As hydraulic fluid circulates through an extrusion press hydraulics 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 be overheating extrusion press hydraulics.
Oil hydraulics of 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.
Causes of overheating hydraulic oil
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 .
The possible reasons for overheating extrusion press hydraulics are following ones [2, 3]:
- Incorrect hydraulic fluid viscosity
- Dirty hydraulic oil
- Low oil level
- Environmental Variables
- Air in oil
- Relief value may be set too high
- Faulty oil cooling system
- Worn or damaged pumps
Incorrect hydraulic oil viscosity
Hydraulic oil 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 and overheating hydraulics of extrusion press.
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.
Low oil level
Low oil level 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.
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. This leads to overheating hydraulics of extrusion press and extremely damages of the hydraulic pump and the system as a whole.
Air in oil
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.
Relief value may be set too 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.
Faulty oil 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, this will result in the oil becoming overheated.
Worn or damaged pumps
As pumps wear, the internal leakage or “slippage” increases. Oil is able to make its way past tight fitting components, which reduces the efficiency of the pump. Oli moves from a high pressure to a low pressure without doing any mechanical work. So potential energy of pressured oil converted into heat .
Improper operation of the press
When all the above mentioned reasons have been eliminated, but the oil overheating continues, it’s time to make sure that your press is being operated correctly.
Over working of the press
According to an recognized authority on aluminum extrusion presses, Al Kennedy, the top causes of oil overheating are direct errors by the personnel operating the extrusion press .
- A common fault of many operators is to give high or full pump volume of oil during the pressure peaking (Figure 4). This causes a large amount of oil to be released through the relief valve. As a result of this oil moving from high pressure to low pressure, it becomes very hot .
- If the system uses a pressure relief valve to maintain pressure, a significantly larger heat exchanger will be needed .
Energy conversions during extrusion
The following energy transformations occur in an extrusion press :
- Electrical motors convert electrical energy into mechanical energy to drive the pomps.
- The pomps then convert this energy into potential energy of high-pressure oil.
- This energy available to do work by extruding the profile.
High-pressure oil discharged over a relief valve does no usuful work. Instead that energy is released in the form of heat.
Operation of a press without overheating oil
Proper operation of the press will reduce the heating of the oil :
- reduce volume of the oil to point corresponding to the flow of the metal through the die
- as the pressure drops, the operator can increase the pump volume until the extrusion speed meets the required velocity.
Good operation of a press can be obtained if :
- For each extruson will be programmed optimal billet temperature and billet length.
- This temperature and length must ensure that each breakthrough to be made without the relief valve actuation and without overheating of the oil.
Peak pressure and relief valve
Under no circumstances should a press be held at peak pressure with relief valve blowing for more than thirty seconds at the most. It is believed that it is much better to cancel the extrusion cycle, reject the billet and stsrt over than to overheat the pomp and oil .
One simple solution
Another authority on aluminum extrusion says about the simple solution of the problem :
- The electronic servo control of modern press system may automatically reduce the pump output as the pressure required for extrusion closely approaches the set discharge point of the relief valves.
Oil pressure and extrusion force
The external force given by the extrusion press determines the press capacity. For successful extrusion, the force applied by the press need be more then the force required for extrusion.
The main factor for aluminium extrusion process
The force required for extrusion depends on the following main factors  (Figure 3):
- the extrusion ratio
- the length of extrusion billet
- the friction condition at the billet container interface
- the friction condition at the die material interface
- the flow stress of the billet material
- the initial billet temperature
- the speed of extrusion.
- Extrusion //M. Bauser, G. Sauer, K. Siegert – 2009
- The Extrusion Press Maintenance Manual // Al Kennedy
- Extrusion of Aluminium Alloys /T. Sheppard – 1999
- Aluminum extrusion technology /P. Saha
- Proper Handling Of Hydraulic Oil In The Extrusion Process / R.L. Ferryman // Aluminum ET Workshop, 1992
- Pressure and Strain Measurement During Hot Extrusion of Aluminium /Per Thomas Moe – PhD Thesis – Norwegian University of Science and Technology – 2005