Split eccentrics?

[Perko7Participant @perko7]

Hi all, another newbie question for a loco with inside valve gear. Just wondering if there is any good reason to not use split eccentrics which can be fitted to the axle once the wheels have been pressed on and quartered. I would imagine they would be made from 2 pieces bolted together and then machined to be a clamp fit on the axle. After the wheels are on then they would be fitted and positioned to give correct valve events and then tightened up and locked in position with taper pins or something similar. Any thoughts? I am a bit uncertain about trying to get both the eccentrics and the wheels correctly lined up when pressing the wheels on. Alternatively would the eccentrics be made in one piece as a push fit on the axle and slipped on before the wheels, then pinned in position to give the correct valve events.

I have not been able to find a description of this process in any of my available resources. Had a look on this forum too but nothing came to light. Apologies if this is already covered somewhere and I could not find it. Directions to a suitable article or two would be appreciated.

 

[Nick Clarke 3Participant @nickclarke3]

There are two references to making split eccentrics in the online Model engineer index:-

1941 vol 84 issue 2089 page 407 LBSC how to make small split eccentrics.

and

1943 vol 89 issue 2211page 296 again by LBSC

I don’t have these scanned but they should be available in a main library.

[WearyParticipant @weary]

To address your questions:

There is ‘no good reason’ not to use split eccentrics which have the advantages that you mention.  However, they are a little more difficult to make than single ‘solid’ eccentrics.  So there is a very good reason to use solid turned eccentrics – especially for ‘tyros’ ( to use LBSC’s terminology).

Solid eccentrics would be fitted exactly as you describe, reamed to axle size, slid-on in correct order before second wheel fitted and fixed with one or two grub-screws through each eccentric (or eccentric boss) to secure them to the axle.  So, eccentrics would be free to turn on the axle until secured in correct position upon setting valves.

You could pin the eccentrics to the axle using either taper or parallel pins as you suggest, however (pointed) grub-screws, or flat-ended grubscrews onto appropriate flats filed on the axle, should be adequate and easier to deal with overall!

Note that there is only one ‘correct’ position for your eccentrics relative to the piston movement which if not shown on your drawings could be derived by calculation rather than the time-honoured & somewhat tedious method of ‘adjustment in situ’.  It may be worth mentioning the loco that you are looking-at/constructing in order to give other owners/builders the opportunity to give you info’ directly relevant to that design?

Regards,

Phil

 

[Clive Brown 1Participant @clivebrown1]

I’ve never made split eccentrics, but I’d see them as a fiddly job. The 4 eccentrics on my Princess of Wales crank-axle are held by grub-screws, as per usual Martin Evans design. This seems to be OK, but I’ve only run on air so far. I’m not intending to pin them later. It’s perhaps helpful, but not essential, if you can make the eccentrics a hand- push fit on the axle which helps initial positioning.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, with reference to Nick Clarke 3’s post, the one in Vol 84 is actually on page 409.

Regards Nick.

[Perko7Participant @perko7]

Thanks for the speedy replies.

My question was prompted by the fact that I am building a 5inch gauge loco of a fairly obscure prototype based on original GA drawings and there is no equivalent published design that I can find to ‘borrow’ information from. I’m in Australia and only have a small collection of ME magazines from the 50’s which belonged to my grandfather. I’ve looked through all the loco build articles by LBSC that exist within the volumes that I have and nowhere is the actual process of fitting the eccentrics described. LBSC appears to assume that builders of his designs either already have the experience, or have access to various previous designs in which he has described it.

The info you gents have provided is most welcome and seems to point me in the direction of solid eccentrics slipped on the axles and fixed in position later. That will be the method I will use.

[WearyParticipant @weary]

Don’t know if you are aware, but, there are a number of regular posters on Model Engineering Clearing House who are Aus/NZ based and have built/are building locomotives who may be helpful in your endeavours.

Phil

[Dave WoottonParticipant @davewootton]

As well as the excellent Model Engineering Clearing House forum mentioned by Weary above, there is a huge amount of valve gear information on the late Don Ashton’s website, well worth a visit. If it is Stephenson’s valve gear you are making his book on model Stevenson’s and Walshaerts gear is a mine of information, including setting the eccentrics using  simple jigs. Works wonderfully, even I can do it!

[Nigel Graham 2Participant @nigelgraham2]

I have often pondered this question. It seems to me if you need replace eccentrics, though they should have a long life, removing and re-fitting press-fitted wheels to do so is not best practice.

Looking at the facsimile above, I can see one possible trap though.

If the bore and the split are not spot-on to very fine limits, tightening the screw to close its side could open the other, maybe by only a “thou” but possibly enough to cause problems.

.

Split eccentrics were common in full-size practice, but not as that ME article.

Instead, referring to the diagram, two screws, actually studs and nuts were used, in the wide parts at right-angles to the single screw shown. There is less metal there so making them reliably, in small scales, won’t be easy; but it would give equal clamping.

To accommodate them the nuts were housed in apertures through the half-sheave, rather than radial holes from the rim; but that may be troublesome on anything smaller than 7.25″ gauge, and not exactly easy there. You’d hit access problems for the spanner with two eccentrics side-by-side in a narrow space.

[I’ve an idea it is no less than S.M.E.E. whose club badge is based on one such eccentric, with its distinctive “windows” .]

.

The large engines’ eccentrics are keyed to the crankshaft, needing correct locations to start with, determined by careful drawing and calculations. I think some workshops used templates to set the key-way cutting.

Given that in principle, Stephenson’s Link Motion is all symmetrical about the crank at dead-centres, especially with a centrally-suspended expansion-link, I am not sure why angular adjustment should be needed, other than on locomotives whose piston-rods and valve-spindles are not radial to the axle. Axial adjustment on the valve-spindle is still necessary, to deal with slight tolerance accumulations.

…

Regarding the eccentric described, it uses the conventional channelled rim to retain the strap sideways. For my engine I opted instead to turn plain surfaces but cut a thin groove in both the cast-iron sheave and mild-steel strap, at mid-thickness, to take a split mild-steel ring like a circlip. Indeed, a commercial circlip with the ends trimmed off might be used. It is fitted to the sheave in a manner similarly to installing a piston-ring.

[Author]

Posts