Mobile hydraulics valve selection guide for crane and lifting applications

The lifting equipment and cranes work under conditions that expose each hydraulic component to the limits, such as dynamic loads, varied operating pressures, and extreme operating cycles, as well as environments that range across urban sites to offshore structures. The key to high-quality lifting performance is the valve for control, and choosing the wrong one for a mobile hydraulics system is among the most significant mistakes in specification the system designer could make.

This guide explains the main types of valves and the performance parameters, as well as the criteria for selecting suitable crane hydraulics and lifting hydraulics.

Why do crane applications need more valves?

Mobile cranes, knuckle boom loaders, marine cranes, as well as aerial work platforms all share the same hydraulic problem: to manage large inertial loads precisely and frequently under load conditions that are asymmetric, while ensuring safety when power is out or the system malfunctions.

Contrary to industrial hydraulics, where loads are mostly static and predictable, crane systems have to deal with suspended loads that shift, swing around, shift, and cause pressure spikes in acceleration and deceleration. The valve system has to manage all this while staying within the size, weight, and contamination limitations for mobile machines.

Types of valves that are used in lifting and crane systems.

Direct control valves (DCVs)

The central control element in every crane circuit. In mobile lifting load-sensing DCVs tend to be preferred over designs with fixed centers since they permit the pump to provide only the amount of flow and pressure that the system requires. This increases fuel efficiency when operating with a partial load and decreases the amount of heat generated, which are both crucial factors when working with heavy-duty cycles.

In the case of cranes, booms, or slew drives, spool-type DCVs with precise metering notches are required to provide operators with the ability to control their speed in a proportional manner. The coarse metering causes jerky movement, which causes shock loads on the structure and decreases the lifespan of the hydraulic cylinders and swing bearings.

Valves for Counterbalance

It is the most crucial valve class for lifting and crane hydraulics. A counterbalance valve (CBV) is installed right on top of the actuator in the form of a cylinder or a hydraulic motor -- which helps prevent runaway due to load. Without CBV, a loaded load could force the actuator to move more quickly than the pump could supply fluid, which could cause an uncontrolled descent or collapse of the boom.

The CBV is able to hold the load by ensuring backpressure is maintained on the motor's or cylinder's return port. It is only opened after positive pilot pressurization from the source side indicates an intention to lower the load is set. For cranes, it is recommended that the CBV set pressure must have 1.3 or 1.5 times that of the highest pressure that is induced by the load. If it is set too high, it can cause an excessive amount of heat and a slow response. Setting it too low could lead to loading drift.

Dual counterbalance valves can be used for situations when loads reverse direction, for instance, in telescopic boom extensions where both cap-end and rod-end circuits are subject to pressure from the load.

Valve for holding load (pilot-operated check valves)

The pilot-operated valves (POCVs) ensure that the load remains in place when no movement is being controlled. They are found on slew cylinders and outrigger pads as well as boom angle cylinders, in which keeping the exact position even under different load conditions is vital. Contrary to counterbalance valves, POCVs don't allow controlled lowering on their own; they're either open or completely closed. When used in crane systems, they're usually paired together with throttle valves or employed in conjunction with CBV.

Electrohydraulic and proportional valves

As crane systems get more automated, including load moment indicators, collision prevention devices, remote control, and load moment indicators proportional to directional valves, they are becoming necessary. These valves receive an electrical signal for control and alter the spool's location to a certain extent, allowing for ramp rates that can be programmed and electronic load limiters in addition to coordinated movement across all axes.

Proportional valves for crane applications need to be identified with a focus on hysteresis as well as repeatability. High hysteresis indicates that the spool's location for a specific command signal varies between decreasing and increasing inputs and can result in an inconsistent response to control, which is a major issue in situations where precise placement of load is needed.

Pressure relief valves and valves for reducing pressure

Secondary relief valves can be positioned to the ports of actuators on crane circuits to guard against sudden pressure surges resulting from shock to the load, sudden acceleration, or external impacts. Port reliefs need to be calibrated and properly set—not too low, or they will open in the wrong way, leading to the load creeping up; or overly large, and they do not safeguard the hose and actuator.

Pressure-reducing valves are employed in pilot circuits as well as in systems where actuators operate at different pressures, like when the slewing system must be restricted independently from the primary lift circuit.

Key choice parameters for lifting applications.

Pressure and flow rating

Every valve should be calibrated to meet the maximum demand for flow and the maximum pressure that can be used by the circuit within which it is operating. When working with crane systems, the highest demands for flow occur when there is coordination of multi-purpose movements, such as when booms extend while slewing, for instance. Insufficiently sized valves for these peak demands result in excess pressure drop, excessive heating, and the loss of control authority.

The rate of leakage

The performance of load-holding is influenced by the leak rate of the valve. In the lifting and crane circuits, actuator drift due to valve leakage can be a safety problem, particularly for platforms for man-riding and precise placement cranes. Select valves with certified leak rates that are appropriate to the holding time required and the acceptable drift tolerance.

Tolerance to contamination

Mobile cranes are naturally polluted by dust and water ingress, as well as wear and tear particles from the winch and slew drives. All affect the purity of hydraulic fluid. Valve selection should take into account this. Clearances for spool and sleeve orifice size within pilot circuits as well as whether there is internal filtration influence how tolerant an individual valve is to contamination levels. The majority of crane-grade hydraulic valves are designed to function reliably at ISO 4406 cleanliness codes of 17/15/12 or higher.

Performance in thermal conditions

The operation of cranes in hot environments—or prolonged duty cycles that do not have adequate heat rejection—can increase the viscosity of the fluid and reduce the valve's response. The valves designed for crane use must be tested across the operating temperature range expected, and counterbalance valves in particular need to be tested at both the cold start and operating temperatures up to maximum, as their pilot ratios and pressures for cracking change with the fluid's viscosity.

Multi-faceted integration

In the current mobile crane design, line-mounted valves are generally replaced by manifolds for valves in cartridges. Manifold integration decreases the complexity of plumbing leak points, as well as installation space, and improves accessibility to service. Port reliefs, valves for counterbalance, and pilot-operated check valves are usually joined into one manifold block that is connected on top of the actuators or boom structure.

The mounting process and security concerns

Standards for the safety of lifting gear, which include EN 13000, which is for cranes that move, as well as ISO 4413 for hydraulic safety, require that load-holding valves be placed as close to an actuator as possible and, most importantly, directly to the motor's cylinder or port. Any hose or pipe that connects the load-holding valve and actuator is a risk of a fault point that could result in an uncontrolled descent of the load if it is breached.

A fail-safe valve design is equally crucial. In the event of an electrical malfunction in an electronic control system, the valves should be set to default to a safe state, which is typically closed or centered. Springs-centered DCVs equipped with work port obstructions fulfill this requirement for the majority of lifting circuits.

The choice of valves for cranes and lifting hydraulics demands treating every valve as a critical safety element and not just a flow control device. The combination of counterbalanced valves, pilot-operated checks, and proportional direction controls as well as port reliefs should be designed as a complete system suited to the pressures in the circuit, the flow requirements, the environment of contamination, and frequency of operation for the particular crane application. Making this right is crucial to both operation and structure of the load-handling system.