How A Ball Screw Works
- Apr 28, 2018-
A. The Ball Screw Assembly
The ball screw assembly consists 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 rotates, 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 exit from the ball return system into the ball screw and nut thread raceways continuously to recirculate in a closed circuit.
B. The 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.
C. 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 used by Barnes Industries.
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 used by Barnes Industries.
For high speed or high load applications Barnes uses a tangential ball return system. 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. Barnes can help you with your high speed or high load design. Call today!
D. 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.