16 Nov 2016

Ball Screw
and Ball Screw Repair
All you need to know

First things first. We owe you an explanation of “What is a Ball Screw?”.

Without them probably you wouldn’t be able to brush your teeth! You can read more about it in our previous article.

What is a Ball Screw?

Ball screw is a mechanical device which does the job of converting revolving motion to liner motion, with a decreased level of friction. They are mainly used in various ranges of machinery such as low precision transport guides as well as high accuracy grade screws. They are also used in robots, machine tools, and precision assembly devices.

Classification of Ball screws

Ball screws are mainly classified into factors such as: axial play, pre-load, lead accuracy, life/load relationship. Axial play refers to the extent to which a ball nut can be relocated in the direction of the screw axis without the revolving motion of either the nut or screw. Preloading denotes the course of raising the stiffness of a ball screw which eliminates backlash. Lead accuracy focuses on the extent to which the shaft’s revolving movements are converted into the runs proportional liner movement. Lead accuracy and axial play are chiefly determined by the manufacturing technique used for the ball screw shaft.

Comparison of Ball Screws with conventional screws

The major advantage, out of the general benefits of ball screws is the fact that they can help raise efficiencies well above the 90% mark. Secondly, there are minimal thermal effects associated with ball screws. Also, ball screws can be conveniently preloaded to eradicate the issue of backlash. Ball screws are moreover known to offer smoother movement over the full travel range.

As compared to other alternatives available, the ball screw offers low friction plus generates higher efficiencies, unlike the acme and lead screws which are only up to 50% efficient. Though the ball screw is the more expensive choice, the high cost can be offset by the decreased power requirements for similar performance.

Ball screws, however, require high levels of lubrication and they must be properly lubricated at all times to prevent corrosion and to reduce friction. Lubrication also extends the operating life, leading to a more efficient level of operation. Care must be taken though to avoid contaminated lubrication which can lead to increased friction and lower efficiency. Ball screws may fail completely if the ball travels over metal chips or the dirt present in the ball thread raceway.

Using lubricants recommended by machine tool manufacturers can help in preventing these issues. Also, the use of telescopic covers and lip seals should be considered too as they provide a tight fit and thus, will keep out any contaminants in the environment. Since ball screws are bulkier ball screws are more prone to damage at the time of installation compared to other conventional screws.

Selecting a Ball Screw

In the process of selecting a ball screw, a few parameters must be considered. One must know the expected load and the operating speed needed as well as the positional accuracy required for a good start to sizing the right ball screw for the application. This information can help in determining the ball screw diameter and lead plus allow for the making of further decisions based on expected life, any special mounting configurations, assembly instructions or special environmental conditions.

Now we will explain to you how a Ball Screw works.

Ball Screw Assembly

The Ball Screw assembly comprises of a screw and a nut, each with matching helical grooves, and balls which roll between these grooves providing the only contact between the nut and the screw. As the screw or nut turns, the balls are deflected by the deflector into the ball return system of the nut and they travel through the return system to the opposite end of the ball nut in a continuous path. The balls then leave from the ball return system into the ball screw and nut thread raceways continuously to recirculate in a closed circuit.

Ball Nut Assembly

The ball nut determines the load and life of the ball screw assembly. The ratio of the number of threads in the ball nut circuit to the number of threads on the ball screw determines how much sooner the ball nut will reach fatigue failure (wear out) than the ball screw will.

Ball Nuts Are Manufactured With Two Types of Ball Return Systems

(a) The External Ball Return System. In this type of return system, the ball is returned to the opposite end of the circuit through a ball return tube which protrudes above the outside diameter of the ball nut.


(b) The Internal Ball Return System (There are several variations of this type of return system) The ball is returned through or along the nut wall, but below the outside diameter.

Example (1): Some manufacturers have one revolution circuits where the balls are forced to climb over the crest of the thread on the screw by the return system. This is known as a cross-over deflector type internal return system. In the cross-over deflector type of ball nuts, the balls make only one revolution of the shaft and the circuit is closed by a ball deflector (B) in the nut (C) allowing the ball to cross over between adjacent grooves at points (A) and (D).


Example (2): The Internal Ball Return System.
In this type of return system, the ball is returned to the opposite end of the circuit through or along the nut wall, but below the outside diameter through a “V” cap.


Example (3): The Tangential Internal Ball Return System.

For high speed or high load applications a tangential ball return system is used. This provides a very smooth flow of balls at any speed in a limited amount of space. This is a very durable ball return system and is also used on high load carrying applications.


Rotating Ball Nut Assembly

When a long ball screw rotates at high speed it can begin to vibrate once the slenderness ratio reaches the natural harmonics for that shaft size. This is called the critical speed and can be very detrimental to the life of a ball screw. The safe operating speed should not exceed 80% of the critical speed for the screw.


Still some applications require longer shaft lengths and high speeds. This is where a rotating ball nut design is needed.

The last step to our “Ball Screw- All you need to know” article is how to uncover a Ball Screw noise.

The uncovering of ball screw noise

Drives that use a ball screw have been deemed as noisy by classification, however, companies which purchase and make use of them are progressively demanding that their machine tools should be able to operate rather quietly to raise the level of work environment through the elimination of noise pollution. Thus, several engineers have looked into the matter of ball screw noise to see if the issue can somehow be resolved for betterment. To make sure that the experts were handling the problems in an appropriate manner, the help of Prof. Dr.-Ing. Paul Helmut Nebeling at Reutlingen University was sought who is a professional in the field of the dynamic, static, acoustic and thermal behavior of machines tools as well as components.

What causes the noise?

The fact that ballscrew noise is somewhat due to the thread geometry was already identified. All of this can be attributed to the undulating motion of the balls in their very paths, the typically huge powers these drives must specifically diffuse, along with the predominantly critical point- the smoothness of the thread surfaces on which the balls run. It was determined by a team of engineers that the high-frequency resonance of moving ball screw drives was the chief source causing the troubling noise in the machine tools.

The ball screws go through a cycle of high acceleration followed by deceleration and this specific tend is the reason behind the disturbing “whistling noise” which can be heard. After the team measured torque and sound emissions followed by the carrying out of suitable FFT investigation. It was identified that minute corrugations on the shaft thread suggestively affect the general sound produced by moving ball screw drives.

Later when the team of engineer investigated the surface of the shaft’s leveled thread under enlargement; they observed minute features measuring about one-half of one thousandth of a millimeter about the same wavelength of that of blue light. The facets apparently come into being because of the vibrations of the grinding wheel when it moves to ground the shaft. The team concluded they were the main cause of the irritating clamor; it was noticed by them that the noise’s incidence peak at 265 Hz matches precisely to that forecasted from the magnitude of the surfaces and the rotation rate of the drive’s shaft. When the bearing balls reel and move over the slight bumps or tiny facets, there is a certain amount of slipping, which causes the distressing whistle sounds which are quite noticeable.

How should ball screw noise be reduced?

As one of the measures to eradicate the noise, it was decided to reduce to grinding speed however that would lead to the problem of the shafts manufacturing becoming less economical. There is another treatment for noise reduction which is still in its testing stage. This is a new oil treatment in the grinding shop where cutting oil is maintained at a constant temperature. Using this procedure, the temperature in the grinding wheel’s contact zone does not increase during machining and the end result is the production of smooth shafts with this method.

Now you know it all! You became a Ball Screw Master!