Stepped shaft assembly

#This invention relates generally to an elongated shaft, and more particularly concerns forming an elongated plastic shaft with a stepped functional surface. Also described is a gear with a stepped hub that runs on the stepped functional surface of the shaft.

#Currently, when molding long plastic shafts a 2-5 degree draft along the length of the shaft is necessary in order for the shaft to be successfully removed from the mold. This means that any gear to be used on the shaft must have its hub formed to allow for the degree of draft of the shaft. Even with the gear designed for the draft of the shaft, there is a sloppy fit between the shaft and gear which results in rattling noise and uneven wear.

#FIG. 1 shows an elongated shaft 10 with a base 12, and first section 14 which has a second section 16 connected thereto. The shaft shown is hollow as indicated by bore 18 and formed by an injection molding or extrusion process, however, the shaft may also be solid. Typical plastic materials include thermoplastic and thermosetting resins. Typical thermoplastic resins include polyethylene, polystyrene, polypropylene, polyurethane, polyvinylchloride, nylons, polycarbonate ABS, as well as fluorcarbons. Typical thermosetting resins include acrylics, phenolics and polyesters. In conventional molding processes in order for the molded shaft to be separated from the mold section 14 must have a draft of 2-5 degrees as shown. Gear 20 runs on first section 14 and the gear's hub 22 is designed with a draft corresponding to the draft of the shaft. These angled surfaces of the shaft and the gear do not allow for a tight fit since precise forming of angled surfaces is difficult. The angled gear and shaft configuration has axial forces acting the surfaces where the gear hub and shaft surface come into contact.

#Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which:

#FIG. 1 is a a cross-sectional view of a prior art shaft formed with a draft and a corresponding gear;

#FIG. 2 is a is a cross-sectional view of a shaft formed with steps and a corresponding gear;

#While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

#FIG. 2 depicts a stepped shaft 50 of the present invention with a base 51, and first section 52 which has a second section 56 attached to it. Section 52 is comprised of two straight edge regions, first straight edge region 53 and second straight edge region 55 with an inclined region 54 connecting the two straight edge regions. This stepped section 52 can be referred to as the shaft functional surface since this is the surface on which a gear may be fitted. A gear 60 has corresponding straight edge regions 62 and 66 with an inclined region 64 connecting the two straight edge regions. The inner diameter of the gear hub 62 and 66 runs on the outer surface of the shaft 53 and 55. As shown, the gear and shaft contact each other only on the straight regions, which leads to a much better fit of the gear onto the shaft and results in more uniform loading along the contact surfaces. The new design of the stepped regions results in an optimal design because there is more material in the fixed end of the shaft, where there is more stress.

#The shaft shown is hollow as indicated by bore 58, however the shaft could also be solid. The gear can be attached to the shaft in a number of different ways. For example, the gear may be attached to the stepped shaft functional surface by a snap fit, held with an e-clip, or held in place by external gearing.