The Story of Hydraulic Contamination

25 August 2021

Modern hydraulic systems produce more power and have finer control than those of the past due to precision engineering and closer fitting parts. The downside is the increased danger of foreign contaminates damaging machines.

Contaminate particles are hard to spot as they can be invisible. A speck of dust you can barely see on your television screen or computer monitor has a diameter of forty microns, making it 40 millionths of a meter. Cat hydraulic systems have clearances of less than thirty microns and even as little as five microns - invisible to the human eye. Particles too small to even see can dramatically affect your system. Contaminate particles can be dust, dirt paint, metal fragments or even cloth fibres, anything foreign to the hydraulic system.

Particles can enter your system in several ways, but the most common by far is by taking a ride in on a cylinder rod. The rod is one of the only sections of a hydraulic system regularly exposed to the outside air. Airborne contaminate particles stick to the rod’s surface while it’s exposed and are carried inside as part of the cycle. Usually this isn’t an issue, as a series of wiper seals on the end of the cylinder catch particles. If the rod is scratched or dented, contaminates can slip under the seals and enter the system. Once within the system, the particles are no longer airborne, become encased within the hydraulic fluid and float freely. As the cylinder draws the rod back in, pressure forces the particles deeper into the system.

Before it enters a sump, for storage while not in use, hydraulic fluid goes through a filter. While filters are highly efficient at trapping and removing contaminates, they will not remove everything in a single pass. Even with a high-efficiency filter, different sized contaminants will make it through and start wearing down internal parts. Over time, these contaminants cause further damage to valves, pumps, and seals, making them less effective and making it easier for more contaminates to enter. Some particles have sharp edges that scrape against walls and moving parts, breaking off sections which can then cause further problems, a process known as abrasive wear. Repeated stress can also cause fatigue wear, with pieces of metal chipping or breaking off. Particles gathering and jamming valves causes adhesive wear, reducing the system’s overall efficiency.


The effects of contamination can be subtle and go unnoticed or unreported for month while your machine slowly and surely becomes less effective. So, what early warning signs might you spot? One is cylinder drift, where a cylinder cannot keep its target extension, either drifting to an extended or retracted position. This happens when a worn seal leaks hydraulic fluid this reduces pressure and causes drift. Jerky or sluggish movements can be a sign of contamination. Contaminated particles can stick to valves in a process known as silting, either collecting in the grooves of the wall or magnetising because of the electromagnetic environment. The build-up can either slow the spool, creating sluggish movements or cause it to hesitate, resulting in jerky motions.   

Hydraulic contamination can also hit you where it hurts - on your bottom line. Slower moving systems get less work done. Machines can be moving up to 20 percent slower before an operator notices the difference in cycle times. More contaminates means more wear, with shorter service intervals and higher operating costs. In a worst-case scenario, catastrophic failure can occur if say a particle completely blocks the movement of a critical valve.

Next month we’ll look at four practical steps you can take to help protect your machines from hydraulic contamination.