Hydraulic clearance

Leakage in narrow clearance, spool valve

Hydraulic clearance. Flow in narrow clearances are of vital importance in hydraulic system 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 pumps, hydraulic motors, and spool valves. Flow is considered to be laminar. The formula below is valid for a spool valve when the spool is steady.[1]

Concentric spool/valve housing position i.e. the height/radial clearance c is the same all around: Units as per SI conventions : Flow Qi = (∆P · π · d · c3) ÷ (12 · ν · ρ · L) where:
Q = volumetric flow rate (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 conditions. It is also valid for gases.

Contact between the spool and the wall, the value that is generally used for practical calculations:

Flow Qe = 2.5 · Qi

## Hydraulic Clearance in Hydraulic Components

Pistons:

The clearance between the piston and cylinder 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 and wear. The piston's design and the material used influence the optimal clearance.[2]

Hydraulic spool valves:

These valves rely on precise clearances to control the flow of hydraulic fluid. 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:

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

1. ↑ Majumdar, S. R. (2006). Oil Hydraulic Systems: Principles and Maintenance (1st ed.). New Delhi: Tata McGraw-Hill. p. 164. ISBN 0-07-463748-7.
2. ↑ "Piston là gì? Cấu tạo và nguyên lý hoạt động - Naltako" (in vi). https://naltako.vn/piston-la-gi/.

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