Cylinder Failures Due to Overpressurization
All hydraulic cylinders are designed to withstand a specified working pressure and are often rated with a maximum pressure rating as well. The maximum pressure rating is the amount of pressure that the cylinder can withstand for a short time without causing damage. This allows the cylinder to absorb minor pressure spikes that jump above the specified working pressure and remain in service for its expected life. However, cylinders that experience pressures above the maximum pressure rating will have a reduced life and may fail immediately due to overpressurization.
The components which make up a hydraulic cylinder are steel and elastomers. Each of these have pressure limitations and will yield (fail) if exceeded. Limiting pressure isn’t as simple as making sure that the pump pressure doesn’t exceed the working pressure of the cylinder. We also need to ensure that the forces acting on the actuator don’t induce overpressure. To do that we need to understand what causes the pressure within the cylinder to spike above the working pressure in the first place and how that relates to cylinder failure.
There are endless configurations of hydraulic systems operating around the globe, and all are unique in some way. One feature that all hydraulic systems have in common is that they all have an actuator. Actuators are either rotary, such as a motor, or linear, such as a cylinder. Regardless of the type, the actuator moves a load of some sort at a given speed. Newton’s second law states that mass times velocity equals force, but how does external force result in a pressure spike inside the cylinder? To understand the correlation, we also need to understand Pascal’s law, which states that “a pressure change at any point in a confined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.” Pascal’s Law is demonstrated in the graphic below, which illustrates the relationship between Force, Area, and Pressure.
In a cylinder that is lifting a load, the pressure within the cylinder is determined by the force applied by the load divided by the area of the cylinder. The larger the cylinder area the greater the force that can be achieved with the same pressure. If the force is increased, the pressure inside the cylinder will increase.
Let’s say that we have a wheel loader that has a rated bucket capacity of 30,000 pounds with a max system pressure of 3000 PSI. If the bucket is loaded to capacity the pressure on the cylinders will be 3000 PSI. The operator drives the loader slowly over some rough ground and the load begins to bounce, causing the pressure in the cylinders to spike to 4000 PSI. If the operator increases the speed of the loader, the load will bounce with greater speed. The mass will increase velocity – which will increase the force. This erratic increase in force will cause repeated jumps in pressure within the cylinders – otherwise known as pressure spikes. In some cases, rough service of a machine can cause pressure spikes of 2-3 times the working pressure. In this example, that would mean the pressure within the cylinders could be as high as 6000 – 9000 PSI, which is easily high enough to fail both seals and steel components in a cylinder designed to work at 3000 PSI.
Usually, the manufacturer builds in a service factor that allows for a certain amount of rough service. This may include features such as port reliefs that are designed to dissipate the pressure spike quickly and protect the cylinder from damage. If the cylinder fails under normal operating conditions, it may be a result of a failed safety valve in the circuit. As with all failures, the cause should be investigated and understood to prevent a recurrence.
RHK Hydraulic Cylinder Services Inc. are experts in hydraulic cylinders in Canada. Whether you have a single acting hydraulic cylinder or a double acting hydraulic cylinder – we can help. Contact us today to learn we can help you – from hydraulic cylinder manufacturing through to rebuilds and repair.