Contamination comes in many forms including air, water, dirt and other particulates. Concentrating on contamination control, system cleanliness and equipment wear protection enable companies to improve the life of their hydraulic system and streamline production while also simplifying maintenance. Shankar Karnik elaborates on various means of contamination and the solutions to prevent contamination in hydraulic system.
For many plant managers, achieving hydraulic system efficiency is an enigma. Growing sophistication in system components and increasing complexity of maintenance strategies have left plant managers puzzled. During this time of technological and procedural innovation, it is imperative to acknowledge that the basic principles of hydraulic systems have not changed. Maintenance professionals are able to unlock the potential for achieving hydraulic system efficiency by focusing on the fundamentals. Concentrating on contamination control, system cleanliness and equipment wear protection enable companies to improve the life of their hydraulic system and streamline production while also simplifying maintenance.
Back to Basics
By definition, hydraulic systems transmit power through a liquid, permitting multiplication of force in accordance with Pascal's law, "a pressure exerted on a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas." Hydraulic systems consist of: Hydraulic fluid, a reservoir to hold the fluid supply, a pump to move the fluid, filters to clean the fluid, valves to regulate the pressure and directional flow of the fluid, and an actuator - generally a cylinder and piston or a shaft.
Air in hydraulic system: Friend or Foe?
Contamination comes in many forms including air, water, dirt and other particulates. While air is not harmful to humans, it can be detrimental to the life of your hydraulic system. Air inhibits a fluid's ability to protect moving components, causing parts to wear at accelerated rates, foam to develop, operating temperatures to increase, premature pump failures and erratic actuator performance to occur. The two most common air-related hydraulic contamination conditions are referred to as the "ations", specifically aeration and cavitiation.
Aeration occurs in the preliminary stages of a hydraulic system. When the pump sucks the oil out of the reservoir, a vacuum is created that moves the oil through the pump and into the hydraulic pipeline. If oil levels are low or pipe fittings are loose during this process, free air will get into the pump and aeration occurs. The pump making noise, the pressure gage becoming spongy or oil foaming are tell tale signs of aeration.
To avoid any potential performance issues, make sure the reservoir is filled to the proper fluid level. In addition, test inlet connections by covering them with shaving cream. If bubbles implode toward the union, the problem needs to be fixed and the air must be bled from the system.
The other "ation" that can occur within a hydraulic system is cavitation. Mineral oil- and synthetic-based fluids contain about eight per cent of dissolved air and when this air is freed from the oil, cavitation occurs. Common causes of cavitation include the vacuum draw on the oil exceeding 4.5 mmHg, an inlet strainer being clogged or collapsed or the diameter of the inlet piping is too small. Each of these factors can lead to foam formation, premature pump failures, accelerated component wear, increased operating temperatures and unscheduled maintenance.
To prevent cavitation, it is imperative to maintain optimal levels of hydraulic fluid in the reservoir and monitor hydraulic pressure so it remains within the acceptable range. When selecting a hydraulic fluid, it is important to partner with a lubricant manufacturer with an in-depth knowledge of hydraulic fluids and oil analysis to help ensure optimal hydraulic performance.
Keep it Clean
Moving along the system, oil's next flow point in the pipeline is at the servo valves. From the simple to the complex, servo valves regulate directional flow and fluid pressure.
There are two critical components of a servo valve - the spool and the control device. The spool guides the oil in the proper direction to provide the actuator with force to work. The clearance around the spool and valve body is extremely finite, in the range of 1-3 micron. To put that into perspective, a human hair is 25 micron. If dirt and varnish collect on the spool, the valve becomes difficult to operate and eventually fails. Maintenance professionals will often mistake problems with servo valves for pump failure. Actually, major pump manufacturers have indicated that a majority of the hydraulic pumps sent in for failure analysis have not failed. To avoid unnecessary maintenance costs or extended periods of downtime, plant managers should evaluate the servo valves before more drastic measures are taken.
To keep the spool loop clean, it is important for maintenance professionals to set up a filtration system. At a minimum, these filters should be as tight as the main hydraulic system filters and more finite (1-3 micron) to keep particulates from the spool.
The control device provides current that moves the valve to regulate pressure. As particulates continue to accumulate, resistance and solenoid temperature of the valve will increase. When conditions reach approximately 150¦F, the epoxy coating around the coil begins to melt and the mechanism shorts out. It is advised to use an infrared heat gun to monitor this condition and prevent failure.
Enhanced Wear Protection = Longer Equipment Life and Better Performance
If hydraulic oil does not provide extended component protection, you will start to see abnormal wear, which will cause more frequent lubricant application, reduced equipment life and lead to potentially catastrophic system failures.
So how can you determine if your hydraulic oil is well suited to protect your valuable equipment?
First, companies should employ hydraulic lubricants in viscosity grades of 32, 46 or 68 with a product profile that delivers superior keep-clean performance, superior wear protection, and superior contamination control.
Second, a key indicator of a hydraulic fluids anti-wear performance is the ability of hydraulic oil to retain it properties over the long haul. Operational longevity is dependent on many factors, however three to five years is an acceptable lifespan of a hydraulic fluid. Higher-performing fluids have achieved service lives in upwards of seven years. As with most lubricant applications, these performance characteristics are best achieved when the oil is clean. Typical hydraulic systems require oil cleanliness in the ISO 16/13 or 14/11 range. To support this level of cleanliness, maintenance professionals should install a filter before the pump, after the pump, a separate servo valve filtration and/or return line filtration. The strainer will protect the pump, the pressure filter will protect the actuator, the servo filter will ensure proper servo operation and the return line filter will make sure the reservoir stays clean. While each filter may have a different cleanliness rating, it is important to work with your lubricant manufacturer to realise the levels of cleanliness needed to maintain proper component functionality and achieve hydraulic system efficiency.
Keep it Simple
Hydraulic systems consist of sophisticated pieces of equipment. However, this does not mean that sustaining optimal performance needs to be complicated. By focusing on the fundamental elements detailed in this article, companies will be rewarded with enhanced hydraulic performance, simplified maintenance procedures and minimized unscheduled downtime.
Keep Clean Case Study
A lumber mill is now saving $12,000 a year on maintenance-related issues for its hydraulic system by switching to Mobil DTE 25. The high-performance hydraulic oil's outstanding anti-wear protection and resistance to varnish and sludge formation helps the company keep its hydraulic system clean and operating longer, eliminating the need for unscheduled valve replacement or oil changes.