A spherical rolling joint is a high-precision ball joint consisting of a spherical outer and inner race separated by ball bearings. The ball bearings are housed in a spherical retainer and roll along both the inner and outer surfaces. This design allows the joint to have very low friction while maintaining a large range of motion and backlash as low as 1 μm. SRJs are often used in parallel robotics applications like a Stewart platform, where high rigidity and low backlash are essential.
Most SRJs are designed with an offset housing, allowing for higher compressive loads in a smaller space. Alternatively, the joint can be assembled backwards for higher tensile load capability but less range of motion.
An alternative to the SRJ is the universal joint, which consists of two revolute joints.[7] By using spherical rolling joints instead of universal, designers can reduce the number of joints to achieve the same result. Using a spherical joint as opposed to a universal joint also eliminates the problematic possibility of a kinematic singularity.Plain spherical bearings can be used in place of SRJs at the cost of increased friction, but offer an opportunity to preload the joint further.
Failure
While there is no exact lifespan that can be put on sealed ball joints, they can fail as early as 80,000 miles (130,000 km) in modern vehicles, and much sooner in older vehicles.[citation needed] Signs of a failing ball joint may start with a sudden burst sound as a result of ball joint dismantling. Then it keeps on with clicking, popping or snapping sound when the wheel is turned and eventually turn into a squeaking sound at the end of a stop, when the gas pedal is used and/or also when hitting bumps. Another symptom could be ‘thud’ noises coming from front suspension when going over bumps. Dry ball joints have dramatically increased friction and can cause the steering to stick or be more difficult.
If a ball joint fails, the results can be dangerous as the wheel’s angle becomes unconstrained, causing loss of control. Because the tire will be at an unintended angle, the vehicle will come to an abrupt halt, damaging the tires. Also, during failure, debris can damage other parts of the vehicle.
