hydraulic-circuit-design

name	hydraulic-circuit-design
description	Hydraulic circuit design — pressure/flow/power analysis, actuator sizing, pump selection, valve types, accumulator design, ISO circuit symbols, safety, efficiency.
metadata	{"priority":7,"promptSignals":{"phrases":["hydraulic circuit","hydraulic system design","hydraulic pump","hydraulic actuator","hydraulic pressure","hydraulic flow"],"minScore":3}}

Hydraulic Circuit Design — Complete Skill

Fundamental Equations

Flow rate: Q = A × v [L/min or m³/s] A = piston area [m²]; v = velocity [m/s] 1 L/min = 1.667 × 10⁻⁵ m³/s

Pressure: P = F / A [Pa = N/m²; bar = 100,000 Pa] Power: P_hyd = Q × ΔP [W; Q in m³/s, ΔP in Pa] P_hyd [kW] = Q [L/min] × ΔP [bar] / 600

Continuity: Q₁A₁ = Q₂A₂ (incompressible flow)

Hydraulic Cylinder Sizing

Extension force: F_ext = P × A_piston = P × π D²/4

Retraction force (rod side): F_ret = P × A_annulus = P × π(D² - d²)/4 (d = rod diameter)

Speed: v_ext = Q / A_piston; v_ret = Q / A_annulus

Cylinder buckling (long stroke): F_cr = π²EI / (L_eff)² (check rod as column for long strokes) Rod diameter: d_rod ≥ 0.2–0.35 × D_bore (for stroke/bore ratio check)

Hydraulic Motor

Torque: T = ΔP × D_m / (2π) [N·m; D_m = displacement per rev in m³/rev] Speed: n = Q / D_m [rev/s] Overall efficiency: η_total = η_vol × η_mech (typically 0.85–0.92)

Pump Selection

Required displacement: D_p = Q / n_pump [m³/rev or mL/rev] Drive power: P_shaft = Q × P_sys / η_pump [W]

Fixed displacement: simple, low cost; flow proportional to speed Variable displacement (axial piston): pressure-compensated; load sensing; energy efficient

Pump types:

Type	Max pressure	Max speed	Notes
Gear pump	250 bar	3000 rpm	Low cost, moderate cleanliness
Vane pump	170 bar	3000 rpm	Low noise
Axial piston	420 bar	3000+ rpm	Highest efficiency, load sensing
Radial piston	700+ bar	1500 rpm	High pressure specialty

Directional Control Valves

4/3 valve: 4 ports, 3 positions (standard D03, D05, D07, SAE mounting) 3-position center conditions:

• Closed center: all ports blocked at rest → load holds; pump unloaded separately
• Open center: pump bypasses to tank; simple but wastes energy
• Tandem center: pump unloads to tank; A&B to tank; load drops

Solenoid-operated: direct or pilot-operated (for high flows) Response time: 20–50 ms (direct); 50–150 ms (pilot)

Pressure Relief Valve (PRV)

Set to maximum system pressure; direct-acting or pilot-operated Cracking pressure: onset of opening Full-flow pressure: typically 10–15% above cracking PRV must handle full pump flow at maximum pressure for safety

Pressure Drop in Lines

Laminar flow (Re < 2300): ΔP = 128 μLQ / (πd⁴) Turbulent: ΔP = f_D × (L/d) × ρv²/2 (Darcy-Weisbach) f_D = 0.316 / Re^0.25 (Blasius, turbulent)

Recommended velocity: pressure lines 3–5 m/s; return lines 1–3 m/s; suction lines 0.5–1.5 m/s

Accumulator Design

Hydraulic accumulator stores energy (nitrogen gas pre-charged)

Sizing for cycle peak flow: V_gas = (P₁/P₂)^(1/n) × V_acc (n = 1.4 adiabatic for fast cycle; n=1 isothermal for slow) Effective fluid volume ΔV = V_acc [1/(P_2/P_0)^(1/n) - 1/(P_1/P_0)^(1/n)]

P₀ = pre-charge pressure (80% of minimum system pressure) P₁ = minimum working pressure; P₂ = maximum pressure

Types: bladder, piston, diaphragm Safety: ISO 4413, pressure vessel codes (ASME VIII or PED)

Fluid Selection

Mineral oil: ISO VG 32–100 (32=cold, 68=standard, 100=high temp) Fire-resistant: phosphate ester (HFD), water glycol (HFC), water-oil emulsion Temperature range: mineral oil –25°C to +80°C continuous; 100°C peak

Filtration: Actuate at β₁₀ ≥ 75 ratio; target ISO 16/14/11 for servo valves Filter location: pressure line (10 μm), return line (25 μm), offline (10 μm)

Circuit Efficiency

Open-circuit efficiency: pump power / mechanical power Energy losses: flow over PRV (throttle loss), heat in lines, cylinder cushion Load sensing (LS) circuit: pump pressure = load pressure + ΔP_margin (15–25 bar) → minimal losses

Safety Requirements (ISO 4413)

Maximum pressure must be limited by PRV set below system component ratings Emergency stop: must apply brakes / lower loads safely Circuit must fail-safe: cylinders clamp (not extend) on power loss where appropriate

Output

Provide: cylinder bore × rod diameter [mm], system pressure [bar], flow rate Q [L/min], pump displacement [mL/rev], pump power [kW], PRV setting [bar], line sizes [mm], accumulator volume [L] and pre-charge pressure [bar].