Many engineers are confused about this topic, especially if they don’t work frequently with ball screws. So let’s take a moment to explain the terminology.
There are generally two load capacities given for a particular ball screw: Dynamic and Static.
Dynamic Load Capacity
The dynamic load capacity (DLC) is simply a load rating. It’s the load for a (theoretical) life of 1 million revolutions (ISO/JIS standard) or for 1 million inches of travel (ANSI standard). The DLC is critical for making lifetime calculations, since the expected life goes as the cube of the ratio of DLC to actual load. So, if the load is only 10% of DLC, then the expected life is 1 billion revolutions.
But that still doesn’t mean you could run the screw with this kind of load and expect a to achieve that life! Why? The normal maximum operating load of a general use ball screw is about 30% of the DLC. (Above that, the elastic deformation of the balls and races is too large, which may cause excessive wear.) So, if you run a ball screw with a load equal to its DLC, you may not get as much life as you expect.
Static Load Capacity
The static load capacity (SLC) indicates the load above which the screw may be damaged. Staying below SLC ensures that balls and races don’t suffer brinelling (or plastic deformation). This is critical! But bear in mind that a ball screw may be damaged in other ways with loading below the SLC: you could easily tear off the flange of the nut, snap the bearing or drive journal of the screw, or even collapse it from exceeding the buckling load.
Please consult a Steinmeyer engineer for an assessment of your application.
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In this case study, learn how Steinmeyer used its full capabilities to meet this challenge. We designed and assembled an innovative, multi-axis system, driven by ground, stainless steel ball screws.
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You will learn how mechanical engineers at a major OEM increase system speed and flexibility to satisfy the demands of high-throughput customers.
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Ball screws classified as “double nut” appear to be aptly named. However, what seems to be a simple distinction varies from one vendor to another. Let’s clarify the true definition of a double nut and the unique features of Steinmeyer designs.
Taken literally, a double nut is composed of two separate nuts that are more or less identical. The purpose of this design is to preload the two nuts against each other and achieve two-point contact on the balls. Preload eliminates play and helps build additional stiffness, keeping the nut centered on the shaft. While there are many ways to achieve preload, Steinmeyer defines double nuts as those that have two-point contact. This makes sense as it describes the double nut from a performance point-of-view, regardless of the design details.
The main design challenge involving double nuts is how to join them. Various solutions have been developed around the industry to solve this issue, but most require complex joining with spacers, shims, fitting keys, pins, and setscrews. We believe in a simpler approach.
Steinmeyer’s patented UNILOCK coupling system avoids complicated hardware by using a special epoxy. It is proven to deliver a robust connection of the two nut halves, making our double nut nearly as compact and stiff as a single-piece design. It also prevents radial slippage of the two halves, so that the UNILOCK double nut cannot be misaligned. The coupling is rugged and absolutely tight to prevent the loss of lubricant.
Contact our motion control engineers to learn the benefits of UNILOCK for your application.
In recent years, ball screws have been looked at more and more favorably as replacements for hydraulics. They offer higher efficiency and speed, and can be controlled more precisely with servo loops. And who wants to deal with the unavoidable maintenance and disposal hassle of hydraulics?
Steinmeyer has been supporting this market shift with its line of Ultra Thrust ball screws. These screws utilize larger balls to support much greater load capacities. More recently, we have introduced innovations in through-the-nut return systems to push performance even higher.
The Ultra Thrust line began with our 50×20 mm nut using 15 mm balls. Our standard nut of that size, with 9.5 mm balls, was rated at a dynamic capacity of about 120 kN (12 tons). But the Ultra Thrust version exceeded twice the capacity, reaching 262 kN. We then expanded the line to include larger nuts. Most of our Ultra Thrust screws in the field are sized at 80×20 or 63×20.
More recently we extended the line with a 100×25 mm nut, using 19 mm balls. These screws have proved themselves in the most demanding applications of injection molding.
Beyond increasing the ball diameter, Steinmeyer has designed new return systems to optimize the load capacity. Our latest innovation uses multiple through-the-nut return paths, shown in the schematic below. With this system, the screw exceeds 1,400 kN of dynamic capacity!