Hydraulic Reservoirs and Filtration

Reservoirs

  • Two main types:
    • Vented (atmospheric)
    • Pressurized (closed)

Vented Reservoirs

  • Air moves in and out as oil heats/cools.
  • Preventative Maintenance (PM) services involve breathers and filters on tanks/dips.
  • These breathers/filters must be maintained to aid cooling in the vented reservoir.

Closed/Pressurized Reservoirs

  • When oil heats and expands, air in the reservoir compresses.
  • This results in different oil levels for cold and hot conditions.
  • Equipment may have two sight glasses indicating hot and cold levels.
  • When the machine is hot, fill the oil to the "hot" level.
  • Steering tanks with accumulators often have running and shutoff levels.
  • Purpose: to accommodate accumulator volume changes.
  • If the tank is filled to the top when the accumulators are full and the machine is shut off, oil may overflow when the accumulators bleed down.
  • This overflow typically occurs through the breather or, in extreme cases, can damage the tank.
  • Pressurized reservoirs prevent pump cavitation by ensuring the inlet is always fed.
  • Pressure caps limit maximum reservoir pressure (10-15 psi), functioning like radiator caps.
  • They allow air to be drawn in as oil cools and expands.

Pressure Cap

  • The pressure cap maintains reservoir pressure.
  • Pressurized reservoirs can use an onboard air system.
  • Always depressurize a reservoir before opening it.

Oil Level Indicators

  • Some manufacturers provide oil level indicators.
  • These prevent large particles from entering the tank during filling.

Line Location

  • Aids in oil cooling, deaeration, and sedimentation.
  • Return lines are typically angled at 45 degrees to redirect oil flow.
  • This reduces heat and allows for better segmentation.

Baffles

  • Minimize oil sloshing, especially in off-road equipment.
  • The 45-degree return line directs fluid outward, avoiding turbulence at the pump inlet.
  • This setup cools the oil and separates air before it reaches the inlet.

Filtration Devices

  • Mesh screens, strainers, and filters are used.
  • Micron rating is a key term related to filter size.
  • Internal clearances in modern hydraulic systems typically range from 5 to 20 microns.
  • 1 micron=1 millionth of a meter1 \text{ micron} = 1 \text{ millionth of a meter}

Contamination

  • Built-in contamination: present from the factory.
  • Ingress: contamination introduced during servicing.
  • Internal wear particles larger than internal clearances can block flow.
  • These particles can cause wear and damage to internal parts.
  • Use proper caps, plugs, and dead ends when disconnecting lines.
  • Avoid using rags to plug hoses, as they can introduce contamination.
  • Tape or zip ties with baggies are better alternatives.

Screen Sizes

  • Screens trap fine particles to prevent contamination.
  • Gear pumps are more tolerant of contamination than piston or vane pumps.
  • Contamination can quickly seize up pistons in piston-style pumps, leading to catastrophic failure.

Particle Sizes

  • Two-micron particles are invisible to the naked eye.
  • Table salt: 100 microns100 \text{ microns}
  • Strand of hair: 70 microns70 \text{ microns}
  • Smallest visible particle: 40 microns40 \text{ microns}
  • Calcium powder: 10 microns10 \text{ microns}
  • Eliminating contamination is critical for hydraulic systems.

Filters

  • Typically have pleated media around a center tube.
  • Strainers and pleated paper filters remove contamination from the oil flow.
  • Filters using nonwoven inorganic fibers have no distinctive size to the opening in the filter media, and they will remove smaller particles.

Beta Rating

  • Compares the number of particles of a given size that enter a filter with the number that come out.
  • The higher the beta rating, the more efficient the filter is at removing particles of a specific size.

Strainers

  • A 200 mesh strainer is often located in the reservoir filler opening.
  • High-pressure filters can be placed after the pump to protect the system.

Control Valves

  • Spring centered.
  • Open center (back to tank).
  • Three positions.
  • Four ways.
  • Double acting cylinder.

Filter Bypass Operation

  • Maintains oil flow if oil viscosity is too high or if the filter is plugged.
  • Prevents damage to the oil pump.
  • A restriction indicator pressure gauge or pressure switch in the cab can signal a plugged filter.