12th March 2007
Given sufficient operating hours, all gearboxes eventually suffer a problem with oil leaks. Over time, paper or cork gasket can relax sufficiently to loosen the joint fasteners. Unless this joint is retorqued during maintenance periods, you can expect leaks to occur. Or the leak might be an 'O' ring seal, where temperature drops can shrink the 'O' ring many times greater than the surrounding metal will shrink. The result could ultimately be static joint failure, and an oil leak will emerge. Most bigger gearboxes have oil vapour escape from the breather, which can also be seen as oil leakage.
Cause of oil seal leaks
However, the most common source of oil leakage is from dynamic oil seals. This can be the result of incorrect selection, improper application, poor installation and inadequate maintenance practices resulting in unsuitable environments for these seals. Regretfully, this problem is often perpetuated by the usage of low quality - read 'low initial cost' - seals, or by poor installation procedures. Sometimes, even the wrong type of oil seal is used. An often overlooked cause of failure relates to the shaft surface on which the oil seal lip runs. This is where DCL Engineering can offer years of expertise, saving expensive downtime.
The seal running area can eventually become worn, due to the action of the seal lip operation. If lubrication is minimal, and dust or other abrasives are present, a groove can be quickly machined into the shaft surface.
Damaged seal running area
Metal sprayed ground area
Fixing the problem
Correct oil seal selection requires the following:
- Shaft diameter and speed in rpm
- Max / min operating temperature ranges
- Details pressure behind the seal
- Shaft surface hardness and finish
- Shaft eccentricity and ovality
- Diameter tolerances
- Up to 5 m/sec is considered slow
- Between 5 and 10 m/sec is moderate
- Between 10 and 15 m/sec is high
A shaft of 100 mm dia at 1475 rpm has a surface speed of 7.7 m/sec. But that same 100 mm dia shaft at 2880 rpm is travelling at 15 m/sec, which indicates care is required in selecting an appropriate seal material.
The temperature in which the seal operates is critical. Continuous usage over about 105°C will require special seal compounds, as the elevated temperatures will cause failure in the rubber compounds of most seal materials.
Seal Temperature Guide
|Nitrile||-30 to 100|
|Chloroprene rubber||-45 to 100|
|Ethylene propylene||-54 to 149|
|Silicone rubber||-55 to 200|
|Fluoro rubber||-20 to 200|
|PTFE (Teflon)||-80 to 250|
It is clear that different materials can withstand differing ranges of temperatures. Please note that the higher temperature capacity of Viton® is accompanied by a higher coefficient of friction, requiring better lubrication and also a harder running surface. Teflon seals also are usually more aggressive and abrasive to the shaft than standard elastomeric lip seals, although they can also run dry. For best results, a shaft hardness of HRc55 to HRc65 can be required, together with a fine surface finish.
The oil pressure being sealed is critical. High speed applications (10 - 15 m/sec) can normally withstand only 3 psi pressure before seal leakage can be expected. This is important where the bearing is being pressure lubricated, e.g. vertical shaft applications such as cooling towers, mixers, agitators, aerators etc.
Shaft Surface Hardness and Finish
Under approx 10 m/sec, a shaft surface finish between 0.25µ and 0.6µ is required.
Over 10 m/sec require a shaft surface finish between 0.25µ and 0.5µ to achieve satisfactory results. This must be polished or ground, with no directional finishing marks, to be successful. For higher speeds, even smoother surfaces may be required.
Hardness of the seal bearing area greater than HRc30 is preferred, but when any abrasives are present in the lubricant or in the environment (such as a dirty or dusty atmosphere), then upwards of HRc65 can be required.
Environmental abrasives can be minimised by the fitting of grease purged labyrinth seals outside the lip seals, which then protects the outer side of the lip as well as keeping the outer lip lubricated.
Other Problem Areas
- Does the shaft still run true to bore?
- Is it misaligned to the seal?
- Is the shaft bent?
DCL Engineering solutions
These are all important areas where DCL Engineering's experience can assist. Many different thermal coatings can be applied, including the popular 420 stainless steel rated at HRc50, and ceramic coatings which start at HRc57. These ceramic coatings will not only resist high abrasion, but possess extreme corrosion proof qualities. There are the even harder coatings such as Tungsten and Chromium Carbides, where surface hardness starts at HRc60. This latter range of thermal coatings requires diamond grinding to finish. These applications are typically a coal or iron ore mine, and most nearly any drives in a steel works where abrasive environments are unavoidable.
Logically, each and every application has to be evaluated on it's merits. At DCL Engineering, we will photograph, strip and inspect each application, before preparing a comprehensive QA report for the customer. This proposal will include the details of the application and failure, as well as the proposed correction. Once accepted, this becomes the scope of work for DCL Engineering to refurbish the oil seal leakage area. To do the job correctly, getting access to these seal bearing surfaces areas requires the disassembly of the gearbox, and replacement of the existing bearings and seals. We also crack test all gears and shafts unless instructed otherwise. Our QA reports will also comment on the condition of other components within the gearbox.