Ball Screw Orientation
Another factor that engineers tend to forget about is ball screw orientation. Ball screws are designed to perform best when their loads are in the axial position. The reason is that there is usually a profile rail, linear bearing, or rail that is supporting the load while the ball screw itself is doing the motion.
Once that system is turned vertical, the load direction becomes one-unit directional with the forces completely downward. That has multiple effects on the design of the system, including how the ball screw wears during movement in both speed and acceleration. As the device moves up and down, the speed and deceleration adds extra load to the system. The result can be an implied impact load at the bottom, so reversing the load becomes critical to the design of the system.
Ball Screw Speed and Acceleration
Speed is another critical factor, but is best broken down into two parts: ball nut speed and screw speed. The first part applies to the screw itself, and refers to how fast the screw will spin. The length of the screw will often define the limits of screw speed. For example, the longer a screw is, the more vibration is possible. Vibration in the system will lead to corrosion and reduced life. Many designers want loads to move as fast as possible in order to reach the desired position as quickly as possible. Unfortunately, there are limitations with the screw that must be addressed.
The second part of critical speed applies to the nut. Here, critical speed refers to how fast the nut can spin within the limits of the return system, and reflects how fast the internal ball bearings recirculate. Miniature metric screw assemblies from PBC Linear have an internal return that is very smooth, quiet, and capable of higher nut speeds.
Ball Screw Duty Cycles
A duty cycle by itself is not overly critical. Usually it lends itself more into a discussion on screw life, which can get extremely complicated when considering a move profile. A move profile is typically a trapezoidal looking movement where there’s the initial acceleration, then constant motion, and finally deceleration. While these are all very critical, acceleration is one of those items that is typically disregarded. In fact, trying to find ball screw acceleration limitations in reference materials is extremely challenging, so it is often limited to a standard one-and-a-half G’s. That number is more of a guideline because actual max speeds, acceleration and deceleration are really application-based and often need to be defined through experimentation.
One of the great things about ball screws is their defined life. International standards clarify how we define the life of a ball screw. For metrics, it's usually a function of a million revolutions, which is our L10 life and where statistically 90% of ball screws are going to achieve this life. In reality, they may reach much more, but now there is an established minimum value.
Ball Screw Travel
With miniature ball screws, there are a couple different factors related to travel. In short travel scenarios of one or two-millimeters, difficulties arise because the balls are not fully recirculating within the nut. Defining ball screw life under these circumstances along with the design and function of the return system will play a critical role on how that's going to perform. For example, a fluid pump requires extremely short travel range of 10 to 100 millimeters. That last one millimeter of travel will experience the most force, creating possible issues when it comes to defining ball screw life.
