Sharing the use and maintenance of piston pumps

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The efficiency of the hydraulic system in construction machinery depends primarily on the volumetric efficiency of the hydraulic pump. When this volumetric efficiency drops to 72%, routine maintenance is required, including the replacement of bearings and worn-out seals, as well as the replacement or repair of friction pairs that have exceeded their allowable clearance, in order to restore their performance. A review by Li Gong from the hydraulic sector: 1. Oil supply methods of hydraulic pumps – Straight-shaft swashplate piston pumps are divided into two types: those that supply oil under pressure and those that can draw in oil by suction. Most pressure-fed hydraulic pumps use a tank filled with pressurized oil; there are also hydraulic pumps that come equipped with a filler pump to supply pressurized oil to the inlet of the hydraulic pump. Self-priming hydraulic pumps have a strong self-priming capability and do not require external oil supply. For hydraulic tanks that are powered by air pressure, it is necessary to wait until the hydraulic tank reaches the required pressure after each start-up of the machine before operating the equipment. If the air pressure in the hydraulic oil tank is insufficient, it can affect the machine, causing the components inside the hydraulic pump to become detached from each other; this in turn can lead to abnormal wear of the return plate and the pressure plate within the pump. For plunger pumps that use an oil feed pump, after 3000 hours of operation, the operator must inspect the plunger pump 1–2 times per day to check whether the operating noise of the hydraulic pump is normal. If a decrease in the speed of the hydraulic cylinder or stalling occurs, the oil feed pump should be disassembled for inspection to check for any scratches on the edges of the impeller and to determine whether the clearance of the internal gear pump is too large. For self-priming piston pumps, the level of oil in the hydraulic tank must not be below the lower limit indicated by the oil gauge; an adequate amount of hydraulic oil must be maintained. The higher the cleanliness of the hydraulic oil, the longer the service life of the hydraulic pump. 2 Bearings for hydraulic pumps: The most important component of a piston pump are the bearings. If there is play in these bearings, it is not possible to maintain the proper clearance between the three friction pairs inside the hydraulic pump. This also affects the thickness of the hydrostatic lubrication film surrounding each friction pair, thereby reducing the service life of the piston pump’s bearings. According to the information provided by the hydraulic pump manufacturer, the average service life of the bearings is 10,000 hours; once this value is exceeded, new bearings need to be replaced. For the removed bearings, it is impossible to determine their clearance without specialized testing equipment; visual inspection is the only option. If scratches or discoloration are found on the surface of the rollers, they must be replaced. When replacing bearings, attention should be paid to the English letters and model of the original bearings. Plunger pump bearings are usually large-capacity bearings, and it is best to purchase products from the original manufacturer that match the original specifications. If another brand is used, it is necessary to consult someone with experience in bearings to determine the appropriate replacement, in order to maintain the bearing’s precision grade and load capacity. 3 Inspection and repair of three pairs of friction pairs: When the various clearances between the piston rod and the cylinder bore exceed the specified values, they can be repaired using the methods outlined below: (1) If the cylinder bore is fitted with copper sleeves, these can be replaced as a way to carry out the repair. First, trim the diameters at a set of plunger rods to a uniform size, then polish the outer diameter using sandpaper graded 1000# or higher. Three methods for installing copper sleeves in the cylinder block: (a) heating the cylinder block for hot fitting, or freezing the copper sleeve at low temperature for forced fitting through interference fit ； (b) Assembly is carried out using Loctite adhesive; this method requires grooves on the outer diameter surface of the copper sleeve ； (c) Tap the cylinder bore, thread the outer diameter of the copper sleeve, apply Loctite, and then screw it in for assembly. (2) For cylinder blocks and copper sleeves bonded by sintering, the repair method is as follows: (a) Use an abrasive rod to grind and repair the cylinder bore manually or mechanically ； (b) Use a coordinate boring machine to re-bore the cylinder block holes ； (c) Use a reamer to repair the cylinder block bore. (3) The “surface engineering technology” is employed, with the following methods: (a) Electroplating technique: Plating a layer of hard chromium on the surface of the plunger ； (b) Brush plating technology: Plating wear-resistant materials on the surface of the plunger ； (c) Thermal spraying or arc spraying or electro-spraying: spraying high-carbon martensitic wear-resistant materials ； (d) Laser cladding: Cladding high-hardness wear-resistant alloy powder on the surface of the plunger. (4) For cylinder blocks that lack copper sleeves, the material is usually ductile iron, and an amorphous film or coating is prepared on the inner wall of the cylinder block. It is because of this special substance on the inner wall of the cylinder hole that a hard-hard friction pair can be formed. If the cylinder hole is ground blindly, removing the surface material from the inner wall of the cylinder hole will also change its frictional structural properties. If a friction pair with its coating removed is used forcibly, the temperature of the friction surface will rise sharply, causing adhesion between the plunger rod and the cylinder bore. In addition, a unique thin film coating is applied to the surface of the plunger rod; this coating possesses anti-friction, wear-resistant, and lubricating properties. In this case, the friction pair consists of a hard material and a soft material. Any change to the coating will disrupt the optimal pairing of materials, and repairing such special plunger pumps requires taking them to a professional repair shop.