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Articles About gear fundamentals
Whether gear engineers have to replace an old gear which is worn out, find out what a gear's geometry is after heat treatment distortion, or just find out parameters of gears made by a competitor, sometimes they are challenged with a need to determine the geometry of unknown gears. Depending on the degree of accuracy required, a variety of techniques are available for determining the accuracy of an unknown gear. If a high degree of precision is important, a gear inspection device has to be used to verify the results. Frequently, several trial-and-error attempts are made before the results reach the degree of precision required.
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.
For heat treatment of tool and alloy steels, the end-user has a wide range of basic types of heat treating equipment to choose from. This article reviews them and details the criteria that must be considered in selecting equipment for a specific application. In making this choice, the most important criterion must be the quality of the tool or part after processing.
Today gear drive operations have several options when selecting the proper lubricant for their gearboxes. As in the past, the primary lubricant used for gearbox lubrication is mineral oil. But with the advances in technology, synthetic hydrocarbons (PAOs) and polyglycols show very specific advantages in certain applications. With gear drives becoming more and more precise, it is now also to the benefit of the gear operator to verify that he or she has the proper additive package and viscosity in the lubricant selected. Fig. 1 shoes that a gear oil is a combination of a base oil and specific additives. The base oils can be either mineral oil, a synthetic or even in some cases a combination of the two.
This section will deal with the use of gear inspection for diagnostic purposes rather than quality determination. The proper evaluation of various characteristics in the data can be useful for the solution of quality problems. It is important to sort out whether the problem is coming from the machine, tooling and/or cutters, blanks, etc. An article by Robert Moderow in the May/June 1985 issue of Gear Technology is very useful for this purpose.
Quality gear inspection means doing the "right" inspections "right." A lot of time and money can be spent doing the wrong types of inspections related to function and doing them incorrectly. As we will discover later, such things as runout can creep into the manufacturing and inspection process and completely ruin any piece of data that is taken. this is one of the most important problems to control for quality inspection.
Can a gear profile generated by the hobbing method be an ideal involute? In strictly theoretical terms - no, but in practicality - yes. A gear profile generated by the hobbing method is an approximation of the involute curve. Let's review a classic example of an approximation.
Flute Index Flute index or spacing is defined as the variation from the desired angle between adjacent or nonadjacent tooth faces measured in a plane of rotation. AGMA defines and provides tolerance for adjacent and nonadjacent flute spacing errors. In addition, DIN and ISO standards provide tolerances for individual flute variation (Fig. 1).
In the quest for ever more exacting and compact commercial gears, precision abrasives are playing a key production role - a role that can shorten cycle time, reduce machining costs and meet growing market demand for such requirements as light weights, high loads, high speed and quiet operation. Used in conjunction with high-quality grinding machines, abrasives can deliver a level of accuracy unmatched by other manufacturing techniques, cost-effectively meeting AGMA gear quality levels in the 12 to 15 range. Thanks to advances in grinding and abrasive technology, machining has become one of the most viable means to grind fast, strong and quiet gears.
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.
Increasingly gear designers and product engineers are capitalizing on the economic advantages of powder metallurgy (P/M) for new and existing gear applications. Powder metal gears are found in automobiles, outdoor power equipment transmissions and office machinery applications as well as power hand tools, appliances and medial components.
It is very common for those working in the gear manufacturing industry to have only a limited understanding of the fundamental principals of involute helicoid gear metrology, the tendency being to leave the topic to specialists in the gear lab. It is well known that quiet, reliable gears can only be made using the information gleaned from proper gear metrology.
In the last section, we discussed gear inspection; the types of errors found by single and double flank composite and analytical tests; involute geometry; the involute cam and the causes and symptoms of profile errors. In this section, we go into tooth alignment and line of contact issues including lead, helix angles, pitch, pitchline runout, testing and errors in pitch and alignment.
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. The process also improves tooth surface finish and eliminates by means of crowned tooth forms the danger of tooth end load concentrations in service.
Fig. 1 shows the effects of positive and negative rake on finished gear teeth. Incorrect positive rake (A) increase the depth and decreases the pressure angle on the hob tooth. The resulting gear tooth is thick at the top and thin at the bottom. Incorrect negative rake (B) decreases the depth and increases the pressure angle. This results in a cutting drag and makes the gear tooth thin at the top and thick at the bottom.
In 1961 I presented a paper, "Calculating Conjugate Helical Forms," at the semi-annual meeting of the American Gear Manufacturers Association (AGMA). Since that time, thousands of hobs, shaper cutters and other meshing parts have been designed on the basis of the equations presented in that paper. This article presents the math of that paper without the formality of its development and goes on to discuss its practical application.
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 purpose of this article is to clarify some terms and methods used in measuring the size of gears. There is also an explanation given of the error induced and how to correct for it in certain cases when the measurement is made using pins instead of balls.
Grinding in one form or another has been used for more than 50 years to correct distortions in gears caused by the high temperatures and quenching techniques associated with hardening. Grinding improves the lead, involute and spacing characteristics. This makes the gear capable of carrying the high loads and running at the high pitch line velocities required by today's most demanding applications. Gears that must meet or exceed the accuracy requirements specified by AGMA Quality 10-11 or DIN Class 6-7 must be ground or hard finished after hear treatment.
In today's industrial marketplace, deburring and chamfering are no longer just a matter of cosmetics. The faster speeds at which transmissions run today demand that gear teeth mesh as smoothly and accurately as possible to prevent premature failure. The demand for quieter gears also requires tighter tolerances. New heat treating practices and other secondary gear operations have placed their own set of demands on manufacturers. Companies that can deburr or chamfer to these newer, more stringent specifications - and still keep costs in line - find themselves with a leg up on their competition.
Most steel gear applications require appreciable loads to be applied that will result in high bending and compressive stresses. For the material (steel) to meet these performance criteria, the gear must be heat treated. Associated with this thermal processing is distortion. To control the distortion and achieve repeatable dimensional tolerances, the gear will be constrained during the quenching cycle of the heat treatment process. This type of fixture quenching is the function of gear quench pressing equipment.
Dear Editor: In Mr. Yefim Kotlyar's article "Reverse Engineering" in the July/August issue, I found an error in the formula used to calculate the ACL = Actual lead from the ASL = Assumed lead.
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.
This is Part II of a two-part series on the basics of gear hobbing. Part I discussed selection of the correct type of hobbing operation, the design features of hobs and hob accuracy. This part will cover sharpening errors and finish hob design considerations.
Metrology is a vital component of gear manufacturing. Recent changes in this area, due in large part to the advent of computers, are highlighted in this article by comparison with more traditional methods.
The finished gear engineer, the man who is prepared for all emergencies, must first of all know the basic design principles. Next he must be well versed in all sorts of calculations which come under the heading of "involute trigonometry."
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.
The quality of a gear and its performance is determined by the following five parameters, which should be specified for each gear: Pitch diameter, involute form, lead accuracy, spacing accuracy, and true axis of rotation. The first four parameters can be measured or charted and have to be within tolerance with respect to the fifth. Pitch diameter, involute, lead, and spacing of a gear can have master gear quality when measured or charted on a testing machine, but the gear might perform badly if the true axis of rotation after installation is no longer the same one used when testing the gear.
The following article is a collection of data intended to give the reader a general overview of information related to a relatively new subject within the gear cutting industry. Although carbide hobbing itself is not necessarily new, some of the methods and types of application are. While the subject content of this article may be quite broad, it should not be considered all-inclusive. The actual results obtained and the speeds, feeds, and tool life used in carbide hobbing applications can vary significantly.
The term "blanking" refers to the initial metal cutting operations in the process planning sequence which produce the contour of a part starting from rough material. The scope of blanking is: To remove the excess material To machine the part to print specifications, except for those surfaces with subsequent finishing operations. To leave adequate machining stock for finishing operations. To prepare good quality surfaces for location and clamping of the part throughout the process.
In a very general sense, increasing the hardness of a steel gear increases the strength of the gear. However, for each process there is a limit to its effectiveness. This article contains background information on each of the processes covered. In each section what is desired and what is achievable is discussed. Typical processes are presented along with comments on variables which affect the result. By reviewing the capabilities and processes, it is possible to determine the limits to each process.
The purpose of gear inspection is to: Assure required accuracy and quality, Lower overall cost of manufacture by controlling rejects and scrap, Control machines and machining practices and maintain produced accuracy as machines and tools wear, Determine hear treat distortions to make necessary corrections.
The use of dimensionless factors to describe gear tooth geometry seems to have a strong appeal to gear engineers. The stress factors I and J, for instance, are well established in AGMA literature. The use of the rack shift coefficient "x" to describe nonstandard gear proportions is common in Europe, but is not as commonly used in the United States. When it is encountered in the European literature or in the operating manuals for imported machine tools, it can be a source of confusion to the American engineer.
The Hobbing Process The hobbing process involves a hob which is threaded with a lead and is rotated in conjunction with the gear blank at a ratio dependent upon the number of teeth to be cut. A single thread hob cutting a 40-tooth gear will make 40 revolutions for each revolution of the gear. The cutting action in hobbing is continuous, and the teeth are formed in one passage of the hob through the blank. See Fig. 1 for a drawing of a typical hob with some common nomenclature.
Worm gears are among the oldest types of gearing, but that does not mean they are obsolete, antiquated technology. The main reasons for the bad experiences some engineers have with worm gearing are misapplication and misuse. No form of gearing works for every application. Strengths and weaknesses versus the application must be weighed to decide which form of gearing to use. For proper application and operation of worm gears, certain areas that may differ from other types of gearing need to be addressed.
This article discusses briefly some common manufacturing problems relating to coarse pitch gears and their suggested solutions. Most of the discussion will be limited to a low-quality production environment using universal machine tools.
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.
There is one dimension common to both members of a pair of properly mating spur gears - the base pitch (BP). This base pitch is equal to the circular pitch of the gear on the base circle (see Fig. 1). For a helical gear, the base pitch can be described in either the transverse or normal plane, and is called the transverse base pitch (TBP) or normal base pitch (NBP), respectively. For parallel axis helical gears, both the TBP and NBP must be the same on both mating gears. For skew axis helical gears, only the NBP must be common.
The phenomena of deterioration of surfaces are generally very complex and depend on numerous conditions which include the operating conditions, the type of load applied, the relative speeds of surfaces in contact, the temperature, lubrication, surfaces hardness and roughness, and the compatibility and nature of materials.
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.
Although gears can be manufactured using a wide variety of profiles, the involute curve is the most commonly used. Here are some of the basics.
What is so unique about gear manufacturing and inspection? Machining is mostly associated with making either flat or cylindrical shapes. These shapes can be created by a machine's simple linear or circular movements, but an involute curve is neither a straight line nor a circle. In fact, each point of the involute curve has a different radius and center of curvature. Is it necessary to go beyond simple circular and linear machine movements in order to create an involute curve? One of the unique features of the involute is the fact that it can be generated by linking circular and linear movements. This uniqueness has become fertile soil for many inventions that have simplified gear manufacturing and inspection. As is the case with gear generating machines, the traditional involute inspection machines take advantage of some of the involute properties. Even today, when computers can synchronize axes for creating any curve, taking advantage of involute properties can be very helpful. I t can simplify synchronization of machine movements and reduce the number of variables to monitor.
"Gear Train" is a new Gear Technology section focusing on training and education in the gear industry. For the first installment, we've focused on AGMA's online and video training programs.