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Abstract: Based on the one - way honing process of inner surface of spherical plain bearing，the principle of one - way honing and finishing process are analyzed，and the principle defects of one - way honing and finishing are illustrated. A method of the spherical round - trip honing and super - finishing technique to replace the one - way honing is put forward，and the function of round - trip honing and working principle and characteristics of super - finishing are expounded．
Key words: spherical plain bearing; inner ball surface; finishing; honing; superfinishing
1. Analysis of one-way honing of inner ring spherical surface
The surface roughness, waviness, and geometric accuracy of the spherical surface of the inner ring of a joint bearing are required to be high, and usually require finishing to meet the technical requirements. Smooth machining is the final fine machining of a product after grinding. At present, the single pass honing method is commonly used for the spherical surface finishing of the inner ring of joint bearings, which is similar to the conventional grinding method.
1.1 Inner Ring Spherical Single Pass Honing Process
Honing is often used for inner hole finishing, and according to its working principle, it can also be used for outer surface finishing. Spherical one-way honing is a common method for outer surface honing. The spherical one-way honing (hereinafter referred to as honing) process uses a honing head to clamp a cup or bowl shaped oilstone (Figure 1), and the end face of the oilstone is trimmed to a spherical surface. It floats and presses on the inner ring spherical surface (hereinafter referred to as the spherical surface) with a certain pressure, creating a circular spherical contact. At the same time, both the oilstone and the spherical surface rotate unidirectionally to achieve low-speed grinding of the spherical surface. The processing principle is shown in Figure 2.
Figure 1 Oil Stone Cross Section
Figure 2 Schematic diagram of single pass honing process for spherical surfaces
1.2 Principle of one-way honing
During honing, the oil stone with a spherical end face is in contact with the spherical surface under a certain pressure, resulting in gaps and overlapping contact points at the contact interface. The overlapping contact points interfere with each other, as shown in Figure 3.
Figure 3 Boundary state of oil stone and spherical contact
When the oilstone moves relative to the spherical surface, on the one hand, the oilstone grinds away the interference points on the spherical surface, and on the other hand, the spherical surface also correspondingly causes the interference points (sharp corners of abrasive particles or the entire abrasive particles) on the oilstone surface to fall off or break, that is, the oilstone surface and the spherical surface are mutually repaired during honing, which is similar to grinding away the interference points when two flat plates are in plane motion. Due to the perpendicular intersection or intersection of the oil stone rotation axis and the spherical rotation axis, their relative rotation is equivalent to the planetary motion of the abrasive particles relative to the inner circle sphere, that is, they rotate around the honing axis while orbiting around the spherical hole axis. The trajectory of each abrasive particle on the oil stone surface on the sphere is a crossed spiral arc. Figure 4 shows the honing pattern generated by the spherical rotation of the oilstone when the ratio of the oil stone speed ny to the spherical speed nq is i=24:1.
Figure 4 Spherical honing mesh lines
Changing the speed ratio i will change the helix angle β And mesh density δ。 The larger i, the more, β The smaller the angle, δ The larger the value, the higher the honing accuracy and the lower the efficiency; On the contrary, the smaller i, β The larger the angle, δ The smaller the value, the lower the honing accuracy and the higher the efficiency. When i is an integer, the mesh patterns of each circle overlap, and the mesh density δ The value remains unchanged, and the mesh pattern is repeated and deepened, making the mesh pattern obvious. The surface roughness value of the sphere increases, and the accuracy decreases; When i is an irrational number, the mesh patterns in each circle do not overlap, and the mesh density δ As the value increases, the dense and evenly distributed mesh will reduce the surface roughness value of the sphere and improve the accuracy of the spherical profile.
1.3 Single pass honing accuracy
Batch (100 pieces) of honing joint bearing GE80ET inner ring spherical surface, randomly select 10 pieces for measurement, and obtain the average surface roughness Ra ≤ 0.2 of the spherical surface μ m. The average spherical profile is less than 0.02 mm. Both indicators are qualified, but there is still room for improvement.
2. Analysis of single pass honing defects on spherical surfaces
2.1 Analysis of Precision Error
Figure 5 shows a grinding ring formed by grinding a certain abrasive particle of an oilstone on the inner ring spherical surface. Grinding point 1 is taken on the rear hemisphere and grinding point 2 is taken on the front hemisphere. The cutting speed vh is distributed in the spatial distribution of the spherical surface. In the figure, vy1 and vy2 represent the tangential velocities of the abrasive particles at positions 1 and 2; Vq1 and vq2 are the tangential rotational velocities of the inner ring spherical particles relative to the abrasive particles at positions 1 and 2; Vh1 and vh2 are the cutting speeds of the abrasive particles at positions 1 and 2.
Figure 5 Distribution of cutting speed vh in spherical space
From Figure 5, it can be seen that the cutting speed vh of the abrasive particles on the spherical surface varies in size and direction. For the convenience of observation, the cutting speed of 8 points on the grinding ring is projected, as shown in Figure 6.
Figure 6 Projection of cutting speed vh
In the formula: vy is the tangential velocity of the oilstone; Vq is the tangential velocity of the sphere; θ For position angle, θ = 0~180 ° refers to the posterior hemisphere region, θ = 180-360 ° is the region of the anterior hemisphere; Dy and Dq are the diameters of oilstones and balls, respectively; Ny and nq are the oil stone and ball speeds, respectively.
Due to the continuous variation of cutting speed vh and circumferential direction, when vhmax=vy+vq, it is located in the rear hemisphere; When vhmin=vy vq, it is located in the anterior hemisphere.
According to the peeling theory, the honing and grinding rate γ Mainly depends on surface pressure p and cutting speed vh, i.e
In the formula: K is the coefficient of operating conditions; m. N is the abrasive wear index, generally taken as m=n=1.
If the working conditions of the front and rear hemispheres are the same in spherical honing, then the coefficient K and surface pressure p are the same. If the cutting speed vh is different, then the grinding rate of the front and rear hemispheres is different γ Different, grinding amount fz= γ T is not equal. The greater the tangent velocity vh, the higher the grinding rate γ The higher, the greater the grinding amount fz; On the contrary, the smaller the cutting speed vh, the higher the grinding rate γ The lower, the smaller the grinding amount fz. Due to the unequal cutting speeds vh between the front and rear hemispheres during one-way honing, the cutting speed in the rear hemisphere is greater than that in the front hemisphere. This will result in a greater amount of grinding in the rear hemisphere than in the front hemisphere, leading to machining errors.
2.2 Cutting fluid splashing phenomenon
When honing, cutting fluid needs to be added to the honing area to improve the surface quality of the spherical surface after honing. The added cutting fluid fills the surface and spherical surface of the oilstone and rotates with the oilstone and spherical surface. The oil stone has a high rotational speed, with a surface tangent speed of 0.25-0.4 m/s, causing the cutting fluid to be thrown out along the tangent line, resulting in splashing of the cutting fluid. This not only worsens the operating environment, but also causes waste.
3. Improvement of spherical one-way honing process
3.1 Spherical two-way honing
3.1.1 Working principle
The most direct improvement method for one-way honing process is to use two-way honing, which means that the oil stone rotates at high speed in one direction, and the spherical surface rotates one circle forward and then another circle backward, repeating the cycle. The process principle is shown in Figure 7.
Figure 7 Principle diagram of two-way honing process
When the two-way honing is turned forward, the grinding amount in the rear hemisphere is greater than that in the front hemisphere; After reversal, the grinding speed of the front hemisphere is greater than that of the back hemisphere, resulting in a grinding amount of the front hemisphere greater than that of the back hemisphere. After a reciprocating stroke, the grinding amount of the front and rear hemispheres tends to be consistent, which can reduce machining errors.
3.1.2 Accuracy
Similar to single pass honing, a batch of 100 pieces of double pass honing were used to process the inner ring spherical surface of the joint bearing GE80ET. Ten pieces were randomly selected for measurement, and the average surface roughness Ra of the spherical surface was obtained to be ≤ 0.09 μ m. The spherical profile is less than 0.008 mm, which improves the accuracy compared to one-way honing.
3.2 Spherical Superprecision Machining Process
3.2.1 Working principle
Spherical ultra precision machining is a method of achieving micro grinding of rotating spherical surfaces by pressing oil stones (Figure 8) towards the spherical surface under a certain pressure and continuously performing short reciprocating swings, as shown in Figure 9. The difference between spherical ultra precision machining and honing lies in the different movement modes of the oilstone.
Figure 8 Spherical Oil Stone Block
Figure 9 Spherical Superprecision Machining
The motion trajectory of any abrasive particle on the surface of the oil stone during ultra precision machining is a sine curve (Figure 10), and its mathematical expression is
In the formula: A is the reciprocating swing amplitude; F is the reciprocating frequency; T is time; λ is the wavelength.
The inner circle spherical ultra fine mesh pattern is shown in Figure 10. Similar to honing, the ultra precision cutting speed vh is synthesized by the tangential velocity vq of the circular motion of the spherical particle in contact with any abrasive particle on the oilstone surface, which is the reciprocating swing speed vy of the abrasive particle. The direction of the synthesized velocity is the cutting direction of the abrasive particle, as shown in Figure 11.
Figure 10 Inner Ring Spherical Superfine Mesh Lines
Figure 11 Changes in Spherical Superprecision Cutting Speed vh
In the formula: α is the cutting angle; β drive the wheel angle for the oil stone.
From equations (7) to (9), it can be concluded that
From this, it can be seen that the maximum value vhmax and minimum value vhmin of ultra precision cutting speed have an area of S=A on the sphere λ The region is alternating, and vhmax is located in the middle of the region with a cutting angle α Reaching maximum grinding capacity; The vhmin is located at the edge of the area and has a cutting angle α Reaching the minimum, the grinding amount is also minimized.
When the swing amplitude A is ＜＜ B (inner circle width) and the reciprocating frequency f is ＞＞ nq, that is, the wavelength λ is very small, then the area S of any abrasive particle is very small and tends towards a point. Therefore, a reciprocating grinding of an abrasive grain is equivalent to a point scraping of a scraper. In a reciprocating swing, the grinding of numerous abrasive particles on the oilstone is equivalent to scraping the spherical surface with numerous scrapers. When the spherical surface rotates once, it is scraped once. Similar to honing, when the speed ratio i=f: nq is an irrational number, the scraping area of each circle point does not overlap, and the mesh density δ as the value increases, the uniform distribution of scraping points reduces the surface roughness value of the sphere and improves accuracy.
Ultra precision machining involves a reciprocating swing, and the cutting edge of the abrasive particles on the oilstone is used alternately on both sides. Compared with single and two-way honing using the cutting edge on one side, the grinding efficiency is significantly improved. In addition, when the oil stone surface swings around the ball axis, there will be no interference between the oil stone surface and the groove surface during the ultra precision machining of the rolling bearing ring groove, that is, there is no principle error in the ultra precision machining of the spherical surface.
3.2.2 Accuracy
Batch (100 pieces) ultra precision machining of the inner ring spherical surface of joint bearing GE80ET, randomly selecting 10 pieces for measurement, and obtaining the average surface roughness Ra ≤ 0.04 of the spherical surface μ m. The spherical profile is less than 0.003 mm, which improves the accuracy of single and double pass honing.
4. Conclusion
Spherical honing can improve the surface roughness of the inner ring spherical surface of joint bearings, thereby achieving the goal of spherical finishing machining. However, due to the inherent principle defects of one-way honing, the surface roughness value of the spherical surface is relatively large. Double pass honing is the most direct improvement to single pass honing of spherical surfaces, which can improve machining accuracy.
The process of spherical ultra precision machining is similar to honing, but its machining efficiency and accuracy are better than honing, and it can partially replace the running in effect (i.e. the friction surface of the product can cross the initial wear stage and directly enter the normal wear stage). Moreover, the small swing of the oilstone will not cause the added cutting fluid to splash, improving the operating environment.
2023 December 3th Week KYOCM Product Recommendation:
KYOCM cylindrical roller bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life. KYOCM cylindrical roller bearings include single row cylindrical roller bearings, double row cylindrical roller bearings and four row cylindrical roller bearings.
Cylindrical roller bearings are also available in sealed or split designs. In sealed bearings, the rollers are protected from contaminants, water and dust, while providing lubricant retention and contaminant exclusion. This provides lower friction and longer service life. Split bearings are intended primarily for bearing arrangements which are difficult to access, such as crank shafts, where they simplify maintenance and replacements.
2023-12-28