Flat gaskets are cut from vulcanised sheet 1-3 mm thick with a sharp punch or knife and fitted to flange joints; O-rings are compression-moulded in grooved steel moulds, with standard cross-sections of 2, 3 and 5 mm and a range of inner diameters kept in stock; cup seals are moulded rubber for hydraulic pistons. Natural rubber must not be used where it will touch oils or petroleum, because it swells and softens in them.
Part of Rubber Processing
The three sealing shapes made from rubber and what each is used for.
Each Seal Shape Does A Different Job
The corpus’s chemistry Gaskets and Seals article covers the cutting, bolt tightening and gland packing in detail, and gives the shape table from which this one is drawn.
| Seal | Where used | Made by |
|---|---|---|
| Flat gasket | Flange joints, covers | Cut from 1-3 mm sheet |
| O-ring | Piston and shaft seals in a groove | Compression moulding or cord |
| Cup seal | Hydraulic pistons | Moulded rubber |
For the O-ring groove depth and squeeze, use the chemistry article. It gives a groove depth of about 75% of the ring’s cross-section, which would compress the ring by about 25% (100 - 75), and also a fit with 10-15% squeeze. Those do not agree and the article does not reconcile them, so test every new ring and groove on a trial fit.
What You Need
- Vulcanised sheet of 1-3 mm, from a batch that passed Quality Testing.
- Punches and a knife, and a flat surface to cut on.
- O-ring moulds, per Compression Molding.
- Rubber cord and raw latex to join cut cord into rings, as the chemistry article describes.
- A drawing or a template of every flange and groove.
Method: A Flat Gasket
Step 1: Trace the flange and bolt holes onto the sheet.
Step 2: Cut the shape and punch the bolt holes.
Step 3: Fit between flanges and tighten the bolts in a star pattern; the chemistry article says to re-check after 24 hours as gaskets settle.
Step 4: Check for seepage after the first pressure cycle.
For O-rings and cup seals, mould or cut to the groove and fit with a thin smear of lubricant.
Worked Example
Stock plan for O-rings. With three standard cross-sections (2, 3 and 5 mm) and, say, 10 inner diameters in each (an illustrative stock size), the stock is 3 × 10 = 30 different rings, each needing its own mould groove or a cord cut to a calculated length. Cord length for one ring is the centre-line circumference π × (inner diameter + cross-section): a 3 mm ring of 50 mm inner diameter needs π × 53 = 166.5 mm of cord. The chemistry article says to add 5% extra length: 166.5 × 1.05 ≈ 175 mm.
Common Mistakes
- Natural rubber in oil. It swells; use leather, waxed cord or tar-coated fibre as the quality-testing article advises.
- Not re-tightening bolts. Gaskets relax.
- Trusting one groove figure. The two squeeze figures above differ; trial-fit.
Safety
Pressure behind the seal
A seal that fails under hydraulic or steam pressure releases fluid in a jet. Depressurise and drain before changing a seal; never judge a seal by putting a hand near a pressurised joint, because a pinhole jet can cut skin. Steam seals leak scalding steam, which is invisible near the leak.
What’s Next
- Learn next: Hoses and Tubing — flexible connections
- Combine with: Hydraulic Systems — where cup seals go