Combination with Resin Parts

Resin materials may cause problems such as breakage and deterioration depending on the components of the oil used in the bearings.
MinebeaMitsumi completes compatibility tests between resin materials and lubricants.

If there is a resin material around the bearing, the resin material may deteriorate or break due to the lack of compatibility with the lubricant filled in the bearing or the rust-preventive oil applied to the outer periphery of the bearing. This phenomenon is called "chemical attack".
In general, chemical attack tends to occur easily with non-crystalline resin, and tends not to occur with crystalline resin. This is probably because non-crystalline resin tends to penetrate lubricant between molecules and break intermolecular bonds more easily than crystalline resin. The resin material is selected in consideration of functionality, productivity and economics, so it may be difficult to only emphasize the countermeasures for chemical attack. Chemical attack tends to occur especially when the resin material is exposed to high temperatures while being subjected to physical stress, so measures such as stress mitigation and temperature reduction must be carefully considered.
Ester-based lubricants , that are commonly used, easily penetrate into the resin and have the tendency to cause chemical attack. Lubricants that do not easily penetrate include those using synthetic hydrocarbon-based, fluorine-based, and silicone-based base oils. MinebeaMitsumi is developing synthetic hydrocarbon grease and oil that are less prone to chemical attack.

The physical stress of resin parts includes those that remain inside during molding and those that are applied later in assembly.
These stresses not only promote the chemical attack, but also contribute to bearing mounting errors and deformation of the bearings themselves, so thorough verification is required to confirm that the stress on the bearing (movable section) has been eliminated. In addition, it is necessary to pay attention to interference with other parts as secondary processing after molding and the force applied to a distant location may have an effect.

Weld lines found in injection molded parts not only reduce the strength of resin parts, but also cause mounting errors and promote chemical attack due to penetration of lubricant. Care must be taken to ensure that there is no inappropriate weld line around the bearing.