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.

Forms of delivery

Various options are available for the delivery of power springs which offer benefits depending on the application.
Power springs can be wound into simple wire retaining rings, into holding discs or directly into housings. There are numerous variants to choose from, up to and including fully assembled assemblies.
The best form of delivery is determined during product development, taking both economic and logistical aspects into account.

Spring wound in wire ring or retaining strip

Delivery in an intermediate carrier, wire ring or retaining strip is the choice if the spring cannot be installed directly in the original assembly unit . The wire ring can only be used once and is disposed of after final assembly. For assembly, the power spring is transferred from the wire ring into the spring installation space.

The wire ring is a simple and inexpensive form of delivery and is therefore often used for sample springs or initial functional samples. The choice of wire diameter should ensure that the retaining ring fits as snugly as possible around the coiled power spring and secures it reliably against radial relaxation during transport. The appropriate inner diameter of the wire ring is determined in the course of prototype release in order to ensure easy assembly of the finished product. For stronger power springs, however, this form of delivery is only of limited use. In particular, moving the spring from the wire ring into the final spring installation space entails a risk of the spring jumping out of the wire ring retainer. Go to the safety instructions

For larger dimensions, retaining strips are often used. These are overlapping strips of steel strip welded to form a ring which, like the wire ring, are used to secure the power spring radially during transport. The thickness and width of the retaining strip material depends on the spring dimension.

The retaining strip generally remains on the coiled power spring after final assembly and is installed together with it in the spring installation space. The design and engineering of the end product should ensure that there is sufficient space for the retaining strip, which spreads radially in diameter to the diameter of the power spring. Due to the larger dimensions and forces, peeling off the retaining strip and repositioning the power spring is no longer possible in most cases.


Spring wound in holding disc or retaining ring

This type of transport lock can be reused multiple times. 
Similar to the design in a wire ring, the power spring must be transferred into the original component during final assembly. It is chosen mainly for products produced in very high volumes.

Compared to wire rings, the holding disc or retaining ring is characterized by a higher precision and stability of the mounting diameter. This assembly aid is defined and optimized during the product development phase for final assembly and transfer of the power spring to the specific structural conditions in the spring installation space.

Spring wound in housing

One of the most cost-effective and frequently used variants is to wind the spring directly into the housing. To do this, the power spring and spring housing need to be matched to each other at an early stage of product development in order to optimize the winding process cost effectively. Spring housings can be made of both metal and a variety of plastics.

When designing the housing, the following aspects will need to be considered:

  • The base of the housing must be flat and level, because a bearing surface (raised area or pin for bearing bushes, axis supports etc.) at the base of the housing will have a detrimental effect on the installation and running of the power spring in series operation.
  • The housing as a whole must have the stability necessary to withstand the spring, and in particular the radial forces.
  • The hole for axis feed-through should be centered where possible.
  • The housing must be closed during use to protect against environmental influences.
  • The depth of the housing must be selected to reflect the width of the spring. If the depth of the housing matches the spring width exactly, this can lead to blocking of the spring coils during operation. If the housing is too deep, individual coils can emerge from the spring assembly, impairing the running of the spring.

For low-friction use of the power spring, the spring cartridge must also be greased, which can be integrated as early as the winding process.


Installed and pretensioned assemblies

The manufacture of fully installed and preloaded assemblies with power springs is possible.

This is done by winding a power spring into a housing and installing other additional components such as axes or covers. For the user, there is no need to transfer the spring to the original component or to dispose of or return the transport lock.

Another advantage is that a fully installed assembly can be supplied with or without pretension (initial tension). A pretensioned assembly then only has to be introduced into the end application, where it is immediately ready for operation. For a pretensioned spring cartridge, the power spring is wound into the spring housing on special winding devices. The spring core (spring axis) is then inserted and a cover (spring retainer) is fitted into place. The spring housing and spring retainer must be connected to each other and secured. A variety of systems such as clips, bent lugs or screw connections are used for this purpose. The resulting assembly is then pretensioned to a specified torque value by a corresponding number of revolutions. The pretensioned assembly must then be locked into place with a retaining bracket (also called a retaining clamp or retaining clip) or with a pin. The required minimum torque is defined within the working range and an appropriate pretension is selected for each batch.

For the cost-effective production and proper functioning of the assembly, early and thorough coordination of the individual components and steps in the process is very important.

Design as a rivet spring

Another form of delivery of power springs is the riveted spring design.
This variant is generally only used for very heavy springs for mechanical engineering.
In this variant, the outermost coil of the power spring is riveted to the ring immediately after the winding process.
This riveting serves as a radial limitation, secures the power spring and remains in place after final assembly. In addition, the riveted ring performs the function of the spring support, ensuring that the rivet spring run smoothly.

This makes any additional transport lock using single or multiple rings unnecessary. Another advantage of this form of delivery is the ease of assembly. The riveted diameter is determined at an early stage, during product development, to ensure that the spring fits perfectly into the spring installation space. The rivet spring then only needs to be inserted into the outer and inner mounting point in the spring installation space. There is no need to transfer the power spring or dispose of the transport lock.

A rivet spring design is not possible for all power spring dimensions. If the spring dimensions are very narrow and thin, there will not be sufficient spring material to ensure stable riveting. With particularly large spring cross-sections, several riveting points can also be provided to ensure sufficient holding force.