Welcome to this installment of Motion Control Classroom, or MC2, covering ball screws.
Recall that ball screws are mechanical devices that translate rotational motion to linear motion. There are three main components of a ball screw; a nut, a screw with grooves and balls, which can be made of steel, ceramic, or hard plastic. The balls rotate between the nut, screw, and grooves when either the nut or screw rotates.
More generally, power transmission applications use one of two primary types of screw drives: those that rely on sliding contact (lead screws), for applications that require corrosion-resistance, self-locking operation, and customizable nut designs, and those that use re-circulating balls (ball screws) or rollers, for applications that require high rigidity, excellent positioning accuracy, and high duty cycles.
Among ball screws, there are two primary divisions based on the manufacturing method; ball screws with ground threads and ball screws with threads formed by rolling. Ground ball screws have traditionally been the choice for high-precision applications, whereas rolled screws offered an economical solution for general industrial and automation applications.
In addition to some basics of ball screws such as the differences between rolled and ground ball screws, the fundamentals of preload, and ball screw standards, this installment of Motion Control Classroom also covers some basic calculations involved with preload as well as ball screw whip and how to account for shock and vibration in ball screw drives. There’s also information on types of seals for ball screws as well as some basics of ball screw operation in harsh environments.
You’ll find these and many more resources on a wide array of motion control components and systems, including other MC Classroom installments, at www.designworldonline.com/mc2/.
Editor, Design World
What’s the difference between ball screw critical speed and characteristic speed?
Ball screw standards: What you need to know about DIN, ISO, and JIS
Ground vs. Rolled Ball Screws: Does manufacturing method matter?
Examining screws from three different angles
Ball screw preload: What you need to know
Ball screw drives incorporate two primary moving components: the rotating screw shaft and the nut that traverses the screw. (The radial bearings that support the screw shaft play a minor role for the purposes of this discussion.) This means that to
If you’re involved in ball screw sizing or selection, you’ve probably noticed that there are several different standards that govern various aspects of ball screw design, from lead accuracy and load capacity to ball nut tolerances and rigidity. Fortunately, in many cases, these standards are in harmony
When a linear motion application requires high thrust force and rigidity along with good positioning accuracy, a preloaded ball screw assembly is often the best choice. Manufacturers offer ball screw preload amounts ranging from just 1 or 2 percent for applications where backlash would be detrimental
The thread design of a power transmission screw—whether a lead screw or a ball screw—plays a critical role in the screw’s function. Aside from the basic thread form (acme, trapezoidal, etc.), the three main aspects of the thread geometry—helix angle, lead angle, and thread angle
There’s much debate among manufacturers, and sometimes among users, regarding the superiority (or lack thereof) of ball screws with ground threads over those with threads that are formed by rolling. Ground ball screws have traditionally been the choice
How to account for ball and lead screw inertia during system design
What is ball screw whip and why does it happen?
How to account for shock and vibration loads in ball screw drives
How to calculate the effect of preload on ball screw axial deflection
When sizing a motion system with a ball or lead screw drive, the first step is to determine the screw diameter and lead that can meet the application requirements for thrust force, speed, and compressive (buckling) loads. Once the screw diameter and lead are determined, the next step is to choose a motor that can deliver the required torque and speed, while also providing sufficient control
Ball screws are often the drive mechanism of choice in applications that require high thrust forces with excellent positioning accuracy and repeatability. But one of the drawbacks of ball screw technology is that speed is inversely related to length — the longer the ball screw shaft, the more likely it is to whip, like a jump rope, as it turns. This behavior limits the maximum travel distance
Calculations of ball screw service life and permissible static load take into account loads and forces that are predictable and quantifiable — thrust loads due to acceleration, process forces, and forces generated when holding a load in place, for example. But some applications are also susceptible to loads caused by shock
In a recent post, we looked at the methods for inducing preload in a ball screw assembly and the effects it has on performance. One of those effects is that preload reduces axial deflection (displacement) and increases rigidity. But preload also increases the applied load
Ball screw seals: Types, options,and applications
Ball screws are used in a wide range of applications, many of which expose the screw and nut assembly to harmful contamination in the form of dust, chips, or liquids. And like any bearing that uses metal balls (or rollers) on metal raceways, ball screws can experience abrasive wear when exposed to solid contaminants can interfere with proper lubrication — with both conditions reducing the expected life of the assembly and potentially causing catastrophic failure.
How to protect ball screws in harsh environments
Harsh environments can wreak havoc on motion system components — especially rolling element systems such as linear guides and ball screws. And while protective devices such as rail covers, wipers, and scrapers are widely available for linear guides, protecting ball screws in harsh environments is a bit more tricky.
What makes miniature ball screws unique?
Ball screws are used in a wide variety of applications, but some of the most challenging are those on the extreme ends of the performance spectrum – from large diameter, large lead screws for machine tools, to screws with small diameters and very fine leads for optical and medical applications. For very small, high-precision movements, designers and engineers often turn to miniature ball screws.
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