The LNER Encyclopedia

I wonder if anyone can help me to clear up a question concerning the angular spacing of the crankpins on 3-cylinder locomotives like Gresley's. If the inside cylinder is inclined at (say) 7.5°, I reckon that the crankpins must be at 120° + 127.5° + 112.5° as a matter of principle to keep the torque and exhaust even and that this principle must apply to all 3-cylinder locos. Can anyone please say yea or nay?

This question originated in a somewhat extended email discussion about the uneven, "loping" exhaust that some observers recorded from the UP 4-12-2s, which are also 3-cylinder machines on similar lines to the Gresley locos (albeit with divided drive). In the course of said discussion, it was said that ALCO made much marketing capital out of the equal 120° (sic) spacing of the crankpins. I find this hard to believe, bearing in mind (a) the inclination of the inside cylinder and (b) that ALCO could surely not have built such an unevenly running loco deliberately. I would be very interested to hear other members' views.

Many thanks

RSR Engineer
Thannhausen
Germany

in OS Nocks book `The Gresley Pacifics` vol 1,page 15
The author gives a brief description of the crank disposition of gresley 3 cyl engines,below is an extract.
"If the centre-line of the inside cylinder was inclined to that of the outside cylinders by an angle x, the spacing of the cranks on the driving axle were made successively,.... 120 deg plus angle x,120 deg minus angle x,and an exact 120 deg.
Thus if the angle of inclination was 1 in 8 or a fraction over 7 deg,the spacing of the cranks would be 127,113 and 120 degrees,the author adds further,
"And this gave rise to the slight inequality of the beats of a gresley 3-cylinder engine".
cheers......Hunslet

Thank you, Hunslet, for the useful info.

Regards

RSR Engineer

Hunslet,

Thank you for answering that question for me as well. Since RSR asked it, I have been stuck for an answer.

I lack that particular book by OS Nock, but I have just bought a copy of his second book in the series on Classic Locomotives, The Great Northern Atlantics. These look to be highly technical, and I am looking forward to getting into it on my next long flight.

Colombo

Hunslet, let me be sure I understand you correctly. Is Nock saying that the adjustment of the crankpin angles does not correspond exactly to the inclination of the inside cylinder?

In a conventional two-cylinder steam locomotive, the exhaust is in precise 4/4 time (ignoring the way in which the valve gear compensates - or doesn't compensate - for the angularity of the connecting rods). In a locomotive with three cylinders all mounted parallel to each other (as, I assume, was the case with the NER class "Z"), the exhaust is in 6/8 time. I suspect that the "Gresley beat" is caused more by exhaust pulses of different strength than by unevenly spaced ones and that Nock may also have confused the two. What do you think?

Regards and many thanks

RSR Engineer

hello again RSR Engineer
whilst not being an authority on the subject,i can only quote the values given by Nock,he does though appear to have rounded down the angles required.
i do myself have an excellent schematic drawing of the crank axle configuration of Gresleys first pacific 1470 Great Northern.
They are as follows........120/112.52/127.8
The inclination of the inside cyl is given as exactly 1in8.
There are several usefull websites relating to the Gresley/Holcroft conjugated valve gear.
An interesting discussion!
thanks ......Hunslet