Hydraulic clearance. Flow in narrow clearances are of vital importance in

hydraulic system Hydraulics () is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counterpart of pneumatics, which concer ...

component design. The flow in a narrow circular clearance of a spool valve can be calculated according to the formula below if the height is negligible compared to the width of the clearance, such as most of the clearances in

hydraulic pump A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy ( hydrostatic energy i.e. flow, pressure). Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. They gen ...

hydraulic motor A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder as a linear actuator. Most broadly, ...

s, and

spool valve Spool may refer to: *Bobbin, a cylinder or reel on which a quantity of thread, yarn or wire is wound for use in a particular machine or device *Cable reel, used to carry various types of electrical wires * Spool (record label), active 1998–2008 ...

s. Flow is considered to be laminar. The formula below is valid for a spool valve when the spool is steady. Concentric spool/valve housing position i.e. the height/radial clearance c is the same all around: Units as per SI conventions : Flow Q_i = (∆P · π · d · c³) ÷ (12 · ν · ρ · L) where:
Q =

volumetric flow rate In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes \do ...

(m^3/sec)
ΔP = P1-P2 = pressure drop over the clearance (N/m^2, Pa)
d = valve spool diameter (metre)
c = clearance height (radial clearance) (metre)
ν = kinematic viscosity for the oil (m^2/sec)
ρ = density for the oil (kg/m^3)
L = clearance length (metre) As can be seen from the formula, the clearance height c has much more influence on the leakage than the length. The formula clearly hints of pure

laminar flow Laminar flow () is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral m ...

conditions. It is also valid for gases. Contact between the spool and the wall, the value that is generally used for practical calculations: Flow Q_e = 2.5 · Qi

Hydraulic Clearance in Hydraulic Components

Pistons A piston is an engineering component of engines and pumps. Piston(s) may also refer to: Science and technology * Misnomer for a hydraulic cylinder * Piston (optics) * Piston (subcellular structure) * Piston valve * Fire piston, an ancient device ...

: The clearance between the piston and

cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

wall is crucial for preventing leakage and maintaining hydraulic efficiency. A tight clearance minimizes fluid loss, while a clearance that is too small can lead to increased

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

and wear. The piston's design and the material used influence the optimal clearance. Hydraulic spool valves: These valves rely on precise clearances to control the flow of

hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backho ...

. The clearance between the spool and valve body affects the valve's responsiveness, leakage rate, and overall performance. Different types of spool valves, such as two-way, three-way, and four-way valves, have varying clearance requirements. Hydraulic seals: Seals are essential for preventing fluid leakage in hydraulic systems. The clearance between the seal and the mating surface is critical for ensuring effective sealing. Different seal materials and designs have different clearance tolerances. Proper clearance is necessary to avoid excessive friction, wear, and seal failure.

Hydraulic cylinders A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment (engineering vehicles), ...

: The clearances within a hydraulic cylinder, such as between the piston and cylinder wall, and between the rod and gland, affect the cylinder's efficiency, service life, and leakage rate. Accurate clearances are necessary for smooth operation and to prevent damage to the cylinder components. Understanding and controlling hydraulic clearance is essential for optimizing the performance, efficiency, and longevity of hydraulic systems.

References

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