When the excavator is working, the boom is lifted slowly. The boom of excavator is lifted by the pressure oil output by the oil pump, which enters the rear chamber of boom oil cylinder through the valve along the pipeline. The piston in the oil cylinder moves under the action of working oil pressure and lifts the boom through the piston rod. Therefore, the speed of boom lifting is related to the pressure and flow of oil entering the cylinder. According to the principle that the power of the input cylinder is equal to the output power of the cylinder, and then according to the fact that the power of the input hydraulic cylinder is equal to the product of the working pressure of the hydraulic cylinder and the flow entering the hydraulic cylinder, the slow boom lifting is mostly due to the reduction of the power of the input hydraulic cylinder, that is, the reduction of the power of the input cylinder. The reasons for the power reduction of the input cylinder are:
(1) Influence of oil pump
According to the proportional relationship between the moving speed of the piston in the oil cylinder and the working flow of the oil entering the oil cylinder, if the output flow of the hydraulic oil pump decreases, the boom oil cylinder will be lifted slowly. Further analysis, the actual flow of the oil pump is equal to the product of the theoretical flow of the oil pump and the total efficiency (the sum of the volumetric efficiency and mechanical efficiency of the oil pump). If the total efficiency of the oil pump decreases, the actual flow of the oil pump must also decrease. The main reasons for the decrease of the total efficiency of the oil pump are the increase of oil pump leakage and mechanical friction loss.
(2) Influence of oil cylinder
The damage of the piston seal ring of the hydraulic cylinder, the pull of the cylinder and the seal ring of the cylinder cover cause oil channeling or oil leakage, which reduces the volumetric efficiency in the cylinder, that is, the efficiency of the cylinder itself, so the output power decreases, which is manifested in the slow lifting speed of the boom.
(3) Influence of oil leakage
The suction section of the oil pump (from the oil pump to the oil tank) sucks air due to poor sealing, which affects the oil pump efficiency; If the pressure range section from the oil pump to the oil cylinder leaks out of the system or leaks back to the oil tank through the control valve, the oil flow into the oil cylinder will be reduced and the pressure will be reduced, resulting in slow boom lifting.
(4) Influence of oil circuit blockage
The working oil of the hydraulic system generally has mechanical impurities, which are blocked in the small part of the oil flow load surface in the oil circuit, reducing the oil flow into the oil cylinder and reducing the pressure at the same time. Therefore, the output power of the oil cylinder is reduced and the boom lifting is slow. The blockage of the filter screen at the oil suction port will increase the vacuum of the oil suction section of the oil pump. When the air pressure in the separated oil is reached, the air dissolved in the oil will be separated and sucked by the oil pump, affecting the output power of the oil pump. According to the fact that the output power of the oil cylinder is equal to the input power of the oil pump, the boom lifting is slow due to the reduction of the power of the oil cylinder. If the oil filter screen or radiator in the oil return circuit is blocked, the back pressure of the oil cylinder will increase, resulting in a small pressure difference on both sides of the oil cylinder piston. According to the proportional relationship between the piston moving speed of the oil cylinder and the pressure on both sides of the piston, when the pressure difference on both sides of the piston decreases, the boom lifting is slow.
(5) Effect of valve
If the set pressure of the safety valve (overload valve or overflow valve) is too low or the valve is not closed tightly, the working oil pressure in the system will decrease. According to the Basque theorem that the pressure is equal everywhere, the pressure in the oil cylinder will also decrease, so the lifting speed of the oil cylinder is slow due to low pressure. The check valve is not tightly closed due to mechanical impurities, or the valve is not tightly closed due to poor sealing between the valve and the valve seat. The result is the same as the above situation. The clearance between the valve core of the directional valve and the mating pair is too large, resulting in part of the pressure oil flowing back to the oil tank during operation, and the result is the same as the above situation.
(6) The oil inlet and outlet pipe joints of the oil pump are reversed, and the oil cylinder does not act.
