A hydraulic seal holds only if the right type is matched to the right fluid and the right groove: leather cups seal harder as pressure rises, an O-ring seals by squeeze, and a stack of V-shaped chevron rings can be tightened in service. The corpus’s two O-ring groove rules disagree, giving a squeeze of 13% from a depth of 0.87 times the cord diameter and 23-24% from a table near 0.75 times it, and natural rubber swells in oil. Find leaks with card or wood and never with a hand.
Part of Hydraulic Systems
Seals keep oil in the cylinder and the pump and dirt out. They are the part that fails first and the one that decides whether a hydraulic system is usable.
Match the Seal to the Job
- Leather cup seals: the oldest and most rebuildable type. Cut thick oil-tanned leather (4-6 mm vegetable-tanned cowhide per Gaskets and Seals), form it into a cup with the lip angled outward about 30° and soak it in oil for 24 hours before fitting. The lip faces the pressure, so the pressure pushes the lip harder against the wall; the seal improves as the load rises.
- O-rings: a ring of round-section rubber in a groove, compressed by the mating surface. They suit static seals and short moving seals.
- Chevron packing: a stack of V-section rings (leather, or fabric-reinforced rubber) between a male and a female adapter, inside a gland that can be tightened. It is adjustable in service.
- Wipers: a lip on the rod end that scrapes grit off before it enters the bore.
The Fluid Chooses the Rubber
The corpus lists natural rubber as swelling in oils (Hoses and Tubing, where natural rubber for oil lines is a listed mistake). Oil-soaked leather does not have that problem, which is one reason the old cup seal is the safe choice with oil. If a salvaged seal came out of a machine that ran brake fluid and you fill it with mineral oil, or the reverse, the rubber may swell and soften or shrink and harden; see Available Fluids. Match every seal to the fluid it will sit in, and test with the fluid before loading.
O-Ring Groove Depth Sets the Squeeze
The squeeze is the fraction of the ring’s cross-section that the groove takes: squeeze = 1 - groove depth / cross-section. The corpus gives two rules that do not agree.
| Source | Groove depth | Squeeze, 3 mm cord | Squeeze, 5 mm cord |
|---|---|---|---|
| Hydraulic Systems carrier | 0.87 x cross-section | 13% (2.61 mm deep) | 13% (4.35 mm deep) |
| Gaskets and Seals table | about 0.75-0.77 x cross-section (2.3 mm for 3 mm, 3.8 mm for 5 mm) | 23% | 24% |
The gaskets article says to aim for a squeeze of 10-15% when fitting a ring, which matches the carrier’s 0.87 rule and contradicts its own table of 0.75. Both can be sound in different uses, since static and moving seals are squeezed differently in common practice, but neither article says so and no source for the split is available here. Settle it on the bench: make the groove, fit the ring, move the part and look for weeping, and choose the lower squeeze for sliding seals.
Make the ring right. The gaskets article makes rings from cord with a spliced joint. A spliced ring has a weak point, and no pressure rating for it is sourced here; use it at low pressure and prefer a moulded ring for megapascals. The groove also needs room for the ring to move and a close clearance behind it, because at high pressure rubber extrudes into the gap. A hard backup ring (wood or metal, as in the gaskets article) on the low-pressure side blocks the gap.
Test Seals Without Using Your Hand
- Fit the seal, bleed the system and raise pressure in steps, watching the gauge.
- Look for a pressure that falls when the pump stops, a cylinder that drifts under load, or oil weeping at a rod.
- To find a leak at a joint, hold a piece of card or wood across the joint and look for oil on it. Never search for a leak with a hand.
Diagnosis by symptom: weeping at the rod points to a scored rod or a worn rod seal; drift under load with no external leak points to a leaking piston seal or check valve; a seal torn at one spot points to a sharp groove edge or an extruded ring.
Common Mistakes
- Using a seal whose rubber is wrong for the oil. It swells, softens and tears.
- A rough bore or a sharp groove edge. It shreds the seal; chamfer the edges and finish the bore (Hydraulic Cylinders).
- Over-tightening packing. It seals but generates heat and wears the rod.
- Dirt in the oil. Grit scores the bore and the seal wears out fast.
Safety
A failed seal at pressure injects oil through skin
A seal that fails at pressure can send a fine jet of oil through a pinhole in a fitting or between parts, too thin to see, and oil can pass through skin and into the tissue. The wound looks trivial, but it is a surgical emergency that can cost the limb. Never search for a leak with a hand; use a piece of card or wood and keep clear of the line of the jet. Relieve pressure before touching a seal, and block any load mechanically before opening a cylinder. Anyone who may have been hit needs urgent surgical care at once.
What’s Next
- Learn next: Control Valves — directing and limiting the oil the seals hold
- Combine with: Rubber Processing — making the rubber for seals