As a pump wears in service, internal leakage increases and, therefore, the percentage of flow available to do useful work (volumetric efficiency) decreases.
If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.
In a condition-based maintenance environment, the decision to change-out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.
Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. It is calculated by dividing the pump’s actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.
Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when stating volumetric efficiency.
For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS.
When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant.
To understand why this is so, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump’s displacement.
Mistake No. 1 Changing the Oil
There are only two conditions that mandate a hydraulic oil change: degradation of the base oil or depletion of the additive package. Because there are so many variables that determine the rate at which oil degrades and additives get used up, changing hydraulic oil based on hours in service, without any reference to the actual condition of the oil, is like shooting in the dark.
Given the current high price of oil, dumping oil which doesn’t need to be changed is the last thing you want to do. On the other hand, if you continue to operate with the base oil degraded or additives depleted, you compromise the service life of every other component in the hydraulic system. The only way to know when the oil needs to be changed is through oil analysis.
Mistake No. 2 Changing the Filters
A similar situation applies to hydraulic filters. If you change them based on schedule, you’re changing them either too early or too late. If you change them early, before all their dirt-holding capacity is used up, you’re wasting money on unnecessary filter changes. If you change them late, after the filter has gone on bypass, the increase in particles in the oil quietly reduces the service life of every component in the hydraulic system costing a lot more in the long run.
The solution is to change your filters when all their dirt-holding capacity is used up, but before the bypass valve opens. This requires a mechanism to monitor the restriction to flow (pressure drop) across the filter element and alert you when this point is reached. A clogging indicator is the crudest form of this device. A better solution is continuous monitoring of pressure drop across the filter.
Mistake No. 3 Running too Hot
Few equipment owners or operators continue to operate an engine that is overheating. Unfortunately, the same cannot be said when the hydraulic system gets too hot. But like an engine, the fastest way to destroy hydraulic components, seals, hoses, and the oil itself is a high-temperature operation.
How hot is too hot for a hydraulic system? It depends mainly on the viscosity and viscosity index (rate of change in viscosity with temperature) of the oil, and the type of hydraulic components in the system.
As the oil’s temperature increases, its viscosity decreases. Therefore, a hydraulic system is operating too hot when it reaches the temperature at which oil viscosity falls below that required for adequate lubrication.
A vane pump requires a higher minimum viscosity than a piston pump, for example. This is why the type of components used in the system also influences its safe maximum operating temperature.
Apart from the issue of adequate lubrication, the importance of which cannot be overstated, operating temperatures above 82 degrees Celsius damage most seal and hose compounds and accelerate degradation of the oil. But for the reasons already explained, a hydraulic system can be running too hot well below this temperature.
Mistake No. 4 Using the Wrong Oil
The oil is the most important component of any hydraulic system. Not only is hydraulic oil a lubricant, it is also the means by which power is transferred throughout the hydraulic system. It’s this dual role which makes viscosity the most important property of the oil, because it affects both machine performance and service life.
Oil viscosity largely determines the maximum and minimum oil temperatures within which the hydraulic system can safely operate. If you use oil with a viscosity that’s too high for the climate in which the machine must operate, the oil won’t flow properly or lubricate adequately during cold start. If you use oil with a viscosity too low for the prevailing climate, it won’t maintain the required minimum viscosity, and therefore adequate lubrication, on the hottest days of the year.
But that’s not the end of it. Within the allowable extremes of viscosity required for adequate lubrication, there is a narrower viscosity band where power losses are minimized. If operating oil viscosity is higher than ideal, more power is lost to fluid friction. If operating viscosity is lower than ideal, more power is lost to friction and internal leakage.
Using the wrong viscosity oil not only results in lubrication damage and premature failure of major components, it also increases power consumption (diesel or electricity) two things you don’t want.
And despite what you might think, you won’t necessarily get the correct viscosity oil by blindly following the blanket recommendations of the machine manufacturer.
Mistake No. 5 Wrong Filter Locations
Any filter is a good filter, right? Wrong! There are two hydraulic filter locations that do more harm than good and can rapidly destroy the very components they were installed to protect. These filter locations which should be avoided are the pump inlet and drain lines from the housings of piston pumps and motors.
This contradicts conventional wisdom: that it is necessary to have a strainer on the pump inlet to protect it from “trash”. First, the pump draws its oil from a dedicated reservoir, not a garbage can. Second, if you believe it’s normal or acceptable for trash to get into the hydraulic tank, then you’re probably wasting your time reading this article.
If getting maximum pump life is your primary concern (and it should be), then it’s far more important for the oil to freely and completely fill the pumping chambers during every intake than it is to protect the pump from nuts, bolts and 9/16-inch combination spanners. These pose no danger in a properly designed reservoir where the pump inlet penetration is a least 2 inches off the bottom.
Research has shown that a restricted intake can reduce the service life of a gear pump by 56 percent. And, it’s worse for vane and piston pumps because these designs are less able to withstand the vacuum-induced forces caused by a restricted intake. Hydraulic pumps are not designed to “suck”.
A different set of problems arises from filters installed on the drain lines of piston pumps and motors, but the result is the same as suction strainers. They can reduce service life and cause catastrophic failures in these high-priced components.
Mistake No. 6 Believing Hydraulic Components are Self-Priming and Self-Lubricating
You wouldn’t start an engine without oil in the crankcase not knowingly, anyway. And yet, I’ve seen the same thing happen to a lot of high-priced hydraulic components.
The fact is, if the right steps aren’t followed during initial start-up, hydraulic components can be seriously damaged. In some cases, they may work OK for a while, but the harm incurred at start-up then dooms them to premature failure.
There are two parts to getting this dilemma right: knowing what to do and remembering to do it. Not knowing what to do is one thing. However if you do know, but forget to do it, that’s soul-destroying. You can’t pat yourself on the back for filling the pump housing with clean oil when you forgot to open the intake isolation valve before starting the engine!
Hydraulic seals are available in a wide variety of orientations and different types. The basic types of hydraulic seals that are used in industrial products are rod seals, flange packing, and U-cups.
When purchasing hydraulic seals, it is necessary to consider the following:
Orientations of Hydraulic Seals
For the various industrial products, the sealing directions are different, which we are discussing below:
Hydraulic Seals Types
Basically they are of two types.
I got a HUGE response from sending out the cheat sheets, many people printing them off for quick reference for when they need them.
I’m glad they were helpful!
How to reduce noise emission from hydraulic machines
It is to be noted that in many industrialized nations, there are rules and regulations that restrict noise levels in the factories and workplace. The regular activities in the industrial areas using hydraulic machines and the resulting high noise emission of hydraulic components means that it is warning the machine operator to do something to reduce the noise of the machine in the working area. To do that you should know, “what exactly is the cause of the noise?”
Three Main Causes
Fluid Borne Noise
Structure Borne Noise
Air Borne Noise
The dominant cause of noise in hydraulic systems is the pump. The hydraulic pump produces fluid-borne noise and structure-borne noise into the system and radiates air-borne noise. All hydraulic pumps have a fixed number of pumping chambers, which operate in a continuous cycle like as opening closing of fluid inflow. The continuous process leads to a corresponding sequence of pressure pulsations, which cause the fluid-borne noise. This results in the downstream components to vibrate. The structure-borne noise is produced by exciting vibration in any component. The transfer of fluid and structure induced vibration to the nearby air mass results in air-borne noise.
How to reduce fluid-borne noise?
While the main cause of fluid-borne noise is pressure pulsation, it can be reduced through hydraulic pump design, though the problem cannot be fully eliminated. In large hydraulic systems or noise-sensitive applications, the fluid-borne noise emission can be reduced by the installing a silencer. The most simple form of silencer is the reflection silencer. This is widely used in hydraulic systems and it reduces sound waves by inducing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.
How To reduce structure-borne noise?
The structure-borne noise is caused by the vibrating mass of the power unit (this includes the hydraulic pump and its prime mover). This can be reduced through the elimination of sound bridges between the the power unit and valves and the power unit and tank. This is normally attained by using flexible connections like rubber mounting blocks and flexible hoses. In some cases it is necessary to introduce additional mass, the force of which reduces the transmission of vibration at bridging points.
How to reduce air-borne noise?
The force of noise radiation from an object is proportional to its area. This force is inversely proportional to its mass. Hence to reduce air borne noise, you can reducing an object’s surface area or increase its mass. For example, build the hydraulic reservoir from thicker plate. The magnitude of air-borne noise created directly from the hydraulic pump can be reduced by mounting the pump inside the tank. If the noise from hydraulic system remains outside the required level even after all the above noise reduction measures have been tried, encapsulation or screening must be considered.
Hydraulic Fluid Energy Storage
Another cause of noise in hydraulic systems is due to the storage and subsequent release of energy in the hydraulic fluid. Hydraulic fluid is not perfectly rigid. When the fluid is compressed, it results in energy storage. However if this compression is not properly controlled, the stored energy dissipates instantaneously. This sudden release of energy moves the fluid very fast, and this creates noise. So while handling hydraulic system, it is necessary to control the energy storage in hydraulic fluid.
Click on the links below to get cheat sheets 5 and 6 of hydraulic symbols. Print them off and use them for reference.
Cheat sheets 5 and 6 are lists of hydraulic symbols like valve hydraulic symbol, solenoid valve symbol, directional valve symbol, servo valve symbol, electric motor symbol, lubricator hydraulic symbol, gauge hydraulic symbol, indicator hydraulic symbol, thermometer hydraulic symbol, thermostat hydraulic symbol, silencer hydraulic symbol, cooler hydraulic symbol, filter hydraulic symbol, heater hydraulic symbol, level gauge hydraulic symbol, flow meter hydraulic symbol, etc.
Click on the links below to see the Working Line-Pressure/Return Hydraulic symbols, Hydraulic Cylinder Symbols, Hydraulic Motors Symbols etc.
Click on the links below to get cheat sheets 3 and 4 of hydraulic symbols. Print them off and use them for reference.
Cheats sheets part 3 and 4 are hydraulic symbols or schematic symbols of hydraulic valves, valves control hydraulic symbols, electric motor hydraulic symbols, reservoir hydraulic symbols, directional valve hydraulic symbols, filter, electro hydraulic servo symbols etc.
Click on the links below to see the Working Line-Pressure/Return Hydraulic symbols, Hydraulic Cylinder Symbols, Hydraulic Motors Symbols etc.
Click on the links below to get 2 cheat sheets of hydraulic symbols. Print them off and use them for reference.
These cheat sheets have a list of common hydraulic symbols and hydraulic schematic diagrams which are useful for reading, understanding and interpreting hydraulic schematics and circuit drawings. Click on the links below to see the Working Line-Pressure/Return Hydraulic symbols, Hydraulic Cylinder Symbols, Hydraulic Motors Symbols etc.
Next week I’ll put together another list of symbols you can print off and use as reference.
The ‘built-in’ inefficiency of every hydraulic system:
Compression of the oil.
A fluid’s compressibility is defined by its bulk modulus of elasticity – which is the opposite of compressibility. Meaning, as the bulk modulus of elasticity increases, compressibility decreases.
Bulk modulus is an inherent property of the oil and therefore an inherent inefficiency of a hydraulic system.
The fluid in the pipeline and actuator must be pressurized, and consequently compressed, before it will move a load.
Because this compression of the fluid requires work at the input – which cannot be converted to useful work at the output – it is lost work and therefore a contributing factor to the overall inefficiency of the hydraulic system.
The larger the actuator and the faster the response time, the higher the inefficiency attributable to bulk modulus.
And in high-performance, closed-loop electro-hydraulic systems, deforming oil volumes affect dynamic response, causing possible stability problems such as self-oscillation.
Unlike viscosity index, bulk modulus cannot be improved with additives. However,hydraulic equipment users can take steps to minimize the inefficiencies and potential control problems associated with compression of the fluid.
The first is to ensure hydraulic equipment doesn’t run hot.
Compressibility of the fluid increases with temperature. Mineral hydraulic oil is approximately 30 percent more compressible at 100°C than it is at 20°C.
Of course, there are many reasons why you should never allow hydraulic equipment to run hot – most of which we’ve already discussed. Reduced bulk modulus is another one.
The second is to prevent conditions that cause aeration.
Air is 10,000 times more compressible than oil. One percent of entrained air by volume can reduce the bulk modulus of oil by as much as 75 percent.
While controlling aeration is largely a design issue – for example, the amount of dwell time the oil has in the tank – proper maintenance also plays an important role.
Dissolved air comes out of solution as temperature increases, which is another reason to maintain appropriate and stable operating temperatures.
Also, oxidative degradation and water contamination inhibit the oil’s ability to release air, often resulting in an increase in entrained air and thus compressibility.
It’s a simple problem really. If you understand percentages – and I know you do, then overheating problems are easy to get a handle on.
You see, it’s a balancing act between the percentage of input power lost to heat and the percentage of heat dissipated by the system – mainly the heat exchanger.
Take a system with an input power of 100 kilowatts.
If it’s 80 percent efficient, it’s creating 20 kilowatts of heat load.
If the exchanger is dissipating 20 kilowatts of heat then a stable operating temperature will be maintained.
If the system starts to overheat, then either:
It’s that simple.
When a hydrostatic transmission is subject to a sudden increase in load, the motor stalls momentarily and system pressure increases until the increased load is overcome or the high pressure relief valve opens – whichever occurs first.
While the motor is stalled, there is no return flow from the outlet of the motor to the inlet of the pump. This means that the transmission pump will cavitate for as long as it takes to make up the volume of fluid required to develop the pressure needed to overcome the increased load (or the high-pressure relief valve).
How long the pump cavitates depends on the output of the charge pump, the magnitude of the pressure increase, its influence on the increase in volume of the pipe or hose, and the decrease in volume of the fluid.
This is called the ‘accumulator effect’.
One way to minimize stalling and pressure spikes and the resulting ‘accumulator effect’ in applications where the load on the transmission varies – in say drill rigs for example, is to install a flywheel between the hydraulic motor and reduction box.
The stored energy in the flywheel assists the hydrostatic drive to maintain speed and torque, and minimize the magnitude of pressure fluctuations resulting from sudden increases in load.