Displacement lubricators operate in a very simple way, in that initially, when cold, they are nearly full of very thick steam oil – usually in the range from 90 to 140 grade (which will be sitting on top of any water – previously condensed steam from an earlier run) – with a small air gap on top level of the outlet pipe.
With the smaller scale of steam engines, where an intermittent dab of oil is needed, a lighter oil like Liquid Bearing Oil can be used to great effect. Being a synthetic oil, it works a lot better than mineral oils which allows the use of a thinner oil with hot steam engines.
Back to the lubricators; having this air gap is necessary as the cool air is compressed as the steam pressure rises, causing steam to enter the top of the lubricator through the single pipe where it is cooled and forms droplets of water, which sink below the oil and pushes some out through the single pipe into the main steam line (could also be coupled up to the regulator box) and the fine droplets are then carried through to the cylinder and valves where the lubrication is needed.
Draining the water from the bottom of the lubricator is needed before a run, possibly once per day, although the amount of water produced, or steam oil used, is quite small.
If you are using one on a newly built engine, then having as much oil as possible going through to your cylinder is a necessity until the internals of your engine have smoothed off. Too little and your engine may well seize up, although generally speaking, as the engine cools down, it can be freed up with a little coaxing.
This is all down to the “piston rings” you are using.
With conventional steel piston rings inside a cast-iron cylinder, then the cast iron itself acts as a lubricant and needs very little oil. The same applies with cast iron rings inside steel, although this needs a little more oil.
For cast iron rings inside a phosphor bronze cylinder, you can use very little oil indeed, as this is the safest way to go, because both metals tend to be self-lubricating.
Steel on steel does not work at all, especially in a steam engine because of the inherent rust problems and the fact that like metals tend to fuse together with friction.
Using “rubberised” rings in any metal cylinder requires a lot of lubrication owing to the flexible nature of the “rubberised” ring as these produce a lot of friction generally speaking, a bit like applying your brakes, and if you are a first-timer or a novice at model engineering, then you need to accurately cut the groove the ring sits in, or use excessive amounts of oil in your cylinder initially.
With “rubberised” rings the cylinder bore must be honed to perfection as any highs or lows in the face of the bore will simply rip pieces off the soft ring you have installed and cause poor sealing.
Other than the minuscule models available, this method is not too clever for the novice steam engine builder, and perhaps the better option is to use tightly wound gaskin between two hard faces (the top and bottom of the piston) to produce a lubricated and tight seal. Gaskin is something like string soaked in thick grease and works well for sealing and packing purposes, although lubrication is still required with this method.
Back to the displacement lubricators…
There are basically two types:
- A stand-alone, where the lubricator sits off-set from the main steam line (or is attached to the regulator box).
- A through-flow system where the main steam line passes through the top of the lubricator.
The first has no control of the steam inlet, and hence oil outlet, making it a very ‘hit and miss’ lubricator.
What you will find is that the fixed amount of oil being displaced is controlled by the size of the hole in the top inlet and outlet bar. If you have a model that requires different amounts of steam, say over a period of a couple of years, once the motor is worn in, then you cannot change the volume of oil being supplied, other than replacing the lubricator itself.
Also, this type is not variable to suit different needs owing to the different speeds and loads on your engine.
The second has a screwed valve running through the outlet tube, making the size of the inlet and outlet hole variable. Much better all around for a stiff, new engine, and can gradually have the size of the hole reduced as time goes by.
The through-flow has problems in that the lubricator needs to be cooler than the main steam line for condensation to occur, and as the steam line is passing through it, the temperature difference is negligible after the first few minutes of running. For this reason, they tend not to deliver the needed oil properly.
You would think that being in the direct flow would work wonders, but I’m sorry to say, that is not the case.
So, all in all, if you want to choose one, I would recommend going down the variable (adjustable) stand-alone route.
The sizes of the reservoir are various, although the majority on offer tend to be for models with up to a one-inch bore. Obviously, having two in tandem of this size along the steam line would help with larger bores. Having the larger reservoir allows longer periods without topping up with steam oil and draining the condensate.
I tend to purchase my displacement lubricators from local model engineering suppliers, but if you wish to make your own, free plans are available for download at the link below.
Hope this helps you to a better understanding of automatic steam engine lubricators and displacement lubricators as they are known in the model engineering world.