How to evaluate scraping contact spot of the hotte

2022-08-02
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How to evaluate the scrape and grind contact spots in the verification of cast iron plate

cast iron plate is an important tool widely used in precision testing, inspection and marking in the machinery industry. In order to prolong the service life of cast iron plate and ensure the service accuracy The quality of scraping contact spot is an important technical index of cast iron plate

at present, the evaluation of scraping contact spots is as follows:

first, check the micro quality of the working surface of the plate, that is, the flatness of small peaks and valleys. The service life of the plate can be guaranteed only if the surface micro quality is high and the wear resistance is good

the second is to prevent small-scale local distortion on the working surface of the plate, so as to ensure the service accuracy of the plate. See the following table for the requirements of jjg

flat plate verification regulation on scrape and grind contact spots: the above table has no specific provisions on the area (single point area, contact area within unit area) and scrape and grind depth of contact spots, resulting in different opinions on the evaluation of scrape and grind contact spots

reasonable scraping and grinding contact spots on cast iron plate can prolong the service life of the plate, which must be realized through repeated tests by downstream automobile manufacturers. This paper discusses this as follows:

first, the influence of friction on the wear rate of scraping and grinding contact spots

because the solid surface is rough, the two surfaces always contact at individual points. Therefore, the actual contact area is much smaller than the theoretical contact area. Because the actual contact surface is small, even when the load is very small, a large unit pressure will be generated. Due to different surface roughness, the wear of contact surface varies with the friction force under the action of unit pressure

under the action of unit pressure, the change curve of repeated friction and wear with time is shown in the following figure:

the curve shows that the amount of wear is large at the initial stage, which is caused by the uneven wear surface, small actual contact area and large specific pressure. For the flat plate, two tests of simply supported beam and cantilever beam can be realized, and the scraping contact spot of the flat plate is always in the initial wear stage, which is one of the main reasons for the scraping contact spot wear. It can be seen from the friction speed and contact pressure wear curve (as shown in the figure below) that when the wear speed is v=0.3 ~ 0.5m/s, the wear amount is the largest, which is called adhesive wear. When the flat plate is in use, the moving speed of the workpiece on the flat plate is also carried out at this stage, which is the second reason for the abrasion of scraping contact spots

in addition, plate material, hardness, environment, maintenance and other factors will also have different effects on the wear of scraping contact spots. In order to study the more popular research directions at home and abroad, including automatic universal electronic testing machine and dynamic testing machine (such as fatigue testing machine and impact testing machine) to study the exact wear conditions of different scraping depths and different contact areas, this paper analyzes and discusses the common test methods and the collected data

II. Determination of scraping depth, area of scraping contact spots and analysis of service life of plates

the following table shows the test data of common Hebei Botou cast iron plates and cast iron plates in other provinces and cities:

1 plate has 25 ~ 27 contact spots, and the contact area is about 20%; The contact spots of No. 2 plate are 25 ~ 29, and the contact area is about 10%

the wear investigation results of 20 flat plates are as follows: two Hebei Botou cast iron flat plates, one of which has been used for more than 5 years and is still in normal use; The other conductor has been used for more than 5 years. Due to poor storage, it was repaired due to corrosion. The remaining 18 cast iron plates scraped and grinded by other provinces and cities have been used for 1 year. After verification, 10 of them have been seriously worn and unqualified, and need to be repaired again; 8 pieces can still be used, but some spots have appeared

after a simple treatment, take the average of the scraping depth. The scraping depth of No. 1 plate is 37.36 μ m, and that of No. 2 plate is 9.06 μ m, with a difference of three times. According to the relationship between the wear amount of the friction curve and the friction time, the use of the flat plate is always in the initial wear stage, and the curve is linear. The wear amount is directly proportional to the reduction of the vehicle weight during friction. According to the investigation of plate wear, the service life of Botou plate in Hebei is 5 times that of Wafangdian plate. Obviously, the impact of the scraping depth on the service life of the flat plate can not be ignored. The JJG flat plate verification regulation does not stipulate the scraping depth, which is undoubtedly a defect

it can also be seen from the above test results that the two contact spots are roughly the same, but the measured contact areas are 20% and 10% respectively, and the difference is very large. From the perspective of use, the service life of the flat plate can be improved by increasing the contact area as much as possible without causing the lapping between the workpiece and the flat plate. JJG flat plate verification regulation does not properly limit this item

in order to improve the service life of the plate and ensure its working accuracy, it is recommended to control the scraping depth above 0.02mm, so as to ensure that the plate has a long service life and verification cycle. According to the introduction of scraping depth and the measured data of scraping depth, the processing requirements of this process can be met

for the control of contact spot area, the British Standard Specification for flat plates and platforms stipulates that the proportion of bearing area of level AA (equivalent to level 0 of JJG regulations) and level a (equivalent to level 1 of JJG regulations) flat plates shall not be less than 20%, and that of level B (equivalent to level 2 of JJG regulations) flat plates shall not be less than 10%. The high points shall be evenly distributed, and the proportion of the bearing area shall not be high enough to cause adhesion of the workpiece. This regulation not only ensures the service accuracy of the plate, but also takes into account the service life of the plate, which can be used for reference. (end)

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