WHEN SHOULD I CHOOSE A BALL SCREW SYSTEM?
One deciding factor is the need for control. An electric driven ball screw is far easier to control compared to pneumatic or hydraulic systems. This is especially true if high accuracy is needed, and ball screws can position with mircron accuracy. Other significant factors, some of which may apply, are high linear speeds up to 2 m/s, long strokes up to 5 m and high efficiency.
WHAT PERFORMANCE IS POSSIBLE?
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.
WHAT AMBIENT CONDITIONS ARE NEEDED?
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.
WHICH INTERNATIONAL STANDARDS APPLY?
The international standards BS ISO 3408 and DIN 69051 are a big help in the selection and application of ball screw systems. They define performance ratings, accuracy grades and the dimensions of screws and nuts. As a general guide, the performance of one system manufactured to these standards will match that of another, although components will not be interchangeable.
WHAT ARE THE STANDARD ACCURACY GRADES?
Based on the standards ISO 3408 and DIN 69051, there are accuracy grades that range from 1 (very high) through 3,5 and 7 to 10 (low). Material transport applications generally require lower accuracies. Typically 5 and 7. Positioning generally requires higher accuracy, typically 3 and 5.
The grades most commonly used are 3,5 and 7. The accuracy grades reflect the precision of machining of the ball screw and are defined by the thread error of a 300 mm length. Even the lowest of these three levels has a tight limit of 52 microns travel error. Class 7 is the least accurate, yet it suits about 80% of industrial applications. Class 5 reduces errors by half and suits precision positioning. Class 3 reduces by half again to give extremely high precision, for example in machine tools.
WHAT ABOUT EFFICIENCY AND ENERGY CONSUMPTION?
Ball screw systems are an efficient way to achieve linear motion with typical system efficiencies around 90%. this is far above linear actuators (typically 40%) and screw jacks (typically 25%). Compared to hydraulic and pneumatic systems, balls screws have the advantage that no energy is used when at a standstill. Fluid alternatives require continued pressurisation and furthermore leakage can be a problem. The high efficiency of ball screw systems means that high duty cycles are possible, even up to 100%.
WHAT LIFETIME SHOULD I EXPECT FROM A BALL SCREW?
Lifetime depends on the actual loads, the rated loads, any shock loading and the speed. Calculation is best left to the manufacturer. Where a target lifetime is specified, the sizing and features of the ball screw can sometimes by adjusted to achieve it.