Hydraulic fluids

Hydraulic fluids

Hydraulic oils are also called hydraulic liquids or hydraulic fluids. They are the medium by which power is transferred in hydraulic equipment. Hydraulic fluids have the primary purpose of transferring potential or kinetic energy (pressure and movements), create volume flow between pump and hydrostatic motor, and reduce the wear of parts that rub against each other. In addition, they protect the system from corrosion and help carry away the heat produced during energy transformation.

The operating practices of yesterday in industry have changed a lot. But steady and dependable, hydraulic fluid technology did not change much for decades. But today, the pressure is on hydraulic systems. Hydraulic systems are expected to deliver optimum performance while operating at higher pressures, temperatures, and tougher operating conditions.

Common hydraulic fluids are based on mineral oil or water. These oils have generally low compressibility. Hydraulic oils are circulation quality oils since they are in continuous use in an enclosed system with practically very little residence time in a reservoir or a storage tank. Hydraulic fluid has to perform the following tasks:

  1. Energy transmission
  2. Lubrication
  3. Heat removal

The primary function of a hydraulic fluid is to convey power. However there are other important functions of hydraulic oils such as protection of the hydraulic machine components. The main functions of hydraulic oil and the corresponding properties of the hydraulic oil which affects its ability to perform the required function are given below:

  • Hydraulic oil as medium for power transfer and control needs low compressibility (high bulk modulus), fast air release, low foaming tendency and low volatility.
  • Hydraulic oil as medium for heat transfer requires good thermal capacity and conductivity.
  • Hydraulic oil as sealing medium must have adequate viscosity and viscosity index as well shear stability.
  • For hydraulic oil to perform the function of a lubricant, hydraulic oil needs viscosity for film maintenance, low temperature fluidity, thermal and oxidative stability,  hydrolytic stability / water tolerance, cleanliness and filterability, demulsibility, anti wear characteristics and  corrosion control.
  • For efficiency of the pump hydraulic oil requires necessary viscosity to eliminate internal leakages and high viscosity index.
  • Other properties expected from hydraulic oil are fire resistance, friction modifications and radiation resistance.
  • Hydraulic oils are to be low in toxicity when new or decomposed and should be bio- degradeable. They should have material combatibility.

During the selection of a hydraulic fluid the following features are very important and are to be considered.

  1. Viscosity
  2. Viscosity Index (VI) and/or Viscosity Grade (VG) viscosity at 40 deg C
  3. Pour point
  4. Shear stability, when polymer VI-improvers are used

For any application the features of the hydraulic fluid must be appropriate to the operating environment of the unit and its components. The following is an explanation of the fundamental features of the hydraulic fluids:

  1. Viscosity – A hydraulic fluid has a low viscosity when it is thin and a high viscosity when it is thick. The viscosity changes with the temperature. If the temperature increases, viscosity is reduced and vice versa. Hydraulic units work under extreme temperature changes. The viscosity range of the hydraulic fluid is extremely important. The hydraulic fluid must be thin enough to flow through the filter, inlet and return pipes without too much resistance. On the other hand, the hydraulic fluid must not be too thin, in order to avoid wear due to lack of lubrication and to keep internal leakage within limits.
  2. Viscosity index (VI) – The viscosity index is a calculated number, which describes the viscosity change of a mineral oil based or a synthetic fluid versus temperature. A high viscosity index means a small viscosity change when the temperature changes while a low index means a large viscosity change when the temperature changes. Most hydraulic fluids have a VI value of 90 – 110. Hydraulic fluids with a VI larger than 110, e.g. between 130 -200, are not as sensitive to temperature change. These hydraulic fluids distinguish themselves by starting up well and having minimal loss in performance at low temperatures. At high temperatures a sufficient sealing effect and protection against wear is achieved by using hydraulic fluids with high viscosity index. The high durability of a hydraulic fluid with a high viscosity index avoids damage and machine breakdown, lowers the operating cost and increases the life of hydrostatic transmissions and units.
  3. Shear stability – Fluids using polymer viscosity index improver may noticeably shear down (> 20 %) in service. This will lower the viscosity at higher temperatures below the originally specified value. The lowest expected viscosity must be used when selecting Hydraulic fluids.
  4. Pour point – The pour point defines the temperature when the hydraulic fluids stop to flow. Start up temperature is recommended to be approximately 15 deg C above hydraulic fluid pour point.
  5. Sealing compatibility – In general NBR (Nitrile) or FPM (Fluorocarbon, Viton) is used as seal material for static and dynamic seals. For most hydraulic fluids both seal materials are suitable, but for some hydraulic fluids only one kind is preferred.
  6. Density – The density has to be specified by the manufacturer of the hydraulic fluid. Using hydraulic fluid with a high density requires the sufficient diameter of the suction line and/or elevated tank to provide positive inlet pressure.
  7. Air in the hydraulic fluid – Free air is considered as contamination as well. Air typically enters the circuit through the suction line if the seals and fittings are not tight. This free air then may be dissolved in the hydraulic fluid. Mineral based hydraulic fluid may contain up to 9 % volume percent dissolved air at atmospheric pressure. If 1 litre of hydraulic fluid is compressed to 100 bar, it may dissolve 9 litres of free air if offered. This is not a problem unless the pressure drops down quickly to a lower level. Then the air becomes free again and bubbles show up. These bubbles collapse when subjected to pressure, which results in cavitation which causes erosion of the adjacent material. Because of this, the greater the air content within the oil, and the greater the vacuum in the inlet line, the more severe will be the resultant erosion. Air increases the compressibility and may also result in a spongy system, slow response time, and poor controllability. Air also creates a loss of transmitted power, higher operating temperatures, increased noise levels, and loss of lubricity. Therefore care must be taken to avoid air to enter the system. If air has entered a system the air release time and foam characteristic becomes important.
  8. Air release – Air release (Fig 1) is a measure for the time needed to release air bubbles (free air) contained in the fluid to the surfaces. Air typically enters the circuit through the suction line if the seals are not tight.
  9. Foaming characteristic – Foaming characteristic (Fig 1) defines the amount of foam collected on the surface in the reservoir and the air bubble decomposition time. Foaming may become a problem when air has entered the circuit through an insufficient tight suction line.
  10. Bulk modulus/Compressibility – While fluids are usually considered incompressible, the pressures that can occur in hydrostatic systems are of a magnitude that fluid compressibility can be significant. In applications that experience system pressure fluctuations resulting in random high pressure rise rates, consideration must be given to fluid compressibility when sizing a charge pump to ensure adequate charge pressure. The amount that a specific fluid compresses for a given pressure increase is related to a fluid property known as the bulk modulus. The bulk modulus is a measure of a fluids resistance to being compressed. It depends on pressure and temperature. The air content is important as well especially below 50-100 bar. The higher the air contents the spongier is the system (lower bulk modulus). For a given pressure increase and fluid volume, a fluid with a large bulk modulus will experience a smaller reduction in volume than a fluid with a low bulk modulus. Another term often used is compressibility. It defines how much a fluid can be compressed. Compressibility is the reciprocal of the bulk modulus.
  11. Cleanliness – The cleanliness level of a hydraulic fluid is determined by counting number and size of particles in the fluid. The number of particles is defined as a cleanliness level according to ISO 4406.
  12. Water content – In a new fluid the water content must be out of the quantitative detectable range. Unless otherwise specified in individual fluid standards the water content for continuous operation must not exceed 0.1 % (1000 mg/kg). The lower the better. In principle water is a harmful contaminant, reducing the life of the hydraulic fluid and the mechanical components. Water in a system may result in corrosion, cavitation, and altered fluid viscosity. Depending on the fluid, water may also react with the fluid to create harmful chemical by-products or destroy important additives. Left unchecked, water contamination may result in microbial growth. At this stage, system components may already have been damaged

Air release and foaming characteristics

 Fig 1 Air release and foaming characteristics in hydraulic fluids

Originally hydraulic fluid used was water. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its lubrication properties and its ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks. Natural oils such as rapeseed are used as base stocks for fluids where biodegradability and renewable source is considered important.

For specialty applications such as fire resistance the hydraulic oils used are of four types. They are dilute emulsions (oil in water), invert emulsions (water in oil), water glycols and synthetic preparations such as chlorinated hydrocarbons, organophosphate easters, polyalphaolefin, propylene glycol and silicone oils.

Usually hydraulic oil would last in service for many years, if it is not for the contamination and degradation. Contamination by other fluids, external dirt, wear metals, rust particles and water reduce the life of the hydraulic oil. The characteristics of the hydraulic oil most likely to change in service are visual appearance, smell, water content, solids content, foaming, acidity and viscosity. Regular quality checks of the hydraulic oil are necessary to identify the potential problems for corrective actions in order to avoid malfunctioning and failure of the hydraulic system.

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