Adding safety into the mix
Trapped Key interlocks are a concrete safety solution which are ideal for retrofitting to industrial pan mixers.
Ready-mix concrete that is prepared at an industrial concrete plant is made by mixing a combination of cement, water, and various aggregates, using specialist machines called 'Pan Mixers' that are capable of accurately measuring the exact proportions of each material to achieve a consistent, high quality mixture on every production cycle.
To ensure that these mixers continue to run at optimal efficiency, operators may need to gain regular access inside the machine for cleaning and maintenance purposes. Operators can gain entry into the pan mixer through a series of hinged inspection doors located around the circumference of the mixers 'hood enclosure.'
However, the rotary mixing arms pose a potential safety risk to any personnel that may come into contact with these moving parts during a scheduled cleaning operation. Therefore, a reliable safeguarding method is required to prevent operators from opening any of the mixer doors before power to the machine has been isolated and the mixers rotary arms have come to a controlled stop.
As these machines do not require machine guarding to be fitted around its perimeter, each hatch door needs to be fitted with a robust safety device that keeps the doors locked closed until the machine has entered a safe state of operation.
Trapped Key interlocks remove the need for wiring, making it easy to install them on the mixers large hatch doors. All locks are fitted with dust covers to protect against a build of dust, generated from the mixing process, clogging up the locks internal mechanisms which could compromise the integrity of the safety system
To prevent operators from opening the hatch doors while the mixer is operational, a purely mechanical Trapped Key system was retrofitted to the existing equipment to create a safe sequence of operation that forces personnel to isolate power to the machine, before the hatch doors can be opened.
All keys within the sequence have been uniquely coded so that they only operate their corresponding locks. meaning that the sequence cannot be circumvented and the risk of intentional defeat is virtually eliminated.
Power to the machine is isolated by turning the electrical disconnect switch to the 'OFF' position. To lock the disconnect switch in place, a mechanical Bolt Module has been retrofitted so that removing the Primary Access Key will fire the 16mm bolt mechanism into the corresponding hole that has been drilled into the electrical disconnects handle, preventing any further movement of the handle.
The remaining two Access Keys can also be removed once the bolt mechanism has been fired and all three Access Keys can be used to unlock any of the three mechanical door locks that have been fitted to each of the mixers hatch doors.
To unlock the Pan mixers hatch doors, an Access Keys must be inserted into the mechanical Door Module and turned to the locked position to release the actuator so that the door is no longer locked closed.
The Access Key remains trapped in the Door Module until the mixer door has been closed and the actuator has been reapplied to the Door Modules head mechanism. However, the machine will only recommence its operation once all three Access Keys have been returned to their corresponding locks on the Bolt Module which enables the bolt mechanism to be retratced, enabling the electrical disconnect switch to be used to restart the power.