Gear Pitch Gauges
One of the most common questions we see is “what gear is this?” – usually when people come in with a gear (or pieces of a gear) and need a replacement gear made. Whether your gear has worn down, shattered, or just doesn’t fit, a gear pitch gauge is one of the best methods of determining what an existing gear is.
What are Gear Pitch Gauges?
Gear pitch gauges are short segments of known gears, usually just a few teeth, that work by meshing with and running on an unknown gear. In order for the gears to mesh, both the gear pitch and pressure angle need to be the same. If you have access to it, the mating gear (whichever hasn’t worn down or broken) could be the perfect gear pitch gauge if its pitch is already known. Since this isn’t often the case – lots of gears aren’t labeled – having a standard set of gear pitch gauges is critical to the reverse-engineering process.
Coincidentally, the key value from the gear pitch gauge isn’t the pitch measurement so much as the pressure angle of the gauge. That’s because the gear pitch can be precisely calculated from its measurements. Dividing the number of teeth by its pitch diameter will give you the diametral pitch of the gear. For English/imperial gears, this will very likely be an even integer; gears with a diametral pitch less than five are incredibly stout. Metric gears have similar measurements, but instead divide the pitch diameter by the number of teeth (1/DP) and convert to millimeters, if needed. These values will be clean decimals ranging from 0.4 for very fine gears to 3+ for coarse gears.
Measuring Pressure Angle with Gear Pitch Gauges
The pressure angle of a gear can be tougher to discern from quick inspection or measurement without the right tool. The pressure angle informs the overall shape of the gear tooth such that when two gears mesh, their teeth meet at the pressure angle. Most diametral pitch gear pitch gauge sets will include both 14.5º and 20º pressure angle gauges so the operator can test both and determine which fits better. Since the pressure angle doesn’t influence the outer diameter or root diameter, which are the most easily measured, only tools that measure along the tooth itself can detect and report the pressure angle. A gear pitch gauge in the hand of a skilled (or patient) operator can readily discount the pressure angle, while (for fine gears) Measurement over Pins measurements might get lost in noise.