Abstract
Magneto-rheological fluid (MRF) consists mainly of iron particles with a diameter of several micrometers suspended in oil. When a magnetic field is applied to MRF, the iron powder forms clusters around magnetic flux line, and shows viscoelasticity to increase resistance force. With good linearity and short response time, MRF is attractive for constructing compact passive devices. Today, MRF has many applications in various fields including automobiles, such as shock absorbers and engine mounts. However, rotary type application such as brakes or clutches is rarely implemented, because of requirement for highly durable MRF seals. The slip distance of a rotary seal is one order of magnitude larger than that of a liner seal. Clearly, existing seals can't satisfy the required life time, resulting in a leak within a short period of time. In this study, we test the durability of existing seals and analyze the cause of MRF leaks. Based on the analysis, we propose a high durability MRF seal suitable for rotary applications. We tested the endurance of conventional oil seals and fluoropolymer (PTFE) seals. PTFE seals have lower performance in oil sealing but expected to be more durable than conventional oil seals. The lip of the oil seal was damaged after 105 km of slip distance. On the other hand, the PTFE seal started to leak iron powder after 792 km (7.5 times compared with baseline oil seal) even without any visible damage on the seal lip. Referring to the appearance and profile of the shaft surface after tested with PTFE seal, deep scratch and deposit of sludge were formed on the shaft surface. The sludge is made up of iron particles both from the MRF powder and wear debris of the shaft, bound by degraded oil. Maximum valley depth of 3 μm and maximum peak height of 4 μm were observed, both of which is larger than the minimum diameter of iron powder particle of 3 μm. It is known that wear debris of the shaft (uncoated iron powder) is easier to coagulate than the MRF iron powder coated with silica, suggesting that the wear debris is the cause of the sludge. To prevent wear debris from forming, we coated a portion of the shaft with DLC (Diamond-Like Carbon) where it contacts the PTFE seal. Because the PTFE seal doesn't have a good performance in sealing oil, we set another oil seal in tandem with the PTFE seal. The combination of PTFE seal and DLC coated shaft achieves high durability against wear as well as low friction. To prove the effectiveness of this structure, we conducted a long term endurance test. We observed that the life of the seal with 1.5 μm DLC coating was extended to 2,270 km, 22 times longer than the baseline conventional oil seal. And with 3.0 μm DLC coating, no leakage was observed after 5,300 km of slip, 50 times longer than the baseline. This structure is promising in extending the life of rotational seals for MRF.
KEYWORDS
Transmission, Brake, Clutch, MRF, Seal