As a gear pair rotates through mesh the number of teeth in contact changes. For a spur gear with a contact ratio between one and two, the number of teeth in contact changes from a single pair, to two pairs, back to a single pair, etc. The angular stiffness between the gears is roughly proportional to the number of teeth in contact. This step change in stiffness contributes to the total transmission error and creates dynamic load at mesh frequency and harmonics of mesh frequency.
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