Features:
The actuator and mechanism are designed to prevent easy cheating of the safety switch. These devices are straightforward to install and are very reliable. They can be used on sliding, hinged and lift-off guards and because of their versatility they are one of the most commonly used types of interlock switches.
For consideration:
The guard mounted actuator needs to remain reasonably well aligned with the entry hole in the switch body. Actuator operated switches can be difficult to clean thoroughly. This may be a problem in industries such as food manufacturing and pharmaceuticals.
Hinge Operated Actuation
The device is mounted over the hinge-pin of a hinged guard. The opening of the guard is transmitted via a positive mode operating mechanism to the control circuit contacts (fig. 60).
Features:
When properly installed, these types of safety switches are ideal for most hinged guard doors where there is access to the hinge center line. They can isolate the control circuit within 3° of guard movement and they are extremely difficult to defeat without dismantling the guard.
For consideration:
Care must be taken on very wide guard doors, as an opening movement of only 3° can still result in a significant gap at the opening edge. It is also important to ensure that a heavy guard does not put undue strain on the switch actuator shaft.
Cam Operated Actuation
This type of arrangement usually takes the form of a positive mode acting limit (or position) switch and a linear or rotary cam. It is usually used on sliding guards and when the guard is opened the cam forces the plunger down to open the control circuit contacts (fig. 61).
Features:
The simplicity of the system enables the safety switch to be both small and reliable. For consideration: It cannot be used on hinged or lift-off guards.
It is extremely important that the safety switch plunger can only extend when the guard is fully closed. This means that it may be necessary to install stops to limit the guard movement in both directions.
It is necessary to fabricate a suitably profiled cam which must operate within defined tolerances. The guard mounted cam must never become separated from the safety switch as this will cause the safety switch contacts to close. This system can be prone to failures due to wear especially in the presence of abrasive materials or with badly profiled cams. It is often advisable to use two safety switches as shown in fig. 62.
Non-Contact (Non-Mechanical) Actuation
With these devices, the guard door is linked to the control circuit contacts of the switch via a magnetic or electronic field. To make these types of devices suitable for interlocking applications, they must incorporate enhancements to ensure their satisfactory operation.
Because they do not have the benefit of true mechanical positive mode operation, they must have other ways of ensuring that they cannot fail to a dangerous condition. This is achieved by either “oriented failure mode” principles or by the use of duplication and monitoring.
The MA Series uses the principle of oriented failure mode. Due to the use of special components, the only safety critical fault likely to occur would be a welding of the reed contacts due to excessive current being applied to the switch. As shown in fig. 63, this is prevented by the non-resettable overcurrent protection device. There is a large margin of safety between the rating of this device and the reed contacts. Because it is non resettable, the switch should be protected by a suitably rated external fuse.
fig. 63
It is important that the switch is only operated by its intended actuator. This means that ordinary proximity devices which sense ferrous metal for example are not appropriate. The switch should be operated by an “active” actuator. The security can be further improved by coding such as on the MC-K Series. This switch uses the same oriented failure mode principle as the MA Series but has higher security due to its coded actuator and sensor.
Features:
Non-contact devices are available in fully sealed versions which makes them ideal for hygiene sensitive applications, as they can be pressure cleaned and have no dirt traps. They are extremely easy to install and have a considerable operating tolerance, which means that they can accept some guard wear or distortion and still function properly.
Sophisticated non-contact devices, such as the MF Series, feature an electronic coding principle. It also has two separate and diverse switching “channels”. A control unit is available which monitors up to six sensor sets and can also monitor the contactors and wiring fig. 64).
For consideration:
For the simpler actuation types, if security is an important issue, it may be necessary to install them as shown in fig. 65 so that they cannot be accessed while the guard is open. It is important, particularly for the non coded types, that they are not subjected to extraneous interference by fields which are the same as their operating principle.
Fig. 70 shows a configuration where the switch will not release the guard until the contactor is off and all motion has stopped.
On the systems shown in figs. 69 & 70, the machine is stopped by its operational control system either manually or automatically. Therefore, these systems are especially useful on machines where tool damage or program loss could be caused by a crash stop due to inappropriate guard opening.
