For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to These pumps come with a straight spur, helical, or herringbone gears. . The drive shaft axis is angular in relation to the cylinder block axis. The unused fluid energy produced by the pump becomes heat that must be dissipated. Environmental factors can significantly affect gear-drive service life and maintenance costs. viscosity will decrease at a lesser rate than comparable fluids. Extreme Pressure (EP) mineral oils, in particular, contain additives. Once the system working fluid pressure . The formula below describes the ambient temperature and precharge pressure relationship to any . Gear Pump.
Hence, for high-temperature applications, the oxidation stability of an oil can have great significance and users should assess it carefully. The thermal stability of a fluid is its ability to resist decomposition due to temperature alone. It establishes the ultimate high-temperature limit for a tribological system fluid that will ensure continual unimpaired service. The most significant change in fluid properties caused by thermal decomposition of organic molecules is an increase in vapor pressure caused by the shearing of molecules into smaller, more volatile fragments.
Modern formulations of lubricating fluids contain vital additive packages to help the fluid satisfy essential operating functions. Unfortunately, high temperature operation can deplete all such additives, but especially rust inhibitors, foam depressants, antioxidants and antiwear ingredients. Another factor that deserves consideration in high-temperature operation is the resistance of the component materials to oxidation.
Positive displacement pumps
However, because the oxidation rate is accelerated at high temperatures and any film built-up in fluid components is exposed to cyclic stresses, the protective coat continually ruptures and flakes off. Thermal cycling also intensifies the situation by causing severe compressive stresses due to differences in the coefficients of thermal expansion of the film and the underlying material.
High fluid temperature can cause a chain reaction leading to total system destruction. High-temperature operation has a pronounced effect on the wear of all bearing type surfaces in a system.
Temperature Stability of Lubricants and Hydraulic Fluids
Engineers can evaluate this effect for a particular fluid by using the Gamma Wear Test System. Notice the impact of increasing temperature on gamma wear. After the system uses the same fluid for a significant length of time, the wear curves at the same three temperatures become seriously elevated see Figure 2. Close comparison of Figure 2 reveals depletion of the antiwear additive in the used oil, significantly reducing its effectiveness.
Also, the fluid viscosity may have been sheared down to the point where the lubrication film thickness has become totally inadequate to prevent asperity contact wear.
Heat Generation and Removal Heat cannot be created, only derived from some other form of energy. Fluid systems generally produce heat by converting mechanical energy or fluid pressure energy. Friction is the conversion process in a fluid type system. Because molecular friction generates heat in a sheared fluid, the higher the viscosity, the more heat this friction produces.
Many points in the system can add heat, particularly points with high frictional resistance. Good examples include such sources as bearings, fluid being pushed through orifices and various restrictions, and frictional drag on the fluid as it courses through restricted passages. The larger the pressure drop, the greater the amount of heat generated. Pressure-activated piston seals create high contact pressures to minimize internal leakage.
The result is that friction is high, thus creating a massive heat generator that elevates fluid temperature. Low-viscosity fluid can also contribute to heat generation because it inherently fails to maintain a crucial lubrication film between moving surfaces. This failure to separate the running surfaces results not only in wear abrasion and adhesion of the two surfaces but also in excessive leakage.
Both factors reduce the efficiency of the system and the lost energy is converted to heat. Engineers often overlook compression heating of aerated fluid as a heat source. The solution in this case is to reduce the amount of air entrained in the fluid.
Intense heat sources can be devastating to hydraulic systems required to operate in their immediate vicinity. A fluid system located near an external heat source or in a place where it cannot receive good ventilation must rely on some artificial means of dissipating system heat.
Such a situation is not only a heat source problem but also a heat dissipation problem. No matter how careful designers of fluid systems are, excessive heat generation sometimes occurs. If a machine like a hydraulic system has an overall efficiency of 80 percent, rough approximations would indicate that the amount of generated heat for an average fluid system is equal to 20 percent of the connected shaft power.Poiseuille's Law - Pressure Difference, Volume Flow Rate, Fluid Power Physics Problems
This heat must be dissipated to the surroundings in some way, otherwise the fluid temperature will keep rising until the system either stabilizes where the heat dissipated to the environment balances the heat generated by the system at some undesired elevated temperature or destroys itself.
The first avenue of escape from generated heat is by natural dissipation. With natural cooling, heat in the system fluid dissipates into the surrounding air, primarily by conduction and convection. All metal surfaces in contact with the fluid serve as heat-transfer surfaces.
If engineers design sufficient heat transfer surface area into the machine and expose the external surface to ambient air sufficiently cooler than the required system temperature, then much or all of the heat the system generates dissipates by natural cooling. Systems utilize heat exchangers or oil coolers to relieve the system fluid of excess heat and lower its operating temperature. Basically, the amount of heat a system must remove and transfer to a cooling medium is equal to the difference between the power input to the hydraulic pump and the power output of all the system actuators.
This implies that the ambient temperature is not adding more heat than is dissipating by natural cooling and that environmental conditions are not adding or subtracting heat from the fluid, which is seldom the case. Vicious Cycle of Thermal Failure Under extreme cold and hot environmental conditions, heat exchangers may be necessary primarily to counteract environmental conditions rather than to satisfy operating conditions needed to maintain the oil temperature within operable limits.
For example, when systems operate in northern climates, users frequently add heat to the fluid to decrease its viscosity. In hot climates or in systems operating near furnaces, users must subtract heat from the fluid to increase fluid viscosity and reduce the temperature.
Oil-to-water heat exchangers require a source of cold water and a means of disposing of the water after the system fluid warms it. This type of exchanger routes the less viscous fluid water through a bundle of tubes and the thick fluid hydraulic oil through the shell or housing. This type of heat exchanger requires some means to regulate the water flow, perhaps a valve controlled by a sensing element in the reservoir. The controller keeps the fluid temperature nearly constant, thus reducing the cyclic variation in performance and water consumption.
On mobile equipment, hydraulics or in other applications where water is not readily available, the use of oil-to-air heat exchangers with a suitable radiator and fan may prove a good choice. As a coolant, air offers several advantages over water. Piping and sewer charges are saved and air is unaffected by freezing weather. It can also be located on the machine without regard to water supply or sewer connections and is suitable for mobile equipment. Air-cooled heat exchangers require that oil temperatures be at least 10 to 15 degrees Fahrenheit above the cooling air temperature.
Oil-to-air heat exchangers are least effective when they are needed most, at high ambient temperatures where their efficiency is lowest.
Mechanical refrigeration systems have found broad application where the amount of space available and the heat generation are not compatible with either water-cooled or air-cooled heat exchangers. In addition, for those applications that require limited cooling for short periods, refrigeration type heat exchangers have proven particularly useful.
The refrigeration system employs a standard liquid chiller that combines the compressor, evaporator, condenser and circulating pump into a completely self-contained unit that is compact and portable. Some materials cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. Examples include liquid crystalsused in LCD displays, and biological membranes.
Applications Liquids have a variety of uses, as lubricants, solvents, and coolants. In hydraulic systems, liquid is used to transmit power.
In tribologyliquids are studied for their properties as lubricants. Lubricants such as oil are chosen for viscosity and flow characteristics that are suitable throughout the operating temperature range of the component.
Temperature Stability of Lubricants and Hydraulic Fluids
Oils are often used in engines, gear boxesmetalworkingand hydraulic systems for their good lubrication properties. Solutions are found in a wide variety of applications, including paintssealantsand adhesives.
Naphtha and acetone are used frequently in industry to clean oil, grease, and tar from parts and machinery. Body fluids are water based solutions.
Liquid - Wikipedia
Surfactants are commonly found in soaps and detergents. Solvents like alcohol are often used as antimicrobials. They are found in cosmetics, inksand liquid dye lasers. They are used in the food industry, in processes such as the extraction of vegetable oil.
The heat can be removed by channeling the liquid through a heat exchangersuch as a radiatoror the heat can be removed with the liquid during evaporation. During perspirationsweat removes heat from the human body by evaporating. In the heating, ventilation, and air-conditioning industry HVACliquids such as water are used to transfer heat from one area to another.
Devices such as pumps and waterwheels have been used to change liquid motion into mechanical work since ancient times. Oils are forced through hydraulic pumpswhich transmit this force to hydraulic cylinders. Hydraulics can be found in many applications, such as automotive brakes and transmissionsheavy equipmentand airplane control systems. Various hydraulic presses are used extensively in repair and manufacturing, for lifting, pressing, clamping and forming.
A thermometer often uses the thermal expansion of liquids, such as mercurycombined with their ability to flow to indicate temperature.
A manometer uses the weight of the liquid to indicate air pressure. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed. October Learn how and when to remove this template message Volume Quantities of liquids are measured in units of volume.
The volume of a quantity of liquid is fixed by its temperature and pressure. Liquids generally expand when heated, and contract when cooled. Liquids have little compressibility. Water, for example, will compress by only