Bearing premature failure of metallographic analysis
DUF bearing in the job, as a result of a variety of complex reasons, such as the structure unreasonable, the material quality is poor, poor performance, work on the surface defects, shock, vibration, improper installation and bad lubrication etc, all can cause bearing early failure. However, in some cases, some bearing was not put into use (run), but users in machine process the original complete appearance is often damaged, lost the applicability, cause bearing premature failure. This paper tries to reveal is belong to the failure form of a practical example.
One, failure background
A certain power plant in assembly S195 diesel engine gear box, the 6205 deep groove ball bearing inner ring gap or continuously happen break-up (block) phenomenon, in quantity of certain proportion (per gear box four sets). According to the investigation on the spot, this batch of bearing total extreamely (-a ly for 3000 sets, the actual have been loaded with 1000 sets, the remaining 2000 sets for user "was" dare not installed.
After the verification, gear box axial dimension conforms to the drawing requirements, bearing installed method correctly, is the fixed journeymen finish. The user had unpacking extraction 40 sets of measurement bearing internal and external ring of hardness, and the DZ - 2000 type large magnetic inspection machine testing, the result hardness in 61 ~ 65 HRC range, in line with the JB1255 standard, visual check not found crack. In view of this, in order to beg a split bearing ring, the reason of the site would be damaged bearing return, the conventional physical and chemical test.
These ring system by box type resistance furnace 840 ~ 860 ℃ heating quenching, 160 ℃ x 2 ~ h low temperature oil tempering treatment.
Second, the test methods and results
1. The macroscopic observation
Figure 1 is installed in the damaged bearing real photos.
After careful observation physical known:
(1) in all of the damaged bearing not seen for machine knock and left traces wounded, the ball and cage the spare parts such as intact.
(2) inner ring raceway of split starting at one side of the center, occupy end diameter 1/2 or 1/4.
(3) fracture in stretch into inside the second longitudinal straight crack, crack is similar to knife cut, through wall.
(4) the break-up appearance rugged, but fracture surface smooth level off. No signs of plastic deformation, only a small amount of tearing crack.
(5) fracture with grey fine grain porcelain shape of brittle fracture.
2. Acid corrosion test
Respectively will split inner ring with complete inner ring with the 1-1 hydrochloric acid aqueous solution of the thermal pickling, temperature is 60 ~ 70 ℃, time 10 min, the results shown in figure 2.
(1) in the break-up place nearby raceway exist inside banded grinding burn and grinding cracks.
(2) grinding cracks are dendritic, along the raceway side distribution, and with grinding direction parallel, just burst along the grinding crack development.
(3) secondary longitudinal straight crack and burst fracture and grinding cracks is roughly "T" type vertical.
(4) the entire ring surface distribution with loose shape black scotoma and pore, its shape is round or oval. In the amplification 40 times the stereoscopic microscope observation, its shape is irregular hole or circular aperture, they can be connected into each other like grain boundary shape, and complete ring surface is quite bright and clean, could not see the above defects.
3. Apparent hardness and microstructure
In order to further understand ring of hardness distribution and quenching tempering microstructure and surface of the microstructure feature, and selected parts of the inner ring and burst fragment, in its upper end fixed macroscopic hardness, and linear cutting method in split place near interception longitudinal metallographic specimen, the result is:
(1) the macro hardness value for 64 ~ 65.2 HRC.
(2) the organization JB1255 quenching and tempering with standard 3 ~ 4 level, local area up to 5 level, for small crystal martensite + aphanitic martensite ten fine acicular martensite + a small amount of residual carbide + residual austenite.
(3) in 100 times and observed under martensitic matrix is obviously black and white banded organization, if according to YB9-59 standard can be judged to be 4 level, as shown in figure 3 shows.
(4) observed raceway surface exist a layer of a crescent shape of the secondary hardening white layer organization and sub-surface of high temperature softening layer organization, namely "after tempering layer". The white layer depth (the surface to link bottom) is about 0.033 ~ 0.056 mm, tempering depth about 0.293 mm, visible grinding burns very serious degree, as shown in figure 4 shows.
Three, result analysis
1. The grinding quality on bearing premature failure of the influence from the above test results can be judged, raceway side of the grinding cracks and grinding burn is a cause of ring break-up, cause bearing premature failure of a direct cause, this example reveals macroscopic crack form and grinding burn the distribution characteristics of instructions, raceway of the original position after partial or roller way R not round, one side of the raceway grinding allowance increases, when the coarse grinding feed stage, grinding speed, the whole raceway is produced not uniform grinding. If not timely grinding wheel dressing, will promote grindings embedded or adhesive cause wheel pore jams and passivation, which will cause the grinding wheel unbalance degree high, the spindle vibration increased, even machine vibration and produce grinding flutter. Obviously grinding force occurs cyclical change, form discontinuity burns banding.
This grinding area rate wipe force and grinding force will inevitably will heighten, produce high grinding heat. When grinding area surface temperature as high as moment or exceed the Acm, raceway local be ground surface will be back austenitizing. The volume expansion, and the low temperature of inner part to restrict its expansion, produce surface compression and lining for expansion of the thermal stress. When the wheel from grinding after surface. In external cooling liquid quenching conditions (even empty cold), the surface has austenitizing thin layer, again carries on the second Cui fire, forming high hardness of the secondary hardening organization, namely "grinding white layer". At the same time because grinding heat inward and its adjacent layer on the penetration of the diffusion effect, it can form a layer of hardness lower black "tempering layer", the volume shrinkage, so in the grinding surface layer is produced in the high temperature gradient, which cause a series of organization change and mechanical properties change. "grinding white layer" system in a pressure stress state, and "tempering layer" is in the largest tensile stress state, so the most easy to crack the core in the initiation, especially "grinding white layer" and "after tempering layer, on the border, because it
Bearing the specific classification
According to the loading direction can be divided into: (1) the radial humanistic bearing, also called radial bearing, bear radial load. (2) the thrust bearing, say again thrust bearing, bear axial load. (3) radial thrust bearing, also called radial thrust bearing, and at the same time, bear radial load and axial load. Work according to the bearing friction properties can be divided into sliding friction bearing (hereinafter referred to as sliding bearing) and rolling friction bearing (hereinafter referred to as rolling bearing) two kinds big.
Deep groove ball bearing
The most representative rolling bearing, wide range of USES
Can bear radial load and two-way axial load
It is applicable to high-speed rotating and requirements of low noise, low vibration occasions
With steel plate dust cover or rubber sealing ring seal type Harbin bearing inner advance filling the right amount of grease’s outer ring has anti-rotation ring or flange bearing, namely easy to axial positioning, and convenient for the installation of the enclosure
Maximum duty bearing size and standard bearing the same, but the inner and outer one place filling tank, increase the loading ball number and improve the rated negative.
The main applicable cage: steel plate stamping cage (waveform, crown... single row; S shape... double column)
Copper alloy or phenolic resin cutting cage, synthetic resin forming cage
Main application: car: rear wheel, gear boxes, electrical installation components
Electrical: general motor, household electrical appliances
Other: instrument, internal combustion engine, construction machinery, railway vehicle, loading and unloading handling machinery, agricultural machinery, all kinds of industrial machinery
Angular contact ball bearing
Ring and ball between contact Angle, the standard contact Angle is 15 ° and 30 ° and °The greater the Angle of contact axial load capacity is bigger also Contact Angle is smaller, the better the high speed rotating Single row bearing can bear radial load and unilateral axial load DB combination, DF combination and double bearing can bear radial load and two-way axial load DT combination suitable unilateral axial load is bigger, a single bearing rated load shortage situation High speed with ACH type bearing ball diameter small, ball number, mostly used for machine tool spindle Angular contact ball SKF bearing is suitable for high speed and high precision rotary The structure for the combination of the two single row angular contact ball bearing common inner ring and outer race, can bear radial load and two-way axial load No filling tank bearing also have sealed typeThe main applicable cage: steel plate stamping cage (bowl... single row; S form, crown... double column) Copper alloy or phenolic resin cutting cage, synthetic resin forming cage Main application: single row: machine tool spindle, high frequency motor, gas turbine, centrifugal separator, small car wheel, differential pinion shaf Double: oil pump, roots blower, air compressor, all kinds of transmission, fuel injection pump, printing machines Four contact ball FAG bearing can bear radial load and two-way axial load A single bearing can replace positive combination on the back or combination of angular contact ball bearing Applicable to inherit pure axial load or axial load component large synthetic load This kind of bearing bear any direction axial load can form one of the contact Angle (alpha, so ring and ball always in a contact line double dealing point contact The main applicable cage: copper alloy cutting cage Main use: the plane jet engines, gas turbine Self-aligning ball bearing
Due to the outer raceway hand deliver spherical, with self-aligning properties, so it can automatic adjustment for shaft or shell of deflection or different heart is caused by the axis Cone hole bearing through the use of fasteners can easily installed in shaft Steel plate stamping cage: chrysanthemum form... 12, 13, 22... 2 rs, and... 2 rs Kwai form... 22, 23 Woodworking machinery, textile machinery transmission shaft, vertical set-contained self-aligning bearing Cylindrical roller bearing
Cylindrical roller and roller way a line contact, radial load capacity, which is designed to withstand heavy loads and shock load, also suitable for high speed rotating N type and NU type can be axial movement, can adapt for thermal expansion or installation error caused by the shaft and housing relative position changes, the most adapt to used for free end bearing NJ type and NF type can bear a certain degree of unilateral axial load, NH type and NUP type can bear a certain degree of two-way axial load inner ring or their cups are separable for easy loading and NNU type and NN type resistance to radial load strong rigidity, mostly used for machine tool spindle
The main applicable cage: steel plate stamping cage (Z), copper alloy cutting cage, pin type cage, synthetic resin forming cage
Main application: medium and large motor, generator, internal combustion engines, gas turbine, machine tool spindle, speed reducer, loading and unloading handling machinery, all kinds of industrial machinery
Entity type needle roller bearing
Have the basic structure of bearing inner ring and NU type cylindrical roller bearing the same, but because the needle, the volume can be reduced, and can withstand large radial load without inner ring of bearing the has an appropriate precision and hardness of the shaft installation surface as raceway surface use
The main applicable cage: steel plate stamping cage
Main application: automobile engine, transmission, pump, excavator crawler wheel, hoist, bridge crane, compressor
Tapered roller bearing
This kind of bearing with frustum of a cone shape roller, roller by inside circle big guard guide
Design makes the inner ring raceway surface and outer raceway surface and roller rolling the each cone vertex intersection in the bearing center online point
Single row bearing can bear radial load and unilateral axial load, double bearing can bear radial load and two-way axial load
Applicable to withstand heavy loads and shock load
According to the contact chest (alpha different, divided into small cone Angle, the cone Angle and the big cone Angle three type, the greater the contact Angle axial load capacity is bigger also
Outside and internal components (inner ring and roller and cage components) separable, easy to put down
Rear auxiliary code "J" or "JR" bearing with international compatibility
This kind of bearing has many use English series products
The main applicable cage: stamping steel cage, synthetic resin forming cage, pin type cage
Main application: car: front, rear wheel, transmission and differential pinion shaft. Machine tool spindle, construction machinery, large agricultural machinery, railway vehicle gear reducer, mill roll neck and reduction gear.