55 - 7947 Equipment Engineering and Design
Hydraulic Problem (7947 PBL)
Background
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 issue you are asked to deal with has occurred in real life. The unloading circuit shows a hydraulic system. When the hydraulic cylinder (item 10) operates it 'Snaps a lock on and off as quickly as possible. The lock is on a large automatic, machine; its purpose is to hold 2 dies together while a product within the dies cures.
The cylinder operates and holds a nominal pressure of 100 bar for approximately 2 minutes, then is off for approximately 2 minutes while the dies are recharged. If raw material stiffness increases, greater pressure can be applied as the relief valve is set to 200 bar, but this would be an abnormal situation.
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 bellow).
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 energise 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 de–energised.
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.
Item Description
1 Suction Strainer
2 Fixed Displacement Hydraulic Gear Pump
3 Fixed speed AC Motor
4 In line Pressure filter with Bypass
5 2 stage pressure relief valve
6 2 Position Directional Control valve
7 Spring assisted NRV
8 Accumulator
9 3 Position Directional Control v
10 Hydraulic cylinder
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 system is having difficulty generating sufficient curing pressure.
Raw material characteristics have not changed.
What are the possible causes of the problems; this may include technical and human issues. Remember, you are being asked to discuss the system fault and that the rise in temperature is a symptom.
What modifications might you make to protect against this occurrence in future?, (You are not expected to suggest hydraulic changes as the circuit generally works well.
What maintenance strategy could be applicable to this piece of equipment?
Full explanations are required as to how you arrive at your conclusion, with any theory being applied to the problem at hand. A understanding of the circuits logic/operation is essential; Do not describe the oil path, this does not imply an understanding of the circuits logic.
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 design issues, management techniques etc are not acceptable and will lose marks; all discussions must be related to the problem at hand.

Assessment criteria –PBL reports, module 7947 equipment engineering & design
See Marking Criteria sheet on Black Board.