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Safety Switch | USA

Interlock Switches

In many cases, serious accidents can occur when people approach machinery and equipment that is operating. And so, safety must be ensured by isolating people from the machinery and equipment in operation. On the other hand, there are times when work must be done inside the machinery and equipment, such as maintenance, setup, and cleaning. In these cases, safety must be ensured by bringing the machine to a complete stop before entering the machinery and equipment (Figure 1). This is called safeguarding by isolation and by shutdown and is the basic concept for ensuring the safety of machinery and equipment.

 
Interlock switches (sometimes referred to as door interlocking devices or guard interlocking devices) are installed on the front door of machine tools or on the door section of machine guard enclosing industrial robots to detect the opening and closing of the door (Figure 2).
In other words, this is a safety-related part of the control system that permits the machine to run only when the door is closed and keeps the machine stopped when the door is open.

Control by Interlock Switches

There are two types of interlock switches: the non-locking type, which allows the door to be opened at any time regardless of the state of the machine, and the locked type, which allows the door to be opened only when certain conditions are met.
First, the basic non-locking interlock switch is shown below.

The interlock switch has a structure where the switch body and the paired actuator are separated as shown in Figure 2.
The switch body is mounted to the machine, and the paired actuator is mounted to the door. The internal contact of the switch body turns on and off in conjunction with the opening and closing of the door (whether the actuator is inserted into the interlock switch body), and the signal is transmitted as information about the opening and closing of the door.
 

The figure on the left side of Figure 3 shows the state of the interlock switch when the door is closed, and the figure on the right side shows the state of the interlock switch when the door is open.


As shown in the figure on the left side, when the door is closed, the actuator mounted to the door turns the internal cam, and the NC contact turns on by spring force, causing current to flow in the circuit. In this state, the control system determines that the door is closed and the machine is ready for operation.

On the other hand, when the door is opened as shown in the figure on the right side, the actuator rotates the internal cam, and the NC contact is pushed down by the cam to turn off, thus cutting off the current in the circuit. In this state, the control system determines that the door is open and puts the machine into a stop state. Because this stop state is maintained, the machine will not start even if the start switch is used.


Structural Features of the Interlock Switch

The following is a description of the structural features of the interlock switch.

Protection against defeating for Interlock Switches

From a safety standpoint, an interlock switch detects the open/close status of the door so that the machine can be run only when the door is closed. However, there are times when the interlock switch is defeated with and disabled so that the machine is left running with the door open, for example, because it is troublesome to open and close the door every time a workpiece is loaded or unloaded, or because the operator wants to perform adjustment work while the machine is running. This is an extremely hazardous situation where a serious accident could occur at any time.

To prevent this kind of defeating for the interlock switch, as shown in Figure 4, the interlock switch is designed so that it cannot be disabled by screws, needles, or plate metal pieces other than paired actuators, or common tools and objects such as keys, coins, and screwdrivers.

In addition to the above, there are other considerations that must be taken into account when mounting the interlock switch body and the paired actuator to the machine to prevent them from being defeated with.
The requirements for protection against defeating are provided in the standard ISO 14119: Safety of machinery — Interlocking devices associated with guards — Principles for design and selection. The following link explains this standard, so please also read through this information.

Group Safety Standard: ISO 14119

 

Direct Opening Action

To ensure the safety of operators and maintenance personnel accessing the inside of the machine, when the door is opened, the NC contact inside the interlock switch must be opened to break the circuit and stop the machine.
For this reason, the NC contact built into the interlock switch must be equipped with a direct opening action.

This direct opening action is a function where the force to open the door is directly transmitted as the force to open the NC contact because the space between the actuator and the NC contact is comprised of inelastic structural material only. And so, even if an NC contact with the direct opening action has become welded (A state where the contact surface melts and then cools and adheres due to an inrush current exceeding the capacity during contact opening and closing) together, opening the door will open all the NC contacts, and the machine can be stopped.

NC contacts with this direct opening action are marked with the arrow symbol shown in Figure 5.

Selecting a Model Based on the Need to Lock Door

There are two main types of interlock switches: non-locking and locking. Also, there are two types of locking switches: locking switches for safety purposes (spring lock type) and locking switches for non-safety purposes (solenoid lock type).
For these three types, Figure 6 shows when to select each type of interlock switch.
 
As shown in Fig. 6 (1), if the hazardous moving parts of the machine stop immediately when the door is opened, and if the door does not need to be locked, select the non-locking interlock switch described so far.

As shown in Fig. 6 (2), if the door does not need to be locked for safety reasons because the hazardous moving parts of the machine stop immediately when the door is opened, but the door needs to be locked for production or other reasons, select a solenoid lock interlock switch (locking type for non-safety purposes).
As shown in Fig. 6 (3), if the hazardous moving parts of the machine do not stop immediately when the door is opened, and there is a possibility of harm to people due to coasting operation or similar hazards, select a spring lock interlock switch (locking type for safety purposes).

Applications for Spring Lock Interlock Switches

Figure 7 shows an example of a spring lock interlock switch mounted on a machine tool door. It is primarily mounted on machines that have a large inertia and cannot be stopped immediately. The door is securely locked while the machine is running, and the door can only be opened after the coasting operation has completely stopped. This enables prevention of harm to people due to the coasting of the machine.

Spring lock interlock switches are primarily used in machines that have a high risk of mechanical hazards, such as large machining centers and lathes that continue coasting even after the stop signal is issued and industrial robots that continue moving until the end of the cycle even after the stop signal is issued. In addition to mechanical hazards, these switches are also used to provide isolation from thermal hazards and other hazards that require time for reducing the risk.

Safety Features of Spring Lock Interlock Switches

The basic operation of locking and unlocking a spring lock interlock switch is shown in Figure 8.

As shown in the left figure of Figure 8 above, when the door is closed and the actuator is inserted, the rod moves to the left by the force of the compression spring and is locked in the recess of the cam. In this state, the cam cannot rotate due to the rod, and so the actuator cannot be pulled out and the door is locked.
To unlock, as shown in the figure on the right, a current is applied to the solenoid to generate a magnetic force, and when the generated magnetic force exceeds the force of the compression spring, the rod moves to the right, and the lock is released, enabling the cam to rotate.

There is a safety reason for using this type of lock/unlock mechanism. This is because this mechanism takes into account the possibility of unexpected power outages and wire disconnections. If the lock were released due to an unexpected power outage or wire disconnection, an operator could get caught in the coasting operation of a machine with a large inertia that could not stop immediately, resulting in an accident.

When a spring lock interlock switch is used, even if an unexpected power failure or wire disconnection occurs, the solenoid cannot be powered, and so it cannot be unlocked and the door cannot be opened. Consequently, an accident will not occur due to a person getting caught in the machine while it is coasting.

For such machines with a high risk due to coasting operation, spring lock interlock switches must be used to ensure safety in case of unexpected power outages or wire disconnections.

Structure of Spring Lock Interlock Switches

The standard structure of a spring lock interlock switch is shown in Figure 9.
 

The left half of Figure 9 is almost identical to the structure of a non-locked interlock switch. When the door is opened or closed, the cam rotates in conjunction with the actuator mounted to the door to open or close the contact C for detecting door opening or closing, and this action opens or closes the circuit between terminal 1 and terminal 2.

On the other hand, the right half of Figure 9 is the part that controls the locking and unlocking of the door. Figure 9 shows the state when the solenoid is de-energized (no voltage being applied) and rod B is locked by compression spring F to engage the recess of the cam.
In order to operate the machine, the door must be closed and also locked as shown in Figure 9. And so, by connecting contact C and contact D in series, the machine is operational only when both are turned on.

Spring Lock Interlock Switches and Door Opening/Closing

Figure 10 shows the relationship between a spring lock interlock switch and the opening and closing of a door.

State 1

The door is closed and locked, and the machine is operational. Looking in more detail, the actuator is inserted into the switch body, and so the NC contact of section C is closed. The solenoid is not excited, the rod is locked by the force of the compression spring engaging the recess of the cam, and the NC contact of section C is also closed. As a result, current flows from terminal 1 to terminal 2. This means that the door is closed and locked, and the machine is operational (safe state).
 

State 2

The machine has stopped, and the door is unlocked. After separately detecting that the machine has stopped due to the end of coasting or other motion, the solenoid is excited, and the NC contact of section D is open. As a result, the current from terminal 1 to terminal 2 is interrupted, and the machine is non-operational.
Note that the solenoid has been excited, and the door has been unlocked, and so the door can be opened.
 

State 3

The door is open. The actuator has been pulled out, and so the NC contact of section C has been opened by the rod. Rotation of the cam holds the rod in the open position of the NC contact of section D regardless of the excitation state of the solenoid. In this way, when the door is open, both the NC contact of section C and the NC contact of section D are open to maintain the machine in a stopped state with high reliability.
 


Then, to restart the machine, the door is closed to return it to state 1.

However, with a few exceptions, simply closing the door and allowing the machine to run may lead to unexpected accidents. Therefore, it is necessary to make sure that the machine can only be restarted by pressing another start button provided in a safe location after the operator has checked the safety of the surrounding area.

Applications for Solenoid Lock Interlock Switches 

Figure 11 shows an example of an interlock switch installed on a machine tool door.
Doors must be closed while the machine is operating to isolate people from potential hazards.
If the machine stops as soon as the door is opened, the door does not need to be locked for safety reasons.
However, it may be necessary to lock the door for production reasons, such as not wanting to stop the line unnecessarily or to avoid damaging the workpiece being machined and discarding it, although it is safe to open the door because the machine stops as soon as the door is opened.

Safety Features of Solenoid Lock Interlock Switches

The basic operation of locking and unlocking a solenoid lock interlock switch is shown in Figure 12. To lock the door, power is supplied to the solenoid after the door is closed and the actuator is inserted. The electromagnetic force causes the rod to lock into the recess of the cam against the force of the compression spring. When the power supply is cut off, the rod moves in the opposite direction of the cam (to the right in Figure 12) by the force of the compression spring, and the door is unlocked.
 

If there is an unexpected power outages or wire disconnection while the door is closed and locked, the lock will be released by shutting off the power to the solenoid, and the door can be opened. Therefore, mistakenly using a solenoid lock interlock switch on a machine with a hazardous coasting operation is extremely dangerous because when the operator opens the door to check the internal state of the machine in the event of a power outages or wire disconnection, an accident could occur if the machine was performing a coasting operation. And so, solenoid lock interlock switches can be used only on machines that do not have coasting operation and that stop immediately when the stop signal is received.

Solenoid Lock Interlock Switches and Door Opening/Closing

Figure 13 shows the relationship between a solenoid lock interlock switch and the opening and closing of a door.

State 1

The door is closed and locked, and the machine is operational. Looking in more detail, the actuator is inserted into the switch body, and so the NC contact of section C is closed. Also, the solenoid is excited and the rod is locked against the force of the compression spring by engaging the recess of the cam, and the NC contact of section D section is closed.
As a result, current flows from terminal 1 to terminal 2. This means that the door is closed and locked, and the machine is operational (safe state).
 

State 2

The solenoid is de-energized so that the rod is released from the recess of the cam by the compression spring, and the cam is able to rotate. In this state, the NC contact of section D is open. As a result, the current from terminal 1 to terminal 2 is interrupted, and the machine is non-operational. The door has been unlocked, and so the door can be opened.
 

State 3

The door is open. The actuator has been pulled out, and so the NC contact of section C has been opened. Rotation of the cam holds the rod in the open position of the NC contact of section D regardless of the excitation state of the solenoid. In this way, when the door is open, both the NC contact of section C and the NC contact of section D are open to maintain the machine in a stopped state with high reliability.
 


Then, to restart the machine, the solenoid must be excited after the door is closed to lock the door and return it to state 1.

Installation Notes for Interlock Switches

There are some points to consider when installing a non-locking interlock switch, a solenoid lock interlock switch, or a spring lock interlock switch on a machine.

For example, there should be an appropriate gap between the actuator and the switch body when the door is closed, or a special stopper should be installed on the door (Figure 14). This will prevent the actuator or the door itself from colliding with the interlock switch body, causing the mounting position to be shifted or damaging the switch.

Also, take notice of the bouncing of the door due to recoil when the door is closed with force. This is because bouncing of the door could lead to the following situations.

● For non-locking interlock switches, bouncing of the door applies a sectional force to the internal cam of the interlock switch, causing only one side of the dual NC contacts to open. As a result, this would be incorrectly recognized as a failure, and the machine would be non-operational.

● For locking interlock switches, bouncing of the door applies a force to the locking mechanism, which generates a frictional force on the movement of the rod, and the door cannot be opened because the rod does not move even when trying to unlock it.

● For locking interlock switches, repeated bouncing of the door applies repeated force to the locking mechanism, which could damage the locking mechanism and/or NC contact opening/closing mechanism.

Conditions under which the machine can be operated 

To run the machine, the machine must satisfy both conditions of the door being closed and also locked. For example, as shown in Figure 9, these conditions can be satisfied by connecting the contact for detecting door opening/closing and the contact for detecting locking/unlocking in series.
Of course, this method is adequate from a safety standpoint, but it requires two NC contacts, one for detecting door opening/closing and the other for detecting locking/unlocking.

 

On the other hand, there are products that have NC contacts in the structure of the interlock switch itself that only turn on when the door is closed and locked. If these products are used, the size of the interlock switch can be relatively small because only one NC contact is required, and the cost can be reduced because the number of contacts is small. These NC contacts, which are only turned on when the door is closed and locked, are indicated by the lock monitoring mark shown in Figure 15. NC contacts with this lock monitoring mark can be used in the safety-related part of the control system for serving as one of the safety conditions for running the machine.
However, there is another point to consider if using this NC contact with lock monitoring mark as a safety condition by itself.
That is, what happens when the locking mechanism breaks? For example, if a static force exceeding the actuator pull-out strength specification value at the time of locking specified by the manufacturer is applied to an interlock switch in the locked state, or if a dynamic force is repeatedly applied to the locking mechanism due to door bouncing or other action, even if the force is less than the actuator pull-out strength specification value at the time of locking, the locking mechanism may be damaged.

And so, an extremely important issue for safety is whether the machine will stop (safe failure) or not (dangerous failure), that is, which failure state occurs if the locking mechanism breaks and the door opens. This is because the interlock switches with the lock monitoring mark are primarily used on machines where the operator may be involved in a serious accident.

And so, when selecting a spring lock interlock switch, it is recommended that you select a product that displays the lock monitoring mark shown in Figure 15 and that you check the failure status in case the locking mechanism is damaged.

If the locking mechanism of the spring lock interlock switch is damaged and the machine fails on the dangerous failure and does not stop, or if it is uncertain whether the machine will fail on the dangerous failure or the failure, a door with a non-locking interlock switch or a safety limit switch must be installed separately from the spring lock interlock switch. If PL=e with Category 4 is needed, in the same way, the safety standards require installation of a door with a non-locking interlock switch or safety limit switch in addition to a spring lock interlock switch.
 

Safety Interlock Product Descriptions