Ball Screws
The ball screw ball is rolling between the screw and nut screw drive components. Ball screw structure type. Divided into two major categories of the outer loop and inner loop the ball robin fashion. Loop back to the beads used in kidney-shaped slot embedded block more outside the circle back to the beads used in intubation more.
Thread-sectional shape of the orbit is divided into a single arc and double arc two cross-sectional shape. Double arc-shaped axial stiffness is greater than the single-arc cross-sectional shape, is commonly used by dual arc-shaped screw.
Pre-load the form of points, can be divided into single nut preload single nut modification lead preload single nut to increase ball radial pretension, double nut, washer preload, double nuts differential gear preload. Double nut thread pretension. CNC machine tools commonly used double nut, washer preload, the preload is generally 1/3 of the axial load.
Ball screw and sliding screw nut has many advantages: high transmission efficiency, high sensitivity, stable transmission: wear, long life; to eliminate axial clearance and axial stiffness.
The ball screw drives are widely used in small and medium-sized CNC machine tool feed drive system. The feed system is also frequently used in the short stroke of the heavy-duty CNC machine tools (6m).
A. The installation of ball screw
CNC machine tool feed system to obtain a higher transmission stiffness, in addition to enhance the stiffness of the ball screw nut, the ball screw of the correct installation and its supporting structure stiffness can not be ignored. Nut seat and bearing in should have sufficient rigidity and accuracy. Usually appropriate to increase the contact area of ??the combination of parts and machine tools, in order to improve the local stiffness and the contact strength of the nut seat, when the conditions allow, the new design of machine tools are often made of the nut seat or bearing seat and the machine body as a whole to increase the stiffness.
Is also very important to select the appropriate order to improve the axial stiffness of the bearing, rolling bearing. The current combination of the two. A radial ball bearings and tapered bearings for combination use, its structure is simple, but the lack of axial stiffness. The other is the thrust bearing Radial thrust bearings and radial bearings combination, the axial stiffness has increased, but increases the bearing friction and heat and increase the structural dimensions of the bearing bracket. Bearing manufacturers at home and abroad in recent years has produced a ball screw bearings, which is a special Angular Contact Ball Bearings able to withstand great axial force, compared with the general thrust to the heart ball bearing, contact angle increases to 60o, an increase of the number of balls and corresponding decreases the diameter of the balls. Of axial stiffness of the bearing of this new structure than the general bearing more than twice, and extremely easy to use, the products sold in pairs, and in the factory matching the thickness of the assembly of the inner and outer ring nut and end cap the inner and outer compression, you can get the factory preload adjusted.
Installation of ball screw is usually the following:
(1) double push a free Figure 8-8a, screw one end of the fixed side free. The fixed end bearings under axial and radial forces. Such a supporting manner for the trip short screw.
(2) double push of a bearing as shown in Figure 8-8b illustrates, screw one end fixed, other end of the bearing. The fixed-side bearing axial force and radial force; end support bearing is exposed only to the radial force, but also a small amount of axial float, to avoid or reduce screw bending due to weight. Screw hot deformation are free to the end of the elongation.
(3) double push a pair of push in Figure 8-8c, screw both ends fixed. Fixed end bearings can withstand axial and radial forces, and this support can be applied to the screw pre-tension, increase the stiffness of the screw bearing, can partially compensate for thermal deformation of the screw.
(4) screw fixed nut rotation transmission mode as shown in Figure 8-8d shown at this time, the nut side of the rotation, one edge of the fixed screw for axial movement: the screw does not move, you can avoid being affected by the critical speed restrictions, to avoid the problems that occur when high-speed operation of the elongated ball screw. Nut inertia is small, flexible movement, to achieve high speed. In this way can be applied to the screw with a large pre-tension, screw bearing stiffness, heat distortion compensation screw.
Two. Protection and lubrication of the ball screw
(1) protection of ball screw ball screw and rolling friction drive device should avoid hard dust or chips dirt to enter and must therefore be equipped with protective devices. If the ball screw is exposed on the machine, it should be closed shield, such as the use of the spiral spring steel casing, the boom and folding
Casing. The installation will shield one end connected to the side of the ball nut and the other end fixed to the bearing seat of the ball screw. If the ball screw in a secluded location, you can ring protection, ring mounted at both ends of the nut. Contact elastic sealing ring with oil resistant rubber or nylon, the hole made into the shape of the raceway to match with the screw threads; effect of dust on the contact ring, but because of the contact pressure, friction torque is slightly increase. Non-contact seals, also known as a labyrinth seal, which is made of hard plastic, contrary to the shape of the bore and screw threaded raceway, and a slight gap, so to avoid the friction torque, but the effect of dust. The work should avoid crashing protective equipment, protective devices, the damage should be replaced.
(2) The ball screw lubrication lubricant can improve the wear resistance and transmission efficiency. The lubricant can be divided into the lubricating oil and grease two categories. The lubricant is generally a total loss system oil: The grease can be used lithium grease. Grease is generally added to the thread roller and mounting nut shell space, space lubricants after the hole on the shell into the nut. Replaced once every six months on the grease on the ball screw, clean the old grease on the screw, coated with the new grease. Lubricating the ball screw, each machine before refueling.
3. Ball screw on the high-speed CNC machine tools
The high-speed machining is the 21st century, a high-tech, high-efficiency, high precision and high surface quality is the basic characteristic, increasingly in the aerospace, automotive, mold manufacturing, photoelectric engineering, and instrumentation industries more extensive applications, and has achieved a major technical and economic benefits, is an important part of contemporary advanced manufacturing technology. In order to achieve high-speed machining, first of all have high-speed CNC machine tools. The high-speed CNC machine tools must also have high-speed spindle system and high-speed feeding system, in order to achieve the speed of material removal process. In order to achieve high-speed feed, domestic and foreign manufacturers continue to take measures to improve the speed performance of the ball screw. The main measures are:
A) appropriate to increase the speed of the screw, lead, and the number of screw head. Commonly used in large lead ball screw nominal diameter with matching lead: 40mm x 20mm, 50mm x 25mm, 50mm × 30mm, the feed rate can reach more than 60m/min. In order to improve the stiffness and bearing capacity of the ball screw, the large lead ball screw is generally double-headed thread in order to enhance effective ball carrying laps.
2) improving the structure, to improve the fluency of ball movement. Improved ball circulation reverse device, optimizing the parameters of back bead groove curve, three-dimensional shape of the guide beads and back bead device, truly along the direction of the lead angle of the internal thread ball introduction of the nut body, so that the ball movement direction of the tangent of the raceway is not intersect. This impact of loss and noise minimized.
3) "hollow cold" technology. The high-speed ball screw at run time due to friction and high temperature, screw the thermal deformation, and a direct impact on high-speed machining accuracy. "Hollow cold" technology is the thermostat cutting fluid to pass into the hollow screw hole of the ball screw, forced cooling to keep the ball on temperature constant. This measure is an effective way to improve, the large ball screw high-speed performance and accuracy.
4) high-speed feeding system for the big trip, can be screw fixed nut rotation of the drive way. At this point, the nut side of the rotation, one edge of the fixed screw for axial movement can be avoided, because the screw does not move, the critical speed limit to avoid the problems that occur when high-speed operation of the elongated ball screw. Nut inertia is small, flexible movement, to achieve high speed.
5) to further improve the manufacturing quality of the ball screw. By using the above measures, to some extent, to overcome some problems of the traditional ball screw. Japan and Switzerland, is an international leader in the ball screw speed has been the largest fast-moving speed up to 60m/min, individual cases even up to 90m/min and acceleration up to 15m/s2. Ball screw has a long history and mature technology, widely used, lower-cost, medium load, does not require a very high feed rate, the travel range is not too large (less than 5m) high-speed machining centers and other still on the economic high-speed CNC machine tools are often used.



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