Moeller easy relays help manufacturer save money

Moeller easy relays, combined with the skill of independent automation expert, Nick Howlett, are making it possible for a major UK manufacturer of car seatbelts to achieve cost savings.

At the heart of every inertia-reel seatbelt is a cassette containing a pre-tensioned spiral spring, the function of which is to retract the belt when it is not in use.

While in the past the cassettes were manufactured in the UK, no suitable machine was available in this country to tension the spring.

It was necessary, therefore, to ship them to Germany for this process, and then to ship them back to the UK for incorporation into the finished seatbelt assemblies.

Since this procedure was costly and inconvenient, Nick Howlett was approached to assist in the design of an efficient and cost-effective tensioning machine that could be produced and used in the UK.

The principal function of the machine is to tension the spiral spring, winding it through a preset number of turns, then inserting a clip to link the tensioned spring to the frame of the cassette.

Several methods of achieving this were considered, including the use of expensive stepper motors and servo drives.

Howlett, however, was able to propose a simpler and less costly solution, based on the use of an ordinary single-phase capacitor run motor to tension the spring, DC injection braking and a turns counting system operating from a sensor on the spring winding motor.

To control this arrangement, he selected intelligent easy control relays from the Moeller Electric range.

The final version of the control system for the spring tensioning machine uses an easy 719 intelligent relay with an easy 618 expansion unit to provide additional I/O capacity, together with an easy 400 power unit.

The panel also incorporates a Moeller ESR4-NO-31 emergency stop/safety relay, which was chosen on the basis of its proven functionality and its competitive pricing compared with other similar devices.

For the operator interface, Moeller's RMQ Titan indicator lamps and pushbuttons are used.

In operation, a 10-way rotary switch is used to tell the easy relay the number of turns required to pre-tension the particular type of spring cassette loaded into the machine.

When the cassette is in position, the operator simply presses the start button, and the easy relay signals the tensioning motor to start.

As the motor rotates, a single proximity sensor sends a pulse to the easy relay for each revolution.

When the required count has been reached, the easy relay removes power from the motor and closes a contactor that applies a DC injection current.

As a result, the motor stops virtually instantaneously.

The easy relay then energises a solenoid valve that inserts a shot bolt, followed by a second solenoid that inserts the spring retaining clip.

In addition, the easy relay monitors the operation of the emergency stop relay and controls its reset function on the basis of the position of the pneumatic cylinders.

It also drives indicator pass and fail lamps so that the machine operator can instantly see that each cassette has been correctly tensioned.

The spring tensioning machine is the first of its kind in the UK, and its introduction means that it is no longer necessary to send the seat belt spring cassettes to Germany for tensioning.

This is a significant cost saving for the seat belt manufacturer, and also eliminates problems with delayed or lost shipments.

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