Hollow steel ball bearings made to save weight

"In an injection valve the movement of a ball causes the valve to open and close," explained Dr Hartmut Göhler, project manager at the IFAM. "The lighter the ball, the quicker it moves. For the first time we have been able to produce metal hollow spheres in the required diameter of just 2 to 10mm. The hollow spheres are 40 to 70% lighter than solid ones." The process begins with polystyrene balls that are lifted up and held in an air current over a fluidised bed while a suspension of metal powder and binder is sprayed onto them. When the metal powder layer on the balls is thick enough, heat is applied so that the organic components, the polystyrene and the binder evaporate and escape through the pores in the metal layer. A ball is then sintered at just below melting temperature, and the metal powder granules bind together, forming a hard and cohesive shell. The sphere is then sufficiently strong to be ground in a machine, but the pressure must not be too high as otherwise the hollow body will deform. The wall thickness can be set to between a few tenths of a millimeter and one millimeter. Göhler sees applications for the technique wherever a low mass inertia is required. "Hollow spheres will create applications which have not been possible up to now." The scientists have already made ground steel spheres but metals such as titanium and various alloys are envisaged for the future.