Basics of Design Engineering - Bearings
Umbra ball screws equipped with Cerbec silicon-nitride bearing balls handle loads to 1,800 kN, axial speeds to 160 m/min, and accelerations to 3 g. The use of Cronidur 30 steel for certain components helps the units resist corrosion.
Electromechanical injection-molding machines that run only when making parts consume about half to a third the energy of hydraulic-based equivalents that idle between cycles. The all-electric machines also generate about 65 to 70% less waste heat, lowering plant air-conditioning costs.
Conventional injection-molding machines use a complex network of hydraulic actuators, hoses, tubes, filters, and valves to shut molds and inject molten plastics. Leaks are common, especially of concern now because of more stringent environmental regulations. In contrast electromechanical types swap out the hydraulics for simpler, cleaner, ball screws and electric drive motors. The ball screws transmit rotary-to-linear motion for injection slides and for clamping molds shut during the injection process.
Umbra Cuscinetti, Umbra Group, Italy, makes ball screws for aerospace and industrial applications and recently applied the technology to molding machines. High loads and lack of proper maintenance and lubrication typify conditions in this application. Helping ball screws survive the rigors are Cerbec silicon-nitride bearing balls from Saint-Gobain Advanced Ceramics, East Granbury, Conn. (www.cerbec.com).
"Alternating large silicon-nitride balls with smaller steel balls cuts adhesion, boosts load capacity and reliability, and extends ball-screw lifetime," explains Umbra Cuscinetti Stress Analyst and R&D Chief Engineer, Luciano Pizzoni. "The arrangement also lets users switch from oil to grease lubrication. Grease is more environmentally friendly and doesn't need replenishing as often."