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, while liquid contaminants can interfere with proper lubrication — with both conditions reducing the expected life of the assembly and potentially causing catastrophic failure.
One way to protect ball screws from contamination is to completely enclose them in a bellows or telescoping-style enclosure. But the suitability of bellows or telescoping covers should be carefully evaluated, because the negative and positive air pressure caused by their contraction and expansion can act to “suck in” particulates. This air flow can also cause otherwise stationary particles in the environment to become airborne, essentially “stirring up” harmful dust and chips.
In most applications, the best way to protect a ball screw against contamination is by using seals on each end of the ball nut.
Manufacturers use different terminology to describe various ball screw sealing options, but most offer three fundamental types of seals: non-contacting, full-contact, and high-performance versions.
Non-contacting ball screw seals
As their name implies, non-contacting seals leave a very small gap between the seal and the screw shaft, with the most common design being plastic labyrinth seals. The lack of contact between the seal and the screw shaft means these designs don’t increase friction and running torque. However, they can allow smaller particulates and liquids to enter the ball nut, so they’re primarily used in environments where contaminants are in the form of larger, solid particles.
Full-contact ball screw seals
There are several forms of seals that make contact with the outer diameter of the ball screw shaft. The simplest is the brush seal. Although brushes don’t provide a tight, continuous seal with the screw shaft, they are very effective at wiping, or brushing away, larger contaminants and preventing them from entering the ball nut. Brushes are often used in conjunction with other types of full-contact seals, such as lip seals or segmented seals with preloaded “fingers.”
Lip seals and segmented seals are both designed to make full contact with the raceway of the screw shaft, providing excellent sealing against both fine particulates, such as ceramic dust, and liquids. Segmented seals typically use a spring mechanism to preload protruding “fingers” against the screw shaft. Lip seals are molded in a profile that matches that of the screw cross-section, ensuring there are no gaps between the seal and the screw shaft.
It’s important to note that although full-contact seals offer the best protection against fine particulates and liquids, they do increase friction and drag torque, which should be taken into account when sizing and selecting the motor. This additional friction also causes heat generation, which can affect lubrication.
Seals are considered a maintenance part in ball screw assemblies, and they’re relatively easy to visually inspect and replace. If the seal material is degrading, or if the fit of a contacting-type seal becomes loose, replacement is a simple, low-cost procedure that can prevent damage or failure of the screw assembly.
High-performance ball screw seals
Each manufacturer has its own offering of “high-performance” seals or seals for special conditions. These range from double- and triple-lipped seals, to combinations of two or more seal types, to full-contact seals specially designed for low-friction operation. And although standard ball screw seals are typically made of Viton (a type of FKM made by Chemours), manufacturers offer other materials, such as EPDM, that can withstand high- and low-temperature extremes and protect against specific chemicals.
Sealing plus lubricating
Sealing and lubrication for ball screws go hand-in-hand, and many ball screw manufacturers now offer sealing units that also store and deliver lubrication to the screw, increasing maintenance intervals while protecting against contamination.