drive tangs of center links of a saw chain, and the side walls further define outer peripheral support surfaces or rails which support the side links of the saw chain. The sprockets have a spline shaped center opening through the thickness of all three sections that receive a drive shaft, e.g. of an adapter driven by the chainsaw's engine. In an example of such a chainsaw, a centrifugal clutch driven by the engine engages the cup and rotates the adapter shaft to rotatively drive the rim sprocket and thus the saw chain mounted on the rim sprocket. The saw chain is thereby driven around a guide bar of the chainsaw for cutting trees or logs and the like.
The drive sprocket is a key component of the chainsaw's drive system and is subject to harsh abuse and rapid wearing. It is desired that the sprocket be made to withstand the severe abuse over many hours of use, e.g. the lifetime of several cutting chains and yet be produced as inexpensively as feasible.
A process has been developed for making such sprockets. A mold tree is formed. The mold tree is a plastic form with a vertical center section made of many interconnected segments having spokes radiating horizontally. Secured to each spoke is a sprocket mold or mold form, that is in the shape of the rim sprocket to be produced. This tree is coated with ceramic and the plastic form is burned away leaving a ceramic mold. Passageways are thereby provided down the center of the mold tree created by the burned out center section (referred to as a sprue) and through the burned out spokes (referred to as gates) and into each sprocket mold cavity. Molten steel is poured through the passageway and into the numerous sprocket mold cavities in a single operation. When cooled, the ceramic mold surrounding the solidified sprockets is removed but, notwithstanding, the sprockets remain interconnected via the steel that has hardened in the gates. The hardened steel formed in the gates is sometimes referred to as stems. As designed, the steel of the stem formed inside the gate and which is connected to the sprocket is small in dimension and the sprocket can be broken away from the hardened metal formed in the sprue. Any nubbin of the stem remaining on the sprocket can be readily ground away to remove any sign of the interconnection, and thus rendering the sprocket ready for final processing e.g. heat treating.
The process as described has a number of critical aspects and as a result there are problems that are herein addressed. The molten steel is desirably poured when at an established molten temperature to ensure complete filling of the mold forms and to ensure a desired steel composition of the end product. The stems generated at the gates should be configured so as to permit a clean break away of the solidified sprockets. The metal throughout the sprocket form is preferably uniformly dense, i.e., devoid of porosity. Other desirable features for the rim sprocket are that the sprockets as produced facilitate wood chip removal during a wood cutting operation, and that the weight of the sprocket be minimized.
The present invention is derived from an investigation into an undesired high scrap rate that resulted particularly when molding larger sized rim sprockets, e.g. larger than 1½″ diameter. It was determined that the higher scrap rate resulted largely due to the metal in the gates becoming solidified prematurely. Thus, it was reasoned that to insure a flow of sufficient metal into the larger cavities and thus avoid porosity, the gates or portals through the spokes needed to be enlarged. However, when enlarged, the larger stem that was formed inside the gate (following solidification of the metal) was more difficult to break away from the sprocket and resulted in occasional chipping out (chip-out) of a portion of the sprocket body.
Further investigation led to an attempt to reduce the metal quantity for the larger sized sprocket so as to retain the smaller portals. The configuration of the star shaped center section and the disc shaped side walls is dictated at least in part by the configuration of the saw chain being driven. The center opening is dictated by the adapter mounted to the cup that drives the sprocket. Thus, the initial attempts at reducing volume was to create channels in the side walls of the side sections. These attempts were successfull in that the metal volume was reduced, resulting in scrap rate reduction and sprockets thus produced were found to retain the desired strength. Such success inspired further attempts to reduce metal quantity and the side walls were provided with openings, in the axial direction between the sprocket teeth, and in a third stage of development the thickness of the side wall over the sprocket teeth was also reduced.
The above described metal removal and follow up testing led to a further discovery which was that certain of the thinner sections forming the rims were often stronger than, or at least as strong as, the predecessor thicker sections. Even further, wear life appeared to increase due to resultant harder surfaces in the stress critical areas. It was determined that the predecessor thicker sections were somewhat more porous and that such porosity was a phenomenon of the metal cooling and solidification process. As molten metal cools it solidifies and in the process it shrinks. Thus, additional molten metal needs to be provided throughout the shrinking process to maintain a more densely filled cavity. If not, interstices are created that produces the undesired porosity and lesser hard surface areas.
From the above trials and errors a critical relationship was discovered, i.e., a relationship of surface area of the sprocket being poured to the mass of the metal needed to fill the cavity of the sprocket mold. More specifically, the ratio of weight e.g. grams, to surface area e.g. square inches, should be on the order of 4 to 1 or less i.e. no greater than about 4 grams of molten metal for each square inch of surface area making up the exterior surface of the sprocket being poured. This desired relationship is achieved by reducing the thickness for the sprocket configuration in non-stress critical areas, and as feasible by increasing the surface area. From observations of the sprockets produced by the present invention, the more rapid cooling and solidification produces a lighter rim sprocket, is less expensive to produce, and furthermore is found to have a longer wear life. The invention will be more fully appreciated and understood with reference to the following detailed description of preferred embodiments of the invention, having reference to the accompanying drawings.
It will be noted that the stem portions 14 which represent the gates or portals of the mold casting are substantially the thickness of side walls 18 of the sprocket mold forms 16. It will be further appreciated that the molten steel (at e.g. 3,000 degrees F. or higher) flows from the sprue openings (12) through the gates (14) and then into the numerous cavity configurations (16) until the sprocket cavities are filled. Such filling requires but a very brief period of time. Then the molten steel cools and as it cools it shrinks and additional molten steel is drawn into the cavity configurations (16) through the gates (14). To the extent that the additional molten steel is available through the gates (14), the density of the steel desired for the sprocket cavity configurations (16) is maintained. Should the molten steel in the gates (14) solidify and thus close off the gates before the molten steel of the sprocket cavity configurations (16) solidifies, the continuing solidification of the steel in the cavities will result in a contraction or shrinking of the steel which generates interstices within the body of the sprocket and thus the undesired porosity.
Reference is now made to
As will be apparent, such material removal accomplishes lowering the material mass of the sprocket while providing newly exposed surface areas, i.e., the area 40 surrounding the through bores 28 as best seen in
As indicated above and again having reference to
The configuration of sprocket 16 would have previously been considered too fragile based on prior experiences in casting rim sprockets. However, as explained above, as a result of the need to reduce mass and the resultant finding that thicker was not always better, i.e., stronger, as applied to these metal castings (porous v. non-porous), it was found that providing through bores 28, in an area of low stress, reduced the mass of metal making up the sprocket. It did so without reduction in the size of the sprocket (i.e., having the same inner and exterior peripheral configurations and land surface area as required to support and drive a saw chain) and without sacrifice of strength. The greater surface area and lesser mass, in particular, enables sprockets of larger sizes, e.g. greater than 1.5″ in diameter, to be produced while maintaining a desired mass to surface ratio. Through extensive experimentation, it has been determined that this ratio is desirably maintained at no greater than about 4 to 1, i.e., 4 grams of weight for every square inch of surface area.
Reference is now made to the alternate embodiment shown in
A third embodiment is shown in
The above disclosure is directed to preferred embodiments and subject to numerous variations and modifications without departing from the invention which is defined by the claims appended hereto, the terms of which are intended to be given their broad and customary meaning in the trade.
Number | Name | Date | Kind |
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4776826 | Scott | Oct 1988 | A |
4816010 | Reynolds | Mar 1989 | A |
4876796 | Calkins et al. | Oct 1989 | A |
4893407 | Lane | Jan 1990 | A |
5098348 | O'Neel | Mar 1992 | A |
5136783 | Bell | Aug 1992 | A |
Number | Date | Country | |
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20060064880 A1 | Mar 2006 | US |