Miniature & Small ball bearings -17. Combination with plastic parts
Some plastics are susceptible to degradation and cracking due to the oil used in ball bearings.
Minebea does chemical tests for plastics and grease compatibility.
Chemical attack
Depending on the chemical compatibility, plastic parts such as housings in some application could be degraded by the grease inside the bearings or rust preventative oils.
This phenomenon is called "chemical attack". Chemical attacks occur in some specific combinations of plastic materials and lubricants. There are many factors that affect the chemical attacks.
When some physical force is applied to the plastics, grease penetration can cause the chemical attacks.
How to evaluate chemical attack
Minebea evaluates chemical attack by using 1/4 elliptical test method. Resin test pieces which are stressed by a test jig are applied to the lubrications or the rust preventive oils, then leave to stand for a given length of time. We quantify the limit strain amount from the point where cracks or crazes generate (see pictures on the right) and judge the compatibility between the resins and the lubricants, or the rust preventive oils.
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Fig.1 LY121 vs PC resin
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Fig.2 LY121 vs PC/ABS resin
The results of evaluating chemical attack (Representative Lubricants)
| Lubrication | Base oil | PC Flame Resistant Filler 30% |
PC/ABS Flame Resistant Filler 30% |
ABS Flame Resistant |
|
|---|---|---|---|---|---|
| Grease | LY121 | Ester | △ | × | × |
| LY551 | PAO | ○ | ○ | △ | |
| Oil | LY460 | Mineral | ○ | △ | × |
| LY650 | PAO | ○ | ○ | ○ |
* The test piece molded by a resin manufacture's recommended method and annealing condition.
* If you molded by a improper method, it may not apply to that result.
* Finally, please judge use propriety based on actual application test result.
Stress of plastic parts
Internal residual stress is the residual stress within injection molded plastic parts, and can be generated by "secondary process after molding" such as subjecting the parts to a bending process or ultra sonic adhesion. Although internal residual stress decreases slowly by gradual alleviation and annealing treatments, it is difficult to remove completely.
Oil and grease
Ester is the common base oil of grease. Compared to other types of oil, chemical attack occurs in ester oil more often, because it tends to penetrate plastics easily. Some types of oil such as "Synthetic Hydrocarbon, Silicone, and Fluorinated" oils resist against oil penetration better.
Weld line
Not only do weld lines seen in injection molded parts reduce the mechanical strength, but also lubricants penetrate easily, and chemical attack occurs at high rate. In the case of a fit with interference, cracking happens sometimes at weld lines regardless of chemical attack.
Plastic materials
While chemical attack tend to occur more in non-crystalline plastics (PS: Polyethylene, ABS, PC: Polycarbonate), crystalline plastics (PP: Polypropylene, PE: Polyethylene, POM: Polyacetal) have stronger resistance against chemical attack. Lubricants penetrate into inter molecular spaces more in non-crystalline plastics, and intermolecular bonding tends to break easily.
Against chemical attack
When selecting plastics, decisions can not be made solely based on chemical attack probability because there are other factors to be concerned about such as performance, productivity, and economy. Chemical attack will be less likely to happen if the internal stress is reduced, and the appropriate grease is selected.
Minebea is developing synthetic hydrocarbon greases, which could offer improved resistance against chemical attack.
