Composition: Mainly composed of screw rod, nut, ball and raceway (ball return device), nut seat, etc.
Working principle: Arc spiral grooves are processed on the screw rod and nut. When they are put together, a spiral raceway is formed, and the raceway is filled with balls. The balls roll along the raceway and circulate through the ball return tube. Both ends of the ball return tube also play the role of ball blocking to prevent the balls from falling out along the raceway.
Features:
●High transmission efficiency: mechanical efficiency can be as high as 92%~98%.
●Low friction: mainly the rolling of the balls replaces the sliding of the ordinary screw nut pair.
●Axial clearance can be eliminated: also due to the function of the balls, the rigidity of the system is improved. The gap can be eliminated after pre-tightening.
●Long service life and high manufacturing cost: Mainly made of high-quality alloy materials, and the surface is heat-treated to obtain high hardness.
There are two ways of ball circulation in the ball screw nut pair: the balls that sometimes break out of contact with the screw during the circulation process become the outer circulation (as shown in Figure b), and the balls that always remain in contact with the screw become the inner circulation (as shown in Figure a).
(1) outer loop
External recirculation is a commonly used recirculation arrangement. In this structure, two holes are drilled in the nut body, tangential to the helical groove and spaced axially by several half-leads, to serve as the ball inlet and outlet. A ball-return groove is then milled on the nut's outer surface to connect the two holes. A ball deflector is fitted at each inlet and outlet inside the nut, and a sleeve is installed around the nut's outer surface, forming a closed recirculating raceway. The external recirculation structure is simple to manufacture and widely used. Its disadvantage is that the raceway joints are difficult to make smooth, affecting smooth ball rolling and even causing balls to jam. Noise is also relatively high.
(2) inner loop
Internal ball recirculation uses deflectors. CNC-machine ball screws use two deflector types. In a cylindrical keyed deflector, the cylindrical section fits into the nut and has a return groove at its end. The cylindrical outer surface and an upper locating key position the return groove to align with the threaded raceway. A flat insert deflector is a flat-key-shaped insert with a semicircular head, fitted into a slot in the nut and provided with a return groove at its end. The latter type is smaller, reducing the nut's radial and axial dimensions.
Anti-backlash of ball screw nut pair
Double nut gasket gap adjustment:
Grinding pad thickness to eliminate gaps
The ball screw nut pair adopts a double nut structure (similar to the double lamellar gear structure in the gear pair). By changing the thickness of the gasket, the nut is displaced axially, so that the two nuts fit against both sides of the screw respectively. When the worktable is reversed, due to the elimination of backlash, the worktable will follow the CNC's movement instructions in the reverse direction without lag.
Double nut thread gap adjustment:
(Source: Dingsheng Machine Tool)