Hobbing is one of the most fundamental
processes in gear manufacturing. Its
productivity and versatility make hobbing
the gear manufacturing method of choice for a majority of spur and helical
gears.
This article also appears as Chapter 1 in the Gleason Corporation publication "Advanced Bevel Gear Technology."
Gearing Principles in Cylindrical and Straight Bevel Gears
The purpose of gears is to transmit motion and torque from one shaft to another. That transmission normally has to occur with a constant ratio, the lowest possible disturbances and the highest possible efficiency. Tooth profile, length and shape are derived from those requirements.
In our last issue, we covered the basic principles of gear shaving and preparation of parts for shaving. In this issue, we will cover shaving methods, design principles and cutter mounting techniques.
Gear shaving is a free cutting gear finishing operation which removes small amounts of metal from the working surfaces of gear teeth. Its purpose is to correct errors in index, helix angle, tooth profile and eccentricity.
NC and CNC machines are at the heart of manufacturing today. They are the state-of-the-art equipment everybody has (or is soon going to get) that promise to lower costs, increase production and turn manufacturers into competitive powerhouses. Like many other high tech devices (such as microwaves and VCRs), lots of people have and use them - even successfully - without really knowing much about how they operate. But upgrading to CNC costs a lot of money, so it's crucial to separate the hype from the reality.
Gear hobbing is a generating process. The term generating refers to the fact that the gear tooth form cut is not the conjugate form of the cutting tool, the hob. During hobbing both the hob and the workpiece rotate in a continuous rotational relationship. During this rotation, the hob is typically fed axially with all the teeth being gradually formed as the tool traverses the work face (see Fig. 1a).
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.
Precision gears play a vital role in today's economy. Through their application, automobile transmissions are more compact and efficient, ships sail faster, and diesel locomotives haul more freight. Today great emphasis is being placed upon the reduction of noise in all gear applications and, to be quiet, gears must be accurate.
Carburized and hardened gears have optimum load-carrying capability. There are many alternative ways to produce a hard case on the gear surface. Also, selective direct hardening has some advantages in its ability to be used in the production line, and it is claimed that performance results equivalent to a carburized gear can be obtained. This article examines the alternative ways of carburizing, nitriding, and selective direct hardening, considering equipment, comparative costs, and other factors. The objective must be to obtain the desired quality at the lowest cost.
A widespread weakness of gear drawings is the requirements called out for carburize heat treating operations. The use of heat treating specifications is a recommended solution to this problem. First of all, these specifications guide the designer to a proper callout. Secondly, they insure that certain metallurgical characteristics, and even to some extent processing, will be obtained to provide the required qualities in the hardened gear. A suggested structure of carburizing specifications is give.
Gears are toothed wheels used primarily to transmit motion and power between rotating shafts. Gearing is an assembly of two or more gears. The most durable of all mechanical drives, gearing can transmit high power at efficiencies approaching 0.99 and with long service life. As precision machine elements gears must be designed.
The forming of gear teeth has traditionally been a time-consuming heavy stock removal operation in which close tooth size, shape, runout and spacing accuracy are required. This is true whether the teeth are finished by a second forming operation or a shaving operation.
The most conclusive test of bevel and hypoid gears is their operation under normal running conditions in their final mountings. Testing not only maintains quality and uniformity during manufacture, but also determines if the gears will be satisfactory for their intended applications.
The following is a general overview of some of the different factors that lead to the specific design. and the selection of the correct tool for a given hobbing application.
Gear shaving is a free-cutting gear finishing operation which removes small amounts of metal from the working surfaces of the gear teeth. Its purpose is to correct errors in index,
helical angle, tooth profile and eccentricity. The process can also improve tooth surface finish and eliminate, by crowned tooth forms, the danger of tooth end load concentrations
in service. Shaving provides for form modifications that reduce gear noise. These modifications can also increase
the gear's load carrying capacity, its factor of safety and its service life.
The approximate tensile strength of any steel is measured by its hardness, Table 1. Since hardness is determined by both
chemical composition and heat treatment, these are the two important metallurgical considerations in selecting gear steels.
Much information has been written on gear inspection, analytical. functional. semiautomatic and automatic. In most
cases, the charts, (if you are lucky enough to have recording
equipment) have been explained.
Experience has proven that the involute provides the most satisfactory profile for spur and helical gear teeth, and fulfills the requirements for transmitting smooth, uniform angular
motion.
Gear shaving is a free-cutting gear finishing operation which removes small amounts of metal from the working surfaces
of the gear teeth. Its purpose is to correct errors in index, helical angle, tooth profile and eccentricity.
