BELT TENSIONER

Abstract: A belt tensioner for a belt and pulley system is disclosed. The tensioner (1) has a camming surface (29) upon which a pulley axle rides, maintaining substantially uniform tension in the belt despite competing forces of the belt tension and a tensioner holding spring (11). The camming surface (29) profile is generated, for example, when using a spring holding mechanism, by differentiating Hooke's Law.

Description: The present invention relates generally to belt drive systems and, more particularly, to a belt tensioner for a reversible pulley system. In many belt drive system applications, it is important to optimize belt tension. The purpose of a belt tensioner is to maintain a substantially constant tension in a drive belt. In most applications, the belt connects stationary pulleys. Hence, belt tension can be set by affixing one pulley to a mount having an adjustable linkage to a fixed mounting surface. Automobile engine belts are common examples of this type of system. However, in other applications, a belt drive system may be used where the belt is used to position another device. For example, a computer may be used to provide drive signals to an ink-jet printer (a high-speed printing process in which thermally excited ink is broken up into electrostatically charged droplets which are controlled by a computer program into positions that form printed characters on a print media). An ink pen carriage is rapidly and repeatedly scanned back and forth across the print media by providing a belt drive device in which the pen carriage is positioned by "riding" the belt. Positioning of the printhead to the print media is, of course, critical to prevent a printing error. Hence, belt tension is an important feature. One example of such a prior art mechanism is shown in FIGURE 1 (prior art). A belt 100 connects a reversible motor 300 to an idler pulley 500. The pulley axle 700 fit slidingly in slots 900 in mounting frame 110. The belt 100 extends through an aperture 130 in frame 110. A pen carriage (not shown) could ride a guide bar 150 and be attached to the belt 100. Since the pulley axle 700 makes a sliding fit and the belt must be long enough to reach beyond end 170 of the mount 110 to encircle the pulley 500 before fit into slots 900, a tensioner 190 is used to achieve the proper tension. After the pulley 500 is fit into the slots 900, the tensioner 190 is inserted such that the axle 700 will ride on tensioner surface 210. Extension tab 230, with tensioning spring 250 surrounding it, is rotatingly slipped into slot 270 of the mount 110. By designing to a close tolerance, the tensioner surface 210 will pull the pulley outward along the slots 900 just until the belt 100 is properly tensioned. One concern is to maintain tension in the belt over a wide rage of tolerances in the assembly of a printer instrument. If the components are molded of plastic in order to reduce costs, tolerances become even more difficult to maintain. Furthermore, wedge type-tensioners 190 fail to compensate for the effect of varying spring 250 lengths and compression factors. Hence, a relatively wide range of belt tensions can result. US-A-4484 485 discloses a similar system in which the tensioner surface is curved with a constant radius of curvature. This represents an improvement, but not a sufficient one. Accordingly, the invention proposes a tensioner for a belt and pulley systems, comprising a handling means for inserting said tensioner into said system; and a pronged body means connected to one end of said handling means, each prong having two substantially flat surfaces, wherein each surface has an edge at least in part forming a substantially curved surface for a riding pulley axle, and the handling means includes a shaft segment and spring means, the tensioner being characterised in that said substantially curved surface has a varying radius of curvature, in that the shaft segment is non-pivotting, and in that the spring means are for pushing said surface in a straight longitudinal direction against the riding pulley axle. It is an advantage of the present invention that it provides compensation for varying tensioning spring deflections by utilizing a varying contact angle provided by the camming profile for the pulley axle. It maintains tension in a belt over a wide range of tolerances in the manufacture and assembly of a system in which it is employed, and provides a design which facilitates assembly into a system. It also provides tighter control of the range of belt tensions, allowing for the use of smaller low-cost motors and pinions. Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features. Figure 1 is a perspective view depiction of a belt tensioner according to the prior art. Figure 2 is a perspective view depiction of the belt tensioner according to the present invention. Figure 3 is a plan view (side) depicting assembly with a pulley axle being captured by one tip of a tensioner prong according to the present invention as shown in Figure 1. Figure 4 is a plan view (side) of the depiction of the present invention as shown in Figure 3 with the tensioner fully inserted into a housing and demonstrating the theory of design. The drawings referred to in this desçription should be understood as not being drawn to scale except if specifically noted. Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventor(s) for practicing the invention. Alternative embodiments are also briefly described as applicable. Referring now to FIGURES 2 and 3, an embodiment of a tensioner 1 in accordance with the present invention is depicted. The tensioner 1 has two primary segments - a handle segment 3 and a pronged body segment 5. In the preferred embodiment as depicted and described, the invention is molded of a plastic material. This should not be taken as any limitation on the method or type of construction of the invention as many alternatives would be recognized by a person skilled in the art. The handle segment 3 has a substantially straight shaft 7. At one extremity of the shaft 7, viz. distal from said body 5, an extension segment 9 forms an elbow with the shaft 7. It is preferred that the elbow form a right or acute interior angle with the shaft 7. At the other extremity of the shaft 7, viz. proximate to said body 5, the handle segment 3 is connected to the body 5. A helical spring 11 encompasses the shaft 7. The length of the spring 11 should be designed using standard engineering principles dependent upon the operational system specifications (e.g., for an ink-jet computer printer) and the design of the housing in which the tensioner 1 is to be used. The handle segment 3 serves several functions. The elbow extension segment 9 serves as a convenient grip during assembly of a system. Furthermore, by intentional design, the inner diameter of the spring 11 can be made smaller than the length x of the extension segment 9 to keep the spring on the shaft 7 when the tensioner 1 is not in the instrument of the particular system for which it is designed. The outer diameter of the spring is less than the smaller cross-dimension of the base of 33 of the body segment 5. Thus, the spring 11 is captured, encompassing the shaft 7 between the extension segment 9 and the body base 33. In the preferred embodiment and as depicted in relief in FIGURE 3, the lower surface of the base 33 can have a raised region 33′ which has a circular periphery which has a diameter slightly less than the inner diameter of spring 11. Thus, when assembled, the spring 11 remains centered against the lower surface of the base 33. The body segment 5 is shown to have two prongs 13, 15 as shown in FIGURE 2. The gap 17 between the prongs 13, 15 should be adequate to allow a pulley to turn without interference, yet narrow enough such that the pulley axle will span the gap 17. In alternative embodiments, additional prongs could be added to adapt the tensioner 1 to uses with more than one pulley. FIGURES 2 and 3 show that the prongs 13, 15 each have two substantially flat surfaces 19, 19′. In the present embodiment as design for a specific mount housing used in an ink-jet printer, one edge 21 of the surfaces 19, 19′ is substantially flat. The opposite edge 23 is substantially curved to form a camming edge surface having a changing radius of curvature. At the extremity 25 of the each prong 13, 15 the radius of edge 23 is relatively small in order to form a pointed tip 27 having a first edge surface 28. Referring to FIGURE 3, it can be seen that the tip can be used to engage the pulley axle 700′ when assembling the belt and pulley mechanism of the instrument for which it is adapted. Proximate the tip edge 28 is second edge surface 23. Edge surface 23 has a changing radius of curvature. Edge surface 23 forms a second camming edge surface 29 for the axle 700′. Proximate the second edge surface 23 and the body base 33 is a third edge surface 35. Edge 35 has a radius of curvature which is greater than said second edge surface 29. In the present embodiment, this edge surface 35 is substantially flat and parallel to edge surface 21. As will be recognized by a person skilled in the art, other embodiments can have various edge surface regions forming edge 23 in accordance with the specific system requirements. The specific edge 23 configuration described above is not intended to be interpreted as a limitation on the scope of the invention. FIGURE 4 depicts the present embodiment of the invention in full assembly. The specific dimensions can be designed in accordance with system requirements. The varying camming edge surface 29 maintains tension in the belt over a wide range of tolerances in the assembly of the instrument for which it is designed. Referring now to FIGURE 4, the changing radius of curvature of edge surface 29 is defined by a unique application of Hooke's Law for springs, viz. F = 2 T o tan ϑ, where F is the compression force exerted by the spring and T o is the tension force exerted by the belt 100. Conventional engineering practice, e.g. for a prior art device such as shown in FIGURE 1, calls for determining a proper spring based upon a fixed theta angle once the tensioner design is established or the converse where the spring is fixed. The present invention provides a variable theta angle which compensates for changing spring forces. The equation is solved, in other words, to generate the profile of the camming edge's 29 changing angle. F = 2 T o tan ϑ tan ϑ = Δx/Δy = F/2T o = Ky/2T o , where K = spring force constant. Thus, dy/dx = 2T o /Ky, is the differential equation for generating the camming edge 29 profile. Therefore, this equation can be used to design a profile in accordance with the specific needs and design of the overall system.

Claims: 1. A tensioner for a belt and pulley systems, comprising a handling means (3) for inserting said tensioner into said system; and, a pronged body means (5) connected to one end of said handling means, each prong (13,15) having two substantially flat surfaces, (19,19′) wherein each surface has an edge (23) at least in part forming a substantially curved surface (29) for a riding pulley axle, and the handling means (3) includes a shaft segment (7) and spring means (11), the tensioner being characterised in that said substantially curved surface has a varying radius of curvature, in that the shaft segment is non-pivotting, and in that the spring means are for pushing said surface (29) in a straight longitudinal direction against the riding pulley axle. 2. The tensioner as claimed in claim 1, wherein said spring means (11) comprises a helical spring (11) encompassing said shaft segment (7). 3. The tensioner as claimed in claim 1 or 2, wherein each said prong has an extremity tip (27) adapted to catch said axle and transfer said axle to ride on said substantially curved surface (29). 4. The tensioner as claimed in claim 1,2 or 3, wherein said substantially curved surface (29) has a profile angle to the plane perpendicular to the belt tension force vector defined by the equation: tan 0 = dx/dy = Ky/2T o , where K = spring force constant, and T o = belt tension force constant. 5. The tensioner as claimed in any preceding claim, wherein said shaft segment (7) has an extension serpent (9) at an end of said shaft distal from said body means, said extension forming an interior angle to said shaft segment less than or equal to ninety degrees. 6. The tensioner as claimed in claim 5, wherein said substantially curved surface (29) terminates at a continuing surface edge region (35) which is in a plane substantially parallel to said shaft segment (7) and which is spaced adjacently to where said pronged body means (5) is connected to said one end of said shaft segment. 7. The tensioner as claimed in claims 2 and 5, wherein said helical spring (11) has an inner diameter less than the length of said extension segment (9). 8. An endless-belt and pulley system, having at least one pulley with an axle slidingly mounted in a mounting means, and comprising a belt tensioner as claimed in any preceding claim, wherein the tensioner handling means (3) is held in mounting means of the system, and said spring means (11) abutted in the system to force the tensioner towards the pulley axle so that said axle rides on said substantially curved surface (29),

Inventors: BEAUCHAMP, ROBERT WARREN, EBERSOLE, ANTHONY WAYNE
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: B41J19
IPC codes: B41J19, F16H7