Technical information

Function principle

All emergency pull-wire switches from steute are provided with a wire-break detection so that the wire must with be mounted with a defined pre-tension force. This value of the pre-tension force varies depending on the different devices. The appropriate value can be found on the data sheet of the emergency pull-wire switch. With an incorrect mounting it cannot be taken in operation, i. e. an unlocking is not possible. By vertically pulling the pull-wire the switching function is carried out. The actuating force is exclusively depending on the spring rate of the reset spring. There are emergency pull-wire switches with one-side and two-side actuatiuon, see drawings below. Emergency pull-wire switches with two-side actuation must always be mounted with two compensation springs. According to EN 60947-5-5 the maximum values of the actuating force F = 200 N and of the actuating travel s = 400 mm must not be exceeded on vertical actuation of the emergency pull-wire switch. In addition, the pull-wire must withstand the 10 times higher vertical pulling force that is required in order to generate the emergency-stop signal.


Maximum pull-wire length

The maximum pull-wire length is mainly limited by two basic conditions. On the one hand by the maximum admissible actuating travel s of 400 mm and on the other hand by the thermal change in length of the pull-wire with a fluctuating ambient temperature that may not lead to an undesired actuation of the switch. Because the first basic condition requires a preferably low and the second requires a preferably high elasticity of the system it is necessary to optimise such systems in respect to both basic conditions depending on the operational conditions. In addition, it must be checked if the actuating force F of 200 N is adhered.

Interrelation of actuating travel / distance wire support

Application of compensation springs / Travel limitation

Compensation springs are applied to compensate thermal changes in lengths of the pull-wire and therefore allow for higher pull-wire lengths. In general the following is valid:

  • Soft compensation spring with a low spring rate can compensate higher thermal changes in length.
  • Though on pull-wire actuation soft compensation springs have a high expansion behaviour and therefore earlier reach the limit of the maximum actuating travel s = 400 mm. Thus the expansion behaviour limitates the maximum pull-wire length at a constant temperature range or the temperature range at a constant pull-wire length.
  • The dimensioning of the compensation spring is determined by the reset spring of the switches (Value of the pre-tension force and spring rate of the rest spring), the pull-wire length (length and elasticitiy of the pull-wire) and the maximum actuating travel of s = 400 mm.
  • With two-side actuation a travel limitation must be installed, see drawing below, in order to prevent overstretching of the tension spring
  • Before mounting the pull-wire, the red PVC sheath must be removed from the the pull-wire in the clamping range of the pull-wire!

An overstress of the compensation spring is in general prevented by a travel limitation. In practice either additional travel limitations are applied or self-protecting compensation springs are used. Additional travel limitations made of catch-ropes are critical when the function relevant length of the travel limitation is set but have a clear advantage in cost in comparison to compensation springs.

Mounting of one-side actuation

Compensation spring with travel limitation

Mounting of two-side actuation

Distance of wire support

The actuating travel required to vertically actuate the switch results from the sum of the spring travels of the switch, pull-wire and where required compensation spring as well as the distance of the wire supports x [m]. This means a larger actuating travel is required with a larger distance of the wire supports when actuating the pull-wire in order to achieve the same actuating distance. Securing a safe switching at a constant pull-wire length the distance of the wire supports must be reduced in order to aim for a wider temperature range.

Type of pull-wire

The expansion behaviouer of the pull-wire is determined by the type of wire. Besides elastic elongation permanent elongations can occur when actuating the pull-wire. Under certain conditions higher pre-tension forces can lead to relaxation processes (temporal pre-tension loss). Statistical spread of the manufacturing process also have an effect on the expansion behaviour.

Therefore it is urgently recommended at least for longer pull-wire lengths to apply pull-wires from steute. These are much tougher and thus optimised for such applications.

Pull-wires from other manufacturers often lengthen gradually because of the creep characteristics of the plastic core (relaxation). If so, it is necessary to regularly check the pull-wire tension and if required to retension the pull-wire. The appropriate security note in the mounting and wiring instructions and the standard application of a tensioner are the prerequisite for a safe function.

Examples of other compensation spring variants

Mounting notes

  • After fitting the wire, pull strongly on it several times, as the pull-wire and the wire thimble will deform.
  • Subsequently, retense the wire using the wire clamp, eye-bolt or tensioner.
  • In order to guarantee safe operation, observe the enclosed mounting and wiring instructions.
  • According to EN ISO 13850, pulleys may only be mounted such that the complete length of the pull-wire can be observed.

Wire thimble deformation