JNTU B.Tech II MINE MECHANISATION-I Previous Question Paper (March - 2022 - R16)

Pneumatic and hydraulic systems are both essential for power transmission in mining machinery, but they differ significantly in their operational characteristics:

• Working Medium: Pneumatic systems use compressed air, which is readily available and clean, whereas hydraulic systems use pressurized incompressible fluids like oil.
• Pressure Range: Pneumatic systems operate at lower pressures (typically 6-10 bar), while hydraulic systems operate at much higher pressures (up to 350 bar or more), allowing for higher force output.
• Speed and Control: Pneumatic systems offer high speed and simple control but lack precision due to air compressibility. Hydraulic systems provide smooth, precise, and constant speed control.
• Safety: Pneumatic systems are inherently safer in explosive mining environments as they do not pose a fire risk. Hydraulic systems carry a fire risk if leaks occur near heat sources, requiring fire-resistant fluids.
• Efficiency: Hydraulic systems are generally more efficient for heavy-duty applications, while pneumatic systems suffer from energy losses during air compression.

Hydraulic fluids are critical for power transmission, lubrication, and heat dissipation. The main types include:

• Mineral-based oils: The most common type, offering excellent lubrication and corrosion resistance. However, they are highly flammable and unsuitable for high-fire-risk mining zones.
• Water-in-oil emulsions (Invert emulsions): Contain water droplets dispersed in oil. They provide better fire resistance than pure mineral oils but have lower lubrication properties.
• Water-glycol mixtures: Offer superior fire resistance and good low-temperature performance. They are widely used in underground mining but can be corrosive to certain metals.
• Synthetic fluids (Phosphate esters): Highly fire-resistant and stable at high temperatures. They are expensive and can be chemically aggressive toward seals and paints.

In comparison, mineral oils are preferred for general efficiency and cost, whereas water-based or synthetic fluids are mandatory in underground coal mines where fire safety is the primary concern.