Crank settings for 3 cylinder locomotives

[34theletterbetweenB&D]

While I was in the NRM earlier this week, took a good look at the two Gresley three cylinder types on display in the main hall to see how the externally visible cranks are set. (4468 and 60008.) As seen from the cab, when moving forward the left hand crank is in advance of the right hand by 240 degrees, so the stroke sequence is left-centre-right. Now, I am a bear of small brain so not wanting to confuse myself I only glanced at the two Stanier types (3 cylinder 2-6-4T and Jubilee) and the wonderfully cutaway rebuilt MN, but they appear to be the same. What of the Peppercorn and Riddles contributions to the oeuvre? Is anyone able to inspect and report?

 

Happily, those Isinglass drawings I possess of 3 cylinder Doncaster types, show precisely this arrangement drawn, although there is no explicit mention on the drawing. So now with my Bach V2 about to be regeared, since a wheel has to come off anyway to exchange an axle gear, I can set it correctly 'thirded' rather than the terrible quartering compromise typical of models. (My test vehicle for this project has been an old Triang chassis, if old crudity has been happy running some years at a 120 degree crankpin setting, modern kit should be too.)

[Horsetan]

Alternatively, the right-hand crank is 120 degrees ahead of the left.....

[PhilH]

Then of course Bulleid pacifics are set at 120, 114 and 126 deg, and Gresley pacifics at 120/113/127 degrees....

[34theletterbetweenB&D]

Alternatively, the right-hand crank is 120 degrees ahead of the left.....

 Or the centre crank ahead of the right crank. But I feel it is easiest to think of the sequential passage of the cranks past any fixed point going in a single direction across the locomotive. Left to right when the loco is moving forward, right to left reverse.

[jamie92208]

Then of course Bulleid pacifics are set at 120, 114 and 126 deg, and Gresley pacifics at 120/113/127 degrees....

That was because the centre cylinder was higher that the other two to allow the con rod to clear the front coupled axle.  They still have a wheel lathe in the Wabtec part of the Plant at Donny that can do the necessary setting up.  Some classes of locos avoided this problem by having a set of cranks on the front  axle that rotate3d in the correct phase with the con rod going between the crank webs at the appropriate point.

 

Jamie

[Jol Wilkinson]

Better to stick with 90 deg quartering. That way you get more equal effort through the rods to the non powered wheels as the axles rotate. Otherwise you could get problems at certain points of rotation and tolerances in the mechanism become more critical.

 

Three cylinder locos had 120 deg settings to even out the energy from the cylinders going into the system. They probably also had drive to different axles for the inner and outer cylinders, which also changed the dynamics.

 

Jol

[Pacific231G]

Better to stick with 90 deg quartering. That way you get more equal effort through the rods to the non powered wheels as the axles rotate. Otherwise you could get problems at certain points of rotation and tolerances in the mechanism become more critical.

 

Three cylinder locos had 120 deg settings to even out the energy from the cylinders going into the system. They probably also had drive to different axles for the inner and outer cylinders, which also changed the dynamics.

 

Jol

It occurs to me that the one thing you basically never see in a model (or a real loco come to that) is both sets of outside motion at once so it's probably makes sense to do whatever makes for smoother running which I'd guess is quartering. 120 deg or thereabouts - the actual dynamics may require some variation- is the norm for three cylinder simple locos but it's different for three cylinder compounds where some engineers set the the outside low pressure cranks set at 90 degrees and the inside high pressure crank at 135 degrees.

[34theletterbetweenB&D]

The set up for 120 degree is a little more demanding than for 90 degree which is indeed the most tolerant setting in respect of small inaccuracies, but not enough to be a worry. If an old dog of a Triang chassis has tolerated it for many years - and that was set by eye - I am pretty confident that current RTR will be no bother. Just drilled the extra crankpin locating hole in my Hamblings wheel press, the most important aspect being that all wheelsets in a chassis are set the same. And if it is problematic I will return to quartered, there's no bridges being burned. This is all for fun, first and foremost.

[buffalo]

It occurs to me that the one thing you basically never see in a model (or a real loco come to that) is both sets of outside motion at once...

Funny how every time this topic comes up, someone makes this claim. Have you never viewed a model, or the real thing, from the front when it is in motion? It's much more obvious on something like an inside cylinder 0-6-0 without any cylinders or pony/bogie wheels in the way, but the relative positions of the crankpins is usually visible from this direction.

 

Nick

[Poor Old Bruce]

90

120 deg or thereabouts - the actual dynamics may require some variation- is the norm for three cylinder simple locos but it's different for three cylinder compounds where some engineers set the the outside low pressure cranks set at 90 degrees and the inside high pressure crank at 135 degrees.

Certainly the Midland and LMS Compounds were set to 90+135+135.

[ozzyo]

90Certainly the Midland and LMS Compounds were set to 90+135+135.

 

Hello all,

 

Poor Old Bruce, my have mislead us a bit, but when I was taught about valve gear it was looking toward the front, so that the L/H side was on the left (sorry). Reading Bruce's  post it sounds like the L/H wheel is at T.D.C. then the centre leads by + 90, then the R/H by +135, then the L/H by +135. Yes it all adds up to 360. Working it out this way it says that the wheel cranks were at 180 deg. to one another.

 

When you look at this drawing I can see why anyone can get confused. As the outside rods are set at 45 degs.

 

When you turn the drawing by 45 degs and the L/H rod is at TDC it now becomes 0 + 90 +135 +135, so the R/H crank leads by 90 degs.

 

If we are going to talk about the setting of cranks, can we please all have the same starting point L/H TDC = 0. plus (+) is R/H lead [as some locos did have L/H lead].

 

You can get 120 deg axles from Slater's so you can set up your L.N.E.R. & L.M.S. locos correct.  I'm not that sure about the S.R. ones though.

 

OzzyO.

 

Edit. to put my numbers the right way around.

[34theletterbetweenB&D]

...when I was taught about valve gear it was looking toward the front, so that the L/H side was on the left ...If we are going to talk about the setting of cranks, can we please all have the same starting point L/H TDC = 0. plus (+) is R/H lead ...

 Now there's useful, someone who has had the training, knows the established convention, and is prepared to educate us knowlessmen. Duly noted on all the three cylinder Isinglass drawings in my possession. There should be more of you Ozzy O.

 

So, the 3 cylinder types I was looking at in the NRM, mentioned in the OP (and ignoring the small deviations in angularity for inside cylinder position) with LH crank at TDC, the RH crank is +120 degrees, the inside crank is 120 degrees. Thus by convention, these are all RH lead designs if I have grasped the principle correctly. 

[buffalo]

 ...So, the 3 cylinder types I was looking at in the NRM, mentioned in the OP (and ignoring the small deviations in angularity for inside cylinder position) with LH crank at TDC, the RH crank is +120 degrees, the inside crank is 120 degrees. Thus by convention, these are all RH lead designs if I have grasped the principle correctly. 

Presumably the right and inside cranks are not in the same position. Did you mean to include a '-' on the inside angle? If so, perhaps better to express all in the same direction relative to the LH crank, so the inside would be at (+)240 degrees.

 

Nick

[ozzyo]

Hello all,

 

I have edited my last post as I did not stick to what I had said,  it should have read, L/H crank 0, R/H crank +90, middle crank +135 (or +225 from 0) then +135 (or + 360 from 0).

 

I've had a dig around in my engine-men's books and come up with the following.

 

Ex L.N.E.R. 3 cylinder locomotive valve gear. This has been drawn showing the R/H side and from above. In the above view you can see that the L/H crank is in one of the front angles.

 

When you look at a close up of the side view you can see that it's in the bottom front angle with the R/H crank in the rear back quarter (or back dead centre).

 

With a bit of a play this is looking at the L/H crank at T.D.C. looking at the loco from the R/H side. So we have the R/H crank leading by approx. +120, then the middle crank by approx +240, then back to the L/H crank 0. Edit the angles between the cranks are as follows, L/H to R/H 120 deg. R/H to centre 112 deg. 52' , centre to L/H 127deg 8'

 

The same as above but with the R/H crank at T.D.C. All of these show the front of the loco to the right.

 

In this drawing I've flipped it, so that the front of the loco is now at the left, so that the dotted crank is now at the front (L/H side) and that the fully drawn crank is at the back (R/H side).

   

 

These two are from an engine man's book produced by the L.M.S. Question 179 on may help a bit!

 

Just to help fill in the topic about three cylinder locos. I thought that I'd let you all see this. The chain driven valve gear that was carried by the M.Cs., W.Cs. and BoBs. Not that much chain in there?

 

If you thinking about on of the above you may want to read this about inside and outside admission valves. On most locos the return crank lead by approx. 15 degs. but on the rebuilt S.R. pacific's it trailed by about 15 degs. I have seen a few good models let down by this. On one that I have seen one side lead and the other one trailed.

 

OzzyO.

[34theletterbetweenB&D]

Redoubling on the good information. This invaluable content deserves to be a 'sticky' somewhere appropriate - 'Prototype'? Even if only for the wonderfully dated "evils of early exhaust and compression"!

[Poor Old Bruce]

OzzyO,

 

Thanks for the clarification and edification.

[billbedford]

Ex L.N.E.R. 3 cylinder locomotive valve gear. This has been drawn showing the R/H side and from above. In the above view you can see that the L/H crank is in one of the front angles.

 

I don't know where that diagram was drawn, but it is wrong for LNER engines. All the GAs I can find have the cranks set at 120deg with a correction for the height of the middle cylinder.

[Titan]

I don't understand, the LNER drawing above clearly shows the cranks equally spaced at 120 degrees - look at the drawing where the crank is at the back quarter, the dotted position of the crank at the other side is not at the bottom quarter where it would be if it was 90 degrees, but considerably towards the front where I would expect it to be if it was at 120 degrees. the middle crank appears to be half way between the two. The drawing may look a little odd due to the middle crank of necessity being of considerably bigger diameter, which may throw the perspective out.

[ozzyo]

I don't know where that diagram was drawn, but it is wrong for LNER engines. All the GAs I can find have the cranks set at 120deg with a correction for the height of the middle cylinder.

Hello Bill,

 

the drawings came from a book that was published by the B.T.C. in the 1950s for loco-men. it maybe out by a degree or so but it was for loco-men to show them the principals of how that engine worked.

Is it is this one?

 

Or this one? Front of the loco to the RIGHT. L/H crank at T.D.C.

 

On this drawing I have put the R/H crank at T.D.C. so that the front of the loco is at the RIGHT, you can then see that the L/H crank trails by approx 124 Degs.

EDIT. for 124 deg. please read 120 deg.

 

On this drawing I have drawn the three approx. positions of the cranks. if you add them up you will see that I'm out by 1 degree, most of that is down to how I have drawn the centre lines. Please remember that this drawing is from the R/H side.

EDIT. the angles should be as follows, 124 deg should be 120 deg. 112 deg. should be 112 deg. 52' 123 deg should be 127 deg 8'.

 

If it still sound wrong please add a scan of the centre drivers of a L.N.E.R. loco, but please also show the plan view.

 

This drawing I have flipped the drawing and placed the L/H crank at T.D.C. So looking at this the R/H crank leads the L/H by approx. +124 deg. then the centre crank at +112 (or +236 deg.) then back to the L/H crank at approx + 123 (or 359 deg).

EDIT. the above should read as follows. the R/H crank leads the L/H crank by 120 deg. then the centre crank at 112 deg 52' (or + 232 deg. 52') then back to the L/H crank at  + 127 deg. 8' (or 360 deg)

 

OzzyO.

[billbedford]

Better yet I've found the A1 motion arrangement. This shows the RH/LH crank angle to be 120deg and the LH/Centre crank angle to be 120deg plus the inclination of the middle cylinder (7deg 8')

 

[ozzyo]

Hello Bill,

 

I'm not sure about what your saying here as all your drawing is saying the same as the one I put up.

Your drawing.

 

What is so different about them?

 

The R/H crank leads by 120 Degs when the L/H crank is at T.D.C.. Have a look at the drawing and starting at the L/H crank, 0, then its + 120 (the R/H crank), that then takes you to the middle crank by + 113 (or +233) then you have the last part +128 (or back to 0) 

 

I'm sure that's what I started to say with in this drawing?

 

OyzzO.

[Horsetan]

There now follows learned argument between Bill and Ozzy as to who's got the better drawings.....

[Pete Harvey]

This File should help to answer the question for both of you.

 

It will run from the zip file and will run on Windows 7.

 

Pete

 

valgr10k.zip

[billbedford]

What is an '.exe' file? Why should I not expect it to be a virus?

[billbedford]

The R/H crank leads by 120 Degs when the L/H crank is at T.D.C.. Have a look at the drawing and starting at the L/H crank, 0, then its + 120 (the R/H crank), that then takes you to the middle crank by + 113 (or +233) then you have the last part +128 (or back to 0) 

 

I'm sure that's what I started to say with in this drawing?

valve gear 012ab.jpg

 

Yep you are right, but your drawing shows an uncorrected 124deg between the wheel cranks.