Power springs

The traditional form of the strip spring is the power spring.  

The special feature of the MAXIMO power spring is that the combination of winding forward and then back increases the power density, making maximum use of the available material properties.


Power spring

The way a power spring works is as simple as it is effective. A piece of spring steel strip is wound up from the stretched state and has a tendency to return to the stretched state. The energy put into the strip to wind it up, minus the low frictional energy, can be used again during the return motion.

Given this property, power springs are used in many areas as a retraction element, for weight compensation or also for torque storage. The hallmarks of a power spring are many working revolutions, increasing torque and a long service life.

Because of its spring length and the large number of spring coils, it forms a spring assembly without coil spacing in the tensioned state. Compared to an open wound spiral spring, this results in a flat rising spring characteristic curve with friction losses during the return motion.

MAXIMO power spring

The MAXIMO power spring is a variant of the power spring in which the spring torque is increased by the specific introduction of internal stresses and deformation. After winding forward, the spring is rewound in the opposite direction to the rolling direction, producing the so-called “MAXIMO effect” – (MAXImumMOment of torque).  These measures give the spring its characteristic S-shape in the free state.

In addition to a higher torque, the MAXIMO power spring produced in this way has a lower hysteresis and improved retraction.
The heat treatment that takes place between winding forward and back improves the spring-back behavior of the power spring.
By introducing a pretension (initial tension) or through pre-deformation, the MAXIMO power spring achieves its MAXImumMOment of torque over the entire length of the strip.

In the production of the traditional power spring, a specific level of internal stress is introduced into the material during the manufacturing process. Heat treatment is used to reduce internal stress peaks in areas of high plastic deformation of the power spring.

With MAXIMO power springs, it should be noted that the level of the achievable MAXIMO effect depends on the material, strength and spring dimensions.

Thanks to the higher torque produced by the MAXIMO effect, a thinner material thickness than with traditional power springs can be used in the same installation space. The lower material thickness has a positive effect on service life and increases the number of total revolutions.

The characteristic curve of the MAXIMO power spring also rises more steeply and higher in the pretension range than with traditional power springs, giving a flatter characteristic curve in the working range.

Here you can watch how a power spring works in a cable reel:

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With a single spring, only a certain number of total revolutions can be achieved. If a particularly large number of working revolutions is required, the spring length has to be increased accordingly. The number of revolutions is usually limited by the limiting outer diameter of the springs.

Parallel connection of power springs

If not enough torque can be generated with one power spring as a result of the limitations of the installation space, two or more springs can be connected in parallel in the axial direction. The number of usable working revolutions is equal to the single spring, the maximum achievable torque is increased. To prevent increased friction and the individual springs getting caught in or blocking each other, it can be helpful to use a continuous spring axis and separate the springs from each other (for example by inserting plastic washers between the springs).

In terms of service life, a parallel  arrangement of the power springs can also allow for improvement.




Series connection of power springs

If not enough revolutions can be generated with a single power spring as a result of the limitations of the installation space, two or more springs can be connected in series. The maximum torque that can be achieved by the series connection is equal to that of the single spring, the usable working revolutions are increased accordingly.

Slip and drag springs

Slip or drag springs are used where a specified maximum winding torque should not be exceeded and the system needs to be protected against overwinding. In the return motion, the spring works like a conventional power spring, which means that it performs a defined number of working revolutions.

A well-known example of this is the wind-up toy car, which is driven by a slip or drag spring and can be wound up at will without overwinding the spring motor so that the outer hook is pulled out of its attachment point.

With slip springs or drag springs, there is no fixed connection between spring and housing. Instead, a second, stronger spring is installed in the main spring at the outer end. Because the material is significantly thicker, the second spring ensures that the main spring in the housing is tensioned and only “slips” when a certain torque is reached.

Due to the installation situation and different friction ratios in the spring housing, the slip torque, i.e. the interaction between main spring and slip spring (drag spring), can only be roughly calculated. The design of a slip spring depends on its application and may need to be defined on the basis of a series of tests.

Here is an example in which the slip spring is inserted into the main spring. The slip spring is held in the main spring by a parallel hook bent inwards. Because the main spring is tensioned by the slip spring in the housing, the spring is given additional support, lowering the hysteresis in the return motion.

Systems like this are found primarily as main springs in watches. The power spring for mechanical watches is also often referred to by watchmakers as the tension spring or main spring. It supplies the drive energy for the watch mechanism.

When the tension spring is tensioned by winding, it drives the train and the time-determining oscillating system in the return motion with balance and lever escapement. To prevent the relatively small spring from being overstressed and damaged during manual or automatic winding , a slip spring ensures that a “slip” occurs when the maximum number of winding revolutions is reached. Spring systems of this kind are designed for up to 20,000 extractions and have been available to watchmakers for decades as a virtually loss-free mechanical energy store.

The striking S-shape of these springs ensures that a nearly constant force is passed on to the watch mechanism over almost the entire length of the spring.