Which hydraulic oil is suitable for use in lifting machinery?

Contamination and improper selection of hydraulic oil are among the key factors causing failures in the hydraulic systems of lifting machinery. Therefore, choosing the right hydraulic oil appropriately is crucial for enhancing the operational reliability and economic efficiency of crane hydraulic equipment, extending the service life of both the system and its components, ensuring safe operation of the equipment, and preventing potential accidents.

Hydraulic Oil Usage Requirements for Lifting Machinery

1. Proper Viscosity: Viscosity is one of the key factors that must be considered in the hydraulic systems of lifting machinery. It serves as an indicator of how easily an oil flows, reflecting the degree of internal friction between its molecules. If the viscosity is too high, it increases resistance to fluid flow within the pipelines, leading to greater energy losses during operation, heightened energy dissipation under no-load conditions, and elevated system temperatures—potentially even causing "cavitation" at the suction side of the main pump. Conversely, if the viscosity is too low, it fails to provide adequate lubrication for mechanical components, accelerating wear on parts and increasing system leakage, which in turn reduces the pump’s volumetric efficiency.

2. Excellent Viscosity-Temperature Characteristics: Viscosity-temperature behavior refers to the degree to which a fluid’s viscosity changes with temperature fluctuations, typically measured by the viscosity index. The higher the viscosity index, the less the oil’s viscosity decreases as temperature rises during operation, ensuring that internal leakage within the system remains minimal.

3. Excellent antioxidant and hydrolytic stability: The ideal operating temperature range for most hydraulic oils is between 30°C and 80°C, as the lifespan of hydraulic oil is closely tied to its working temperature. Once the oil temperature exceeds 60°C, for every additional 8°C increase, the oil’s service life is halved. For instance, at 90°C, the oil’s lifespan drops to roughly 10% of what it is at 60°C—this decline is primarily due to oxidation. Under atmospheric pressure, every liter of oil contains slightly less than 0.1 liters of air. In fact, oxygen is always present, reacting with the hydrocarbons in the oil and gradually causing it to oxidize. Over time, this leads to a darkening of the oil’s color and an increase in its viscosity. Eventually, the oxidation products no longer dissolve in the oil but instead accumulate as a brown, viscous layer somewhere within the system. This can result in valve sticking, increased friction between components like ball bearings, spool valves, and hydraulic pump pistons, and even accelerate corrosion as acidic byproducts form, ultimately damaging critical hydraulic components.

4. Flame Resistance and Shear Stability: To improve the viscosity index of hydraulic fluids, high-molecular-weight polymers such as polymethyl methacrylate and polyisobutylene are often added. These substances have relatively long molecular chains that experience significant shear forces as they flow through the narrow gaps within hydraulic components, frequently leading to chain scission and a subsequent decline in the fluid’s viscosity-temperature characteristics. During operation of lifting machinery, the frequent directional changes performed by directional control valves and the throttling actions carried out by flow-control elements both demand that hydraulic oils exhibit robust shear resistance.

5. Compatibility with Sealing Materials and the Environment: Hydraulic oil can cause swelling, softening, or hardening in sealing components it comes into contact with, ultimately compromising the sealing material's ability to maintain a tight seal. Additionally, hydraulic systems may experience leaks or sealing failures, leading to hydraulic oil escaping into the environment. If the hydraulic oil is incompatible with its surroundings, it could result in significant environmental contamination.

6. Excellent wear resistance and lubricity: The goal is to reduce mechanical friction and ensure the service life of the main unit.

7. Good fire resistance: Hydraulic oil should have a high flash point, ignition point, and auto-ignition temperature.