55 - 7947 Equipment Engineering and Design
Hydraulic Problem (7947 PBL)
You are the factory engineer working in a highly automated modern factory that has been in operation for some years. You are in charge of the equipment described bellow. The unloading circuit shows a hydraulic system. The hydraulic cylinder (item 10) is part of on a large fully automatic, machine; its purpose is to hold 2 dies together while a product within the dies cures at ambient temperature.
The cylinder operates and holds a nominal pressure of 100 bar for approximately 5 minutes, then is off for approximately 2 minutes while the dies are recharged. The relief valve is set to 130 bar. Materials for the production process are rigorously checked and are very consistent.
When pressure is required for the curing process, the Programmable Process controller (PLC) energises the 2 position, 2 port spring return solenoid valve (item 6 in diagram below).
When the correct curing pressure has been achieved, the pressure switch detects system pressure as being correct and the resulting signal is used by the PLC to deenergise the solenoid on valve 6 (in diagram bellow) to unload the pump. An accumulator is fitted to accommodate any equipment internal leakage.
During the curing process; If the pressure switch sees a drop in pressure bellow the pressure switch differential setting; valve 6 is re–energised, until correct curing pressure is regained.
The machine is capable of operating continuously as production requirements demand. The Hydraulic power-pack is outside the main machine guard and can be adjusted and visually inspected safely while the machine operates.
BS 2917-1:1993 ISO 1219-1 May prove useful.
Remember circuits are always drawn in an 'at rest position'.
A fixed speed AC motor is used to power the system.
1 Strainer (Suction)
2 Fixed displacement gear pump approx. 5 lt/ min
3 Fixed speed Motor (AC)
4 Pressure filter with bypass (opens at 2bar)
5 2 stage pressure relief valve
6 2 Position Directional Control valve
7 Non Return Valve
8 Accumulator:- Nitrogen filled bag type
9 3 Position Directional Control Valve
10 Hydraulic Cylinder
11 Pressure switch activated 100 bar, differential 5 bar
When you arrive in the morning the oil reservoir is so hot you cannot hold your hand on it, this is not the normal state. The tank is usually at ambient temperature. The oil tank appears to be full and the oil looks clean and there appears outwardly to be no problems.
The system seems to be losing its capability to hold its curing pressure.
What are the possible causes of the problems; this may include technical and human issues.
What modifications might you make to protect against this occurrence in future? You are not expected to suggest major hydraulic changes as the circuit generally works well.
What maintenance strategy could be applicable to this piece of equipment? 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 you identify have resulted in the symptoms being experienced.
Use your experiences gained in labs, seminars as well as reading to form your answer.
All Theory and discussions must be directed and related to this problem. Lists of general/generic ideas, issues, management techniques 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 used.
Assessment criteria –PBL reports, module 7947 equipment engineering & design
See Marking Criteria sheet on Black Board.