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What is the life of a ball screw ? (2)

The life of every mechanical element is defined by using various units (time, distance, number of times).
The life of ball screws is defined by the total number of rotations, but converted into total operating time, travel distance, and the like to make the study of specifications easier.

Then, how reliable is the life obtained from the life formula?

According to the definition, the reliability is expressed by "rated life of 90%". However, this life has the following prerequisites:
"when used in an environment without dust and/or contaminants, there is no lubricant degradation, and is correctly mounted (that is, without mounting error)". 

The following explains the points to "maximize the life of ball screws" in order of importance from the viewpoint of design, assembly, and lubrication.

Important points from the viewpoint of design

(1) Protect the ball screw from dust and/or contaminants.

A ball screw works by rolling balls through the screw groove in-between the screw shaft and nut. If contaminants such as solid sand grains or iron powder gets in, scratches and pitting may form on the surface of the screw shaft and balls.
Those damages become the origin of premature flaking of the screw groove and balls.
Furthermore, powders such as sawdust and flour may lead to seizure and malfunction since when they mix with lubricants such as grease and hydraulic oil, they drastically degrade the lubricant performance, preventing the formation of oil film.
Some ball screws are equipped with wipers. However, since the wipers alone cannot completely protect the ball screw, a mechanism other than the ball screw such as a cover, bellows, and the like is necessary to cover the entire ball screw.

(2) Try to uniformly distribute the load.

A ball screw is a mechanical element that works with axial load (load in the axial direction). Therefore, it cannot receive radial load or moment load.
If the ball screw is subject to these kinds of load, its life will be greatly shortened due to load concentrated on a certain portion of balls and screw groove.

Important points from the viewpoint of assembly

(1) Accuracy of assembly is very important.

Improper alignment of the ball screw shaft, bearing, guide, nut, and housing and improper angularity may cause the ball screw to be subject to moment load, a load that the ball screw is not prepared for.
Consequences of the above include malfunction, abnormal noise, and vibration, which lead to shorter life in addition to the possibility of serious accidents such as breakage of the screw shaft due to rotating bending fatigue.

(2) Use within the operating temperature limits.

The operating temperature of ball screws for use in normal environments is normally set to 60ºC because bond may be used to attach ball circulation components, and resin and the like for wipers.
Furthermore, even when neither bond nor resin are used, attention is necessary when using in environments exceeding 100ºC since life may be shortened due to decreased hardness as the rated load, the basis of the life formula, is assumed to be 100ºC or lower where quenched materials can keep their hardness.

Important points from the viewpoint of lubrication

(1) Lubricant is essential.

In a ball screw, steel balls circulate in between the screw shaft and nut, which are also made of steel. Therefore, lubrication is essential to avoid direct contact between metals.
If you fail to refill oil, the oil film that prevents the contact between metals cannot be formed, resulting in seizure due to direct contact between the screw groove and balls.
The need for frequent maintenance and use of a lubrication unit to prevent lack of the oil film in between the balls and screw groove do not apply only to ball screws, but also to all mechanical elements that roll or slide.

(2) Choose a lubricant in accordance with the specifications.

When choosing a lubricant for ball screws, choose one in accordance with its specifications.
For example, an anti-rust oil will not serve as an hydraulic oil because the oil film formed when used on a ball screw will prevent rust from forming but it will not help the balls to roll or slide smoothly.
Choosing the right oil for the right place is very important to maximize the ball screw life.
For a detailed description, see "What is the purpose of lubrication?".

Other additional precautions to use ball screws safely are available in our BALL SCREW CATALOG. Be sure to read through it before using.

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