Multi-grade oils and single-grade oils Multi-grade oils can span several viscosity grades. Taking 15W/40CD diesel engine oil as an example, 15W/40 may include 6 viscosity grades such as 15W, 20W, 25W, 20, 30, 40, and so on, so it is called multi-grade oil. The 15W/40 viscosity grade has both a low temperature viscosity of 15W oil and a high temperature viscosity of 40 oils, so that an oil can be used in winter and summer, and it is not necessary to change the oil due to seasonal changes, so the multi-grade oil is also a four seasons universal engine oil. Multigrade oils are selected from lighter fraction base oils by adding viscosity index improvers, pour point depressants and other functional additives.
Compared with single-stage oils, multi-grade oils have better viscosity-temperature properties (the so-called viscosity-temperature property is the property of oil viscosity changes with temperature, usually measured by the viscosity index, and the high viscosity index indicates the viscosity of the oil with temperature. The change is small. It can be seen that, in the case of the same viscosity of 100, the multi-stage oil has a lower low-temperature viscosity and a higher high-temperature viscosity. Furthermore, in the multi-grade oil product specification, the results of low temperature viscosity (CCS) and boundary pumping temperature (MRV) should be provided to ensure good low temperature performance during the use of the oil.
The low-temperature performance of lubricating oil is mainly measured by the low-temperature viscosity and the boundary pumping temperature. The low-temperature viscosity of the oil mainly affects the cold startability of the engine. If the low-temperature viscosity of the lubricating oil is too high, the torque of the lubrication system at the time of crankshaft rotation is indicated. If it is too large, it may fail to reach the prescribed minimum rotational speed and it is difficult to start, and because the viscosity of the lubricating oil is too large, it is difficult to form an effective lubrication and wear is increased. Therefore, the viscosity limit of lubricants at low temperatures should be regulated. The boundary pumping temperature of the lubricating oil is the lowest temperature at which the engine oil can be continuously and fully supplied to the inlet of the engine pump.
Because the engine's starting process is very short (especially at low temperatures), the oil in the crankcase is too late to be pumped to the friction surfaces. In order to ensure that the engine friction surface can be timely and effective lubrication, the engine must be started In a short period of time, the oil pressure of the lubricating oil system is normal. Since single-stage oils use heavier base oils and no viscosity index improvers are added, there are no requirements for low-temperature viscosity and boundary pumping temperature in the oil specifications. Therefore, high- and low-temperature performance is not as good as multi-grade oils. It can also be seen that if the 100 viscosity is the same, the multi-grade oil has a lower low-temperature viscosity and a higher viscosity of 150 and 230, which is favorable for the engine's low temperature start and high temperature anti-wear.
In addition, the normal operation of the engine, due to the high shear rate (106s-1), the oil has a temporary viscosity loss, generally not less than 3.5mPa.s, the viscosity is too small, the oil film is easily destroyed, the sealing effect is not good, the oil consumption increases , will also produce wear. The operating temperature difference of the engine in use is quite large, from atmospheric temperature to 300, so the viscosity and temperature performance of the oil is particularly important. Through the above analysis, it can be seen that the multi-grade oil viscosity performance is significantly better than the single-grade oil.
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