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Bearings / Rails
1.Introduction to the Bearing Industry
Bearing is used to determine the relative movement position of the rotating shaft and other parts and plays a role of supporting or guiding. Its performance, accuracy and reliability plays a decisive role to the normal work of the main body.
China is a big producer of bearings, but there is a big gap between China and the big bearing countries. The main difference is that China has fewer products with high precision, high technology content and high added value and still depends on import to meet its demands for such products. More than 75% of the global market share is concentrated in eight multinational bearing groups, namely: SKF in Sweden, INA and FAG in Germany Schaeffler Group, NSK, NTN, KOYO and Minebear in Japan, and TIMKEN in the United States.
In addition, the well-known enterprises in China are: C&U Bearings, Wanxiang Qianchao Co., Ltd., Wafangdian Bearing Group Co., Ltd., Luoyang LYC Bearing Co., Ltd., Tianma Holding Co., Ltd., Zhejiang XCC Group Co., Ltd., CW Bearing, Dalian Metallurgical Bearing Group Co., Ltd., Hubei New Torch Technology Co., Ltd., Shanghai Prime Machinery Co., Ltd. and Xiang Yang Automobile Bearing Co., Ltd.
At present, most foreign companies use CBN abrasion wheels for grinding and most domestic enterprises use ordinary corundum wheels, but with the promotion and gradual application of high-performance machine tools in the bearing industry, ceramic bond CBN sand wheels for internal grinding of bearings are gradually adopted by large bearing enterprises in China. Its main advantages are: long service life, high efficiency, high precision and good roughness.
2.FAQs and Solutions about Abrasive Machining in the Bearing Industry
(A) The Formation of Spiral Line: Main reasons: 1. Bad sand wheel trimming, corners are not chamfered, no use of coolant for trimming; 2. Too much lubricating oil on the workbench makes the workbench float; 3. Poor machine tool precision; 4. Too large grinding pressure.
Specific Reasons for the Formation of Spiral Line: 1. Poor V-shaped guide rail rigidity. The sand wheel shifts when grinding and only the edge of the sand wheel contacts with the work surface; 2. When trimming the sand wheel, the reversing speed of the workbench is not stable and the precision is not high, so the trimming of a certain edge of the sand wheel is slightly less; 3. The rigidity of the workpiece itself is bad; 4. The sand wheel has broken sand granules to peel off, and the scrap iron ground from the workpiece accumulates and attaches on the surface of the sand wheel, so the trimmed sand wheel should be flushed or brushed clean with cool water; 5. The sand wheel is not well trimmed and some places bulge out.
(2) Fish Scale Surface: The main reason for the appearance of fish scale marks on the surface is the cutting edge of the sand wheel is not sharp enough, so “biting” phenomena occur during grinding, when will produce larger vibration. The specific reasons for the production of fish scale like marks on the surface of the workpiece are: 1. The sand wheel surface has waste and oil dirt; 2. The sand wheel is not trimmed round; 3. The sand wheel becomes blunt or is not trimmed sharp enough; 4. The diamond fastening frame is not firm, diamond shakes or diamond quality is not good and the diamond is not sharp; 5. The sand wheel hardness is not uniform. The main reason for the burr on the work surface is that coarse abrasive grains fall off and are sandwiched between the workpiece and sand wheel.
(3) The surface of the workpiece is brushed when grinding and the specific reasons are: 1. Marks left during coarse grinding are not ground during accurate grinding; 2. The coarse and tiny abrasive particles in the coolant are not filtered clean; 3. Abrasive particles will fall off easily when the coarse sand wheel is just trimmed; 4. The period of validity of material toughness is short or the sand wheel is too soft; 5. Abrasive particle toughness and workpiece material toughness do not match well. The surface of the workpiece has straight wave marks. Cut the ground workpiece vertical to the axial lead and amplify, we can see its circum is similar to sine wave. Have its center make no rotation or translation along the axial lead, the track around the sine wave is a waveform cylinder, which is also named polygon.
(4) Product straight wave form traces: The reason is the movement of sand wheel relative to the workpiece or the grinding pressure of sand wheel on workpiece produces periodical changes and causes vibration. This vibration may be forced vibration and also may be self-excited vibration, so the straight wave frequency on the workpiece is often more than one. Specific reasons for straight wave marks are: 1. Sand wheel main shaft gap is too large; 2. Sand wheel hardness is too high; 3. Static balance of the sand wheel is not good or the sand wheel become blunt; 4. The workpiece speed is too high; 5. Horizontal knife is too large; 6. Sand wheel main shaft bearing is worn and the matching gap is too large, resulting in radial runout; 7. Sand wheel hold-down mechanism or workbench “crawl”.
(5) the surface of the workpiece is burned in the grinding process: the workpiece surface in the grinding process often can be burned and there are several types of burns, 1. burns along the sand wheel processing direction, and turn dark black spots; 2. Burns turn line or broken and continued line shapes. The surface of the workpiece is burned in the grinding process and the reasons are summarized as follows: 1. The sand wheel is too hard or the particle size is too fine, and the tissue is too dense; 2. the feed is too large, cutting fluid supply is insufficient, and cooling conditions are poor; 3. The workpiece speed is too low, the sand wheel speed is too fast; 4. sand wheel vibration is too large, changes in the depth of grinding cause burns; 5. Sand wheel trimming is not timely or bad; 6 The diamond is sharp, the sand wheel trimming is bad; 7. Burns are too deep during coarse grinding of the workpiece and are not ground off during fine grinding. 8. The workpiece’s clamp force or suction force is not enough; and the workpiece stalls under the action of grinding force. Then how to get whether the workpiece surface is burned black during grinding? It can be inspected by regular acid pickling. After acid pickling, and when the workpiece surface is moist, inspect it visually immediately under the floodlight. If normal, the surface turns uniform gray. If the workpiece is soft, it has cloud-shaped dark spots and the circum is not regular. If decarburized, it turns pale or dark black flower spots; if there are cracks, the surface shows polygons; if there are burns, (1) the surface will turn dark spots along the sand wheel processing direction and (2) the surface will show lines or broken continued lines. If burns appear during grinding process, you must analyze the reason timely and take effective measures, so as to eliminate bulk burns.
(6) The surface roughness does not meet the requirements: There are certain standards and process requirements to the surface roughness of the bearing parts. But during grinding and super-fine grinding, the standards and requirements always cannot be met due to different reasons. The main reasons are: 1. Grinding speed is too low, the feed speed is too high and the feed amount is too large, and grinding time without feed is too short; 2. The workpiece speed is too high or the workpiece shaft and sand wheel spindle vibration is too large; 3. The sand wheel particle size is too large or too soft; 4. The sand wheel trimming speed is too fast or the trimming mechanism gap is too large; 5. The diamond trimming the sand wheel is not sharp or the quality is not good; 6. Quality of oilstones for super-fine grinding is not good and the installation position is not correct; 7. The quality of kerosene for super-fine grinding cannot meet the requirements; 8. The super-fine grinding time is too short, etc.
In recent years, on the basis of the understanding of the characteristics of hole processing in the bearing industry and the combination of sand wheel using conditions of machine tools, we have successfully developed a series of ceramic bond CBN products, which match KOYO and TOYO machines of Japan, domestic new machines and internal grinding machines produced by Wuxi Machine Tool Factory.