Challenges of gear measurement and the instruments to solve them
January 17, 2019
Within the geometrical dimensioning and tolerance (GD&T) measuring process for production quality assurance, the subset of gear measurement presents a particular group of challenges that are very difficult to tackle with conventional measuring tools. Gears are defined as rotating machine parts which have “teeth” which mesh with another toothed part in order to transmit torque force power from a rotating source, and two meshing gears must have the same shape, or profile. Then, with different combinations of gears working in sequence we get a transmission or gear train; a gear may also be part of a linear piece, producing linear movement (or traslation) instead of rotation, and we call this a rack. Gears are analogous to pulleys, but with the advantage that teeth prevent slippage.
Types of gears and measurements
The general issue with gear dimension measurement is the intrinsic complexity of what a gear (or cogwheel) is, the main problem being that there are several different type of gears; first and foremost, gears may be internal or external, meaning that the position of the teeth is either inside or outside of the cylindrical dimension of the gear, then we have basically the following types:
Spur gear: where the teeth projecting radially from the rotation axis of the cylinder geometry. This is the most simple type of gear
Helical gear: where the edges of the teeth are in an angle from the rotation axis, making the shape a part of a helix. This gear may be set in crossed orientations, where the shafts are 90° angled from each other, thus changing the force application direction. However, helix gears are noisy at high rotation speeds, since they generate a lot of friction and thrust along the gear’s axis. A solution to this problem are the skew helix gears, where the angle of the teeth is opposite, reducing the thrust.
Bevel gears: these gears are shaped over a cone configuration, either straight or spiral shaped, allowing positioning angles between 0° and 180°.
Hypoid gears: similar to bevel gears, but the shaft axis do not intersect, and the gear shape is actually a hyperboloid.
Worm gears: these are a mix of a screw meshed with a helical gear. A very simple way to achieve high torque on a small gear ratio. The gear on the screw part (or worm) is basically a helical gear, but with a very large angle, close to 90°. One useful advantage of worm threads is that it is actually difficult for the gear to drive the worm, so some configurations are self locking.
We may mention more complex configurations, such as harmonic, planetary, epicyclic, clycloideal and even non-circular, but this should be enough to realize the complexity of gear geometrics.
Among the many characteristics that are expected to be measured by the quality department GD&T team we find: circular pitch, circular thickness, tooth height, tooth flank and face profile, pitch surface, and then the geometrics related to regular cylindrical measurements, such as run-out or concentricity. Many of these are a composition of many specific separate measurements, and while complex machines such as CMM may provide many of this measurements, the intrinsically complexity of the gear shape demands specialized equipment in order to obtain the optimal results.
Osaka Seimitsu Kikai products
Within these types of machines the Osaka Seimitsu Kikai (OSK) products excel on delivery quick, accurate and clear information on the gear geometry, no matter what type you are measuring; profile deviation, helix deviation, nick, runout, circular display, eccentricity, diameter, composite deviation, all this data is accurately presented by the OSK machine line-up. With constant research and development and collaboration with universities and industrial associations, OSK continues to establish the lead precision and evaluation methods aiming to provide high quality products for high precision measurement.
The GTR Series line-up for double flank gear rolling testing is a versatile group of models that allow gear measurements of 25mm to 600mm center distanced gears. Capable of measuring either pinion or wheel type gears and automatically doing pass-or-failed judgement on the workpiece through a linear variable differential transformer (LVDT) that reads the center distance changes of mating gears .
The CLP Series works with bigger gears with a max. weight of 150kg allows to verify tooth profile, lead, single pitch, adjacent pitch and cumulative pitch, runout (both helical and spur) whether internal or external, with a resolution of 0.0001mm and a diameter up to 350mm, and shaft lengths up to 800mm (with optional attachments) and tooth widths up to 400mm. The CLP Series versatility is so good that you may measure other items that, while not gears, they are used for gear cutting tools, such as shaving, pinion or hobbing cutters, or other type of toothed meshing items such as automotive transmission parts.
Whatever your particular needs Osaka Seimitsu Kikai is the optimal solution for you gear measurements needs. Contact your Yamazen representative in order to select the best measurement machine configuration for your solution.