The hydraulic system of the extrusion press
Below is an overview of the maintenance of the hydraulic system of the extrusion press of the aluminum extrusion line from the famous American "Maintenance manual for the aluminum extrusion press" by Al Kennedy (see. link at the end of the article). This overview concerns the main elements of the press hydraulic system: pumps, pipelines, valves, filters, the main oil tank, oil and the like,. However, here not affect the principle of the hydraulic system and control it. These aspects are disclosed in the special courses to train staff.
hydraulic fluid – hydraulic oil
In most presses for the extrusion of aluminum in the industrial oil is used as hydraulic fluid. Therefore, below interchangeably the terms "oil" and used "liquid". However, it must be borne in mind, in the world that a lot of the press is operating in the water, and, Moreover, for a long time are being developed to obtain a non-flammable hydraulic fluid, that would be suitable for use in extrusion presses.
Dirty oil - 75 % issues
It is believed, what 75 % all the problems the hydraulic extrusion press is a direct result of oil pollution.
Foam or air bubbles indicate oil aeration, which can cause cavitation and premature failure of the pumps. If the oil becomes darker, it may indicate, that it has been overheated. The appearance in-oil "clouds" indicates an increase in its water content.
oil pollution it difficult to perform four basic functions, as a hydraulic fluid:
- act as a medium for the transmission of energy;
- lubricate the internal moving parts of the press components;
- serve as a medium for heat transfer;
- seal small gaps between the moving parts.
oil contamination may be present in many forms, most often - in the form of solid particles, water and entrapped air.
Particulate matter pollution
Solid particles cause problems, associated with wear and tear, and, Moreover, lower lubricity hydraulic fluids. As a result, the internal parts, such as impeller pumps, and various valves, that control or regulate flow of oil pressure, will work with deviations from normal operation. Such sensitive components such as servo-valves can become clogged and completely fail.
The gaps between the moving parts of the press and accumulate along fine dirt particles. Such contamination is the most common cause of failure of the solenoid valves, their inaccurate positioning and general wear and tear.
Particle size, close to the size of the gap, violate the lubrication process and to accelerate the abrasion. They also contribute to the so-called "chain reaction of wear", when the abrasive action helps to create new particles from the surfaces of the equipment component. This phenomenon accelerates wear until, until a catastrophic failure.
Larger particles restrict or block flow through gaps and openings, which leads to malfunction, Higher temperature operation, strong pressure drop and often catastrophic damage.
The size of the "harmful" particles generally constitute at least 40 micrometers and are therefore not visible to the naked eye. For comparison, normal thickness of a human hair is about 70 micrometers.
The purity of the hydraulic fluid
fluid purity level is measured in terms of the number of particles of various sizes in the specific volume of fluid. To count the number of particles used are very sensitive light microscopes.
To assess the purity level is used a special code ISO. This ISO code consists of two numbers: the first refers to the number of particles, a resolution which 5 m, per milliliter, second - the number of particles, is greater than the size of the 15 m, per milliliter. The numbers in ISO code - it is not the number of particles, this number is specified in a special table actual amount of particles. for instance, ISO codes 17/14 means, that the liquid comprises 1300 particle size greater than 5 microns per milliliter and 160 particle size greater than 15 microns per milliliter.
It must be remembered, new oil that is not necessarily a pure oil. Usually, the new oil pollution has to ISO 18/15 and higher. Therefore, all the oil before filling in the work should be filtered.
Water is a common contaminant of hydraulic oil. This contamination can cause damage of the following types:
- corrosion of metal surfaces;
- accelerated abrasive wear;
- fatigue failure of bearings;
- inhomogeneous viscosity.
Water can get into the hydraulic system in different ways. It can enter through the worn seals of the master cylinder and through the holes of the main oil tank. A constant source of water is the moisture, which condenses on the walls inside the oil tank.
Moreover, that water is the cause of rust and corrosion, it leads to the growth of microorganisms, which reduce the service life of oil, injure the working surfaces and disrupt the uniformity characteristics of the oil.
Each liquid has a water saturation point, above which it can no longer dissolve it. Any "extra" water is in the oil in a free state or as an emulsion. It looks like a whitish or discolored oil. When the content of water only 0,03 % it can already be achieved oil saturation.
Contamination of water to 2 % generally may be removed by filters with absorbents, which are placed in standard filters. If the water content is more than 2 %, the special oil treatment is required, for example, using special centrifugal devices.
oil pollution prevention
Oil contamination control is divided into two areas: exclusion and removal. It is necessary to eliminate as far as possible any contamination from entering the hydraulic system.
- oil reservoir dampers must have filters, that need to be changed periodically.
- Gaskets on the press cylinder must undergo regular maintenance and kept clean, to effectively detain pollution.
- During maintenance of hoses and pipes must be plugged.
- Flush out the system before starting up and after the failure of the machine.
- Filter all the liquid before starting their work.
- Often clean the entire system - wash, remove foreign objects (rags, etc.). P.).
- Let me know when new oil leaks and prevent getting in any impurities.
oil filtration system must operate continuously, even when the press is not working, for example, on weekends.
All filters should be specified, when should be the next replacement of cartridges. Moreover, each time you change the cartridge is always necessary to drain all the oil out of the filter housing and clean it.
Some experts recommend annual cleaning of the main oil tank of the press (oil accumulator) with preliminary pumping of oil into the auxiliary tank.. In this case, when returning the oil in the oil tank clean it must pass through the filters and the device for removing water.
Figure 1 - Oil filters in the line of the hydraulic system
(Filters - Filters; Oil circulation pump - Oil circulation pump)
The oil temperature must be maintained no higher than 60 oC. Temperatures above this level leads to deterioration of oil, loss of their lubricating properties and increased wear of the pumps.
Periodically check the heat exchangers to oil leakage or water, especially for water ingress into oil, in the absence of excessive heat, and the operation of the water supply system to maintain the oil temperature is below the set temperature,. When the press stops for repair or maintenance work is necessary to drain the oil from the heat exchanger and check for corrosion. If necessary, replace or clean the zinc anodes.
Figure 2 - The heat exchanger type “annular pipe”
Figure 3 - Plate heat exchanger
The oil level in the oil tank should be checked every day and topped up if necessary.. The oil level should be checked, when the master piston is in the same position. Depending on, where the main piston with respect to the master cylinder, It can be a significant difference in the level of oil in the oil tank. Low oil level may be the result of excessive heat generation or air into the system.
Most of the contaminants in the hydraulic oil can not be seen with the naked eye. particles, which can harm the hydraulic system, in size from 5 to 40 micrometers, and the limit of the human eye - it is 40 micrometers. Moreover, acid, water and oxidation products can not be detected by human senses. It is therefore necessary to select a sample of the oil and carry out their analysis.
Based on the results of the analysis, the following data are established:
- Spectrochemical analysis of metal wear particles and additives.
- Determination of the number of particles of various sizes (ISO code).
- Viscosity at 38 ºС (100 ºF).
- The water content in percentage by volume.
The air valve, who stand on the home oil tank, must be a sufficient amount of oil, the same, which is used in the hydraulic system. It is necessary to clean the valves and fill them with oil every month and more, If required by the local conditions of the surrounding air. environment.
Most hydraulic pumps, which are used in extrusion presses, are radial and axial piston pumps. To achieve maximum pump life and performance, the following must be done:
- High oil quality. The oil must be filtered, chilled, clean and without water. The purity of the oil - is the most critical factor for the long life of the pump.
- avoid cavitation. When air bubbles, which fall into the pump, collapse, this leads to the erosion of metal surfaces.
- motor and pump concentricity. When the motor is changed or pump always produce alignment and concentricity pump motor.
Figure 4 - Three main pump and the auxiliary pump
Valves are used to control the passage of the working fluid in a predetermined direction, and components of the hydraulic system in a predetermined sequence. The hydraulic system of the extrusion press uses the following types of valves:
- safety valves;
- check valves;
- control valves;
- filling valves.
In case of valve problems, the first thing to check are two possible causes: oil contamination and oil condition.. Dirt or foreign matter in the system may interfere with the normal closing / opening valves. Excessive heating oil may be a cause misalignment of the valve body. Carbonization leads to oil sticking of moving parts, valves.
When the damper is installed to avoid the deformation of its body. Pipelines should be free, no excessive force be connected to the valves. If necessary, the pipe bend and welded. If the valve stem has a free running cold, it does not mean, he will so move the operating temperature.
Figure 5 - Distribution unit with control valves
all pipeline flanges should be checked for leaks daily, pipe fittings, welds and collectors. It is necessary to check the oil connected to the cylinders, watch out for oil leakage around the valve. Valve fastening bolts tighten every month. Note, that oil leaks easier to see in ultraviolet light.
Loose or broken pipe clamps must be repaired immediately.. Their function is to minimize pipe forces..
Before replacing the pipes oil pipes they must be thoroughly cleaned of all contaminants, welding slag, zausencev, chips and the like. All welded or bent pipes need to be etched, washed and oiled before installing the hydraulic system. For cleaning the pipes should be used rags, which does not leave lint and fibers. The pipes should be made of stainless steel. It is not allowed to use hemp for sealing pipe connections, putty and sealing tape. As far as possible avoid hoses..
The main oil tank
The roof of the oil tank must be maintained in a clean condition. During maintenance of the tank is necessary to ensure, that all openings are securely closed, to prevent contaminants from entering them. All bolts on the tank are tightened monthly.
Some experts recommend cleaning the main oil tank annually with a preliminary drain of oil into the auxiliary tank. In this case, when returning oil to the main tank, it is passed through filters to remove solid contaminants and water.
Hydraulic system documentation
An accurate hydraulic system of the extrusion press is indispensable for competent maintenance.. This chart needs to be carefully studied., to understand the function of each valve. It is also important to have technical documentation of manufacturers of all valves and other components of the hydraulic system, so that you can order spare parts for the hydraulic system without any problems.
It often happens, that such technical documentation is not in full. It is vitally necessary to restore it as soon as possible, for example, with the help of equipment suppliers and specialized organizations.
Source: Al Kennedy, Aluminum extrusion press maintenance manual