Ball screw / ball bearing screw / recirculating ballscrew Mechanism

Ball screw / ball bearing screw / recirculating ballscrew Mechanism:-


It consists of a screw spindle, a nut, balls and integrated ball return mechanism a shown in Figure below. The flanged nut is attached to the moving part of CNC machine tool. As the screw rotates, the nut translates the moving part along the guide ways. However, since the groove in the ball screw is helical, its steel balls roll along the helical groove, and, then, they may go out of the ball nut unless they are arrested at a certain spot. Thus, it is necessary to change their path after they have reached a certain spot by guiding them, one after another, back to their “starting point” (formation of a recirculation path). When the screw shaft is rotating, as shown in Figure, a steel ball at point travels 3 turns of screw groove, rolling along the grooves of the screw shaft and the ball nut, and eventually reaches point. Then, the ball is forced to change its pathway at the tip of the tube, passing back through the tube, until it finally returns to point. Whenever the nut strokes on the screw shaft, the balls repeat the same recirculation inside the return tube. Thus, it is necessary to change their path after they have reached a certain spot by guiding them, one after another, back to their “starting point” (formation of a recirculation path). When the screw shaft is rotating, as shown in Figure, a steel ball at point travels 3 turns of screw groove, rolling along the grooves of the screw shaft and the ball nut, and eventually reaches point. Then, the ball is forced to change its pathway at the tip of the tube, passing back through the tube, until it finally returns to point. Whenever the nut strokes on the screw shaft, the balls repeat the same recirculation inside the return tube.

Characteristics of ball screws:-

1. High mechanical efficiency


In ball screws, about 90% or more of the force used to rotate the screw shaft can be converted to the force to move the ball nut. Since friction loss is extremely low, the amount of force used to rotate the screw shaft is as low as one third of that needed for the acme thread lead screw.

2. Low in wear

Because of rolling contact, wear is less than that of sliding contact.Thus, the accuracy is high. Ball screws move smoothly enough under very slow speed. They run smoothly even under a load.

3.Thread Form

The thread form used in these screws can either be gothic arc type (fig.a) or circular arc type (fig.b). The friction in this kind of arrangement is of rolling type. This reduces its wear as comparison with conventional sliding friction screws drives. Recirculating ball screws are of two types. In one arrangement the balls are returned using an external tube. In the other arrangement the balls are returned to the start of the thread in the nut through a channel inside the nut.

4. Preloading

In order to obtain bidirectional motion of the carriage without any positional error, the backlash between the nut and screw should be minimum. Zero backlash can be obtained by fitting two nuts with preloading (tension or compression) or by applying a load which exceeds the maximum operating load. 

Effects of preload

Zero backlash: It eliminates axial play between a screw shaft and a ball nut. It minimizes elastic deformation caused by external force, thus the rigidity enhances. In case mounting errors, misalignment between the screw shaft and the nut may occur this further generates distortion forces. This could lead to the problems such as, 

Shortened service life

 Adverse effect on smooth operation

 Reduced positioning accuracy 

Generation of noise or vibration

 Breakage of screw shaft

Advantages of ball screws:-  


 • Highly efficient and reliable.

 • Less starting torque. 

• Lower co efficient of friction compared to sliding type screws and run at cooler temperatures 

• Power transmission efficiency is very high and is of the order of 95 %.

 • Could be easily preloaded to eliminate backlash.

 • The friction force is virtually independent of the travel velocity and the friction at rest is very small; consequently, the stick-slip phenomenon is practically absent, ensuring uniformity of motion. 

• Has longer thread life hence need to be replaced less frequently.

 • Ball screws are well -suited to high through output, high speed applications or those with continuous or long cycle times.

 • Smooth movement over full range of travel.

Disadvantages of ball screws :-


• Tend to vibrate. 

• Require periodic overhauling to maintain their efficiency.

 • Inclusion of dirt or foreign particles reduces the life of the screws. 

• Not as stiff as other power screws, thus deflection and critical speed can cause difficulties.

 • They are not self-locking screws hence cannot be used in holding devices such as vices. 

• Require high levels of lubrication.

Applications of ball screws:-


• Ball screws are employed in cutting machines, such as machining center and NC lathe where accurate positioning of the table is desired

• Used in the equipment’s such as lithographic equipment or inspection apparatus where precise positioning is vital

• High precision ball screws are used in steppers for semiconductor manufacturing industries for precision assembly of micro parts.

• Used in robotics application where precision positioning is needed.

• Used in medical examination equipment’s since they are highly accurate and provide smooth motion.

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