ATEX: Inertia vs Pre-engaged Starter Motors

May 1st, 2015

When it comes to choosing a start system for diesel powered equipment, operators will often be faced with a choice between inertia and pre-engaged starter motors. If the equipment is to be used in a hazardous environment (such as a mine or offshore platform) the starter motor will require ATEX approval. In this situation a pre-engaged starter motor is the only choice.

But, what is the difference between an inertia and pre-engaged starter motor? And, why do inertia starters struggle to achieve ATEX approval? Let us explain by outlining how each system works.

Inertia Starting Systems

Inertia starters have long been the tried and tested method of engine starting. They work by rotating the pinion at high speed. Centrifugal forces then moves the pinion forward until its reaches the ring-gear. As it spins, the teeth of the pinion lock onto the ring-gear and crank the engine.

As soon as the firing speed has been reached, the pinion returns to its disengaged position. This method of engagement is simple, proven and reliable. However it does have the potential to cause a spark when the already spinning pinion makes impact with the stationary ring-gear. This makes it difficult to class inertia starters as safe in a hazardous environment according to ATEX regulations.

Pre-engaged Starting Systems

For a starter motor to be ATEX approved there can be no possibility of sparks. A pre-engaged starter motor achieves this by engaging the pinion with the ring-gear before it rotates. The starter motor is fitted with a relay valve which controls the movement of the pinion. When the starter motor is activated the pinion moves along the shaft until it comes into contact with the toothed ring-gear.

The pinion is then interlocked with the ring-gear before rotating to turn the engine.

As the engine gets up to speed the pre-engaged starter motor is protected from overspeed by a free wheel clutch (like the free wheel gear of a bicycle). At this stage the return spring retracts the pinion to its disengaged position.
Safety is critical when working in hazardous environments and IPU’s pre-engaged starter motors conform to ATEX EN1834-1 and 2.

For more information, contact our team on +44 (0) 121 511 0400 or contact us using this link.

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