What Materials Are Hydraulic Systems Made Of, and Why Does Material Selection Matter?

A hydraulic system is not made from one “hydraulic material”. A cylinder tube, piston rod, valve body, hose, seal and reservoir perform completely different jobs, so their materials are selected for different reasons.

Depending on the component, the important properties may include pressure strength, fatigue life, wear resistance, corrosion resistance, weight, machinability, temperature capability and compatibility with the hydraulic fluid.

The strongest material is not automatically the best hydraulic material. The correct material is the one that can survive the real mechanical, chemical and environmental conditions of its location in the machine.

Hydraulic cylinders use several materials in one component

A hydraulic cylinder is a good example of why a simple material list is not enough. The barrel, piston rod, piston, gland, bearing elements and seals all have different requirements.

Cylinder barrel: Steel is widely used because the barrel must withstand internal pressure, repeated pressure cycles and mechanical loading. Internal surface quality is also important because it affects sealing, friction and service life.

Piston rod: The rod requires mechanical strength, a suitable surface finish and resistance to wear and corrosion. Steel rods may use surface treatments or coatings depending on the application and environment.

Piston and end components: Steel, cast iron and other suitable alloys may be used according to cylinder design, pressure and mechanical loading.

Cylinder design is not determined by pressure alone. Rod buckling, side load, stroke length, mounting arrangement and operating environment also matter. For the working principle, see hydraulic cylinder operation.

Pump housings may be cast iron, aluminium or other engineered materials

Hydraulic pumps and motors contain highly loaded rotating or reciprocating parts, close internal clearances and pressure-containing housings.

Cast iron is widely used in many industrial pump and motor housings because it can provide useful combinations of rigidity, castability, machinability, wear behaviour and vibration damping.

Aluminium alloys may be used where lower weight and compact construction are important, particularly in some mobile and smaller hydraulic components.

That does not mean an aluminium housing and a cast-iron housing of similar appearance have the same pressure capability. The allowable pressure depends on the complete component design, alloy, wall thickness, temperature and fatigue requirements.

For the role of the pump in the hydraulic circuit, see hydraulic pumps.

Valve bodies and hydraulic manifolds need more than simple strength

Directional, flow and pressure-control valves can use cast iron, steel or suitable aluminium alloys depending on pressure class, size and application.

The material of the valve body is only one part of the design. Spools, poppets and other precision internal elements depend heavily on surface hardness, dimensional tolerance, finish and wear resistance.

In high-performance valves, extremely small internal clearances also make contamination control critical. Excellent materials cannot compensate for dirty hydraulic fluid.

Why is steel tubing common in industrial hydraulics?

Properly specified steel tube and pipe are common choices for fixed high-pressure industrial hydraulic circuits. They can be designed for high internal pressure, mechanical support and repeated pressure cycling.

The outside diameter is only part of the specification. Wall thickness, material grade, fitting system, working pressure and fatigue conditions all influence whether the line is suitable.

Stainless steel tubing may be selected for corrosive, marine, chemical or other demanding environments. It should not, however, be treated as automatically superior. Cost, fabrication method, fittings, fluid compatibility and actual corrosion conditions still matter.

Is copper tubing normally used for high-pressure hydraulics?

Copper tubing can be suitable for certain fluid-service and lower-pressure applications, but it should not be presented as the standard tube material for high-pressure industrial or mobile hydraulics.

Hydraulic pipework can experience vibration, pressure pulsation, mechanical loading and millions of pressure cycles. The selected material and connection method must therefore be approved for those conditions.

Corrosion resistance alone is not enough to qualify a tube for a hydraulic pressure circuit.

Are brass fittings common in hydraulic systems?

Brass can be useful in certain instrumentation, auxiliary-fluid and lower-pressure connections because of its machinability and corrosion behaviour.

High-pressure industrial hydraulic fittings, however, are very commonly made from steel or stainless steel. It would therefore be misleading to describe brass as the general fitting material for hydraulic systems.

A fitting is suitable only when its pressure rating, material, sealing method, thread or connection standard and fluid compatibility match the application. A fitting should never be selected simply because the thread appears to fit.

A hydraulic hose is a multi-layer engineered component

Hydraulic hoses are not simply rubber tubes. A typical hose can contain an inner tube compatible with the hydraulic fluid, one or more reinforcement layers for pressure strength and an outer cover for environmental protection.

Depending on the design, reinforcement may use steel-wire braid, spiral steel reinforcement or textile reinforcement. Thermoplastic high-pressure hoses are also used in suitable applications.

The outer cover may be selected for resistance to oil, abrasion, ozone, weather exposure or other environmental conditions.

Hose selection involves much more than the pressure number

Maximum working pressure is essential, but it is not the only requirement. Hydraulic hose selection can also depend on:

Fluid compatibility

Minimum and maximum temperature

Minimum bend radius

Pressure impulses and duty cycle

Abrasion and external wear

Movement and vibration

Compatibility between the hose and its fittings

An incorrect hose specification is not merely a reliability problem. In a high-pressure circuit it can become a serious safety hazard.

Seal materials must match both the fluid and the operating conditions

Hydraulic systems can use sealing materials such as NBR, FKM, polyurethane, PTFE and other elastomers or engineering polymers.

No single seal material is ideal for every hydraulic fluid, pressure and temperature range.

Seal selection can depend on:

Hydraulic-fluid chemistry

Operating temperature

Pressure

Sliding speed

Friction requirements

Static or dynamic sealing duty

An incompatible fluid can cause some elastomers to swell, soften, harden or lose strength. This becomes particularly important when changing away from conventional mineral oil. For the fluid side, see hydraulic fluid selection and compatibility.

Why is aluminium attractive in mobile hydraulics?

Aluminium alloys can provide useful weight reduction and machinability. They are therefore used in some manifolds, valve bodies, reservoirs and compact hydraulic components.

Weight saving can be valuable in vehicles and mobile machinery where every kilogram affects the overall machine.

However, low density does not mean an aluminium component is automatically suitable for high pressure. Alloy selection, wall section, thread design, temperature and fatigue loading all remain important.

When does stainless steel become worthwhile?

Stainless steel can be useful for pipework, fittings, piston rods and other components exposed to marine environments, chemical processes or aggressive outdoor conditions.

The phrase “stainless steel” does not describe unlimited corrosion resistance. Grade, surface condition, temperature and the chemistry of the surrounding environment still determine performance.

Using stainless steel where ordinary steel with appropriate protection would perform adequately can also add considerable cost without improving machine operation.

Engineering plastics and composites also have hydraulic roles

Polymers in a hydraulic system are not limited to seals. Engineering plastics may be used for wear rings, guides, bushes, bearing elements, filter parts and protective components.

Their suitability may depend on temperature, creep, wear, fluid absorption and chemical compatibility in addition to mechanical loading.

Reservoir material affects more than structural strength

Welded steel reservoirs are common in industrial hydraulic power units. Aluminium and other suitable materials may be used where weight, corrosion or machine architecture justify them.

Reservoir design must also consider internal cleanliness, corrosion protection, weld quality, heat transfer, accessibility for maintenance and compatibility with the hydraulic fluid.

A poorly cleaned reservoir can release welding debris, scale or contamination into the circuit and damage components regardless of how strong the tank material is.

Pressure rating is not the same as long-term durability

As hydraulic pressure rises, mechanical stress in tubes, housings, hoses and fittings also increases. But hydraulic components are not designed only for one static pressure event.

Real machines experience pressure spikes, load reversals and repeated operating cycles. A component may survive one high-pressure test but still develop fatigue damage after millions of cycles.

This is why material strength, geometry and fatigue design must be considered together.

Pressure pulsation makes fatigue especially important

Presses, mobile machinery and high-cycle production equipment can repeatedly move between low and high pressure throughout their operating life.

Those cycles can affect pipework, manifolds, threaded connections and other pressure-containing components even when every individual pressure peak remains below the stated maximum.

A maximum-pressure number alone does not describe the complete service life of a hydraulic component.

Temperature changes material behaviour too

Temperature changes hydraulic-fluid viscosity, but it also affects hoses, seals and engineering polymers.

A seal that performs well at normal factory temperature may behave differently at very low or high temperature. Hose pressure capability can also depend on temperature.

The component manufacturer's pressure and temperature limits should therefore be considered together rather than independently.

Corrosion can become a functional failure

Corrosion is not merely an appearance problem. Corrosion on a piston rod can damage the sealing surface, while corrosion in tubing or fittings can reduce mechanical integrity.

Agricultural, marine, mining and outdoor equipment may be exposed to moisture, salt, mud, chemicals and temperature cycling.

Material choice can be combined with coatings, paint, surface treatment and suitable maintenance to control these risks.

Hydraulic-fluid compatibility must be checked before changing fluid family

A component designed around conventional mineral oil should not automatically be assumed compatible with water glycol, phosphate ester, polyglycol or another special hydraulic fluid.

Elastomer compatibility is often one of the first concerns, but coatings, paints, hoses and other polymer components can also be affected.

Changing hydraulic fluid can also be a material-compatibility change. Similar viscosity numbers do not prove chemical compatibility.

Material weight matters differently in stationary and mobile machines

A few additional kilograms may have little importance in a stationary factory power unit. The same mass can matter significantly in a mobile machine, vehicle or moving mechanism.

This is one reason lightweight aluminium manifolds or housings can make sense in mobile equipment while heavy industrial machinery may place more emphasis on rigidity, durability and long service life.

Material choice influences maintenance cost

The lowest-cost material at the time of manufacture may not produce the lowest lifetime cost.

Premature corrosion, short hose life or repeated seal replacement can cost far more than the original component price once downtime and labour are included.

The opposite is also true. Specifying stainless steel or expensive special alloys where they provide no practical benefit can unnecessarily increase machine cost.

For inspection and component-life considerations, see hydraulic system maintenance.

Operating environment should be defined before the material is selected

A hydraulic press inside a clean factory and a hydraulic system working on a ship, farm or quarry do not experience the same environment.

Moisture, salt, UV exposure, mud, stone impact, chemical vapour and temperature cycling can all change the requirements for pipework, hoses, coatings and exposed metal surfaces.

Solving corrosion after the machine has entered service is usually more expensive than including the environment in the original material specification.

Questions to ask when selecting hydraulic materials

What is the maximum continuous working pressure?

What pressure spikes and load cycles can occur?

What are the minimum and maximum temperatures?

Which hydraulic fluid will be used?

Are seals, hoses and coatings compatible with that fluid?

Is corrosion or chemical exposure expected?

Will the component move, flex or remain fixed?

Is vibration or impact loading present?

Is weight important?

What would happen if the component failed?

Can the material and replacement parts be serviced economically?

Material selection is also a hydraulic safety decision

Failure of a pressure-containing tube, hose, fitting or housing can release hydraulic fluid suddenly and at high energy.

For this reason, unknown materials, unverified fittings or components selected only because their dimensions appear correct should not be used in high-pressure hydraulic circuits.

The system's intended working pressure and component ratings must remain compatible. For pressure limits, see hydraulic pressure control; for service precautions, see hydraulic system safety.

Material selection starts with the job of the component

Hydraulic systems can contain steel, cast iron, aluminium, stainless steel, elastomers, thermoplastics and other engineering materials in the same machine.

The correct choice depends on more than simple strength. Fatigue, temperature, corrosion, wear, fluid compatibility, weight and maintenance conditions all influence the final design.

A material that is an excellent choice for a pump housing may be completely unsuitable for a moving hose, piston-rod seal or corrosive marine installation.

Hydraulic material selection is not a search for the strongest material; it is the process of matching each component to the pressure, movement, fluid and environment it will actually experience.

Related hydraulic topics

Fluid and compatibility: Hydraulic fluid selection · Hydraulic maintenance

Pumps and actuators: Hydraulic pumps · Hydraulic cylinders

Pressure and safety: Hydraulic pressure control · Hydraulic system safety