More on Sintered Bronze
Mike DeHart, South Jersey
>I formerly worked in a tiny little division of Dana Corporation that made sintered powder-metal (PM) bearings used in clutches and brakes. You mentioned that to add a grease fitting you would need to drill through the bearing. This is certainly a PM bearing if the oil hole is not already drilled through. As was said below, PM is made by pressing fine metal powder into shape then heating (sintering) it to just below the melting temperature for a period of time, which structurally fuses the powder particles into a single part. The nature of compacted powder dictates that there will be interconnected pores throughout the finished part. This is good for oil. Oil will absorb into the porosity until the bearing is approximately 15%-20% oil. When the shaft rotates, the frictional heat drives a film of oil to the bearing surface. When the shaft stops and the assembly cools, the oil wicks back into the bushing. This works great as long as the bushing is saturated with oil. Since some oil is lost in use it need to be replaced. A drop or two each time you use it will keep the bearing saturated and working properly. If the resevoir has oil in it, the bushing is already saturated. The key here is to keep the bearing saturated. If the bearing is dry and you put a couple drops in the oil hole then start the machine you will kill the bearing since the oil cannot possibly wick through the pores fast enough to lube the shaft. Similarly if you put a drop of oil directly on the shaft, it will absorb into the bearing leaving the shaft dry and also ruining the bearing. There is a simple test to see if the bearing is saturated. Blow a heat gun on the bearing. You should see tiny droplets of oil form on the heated surface. If the bushing is dry due to use, abuse, or storage, be sure to saturate it before you run it. The quick industrial way to saturate a PM bushing is to place it in a jar of oil, draw a vacuum on the jar, hold the vacuum for a short time, then release the vacuum and let the part soak in the oil for a while. The air gets sucked out of the parts and the oil gets slowly sucked back in. The home method would be to keep the oil resevoir filled until the bushing soaks up all it can hold.