ISO 18752 Standard: Hydraulic Hose Classes and Grades

5. ISO 18752 Testing Requirements

A hydraulic hose cannot be classified under ISO 18752 solely from its construction, reinforcement type, or calculated pressure capacity. It must satisfy a series of physical and performance tests applicable to its pressure class, grade, type, and nominal size.

These tests evaluate how the hose and hose assembly perform under internal pressure, repeated pressure cycles, temperature extremes, fluid exposure, bending, and environmental conditions.

Dimensional inspection

Before performance testing, the hose is inspected to verify that its dimensions comply with the requirements for its designated type and nominal size.

Important dimensions include:

• Hose inside diameter
• Reinforcement diameter
• Outside diameter
• Concentricity
• Cover thickness
• Minimum bend radius

Dimensional control is particularly important for compact hose types because they are intended to provide smaller outside diameters and tighter routing than equivalent standard types.

The inside diameter also affects fluid velocity and pressure drop. A hose that is too small can create excessive heat, turbulence, noise, and energy loss even if its pressure rating is adequate.

Hydrostatic proof-pressure test

The proof-pressure test verifies that a hose assembly can withstand a pressure above its maximum working pressure without leakage, rupture, fitting movement, or another visible failure.

During the test, the assembly is filled with a suitable liquid, vented to remove trapped air, and gradually pressurized. Pressure is held for the specified period while the assembly is inspected.

A hose assembly must not exhibit:

• Fluid leakage
• Hose rupture
• Fitting separation
• Cracking or blistering
• Unacceptable deformation
• Other evidence of structural failure

Proof pressure is a qualification or production-verification pressure. It must not be interpreted as an allowable continuous operating pressure.

Testing should always be performed using proper guarding and remote pressurization. Stored hydraulic energy can make hose failure extremely dangerous.

Minimum burst-pressure test

The burst test determines whether the hose assembly can withstand the specified minimum burst pressure.

Pressure is increased at a controlled rate until the hose fails or reaches the required test pressure. The actual failure location and mode are recorded.

Possible failure modes include:

• Rupture through the hose wall
• Reinforcement breakage
• Leakage near the fitting
• Hose pullout
• Fitting fracture
• Separation between hose layers

Burst pressure provides a safety margin above maximum working pressure, but it is not a usable system pressure. A hose should never be intentionally operated near its minimum burst-pressure rating.

The burst test is also destructive. A tested hose assembly must not be returned to service.

Pressure impulse test

The impulse test is one of the most important qualification requirements in ISO 18752. It evaluates resistance to fatigue caused by repeated pressure fluctuations.

The hose assembly is installed in a specified test configuration and exposed to:

• Repeated pressure cycles
• A defined pressure waveform
• A prescribed impulse pressure
• A controlled fluid temperature
• A minimum number of cycles

The required cycle count depends on the hose grade:

For most applicable classes and grades, the impulse-test pressure is 133% of maximum working pressure. Grade C hoses in Classes 350, 420, 490, and 560 are tested at 120% of maximum working pressure.

The test specimen must complete the specified number of cycles without failure. A test failure may involve:

• Leakage through the hose body
• Leakage at the fitting
• Hose rupture
• Reinforcement failure
• Fitting separation
• Another loss of pressure integrity

Impulse qualification applies to the tested hose-and-fitting combination. A hose that passes with one fitting design may not achieve the same performance with an unqualified fitting.

Change in length under pressure

Internal pressure can cause a hydraulic hose to lengthen or shorten. ISO 18752 controls this dimensional change because excessive movement can place additional loads on fittings, clamps, adjacent equipment, and hose bends.

Percentage change in length can be calculated as:

ΔL=L1​L2​−L1​​×100

where:

• $\Delta L$ = change in length, expressed as a percentage
• L1​ = reference length before pressurization
• L2​ = length at the specified test pressure

A positive result indicates elongation, while a negative result indicates contraction.

Hose movement under pressure must be considered during routing. An assembly should not be installed tightly between two fixed connections without sufficient allowance for length change and machine motion.

Leakage test

Leakage testing evaluates the integrity of the hose assembly and its end connections at a specified pressure.

The test may reveal:

• Poor fitting attachment
• Damaged inner tubes
• Incorrect crimp dimensions
• Reinforcement damage
• Defective fitting components
• Incompatible hose-and-fitting combinations

External leakage is unacceptable. However, the absence of visible leakage during a short test does not prove that the assembly will survive long-term impulse loading.

Cold-flexibility test

Hydraulic hoses used outdoors or in unheated environments may be bent or moved at low temperatures. Rubber compounds generally become harder and less flexible as temperature decreases.

The cold-flexibility test evaluates whether the hose can be bent under specified low-temperature conditions without unacceptable:

• Cracking
• Cover damage
• Inner-tube damage
• Delamination
• Loss of pressure integrity

A hose may have an adequate pressure rating but still be unsuitable if it becomes excessively stiff at the minimum operating temperature.

Ozone resistance

Ozone can attack rubber compounds and create small surface cracks, particularly when the rubber is stretched or exposed outdoors.

During ozone-resistance testing, a hose specimen is exposed to a controlled ozone concentration under defined conditions. The surface is then inspected for cracking or deterioration.

Ozone resistance is important for hoses installed near:

• Electric motors
• Welding equipment
• High-voltage systems
• Outdoor machinery
• Equipment exposed to sunlight and weather

The outer cover provides the primary environmental protection, but proper storage and installation remain necessary.

Fluid resistance

The hose inner lining must resist the hydraulic fluid without excessive swelling, softening, hardening, cracking, or loss of mechanical properties.

Fluid-resistance testing commonly evaluates changes in properties after controlled exposure. Depending on the test method, evaluation may include:

• Volume change
• Hardness change
• Tensile-strength change
• Elongation change
• Visual deterioration

Even when a fluid belongs to a category covered by ISO 18752, users should confirm compatibility with the hose manufacturer. Fluid additives and formulations can differ significantly.

Abrasion resistance

Abrasion occurs when a hose rubs against another hose, a machine frame, a bracket, the ground, or other equipment. Continued abrasion can wear through the outer cover and expose the reinforcement.

Where applicable, abrasion testing evaluates the resistance of the hose cover to controlled mechanical wear.

A hose that meets the abrasion requirement can still fail if it is poorly routed. Protective sleeves, clamps, guards, and abrasion-resistant covers may be necessary in severe service.

Adhesion between layers

Hydraulic hoses contain several bonded layers. Adequate adhesion is required between:

• Inner tube and reinforcement
• Adjacent reinforcement layers
• Reinforcement and outer cover

Poor adhesion can produce separation, blistering, movement between layers, and premature fatigue failure.

The adhesion test measures the force required to separate specified hose layers. Acceptable adhesion helps the hose behave as one integrated structure under pressure and bending.

Test results and field performance

Passing ISO 18752 tests demonstrates that representative products meet defined laboratory requirements. It does not guarantee unlimited hose life or eliminate the need for correct application practices.

Field performance still depends on:

• Proper hose selection
• Qualified fittings
• Correct assembly procedures
• Appropriate routing
• Clean hydraulic fluid
• Temperature control
• Regular inspection
• Timely replacement

ISO qualification establishes a consistent minimum performance level, while safe service depends on the complete hose assembly and hydraulic system.

---

6. Hose Construction and Performance Requirements

ISO 18752 does not require every hose within a class to use an identical reinforcement design. Manufacturers may use different materials and constructions, provided the completed hose satisfies the applicable dimensional and performance requirements.

A typical hydraulic hose contains three functional sections:

1. Inner tube
2. Reinforcement
3. Outer cover

Each section performs a different role and must remain compatible with the other layers throughout the hose’s service life.

Inner tube

The inner tube forms the fluid-carrying passage. It must resist the hydraulic medium and maintain pressure integrity over the specified temperature range.

Typical inner-tube materials include synthetic rubber compounds formulated for particular hydraulic fluids and temperature conditions.

An effective inner tube must provide:

• Hydraulic-fluid compatibility
• Low fluid permeation
• Resistance to swelling and softening
• Resistance to hardening and cracking
• Adequate flexibility
• Strong adhesion to the reinforcement
• A reasonably smooth flow passage

Chemical incompatibility can cause the inner tube to swell, crack, separate, or partially dissolve. Degraded material may enter the hydraulic fluid and contribute to filter blockage, valve sticking, or component damage.

Fluid compatibility must therefore be evaluated using the specific fluid formulation rather than only a general description such as “hydraulic oil.”

Reinforcement layer

The reinforcement carries most of the force produced by internal pressure. Depending on the pressure class and performance requirements, it may consist of:

• Textile braid
• Steel-wire braid
• Multiple wire-braid layers
• Spiral steel-wire layers
• A combination of reinforcement materials

ISO 18752 permits construction flexibility because the standard focuses on verified performance. The number of reinforcement layers alone does not determine the pressure class or performance grade.

Textile reinforcement

Textile reinforcement may be used for lower-pressure or highly flexible hose designs. It generally provides:

• Low weight
• Good flexibility
• Resistance to flexing fatigue
• Easier handling

Its pressure capability is normally lower than that of steel-wire-reinforced constructions.

Wire-braid reinforcement

A wire-braid hose contains steel wires woven around the inner tube. Braided construction can provide a useful balance among:

• Pressure capacity
• Flexibility
• Bend radius
• Hose weight
• Impulse resistance

One or more wire-braid layers may be used, depending on the required performance.

Spiral-wire reinforcement

Spiral hose construction uses multiple wire layers wound helically in alternating directions. This arrangement is frequently used for high-pressure and severe-impulse applications.

Spiral reinforcement generally provides:

• High pressure capacity
• Strong resistance to pressure expansion
• Good impulse performance
• Stability under severe hydraulic loading

However, spiral-wire hoses may be heavier and more difficult to bend than lower-pressure braided constructions.

Outer cover

The outer cover protects the reinforcement from the surrounding environment. It does not normally carry the main internal-pressure load, but damage to the cover can expose the reinforcement and dramatically shorten hose life.

Cover compounds may be formulated to resist:

• Abrasion
• Ozone
• Weathering
• Hydraulic oil
• Heat
• Ultraviolet exposure
• Flame
• Chemicals
• Mechanical impact

No single cover material is ideal for every application. A hose installed in a steel mill may require enhanced heat and flame resistance, while one used on construction equipment may require greater abrasion and weather resistance.

Standard and compact constructions

ISO 18752 separates hoses into standard and compact types.

Standard types include:

• AS
• BS
• CS

Compact types include:

• AC
• BC
• CC
• DC

Compact hoses generally have smaller outside diameters and tighter minimum bend radii than standard types of comparable pressure and performance.

This can provide important installation advantages:

• Reduced space requirements
• Easier routing through machine structures
• Smaller hose bundles
• Lower weight
• Improved flexibility
• Reduced interference with moving equipment

Compact does not mean that the hose may be bent without restriction. Every hose still has a specified minimum bend radius that must be respected.

Minimum bend radius

The minimum bend radius is the smallest radius at which the hose may be installed without excessive stress or deformation.

Bending a hose more tightly than permitted can cause:

• Inner-tube flattening
• Kinking
• Reduced flow area
• Reinforcement separation
• Wire fatigue
• Increased pressure drop
• Localized heating
• Premature rupture

Bend radius is normally measured to the inside of the hose curve unless otherwise specified by the manufacturer.

A hose should also be kept reasonably straight immediately behind the fitting. Beginning a bend too close to the fitting concentrates stress at the fitting transition and can lead to reinforcement fatigue or hose pullout.

Hose flexibility

Flexibility depends on more than minimum bend radius. It is influenced by:

• Hose diameter
• Reinforcement material
• Number of reinforcement layers
• Wire angle
• Rubber compound
• Operating temperature
• Internal pressure
• Hose construction

A hose may have a relatively small permitted bend radius but still require considerable force to bend. Designers should therefore consider both minimum bend radius and bending stiffness.

Movement should occur through gradual hose flexing rather than twisting. Hydraulic hoses are designed primarily to bend, not to withstand continuous torsional loading.

Pressure-induced movement

A hose can change length or slightly rotate when pressurized because of the behavior of its reinforcement layers.

If both ends are rigidly fixed without sufficient allowance, pressure-induced movement may create:

• Axial loads on fittings
• Excessive tension
• Hose buckling
• Abrasion against nearby surfaces
• Movement of clamps
• Premature fatigue

The assembly should include enough slack to accommodate pressure changes and machine motion, but not so much that the hose can catch, whip, or rub against other components.

Temperature performance

Temperature affects the inner tube, reinforcement adhesion, cover, and hydraulic fluid.

ISO 18752 differentiates hose types by operating-temperature capability for specified oil-based hydraulic fluids:

For specified water-based fluids, the covered range is generally −40°C to +70°C. For water, it is 0°C to +70°C.

These temperature limits refer to the hydraulic medium under the conditions defined by the standard. External ambient temperature and radiant heat must also be considered.

High temperature can shorten hose life by accelerating rubber aging. Low temperature can increase stiffness and reduce flexibility. Systems operating near the upper temperature limit may require shorter inspection or replacement intervals.

Hydraulic-fluid compatibility

The hose must be compatible with both the fluid passing through the inner tube and any fluid that may contact the outer cover.

Compatibility should be confirmed for:

• Petroleum-based hydraulic oils
• Water-glycol fluids
• Oil-in-water emulsions
• Water-in-oil emulsions
• Biodegradable hydraulic fluids
• Synthetic hydraulic fluids
• Fire-resistant fluids
• Cleaning chemicals
• External lubricants and fuels

The inner tube and outer cover may use different compounds. A hose compatible internally with a fluid may not tolerate prolonged external exposure to the same substance.

Vacuum resistance

Some hydraulic hoses may experience negative pressure at pump suction lines or during particular machine conditions. A pressure hose with adequate positive-pressure capacity may collapse under vacuum if its construction is not designed for suction service.

When vacuum is possible, the designer must verify:

• Allowable vacuum rating
• Hose size
• Bend radius
• Operating temperature
• Reinforcement stability
• Need for an internal support helix

ISO 18752 pressure classification should not be interpreted as an automatic vacuum rating.

Electrical conductivity

Hydraulic hoses may be conductive, non-conductive, or designed with controlled electrical resistance. The appropriate construction depends on the application.

Electrical properties may be important near:

• High-voltage equipment
• Electrical utility machinery
• Flammable fluids or gases
• Static-sensitive processes
• Mining equipment
• Paint-spraying systems

A general ISO 18752 pressure classification does not automatically establish suitability for a specific electrical application. The required electrical resistance or conductivity must be separately verified with the manufacturer.

Fitting compatibility

The fitting is an integral part of hose assembly performance. A reliable assembly depends on compatibility among:

• Hose inside diameter
• Reinforcement structure
• Cover thickness
• Fitting stem
• Ferrule design
• Crimp diameter
• Assembly equipment
• Installation procedure

Fittings should not be selected solely because they physically fit inside the hose. Only manufacturer-approved or properly qualified hose-and-fitting combinations should be used.

Incorrect combinations can cause:

• Leakage
• Inner-tube damage
• Wire displacement
• Insufficient fitting retention
• Hose pullout
• Premature impulse failure

Performance-based construction flexibility

The performance-based nature of ISO 18752 allows manufacturers to improve hose designs without being restricted to one traditional construction.

For example, a manufacturer may reduce hose diameter or weight by using:

• Higher-strength reinforcement wire
• Improved wire geometry
• Advanced rubber compounds
• Better layer adhesion
• Optimized braid or spiral angles
• More efficient cover materials

However, the finished hose must still meet all applicable requirements for its declared class, grade, type, and size. Marketing descriptions such as “four-wire hose” or “compact hose” alone do not demonstrate ISO 18752 compliance.

7. ISO 18752 vs. SAE and EN Hose Standards

ISO 18752, SAE J517, and the European EN hydraulic hose standards all establish minimum requirements for hydraulic hoses. However, they do not classify hoses in exactly the same way.

The main difference is that ISO 18752 emphasizes performance and constant-pressure classification, while many traditional SAE and EN specifications identify hoses primarily by their construction.

Performance-based vs. construction-based standards

ISO 18752 allows manufacturers greater flexibility in selecting materials, reinforcement arrangements, and manufacturing methods. The completed hose must meet the dimensional and performance requirements for its declared pressure class, grade, and type.

Traditional specifications often define construction more explicitly, including:

• Number of wire-braid layers
• Number of spiral-wire layers
• Textile reinforcement
• Cover configuration
• Dimensional limits
• Pressure rating for each hose size

The basic distinction can be summarized as follows:

This does not mean ISO 18752 ignores construction or that SAE and EN standards ignore performance. All of them include performance requirements. The difference is the way compliant hoses are grouped and identified.

ISO 18752 vs. SAE J517

SAE J517 is one of the most widely recognized hydraulic hose standards, particularly in North America. It includes several hose types identified with designations such as:

• SAE 100R1
• SAE 100R2
• SAE 100R3
• SAE 100R4
• SAE 100R5
• SAE 100R6
• SAE 100R7
• SAE 100R8
• SAE 100R12
• SAE 100R13
• SAE 100R15
• SAE 100R16
• SAE 100R17
• SAE 100R19

Many of these designations are associated with recognizable constructions. For example, SAE 100R1 and SAE 100R2 are commonly associated with one-wire- and two-wire-braid hydraulic hoses.

Under several SAE hose specifications, maximum working pressure varies according to nominal hose size. A smaller hose may have a higher pressure rating than a larger hose of the same SAE type.

ISO 18752 instead begins with a pressure class. Every approved size within that class has the same maximum working pressure.

Example of the selection difference

Under a construction-based system, an engineer may begin by specifying a two-wire-braid hose and then check whether the selected size provides sufficient working pressure.

Under ISO 18752, the engineer may begin by specifying:

• Required maximum working pressure
• Required impulse grade
• Standard or compact configuration
• Nominal size for the required flow

The manufacturer can then provide a hose construction qualified for those performance requirements.

SAE constant-pressure hoses

Not every SAE hose specification uses a strongly size-dependent pressure rating. SAE J517 also contains constant-pressure or compact hose types, including certain widely used R-series hoses.

Therefore, it is inaccurate to describe all SAE hoses as variable-pressure products. The important point is that SAE J517 contains many separate hose specifications, whereas ISO 18752 organizes compliant hoses within one unified pressure-class and performance-grade system.

Some ISO 18752 hoses may meet or exceed the requirements of a particular SAE type. However, this does not make the two designations automatically equivalent.

ISO 18752 vs. EN 853

EN 853 traditionally covers wire-braid-reinforced hydraulic hoses, including familiar types such as:

• 1ST
• 2ST
• 1SN
• 2SN

The designation indicates a particular hose construction and dimensional category. Maximum working pressure generally varies with nominal bore size.

An ISO 18752 hose may use a similar wire-braid construction, but its compliance is established through its ISO pressure class, grade, and type rather than an EN 853 construction designation.

ISO 18752 vs. EN 856

EN 856 covers rubber hydraulic hoses with spiral-wire reinforcement, including types such as:

• 4SP
• 4SH
• R12
• R13
• R15

These hoses are generally used for high-pressure or severe-duty applications. Their specifications address construction, dimensions, pressure capability, and impulse performance.

High-performance ISO 18752 Grade C or D hoses may also use spiral-wire reinforcement. However, ISO 18752 does not require every high-grade hose to carry a particular EN 856 designation.

A hose may comply with both standards if it is separately designed, tested, and declared to meet both sets of requirements.

ISO 18752 vs. EN 857

EN 857 includes compact wire-braid hydraulic hose types, such as 1SC and 2SC. These hoses typically have smaller outside diameters and tighter bend radii than traditional standard-cover braided hoses.

ISO 18752 compact types serve a similar general objective:

• AC
• BC
• CC
• DC

However, the letter C in an ISO 18752 hose type identifies a compact dimensional category and should not be treated as a direct replacement for the SC designation in EN 857.

Why cross-reference charts require caution

Manufacturers and distributors sometimes publish cross-reference charts comparing ISO, SAE, and EN hose types. These charts are useful for initial screening, but they should not be used as automatic substitution tables.

Two hoses may have the same nominal size and working pressure but differ in:

• Outside diameter
• Minimum bend radius
• Impulse-test cycles
• Impulse-test temperature
• Fluid compatibility
• Cover abrasion resistance
• Electrical properties
• Fitting compatibility
• Vacuum capability
• Temperature rating
• Change in length under pressure

A cross-reference should therefore be confirmed against the current technical data for the specific products.

Similar pressure does not mean interchangeability

Consider two hoses that are both rated for 350 bar. One may be an ISO 18752 Type BC hose, while the other may be an EN spiral-wire hose.

Although their working-pressure ratings are equal, they may not have the same:

• Impulse endurance
• Bend radius
• Outside diameter
• Reinforcement design
• Fitting system
• Temperature capability
• Expected service life

The pressure rating is only one selection parameter.

Dual-certified hoses

Some manufacturers offer hoses marked as complying with more than one standard. A product might be qualified to an ISO 18752 classification and also satisfy a particular SAE or EN hose specification.

Dual certification can provide several benefits:

• Easier use in international equipment
• Greater replacement availability
• Compliance with different customer specifications
• Simplified global inventory
• Easier approval for export machinery

The manufacturer’s declaration and test documentation should clearly identify the applicable standards, editions, hose sizes, pressure ratings, and qualified fittings.

Which standard should be used?

The appropriate standard depends on:

• Equipment specification
• Customer requirements
• Regional industry practice
• Replacement availability
• Required pressure and impulse performance
• Fitting system
• Regulatory or contractual requirements
• Original equipment manufacturer approval

ISO 18752 is particularly useful when designers want a performance-based, constant-pressure hose specification. SAE and EN standards remain important when a particular construction or established hose series is required.

One standard is not universally superior to the others. The correct choice is the standard that fully covers the application and is accepted by the equipment manufacturer, end user, and relevant technical specification.

---

8. Selecting and Applying an ISO 18752 Hose

Selecting an ISO 18752 hose requires more than choosing a pressure class. The complete application must be evaluated, including size, temperature, fluid, pressure, fittings, routing, movement, environmental conditions, and service expectations.

The STAMPED hose-selection method provides a useful framework:

• S — Size
• T — Temperature
• A — Application
• M — Material or media
• P — Pressure
• E — Ends
• D — Delivery

Step 1: Determine the required hose size

Hose size is primarily selected according to required flow rate and acceptable fluid velocity.

A hose that is too small can cause:

• Excessive pressure drop
• Increased fluid temperature
• Higher energy consumption
• Turbulence and noise
• Cavitation at pump inlets
• Reduced actuator performance

A hose that is unnecessarily large can increase:

• Cost
• Weight
• Fluid volume
• Bend radius
• Installation-space requirements

Fluid velocity can be estimated from:

v=AQ​

where:

• $v$ = average fluid velocity
• $Q$ = volumetric flow rate
• $A$ = internal flow area

For a circular hose bore:

A=4πD2​

Combining the equations gives:

v=πD24Q​

where $D$ is the hose inside diameter.

The selected nominal size should be verified using the manufacturer’s actual inside diameter because the nominal designation may not equal the exact bore measurement.

Step 2: Determine maximum system pressure

The selected ISO 18752 class must have a maximum working pressure equal to or greater than the highest pressure the hose assembly can experience.

Pressure evaluation should include:

• Normal operating pressure
• Maximum pump pressure
• Relief-valve setting
• Pressure-compensator setting
• Pressure spikes
• Load-induced pressure
• Pressure intensification
• Thermal expansion of trapped fluid

For example, a circuit operating normally at 260 bar may appear suitable for Class 280. However, if measured transients reach 310 bar, Class 280 is insufficient.

Where pressure spikes are unknown, the system should be measured with instrumentation capable of capturing fast transient events. A conventional pressure gauge may not respond quickly enough to show short-duration peaks.

Step 3: Select the appropriate performance grade

After selecting the pressure class, the designer must determine the required impulse grade.

This table is only a general guide. The final selection must follow equipment requirements and manufacturer recommendations.

A higher grade should be considered when:

• The machine operates continuously
• Pressure changes occur rapidly
• Hydraulic temperatures are elevated
• Hose replacement is difficult
• Downtime is expensive
• Failure consequences are severe
• The original equipment manufacturer specifies a high impulse level

Step 4: Choose standard or compact type

ISO 18752 standard types include AS, BS, and CS. Compact types include AC, BC, CC, and DC.

Compact hoses may be preferable where:

• Routing space is limited
• Small bend radii are required
• Several hoses are bundled together
• Lower hose weight is beneficial
• Equipment movement requires improved flexibility

Standard hoses may be suitable where there is adequate installation space and compact dimensions are not required.

Compact hoses must still be installed above their specified minimum bend radius. “Compact” does not mean unlimited flexibility.

Step 5: Check temperature conditions

Both internal fluid temperature and external ambient temperature must be considered.

For specified oil-based hydraulic fluids, ISO 18752 covers:

• AS, AC, BS, and BC: −40°C to +100°C
• CS, CC, and DC: −40°C to +120°C

The hose may experience additional external heat from:

• Engines
• Exhaust systems
• Furnaces
• Hot process equipment
• Radiant surfaces
• Welding operations
• Direct sunlight

When external heat cannot be avoided, the installation may require:

• Heat shields
• Insulating sleeves
• Reflective protection
• Increased clearance
• Alternative routing
• A higher-temperature hose construction

Continuous operation near the upper temperature limit can shorten hose life, even when the hose remains within its stated rating.

Step 6: Confirm fluid compatibility

The inner tube, cover, fittings, and seals must be compatible with the hydraulic fluid and surrounding environment.

Compatibility should be checked for the exact fluid, including its:

• Base oil
• Additive package
• Concentration
• Operating temperature
• Water content
• Contamination
• Cleaning chemicals

Particular attention is required for:

• Water-glycol fluids
• Phosphate ester fluids
• Biodegradable oils
• Synthetic fluids
• Fire-resistant fluids
• High-water-content fluids
• Aggressive chemical mixtures

A general statement that a hose is “oil resistant” is not sufficient for every hydraulic fluid.

Step 7: Select compatible fittings

The hose must be assembled with a qualified fitting system. The selected fittings should match the hose’s:

• Inside diameter
• Outside diameter
• Reinforcement structure
• Cover thickness
• Pressure rating
• Assembly method

The complete assembly should use the manufacturer’s specified:

• Fitting stem
• Ferrule
• Crimp diameter
• Crimping machine
• Tooling
• Inspection procedure

Mixing hoses, fittings, ferrules, or assembly specifications from different manufacturers can create an unverified combination. Components that appear to fit physically may not provide adequate impulse life or fitting retention.

Step 8: Verify connection ends

ISO 18752 does not specify all requirements for connection ends. The designer must separately select suitable:

• Threaded fittings
• Flanges
• Adapters
• Quick couplings
• Sealing methods
• Port connections

Important considerations include:

• Thread type and size
• Sealing mechanism
• Working-pressure rating
• Material compatibility
• Corrosion resistance
• Orientation
• Installation torque
• Space for assembly tools

Common thread systems such as NPT, BSPP, BSPT, JIC, ORFS, and metric connections are not automatically interchangeable.

Step 9: Evaluate hose routing

Correct routing allows the hose to flex naturally without excessive bending, twisting, stretching, rubbing, or compression.

Good routing practices include:

• Keep bends above the minimum bend radius
• Avoid bending immediately behind fittings
• Prevent hose twisting
• Allow for pressure-induced length changes
• Use suitable clamps and supports
• Protect hoses from abrasion
• Keep hoses away from hot surfaces
• Avoid sharp edges
• Provide sufficient movement for articulating equipment
• Prevent hose bundles from rubbing together

When a hose bends in more than one plane, an elbow or suitable adapter may be needed to prevent twisting.

Step 10: Account for hose movement

A moving hose should flex through a controlled bend rather than being stretched or twisted.

The routing design should consider:

• Full cylinder travel
• Steering movement
• Suspension movement
• Machine articulation
• Rotating equipment
• Vibration
• Pressure-induced movement

The complete motion envelope should be checked in all machine positions. A hose that appears correctly routed while the equipment is stationary may become overstretched or sharply bent during operation.

Step 11: Protect against abrasion and impact

Abrasion is one of the most common causes of premature hydraulic hose failure. If contact cannot be eliminated, protection may include:

• Abrasion-resistant hose covers
• Textile sleeves
• Plastic spiral guards
• Spring guards
• Clamps
• Protective channels
• Machine-frame shields

Protective sleeves should not hide leakage or damage so completely that routine inspection becomes ineffective.

Step 12: Consider application-specific hazards

Additional requirements may apply when hoses are used in:

• Electrically insulated equipment
• Flame-resistant environments
• Mining machinery
• Food-processing equipment
• Marine systems
• Offshore installations
• Very-low-temperature service
• High-abrasion environments
• Suction or vacuum lines
• Safety-critical lifting systems

ISO 18752 classification alone does not confirm suitability for every special application. Additional standards, approvals, or manufacturer qualifications may be required.

Practical selection example

Consider a mobile machine with the following conditions:

The initial selection would be:

• At least Class 350 because the maximum pressure is 345 bar
• Grade C because the fluid temperature exceeds 100°C
• Compact type because installation space is limited

A Type CC, Class 350 hose in nominal size 19 may therefore be a suitable starting point.

Before final approval, the designer must still confirm:

• Availability of the exact size and classification
• Fluid compatibility
• Minimum bend radius
• Qualified fittings
• Connection-end pressure rating
• Routing and abrasion protection
• Manufacturer approval

Final selection checklist

Before installing an ISO 18752 hose, verify the following:

• Correct pressure class
• Correct performance grade
• Standard or compact type
• Correct nominal size
• Acceptable pressure drop
• Fluid compatibility
• Internal and external temperature limits
• Minimum bend radius
• Hose length and motion allowance
• Qualified hose-and-fitting combination
• Correct end connections
• Adequate abrasion and heat protection
• Suitable inspection and replacement plan

A properly selected ISO 18752 hose should meet the pressure requirement while also providing the flexibility, impulse resistance, temperature performance, and durability required by the application.