Failures & Fundamentals: Hydraulic Systems Expert Overview

In this article, machinery experts at Robson Forensic provide an introduction to hydraulic systems and the hazards they present. Hydraulic systems are used everywhere, from forklifts to landing gear to industrial systems and the largest earth-moving machinery. Some of the most complex and critical life safety hydraulic systems are found on board ships and aircraft. Expertise in those environments translates well to industrial applications on terra firma. Required to be powerful and reliable, hydraulic systems are not, however, immune to malfunction and failure.

Failures & Fundamentals: Hydraulic Systems - Expert Overview

In a typical hydraulic system, a motor driven pump dramatically raises the pressure in the working fluid (usually hydraulic oil). For example, where typical residential water pressure is less than 60 psi (pounds per square inch), hydraulic system pressure can be 50 or 100 times that.

The pump takes suction from a low pressure reservoir and transfers the now high pressure hydraulic fluid to a directional control valve, usually under the control of an operator. The directional control valve directs flow as appropriate to a piston type actuator or hydraulic motor. The flow is directed to initiate or rescind a desired action of whatever is attached to the actuator or motor. This could be the turning of a ship’s rudder, stroking of a baler/compacter, or positioning of a safety critical machine guard. Hydraulic fluid is returned in the system from the actuator back to the reservoir under relatively low pressure. For every square inch of piston area exposed to high hydraulic pressure, thousands of pounds of force are exerted by the actuator’s piston to whatever its rod end is attached. Users most often interface with a hydraulic system or machine at its “business end,” i.e. the working part or assembly attached to the rod end.

Figure 1: Basic Hydraulic System

The diagram above depicts the following components typically found in a basic hydraulic system:

  • Reservoir: container that stores the hydraulic fluid in a system with allowance for fluid expansion and release of entrapped air.
  • Strainer and filter: protects the hydraulic system from damage due to contamination.
  • Pump: converts the mechanical energy of the drive motor to a hydraulic energy of flow and pressure that is used as the motive force to move actuators and rotate hydraulic motors.
  • Relief valve (RV): limits the hydraulic system pressure to a specific pressure thus providing overpressure protection for the pump.
  • Directional control valve (DCV): provides high-pressure hydraulic fluid to the desired port on the actuator or hydraulic motor with a return port for the low-pressure hydraulic fluid. DCVs control the start, stop and change in direction of the hydraulic fluid.
  • Actuator (linear motion): converts hydraulic energy into mechanical energy to generate a linear motion where the output force is proportional to the hydraulic fluid pressure and the actuator piston area.
  • Hydraulic motor (rotary motion): converts hydraulic energy into mechanical energy to generate the required torque and rotational speed.
  • Connecting piping, tubing or flexible hose: connects individual components within a hydraulic system.

Hazards Associated With Hydraulic Machinery

Using the example of a hydraulic press, these machines typically have the piston type actuator oriented vertically and fixed at the top of the press. The piston rod end is attached to the movable upper platen to which forming or shearing dies may be attached. The press is made to stroke manually, semi-automatically, or automatically. The operator’s hands need to be inside the die space to place or remove work pieces being pressed, formed, sheared, or trimmed by the press.

Often two-hand control and/or light curtains are utilized to prevent inadvertent stroking of the press while the operator’s hands are in the die space. However, Robson Forensic experts have provided expert analysis and opinions involving catastrophic injuries that occur when the connection between the rod end of the actuator fails, and the platen crashes down on the hands of the operator while the press is idle.

The same hazard exists when hydraulics are used to raise or hold a device that is connected to the rod end of the actuator, such as a hatch or safety critical guard. Because of the high pressures, hydraulic system components must be compatible and rated for the conditions of service: failures in the connections within the hydraulic system can turn components into missiles and pieces into shrapnel. Internal leaks in the system can cause unintended and hazardous motion of the driven components. Air or water being introduced into the system can cause the system to behave erratically.

Hydraulic System Failure Investigations

Catastrophic failures of hydraulic systems can be especially nefarious, because absent proper preventative maintenance (especially periodic replacement of hydraulic hoses), such failures often occur suddenly and with little or no proximate warning. Persons in the vicinity of small external leaks in the high pressure circuit can be injected with hydraulic fluid. This can lead to necrosis and infection and is often quite a serious injury. Small high pressure leaks cause the hydraulic oil to atomize, or “mist,” which creates an inhalation hazard and can even precipitate a particulate explosion and/or fire.

Cases involving complex machinery and catastrophic losses call for experts with both industry and forensic experience. To determine which of our experts is best suited to address the technical issues in your case, contact the author of this article or submit an inquiry.

Featured Expert

John G. Green II, Mechanical Engineer & Industrial Machinery Expert

John G. Green II, P.E.

Mechanical Engineer & Industrial Machinery Expert
Jack Green is an expert in industrial machinery, process systems and construction site safety. His expertise includes the design, operation, maintenance, and inspection of machinery; he has specific… read more.


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