Part of my Victorian Steam engine restoration is fitting the eccentrics on the main shaft for the slide valves, obviously these need to be positioned correctly along the shaft.

The eccentrics are held in place by a flat, wedged metal key that fits in a tapered recess in the eccentric and against a flat machined on the main shaft. This key is hammered into place when the eccentric is correctly positioned. Fine so far.

There is also a screw in the eccentrics that locates onto the key. What pupose does this serve? It can't be for intial or subsequent later adjustment as once the wedge of the key is hammered into place nothing is going to shift! Totally baffled – or am I misunderstanding something obvious? Any thoughts appreciated

Photo – best I can do in limited space – shows the eccentric with its screw, the flat on the shaft ( To the left of the eccentric at the moment – which will be roughly where the eccentric finally ends up ) and the key just loosley placed on the shaft to give scale / relationship. Ignore the adjacent vee pulley.

Thanks for all the ideas chaps – much appreciated, but I'm still not getting my head around this concept!

IAN Perfectly valid suggestion!

JASONB There isn't any radial adjustment as the key and the keyway are close tolerence and the flat on the shaft of course is static, so even with the screw loose you cannot adjust the position radially.

PAUL The key themselves are a flat wedge ( like a cheese slice ) close fitting in similarfly tapered ways in the eccentric they have a slope of about 1 in 12 to my reckoning.

HOWARD They are very close fitting, but when in situ, even just pushed in finger tight, there is no gap at the top at all, so would not appear to be able to lift as you suggest.

ROGER so why do they use a wedged key? Surley a flat one that the screw then locked in position would be adequate? In some ways better as once the ' wedge ' becomes even the tiniest bit loose they key displaces and becomes ineffective.

CLIVE Following all the thoughts here, I'm starting to wonder, as per your comments, if these keys should NOT be ' hammered ' in tight, but just ' firmly inserted ' instead and then the screw tightened. Whilst not giving any radial freedom to adjust, you would find it easier to position on the shaft to accurately line it up with the rods activating the slide valves on the cylinders?

When I first received the engine it was in a dire state and everything was rusty, siezed solid or corroded so evary part had to be hammered off, so I have no idea what the original fit might have been like.

Any further thoughts or observations will be much appreciated. Rather cool, grey Bank Holiday weather here in the Midlands today, so don't think I will be venturing far or wide , so probably a workshop day anyway!

All the best. Mike