| description abstract | Previous investigations have demonstrated the principle and feasibility of quasi-hydrodynamic lubrication using powders. With conventional lubricants, the hydrodynamic behavior of a journal bearing can be summarized in the form of a plot of friction coefficient against a nondimensional load parameter, usually referred to as a Stribeck curve. One of the main features of the curve is that, as the thickness of the lubricant film increases, the friction coefficient initially decreases until it reaches a minimum and then increases due to shearing of the lubricant film. Previous tests on a three pad journal bearing using molybdenum disulfide powder as the lubricant showed that its friction/load characteristics followed the Stribeck form. The experimental work reported herein describes further tests carried out on a three-pad journal bearing to evaluate its thermal stability and long term wear characteristics using tungsten disulfide powder as the lubricant. Dry tungsten disulfide powder lubricant has been shown to provide excellent wear resistance. Its friction properties are similar to molybdenum disulfide but it can be used at much higher temperatures. Bearing tests were carried out at speeds up to 30,000 rpm and loads up to 236 N. Over the load/speed range stable thermal operation was achieved. From friction torque measurements it was found that the coefficient of friction for the bearing was essentially constant over the range of loads and speeds. Unlike liquids, powders have a limiting shear strength property. When the limiting shear stress is reached in a powder flow, the powder essentially shears along a plane and does not flow in a viscous manner. The constant friction coefficient seen with the tungsten disulfide powder suggests that the powder had a low limiting shear strength and that this dominated the operation of the bearing in the tests. The total accumulated test time was more than eighteen hours. Periodically, the test journal and bearing pads were inspected but there was no measurable wear or visible damage. | |
