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Gears, while being a fundamental means of transferring motion and power and being simple in principle, are components with complex geometries that must be made with great precision to avoid premature component failure and high warranty costs. In recent years, tools and processes for measuring gears have come under great scrutiny by manufacturers, as well as reputable third parties such as NIST. In all the commotion, we sometimes forget that good solutions to seemingly complex measurement problems frequently involve the straightforward use of hand tools.
Depending on the manufacturing process, tolerance and volume of gears produced, there are many tools available for inspecting important characteristics. They range from simple but dedicated hand tools to elaborate analytical testing equipment.
In high-volume industries, semi-automatic and automatic gages have proliferated to keep up with checks essential for dimensional, functional and analytical process validation of gears. Hand tools, however, are both practical and fundamental, and they play an important role in setting up these high-volume manufacturing and inspection processes and performing some types of in-process manufacturing checks. Compared to expensive CMMs, generative analytical testers or gear roll testers, “off-the-shelf” hand tools have several important advantages: They cost much less, are accessible to more people, are portable and deliver results faster. Here are some basic tools that might provide some of the answers you need.
Gear size (DOB, DOP or DOW for dimension over balls, pins or wires) is the most frequently used method to monitor gear manufacturing processes for pre-sizing and/or tooling wear. Pre-sizing tests are used to control for additional processes such as shaving or grinding. Tool wear monitoring is one of the most common ways to track processes such as hobbing, shaping, broaching, powder metal, injection molding and “near-net” gear forging. These tests are often used as a general indicator for specific undesirable size variations, including irregularities requiring other measurements or calculations.
Runout is defined by the MAAG Gear Handbook as “The maximum radial position change of a probe, measured in any transverse plane, with a probe placed in successive tooth spaces while the gear is rotating manually or automatically about its axis. The reading corresponds to the probe depth position perpendicular to the gear axis of rotation.” Runout can often be used as a gross indicator for specific undesirable variations. A sound way to inspect runout is to use a measurement stand with a single probe and a single ball with a diameter that makes contact with the sides of the adjacent teeth at the area of the pitch circle.
Tooth thickness is another dimension important to a gear’s function, easily checked with a hand tool. Standard vernier calipers can be adapted to make this check easy for a direct measurement. The calipers have a small fixture that allows the top of the gear to be used as a reference point so that the ball contacts of the tool align on either side of the tooth precisely at the pitch point (for example, the widest point on the tooth).
Average tooth thickness can also be derived from the measurement of the span chord over a number of gear teeth without the need for any specific reference point. This is great for in-process or on-the-machine testing. The measurement is made with a standard vernier caliper or micrometer adapted with disc-type contacts. The contacts are placed with one on the right and the other on the left flank of the most distant teeth in the span to be measured. (See illustration.) Average tooth thickness is then determined using a standard formula.
These examples of checks commonly used in the monitoring of gear production demonstrate how hand tools can provide simple solutions to seemingly complex gear measurement problems.
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