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The North Eastern Railway (NER) Class Z and Z1 (LNER Class C7) 4-4-2 locomotives were comprised of a development of the V and V/09 classes designed by Wilson Worsdell, Chief Mechanical Engineer, by his successor Vincent Raven.

Overview
On 4 August 1910 the board of the NER authorised Raven to obtain tenders from locomotive builders for the construction of 12 express passenger locomotives. The North British Locomotive Company's tender of £4,485, the second lowest in price, was accepted, and an order was placed for 20 locomotives of Atlantic type, of which 10 were to have boilers fitted with Schmidt superheaters. p 24 It is not known why external contractors were used, instead of the NER's Darlington Works. At this time Darlington were constructing the Y class of 4-6-2 tank locomotives, and may not have had sufficient capacity to undertake the work. p24. In line with the NER's alphabetical designation of classes the new locomotives became Class Z, with the superheated type being classified as Class Z1.

Each tender quoted two prices, the first for locomotives without superheaters and the second fitted with Schmidt superheater. The tender from NBL, the second lowest in price at £4,485 per locomotive for either type, was accepted. p.24 The order was increased to 20, 10 of each type, which were delivered from July to December 1911 p32 The class was allocated class letter Z, with the superheated engines being distinguished as Z1. p25 By the end of 1914 the ten Z class locomotives were also fitted with superheaters, and the Z1 class was redesignated as Z.

Although similar in appearance to the earlier V/09 engines of 1909 the Z/Z1 class can be distinguished by the drive to the leading coupled axle.

Numbering
All 20 locomotives passed to British Railways in 1948 and they were numbered 69770-69789.

Preservation
The A7s were withdrawn between 1951 and 1957 and none has survived into preservation.

refs
Valve setters presented with similar problem to Princess Victoria; expansion of outside valve spindles. The two levers are positioned just below where the char is shovelled out. If small door for lubrication access is left open and seals on pin joints not fitted properly abarsion. p135 Initial tendency for 2:1 lever to oevrrun at high speeds., distoring valev events so that middle valave did more the work. p136

On D49 positioned behind the cylidners as all on in line and driving leading axle. p 136

How Steam Locomotives Really Work P.W.B. Semmens and A.J. Goldfinch Oxofrd U P 0-19-856536-4

The Gresley conjugated valve gear was a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers (the "2 to 1 lever" and "equal lever"). The gear is sometimes known as the Gresley-Holcroft gear, acknowledging Holcroft's major contributions to its development.

Holcroft Patent
Harold Holcroft's patent 7859 of 1909 described the use of two valve gears to drive three cylinders. Each of the outside cylinder valve rods has a rocking lever attached, acting as a first-order lever with the load and the effort on opposite sides of the fulcrum. The arms of the levers are in the proportion 1:2, the longest being the arm opposite the valve rod, which is attached to a combination lever, whose fulcrum is attached to the middle valve rod. Holcroft described the modifications necessary for outside admission valves and for mixtures of the two.

Holcroft's patent 159541 of 1919 described the use of two valve gears to drive four cylinders. In this case the ends of the valve stems are connected to the levers and links. Holcroft's gear as fitted to the three-cylinder N1 and U1 classes of the SR derived its motion from the combination link of one outside cylinder, linked to the base of the intermediate valve stem.

Holcroft and Maunsell
Holcroft valve gear The valve gear assembly originally designed by Holcroft for use on the N1 class regulated the timing of the valve events that admitted steam into the cylinder, increasing efficiency whilst reducing mechanical wear and the effects of hammerblow on the track.[8] Holcroft incorporated a new method of driving the middle cylinder from the right-hand set of the two outside Walschaerts valve gear without the need to use a separate set of valve gear between the frames, saving overall weight.[9] This was achieved by using the combination lever assembly that controlled the fore and aft movement of the valve spindle, which admitted "live" steam into the cylinder and ejected "spent" steam through the ports.[8] Holcroft’s process of driving the middle cylinder directly from the combination lever contrasted to the approach taken by Sir Nigel Gresley, who used an extension of the valve spindles on the outside valve gear to operate the middle cylinders of his locomotives.[9] This variant therefore held an advantage over Gresley’s design, as the Holcroft valve gear would be immune to variations in valve events brought about by heat expansion of the valve spindles and flexing of the conjugation levers when in heavy use.[9] It had the disadvantage that the motion was taken only from the combination lever, and the valve events of the middle cylinder did not match those of the two outside cylinders.

Gresley's Patent
Gresley's patent of 1915 No 15,769 described a simpler setup, two transverse rods or levers of unequal length The longer one being attached by pivot to one of the outer cylinder valve spindles, and pivots at a fixed point near the centre line. The shorter one is attached to the longer one at the opposite end by a pivot,and is pivotally connected to the other outside spindle and the intermediate one. A second scheme, more complex, is also outlined.

1915 patent of Gresley for conjugated valve gear operating three sets of valves from two sets of gear. (p 13) 1918 application on 2-8-0 no 461 used transverse rocking shafts and cranks because all 3 cylinders inclined. Holcroft pointed out that if cranks were not at 120 degrees but at 120+ alpha, 120- alpha, 120, the outside cylinders could be horizontal (p15) and original design in patent could be used. On 1000 Class Moguls the valve travel in full gear was made longer, but as slackness developed in the pin joints of the conjugated valve gear linkage, plus whip from oscillating levers caused longer cutoffs in middle cylinder, hence doing more work, valve spindle crosshead occasionally overran to hit the steamchest cover.(p 22)