Today's KNOWLEDGE Share :Wear is a common failure mode for plastic gears.

Today's KNOWLEDGE Share

Wear is a common failure mode for plastic gears.

It affects the transmission error and the resulting NVH, and in its final stages leads to a complete failure of the gear.

Wear control is therefore a standard step in a rating procedure of a new design of a plastic gear pair, or in an optimization process of an existing one.

Contact conditions between two meshing gears are very complex. There is:

⚙️ changing rolling-sliding ratio,

⚙️ load sharing (affected by the load-induced contact ratio increase),

⚙️ change in the direction of friction.

These complex conditions are challenging to replicate by any tribological test. Wear factors generated by gear-on-gear tests prove to provide the most reliable wear prediction calculations.

A quick overview of a gear meshing process :

The theoretical path of contact for the involute gears has the shape of a straight line. The gears start to mesh in point A; this is point A1 on the flank of drive gear and point A2 on the flank of the driven gear. In the meshing area A-B, two pairs of teeth are in contact, wherefore the transmitted load is divided between them. Point B is the highest point of single-tooth contact for the driven gear. In the area B-D, the total load is transmitted only through one pair of teeth hence the contact pressure and the stress in the material increases. Point D is the lowest point of single-tooth contact for the driven gear; at this point the next pair of teeth comes into contact and the load in the area D-E is again transmitted via two pairs of teeth. Meshing ends at point E; this is point E1 on the flank of the drive gear and point E2 on the flank of the driven gear. The load on a single tooth is not constant during meshing along the path of contact.

Rolling and sliding motion are present between the surfaces in contact. When the gears mesh from A to C, the flank part A1C1 on the drive gear meshes with the flank part A2C2 on the driven gear. Due to the different lengths of the flank parts in contact, specific sliding occurs between the surfaces in contact. Analogously, the same happens in the meshing part from C to E, except that when passing through the pitch point C (also kinematic point), the direction of sliding is reversed. In theory, there is no sliding in the pitch point C, only pure rolling; in reality however, due to tooth deflection, sliding also occurs in point C. The direction of sliding and the frictional force are reversed when passing through the pitch point C. On the driven gear, the direction of sliding always points towards the pitch point C, so the kinematic line is usually clearly visible on the worn gear surface.

source:Damijan Zorko