What Does Hydraulic System Pressure Mean on a Heavy Duty Forklift?

When checking a heavy duty forklift specification or discussing a customized hydraulic function, buyers may come across figures given in:

MPa, bar or psi.

These figures describe hydraulic pressure.

It is easy to assume that a forklift with higher hydraulic pressure must be able to lift a heavier load.

That is not how we look at it from the factory.

Hydraulic pressure is an important part of the lifting system, but it is only one part of the complete forklift design.

A forklift hydraulic system uses pressurized hydraulic oil to operate functions such as:

• Mast lifting
• Mast tilting
• Fork positioning
• Side shifting
• Hydraulic attachments

The hydraulic pump supplies oil to the system, while control valves direct that oil to the required cylinder or hydraulic function.

Pressure develops when the hydraulic system works against resistance.

For lifting, that resistance comes from the load acting through the mast and hydraulic cylinders.

Inside a hydraulic cylinder, pressure acting over the piston area produces force.

This is why hydraulic pressure is directly related to the amount of force the cylinder can generate.

But this does not mean:

Higher hydraulic pressure = higher forklift rated capacity.

Forklift capacity is determined by much more than cylinder force.

The complete machine also needs enough:

• Mast strength
• Chassis strength
• Counterweight
• Axle capacity
• Fork strength
• Stability

A hydraulic system might theoretically generate enough force to begin lifting a load that the rest of the forklift is not designed to handle.

That does not make the load safe.

This is one of the most useful distinctions for buyers.

Pressure mainly relates to force.

Flow mainly relates to movement speed.

If the hydraulic pump supplies more oil per unit of time, a cylinder can generally move faster, provided the rest of the system is designed for that flow.

This is why hydraulic pressure and lifting speed should not be treated as the same specification.

A forklift can have adequate pressure but still have relatively slow hydraulic movement if the available flow is low.

Similarly, increasing flow does not automatically increase the maximum force the cylinder can produce.

Hydraulic systems need a way to prevent pressure from rising beyond the intended limit.

This is one purpose of a pressure relief valve.

When system pressure reaches the set limit, the relief system allows the hydraulic circuit to control excessive pressure rather than continuing to build pressure indefinitely.

Hyster technical specifications, for example, list a defined auxiliary hydraulic pressure relief setting for attachment circuits. Cascade also specifies maximum allowable hydraulic pressure for its attachments and requires pressure regulation when truck pressure exceeds the attachment requirement.

This is important because every hydraulic component is designed to operate within a specified pressure range.

This is not something we recommend treating as a simple adjustment.

Increasing the relief-pressure setting may increase hydraulic force in part of the system, but it does not upgrade the rest of the forklift.

The mast, forks, cylinders, hoses, valves, chassis and stability limits remain part of the original design.

Excessive pressure can also place unnecessary stress on:

• Hydraulic hoses
• Cylinder seals
• Control valves
• Pumps
• Structural components

For this reason, rated forklift capacity should never be increased simply by turning up hydraulic pressure.

If a customer needs to handle a heavier load, the correct solution is to review the complete forklift configuration.

Hydraulic pressure becomes especially important when adding a customized attachment.

Examples include:

• Clamps
• Rotators
• Jib functions
• Specialized fork systems
• Other hydraulic load-handling devices

An attachment may require a specific:

pressure range

and

oil-flow range.

Too little pressure may prevent the attachment from producing the required force.

Too much pressure can damage the attachment or apply excessive force to the cargo.

Cascade, for example, offers attachment configurations for different truck hydraulic-pressure ranges and specifies both pressure and flow requirements for individual attachments.

This is why attachment selection should include a hydraulic compatibility check rather than only confirming that the attachment physically fits the carriage.

Suppose a hydraulic attachment has sufficient pressure but receives too little oil flow.

It may still operate, but very slowly.

The opposite situation can also create problems.

If the truck supplies more flow than the attachment is designed to use, flow-control components may be required.

For customized heavy duty forklifts, we therefore check both:

working pressure

and

hydraulic flow requirement.

This is particularly important when additional hydraulic functions are added before production.

Large forklifts regularly handle loads where smooth hydraulic control is more useful than aggressive movement.

When positioning a heavy machine, steel component or mold, the operator may need to raise or adjust the load only a small amount.

The hydraulic system therefore needs to provide both:

enough force

and

controlled movement.

This is why our factory testing does not simply check whether the mast can lift.

We also pay attention to hydraulic response, leakage and the operation of additional hydraulic functions before shipment.

For a standard forklift using its standard mast and fork functions, buyers normally do not need to choose a hydraulic-pressure figure themselves.

The manufacturer determines the appropriate system setting.

It becomes more important when:

• A special hydraulic attachment is required
• An existing attachment will be installed on a new forklift
• Additional hydraulic circuits are needed
• The customer has specific operating-speed requirements
• The hydraulic system is being modified

In these situations, providing the attachment model and its hydraulic requirements is much more useful than simply asking for "higher hydraulic pressure."

When a customer asks:

"Can you increase the hydraulic pressure?"

our first question is normally:

What problem are you trying to solve?

If the customer needs faster movement, the issue may be hydraulic flow rather than pressure.

If an attachment does not clamp strongly enough, we need to check the attachment's pressure requirement and circuit configuration.

If the customer wants to lift a heavier load, we need to review the complete forklift capacity rather than only the hydraulic system.

The correct solution depends on the actual requirement.

Hydraulic pressure is an important forklift specification, but it should not be interpreted as a shortcut for lifting capacity.

A simple way to remember the difference is:

Pressure helps create hydraulic force.

Flow affects how quickly hydraulic functions move.

Rated forklift capacity depends on the complete machine.

For heavy duty forklifts, the hydraulic system should therefore be matched to the mast, load, attachment and working condition rather than simply set to the highest possible pressure.

References

1. Hyster - H80–120FT Technical Guide: Auxiliary Hydraulic Pressure Relief for Attachments.
2. Cascade Corporation - E-Series Fork Positioner Service Manual: Truck Hydraulic Pressure and Flow Requirements.
3. Cascade Corporation - Hydraulic Valve Kits: Pressure Relief and Flow Control Requirements for Forklift Attachments.
4. Cascade Corporation - J-Series Hydraulic Attachments: High- and Low-Pressure Hydraulic System Compatibility.