cotswold
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Post by cotswold on Apr 3, 2008 10:33:19 GMT
The first thing that I have had to do on completion of my pride and joy is a major strip down.The reason is a shifted valve gear eccentric.
Regardless of how I might do it now, each eccentric is currently held in place by a single 4BA grub screw that impinges upon the axle. This time I shall set the eccentrics off the loco, and by calculation, but the concern remains as to how long they will remain in their appointed positions. I have a vague recollection of reading somewhere that in the absence of hardened grub screws it is advisable to put a slug of hard metal under the grub screw. I imagine that this slug would be a short length of a broken drill or a suitable size hardened ball. If a broken drill is used then it would seem reasonable to grind the 'business end' to a point.
Can anyone words of advice and particularly of experience?
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Post by weldsol on Apr 3, 2008 10:37:36 GMT
Have you drilled the axle to locate the grub screw or are you just relying on the grub screw to grip the shaft
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cotswold
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Post by cotswold on Apr 3, 2008 10:40:47 GMT
Have you drilled the axle to locate the grub screw or are you just relying on the grub screw to grip the shaft That is one of the options that I might consider now but not what 'the book told me to do' then.
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Post by circlip on Apr 3, 2008 11:13:53 GMT
If you're using an "Allen" grub screw then it's already hardened, and if it's a cup point then it's surprising that it's slipped. If you set the eccentric, mark the position of the screw and then dimple the shaft, after applying a thread locking compound it shouldn't slip again.
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cotswold
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Post by cotswold on Apr 3, 2008 12:31:45 GMT
If you're using an "Allen" grub screw then it's already hardened, and if it's a cup point then it's surprising that it's slipped. If you set the eccentric, mark the position of the screw and then dimple the shaft, after applying a thread locking compound it shouldn't slip again. Thanks for that one! 'Loctite' is so obvious that I had overlooked it. This time I shall be setting the eccentrics off the loco and by measurement (using a setting jig that I have dreamed up). Therefore I can even take the opportunity of temporarily moving the eccentrics to one side and de-greasing before repositioning and tightening up.
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Post by freddo on Apr 3, 2008 12:44:22 GMT
Drill an oblique hole through the eccentric and axle, taper ream it and pop in a taper pin and file off the excess from the taper pin.
Freddo
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Post by havoc on Apr 3, 2008 15:39:38 GMT
To make sure with grub screws you need 2 of them at right angles. Best have a flat where the point grips as well or taking apart can be difficult.
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cotswold
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Post by cotswold on Apr 3, 2008 15:43:24 GMT
To make sure with grub screws you need 2 of them at right angles. Best have a flat where the point grips as well or taking apart can be difficult. That's probably my first choice next time (if I live that long!) - but I don't see it as a realistic option on a completed engine.
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Post by albert on Apr 3, 2008 18:49:31 GMT
Hello Cotswold, All the eccentrics on my T/E's are held tight by two * 2ba headless allen grub screws (about 45 degrees apart in the deepest part of eccentric) with the shaft drilled and tapped to half the screw length, NO locktite as removal may be required at a later date.No problems yet. Bye Albert
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DaveD.
Involved Member
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Post by DaveD. on Apr 3, 2008 19:59:00 GMT
Hi Cotswold Circlip is right. The cup screws very seldom come loose, you can tell one 'cos it leaves a small circular 'witness' mark. When I was in the early days of IT stuff, on Tabulators etc., they were invariably used, not the pointed things. Also, if you have the room to get one in, a further screw was put on top of the lower one, bit like a locknut. That was another common occurence. A bit baffling the first time it was encountered Dave dee
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paul
Member
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Post by paul on Apr 3, 2008 20:38:16 GMT
I've used the slug (free pin) under a grub screw several times now - for anyone building little engines it's a handy technique as it's easier than threading a long hole or just the end of a long hole. None have come loose yet but I don't know how much cop it'd be on a 'proper' engine. Source: Elmer's 'Scotty'
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Post by Shawki Shlemon on Apr 5, 2008 7:40:35 GMT
I do what Albert does but I only mark the shaft and use 222 loctite which not hard to undo . The two grub screws give the advantage set the valve gear and when one is happy then remove one at a time and mark the shaft and lock with no chance of the eccentric to move or slip.over the last 28 years i never had one moving or slipping .
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cotswold
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Post by cotswold on Apr 5, 2008 8:45:57 GMT
I do what Albert does but I only mark the shaft and use 222 loctite which not hard to undo . The two grub screws give the advantage set the valve gear and when one is happy then remove one at a time and mark the shaft and lock with no chance of the eccentric to move or slip.over the last 28 years i never had one moving or slipping . Bearing in mind that the difference between full gear lead of -0.005" and +0.005" is less than 1.5 degrees of change of eccentric angle, the idea of two grub screws is very appealing and I would certainly do so in future. But I do not relish the idea of removing wheels and carefully quartered cranks in order to put second grub screws into these eccentrics on this loco. So a single grub screw it shall remain.
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abby
Statesman
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Post by abby on Apr 5, 2008 17:31:30 GMT
The only problem I have encountered with grub screws is the fact that they mark the shaft , during setting the marks can interfere with the smooth rotation of a reamed eccentric on the shaft whilst making small adjustments , I use split collar clamps which can be moved easily for adjustment and never mark the shaft , and they grip !
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Post by chris vine on Apr 5, 2008 18:03:51 GMT
To avoid marking the shaft with a hardened grub screw and making it easy to move the eccentric or remove it, it is possible to machine a very small groove around the shaft. The damage then occurs below the surface and so does not get in the way.
In full size I am sure this would be frowned upon as it would weaken the shaft. However in our sizes everything is an order of magnitude stronger than it needs to be.
Chris.
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