Main parameters and common problems of hydraulic pump

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Main parameters and common problems of hydraulic pump

1.6.8 hydrostatic pressure balance

In the hydraulic pump, usually one side is high pressure, the other side is low pressure, so it will produce an unbalanced hydraulic force. This force acts on the rotor, bearing and other corresponding components, which aggravates the wear, shortens the service life of the pump and destroys the normal work. Therefore, hydrostatic pressure must be used to balance. The way of balance varies with the structure of the pump. For example, the external gear pump adopts the radial force balance structure that leads the high-pressure oil to the low-pressure area, or the structure that the oil discharge port is smaller than the oil inlet port; the vane pump adopts the structure that balances the radial force of the rotor by arranging the oil suction and discharge windows in pairs. For some hydraulic pumps (such as axial piston pump) whose forces caused by high and low pressure oil can't balance each other, it must be solved by using thrust bearing, which increases the structural size of the pump.

1.6.9 friction pair and measures to improve its wear resistance

(1) The types and common requirements of friction pairs stator, rotor and squeezer are the three main components of positive displacement hydraulic pump. When the hydraulic pump is working, the squeezer moves in the stator, and the rotor moves relative to the squeezer and the stator; or the squeezer moves in the rotor, and the squeezer and the rotor move relative to the stator together. Therefore, a pair of friction pairs is formed between any two parts with relative motion. In gear pump, such as tooth and tooth, tooth top and shell, gear and side plate; in vane pump, such as blade and rotor, rotor and port plate, blade and stator; in swash plate axial piston pump, such as plunger and cylinder bore, cylinder and port plate, slipper and swash plate; in screw rod pump, such as thread and thread, screw and shell, are all kinds of different friction pairs.

As the key parts of these friction pairs are generally in the friction condition of high relative speed and high contact pressure, their friction and wear directly affect the volumetric efficiency, mechanical efficiency, working pressure and service life of the pump, so they have the following common requirements.

① It has good sealing performance to ensure no leakage or small leakage and improve the volumetric efficiency of the pump.

② Play a supporting role, transfer a certain amount of force.

③ It works reliably in the movement, with small wear, good wear resistance and long service life.

④ Low heat, low noise, no cavitation and impact resistance.

(2) The main wear reasons of friction pair are adhesive wear and abrasive wear.

① Adhesive wear refers to the wear between friction pairs without complete lubrication. The mechanism of this kind of wear is that the two metal surfaces are uneven and the contact area is very small, so the contact specific pressure is large enough to exceed the yield limit of the material and cause plastic deformation, which makes the concave and convex metal surfaces adhere. When the two metal surfaces slide relatively, the concave and convex surfaces with lower shear strength are cut off and cause wear.

The main measures to reduce adhesive wear and improve wear resistance are as follows: selecting metal pairs that are not easy to adhere to each other, such as iron silver or iron tin pairs, or even iron and plastic pairs to form friction pairs, the wear is small (for example, using zqai-4 bronze cylinder block and 38CrMoAl valve plate to form friction pairs, the wear is very small); when iron iron or iron manganese pairs form friction pairs, the wear is small In order to reduce friction, a thin film with low shear strength is formed on the metal surface, such as oxidation, phosphating and nitriding, which are commonly used in production, to prevent corrosion and reduce friction; the surface roughness is reduced, but the appropriate roughness is also beneficial to the formation of lubricating oil film; the hardness is improved.

② Abrasive wear refers to the wear caused by hard abrasive particles such as metal particles, dust or other impurities such as oxides, fibers and resins. This kind of wear occurs in almost all friction pairs, especially in those with a certain oil film thickness. The wear particles will aggravate the surface wear, worsen the performance and shorten the service life of the friction pair.

The main measures to reduce or prevent abrasive wear are to strengthen filtration and prevent hydraulic medium from being polluted. The average pore size of the filter should be less than the friction gap to be protected.

In addition, proper lubrication should be ensured for all friction pairs. As we all know, friction pairs such as gear mesh of gear pump, between rotor and valve plate of vane pump, between plunger and cylinder bore of swash plate axial piston pump all have self lubrication performance, which is one of the advantages of hydraulic technology. However, some components in the pump, such as bearings, universal hinges, need to take certain structural measures to achieve lubrication.

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