Structural, Pneumatic and Hydraulic Systems
Forces on Structures
Every structure must support and transmit loads — the forces acting on it. Engineers classify loads into two main categories. A dead load is the permanent weight of the structure itself (the steel frame of a bridge, the concrete deck). A live load (also called an imposed load) changes over time — the vehicles crossing a bridge, the people standing on a floor, the goods stacked in a warehouse.
A dynamic load is applied suddenly or changes rapidly — a machine vibrating at high speed, a vehicle braking sharply, an impact during a crash. Dynamic loads are often more damaging than static loads of the same magnitude because the rapid change creates additional stress within the material.
Dead load = permanent weight of the structure (e.g. steel beams, concrete slab). Live load = variable load from use (e.g. vehicles, people, stored goods). Dynamic load = suddenly applied or rapidly changing force (e.g. impact, machine vibration). Cables in a suspension bridge carry tension — NOT compression. This is a common exam error.
Types of Internal Force
When a load is applied to a structural member, it creates internal forces. You need to identify five types:
- Tension — a pulling force that stretches a member. Cables, suspension bridge hangers and tent guy-ropes are in tension. Cables can only carry tension; they buckle and go slack under compression.
- Compression — a pushing force that squeezes a member. Stone arch bridges, concrete columns and walls are in compression. Brittle materials (concrete, stone, brick) are strong in compression but weak in tension.
- Bending — a force that causes a member to curve. A beam supported at both ends and loaded in the middle bends, putting the top surface in compression and the bottom surface in tension.
- Torsion — a twisting force along a shaft’s axis. A car driveshaft experiences torsion as it transmits engine torque to the wheels.
- Buckling — sudden sideways collapse of a long, slender compression member (column or strut) under compressive load. A thin aluminium column buckles before the material crushes in compression.
A space frame is a three-dimensional framework of interconnected bars arranged in triangles. The triangle is the only shape that is inherently rigid without requiring rigid joints — any force applied to a triangle is resolved into tension or compression along the members. Electricity pylons, roof trusses and racing car roll cages are all space frames.
A monocoque structure carries loads through its outer skin rather than an internal frame. Monocoque is French for “single shell.” Aircraft fuselages and Formula 1 carbon fibre chassis are monocoque — the shell itself provides rigidity and strength without a separate skeleton inside. This produces very low weight for a given stiffness.
Pascal’s Law and Pressure Systems
Pneumatic and hydraulic systems both use fluid (gas or liquid) under pressure to transmit force. The underlying principle is Pascal’s law: pressure applied at one point in a sealed fluid is transmitted equally in all directions throughout the fluid. This means a force applied at a small piston can produce a much larger force at a larger piston — force amplification.
The pressure equation is: $$P = \frac{F}{A}$$ where \(P\) is pressure in pascals (Pa), \(F\) is force in newtons (N) and \(A\) is area in square metres (m2). Because pressure is equal throughout a sealed fluid, we can write: $$\frac{F_1}{A_1} = \frac{F_2}{A_2}$$ where subscript 1 is the input piston and subscript 2 is the output piston. Rearranging: $$F_2 = F_1 \times \frac{A_2}{A_1}$$
Pascal’s law: pressure in an enclosed fluid is transmitted equally in all directions. Force amplification: \(F_2 = F_1 \times \frac{A_2}{A_1}\). A hydraulic system with an output piston ten times larger than the input piston produces ten times the output force. The trade-off: the output piston moves a much shorter distance than the input piston (the fluid volume is conserved).
Pneumatic vs Hydraulic Systems
Both systems use fluids under pressure, but the type of fluid determines the application:
- Pneumatic systems use compressed air. Air is compressible, so pneumatic systems are often used for fast actuation. High forces need larger cylinders or higher pressure. They are clean (no fluid spills) and can suit food and pharmaceutical environments, but compressing air uses energy. Typical applications: robot grippers, packaging machines, pneumatic drills, spray painting robots, door openers on trains.
- Hydraulic systems use hydraulic fluid (oil). Liquids are virtually incompressible, which means the force transmitted is very precise and very high. Hydraulic systems can produce enormous forces in compact cylinders. Applications: JCB excavator arms, car brake systems, aircraft landing gear, forklift truck mast, heavy press machines.
A double-acting cylinder has two air inlets — one on each side of the piston. Compressed air directed to the back drives the piston forward (extend); air directed to the front drives it back (retract). This gives positive force in both directions, unlike a single-acting cylinder that uses a spring to retract.
A 5/2 directional control valve is the standard valve used to control a double-acting cylinder. The “5” means five ports (air inlet, two cylinder ports, two exhaust ports); the “2” means two switching positions (extend or retract). Shifting the valve from one position to the other sends compressed air to one side of the cylinder and vents the other side to exhaust. In an automated system, the 5/2 valve is actuated electrically (a solenoid moves the spool inside the valve) rather than manually.
Worked Calculation: Force Amplification
Daleside Pumps manufactures a hydraulic press for forming sheet steel. The input piston has an area of 0.002 m2. The output piston has an area of 0.04 m2. The operator applies a force of 150 N to the input piston. Calculate the output force.
$$F_2 = F_1 \times \frac{A_2}{A_1} = 150 \times \frac{0.04}{0.002} = 150 \times 20 = 3000\text{ N}$$ The hydraulic press amplifies the input force by a factor of 20, producing 3000 N at the output piston. Note: the output piston moves 20 times less distance than the input piston (volume is conserved).