Ball screw systems
Fast & precise linear motion with ball screws
Ball screw systems consisting of a ball screw and a ball nut achieve linear motion with high precision and/or high speeds. Efficiency is high at about 90% and ambient temperatures up to 110 degrees are possible. Compared to mechanical belt driven systems they are more precise. Compared to electric linear actuators they are faster and more precise. Compared to pneumatic and hydraulic systems they offer advantages of precision, controllability and efficiency. When combined with a motor drive they make up a complete linear motion system.
HOW THEY WORK
The ball screw is an accurately machined threaded shaft with specific levels of precision and usually with customised shaft ends.
Ball nuts consist of a steel body, a quantity of balls, ball recirculation elements and wiper seals.
Rotation of the shaft causes the balls to roll in the ball screw threads and the matching threads inside the nut. The rolling action achieves high efficiency linear motion. The balls make a single, or several, revolutions inside the nut and then a recirculation element diverts the balls back to the start of their path. DIN69051 defines standard nut dimensions although specials are possible.
Ball screws are machined from 42/50CrMo4 steel hardened to 58-61 Hrc, on request with chrome plating for severe environments. Accuracy grades are IT7 for transport and general duties, IT5 for positioning and IT3 high precision.
Ball nuts are machined from 18NiCrMo5 steel hardened to 58-61 Hrc. Recirculation elements are plastic or steel for high temperature applications
Ball screws can be used in ambient temperatures from -20 to +90°C. Higher temperatures up to 110°C are possible by changing the material of the liners which divert the balls from their tracks.
Lubrication is required and an oil splash environment is optimum. However many applications use grease lubrication where the grease is periodically recharged at the ball nut. Although the ball nut can be provided with wipers, use in conditions with severe dust and dirt needs careful consideration.
Typical dynamic load ratings for industrial ball screws are from 1 to 150 kN although operating factors will reduce these loads in real life. Performance is usually a compromise between speed, load, duty cycle and lifetime. Dynamic load ratings are based on 1 million revolutions at that load. Linear speed depends on the rotation speed of the screw and the screw pitch which is typically in the range 5 to 40 mm. Up to 2 m/s linear speed is possible for light loads. All loads should be in-line with the screw as radial loads are detrimental to lifetime.