Exercises: Pneumatic and hydraulic circuits

Explain the function of each basic block of a pneumatic circuit: compressor, air service unit (filter, regulator, and lubricator), directional control valve, and cylinder. Which one supplies energy and which one transforms it into motion?

Describe the difference between a single-acting cylinder and a double-acting cylinder: how many connections each has, how the piston rod returns, and a typical application of each type.

The circuit diagram in the figure shows a pneumatic circuit driving a double-acting cylinder, but one of its components is covered. State which component is missing, what function it performs in the circuit, and what would happen to the cylinder if that component were not there.

Draw or describe the standard symbol for a single-acting pneumatic cylinder with spring return: how many air connections it has, what the spring does, and an application where spring return is sufficient.

You can do it on paper and send a photo with your answer.

Explain what a $3/2$ valve is (three-way, two-position): what three connections it has (supply, actuator outlet, exhaust) and why it is the typical valve for single-acting cylinders.

A flow regulator installed at the cylinder outlet controls its speed. Explain how it achieves this (variable flow restriction) and why exhaust flow regulation is usually preferred over inlet flow regulation (more stable motion against load variations).

The air service unit combines a filter, pressure regulator, and lubricator. Explain the function of each and why they must be installed in that order. What happens to a cylinder if the filter is saturated with water?

Pneumatic valve exhausts are fitted with silencers. Explain what problem they solve (noise from expanding air, entry of dirt through the exhaust) and what happens to cylinder speed if the silencer becomes blocked.

A push-button-actuated $5/2$ directional control valve controls the double-acting cylinder of an automatic door. Explain what the numbers "5" and "2" mean (ports and positions) and describe the path of the air when the push-button is pressed and when it is released.

In a hydraulic circuit, the pump delivers oil at $6\ \text{MPa}$ to a cylinder whose piston has a surface area of $80\ \text{cm}^2$. a) Calculate the theoretical maximum force the cylinder can exert ($F = p \cdot S$; mind the units: $1\ \text{MPa} = 100\ \text{N/cm}^2$). b) Explain which circuit component limits the pressure if the load jams (pressure relief valve) and what happens to the oil in that case.

A pneumatic workshop mounts three cylinders powered by a single compressor. Explain what happens to the pressure and speed of the cylinders when all three operate simultaneously, what role the receiver tank plays, and how you would size the compressor.

A double-acting cylinder with a piston of $40\ \text{cm}^2$ receives a flow rate of $24\ \text{L/min}$. a) Calculate the extension speed of the piston rod ($v = \frac{Q}{S}$) in $\text{m/s}$. b) Explain why the retraction speed is higher for the same flow rate (the piston rod reduces the cross-sectional area in the rod-side chamber). c) What minimum pressure is needed to overcome a load of $1\,000\ \text{N}$?

The diagram shows the symbol for a $5/2$ directional control valve with its five ports numbered. Identify each port (supply, two cylinder outlets, and two exhausts) and explain the path of the air in each of the spool's two positions.

In an automatic bus door, the cylinder must open slowly and close quickly. Explain how different speeds in each direction are achieved using unidirectional flow regulators (throttle valves with non-return valves) and where they are placed in the circuit.

A shuttle valve (OR logic function) allows a cylinder to be controlled from two different locations. Explain how it works (it lets the signal through from either push-button) and draw the circuit for a single-acting cylinder actuated from two push-buttons at opposite ends of a machine.

You can do it on paper and send a photo with your answer.

A workshop uses a cylinder with a diameter of $80\ \text{mm}$ performing $10$ cycles per minute with a stroke of $200\ \text{mm}$ at $6\ \text{bar}$. a) Estimate the volume of compressed air consumed per minute (volume of both chambers per cycle, ignoring the piston rod). b) Convert to free air drawn in by the compressor (multiply by the pressure ratio, about $7$ times). c) Comment on the energy cost of compressed air compared to other energy sources.

In a pneumatic marking press, a simple two-hand control was installed by connecting two $3/2$ pushbuttons in series to the pilot port of the cylinder's directional control valve. However, during an occupational safety inspection, the machine is sealed off because an operator has permanently blocked one of the pushbuttons with a cable tie, operating the press with only one hand.

1. a) Technically demonstrate why neither the series connection nor the isolated use of a two-pressure valve ("AND") guarantees true two-hand safety against this improper tampering.
2. b) Design the complete pneumatic circuit (double-acting cylinder, directional control valves, and control elements) that implements a safe two-hand control with timing, so that the cylinder only extends if both pushbuttons are actuated with a time delay of $\Delta t \le 0,5\text{ s}$, preventing the cycle if one of them is left permanently actuated. Justify its operating principle.

The hydraulic circuit of a public works machine feeds two cylinders in parallel with a single constant-displacement pump of $60\text{ l/min}$ and a relief valve set to $200\text{ bar}$. Cylinder 1 (boom lifting) supports a load that requires $140\text{ bar}$, while cylinder 2 (bucket tilting) only requires $50\text{ bar}$.

1. a) If the operator simultaneously and completely opens both spools of the directional control valve, reasonably predict the dynamic behavior of both actuators (which one will move first and what will happen to the pressure in the common line).
2. b) Justify why cylinder 1 remains completely immobilized while cylinder 2 is in motion.
3. c) Propose and explain the hydraulic design principle (such as pressure-compensated directional control valves or Load Sensing systems) that allows the simultaneous and independent movement of actuators subjected to disparate loads in real machinery.

An industrial hydraulic power unit has a constant-displacement pump of $45\text{ l/min}$ and a pressure relief valve set to $180\text{ bar}$. In its duty cycle, an actuator performs a static pressing phase lasting $40\text{ s}$, during which it remains at a mechanical stop demanding only a residual leakage flow rate of $2\text{ l/min}$ at $160\text{ bar}$.

1. a) Calculate the hydraulic power that is entirely dissipated as heat through the relief valve during each pressing phase.
2. b) Reasonably explain the chain effect that this overheating causes on the kinematic viscosity of the oil, internal leakage, and the volumetric efficiency of the pump.
3. c) Design and compare two technical alternatives to solve this energy problem: one based on a high/low pressure circuit with an accumulator and tank unloading ($p \approx 0\text{ bar}$), and another using a variable displacement pump with a pressure compensator. Evaluate the feasibility and efficiency of each.

A pneumatic stamping machine must press cast parts of irregular thickness. For this reason, it is not feasible to use a mechanical position limit switch to command the retraction, since if the part is thicker the press would not complete the stroke, and if it is thinner it would not reach the required forming force.

Design the complete pneumatic schematic to meet the following functional requirements:

1) The double-acting cylinder advances upon pressing a start button.

2) Retraction is not commanded by geometric dimension, but when the rear chamber of the cylinder reaches a forming pressure of exactly $5\text{ bar}$ (detected by means of a sequence valve), additionally guaranteeing a pressure dwell time of $3\text{ s}$ using a pneumatic timer.

3) Once the time has elapsed, the return stroke must be at a regulated speed independent of the advance stroke.

Justify the selection and connection of each component using standard port and pilot designations.

To automate a machining station, the pneumatic sequence $A+ B+ B- A-$ is required.

1. a) Analyze the sequence step by step and identify the exact moment when a blockage occurs due to opposing or permanent signals (pilot interference on the same directional control valve) if you attempt to connect the circuit using a direct intuitive approach.
2. b) Apply the cascade method: break down the sequence into the minimum number of pressure groups, define the limit switches that trigger group changes versus those acting within each group, and draw the corresponding pneumatic schematic.
3. c) Justify how the pressure line reversing valve physically clears the residual pilot signal that was preventing the cylinder reversal.

A double-acting pneumatic cylinder driving an industrial shear was operating properly at high speed, but it has suddenly started to move extremely slowly in both the advance and retraction strokes, while maintaining the same resistive load.

The technical team proposes three independent hypotheses:

• Hypothesis 1: Massive internal leakage due to deterioration of the piston seals.

• Hypothesis 2: Exhaust silencer of the $5/2$ directional control valve clogged with dirt.

• Hypothesis 3: Severe clogging in the filter element of the inlet FRL unit.

Design a systematic differential diagnostic protocol: explain which experimental tests or pressure measurements you would perform to unambiguously confirm or rule out each hypothesis without removing the cylinder from its housing, justifying the physical result you would expect to observe in each test.

An industrial plant has a $55\text{ kW}$ screw compressor that operates $4500\text{ h}$ per year at a mains pressure of $8\text{ bar}$. The average cost of electrical energy is $0,15\text{ €/kWh}$. An energy audit reveals two facts: $20\,\%$ of the produced flow rate is lost in leaks and the working pressure can be reduced to $6,5\text{ bar}$ (knowing that each bar of overpressure increases compressor consumption by approximately $7\,\%$).

1. a) Calculate the total annual economic savings that would be obtained by completely eliminating the leaks and reducing the pressure to $6,5\text{ bar}$.
2. b) Operating on that line is a cylinder that must lift a net load of $1100\text{ N}$ with a piston of $50\text{ mm}$ in diameter. Considering a mechanical efficiency of the cylinder of $85\,\%$ and accumulated head losses in pipes and valves of $0,7\text{ bar}$, calculate the minimum gauge pressure that must be in the network and justify whether the cylinder will operate reliably after the proposed reduction.

A solvent dosing chemical plant classified as ATEX Zone 1 (explosive atmosphere with the presence of flammable vapors) needs to automate a sequential transfer cycle of two cylinders.

Two technical proposals are presented:

• Proposal A: Conventional electropneumatic logic (standard PLC, magnetic reed switch sensors, and $24\text{ V}$ solenoid valves).

• Proposal B: Pure pneumatic logic (pneumatic limit switch sensors, timers, and processing using logic valves).

1. a) Analyze the regulatory feasibility and the technical extra costs of intrinsic safety (Zener barriers, flameproof Ex d enclosures, etc.) that the electropneumatic proposal would require to be legally compliant in Zone 1.
2. b) Evaluate the behavior of both solutions in an environment with high condensation and corrosive vapors in terms of failure rate and preventive maintenance cost.
3. c) If the company requires integrating the cycle into the plant's overall SCADA system in the short term, propose and defend a technical compromise solution that combines ATEX safety and digital connectivity at the lowest cost.

Topic Challenge. How would you safely lift an electric commercial vehicle? Design and size the hydraulic system for a workshop car lift.

1. Data collection: Check the official IDAE vehicle database (or the manufacturer's technical specifications sheet) and write down the maximum mass ($MMA$) of an electric van in $\text{kg}$. Assume a nominal working pressure of $p = 160\,\text{bar}$ ($16\,\text{MPa}$).

2. Physical calculations: Use LibreOffice Calc to determine, using Pascal's principle ($p = F/S$), the minimum inside diameter of the cylinder required to overcome the load ($g = 9,8\,\text{m/s}^2$). Also calculate, using the continuity equation, the volumetric flow rate ($Q$) in $\text{L/min}$ required to raise the vehicle $1,8\,\text{m}$ in $20\,\text{s}$.

3. Circuit design: Prepare a brief report including the standard functional diagram (ISO 1219 symbology) using LibreOffice Draw: integrate the power unit, pressure relief valve, directional control valve, and pilot-operated check valve to prevent unexpected drops.

Evaluation criteria:

• Mathematical correctness in applying Pascal's principle and continuity, with rigor in SI units.

• Technical precision and standardization of the hydraulic circuit symbols.

• Reasoned justification of the selected safety and control elements.