Hydraulic fluid has to transmit pressure without compressing, lubricate moving parts, carry away heat and stay clean, and of those jobs cleanliness is the one that kills systems: a 25 μm particle is as big as the 20-50 μm clearance of a hand-built pump piston. Screen the oil at the fill port, filter the return line, keep air and water out, and treat every leak as a hazard, because a fine jet of oil under pressure injects through skin.
Part of Hydraulic Systems
The fluid is the working medium of a hydraulic system. Choosing it, keeping it clean and replacing it on schedule decide how long the pumps, valves and seals last.
What the Fluid Has to Do
- Transmit pressure without compressing. Its low compressibility is what makes the system stiff; see Hydraulic Principles.
- Lubricate the piston, spool and pump bearings.
- Seal the small clearances in pumps and valves by being thick enough not to leak out.
- Carry heat from the relief valve and the throttles back to the reservoir.
- Not attack the seals, which is why the choice of oil and the choice of rubber go together; see Available Fluids.
A thin oil leaks past clearances and a thick oil resists flow and starves the pump on cold starts. The corpus gives no viscosity figure for hand-built systems, so test the oil in your own pump at your own working temperatures.
Contamination Is the Main Killer
The carrier calls contamination the number one killer of hydraulic systems and says particles as small as 25 μm cause seal wear and valve sticking. It matters because a 25 μm particle is the same size as the clearance between a piston and its bore (0.02-0.05 mm, or 20-50 μm, per the pump article in Single-Acting Pump), and as the clearance of a spool in its bore. A grain that size scores the bore, lifts a check valve off its seat or jams a spool.
The carrier gives three stages of filtration:
| Stage | Where | Filters down to |
|---|---|---|
| Screen | Reservoir fill port | 100 mesh, 150 μm |
| Felt element | Return line | 25-50 μm |
| Wound cotton or paper | Pressure line | 10 μm |
A filter in the pressure line must be rated for the full working pressure. No filter rating for hand-made elements is sourced here, so inspect and clean them on a schedule and check whether the oil on the filter is darkening.
Keep Air and Water Out
- Air compresses, so a bubble makes the system spongy and jerky. Fill through a screen, keep the suction line below the oil level, and bleed the high points.
- Water corrodes parts and, in a cold spell, freezes. Milky oil means water; replace it. Keep the reservoir covered, with a breather that filters air.
Change and Dispose Properly
The carrier suggests changing filtered vegetable oil every 6-12 months. Do not mix different oils or fluids without flushing, because additives and base fluids can react and some fluids attack seals made for others. Oil that has been in service is dirty and may be hot; do not pour it into water or soil.
Common Mistakes
- Pouring oil from a dirty can. The pump eats the grit.
- A reservoir with the return line above the oil. The falling stream whips in air.
- Leaving the reservoir open. Dust and rain get in.
- Mixing brake fluid, transmission fluid and vegetable oil because the corpus calls them all “hydraulic fluid”; see the seal compatibility notes in Available Fluids.
Safety
A pinhole jet of oil injects through skin, and oil burns
Oil leaking from a pressurised line can leave a fine jet too thin to see, and it can penetrate skin and inject oil into the tissue. The entry wound looks trivial and may not hurt, but this is a surgical emergency that can cost the limb. Never search for a leak with a hand. Pass a piece of card or wood across the suspect fitting and look for oil on it, with the pressure low and everyone out of line of the jet. Anyone who might have been hit needs urgent surgical care at once; do not wait for pain or swelling and do not try to squeeze the oil out. Hydraulic oil burns: a spray from a burst line near a forge or stove can ignite, so keep sand or an extinguisher nearby and fit a guard over lines close to heat.
What’s Next
- Learn next: Available Fluids — mineral, vegetable and water-glycol fluids and what each does to seals
- Combine with: Seal Systems — the rubber and leather the fluid touches