Chris. you should use a bronze, like leaded bronze for bearings. Brass is not really a bearing material.

Sounds like a design error.  Bearings are almost never the same metal both sides.  Tends to rub, get hot, gall and seize.

Best practice is for a hard shaft to run in a soft bearing shell, two different materials, with lubrication.  Lots of combinations don’t work, such as steel and aluminium.  Others work extremely well, like the hardened steel pivot and ruby found in mechanical watches.  Easier to copy what others have proved works than to experiment.

Steel in cast-iron is OK for lightly loaded slow turning bearings because most cast-irons are full of slippery graphite.  Popular in old sewing machines, many still in good order a century later.

Bearing shells were originally called ‘Brasses’, because they were made of soft brass, the shaft probably being wrought iron, and lubricated with tallow or animal fat.  Brass is a reasonably good bearing metal for steel, and with a little oil will take moderate speeds and loads.  Bronze will take a bigger beating than Brass.

So far, these are all Plain Bearings, which aren’t ideal, even when souped-up with White Metal and a high-pressure oil pump. They work by the shaft riding on an oil layer, avoiding metal on metal as much as possible. Unfortunately metal on metal occur when the shafts in plain bearings stop and start, and  – much worse – if high speed or an excessive load breaks the oil layer.  `

Plain bearings run within their limits take decades to wear out, but running them too fast takes years off their lives.

The main disadvantage of plain bearings is they are high-maintenance.  Gone are the days when a sailor would spend a watch next to a propeller shaft to make sure a bearing was OK, constantly checking the oil, and periodically hosing it down with sea-water to keep it cool.

Roller and taper bearings have mostly replaced plain bearings because they can do high loads and high speed, don’t need TLC,  and are replaceable.

For lightish duty, almost all my home-made bearings are Steel running in Brass with a simple oiling hole that I must remember to keep topped up.  A more sophisticated variant would be an oil bottle with a wick or needle.  A cast-iron bush is an alternative, but I avoid cast-iron because of the dirt.  Oilite bushes are an option too.

Dave

The designer most definitely should have known better Chris.

Steel shafting running on the more common alloys of aluminum as sleeve bearings doesn’t work for well known engineering reasons. Automotive crank shaft or rod bearings can because in that application they would be using a pumped and pressurized lubrication system. So under rotation, the bearing journals are fully supported by the wedge of oil and don’t touch the bearing itself at all other than when the engine is turned off or during the very first few rotations when it’s started. Yes those aluminum alloys are used and they would be a fairly specialized alloy mixed with a few other metals to increase there durability during those engine starts and not one of the more common alloys of machinable aluminum we might have. In the past, the bearing materials were always much softer than any aluminum without any real issues, but even the relatively slow rpm steam engines still worked and supported the bearing journals on that oil wedge. Go back far enough and the use of dead soft Babbit was very common. With mostly random drip fed oiling, any aluminum alloy most of us would have would still be a very poor choice.

Using bronze as low rpm sleeve type bearings often gets mentioned. Unfortunately it’s not quite as simple as that with any non pressure lubricated systems. Most designated as an actual bearing bronze are still a fairly hard material. For good engineering reasons and longest life, then ideally the shaft itself should be hardened. For model engineering purposes that might be of less or of no real concern, but still worth knowing about in my opinion. In most full scale designs, bearings or bushings would be considered the cheapest or consumable item. So it’s still good engineering to have them wear first and then be replaced instead of the usually more expensive or harder to replace component that’s rotating within or on them.

All materials will have what’s known as a coefficient of friction. https://www.engineeringtoolbox.com/friction-coefficients-d_778.html And in general, alike materials shouldn’t be used together due to that serious galling or much higher wear rate issues. Cast iron on cast iron is one of the few where it’s possible and machine tools have certainly used it for a very long time, but it would still a bit less than ideal for the best durability. So hardened ways and even specialized low friction plastics are now being used on the more expensive machine tools. At low rpm and loads, and for most of us, items not being used all that often, then common brass should still work well enough.

I am halfway through a sparey diesel which has a conrod in gauge plate with an en8 journal and looking at various write ups people are not recommending it as they are saying that they have have had siezures so have gone the aluminium rod route. perhaps this proves the point. Out of interest I have made mine from 7075 t6 and a nice piece of material it is.