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The Right Design

 
 
 
 
 
 
 
 
 
Maintaining elastomeric components is crucial to the safe operation of the aircraft and for prolonging component life. Understanding the maintainability of these components begins right at the design process. LORD Corporation, a leader in elastomer technology, works very closely with helicopter design engineers to determine the correct size and life cycle needed for the aircraft.

The goal is to not only design the best performing components but to also assure that the components are maintainable. To achieve this, maintenance personnel are consulted for their opinion on a component’s design and maintainability. This helps to ensure that everyone understands how to operate the helicopter in the most cost-effective manner.

LORD Warranty for Bell 407 Main Rotor Hub Components to 2,500 Hours!

LORD Warranty for Bell 407 Main Rotor Hub Components to 2,500 Hours!
Bell Helicopter approved the upgrade to LORD SPE IIA elastomer on the lead lag and shear bearings in 2014. SPE IIA is LORD Corporation's newest generation of elastomer that was specifically formulated for helicopter bearing applications. Comparative endurance tests validate a significant increase in test life for the SPE IIA bearings. LORD conservatively predicts a minimum of 30 percent increase in actual service life.

 

Elastomeric Components

Elastomeric components are comprised of bonded-rubber elastomeric elements that are specially designed to eliminate certain types of vibration. Compared to coiled metal springs, elastomeric elements allow components to be easily configured in different sizes and shapes, and can be readily integrated into various helicopter designs. Elastomeric parts are multi-directional, simple, lightweight, reliable, and can be designed into a system at a moderate cost compared to more complex methods. They also offer the advantage of an extremely long operating life.
These components are generally maintained through visual inspection, and it is essential for the mechanic to understand how these parts operate, the causes and effects of degradation, and the best maintenance practices. A typical helicopter will have several elastomeric components which includes: main rotor lead lag damper, main rotor spindle bearing, main rotor CF bearing, tail rotor trunnion and many more.   All of these parts and others must be periodically inspected and maintained to guarantee a safe, vibration-free ride.
 
Elastomer Degradation
Elastomeric components will degrade due to conditions such as fatigue, overload or tension loading, fluid contamination, and environmental attack from heat and ozone. Typically a combination of these degradation conditions will occur, accelerating the component damage. By thoroughly inspecting the parts, a knowledgeable mechanic will be able to assess when an elastomeric part needs to be overhauled or replaced.

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).

Elastomeric care

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:
  • Remove the elastomeric device if it allows excessive motion or vibration
  • Remove the elastomeric device if any of the rubber-to-metal layers are touching each other or are cracked
  • Remove the elastomeric device if more than 25 percent of the bonded area has separated
  • Superficial cracks due to ozone, heat, or sunlight do not warrant removal of the part
  • Any slight oil contamination may be cleaned and the component returned to service
  • Severe oil contamination is always cause for removal of the device
  • Any unusual damage condition should be evaluated by OEM product support

Keeping up with maintenance

To keep up with changes in maintenance techniques, mechanics should seek out continuing education courses given by aircraft manufacturers, OEMs, and component suppliers. Many organizations offer maintenance conferences where mechanics can receive certification to maintain their FAA Inspection Authorization (IA) accreditation.

The key to properly maintaining elastomeric components is periodic visual inspection. Elastomeric parts are basically “maintenance-free” — the only requirement is visual inspection for degraded elements and then a determination to have the part overhauled or replaced. These periodic checks will assure that the aircraft is kept in safe working condition, while providing a vibration-free ride for the pilot and passengers.

Each elastomeric part will degrade differently over time in different environments, so it is important to know the difference in appearance between a properly working part and one that is starting to degrade. Establishing a time frame for inspections will ensure that all parts are kept in good working order. Unscheduled maintenance events cost time and money, but can be avoided with proper inspection of elastomeric components.
Canadian Air Parts Ltd.
4193 104 St, Delta, BC  V4K 3N3

Phone: 604.596.7007
parts@cdnairparts.com