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In the process of developing gear trains, it occasionally occurs that the tip of one gear will drag in the fillet of the mating gear. The first reaction may be to assume that the outside diameter of the gear is too large. This article is intended to show that although the gear dimensions follow AGMA guidelines, if the gear is cut with a shaper, the cutting process may not provide sufficient relief in the fillet area and be the cause of the interference.
The paper describes a procedure for the design of internal gear pairs, which is a generalized form of the long and short addendum system. The procedure includes checks for interference, tip interference, undercutting, tip interference during cutting, and rubbing during cutting.
In principal, the design of internal helical gear teeth is the same as that for external helical gears. Any of the basic rack forms used for external helical gears may be applied to internal helical gears. The internal gear drive, however, has several limitations; not only all those which apply to external gears, but also several others which are peculiar to internal gears. As with external gears, in order to secure effective tooth action, interferences must be avoided. The possible interferences on an internal gear drive are as follows: 1. Involute interference. To avoid this, all of the working profile of the internal tooth must be of involute form.
Since size and efficiency are increasingly important considerations in modern machinery, the trend is gear design is to use planetary gearing instead of worm gearing and multi-stage gear boxes. Internal gearing is an important part of most of planetary gear assemblies. In external gearing, if the gears are standard (of no-modified addenda), interference rarely happens. But in an internal gearing, especially in some new types of planetary gears, such as the KHV planetary, the Y planetary, etc., (1) various types of interference may occur. Therefore, avoiding interference is of significance for the design of internal gearing.
Point-surface-origin (PSO) macropitting occurs at sites of geometric stress concentration (GSC) such as discontinuities in the gear tooth profile caused by micropitting, cusps at the intersection of the involute profile and the trochoidal root fillet, and at edges of prior tooth damage, such as tip-to-root interference. When the profile modifications in the form of tip relief, root relief, or both, are inadequate to compensate for deflection of the gear mesh, tip-to-root interference occurs. The interference can occur at either end of the path of contact, but the damage is usually more severe near the start-of-active-profile (SAP) of the driving gear.
The first part of this article included abrasive wear with two bodies, streaks and scoring, polishing, and hot and cold scuffing. This part will deal with three-body wear, scratches or grooves, and interference wear. Normal, moderate, and excessive wear will be defined, and a descriptive chart will be presented.
One of the most frequently neglected areas of gear design is the determination of "form diameter". Form diameter is that diameter which specifies the transition point between the usable involute profile and the fillet of the tooth. Defining this point is important to prevent interference with the tip of the mating gear teeth and to enable proper preshave machining when the gear is to be finished with a shaving operation.
This paper presents a unique approach and methodology to define the limits of selection for gear parameters. The area within those limits is called the “area of existence of involute gears” (Ref. 1). This paper presents the definition and construction of areas of existence of both external and internal gears. The isograms of the constant operating pressure angles, contact ratios and the maximum mesh efficiency (minimum sliding) isograms, as well as the interference isograms and other parameters are defined. An area of existence allows the location of gear pairs with certain characteristics. Its practical purpose is to define the gear pair parameters that satisfy specific performance requirements before detailed design and calculations. An area of existence of gears with asymmetric teeth is also considered.
News Items About interference
1 Schunk?s New Toolholder Designed for Low Interference Applications (April 19, 2006)
The TRIBOS-RM from Schunk is designed for cutting speeds up to 50,000 rpm while offering run-out accuracy of less than three microns. The... Read News