Overview
ISO 3939:1977 - Fluid power systems and components - Multiple lip packing sets - Methods for measuring stack heights - defines standardized measurement methods for the stack height of multiple-lip fluid power packing (sealing) sets. It specifies three acceptable measurement methods so suppliers and users can determine whether packing sets meet an agreed dimensional standard. The standard does not prescribe packing dimensions or component sizes; it focuses only on measurement procedures and test setup.
Key topics and requirements
- Scope
- Applies to homogeneous and fabric‑reinforced fluid power lip seals (V, Y, U, W profiles) used in hydraulic and pneumatic systems.
- Intended to give reproducible stack-height values for inspection, design and acceptance.
- Three measurement methods
- Method 1 - Unconfined (preferred for diameters up to ~200 mm): Use flat parallel plates and weights. Simple setup for small diameters; widely used by suppliers and users.
- Method 2 - Confined at one point (recommended for diameters over ~200 mm): Uses two parallel pillars/rods on a rigid base with male/female adapters and follower ring to simulate the seal housing.
- Method 3 - Confined I.D. and O.D.: Uses accurate I.D. plugs, O.D. rings and follower to reproduce actual housing constraints; suitable for initial testing and correlation with other methods.
- Checking load and preload
- A checking load (P) is required for each method. Its magnitude depends on seal geometry and material and must be agreed between supplier and user.
- For Methods 1 & 3, the checking load is calculated using the packing set mean diameter and radial cross‑section together with a specified load factor. For Method 2 the load calculation uses the radial cross‑section and a specified load factor.
- Preload is required for Method 3 and optional for Methods 1 and 2 to seat components before applying the checking load.
- Good practice
- Minimize air entrapment and friction (provide airways in gauges, match surface finishes), take multiple equidistant measurements, and agree measurement counts and loads between supplier and user.
Applications and users
- Who uses ISO 3939: suppliers of packing sets, quality control/inspection personnel, cylinder/valve/pump designers, and end users who need to verify received sealing sets.
- Practical uses include incoming inspection, supplier acceptance testing, production quality control, and providing consistent stack‑height data for design and assembly.
Related standards
- ISO 5598 - Fluid power systems and components - Vocabulary (referenced for definitions).
- Developed by ISO/TC 131 (Fluid power systems and components).
Identification statement for reports: “The method of measuring conforms to method … of ISO 3939, Fluid power systems and components - Multiple lip packing sets - Methods for measuring stack heights.”