What to consider when selecting a ball screw
There are very many factors that influence ball screw selection. The following are a few key areas to consider.
Axial force required
All ball screws require guidance and can only ‘see’ axial forces. The required axial force will be a function of the mass of the load and the coefficient of friction of the guide system to be used. Orientation of the application should also be considered, i.e. whether vertical or horizontal.
Once the working load is calculated, then a provisional selection can be made by selecting a ball screw nut with a dynamic load rating that is a minimum of 3 times the working load. The working load s
hould include a factor for the working conditions.
Lead accuracy
This can be quite an involved aspect of the specification; reference should be made to catalogue information and explanations of fluctuation, target value for reference travel distance, etc.
Rolled ball screws typically have accuracies of G9 to G5. Essentially, the lead accuracy is the amount of fluctuation against a given thread length of 300mm. Higher grades are offered by ground ball screws
Axial clearance & preload
Axial clearance is normally in the range of 50 to 100 micr
ons for rolled ball screws. Reduced clearances are possible. Backlash can be eliminated by ball selection.
Preload may be required to eliminate axial clearance and to minimize displacement under an axial load. Preload can be achieved using a variety of methods, e.g. pitch shifting or use of spacers. Highly accurate positioning systems would normally have a preload.
Critical Speed
This refers to the maximum rotational speed of the shaft. It is dependent on the mounting distance, the shaft root diameter and how the shaft is mounted (fixed-free, fixed-supported or fixed-fixed)
Rotational speed of the nut
The maximum rotation speed of the nut is determined by its DN value and the ball circle diameter. Typical DN values for a rolled ball screw are 50,000 and 70,000 for a ground screw or high lead rolled ‘screw. Recent advances in manufacturing technology have led to increases in these values.
It is important to remember that the maximum rotational speed of the ball screw assembly is the lower of the max nut speed and shaft speed.
Ball screw life
The rated life will determine how many revolutions the ball screw will complete before fatigue life. It is a function of the basic dynamic load rating, axial load and a factor for the working conditions.
The above are just some of the considerations to be taken into account when selecting a ball screw. There are other factors involved.
At Acorn we offer high quality ball screw (& roller screws) from THK and SKF. We are always happy to provide assistance with ball screw selection and specifications.