Hydraulic fluid: water or oil
Hydraulic fluid carries energy in form of pressure and converts it into force with linear motion. It has a job to do in a hydraulic system. Hence it must be selected in such a way that it matches with duty that it has to perform. Performance of a hydraulic system depends upon performance of a hydraulic fluid .
When hydraulic extrusion presses were first built, the operating medium was exclusively water. Oil hydraulic presses become popular approximately since 1950. In contrast to water, oil can be used as the operating medium without packings and thus without expensive seals, because of its viscosity and its lubrication effect, for the pump pistons of the high-pressure pumps and for the directional valves . At the same time, water hydraulic systems have a number of advantages and they are successfully used in special extrusion presses .
Oil hydraulic extrusion presses
Typical schematic arrangement of an oil hydraulic system for an extrusion press is shown in Figure 1 .
The four main functions of hydraulic fluid are :
- to act as an energy transfer medium
- to lubricate internal moving parts of components
- to act as a heat transfer medium
- to seal small clearances between moving parts.
Overheating of hydraulic system
As hydraulic fluid circulates through an hydraulic system it both produces heat by doing work and transports that heat away from working components. However, if the heat dissipation is inadequate, or if the hydraulic oil does not have the right heat dissipation properties, then it can overheat. Build-up of dirt, debris and clogged hydraulic filters can also affect a system’s ability to dissipate heat and lead to hydraulic fluid becoming too hot, leading to loss of fluid viscosity and reduced lubrication and pump efficiency.
Oil temperature must be maintained below a maximum of 60 ºC). Temperatures above this level lead to deterioration of the oil, loss of lubricating properties, and excessive wear to pumps .
Below are the main reasons for such overheating [4, 5].
Incorrect hydraulic fluid viscosity
Hydraulic fluid viscosity refers to the thickness or “resistance to pouring” of your hydraulic fluid. This is very important to the correct operation of your pump. The fluid not only transmits the power that moves your drives and actuators. It also lubricates internal components and removes heat from the system. Hydraulic fluid is designed to operate at a specific temperature range. As it heats, it becomes thinner and eventually it will lose the ability to lubricate moving parts. The increased friction may cause the pump to heat up, and naturally increased wear will be taking place when this is happening. On the other hand, hydraulic fluid that is too thick flows less efficiently within the system, which also results in heat buildup.
Dirty hydraulic fluid
Fluid that is contaminated with dirt, debris, water and other impurities may cause heat build up in a few ways. Blocked fluid filters, pipes and strainers place undue load on the pump or even lead to pressure drops on the back side of filters that cause cavitation.
Low fluid levels can result in a condition in which not enough flow is reaching the critical hydraulic components and moving parts. This is known as oil starvation and it will increase metal-on-metal friction and lead to increased heat and wear. Oil starvation can also be caused by clogged hydraulic filters, incorrect fluid reservoir design.
Cavitation is the rapid formation and implosion of air cavities in the hydraulic fluid. When these air cavities collapse under pressure, they generate a lot of heat. In fact, temperatures can reach up to 2700 degrees C at the point of implosion! Not only does cavitation compromise the lubrication properties of the oil, the excessive heat that is generated is extremely damaging to the hydraulic pump and the system as a whole. Attacking hoses and seals and causing metal components to expand and wear.
Air in fluid
This happens when air makes its way into the system via air leaks at points like pump seals, and pipe fittings. Air generates heat when compressed, which naturally leads to an increase in temperature. In extreme circumstances compressed air bubbles explode in the same process that powers diesel engines. This is not good and leads to degradation of the fluid and damage to system components through loss of lubrication and burning of seals.
Relief value may be set to high
Pressure relief valves are there to ensure pressure does not rise beyond prescribed limits. If the relief valve is set incorrectly, it will not open soon enough and pressure will increase, which generates significant heat. This can have a detrimental effect on many parts of hydraulic system and contributes to fluid degradation, loss of lubrication and lower viscosity.
Faulty fluid cooling system
The design of any hydraulic system must allow oil to dissipate heat in order to operate efficiently. Some systems may do this with an oil reservoir or heat exchanger or combination of both. If the system design does not allow for sufficient heat dissipation or if the system is clogged or faulty, this will result in the fluid becoming overheated.
- Assess the design of hydraulic circuit and, if necessary, install an oil cooler or increase the oil reservoir capacity.
- Check for obstructions to airflow around the reservoir, such as a build-up of dirt or debris.
- Inspect the heat exchanger and ensure that the core is not blocked.
- Check the performance of all cooling circuit components and replace as necessary.
Worn or damaged pumps
As pumps wear, the internal leakage or “slippage” increases. Essentially, fluid is able to make its way past tight fitting components, which reduces the efficiency of the pump, but in addition, as this occurs, fluid moves from a high pressure to a low pressure without doing any mechanical work, since according to the laws of physics energy cannot be destroyed, it is instead converted into heat .
Modernize the old hydraulic system
- The exact list of changes will depend on the state of the old hydraulic system.
- A study of your system by a qualified hydraulic system engineer is recommended when planning the project.
Some possibilities include :
- Replace pneumatic servo controls with modern electronic servos.
- Improve filtration to increase component life and reduce downtime.
- Modernize pumps with newer models, for improved performance and better availability of parts and service support.
- Increase pumping capacity in order to decrease the press dead cycle.
- Install manifolds and re-pipe the press, to reduce leaks and improve performance.
- Improve oil-cooling capacity.
- Consider converting to variable-frequency motors, with either fixed-volume or variable volume pumps.
- Study of Hydraulic Seals, Fluide Conductor and Hydraulic oil/Design and Manufacturing of Hydraulic Pressess // Q.S. Khan – TANVEER PUBLICATIONS
- Extrusion //M. Bauser, G. Sauer, K. Siegert – 2009
- Water Hydraulic Press Control /Steve DeMar – Extrusion Technology Seminar – 2008
- The Extrusion Press Maintenance Manual // Al Kennedy