Isn’t the piston speed of the 5AT going to be too high?

Note: With 30 inch piston stroke and 6′ 2″ wheels, the piston speed at the max. design speed of 200 km/h (125 mph) will be 23.8 m/s or 24.8 m/s with worn tyres.
David Wardale responds as follows:

  • Whatever the mean piston speed, a piston always moves slowly close to the end of its stroke and is in fact stationary, relative to the cylinder, at dead centre. Cylinder liners are known to wear more at the ends than the centre. These 2 factors are linked. Near the ends of the stroke there is too little velocity for hydro-dynamic lubrication to occur – with any piston speed. Conversely where the piston is moving fast, near the centre of the stroke, hydrodynamic lubrication does occur and wear is less. The high velocity of the 5AT piston in this zone will merely accentuate the trend to good hydrodynamic lubrication and low wear near the centre of the stroke, whilst doing nothing to make it worse at the end of the stroke where the velocity is always at or near zero.
  • The piston head does not touch the liner, therefore contact is limited to the piston rings and liner.
  • The rings are “barrelled” on their outside diameter and can to a limited extent tilt in their grooves, both of which favour full hydrodynamic lubrication.
  • The rings are very elastic, therefore their pressure on the liners is low when steam pressure is low, e.g. when drifting.
  • Drifting has to be with a limited amount of drifting steam which carries away friction-generated heat which would otherwise build up during high-speed drifting.
  • The rings and liners are perlitic chromium cast iron of high quality, which gave excellent results in 3450 (Red Devil“).
  • Cylinder lubrication will be such as to guarantee oil reaching the cylinder liner walls and not simply blown to exhaust which all too often happened in the past.
  • Cylinder oil may have colloidal graphite added, which greatly reduces friction and wear, or perhaps synthetic oil specifically for the conditions in a locomotive’s cylinders and valves may be used.
  • Drifting steam ensures no abrasive particles are carried into the cylinders from the blast nozzles, because of no vacuum occurring in the cylinders. (Note: There will in any case be minimal such particles would at any rate be minimal with oil firing). Likewise antifoam ensures no abrasive scale particles can form in the steam in the superheater, because no water carrying dissolved solids leaves the boiler.
  • The reciprocating masses are supported by the crosshead/slidebar bearing and tail rod, which are easy to lubricate as they are not subject to steam temperature (and the piston etc. mass is also very low for reasons of reducing the reciprocating masses, therefore low bearing pressures).

The above 10 reasons are given to indicate why no lubrication problem is expected on the 5AT despite its high piston speed (and it is not so much higher than what has been achieved before, e.g. the N & W J Class 4-8-4s apparently ran regularly at 90 mph with 70 inch drivers giving the same piston speed that the 5AT would have at 97mph – see note below). The key, apart from the fundamental item No. 1 in the list, is the taking of so many factors (e.g. light piston, use of a tail rod, flexible rings, drifting steam etc.) all of which act to improve lubrication. If you didn’t have all these factors coming together you would have trouble if for any reason (e.g. oil starvation or excessive bearing pressure) full hydrodynamic lubrication (i.e. the forming of a proper load-bearing oil film) were not present during the entire piston stroke. That this may well have happened with first-generation locomotives is why some idea of limiting piston speed probably developed, but, e.g. like using 450oC steam (where 400oC was thought the limit in the past), such previously considered limitations no longer apply where proper means are taken to overcome them.

Note: According to Philip Atkins in his book “Dropping the Fire”, page 24, one of the Class Js attained a speed of 110 mph whilst experimentally running on the Pennsylvania Railroad in 1944 with half worn (5ft 8½ inch) tyres.  And of course the boiler pressure of the Js in final form was 300 psi.  The 110mph speed reached by a J is further corroborated in David P. Morgan’s book “Steam’s Finest Hour” p61: “As for the J, need anything more be said than that a comparatively low-drivered 4-8-4 proved herself not only equal to the mountains but also capable of whipping a 1025-ton test passenger train up to 110 miles per hour across the Virginia swamps”.