Proper Application of Rubber Seals in Hydraulic Systems

      Seals (including O-rings, lip seals with metal cases, gaskets, etc.) are critical components used in agricultural and construction machinery for containing oil, water, and air/gas. Industry experience shows that many mechanical failures stem from the failure of rubber seals. Specifically, in hydraulic systems, approximately 80% of failures are attributable to rubber seal malfunction. Consequently, the proper application of rubber seals to extend their service life is of paramount importance.

1. Selection of Qualified Seals

Nitrile rubber (NBR) exhibits excellent resistance to oil, heat, cold, compression, and water, along with outstanding wear resistance. Its typical operating temperature range is approximately 40°C to 120°C. Due to its ease of molding into complex geometries using metal dies, NBR is ideally suited for hydraulic seals in cylinders operating at pressures not exceeding 32 MPa. Polyurethane rubber (PU) offers the highest tensile strength among synthetic rubbers. It possesses superior resistance to oil, heat, cold, compression, and abrasion, with a typical operating range of -40°C to 80°C. At room temperature, PU demonstrates superior sealing characteristics compared to NBR, making it particularly well-suited for seals used in medium-, high-, and ultra-high-pressure hydraulic cylinders.

Additionally, when selecting seals, verify that their inner/outer diameters and widths conform to specifications. Seal surfaces must be smooth, flat, and free of aging, cracks, or defects. The chosen seal must deliver reliable performance under its intended service conditions. Provided operating pressures and temperatures remain within specified limits, its sealing integrity shall be maintained.

2. Selection of Qualified Shafts

Shaft surface roughness must be appropriately controlled. A roughness exceeding Ra 0.8 μm prevents the rubber seal from adequately conforming to surface irregularities, leading to oil leakage through the microscopic valleys. Conversely, an overly rough shaft accelerates wear at the seal lip contact zone, increasing the clearance between the shaft and the lip, thereby exacerbating leakage. A shaft roughness below Ra 0.1 μm hinders the formation of a stable lubricating film between the seal lip and the shaft surface. This results in boundary friction or even dry friction within the contact zone, causing accelerated wear and temperature rise. These conditions can lead to seal tearing, premature aging, and subsequent oil leakage. Therefore, an optimal surface finish for the shaft sealing area falls within the range of Ra 0.4 to 0.8 μm.

3. Critical Installation Clearances

(1) O-Ring Seals (Compression Type):
    O-rings function as compression seals. Insufficient compression – caused by an excessively deep groove on the shaft or within the bore housing – will result in leakage. Conversely, if the groove is too shallow or the O-ring outer diameter is oversized, creating minimal clearance, high-pressure fluid will extrude the O-ring into the mating gap, leading to shear failure and leakage under repeated pressure cycles. As a result, for dynamic O-ring seals, the groove width should exceed the O-ring cross-section diameter by approximately 25% to ensure optimal compression and prevent extrusion.

(2) Lip Seals with Metal Casings:

    Housing Fit: Install the seal by evenly pressing the metal casing into the bore around its entire circumference. Prevent casing distortion (bending, denting, or tilting), as misalignment readily induces lip deformation and leakage.

    Installation Practice: Manufacturers in the US, Japan, and elsewhere often apply a sealant to the casing OD during installation. This practice, worthy of adoption, enhances sealing between the casing and bore while securing the seal against operational misalignment.

    Lip-to-Shaft Clearance: Maintain appropriate radial clearance between the seal lip and shaft. Excessive or insufficient clearance promotes leakage (mechanism similar to O-rings).

    Garter Spring Function: The integral garter spring applies constant radial force, pressing the lip against the rotating shaft. This spring compensates for minor lip wear during operation, maintaining effective sealing contact. As a result, when selecting or inspecting lip seals, it’s critical to verify the garter spring's condition. Immediately replace any spring found to be broken or exhibiting insufficient tension. Often, replacing a faulty spring can restore sealing functionality to a leaking seal.

Important Note: Seal Break-In Period
    Similar to metal components, rubber seals require a break-in period (typically several hours). During this phase, the lip beds-in against the shaft, forming a continuous contact band. Minor leakage is normal during initial operation and should not prompt immediate disassembly. As for the Post-Break-In Check, which usually takes splace after 5-6 hours of operation, If minor leakage ceases, the break-in is complete, in contrast, if leakage persists or increases, this indicates seal failure, necessitating disassembly and inspection.

4. Maintaining Fluid Cleanliness

(1) Prohibit Use of Degraded Oil:
    Hydraulic fluids and lubricating oils undergo chemical degradation over extended service periods, characterized by increased acidity, elevated contaminant levels, and diminished lubricating properties. Degraded oil not only damages metallic components but also accelerates corrosion of rubber seals. Critically, it contains abrasive oxide particles (typically 0.1–3 μm in size) which severely accelerate wear and aging of already compromised seals. Therefore, regularly monitoring fluid condition and promptly replacing degraded oil are essential measures for extending seal service life and preventing leakage.

(2) Prevent Fluid Contamination:
    Oil contaminated with particulate matter similarly accelerates seal wear and degradation. Consequently, oil filtration elements must be routinely cleaned or replaced to prevent ingress of contaminants such as dust and dirt into the system.

5. Preventing Seal Damage During Installation

Protection Against Sharp Edges:
    When installing seals that must pass over sharp features such as keyways or shaft chamfers, protective sleeves or installation cones must be used to guide the seal and prevent cutting or tearing. An established practice involves applying a thin plastic film (0.076-0.25 mm thick) over these critical areas. The film is coated onto the sharp surfaces, remains during seal installation, and is removed afterward. If dedicated tools are unavailable, a used bearing outer ring (with an outer diameter slightly smaller than the seal's inner bore) can serve as an effective improvised sleeve.

Ensuring Mating Surface Cleanliness:
    Absolute cleanliness of sealing surfaces is imperative. All oil residue, contaminants, abrasive particles, or metal shavings must be completely removed prior to assembly. Residual debris will damage component surfaces and compromise the seal. As for thread Cleaning, contaminated threads on bolts or in threaded holes should be cleaned by rethreading (using a die) or tapping (using a tap). Debris from the bottom of blind threaded holes should be removed by using a bottoming tap followed by compressed air to blow out loosened particles.