romtrt
07-01-2008, 01:07 AM
I read it from vwvortex.com forum it came from a book Chassis/Engineering design.
URETHANE BUSHINGS
In recent years, several companies have offered urethane suspension bushings. These plastic bushings look great on the parts shelf. Unfortunately, the urethane used in the products we have evaluated is not a material well suited for suspension bushings.
Disadvantages
Depending on the application, suspension bushings must allow movement in different planes. The most common plan e of movement is simple rotation. Rubber bushings allow rotation by the internal shear of the rubber itself. This means there is no sliding motion between any of the members. The rubber flexes allow the inner and outer sleeves to rotate relative to each another. Since there is no sliding motion, there is no friction-caused wear and no need for lubrication. Because the rubber is molded to the inner and outer sleeves, there are no critical tolerances to maintain during manufacturing. This is one of the features that allows rubber bushings to be made inexpensively.
When a steel or urethane suspension bushing is used, the bushing material cannot deflect. There must be some sliding motion to permit rotation between the inner and the outer sleeve.
Many people who buy aftermarket urethane bushings believe that the urethane behaves like hard rubber, but most urethane suspension bushings are so hard that they have to be considered solid, because they offer little ability to absorb rotation shear within themselves. The only way a rigid suspension bushing can allow rotational movement is if one of the mating surfaces slides on the other. This sliding action demands close manufacturing tolerances and lubrication for satisfactory operation. The aftermarket urethane bushings we've tried did not meet these requirements.
Greasing the bushings before assembly works for a few weeks until th grease is forced out or washed away. Once the grease is gone, the urethane is again able to bind to the steel sleeves, and the driver hears squeaks and moans. Without lubrication the urethane can stick to the steel, and the suspension does not move smoothly.
Urethane bushings could be customized to provide better service by hand-fitting the bushings to obtain proper clearances and installing grease fitting for regular lubrication.
URETHANE BUSHINGS
In recent years, several companies have offered urethane suspension bushings. These plastic bushings look great on the parts shelf. Unfortunately, the urethane used in the products we have evaluated is not a material well suited for suspension bushings.
Disadvantages
Depending on the application, suspension bushings must allow movement in different planes. The most common plan e of movement is simple rotation. Rubber bushings allow rotation by the internal shear of the rubber itself. This means there is no sliding motion between any of the members. The rubber flexes allow the inner and outer sleeves to rotate relative to each another. Since there is no sliding motion, there is no friction-caused wear and no need for lubrication. Because the rubber is molded to the inner and outer sleeves, there are no critical tolerances to maintain during manufacturing. This is one of the features that allows rubber bushings to be made inexpensively.
When a steel or urethane suspension bushing is used, the bushing material cannot deflect. There must be some sliding motion to permit rotation between the inner and the outer sleeve.
Many people who buy aftermarket urethane bushings believe that the urethane behaves like hard rubber, but most urethane suspension bushings are so hard that they have to be considered solid, because they offer little ability to absorb rotation shear within themselves. The only way a rigid suspension bushing can allow rotational movement is if one of the mating surfaces slides on the other. This sliding action demands close manufacturing tolerances and lubrication for satisfactory operation. The aftermarket urethane bushings we've tried did not meet these requirements.
Greasing the bushings before assembly works for a few weeks until th grease is forced out or washed away. Once the grease is gone, the urethane is again able to bind to the steel sleeves, and the driver hears squeaks and moans. Without lubrication the urethane can stick to the steel, and the suspension does not move smoothly.
Urethane bushings could be customized to provide better service by hand-fitting the bushings to obtain proper clearances and installing grease fitting for regular lubrication.