Interlocked guards

Where guards need to be moved or opened frequently and it is inconvenient to fix them, they can be interlocked mechanically, elec-trically, pneumatically, etc., to the machine controls. Two basic criteria must be observed: until the guard is closed the machine should not be capable of being started; and the machine should be brought to rest as soon as the guard is opened. Where there is a run down time, the guard may need to be fitted with a delay release mechanism. The interlock system can provide either control interlocking which acts through the machine controls, power interlocking that operates by interrupting the primary power supply or by mechanical disconnection of the machine from its power source. Different arrangements of interlocking systems are reviewed in BS EN 1088⁵.

An essential feature of an interlock or safety circuit is that it must be completed before the machine can start and that any break in it trips the controls and brings the machine to rest. With hydraulic and pneumatic safety circuits, the circuit must be pressurised in the safe condition, any loss of pressure causing the system to trip.

Interlocked guards can be hinged, sliding or removable but the integrity of the design of the interlocking mechanisms is crucial. The mechanisms must be reliable, capable of resisting interference and the system should fail safe.

Interlocking guards allow ready access while ensuring the safety of the operator. However, there are circumstances that may require the machine to be moved when the guard is open, i.e. for setting, cleaning, removing jams, etc. Such movement is only permitted under the following circumstances:

1 As part of a ‘permit-to-work’ system.

2 On true inch control.

3 On limited inch control with each movement of the producing part not exceeding 75mm (Sin) at a predetermined minimum speed.

4 If for technical reasons the machine or process cannot accept inter-mittent movement, a continuous movement is permitted provided it is

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only at a predetermined minimum speed and is controlled by a hold-on switch, release of which causes the machine to stop immediately.

Also, there should be only one such control operable on a machine at any one time.

The choice of interlocking method will depend on power supply and drive arrangement to the machine, the degree of the risk being protected against and the consequences of failure of the safety device. The system chosen should be as direct and as simple as possible. Complex systems can be potentially unreliable, have unforeseen fail-to-danger elements and are often difficult to understand, inspect and maintain, and can have low operator acceptability.

3.3.6.1 Types of interlocks

(a) Direct manual switch or valve interlocks (Figure 3.3) where the switch or valve controlling the power source cannot be operated until the guard is closed, and the guard cannot be opened at any time the switch is in the run position.

(b) Mechanical interlocks provide a direct mechanical linkage from the guard to the power transmission shaft. The most common application is on power presses (Figure 3.4).

(c) Cam-operated limit switch interlocks are versatile, effective and difficult to defeat. They can be rotary (Figure 3.5) or linear (Figure 3.6) and in each case the critical feature is that in the safe operating position the switch is relaxed, i.e. the switch plunger is not depressed. Any movement of the guard from the safe position causes the switch plunger to be depressed, breaking the safety circuit and stopping the machine. This is the ‘positive mode’ of operation (Figure 3.6(b)) and must be used whenever there is only one interlock switch. ‘Negative mode’ of operation (Figure 3.6(a)) occurs when the switch plunger is depressed as the guard moves to the safe position and is not acceptable for single switch applications. However, a combination of the two in series is used on high risk machines such as injection moulding machines. This arrangement can incorporate a switch condition monitoring circuit as shown in Figure 3.7. The type of electrical switch used in interlocking is important. They must be of Safe use of machinery 4.81

the positive make-and-break type (known as ‘limit’ switches) that fail to safety and have contacts capable of carrying the maximum current in the circuit. The principle of operation of such switches is shown in Figure 3.8. Micro-switches relying on leaf spring deflection for contact breaking are not acceptable.

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(d) Trapped key interlocks (key exchange system) work on the princi-ple that the master key which controls the power supply to the machine through a switch at the master key box, has to be turned OFF before the keys for individual guards can be released. The master switch cannot be turned to ON until all the individual keys are replaced in the master box. Each individual key will enable its particular guard to be opened, releasing the guard key which the operator should take with him when he enters the machine. The

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individual key is trapped in the guard lock until the guard is replaced and locked by the guard key. A diagram of a typical installation is shown in Figure 3.9.

(e) Captive key interlocking involves a combination of an electrical switch and a mechanical lock in a single assembly where usually the

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key is attached to the movable part of the guard. When the guard is closed, the key locates on the switch spindle. First movement of the key mechanically locks the guard shut and further movement actuates the electrical switch to complete the safety circuit (Figure 3.10).

(f) The type of magnetic switch shown in Figure 3.11 which uses a number of magnets uniquely configured to match components of the switch part, provides a high degree of protection. It also has the advantage, since it is encapsulated, of withstanding washing, a benefit in the food industry. Other non-contact switches work through inductive circuits between an actuator and the switch.

(g) Time delay arrangements are necessary when the machine being guarded has a large inertia and, consequently, a long rundown time on stopping. An electromechanical device is shown in Figure 3.12 where the first movement of the bolt trips the machine, but the bolt has to be unscrewed a considerable distance before the guard is released. A solenoid-operated bolt can also be used in conjunction with a time delay circuit that is actuated from either the machine controls or the trip circuit.

(h) Mechanical scotches are required on certain types of presses to protect the operator when reaching between the platens. These

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scotches can be linked to the guard operation so that they are automatically positioned each time the guard is opened. Similar devices are needed to restrain the raised platform of a tipper lorry when work is done on the chassis. However, with scissor lifts the scotch must be inserted between the bottom rollers and the base frame.

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