There are two common circulation modes of ball screw: external circulation and internal circulation. In the process of circulation, the ball sometimes breaks away from the contact with the screw, which is called external circulation; Always in contact with the lead screw is called internal circulation. Each closed cycle of the ball is called a column, and the number of leads contained in each closed cycle of the ball is called the number of turns. Each nut of the internal circulation ball screw pair has two rows, three rows, four rows, five rows, etc., and each row has only one turn; Each column of the outer ring has 1.5 turns, 2.5 turns and 3.5 turns.
1) external circulation: external circulation means that the ball returns to the lead screw nut through the spiral groove or cannula on the outer surface of the nut to re-enter the circulation. The return modes of external circulation ball screw nut pair mainly include end cover type, intubation type and spiral groove type. The common external circulation mode is end cover type; Intubation type; Spiral groove type. End cap type, a longitudinal hole is machined on the nut as the return channel of the ball, and the cover plates at both ends of the nut have the return ports of the ball, and the ball enters the return pipe from the return port to form circulation. Intubation type, using elbow as return pipe, has good manufacturability, but the radial size of the pipe is larger because it protrudes from the nut body. Spiral groove type, a spiral groove is milled on the outer circle of the nut, and through holes are drilled at both ends of the groove, which are tangent to the threaded raceway to form a reflux channel. The radial dimension of this structure is smaller than that of the cannula structure, but it is more complicated to manufacture. The external circulation structure and manufacturing process of the external circulation ball screw are simple and widely used. Its disadvantage is that the seam of the raceway is difficult to smooth, which affects the smoothness of the ball raceway.
2) Internal circulation: The internal circulation adopts commutator to realize ball circulation, and there are two kinds of commutator. The cylindrical part of the cylindrical convex key reverser is embedded in the nut and has a reverse groove 2 at the end. The reverse groove is located by the cylindrical outer surface and the round key 1 at its upper end to ensure alignment with the thread raceway direction. The reverser of the oblate insert is a common round-headed Ping Jian insert. The insert is embedded in the cutting groove of the nut, and the end of the insert is provided with a reversing groove 3, which is positioned by the outer contour of the insert. Compared with the two kinds of reversers, the latter is smaller in size, thus reducing the radial size of the nut and shortening the axial size. However, the dimensional accuracy of the outer contour of this reverser and the groove on the nut is very high.
Selection of ball screw type;
Ball screw nuts can be divided into elbow type, circulator type and end cover type according to the circulation mode of steel balls. The particularity of these three modes of circulation.
Elbow type
(SBN, BNF, BNT, BNFN, BIF and BTK) These models can be found by searching.
Circulating guide plate (HBN type)
These models are the most typical nuts, and the steel balls are circulated with bent pipes. The steel ball is scooped out from the groove of the spiral shaft, enters the bent pipe, and then returns to the groove for infinite circular motion.
Circulator type
(Danish, DKN, DIK, JPF and DIR types)
These models are the smallest nuts, which change the direction of the steel ball through the circulator, pass through the outer diameter of the screw shaft, return to the original position, and do infinite circular motion.
End cover type
(SBK, SDA, SBKH, WHF, BLK, WGF, BLW, WTF, CNF and BLR)
These models are the most suitable nuts for high-speed feeding. The steel ball is scooped from the groove of the screw shaft to the through hole of the nut by the end cover, and then returns to the groove through the through hole for infinite circular motion.
Features of ball screw:
1, low friction loss and high transmission efficiency.
Because there are many balls rolling between the screw shaft and the screw nut of the ball screw pair, high movement efficiency can be obtained. Compared with the sliding screw pair in the past, the driving torque is below 1/3, that is, the power required to achieve the same motion result is 1/3 of the sliding screw pair. This is very helpful for saving electricity.
2, high precision
Ball screw pairs are generally produced with the highest level of mechanical equipment in the world, especially in grinding, assembly and environmental inspection of factories in each process. The temperature and humidity are strictly controlled, and the accuracy is fully guaranteed due to the perfect quality management system.
3. Possibility of high speed feed and micro feed.
Because the ball screw pair adopts ball motion, the starting torque is very small, and there will be no crawling phenomenon like sliding motion, which can ensure accurate micro-feed.
4. High axial stiffness
The ball screw pair can be preloaded, because preloading can make the axial clearance reach a negative value, and then obtain higher stiffness (when the ball screw is actually used in mechanical equipment, the stiffness of the female part can be enhanced due to the repulsive force of the ball).
5, no self-locking, reversible transmission.