The reason motion system designers often specify ball screws instead of guide screws on ball screws is the ability to operate continuously at higher speeds. However, the ball screw itself has a speed limit. Understanding these will help you optimize ball screw assembly performance in applications ranging from small laboratory fluid pumps to large overhead gantry frames and high performance machinery.
Basic knowledge of ball screw
The ball screw assembly consists of a screw and a nut with a circulating ball bearing. The connection between the screw and nut is achieved by ball bearings, which roll into a matching ball form within the nut and screw shafts. This distributes the force across many bearings, resulting in a relatively low load per ball bearing and a low coefficient of friction, which helps to achieve higher operating speeds (Figure 1)
- The friction coefficient of the ball screw is very low because the load is distributed on multiple bearings, and these bearings circulate as the shaft rotates.
Screws and nuts often have unique maximum RPM limits, so selecting the most effective ball screw assembly requires attention to both.
Design characteristics of ball screw
The main screw shaft design features that have the greatest impact on RPM are end mount, screw lead, and screw diameter.
Each ball screw has an inherent frequency at which it vibrates excessively, which is called the critical velocity. When the applied velocity is consistent with the critical velocity, the screw vibrates harmoniously (Figure 2). The screw’s inherent imbalance then resonates with it, which can cause excessive bending, vibration, and eventually failure.
- The lead screw shaft may vibrate at high RPM.
End installation. Installation options include fixing one end in a thrust bearing, supporting both ends in a floating bearing, fixing one end and supporting the other end in a floating bearing or fixing both ends in a thrust bearing. Maximum speed can be achieved when both ends are equipped with ball screws. The fixed end is subjected to a moment load, so the ball screw tries to remain perpendicular to the plane of the supporting bearing.
However, fixed ends can be more expensive and difficult to install than other installation options. When fixed at both ends, the distance between the support blocks is not adjustable and may make it difficult to align the mounting holes.
The screws. Assuming a constant input speed, a higher lead will increase the wire speed by making the nut move faster along the screw. When comparing two screws with different leads with constant linear velocity, a higher lead will reduce the speed of the input motor. However, higher leads do increase the torque required to rotate the screw.
Diameter of screw. Larger diameter screws increase load capacity and maximum RPM rating. Smaller diameter long screws will have lower RPM. A larger diameter screw will have a higher speed rating if the same wire is used.
What limits the maximum RPM?
The maximum RPM of the ball nut is limited by the speed of the ball bearing circulating in the ball nut. Exceeding the maximum RPM of the ball nut may cause permanent damage to the ball cycling components. Ball bearings may rupture or become stuck from the return system, which may result in complete equipment failure.
- Compare the ball return system (left to right) : inside, button and outside.
There are three main types of return systems: internal, button, and external (Figure 3). The purpose of backflow is to recirculate the ball bearing in the nut. The design of the reflux system will affect the speed limit of the nut.
External return systems usually deflect the ball into the return path using a pinky protruding into a spherical groove. At high speeds, the finger may bend against the ball bearing and eventually break. Buttons and internal designs are generally more robust than external return systems and can often be operated at higher speeds.
Speed of optimization
A system optimized for speed might need to meet the following conditions:
▪ Fixed end bracket for screw assembly
▪ Higher leads will increase linear speed
▪ Large diameter screws increase load capacity and RPM rating
▪ Internal oil return system for ball nuts
However, many other factors come into play, including load characteristics, position accuracy, repeatability, desired life expectancy, size limitations, input power requirements, environmental conditions, and available budgets.
It is possible to calculate the effects of most factors that affect speed. Ball screw manufacturers typically recommend the highest speed for their products and provide tools that help you accurately measure physical trade-offs.
