Some of the causes and effects of elastomeric component degradation are:
Fatigue – Elastomer fatigue is the normal degradation mode of a bearing in an oscillatory (dynamic) loading application. A part can experience a considerable amount of crumbing, cracking, and rubber loss before there is noticeable performance impact. Signs of fatigue include “eraser” crumbs, elastomer dust, fuzzy flex cracks, and extrusion of small elastomer segments.
Overload or tension loading – Typically, elastomeric parts are designed to operate in compression and/or shear modes. The main rotor bearings normally carry loads in compression and accommodate motion in shear. Problems can develop due to extreme or inappropriate loading of a healthy bearing or normal loading of a weakened bearing (from oil contamination or fatigue). Damage can also be caused by foreign objects such as tools, fasteners, clamps, and other devices. To determine overload or tension loading fatigue, mechanics should look for the bulk extrusion of elastomer between shims; large, clean cracks in the elastomer; bent or cracked shims; and bent, cracked or gouged major metals.
Fluid contamination – Although elastomeric materials have been developed to improve resistance to various aircraft fluids, solvents, and detergents, there are still some fluids that can cause degradation problems. Petroleum-based oils, fuels, hydraulic fluids, cleaning solvents, and greases can have a devastating effect on the elastomer, and can very quickly cause weakening, swelling, and extrusion. Prolonged fluid attack will degrade the rubber-to-metal bond. Signs of fluid degradation include slick, swollen elastomer; wavy edges; and clean debonding. Parts that have been detrimentally exposed to oils, greases, and fluids should be washed in mild detergent and water or denatured alcohol to prevent further damage. If the parts have been exposed to more volatile fluids such as fuel, solvents, or cleaning fluids, the parts should be dried and the fluid allowed to evaporate.
Environmental degradation – Daily exposure to sunlight, heat, and ozone can cause surface cracking, often appearing perpendicular to the elastomer load. These surface cracks are usually small and do not require the part to be removed. As the part is cycled, the cracks may become sites for initiation of the expected fatigue cracks, so they should be periodically inspected. Most elastomers contain waxes that offer ozone protection. The waxes are designed to reach the surface of the part over time and provide a protective layer, called a “wax bloom.” The wax bloom is gray or brown in appearance, and may flake or crumb as the part is used. Excessive heat will cause most bearings and isolators to become brittle on the exposed surfaces, although heat will not usually cause the elastomer to soften. The rubber-to-metal adhesives are also sensitive to heat, and performance of the part can be degraded as a result of prolonged exposure to high temperatures (above 125 F).
All three types of elastomeric components (bearings, isolators, and dampers) have similar inspection and care guidelines. Keep the surfaces clean and clear of dirt, sand, turbine soot, or dust from condensation (soot, dirt, or dust mixing with water vapor). Keep the components free of grease, oil, fuel, hydraulic fluid, or any petroleum contamination.
To avoid prolonged exposure to hostile fluids, cover the elastomeric surface area if cleaning fluids are being used in an adjacent area. To avoid excessive heat, use heat shields on elastomeric components. If cracks or separations in the elastomeric surface area are more than 0.125-inch deep, the part should be removed from service and overhauled.
Follow the maintenance manual at all times to ensure that the correct procedures are utilized during service operations. If there are no specific criteria, the following general guidelines can be followed:
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