Slip Rings

Slip Rings

The slip ring provides a means of getting the secondary voltage from the rotating armature coil via a spring loaded carbon brush through the pickup and along the HT lead to the spark plug.

With a single cylinder magneto (and multi cylinder magnetos which use a distributor to connect to each spark plug in turn) there is a full circle of brass in the centre. With twin cylinder magnetos, there is a brass segment which covers only part of the circumference. Where this segment is positioned relative to the HT boss depends on the design of the magneto and the direction of rotation. This picture shows Lucas slip rings, left to right, for an anticlockwise K2F, a K1F and a clockwise K2F.

Twin cylinder magnetos have two brushes and the brass segment makes contact with each one in turn to feed the two spark plugs. When the brush is not on the brass segment it will be running on the insulator and this can cause problems if the wrong type of brushes are fitted.
In this picture, the slip ring on the left shows the problem caused by using brushes which are too hard and/or springs which are too strong. The insulator wears considerably quicker than the brass and after a relatively short time, the brush wears a channel in the insulator. As a result, the brush has to bump up and down the step created at the start and finish of the brass segment.

The slip ring on the right shows the problem caused by using brushes which are too soft. A layer of carbon has been left all round the slip ring. With a twin, when one cylinder is under pressure on the compression stroke, the brass segment will be under the brush and pickup for that cylinder but, being an electrical conductor, the carbon deposits provide a track around the slip ring to the other brush. The high voltage will always follow the easiest path and so the spark is most likely to appear at the spark plug on the second cylinder which is on the exhaust stroke and therefore at a much lower pressure.

Broken slip rings are common – they are very fragile. Always check for and remove any safety gap screws before removing the armature from the magneto body and NEVER lever against the slip ring in an attempt to remove the inner bearing track – use a proper extractor.

Even once the bearing track is off, the slip ring may prove stubborn. If this is the case, gentle heat with a hot air gun will soften the ring and make it easier to pull off. Remember that, once it is hot it will be very easy to distort the slip ring so straighten everything out before it cools down.  Once off, it is not unusual for the slip ring material to ‘relax’ and close up the centre hole a little. If the slip ring is good enough to be reused, it may be necessary to open the hole out slightly with an adjustable reamer.

Before attempting to remove the slip ring, always have a close look at the outer ring. If it has a knurled or serrated edge do NOT attempt to pull the slip ring off. Some early slip rings – especially from Bosch magnetos – were multi-part units with the outer ring screwed onto the armature shaft as shown in these pictures.

Bosch also used other designs of three part slip rings. Here, the centre brass ring is keyed to the side shown on the left. When this design is used for twin slip rings, the centre ring (now made from an insulated ring with a brass segment) can be turned over to convert a clockwise slip ring to an anticlockwise one.

Slip rings come in all shapes and sizes.......

This final picture shows a slip ring which started to leak electrically but was not noticed by the driver so the short circuited spark continued to burn away the insulation until it was so bad, daylight could be seen through it! Normally of course, a magneto with this problem would cause misfires and eventually stop the engine long before it got this bad. This one was not noticed because it was in one of a pair of magnetos fitted to a twin mag/twin plug Bentley. When this magneto started to fail, the second magneto kept the engine running.
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