Hydraulic cylinders are a vital part of so many different types of machinery in a wide variety of industries, and when they fail, the results can range from annoying to dangerous. It is important to understand the nature of hydraulic cylinder failure, including what causes it and how to best address it once it happens.
Hydraulic cylinders (which you may see referred to as linear hydraulic motors) are hydraulic actuators used to achieve linear motion by taking advantage of the efficient transfer of force via a virtually incompressible hydraulic fluid. They can be found in just about every industry, ranging from massive earthmoving machines like hydraulic shovels to the aerospace industry where they can be found on aircraft as well as ground support equipment. Hydraulic cylinders are normally a critical component of whatever system they are found in, which means their failure can lead to problematic downtime. That is why it is important to understand what causes hydraulic cylinders to fail and what can be done to prevent these failures.
Components in a Hydraulic Cylinder
The most important components in a hydraulic cylinder include the barrel (which encases most of the parts), the piston rod (which is the part that most people see), the piston (which transfers motion to the piston rod), the cylinder cap, the cylinder head, and a variety of seals and bearings. The failure of any one of these parts will affect the hydraulic cylinder as a whole.
Hydraulic cylinders are often used in environments where they may come into contact with corrosive materials. When such an environment is present, it is usually the cylinder seal that is affected first (depending on what type of material the cylinder seal is made from). If signs of a chemical attack on the seal are found, then the seal not only needs to be replaced but it needs to be replaced with a seal material that is compatible with the corrosive chemicals in the working environment. If purchasing a new hydraulic cylinder system, it would be wise to contact the manufacturer about potential chemical issues with the seals.
Contaminated Hydraulic Fluid
Contaminated hydraulic fluid can take two problematic forms in hydraulic cylinders: contamination with air or water, and particle contamination. The presence of either air or water in the hydraulic fluid will affect the performance of the cylinder, affecting the loads that it can handle and the power of the stroke. On the other hand, particulate contamination can destroy seals and lead to other critical issues. Most particulate contamination takes the form of abrasive particles, and once those particles find themselves trapped between the surface of the piston rod and the piston rod seal, the seal will be damaged and begin to leak. Most contamination is introduced to a hydraulic seal via a faulty wiper seal, but it can also be introduced during maintenance and repair if clean procedures are not followed.
Corrosion Inside the Barrel
If water is allowed access inside the hydraulic cylinder, it can cause internal corrosion within the barrel. This type of damage is difficult to repair and often involves the replacement of affected parts. The best way to prevent this type of damage is to prevent the ingress of water, which often occurs as a result of damaged seals. On the other hand, such damage could also be a sign that the cylinder is not of the appropriate type for the environment it is being used in.
Damaged Eye Bearings
The eye bearing will fail under one of two circumstances: (1) when the cylinder is subjected to a load above its specification or (2) when the cylinder is subject to a sudden impact or shock loading. If a load exceeds the specified loading for a cylinder, something is going to fail and it is usually going to be the eye bearing.
Damaged Rod Bearings and Piston Rods
The cause of damaged rod bearings and bent piston rods is usually improper alignment between the cylinder and the load but can also be caused by loading the hydraulic cylinder beyond the load it is specified for. When the alignment is not correct, there will be a sideways loading or bending induced that the rod bearings are not designed to carry.
Dented Rods and Barrels
The barrel or rod of the hydraulic cylinder can be dented deeply enough to cause performance issues and necessitate their replacement. This type of damage is usually the result of impact with another piece of equipment or a very heavy object. It usually manifests when the rod cannot be fully extended or retracted (i.e., the cylinder is not able to fully stroke). Attempts to continue using a cylinder with a dented rod or barrel will cause damage to other components, including bearings and seals.
Most instances of excessive wear can be traced back to misalignment of the rod and cylinder. When these two components are not aligned correctly, there will be additional forces induced that are perpendicular to the motion of the rod, known as a thrust load or side load. These forces eventually lead to damaged bearings, pistons, rods, and barrels.
Another major cause of hydraulic cylinder failure is extreme operating temperature — both too cold and too hot. When operating temperatures are too hot, it may lead to premature failure of the seals. In addition, at high temperatures, the ability of a fluid to adequately lubricate critical parts can also become an issue and lead to premature wear and significantly reduced efficiency. This can often be seen with bearings that fail too soon. Excessive temperatures can also speed up the natural degradation of hydraulic fluid and lubricants.
On the other hand, if the operating temperatures are too low, the lubricant and hydraulic fluid will become too thick to work properly and damage to the hydraulic cylinder can result. Cold temperatures can also cause seal materials to behave in a brittle manner, leading to cracked sides. If the seals are being affected by extreme temperatures, the affected seal should be replaced with one made of a material that is compatible with the operating temperatures.
If the hydraulic fluid and lubricants are being adversely affected by operating temperatures, then another type that is rated for the expected operating temperatures should be used.
Another major problem that can occur with hydraulic cylinders is seal extrusion, which usually occurs between the barrel and head or cap. As the lip of the seal begins to protrude into the extrusion gap, it will experience extreme wear. Pieces of the seal lip may be torn away, thus introducing contamination into the system and leading to a leaking seal. When there is evidence that a seal has begun to protrude into the extrusion gap, then a different type of seal material or seal geometry becomes necessary.
The most common cause of hydraulic cylinder failure is leaking seals, which not only allow critical fluids to leave the system (including lubricants) but allow damaging contaminants (water, air, and particles) to make their way inside the hydraulic cylinder. Most causes of seal failure are the use of the wrong size seal, inappropriate clearances between the seal and the surface, corrosion of the seal, excessive wear because of abrasive fluid contamination, poor alignment between the rod seal and the rod, and improper installation. However, keep in mind that seals will inevitably wear over time and reach a point when they must be replaced.
Most hydraulic cylinder failures are the result of issues related to the seals (including leaks, contamination, and extrusion), bearings (including the rod bearing and eye bearing), and dents in the rod or barrel. The most common causes of hydraulic cylinder failure are misalignments, extreme temperatures, normal or excessive wear, chemical attacks, corrosion, and hydraulic contamination. The secret to preventing the majority of these failures is quite simple: regular preventative maintenance. For example, fluid testing can reveal a number of issues (e.g., air or water contamination, particulate contamination, corrosion, chemical attack, and degradation to temperature). Visual inspection can reveal the presence of misalignment, leaking seals, corrosion, and dented rods and barrels.
At MAC Hydraulics, our team of technicians can not only repair hydraulic cylinders but provide on-site maintenance and testing that will reduce your M&O costs, downtime, and repair costs. Whether it is a worn-out bearing, a misaligned rod, or a failed seal, our skilled team will perform the necessary repairs in a minimum amount of time. Not only will they address the parts that need replacement or adjustments, but they will track down the root cause of the failure and make recommendations so you can prevent it from happening again. We also polish rods, hone tubes and replace all seals when we perform a repair. Contact us today at MAC Hydraulics to find out what we have to offer in the way of repair, maintenance, and testing of hydraulic cylinders!