Hydraulic problem
Background
You are the factory engineer working in a highly automated modern earthmoving equipment factory that has been in operation for some years. You are in charge of the equipment described below. The circuit shows a hydraulic system. The hydraulic cylinder (item 10) is one of several fully automatic clamping stations that hold large pieces of steel together while they are welded together.
The cylinder operates and holds a nominal pressure of 200 bar for approximately 15 minutes, then is off for approximately 2 minutes while the welded component is removed and re charged with un-welded components. The relief valve (item 5) is set to 230 bars. Materials for the production process are rigorously checked and are very consistent.
When pressure is required for the welding process, the Programmable Logic controller (PLC) energises the 2 position, 2 port spring return solenoid valve (item 6).
When the correct clamping pressure has been achieved, the pressure switch detects system pressure as being correct and the resulting signal is used by the PLC to signal valve 6 via its solenoid to unload the pump. An accumulator is fitted to accommodate any equipment internal leakage.
During the welding process; if the pressure switch (item 11) sees a drop in pressure bellow the pressure switch differential setting; valve 6 is re-energised, until correct clamping pressure is regained.
The machine is capable of operating continuously as production requirements demand. The Hydraulic power-pack is outside the main machine guard, can be adjusted and visually inspected safely while the machine operates and is away from the welding process. In general the plant is so well automated personnel may manage a number of different machines at any time.
A fixed speed AC motor is used to power the system.
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Item
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Description
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1
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Strainer (In tank Suction)
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2
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Fixed displacement gear pump approx. 5 It/ min
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3
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Fixed speed Motor (AC)
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4
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Pressure filter with bypass (opens at 2bar)
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5
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2 stage Pressure Relief Valve cracks at 230 bar
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6
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2 Position Directional Control valve
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7
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Non Return Valve
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8
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Accumulator:- Nitrogen filled bag type
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9
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3 Position Directional Control Valve
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10
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Hydraulic Cylinder
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11
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Pressure switch activated 200 bar, differential 10 bar
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The problem
When you arrive in the morning the oil reservoir is so hot you can not hold your hand on it, this is not the normal state. The tank is usually at ambient temperature.
The oil level appears to be correct and the oil looks clean and there appears outwardly to be no problems.
The system seems to be losing its capability to achieve its welding pressure.
What are the possible causes of the problems; this may include technical and human issues.
Suggest any improvements to the circuit that might protect against this occurrence in the future?, You are not expected to suggest major hydraulic changes as the circuit is a standard circuit that works well.
As an aside what maintenance strategy could be applicable to this piece of equipment and why? A list of detailed maintenance activities would be inappropriate and could lose marks.
Full explanations are required as to how you arrive at your conclusion, with any theory being applied to the problem at hand. You must tell me how the issues/faults you identify have resulted in the symptoms being experienced. Please be brief, succinct and to the point. I see this as a technical report.
Use your experiences gained in labs, seminars as well as reading will be required to inform your answer.
All Theory and discussions must be directed and related to this problem. Lists of general/generic ideas, issues, theories etc. are not acceptable and will lose marks; all discussions must be related to the problem at hand.
You must use the correct terminology, such as relieving, unloading etc. and when referring to equipment in the case study, ensure the correct name are identification numbers are always used.