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The arc spring (also known as - bow spring, curved spring, circular spring or "banana" spring) is a special form of
coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fi ...
which was originally developed for use in the
dual-mass flywheel A dual-mass flywheel (DMF or DMFW) is a rotating mechanical device that is used to provide continuous energy ( rotational energy) in systems where the energy source is not continuous, the same way as a conventional flywheel acts, but damping a ...
of internal combustion engine drive trains. The term "arc spring" is used to describe pre-curved or arc-shaped helical compression springs. They have an arc-shaped coil axis.


Function

Like other technical springs, arc springs are based on the fundamental principle of storing
mechanical work In physics, work is the energy transferred to or from an object via the application of force along a displacement. In its simplest form, for a constant force aligned with the direction of motion, the work equals the product of the force stren ...
in the form of
potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...
and the ability to release this energy again. The force is applied through the ends of the spring. A torque M=F\cdot r can be transmitted around an axis via the force F directed along this helical axis and the lever arm to the system center point r. The wire of the arc spring is mainly subjected to torsional stress.


Support 

An arc spring requires suitable support to transmit
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of the ...
. The support is usually provided from the outside in the form of an arcuate channel (sliding shell) or radially shaped support plates. This prevents buckling of the arc spring. Another result of this support is a
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
between the loading and unloading curves in the characteristic curve. This results from the friction of the spring on the radial support and is an intended effect to achieve
damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples inc ...
in the system.


Arc spring systems

As with compression springs, spring systems can also be used for arc springs. The main designs are series and parallel connection. With these, single-stage or multi-stage spring characteristics can be achieved. In order to make optimum use of the available space, systems consisting of inner and outer arc springs are often used. In addition, the spring characteristic can be influenced by other parameters such as the cross-sectional geometry of the wire, the coil diameter or the number of coils. CAD configurators, which generate a CAD model after entering certain parameters, can contribute to optimal design.


Applications

The arc spring is suitable for static and quasi-static as well as dynamic applications. Examples include: * Dynamic applications: **
Dual mass flywheel A dual-mass flywheel (DMF or DMFW) is a rotating mechanical device that is used to provide continuous energy (rotational energy) in systems where the energy source is not continuous, the same way as a conventional flywheel acts, but damping any ...
**
Torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power ...
**Arc spring clutch ** Belt tensioners, pulley decouplers * Static and quasi-static applications: ** Center position return in a robot joint ** Elastic element in exoskeletons ** Reset of tailgates or backrests


Materials and their standardization 

In principle, the spring steels used for ordinary coil springs can also be used for arc springs. These are: * DIN EN 10270-1 Patented-drawn unalloyed spring steel wire                        * DIN EN 10270-2 Oil tempered spring steel wire                                            * DIN EN 10270-3 Stainless spring steel wire


Important parameters


References

{{Reflist Springs (mechanical)