Another Gresley Thread, middle big end

I'm not sure of the actual chemical, but mechanically there is this in the Science Museum:



https://collection.sciencemuseumgro.../aniseed-bomb-for-inside-big-end-aniseed-bomb

Tom

 

Oh come on Tom…you know what I meant…

 

Well, clearly not. The OP's question is explicitly about the middle big end, and whether Bulleid's big end was a similar design to Gresley's; to which you replied he didn't use it! Which is a rather strange statement. Clearly Bulleid's pacifics had a middle big end; what I don't know is whether the design was substantially similar or different to that used by Gresley.

OK, after about three re-readings, I think I can see what you are saying - that the designs are different.

Tom

 

So I believe, but Jena was in the East!

 

I believe also used on some LMS locos according to E. A. Langridge.

 

GWR at Swindon used Zeiss optical equipment prior to WWII and subsequently. Cook and Doncaster used the British made Taylor Hobson optical equipment after WWII. The Taylor Hobson version was an improvement on the Zeiss and is still available in both it's original optical form and in the newer laser form.

Several heritage railways have and use optical equipment on locomotives, WSR, SVR, MHR, LSL are a few.

Here are a few photographs of my own Taylor Hobson equipment being used on various locomotives:-

The telescope mounted centrally in the LH cylinder of 4110 at Cranmore. The telescope if centralised using an internal micrometer at four positions off the cylinder bell mouth.


A glass optical "target" held in an aluminium adaptor in the gland hole of the rear cylinder cover. Again 4110.


The view of an optical "target" through the eyepiece of a telescope. The cross hairs are the centreline of the telescope line of sight. the small black dot is the centre point of the optical "target". The dot is 0.011" diameter. 45551 at The Railway Age Crewe.


A 6 foot inspection grade straight edge is mounted in the horn opening and a precision optical mirror "target" is attached to the straight edge. Using the precision mirror a scale within the telescope can be viewed as a reflection in the mirror, but only if the mirror is exactly at right angles to the line of sight. 3850 checking re-machined horn squareness.


When rear cylinder covers are not fitted a "target" can be held central in the rear bell mouth using the Taylor Hobson spider fixture. The legs are adjustable and the rotating dial gauge ensures that the 'target" is central to the accuracy of the dial gauge. Again 3850, with new cylinders which had previously been aligned with the same system.


Andy.

 

It is indeed in Bill Harvey's "60 Years in Steam" which has excellent explicative diagrams. Peter Townend has in fact written several books, Both writers with valuable running shed insights being shed masters at Top Shed by Kings Cross and Norwich.

 

There's no doubt that whip and other errors in the conjugated gear led to the middle cylinder doing more work than the outside cylinders. You only have to look at indicator diagrams to see that. However, the greatest cylinder steam force applied to a big end is when the piston is subject to full boiler pressure. You can't exceed this and it is independent of the valve gear. The other forces applied to the big end are dynamic and relate largely to acceleration and deceleration of the Conrod, crosshead and piston. All these should be taken care of at the design stage and are also independent of the valve gear.

 

Is this why Gresley limited the cut-off to 65% Steve?

 

I understood it was an attempt to reduce slipping at starting.

 

My understanding is that Gresley thought that with three cylinders 75% was unnecessary and limiting the cut off to 65% would be more economical as it gave more expansive working in full gear.
As an aside, the stated theoretical tractive effort continued to use an m.e.p. of 85% of maximum pressure, which would no longer be the case.

 

That is a really interesting point. All of my tables with statistics for Gresley's conjugated locomotives state the 85% figure and not an equivalent figure at 65%. Is quoting the 85% figure "wrong" even though it's the official LNER record for the class?

 

I'm not sure on that, Steve. The Nominal T.E. assumes the engine is stationary, so the pressure in the cylinder would eventually reach boiler pressure regardless of the port opening. Most engines' maximum cut-off was around 75%, not the 85% used in the calculations.

 

Why do we use 85%? Historical practice?

 

Might another reason be that (assuming the same maximum valve travel) a shorter maximum cut off means wider port openings (and therefore better steam flow) at any given shorter cutoff such as 30%?

 

All true, but the most unfavourable conditions for a big-end with marginal lubrication could be when the speed is high and the force from the piston is larger than would be normal for that speed.

 

The two concepts are different, albeit not unrelated. Cut-off is the % of the stroke at which the valve closes. If one assumes the TE constant is a proxy for MEP, then in two dimensional indicator diagram terms, the MEP is the area under the indicator curve divided by the area of the cylinder (diameter x stroke). It seems to be commonly overlooked that lap causes the valve open to exhaust before the end of the stroke, so for a V2 (from the P&E report), the valve opens to exhaust at just under 90% of the stroke at which point the pressure drops steeply. i.e. you can't just say that cut-off is at 65% therefore MEP = 65% of boiler pressure. 85% feels too large a constant and different constants were used elsewhere, but having said that, it you extrapolate the TE curves in the P&E reports, the numbers are not vastly different (TE appears not to have been measured at very low speeds).

I don't understand your comment. The MEP is the important number, not a transient admission pressure, and as noted above is the area under the indicator diagram curve, and it cannot rise to 100% for 100% of the stroke.

 

p.12 of Part 2A refers to the alteration to 65% on the A1s because of overrun and the valve clouting the end of the steam chest.

 

I am not sure I understand your query although you might be asking for a treatise on the advantages of long lap! The port opening at 25% cut-off on a V2 was approx. 5/16" (9" dia valves) whereas for instance on a Standard Class 5 (11" dia. valves) it was approx 3/8". The port area was also influenced by the diameter of the valve and the actual dimensions of the openings in the valve liners etc.

 

I will go read this. I have a vague memory that I thought this originally and then changed my mind for some reason. Too many notes to go through!