Technical information

Aplicação

As Chaves de Emergência acionadas por cabo são de grande importância para a interface Homem-máquina em aplicações industriais. Elas são usadas, por exemplo, em transportadores por correias. Após sua atuação o sistema é paralisado.

Quando a nova norma europeia harmonizada EN 60947-5-5 referente a aspectos funcionais e orientações para dispositivos de parada de emergência tornou-se efetiva, novos requisitos tiveram que ser contemplados nestes dispositivos de comando. Todas as chaves de emergência acionadas por cabo neste capítulo atendem estes requisitos.

Design e princípio operacional

Em chaves de emergência acionadas por cabo o comando de parada de emergência pode ser acionado de qualquer ponto ao longo do cabo. Elas possuem uma ligação positiva entre os contatos NF e o cabo. As chaves de emergência acionadas por cabo são colocadas em condição de acionamento primeiramente realizando-se o pré-tensionamento do cabo, quando então os contatos NF e NA podem passar ao estado de repouso. Isto ocorre pois todas estas chaves são equipadas com detecção de quebra de fio. No capítulo “Acessórios” podem ser encontrados os produtos para a instalação adequada.

Chaves de emergência acionadas por cabo sem travamento mecânico VD ou VS não atendem a EN ISO 13850 e a IEC/EN 60947-5-5. É possível atender os requisitos de ambas as normas através de um circuito adequadamente feito e tecnologias de controle.Existem dispositivos para atuação uni ou bilateral. O comprimento, número de contatos e posição de montagem, no meio ou de um lado do sistema, são as principais características para a seleção de uma chave de emergência acionada por cabo.Todas as chaves de emergência acionadas por cabo levam a marca CE de acordo com a diretiva de máquinas 2006/42/EC.

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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