Seems there is still a lot of confusion on load and life ratings when it comes to ball screws. Many people are familiar with the classic L10 life equation L10 = (Ca/P)^3 x 10^6 revolutions where Ca is the dynamic capacity in N and P is the mean applied axial load in N. Recently we fielded several inquiries on applications where an engineer wanted to know whether s/he can apply a force equal to Ca and achieve 1 million revolutions of life. Or even higher than Ca and achieve less than 1 million revolutions. The basic answer is NO! Here is why:
Most industrial ball screws are designed for a Ca/P ratio greater than 3. In many machine tool applications, the Ca/P ratio is actually as high as 10, bringing the calculated life expectancy close to 10^9 revolutions. That is the load range for which most industrial ballscrews are designed, and for which the life equation has been proven in thousands of applications. There are other effects that limit life (such as micro-welding and/or abrasion which cause loss of preload over time) but these are not factored into the classic life equation, and come into play mainly at the high end of the life calculation, that is beyond 1 billion revolutions. A ball screw that failed after 20,000 or 30,000 hours is typically not going to cause many questions to be asked as to what the reason was. So, for practical reasons, don’t rely on calculated life expectancies beyond 10^9 revolutions unless everything, including lubrication and protection from contamination, has been reviewed carefully.
If the load is higher than one-third of the dynamic load capacity, the contact forces between balls and races might become greater than was anticipated in the design of the ball screw. This could lead to accelerated wear, reduced smoothness and premature failure in general. In other words, the life equation is no longer valid in this range.
So, in our experience, the reliable scope of the equation is between 30 million and 1 billion revolutions. This means the allowable applied load is between about 10 and 30% of Ca – again, this is for most industrial ball screws.
It is possible to design a ball screw for operation outside this range – we do it all the time. But certain design parameters of the ball screw must be adapted to such usage. Lubrication can also get tricky at either end of the load range. For example, some frequently used lubricants may not be able to handle the contact pressures of a ball screw operated at or near its dynamic load capacity. Best to contact a ball screw engineer to review it.
Life of a ball screw is determined by material fatigue and wear from surface erosion of balls and races. While surface erosion is hard to predict, fatigue can be calculated. The equations that I will use here are the ones from the ISO 3408 standard, which is widely used. Continue reading