These devices are suited to machines with run down characteristics but they can also provide a significant increase in protection level for most types of machines.

They provide interlocking of the hazard power source with guard movement and also prevent the opening of the guard until it is safe to do so.

These devices can be divided into two types: Unconditional and conditional guard unlocking. Unconditional Guard Unlocking These devices are manually operated and the guard can be opened at any time. A handle or knob which releases the guard lock also opens the control circuit contacts.

On a device such as the BL6009 time delay bolt switch, a time delay is imposed (fig. 66). The bolt which locks the guard in place also operates the contacts. The bolt is withdrawn by turning the operating knob. The first few turns open the contacts but the locking bolt is not fully retracted until the knob is turned many more times taking up to 20 secs.).

The Prosafe trapped key system see power interlocking section) can be used to provide unconditional guard locking.

Features:

These devices are simple to install and they are extremely rugged and reliable. The time delay bolt switch is suitable mainly for sliding guards.

For consideration:

The stopping time of the hazard being guarded must be predict-able and it must not be possible for the bolt to be withdrawn before the hazard has ceased. It must only be possible to extend the bolt into its locked position when the guard is fully closed. This means that it will usually be necessary to install stops to restrict the travel of the guard door.

Conditional Guard Unlocking

With these devices, the guard can only be opened on receipt of a signal showing that:

• the contactor is OFF.
• a pre-set time interval has elapsed or alternatively that dangerous motion has ceased.
  

These signals are usually derived from the auxiliary contacts of the contactor and either a fail safe timer unit such as the SMT01 or a stopped motion detection unit such as the SMD02 or SMD03.

They provide interlocking of the hazard power source with guard movement and also prevent the opening of the guard until it is safe to do so. Because an externally generated signal is required for lock release, it makes them particularly suitable for use with PLC or other programmable electronically controlled machines. The usual type of interlocking device for these purposes is a solenoid operated locking switch such as the TL8012 (fig. 67), TL8018, or TL5012.

(fig. 67)

For less frequently accessed applications, the Prosafe trapped key system (see power interlocking section) can be configured to operate in this way. fig. 68


In the example shown in fig. 68, the hazard ceases as soon as power is isolated by switching the machine off at the normal controls. The guard cannot be opened until the contactor has switched off. Once the guard is opened the control circuit contacts ensure that the contactor cannot be re-energized until the guard is closed and locked. With the configuration shown in fig. 69, the switch will not release the guard until the contactor is off and a pre-set time interval has elapsed. The time range can be set from 0.1 seconds to 40 minutes. The longest stopping time of the machine should be predictable and constant. It should not rely on braking methods which may degrade with use.

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.