Maintenance and troubleshooting
After reading this chapter the student will be able to:
• Understand and explain the various causes for failure in a hydraulic system
• Carry out preventive maintenance of system
• Understand the functions and importance of sealing devices in a hydraulic system
• Carry out preliminary troubleshooting activities for determining the causes of malfunctioning in a hydraulic system.
In the early years of fluid power systems, maintenance was frequently performed on a hit or miss basis. The prevailing attitude then was to fix the problem only after the system broke down. With today’s highly sophisticated machinery and with the advent of mass production, industry can no longer afford a failure, as the cost of downtime is prohibitive. In this chapter we shall try and identify some of the common causes of hydraulic system failures and also examine the various maintenance practices to be followed in a hydraulic system along with the essentials of effective troubleshooting.
Common causes for hydraulic system breakdown
The most common causes of hydraulic system failures are:
• Clogged and dirty oil filters
• An inadequate supply of oil in the reservoir
• Leaking seals
• Loose inlet lines, which cause pump cavitations and eventual pump damage
• Incorrect type of oil
• Excessive oil temperature
• Excessive oil pressure.
A majority of these problems can be overcome through a planned preventive maintenance regime. The overall design of the system is another crucial aspect. Each component in the system must be properly sized, compatible with, and form an integral part of the system.
It is also imperative that easy access be provided to components requiring periodic inspection and maintenance such as strainers, filters, sight gages, fill and drain plugs and the various temperature and pressure gages. All hydraulic lines must be free of restrictive bends, as this tends to result in pressure loss in the line itself.
The three maintenance procedures that have the greatest effect on system life,performance and efficiency are:
1. Maintaining an adequate quantity of clean and proper hydraulic fluid with the correct viscosity
2. Periodic cleaning and changing of all filters and strainers
3. Keeping air out of the system by ensuring tight connections.
A vast majority of the problems encountered in hydraulic systems have been traced to the hydraulic fluid, which makes frequent sampling and testing of the fluid, a vital necessity. Properties such as viscosity, specific gravity, acidity, water content, contaminant level and bulk modulus require to be tested periodically. Another area of vital importance is the training imparted to maintenance personnel to recognize early symptoms of failure. Records should also be maintained of past failures and the maintenance action initiated along with data containing details such as oil tests, oil changes, filter replacements, etc.
Oxidation and corrosion are phenomena which seriously hamper the functioning of the hydraulic fluid. Oxidation which is caused by a chemical reaction between the oxygen present in the air and the particles present in the fluid, can end up reducing the life of the fluid quite substantially. A majority of the products of oxidation are acidic in nature and also soluble in the fluid, thereby causing the various components to corrode.
Although rust and corrosion are two distinct phenomena, they both contribute a great deal to contamination and wear. Rust, which is a chemical reaction between iron and oxygen, occurs on account of the presence of moisture-carrying oxygen. Corrosion on the other hand is a chemical reaction between a metal and acid. Corrosion and rust have a tendency to eat away the hydraulic component material, causing malfunctioning and excessive leakage.
The phenomenon of wear due to fluid contamination
Excessive contaminants in the working fluid prevent proper lubrication of components such as pumps, motors, valves and actuators. This can result in wear and scoring which affect the performance and life of these components and leads to their eventual failure. A typical example of this is the scored piston seal and cylinder bore of cylinders causing severe internal leakage and resulting in premature cylinder failure.
Problems due to entrained gas in fluids
Entrained gas or gas bubbles in the hydraulic fluid is caused by the sweeping of air out of a free air pocket by the flowing fluid and also when pressure drops below the vapor pressure of the fluid. Vapor pressure is that pressure at which the fluid begins changing into vapor. This vapor pressure increases with increase in temperature. This results in the creation of fluid vapor within the fluid stream and can in turn lead to cavitation problems in pumps and valves. The presence of these entrained gases reduces the effective bulk modulus of the fluid causing unstable operation of the actuators.
The phenomenon of cavitation is in fact the formation and subsequent collapse of the vapor bubbles. This collapse of the vapor bubbles takes place when they are exposed to
the high-pressure conditions at the pump outlet, creating very high local fluid velocities, which impact on the internal surfaces of the pump. These high-impact forces cause flaking or pitting on the surface of components such as gear teeth, vanes and pistons leading to premature pump failure. Additionally the tiny metal particles tend to enter and damage other components in the hydraulic system. Cavitation can also result in increased wear on account of the reduced lubrication capacity.
Cavitation is indicated by a loud pump noise and also by a decreased flow rate as a
result of which the pressure becomes erratic. Air also tends to get trapped in the pump line due to a leak in the suction or on account of a damaged shaft seal. Additionally it has to be also ensured that air escapes through the breather while the fluid is in the reservoir or otherwise it tends to enter the pump suction line. To counter the phenomenon of cavitation in pumps, the following steps are recommended by manufacturers:
I. Suction velocities to be kept below 1.5 m/s (5 ft/s)
2. Pump inlet lines to be kept as short as possible
3. Pump to be mounted as close to the reservoir as possible
4. Low-pressure drop filters to be used in the suction line
5. Use of a properly designed reservoir that will help remove the trapped air in the fluid
6. Use of hydraulic fluid as recommended by the manufacturer
7. Maintaining the oil temperature within prescribed limits, i.e. around 65 °C or 150 °F.