Oil chiller　Technical Information　1-4. Hydraulic oil

This section describes the role of hydraulic oil, the characteristics of the different types of hydraulic oil and the need for cooling it.

What is hydraulic oil?

The lubricating oils used in industrial fields include general purpose oil, gear oil, spindle oil, turbine oil, bearing oil, etc., each with various characteristics according to the speed of rotation of the object to be lubricated and the size of the load.
Hydraulic oil is one lubricating oil. In addition to machine lubricating and rust prevention effects, hydraulic oil also has the function of transmitting the kinetic energy generated by the hydraulic pump to the actuator (hydraulic cylinder, hydraulic motor, etc.).
The output of the hydraulic pump is large and much heat is generated by adiabatic compression and friction within the hydraulic equipment, so the thermal load on the hydraulic oil is large. Because of this, among the many types of lubricating oils, hydraulic oils have the characteristic of often requiring cooling measures.

The role of hydraulic oil

Hydraulic pumps, hydraulic actuators, electromagnetic valves, etc., are controlled and operated at high speeds and high pressures. In addition, given such conditions as the oil temperature during operation, the surrounding atmosphere and the materials of the parts used in the machine, the hydraulic oil should have the characteristics listed below.

1. It should have a suitable viscosity and the viscosity should not change much with changes in temperature (lubricity and wear resistance).

   Heat is generated when hydraulic devices are operated, and the oil's viscosity should not change much over a wide temperature range in order to prevent machine wear and maintain smooth operation despite changes in temperature.

2. It should not corrode metal and should have a rust prevention effect.
3. It should not affect the materials used in the hydraulic device's parts (rubber, coatings, etc.).
4. It should not oxidize even at high temperatures.

   As oxidation of hydraulic oil progresses, the hydraulic device itself also oxidizes more easily, so the oil should have a high oxidation stability.

5. It should not compress.

   When accurate operation or highly precision positioning is required, compression of the hydraulic oil can cause errors equivalent to this compression.

6. It should be highly water-separable (low emulsifiability, lower specific gravity than water)

   Moisture mixing with the hydraulic oil harms the non-compressibility of the hydraulic oil and its rust prevention and lubricating characteristics. In order to prevent the moisture content in the system from mixing with the oil and to expel it from the bottom of the tank, the hydraulic oil should not emulsify easily, and its specific weight should be low so that the moisture content quickly sinks to the bottom of the tank.

7. It should be fire resistant.

   Even if the oil heats up due to operation of the machine, it should be safe and not catch fire if ignited.

Types of hydraulic oils

There are three main types of hydraulic oils, as described below.

General hydraulic oils

These are designated hydraulic oils used in general hydraulic devices.
Their friction prevention properties, viscosity shear stability and oxidation stability make them suited for hydraulic devices.

Abrasion resistant hydraulic oils

These are designated hydraulic oils that are suited for high pressure, high speed hydraulic devices.
They prevent wear, burning, etc.

Fire resistant hydraulic oils

These are hydraulic oils used in hydraulic systems in which general petroleum base hydraulic oil risks igniting and causing fires. They include the phosphate-ester base oils used in aircraft and water-glycol oils.
Depending on the type of hydraulic oil, special materials, sealant packings and coatings may be used.

Types and characteristics of fire resistant hydraulic oils

One problem of hydraulic oil is that it can burn. When a mineral base hydraulic oil is used for a hydraulic device in a workplace where a high temperature heat source is present, there is a risk of fire because the oil can burn. In these conditions, a fire resistant hydraulic fluid that has been made hard to burn is used as the hydraulic oil.

Hydraulic oils can be made harder to burn by using fluids that do not combust easily (synthetic type) or using water in the base oil of the hydraulic oil (hydrous type). Table 1 shows the types of fire resistant hydraulic fluids.

Table 1 - Classification of fire resistant hydraulic oils

With phosphate-ester oils, it is necessary to check whether the seals and packings in the hydraulic cycle are compatible. Also, phosphate-ester oils are extremely expensive, but they are frequently used as the hydraulic oil in aircraft.

Fatty acid esters offer excellent lubricity and stability, have good compatibility with the seals and packings used in hydraulic cycles and can easily replace mineral oils. However, their fire resistance is not as good as that of phosphate-ester base oils.

Water-glycol oils are used for a wide range of applications. In addition, the water content must be 30% or greater in order to maintain their fire resistance, and because metallic soap is added as an anti-wear agent and an alkalinity control agent is added for stability, they are alkaline. Because of this, it is necessary to check whether the metals, seals and coatings used in the hydraulic cycle are compatible.

W/O emulsions are extremely inexpensive, offer good compatibility with the hydraulic cycle and the wastewater is easy to treat, so there are no major problems in terms of practical performance, and with the current strengthening of restrictions to the total quantities of factory wastewater, they are increasingly replacing water-glycol oils.

O/W emulsions have a high water content of 90 to 95% and are rarely used.

Why hydraulic oils must be cooled

The power source for operating hydraulic actuators is a motor. Most of the power of the motor is transmitted as motion or force for operating the actuators, but part of it is lost as heat which increases the temperature of the oil. As shown on the diagram below, the major source of heat is friction in the pump and piping.

Hydraulic devices are normally equipped with a cooling mechanism for removing the heat generated in this way, but if cooling is insufficient and the oil temperature rises, the problems listed below can occur, and in some cases can cause the equipment to stop operating.

1. Stopping of equipment in case of abnormality

   - To prevent the major problems related to operation of the equipment described in items 2 to 4 below, operation of the equipment is stopped by an interlock system if the oil temperature reaches a certain level. Normally the interlock system is activated at temperatures of between 60 and 70 ℃, at which the rise of the oil temperature can cause adverse effects.

2. Decrease in the hydraulic oil's viscosity

   - A decrease in the viscosity reduces the lubricating performance, causing wear of sliding parts and in the worst of cases burning, which could cause the equipment to stop operating.

   - A decrease in the viscosity increases the fluidity of the hydraulic oil, increasing the speed of operation of the actuators, which could speed up wear and deterioration and induce critical damage.

3. Deterioration of packings

   - Deterioration of rubber parts in the sliding and rotating sections and the packings used at the joints of pipes can lead to oil leaks that could affect operation of the pump and actuators.

4. Deterioration (oxidation) of the hydraulic oil

   - If the temperature of the hydraulic oil itself remains high for an extended amount of time, the oil oxides, turning dark reddish-brown.
   Oxidation decreases the various elements of the hydraulic oil's performance and could corrode the device, so oxidized oil must be replaced. More frequent oil replacement increases running costs and the operation workload.

Cooling cutting oils

The hydraulic oil must be cooled to prevent the adverse effects described in the previous item. Ideally the hydraulic oil should be kept within the temperature range of 35 to 55 ℃, as shown on the diagram below.

The three methods below are used to cool hydraulic oil.

1. Air-cooled heat exchangers

   Also sometimes called air-cooled radiators or air-cooled oil coolers, these circulate the hydraulic oil in pipes and use fans to blow the surrounding air on them, cooling the oil by the difference in temperature. This method is excellent in terms of cost because it uses the surrounding air, but in summer when the surrounding air reaches above 35 ℃ the temperature difference is insufficient and the hydraulic oil's temperature can rise.

   Reference: 2-1. Air-cooled heat exchanger (liquid to gas)

2. Liquid-cooled heat exchangers

   Plate type heat exchangers and shell and tube heat exchangers are often used as the heat exchangers.
   Using industrial water or cooling tower water as the cooling water provides substantial cooling capacity, but as the water temperature rises in the summer, the temperature of the hydraulic oil also tends to rise.
   Using a chiller is used to chill the water makes it possible to obtain a stable supply of cooling water whose temperature is lower than the outside temperature even in the summer, and if the cooling capacity is sufficient this is an excellent way to stabilize the oil temperature.

   Reference: 2-2. Liquid-cooled heat exchanger (liquid to liquid)

3. Direct cooling with an oil chiller

   Immersion and direct circulation type chillers for hydraulic oil are also available on the market (the Apiste VSC Series consists of direct circulation type chillers).
   Oil chillers offer the advantages that they are compact and installation is simple, so they can be introduced quickly.
   It should be noted, however, that in order to protect the compressor, oil chillers cannot be used with high oil temperatures (about 50 to 60 ℃ or greater).

   Reference: 2-3. Direct cooling by chiller

 

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