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01 July 2004

More than a well balanced breakfast

By Ian Williams

Achieving a balance between the requirements of a more demanding production schedule and the need for high levels of operator safety is never easy, but cereal maker Dailycer has achieved it on two mixing/stirring vessels with a simple system based on trapped key interlock switches. The mechanical safety system helps overcome costly interruptions to production by allowing addition of dosing and additives during the mixing operation. At the same time, it safeguards the machine operators by helping to prevent accidental contact between personnel and the moving agitators and shaft.

Dailycer Ltd. is one of the largest suppliers of cereals to major U.K. supermarkets. Dailycer must consequently meet demanding production schedules.

Central to Dailycer's operations are two large vessels used for mixing syrups. Entry points on top of each vessel give personnel access to add extra ingredients, such as sugar, to the mixture. Workers need to add these extra ingredients while the mixer is in operation, otherwise the ingredients settle on the bottom of the vessel. When this happens they do not always dissolve properly, resulting in quality problems with the finished product.

Until recently, adding ingredients during machine operation was not an option due to safety concerns, which centered on the fact that it was possible for the rotating agitators and shaft within the mixing vessel to contact the limbs of any personnel adding ingredients. A very proactive company in terms of safety, Dailycer actively pursued a solution to this problem and entrusted engineering manager Gwilym Jones with the job of finding a solution.

"We were fully aware from the outset of this project that by finding an answer to the safety problem we could also improve our production," said Jones. "The problem was finding a balance between the two. Any solution had to be simple, because in our experience, factory personnel bypass complicated guarding systems very quickly."

"A second major consideration was the environment. The atmosphere around the mixing vessels is extremely steamy due to the boiling of syrups, so we really wanted to avoid using any safety system that was electrically based or susceptible to corrosion.

"The solution we came up with takes into account all the above considerations. To overcome the problem of stopping the machine, we've designed a basket that allows us to add material with no possibility of catching or trapping hands or arms. The basket is square in section with sides of 3 millimeter stainless steel." A hole is drilled through one side and also extends through the body of the mixing vessel itself. Welded to the basket is a chain, complete with its attached actuator bolt, from a single key chain interlock system. When the basket is in place, the actuator bolt pushes through the hole in its side and through the side of the mixing vessel. With the bolt in place, the trapped key that resides in the key interlock switch releases and subsequently inserts into the isolator that allows the mixing vessels to start. If the key leaves the isolator at any time, the mixing operation stops. You can then insert the key into the interlock switch to allow removal of the bolt and, subsequently, the basket itself.

"We installed the system ourselves, and since installation it is working really well," Jones said. The trapped key interlock system allowed the company to avoid electronics and also have a stainless steel construction—untroubled by the steamy environment. "We can also get replacement coded keys in the event that we lose one, which is very important," she said. Combining a simple design with a safety interlock system offered enhanced levels of safety and uninterrupted levels of production.

Behind the byline

Ian Williams is marketing manager at EJA Ltd., an Allen Bradley machinery safety company in Wigan U.K.

A stainless steel basket helps protect employees from moving parts inside the vessel.

The actuator bolt attached to the basket is pushed through holes in the basket and mixing vessel.

The actuator bolt operates this switch from inside the vessel, releasing key "K."

The trapped key interlock system allows power to be turned on only when key "K" is released from the mixing vessel and inserted into this isolator.

Dailycer employees can now safely add ingredients to cereal mixtures while the agitators are running.