When a seal leaks, the usual response is to fit a new one. If the cause has not been found, the new seal tends to fail in the same way. A few minutes spent examining the old seal and the hardware often shows what went wrong. This article describes typical failure appearances and their likely causes. These are indications rather than firm diagnoses, since several causes often act together.
Read the failed seal before discarding it
| What you see | Likely cause |
|---|---|
| Hard, glazed or cracked sealing lip | Excessive temperature at the lip, or loss of lubrication |
| Swollen, softened or sticky material | Chemical incompatibility with the fluid |
| Cuts, nicks or a torn lip | Damage during installation |
| Wide, heavily worn contact band; grooved shaft | Abrasive contamination, a rough shaft or poor lubrication |
| Wear on one side only | Misalignment, shaft runout or side loading |
| Nibbled or frayed edge on the low-pressure side | Extrusion into the clearance gap |
| Flattened cross-section that does not recover | Compression set from heat, age or an unsuitable compound |
| Seal tilted or displaced in the housing | Incorrect fitting, or pressure the seal was not designed for |
Installation damage
A large share of early failures begin at fitting. Typical causes are:
- the lip dragged over an unprotected keyway, spline, thread or sharp shoulder;
- no lead-in chamfer on the shaft or housing;
- the seal driven in unevenly so that it sits tilted in the bore;
- the seal fitted the wrong way round;
- a dry lip at first start-up;
- dirt introduced during assembly.
These failures tend to show up early, often as leakage from the first day. Protective sleeves, clean working conditions, light lubrication of the lip and a fitting tool that presses the seal in squarely prevent most of them.
Heat and chemical attack
Heat hardens most elastomers. The lip loses flexibility, stops following the shaft and eventually cracks. The source may be the process, the surroundings, or the seal's own friction at high speed, under pressure or with insufficient lubrication.
Chemical attack usually shows as swelling and softening, though some fluids cause shrinkage or hardening instead. It often follows a change of lubricant, additive or cleaning agent. The two effects reinforce each other, because a fluid that is tolerated at moderate temperature can become aggressive when hot. If either is suspected, review the compound using our guide to choosing the right seal material.
Contamination and hardware condition
Abrasive particles trapped under a lip wear both the seal and the shaft. Once the shaft is grooved, a new seal running in the same track will not last. Check the following before fitting a replacement:
- the shaft or rod surface in the contact zone, for grooves, scratches, corrosion and plating damage;
- spiral machining marks on a rotating shaft, which can pump oil past the lip;
- bearing wear that allows the shaft to run out of true;
- alignment between the shaft and the housing bore;
- the condition of wipers, breathers and filters, which keep contamination away from the seal;
- blocked vents that let pressure build up behind a seal not designed for it.
Pressure and extrusion
Under pressure, a seal is pushed towards the clearance gap on the low-pressure side. If the gap is too large for the material and the pressure, the edge of the seal is forced into it and nibbled away, a little more with each cycle. Pressure peaks, worn guide elements that open up the gap, and material softened by heat all make extrusion more likely. The usual remedies are a more extrusion-resistant material, back-up rings or a reduced clearance; the right combination depends on the seal design and the application.
Send us the evidence
When a seal has failed, keep it, note its position and orientation, and record how long it ran and what the leakage looked like. Our sales engineers can review the failed seal alongside the application and suggest a suitable replacement from our hydraulic and rotary seal ranges. Send the shaft, housing and width dimensions, the media, temperature, pressure and speed, photographs, and a drawing or the failed seal as a sample, through the enquiry form.
