How to properly size hydraulic hoses and fittings?

Hydraulic hose size refers to a set of standardized parameters used to describe the geometric characteristics and operational capacity of a hydraulic hose. Hose size is typically represented by two main dimensions: the inner diameter (ID), which is the actual opening through which the fluid flows, and the outer diameter (OD). Selecting the correct hose size is crucial, as it not only affects fluid flow but also impacts pressure ratings and the overall efficiency of the system.

How Hose Size Affects Fluid Flow and System Efficiency

Hose size directly affects the flow rate and velocity of fluid in a hydraulic system: an undersized inner diameter increases flow resistance, leading to pressure loss and energy waste, which reduces system efficiency, while an oversized inner diameter can result in higher costs and limited installation space. Proper selection of hose size ensures smooth hydraulic fluid flow, maintains stable system pressure, and thereby improves the overall operational efficiency and reliability of the hydraulic system.

Common Terms in Hydraulic Hose Size Measurement

Inner Diameter (ID): The internal dimension through which fluid flows in the hose. It is one of the most critical factors in determining the hose’s fluid flow capacity.

Outer Diameter (OD): The external dimension of the hose, important for fitting compatibility and understanding how the hose will be installed within the system layout.

Rated Pressure: The maximum pressure that the hose can safely withstand. The rated pressure of a hydraulic hose varies, typically depending on the hose's size and material.

Reinforcement Layer: Material layers inside the hose (such as stainless steel or textile) that provide strength and support to withstand internal pressure and external forces.

Size Hydraulic Hoses and Fittings

To properly measure and select hydraulic hoses and fittings, you need to consider the hose’s geometric dimensions, pressure rating, and flow requirements. The specific steps are as follows:

Determine Flow and Velocity Requirements

Calculate the required hose inner diameter (ID) based on the hydraulic system's flow requirements.

Recommended hydraulic oil flow speeds:

• Working lines: 2–6 m/s

• Return lines: 1–3 m/s

• Too high a flow speed → increased pressure loss and heat generation

• Too low a flow speed → slow system response

Reference formula:Q=V×A

• Q = flow rate (m³/s)

• V = flow velocity (m/s)

• A = cross-sectional area of the inner diameter (m²)

Measure Hose Inner Diameter (ID)

Measure the actual inner diameter of the hose using a caliper, either at the bare hose end or an existing hose.
The inner diameter determines flow rate and velocity.

Measure Hose Outer Diameter (OD)

The outer diameter affects fitting compatibility and system layout.
Use a caliper to measure the outermost diameter of the hose.

Determine Hose Length

Measure the actual required length according to the equipment layout.
The hose should have some allowance to accommodate bends and movement, but should not be excessively long.

Determine Pressure Rating

Check the system's maximum working pressure and select a hose with working pressure ≥ system maximum pressure, typically with a 25–50% safety margin.
Note that the pressure rating depends on the hose material and the number of reinforcement layers.

Select Fitting Specifications

Ensure the hose and fittings are compatible:

• Inner diameter matches the fitting barb diameter

• Thread type matches (BSP, NPT, SAE, Metric)

• Crimping method (crimp, ferrule, etc.) meets standards

Verification

Check whether the selected hose and fittings meet:

• Flow requirements

• Pressure requirements

• Installation space requirements

• System safety requirements