Flexure bearing
Encyclopedia
A flexure bearing is a bearing
which allows motion by bending a load element.
A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame. Another example is a rope swing, where the rope is tied to a tree branch.
Flexure bearings have the advantage over most other bearings that they are simple and thus inexpensive. They are also often compact, light weight, have very low friction, and are easier to repair without specialized equipment. Flexure bearings have the disadvantages that the range of motion is limited, and often very limited for bearings that support high loads.
A flexure bearing relies on the bearing element being made of a material which can be repeatedly flexed without disintegrating. However, most materials fall apart if flexed a lot. For example, most metals will fatigue
with repeated flexing, and will eventually snap. Thus, one part of flexure bearing design is avoiding fatigue. Note, however, that fatigue is important in other bearings. For example, the rollers and races in a rolling-element bearing
fatigue as they flatten against each other.
Flexure bearings can give very low friction and also give very predictable friction. Many other bearings rely on sliding or rolling motions, which are necessarily uneven because the bearing surfaces are never perfectly flat. A flexure bearing operates by bending of materials, which causes motion at microscopic level, so friction is very uniform. For this reason, flexure bearings are often used in sensitive precision measuring equipment.
Flexure bearings are not limited to low loads, however. For example, the drive shafts of some sports cars replace cardan universal joint
s with an equivalent joint called a rag joint
which works by bending rubberized fabric. The resulting joint is lighter yet is capable of carrying hundreds of kilowatts, with adequate durability for a sports car.
Many flexure bearings are combined with other elements. For example, many motor vehicles use leaf spring
s. The spring both holds the position of the axle as the axle moves (flexure bearing) and provides force to support the vehicle (springing). In many cases it is not clear where flexure bearing leaves off and something else takes up. For example, turbine
s are often supported on flexible shafts so an imperfectly-balanced turbine can find its own center and run with reduced vibration. Seen one way, the flexible shaft includes the function of a flexure bearing; seen another, the shaft is not a "bearing".
Bearing (mechanical)
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
which allows motion by bending a load element.
A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame. Another example is a rope swing, where the rope is tied to a tree branch.
A flexure bearing relies on the bearing element being made of a material which can be repeatedly flexed without disintegrating. However, most materials fall apart if flexed a lot. For example, most metals will fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
with repeated flexing, and will eventually snap. Thus, one part of flexure bearing design is avoiding fatigue. Note, however, that fatigue is important in other bearings. For example, the rollers and races in a rolling-element bearing
Rolling-element bearing
A rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing round elements between the two pieces...
fatigue as they flatten against each other.
Flexure bearings can give very low friction and also give very predictable friction. Many other bearings rely on sliding or rolling motions, which are necessarily uneven because the bearing surfaces are never perfectly flat. A flexure bearing operates by bending of materials, which causes motion at microscopic level, so friction is very uniform. For this reason, flexure bearings are often used in sensitive precision measuring equipment.
Flexure bearings are not limited to low loads, however. For example, the drive shafts of some sports cars replace cardan universal joint
Universal joint
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...
s with an equivalent joint called a rag joint
Rag joint
A rag joint refers to certain flexible joints found on automobiles. They are typically found on steering shafts that connect the steering wheel to the steering gear input shaft, usually at the steering gear end. They provide a small amount of flex for a steering shaft within a few degrees of the...
which works by bending rubberized fabric. The resulting joint is lighter yet is capable of carrying hundreds of kilowatts, with adequate durability for a sports car.
Many flexure bearings are combined with other elements. For example, many motor vehicles use leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...
s. The spring both holds the position of the axle as the axle moves (flexure bearing) and provides force to support the vehicle (springing). In many cases it is not clear where flexure bearing leaves off and something else takes up. For example, turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
s are often supported on flexible shafts so an imperfectly-balanced turbine can find its own center and run with reduced vibration. Seen one way, the flexible shaft includes the function of a flexure bearing; seen another, the shaft is not a "bearing".
External links
- NASA Tech Brief
- http://www-personal.umich.edu/~awtar/PHD/Diaphragm.pdf
- Kinematic Models for Design Digital Library (KMODDL) - Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.
- Weinstein, Warren D., "Flexure-Pivot Bearings", Machine Design, Part 1, June 10,1965, Part 2, July 8, 1965 - Spring rates, bearing types, single and multi-strip design, material types, hysteresis and fatigue
- Weinstein, Warren D., "Microperformance of Metals". Machine Design, September 11, 1969 - Material relaxation and rolamite