This may seem to contradict the points made in previous Best Practice articles, but it actually aligns with them. About one-fifth of a hydraulic motor’s energy is lost to heat, which is significantly influenced by fluid properties impacting mechanical and volumetric efficiency. Thus, energy efficiency becomes a crucial focus.
Even if mechanical components run continuously, if the pump’s overall efficiency is only 65%, then 35% of the energy used to drive the pump is wasted. The cost of energy losses due to poor efficiency can easily surpass the cost associated with poor long-term component reliability. Nonetheless, the condition of components does affect overall system efficiency.
This article primarily discusses selecting fluids that maximize overall efficiency. Future articles will address system condition and contamination control. This month, we’ll cover:
– Explanation of system efficiency
– System fluid requirements for optimum efficiency
– Hydraulic fluid performance criteria:
– Viscosity
– Viscosity index
– Cold flow limits
– Fluid additive classification (R&O, AW)
– Selection of the best-fit fluid option based on known performance criteria
Hydraulic System Efficiency
Car Lift Repair Tampa Florida Hydraulic pump efficiency is crucial for the performance of hydraulic systems. If operating conditions, including fluid viscosity, do not allow the pump to function at optimal efficiency, the driving motor must work harder to compensate for system losses. Systems with fixed-capacity electric motors can become sluggish with poor control and response. In the case of a combustion engine, more fuel must be burned to maintain output, exacerbating costs. This is particularly impactful for engine-driven hydraulic systems used in outdoor construction and mining machines, where operating cost premiums can be substantial.
Overall efficiency is the result of both mechanical efficiency and volumetric efficiency.
– **Car Lift Repair Tampa Florida Mechanical efficiency** depends on frictional losses and the energy needed to overcome these losses, which come from metal surfaces rubbing against each other and fluid friction (resistance to flow). In a normal operating state, where oil flows continuously through the pipes, most frictional resistance arises from fluid friction.
– **Volumetric efficiency** depends on the flow losses within the system. Both types of efficiency are influenced by fluid viscosity.
Mechanical efficiency improves as oil viscosity decreases (the oil becomes thinner). In contrast, volumetric efficiency improves as oil viscosity increases. Therefore, a trade-off exists between mechanical and volumetric efficiency, since optimal conditions for one often contradict those for the other.
Car Lift Repair Tampa Florida System designers aim to establish and maintain an optimal viscosity range, encouraging users to operate within this range as much as possible. As volumetric efficiency increases, mechanical efficiency decreases, necessitating a balance between the two, as illustrated in Figure 1.
If a Car Lift Repair Tampa Florida system’s operating temperature could be kept constant throughout its operational cycles, the system would achieve peak efficiency. However, any factor that affects the oil’s operating temperature in the sump also affects overall system efficiency. Changes in temperature, and consequently viscosity, present a challenge. Operating temperature is influenced by various routine factors, including:
– Changing ambient temperatures (cold and high atmospheric temperatures)
– Changing process temperatures
– Changes in load-state
– Changes in the amount of air and water vapor in the oil
– Changes in the mechanical condition of system components
OEM Viscosity Recommendations
Car Lift Repair Tampa Florida Hydraulic system component suppliers provide extensive information on the safe operating limits of system components, with a strong emphasis on viscosity and system performance limits imposed by the pump. Valves, cylinders, motors, actuators, and other components also impact overall system function and should not be overlooked. Viscosity limits for system pumps are a key focus in new system installations and vary based on pump type (gear, vane, axial piston, radial piston) and operating speed.
Figure 2 shows the viscosity ranges, maximum viscosities, and target efficiency values for various common pump models. Figure 3 summarizes the viscosity parameters for four common component types.
As shown in Figure 3, the viscometric requirements for various applications reveal that the optimum viscosities, maximum startup, and possible working ranges are more alike than different.
Lubricant Properties
A reliability engineer should be familiar with several key lubricant properties and understand how to apply them when selecting a new hydraulic oil. These properties include:
– **Viscosity**: This is a fluid’s internal resistance to flow and is the most crucial characteristic when selecting a lubricant, especially for hydraulic applications. The most widely used viscosity measurements are kinematic units, designated in centistokes (cSt) and centipoises (cPs). Centistokes represent resistance to flow due to gravity, while centipoises represent resistance to flow under dynamic force. Heavier oils take longer to complete viscosity tests than lighter oils. For instance, an oil with a viscosity of 100 cSt takes longer to pass the test than one with 10 cSt.
Figure 4 illustrates the 18 viscosity grade range values according to ISO Standard 3448, which will be discussed in detail in a future article.
Centipoise units better represent hydraulic system operating characteristics. Some lubricant suppliers provide both centipoise and centistokes values, while others do not. To estimate centipoise values, multiply the centistokes value at 40°C by the lubricant’s specific gravity. For example, an oil with 100 cSt and a specific gravity of 0.85 has a dynamic viscosity of 85 cPs (100 x 0.85). The charts later in the article use centistokes increments, but these can be converted to centipoise values.
– **Viscosity Index (VI)**: This number indicates how much an oil’s thickness changes with temperature. Higher VI values mean the oil will perform effectively over a wide temperature range, maintaining efficient system performance and safe startup at cold temperatures.
Car Lift Repair Tampa Florida Mineral oil-based lubricants typically have VI values between 95 (API Group I) and 130 (API Group III), with some high-performance products achieving better results. High VI products are desirable as they allow for the selection of a thinner oil while providing an additional margin of surface protection for components operating under extreme temperatures.
The pour point of Car Lift Repair Tampa Florida hydraulic fluid refers to the lowest temperature at which the oil can flow after being cooled under controlled conditions. This limit can be influenced by the formation of wax crystals as temperatures drop or by the thickening of viscosity to a point where flow ceases entirely. Additives can be used to improve the pour point of oils containing waxy components left over from refining processes. These additives interfere with wax crystal formation, thus lowering the pour point temperature. However, oils that are already wax-free do not benefit from such additives.