Sintered bearings behave much like a sponge by storing oil, and then releasing whenever it is needed. This happens when the joint starts to spin and causes heat. Heat causes the bearing to release oil and lubricate the running joint. A neat idea - until it is a quater of a centuary old!
The assembly consists of two sleeve bearings inserted at both ends of a cast metal holer. First the bearing is checked visually and everything is in good shape. Swabbing with cotton buds and alcohol does not show any dirty lubricant. The assembly is then inserted into a oven. Heat is then slowly and carefully applied to the joint to ensure that expansion is as linear as possible.
Our oven has a grill function will allows the heat to circulate. This ensures that the heat is even at all parts of the oven - useful!
After about 15 minutes at 250 degrees Celsius, thick , dark brown oil starts to drip out from the bearings! This is caused by the heat which causes expansion of the bearing which releases the oil.
The bearing is then slowly cooled down, then heated up again. This causes a small quantity of brown oil to be collected on the foil.
On the right, you can see the some of the dirty oil on a kitchen towel. (Thanks Wife for the oven and towel!)
Next the assembly is immersed in solvent (like common paint thinner in an open ceramic or glass bowl) and then set back into the over at a lower temperature of about 130~150 degrees Celsius.
This has to be done in a well ventilated area because we are dealing with heat and solvent. The bowl is then taken out and let to cool in outside air. Because the assembly is immersed, it will take some time to cool down. The assembly is then set back into the over for more heating. Doing this about twice turns the clear solvent into a brownish mixture which contains more used oil.
I then change the solvent and perform the heat and cool cycle until the solven runs near clear. Finally I see some clean oil which is the unused oil that was deep inside the bearings. The dirty oil would have clogged the pores and prevented the remainder of the clear oil from lubricating the bearings. On the picture on the right shows the solvent mixed with brown oil. Some of the clear oil is visible at the bottom of the bottle. The dirty oil comes out first and now the clear oil is able to exude from the bearing. However, now the bearing is void of oil and needs to be filled up with new and proper oil.
I will have to find out the best oil with the correct number for lubricating this grade of sintered bearings. Too viscous and not enough will enter the bearing. Too runny and the bearing will not soak and hold the oil.