Lubrication and Tribology
Lubrication and Tribology
The Oxford Dictionary defines tribology as the branch of science and technology concerned with surfaces in relative motion, as in bearings. It is therefore inseparably associated with the subject of lubrication.
Ing. L.D. Porta was the first engineer to apply the science of tribology to the design of steam locomotives, seeing it as an essential function in the application of the high temperature steam that is needed to deliver improved cylinder performance.
Porta studied the topic extensively, researching papers and records relating to the application of tribology in the development of the modern Internal Combustion (IC) engine. Porta recorded his findings and his theories in two papers, the first written in 1975 and the second, an updated version of the first, in 1995.
Porta introduces the second of these papers by pointing out that steam locomotives traditionally achieved only around 1000 hours’ life for piston rings whereas large marine diesel engines regularly achieve 20,000 hours despite working at pressure five times higher than steam, and at temperatures of 2000oC (as compared to ~400oC). His paper attempts to address the differences between the two technologies and puts forward recommendations for improvements to steam locomotive piston and valves, many of which have been applied and field-proven by David Wardale in his iconic Class 26 rebuild, No 3450 “The Red Devil”.
Porta’s paper focuses mainly on locomotive piston valve rings since these are subjected to the most demanding service, both in terms of high temperature and of low velocity in that most of their movement is too slow to generate the hydrodynamic conditions on which good lubrication depends. The paper also makes recommendations for piston rings for which conditions are less demanding, temperatures being significantly lower and rubbing speeds higher.
His tribology papers include some very instructive diagrams. A particularly useful one shows the variation in temperature over the surfaces of valve and cylinder liners as measured on his experimental locomotive No 1802. The diagram is reproduced below, illustrating the very high temperatures experienced by the liner, valve heads and valve rings on the admission side of the port:
Porta offers several principles for improving both the life and seal of valve and piston rings, all of which have proven successful on his own locomotive modifications and on others’ (notably Wardale’s in South Africa).
1: Minimizing the temperature of the oil film:
- Do not mix atomized oil with live steam. Inject oil in liquid form directly onto rubbing surfaces.
- Arrange the piston valve heads so that some of the exhaust rings rub over the admission edge of the liner port to facilitate transportation of heat from the hot (admission) side to the cold (exhaust) side of the liner;
- Use light-weight (small section) rings of diesel engine quality, to maximize the conduction and transport of heat as described above;
- In the case of valve heads, ensure that oil is injected “between the rings” (the coolest zone) from where it can spread over the liner surface;
- Use relatively heavy bridge bars to limit temperature rise from contact with live steam and to better transport heat away from the hottest valve rings. [Note: A wide bridge bar is required at the bottom of each liner to support the valve head and ring joints.]
- Cool valve liners (locally) using saturated steam –e.g. by passing it through grooves formed in the outer faced of the liner – see Porta’s A1 valve diagram on the Valve Design page and the following images taken from Wardale’s book and the 5AT FDCs.
- Keep live steam in the steamchest separated from the outer surfaces of the main cylinders.
2: Maximizing the seal between rings and liner:
- Fit as many rings as possible to each piston and valve head;
- Use light-weight (small section) rings of diesel engine quality to better conform to the shape of the liner.
- Minimise the gap between the (valve or piston) crown and liner and minimise the ring gaps.
- Do not use tail rods on valve pistons. Fit ring retainers to keep ring gaps aligned along the bottom of the valve head – i.e. at the point of contact with the liner – to prevent steam leakage through the gaps.
- Use articulated valve spindles so that both valve heads rest evenly on the liner, contact being between the bottom of the valve head and the liner to ensure zero steam leakages through the ring gaps which are aligned along this point of contact.
- In the case of pistons, tail rods should be fitted to prevent direct contact between piston and liner. Where no tail rod is fitted, piston crowns should have bronze facings to minimize wear. Rings should be left free to rotate on the piston crown.
3: Maximizing lubrication:
- Provide an sufficient allocation of oil to ensure good lubrication: German State Railways’ allocation of 1.4 g/km for both valves and cylinders should be taken as the absolute minimum; Porta’s C16 1802 with an allocation of 11 g/km is a safer quantity.
- Small reductions in surface temperature deliver large gains in lubricant life and performance;
- Use a mixture of bronze and cast-iron rings, bronze rings providing a polishing action on the liner surface;
- Weld bronze facings to valve crowns that rub directly on the bottom of their liners; also to piston crowns where tail rods are not fitted.
- Use high-viscosity paraffinic oils, bearing in mind that there is a trade-off in that high molecular weight oils decompose more rapidly that lighter oils.
- Use additives such as neatsfoot to aid emulsification of condensate, and colloidal graphite (in pumped lubrication systems) to enhance high temperature lubrication and to fill scratches in liner surfaces.
- Drive oil pump(s) from top of the combination lever to make the rate of oil delivery proportional to cut-off.
- Provide means to (manually) increase oil delivery whenever priming occurs.
4: Minimize Abrasion
- Avoid use of oils that carbonize rapidly at high temperatures.
- Avoid situations were oils might carbonise from being in contact with very hot surfaces for long periods.
- Prevent the ingress of abrasive material either from steam contaminants (through the use of high-performance antifoams, or from smokebox ashes being drawn down though the blast nozzle (through the technique of drifting in mid-gear with the throttle “cracked open“.
- Avoid gum formation through oxidation as a result of oil contact with air. For instance, avoid the use of snifting valves or other means of allowing air to enter the steamchest or cylinders.