Purpose of screw on keyway

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.

Belt and braces?

Such screws are said to make for more positive key location and, when used with drive in keys, resist fretting. Not sure that I follow the engineering justification when tight drive in keys are use but it has been regularly done.

More usual to find them where ordinary feather keys are used to retain pulleys and the like as such keyed systems are usually a smidgin slack in the pulley so assembly can be done by a firm hand push. Generally the keyway in a pulley is a little deeper than the key projection so the screw has obvious utility in keeping the key fully home in the shaft slot.

Clive

Screw quite possibly used for initial setup to get the timing right and then when happy the eccentric is keyed, screw can also help stop sideways movement with the key providing drive and correct angular position.

Keys are usually defined as side fitting, therefore the screw stops the key from lifting. That is supposing all the sizes are to the standards.

Location along the axis?

I don't see anything else to stop it wandering in this case.

Taper keys (usually gib head) as long as the taper is slow enough (off the top of my head shallower than 1 in 9) fit on all four faces and are designed to be self locking. If you ever have cause to remove one on say an old line shaft pulley or steam engine flywheel you will see how well! The top of the keyway is normally cut at the same angle as the key. Usually a quick blue check prior to driving properly home to verify. I fitted the flywheel on my traction engine so, it was going on temporarily to come off later and I only gave it a light tap to seat, was quite a fight to remove it again!

Paul.

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

Mike, I think you misread my comment. Screw used for initial adjustment on a plain shaft, once happy with the correct timing THEN the flat/keyway is cut so no risk of eccentric slipping out of position radially

"Screw quite possibly used for initial setup to get the timing right and then when happy the eccentric is keyed"

If the angle is 1:12 then i's a bit steep to be self locking. Gib head keys are 1:96 or 1:100 so screw will be needed for axial location, and drive only from the key.

"screw can also help stop sideways movement with the key providing drive and correct angular position"

I think the reason is that the end of the screw bears on a flat surface that can accept the slight gourging that the end of say an allen type grub screw will leave on the top surface of key. If the grub screw is oriented anywhere else it will score the shaft, making removal much more difficult.

Alan

Thanks for the follow-ups. & I get your thinking now Jason.

regards Mike

Just a thought, but the wedge and screw imply to me that the joint is meant to come apart without too much bother for adjustment or maintenance. For that reason not many engine parts are permanently fixed. Although blown head gaskets could be avoided by welding the head to the cylinder block, makes it very difficult to replace piston rings, or regrind valve seats etc. Engines are designed with maintenance in mind, though judging by what's under the bonnet of my car, home maintenance comes bottom of the list.

I suggest although the coarse wedge is tapped in tight, the joint can be broken if need be. It's like a Jacobs Chuck taper; semi-removable. The screw protects the wedge from coming loose due to single-cylinder vibration, which has a powerful loosening effect on joints.

On a lightly loaded model engine fired up occasionally for amusement rather than hard work, the precaution is probably unnecessary but I'd put thread-locker on the screw if the engine was going to be run for more than a few hours. Scale matters: many design features make more sense on a full size engine than they do on a model, and vice versa!

Dave

Yes Dave, I tend to agree now after reading all the other postings and have thought some more about it. There isn't a need for a permanent fixing just as log as it stays in position in use and I suppose can be removed if it wears. The slots in the cheese headed screws were pretty chewed which may have indicated lots of usage over many years - or possibly just one ham-fisted user?

Regards Mike