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Ball screw pair equipment
Source: Bearing network time: 2014-07-02
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According to the cross-sectional shape of the thread path, it is divided into two types: single arc and double arc. Because the axial stiffness of the double arc is larger than that of the single arc, the current double-arc-shaped screw is used.
According to the pre-loading method; it can be divided into single nut without pre-tightening, single nut displacement lead pre-tightening, single nut to increase steel ball radial pre-tightening, double nut gasket pre-tightening, double nut differential tooth pre-tightening, Double nut thread preloading. Double nut washer preloading is often used on CNC machine tools; its preload is usually 1/3 of the axial load.
Ball screw pair and sliding screw nut pair have many advantages: high transmission power, high sensitivity, stable transmission: small wear, long life, can eliminate axial clearance; improve axial stiffness and so on.
Ball screw nut drive is widely used in the feed transmission system of small and medium-sized CNC machine tools. It is also often used in the short stroke (6m or less) feed system of heavy-duty CNC machine tools.
1. The feed system of the CNC machine tool of the ball screw pair must achieve higher transmission rigidity; in addition to strengthening the rigidity of the ball screw nut itself; the correct equipment of the ball screw and the rigidity of the layout of the support are also neglected. Both the nut seat and the support seat should have the required rigidity and accuracy. It is usually appropriate to increase the touch area of ​​the parts connected to the machine tool; to improve the rigidity and touch strength of the nut seat; the newly depicted machine tool often puts the nut seat when the process conditions are agreed. Or the support base and the machine body are made to increase the rigidity.
In order to improve the axial stiffness of the support; it is also very important to select the appropriate rolling bearing. The two primary methods are currently used in China. One is to combine the radial bearing and the tapered bearing; the layout is simple; but the axial stiffness is lacking. The other is the combination of thrust bearing or radial thrust bearing and radial bearing; its axial stiffness has improved; but it has increased the conflict resistance and heat of the bearing and added the layout scale of the bearing bracket. External bearing manufacturers have produced a special ball screw bearing; this is a special radial thrust ball bearing that can accept large axial forces; compared with the usual radial thrust ball bearings; the touch angle is increased to 60o; added the number of balls and correspondingly reduced the diameter of the balls. This new layout of the bearings has more than doubled the axial stiffness of the usual bearings; and the use is extremely convenient; the products are sold in pairs; and now at the factory The thickness of the inner and outer rings is selected. When the equipment is pressed with the nut and the end cap, the inner ring and the outer ring are pressed tightly; the pre-tightening force that has been adjusted at the factory can be obtained.
There are usually the following methods for ball screw sub-equipment:
(1) Double push and one free method are shown in Fig. 8-8a; the screw is fixed at one end; the end is free. The fixed end bearing together receives axial force and radial force. This support method is used for short lead screw with small stroke.
(2) Double push and one support method are shown in Figure 8-8b; the screw is fixed at one end; the other end is supported. The fixed end bearing receives axial and radial forces together, and the bearing at the end receives only radial force; A small amount of axial lifting; can prevent or reduce the bending of the lead screw due to its own weight. Together with the hot deformation of the screw can be freely extended to one end.
(3) Double push and double push method is shown in Figure 8-8c; both ends of the screw are fixed. The fixed end bearings can accept axial and radial forces together; this support method; can apply appropriate pre-measure to the lead screw Pulling force; progressive screw support stiffness; can compensate for the thermal deformation of the screw.
(4) The transmission method of screw fixing and nut rotation is shown in Figure 8-8d; at this moment; the nut is changed on one side, and the screw fixed at the edge is moved axially: because the screw does not move; it can prevent the critical speed Constraint; prevent various problems presented by the high-speed operation of the slender ball screw. The nut has small inertia and is sensitive to motion; the rpm can be completed. This method can apply a large pre-tension to the lead screw; Respond to the thermal deformation of the screw.
2. Ball screw pair protection and smooth
(1) The ball screw pair of the ball screw pair is the same as other gears with tumbling conflict; it should prevent hard dust or chip dirt from entering; therefore it is necessary to install protective equipment. If the ball screw pair is exposed on the machine tool The closed protective cover should be used; if the coil spring steel sleeve, elastic sleeve and folding sleeve are used, the end of the shield is connected to the side of the ball nut; the other end is fixed to the ball screw. On the support seat. If the ball screw pair is in the shaded orientation; the seal ring protection is optional; the seal ring is mounted on the two ends of the nut. The touch type elastic seal ring is made of oil resistant rubber or nylon; The shape of the screw thread is similar to that of the screw thread. The touch seal has good dustproof effect; however, because of the touch pressure; the conflicting torque is slightly added. The non-touch seal is also called the labyrinth seal; it is made of hard plastic. The inner hole is opposite to the shape of the screw thread raceway; and there is a slight gap; this can prevent the conflicting torque; but the dustproof effect is poor. The impact protection equipment should be prevented during the operation; Should be promptly replaced.
(2) Smooth smoothing agent of ball screw pair can improve wear resistance and transmission power. Smoothing agent can be divided into two categories: smooth oil and smooth grease. Smooth oil is usually used for total loss system oil: smooth grease can be selected from lithium base Smooth grease. Smooth grease is usually added to the shell space of the thread raceway and equipment nut; smooth oil is written into the nut space through the oil hole on the housing. Replace the smooth grease on the ball screw once every six months. Remove the old smooth grease on the lead screw; apply a new smooth grease. Use a smooth oil smooth ball screw pair; you can refuel before each machine operation.
3. High-speed machining of ball screw pair on high-speed CNC machine tools is a high-tech skill for the 21st century; it is characterized by high power, high precision and high surface quality; in aerospace, automotive industry, mold making, optoelectronic engineering And the instrumentation and other occupations have become more and more widely used; and has achieved serious skills and economic benefits; is an important component of the current leading production skills. In order to complete high-speed machining; the first must have high-speed CNC machine tools. High-speed CNC machine tools It is necessary to have a high-speed spindle system and a high-speed feed system together; to complete the high-speed data cutting process. In order to complete the high-speed feed; the relevant manufacturers at home and abroad continue to adopt methods; improve the high-speed function of the ball screw. The first methods are:
1) Properly increase the speed, lead and number of threads of the lead screw. The matching between the nominal diameter and the lead of the commonly used large lead ball screw is: 40mm × 20mm; 50mm × 25mm; 50mm × 30mm; Speed ​​can reach more than 60m / min. In order to improve the rigidity and load bearing capacity of the ball screw; the large lead ball screw usually uses double-head thread; to improve the useful bearing ring number of the ball.
2) Improve the layout; Improve the smoothness of the ball movement. Improve the ball cycle reversal equipment; Optimize the curve parameters of the return bead; Select the three-dimensional shape of the bead tube and the ball returnor; Really follow the lead of the internal thread The ball is introduced into the nut body in the angular direction; the direction of the ball movement is tangent to the raceway rather than intersecting. This minimizes impact loss and noise.
3) Select the "hollow strong cold" skill. The high-speed ball screw will have high temperature due to conflict during operation; form the thermal deformation of the screw; directly affect the machining accuracy of the high-speed machine tool. Select the "hollow strong cooling" skill; Pass into the hole of the hollow screw; forced cooling of the ball screw; adhere to the stability of the ball pair temperature. This method is a useful way to improve the high-speed function and precision of the large and large ball screw.
4) About the high-speed feed system of large stroke; the transmission method of screw fixing and nut rotation can be selected. At this moment; the nut is changed while the edge is fixed, and the screw is fixed in the axial direction; because the screw does not move; The speed limit is prevented; the various problems presented by the slender ball screw at high speed are prevented. The nut has small inertia and is sensitive to motion; the rpm can be completed.
5) Further improve the quality of ball screw production. After selecting all the above methods; it can overcome some of the problems of traditional ball screw to a certain extent. Japan and Switzerland have always been in the international leading position in the high-speed ball screw; The maximum speed can be moved up to 60m/min; in a single case, it can reach 90m/min; the acceleration can reach 15m/s2. Because the ball screw has a long history, sophisticated craftsmanship, wide use and low cost; Feed speed requirements are not very high, the stroke size is not too large (less than 4 ~ 5m), usually high speed machining intermediate and other economical high-speed CNC machine tools are still often used.
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