Ron you couldn't do much better.

Ian S C

Much of the industrial revolution relied on this fortuitous property of cheap cast iron. But that doesn't mean bronze or whitemetal isn't better.

Some well known lathes were made initially with a steel spindle running directly in the CI headstock. Then the manufacturers would bring in an uprated model with the same steel spindle but modified headstock to provide the improved capabilities of a bronze or whitemetal bushes.

When a plain bearing is running normally there shouldn't be any metal to metal contact at all. Instead the spindle rides on a thin cushion of oil. In this condition it doesn't matter at all what metals the bearing is made of.

The metals in a bearing matter most when they come into contact, usually when not turning:

• When at rest a bearing should not deform – it needs to be strong enough to take the weight, which tends to wipe out metals that creep.
• When starting up from rest a plain bearing has no oil between the metal surfaces. Metal to metal contact means friction, wear and damage hence the need for one metal in the bearing to be softer than the other to provide some slip. Once the spindle is turning fast enough, oil penetrates and metal to metal contact ceases.
• Stopping is the reverse of starting. As the spindle slows down the oil pressure drops. Eventually the film fails and the spindle crashes into the bearing and grinds to a stop.

Much depends on the service the bearing is subjected to. Cast iron is used because embedded graphite makes it naturally slippery and because it's cheap. A disadvantage is that it's softer than many alternatives, and – of course – not all cast irons are slippery (some are packed with abrasive slag). As such cast iron is best used for relatively lightly loaded bearings that don't stop and start very often. They were common when domestic machines were made of cast iron and bearings could be formed simply by drilling holes in it. It's a sign of cheapness, which is good unless the bearing wears out early.

More expensive alternatives like Brass, Bronze, Teflon, Oilite etc. provide better service. Because there are better alternatives, brass and cast iron aren't much used today.

Ball and roller bearings and oilite/plastics have mostly edged plain bearings out of modern production. Although initially more expensive than a plain bearing, rolling bearings don't develop excessive friction when stopping and starting. In many applications this saves energy and provides better acceleration. Low wear and no need for constant oiling makes them low maintenance, which matters when you have to pay for the labour.

The only reason I can think of for a cast-iron bearing outlasting bronze is poor maintenance. If a bronze bearing isn't oiled or is allowed to fill with grit, it might well wear faster than cast-iron. But if long-life and low maintenance is the goal, a sealed roller bearing would be a better choice.

With models, the best bearing might be the one that's easiest to make and fix. If you can get away with a simple steel axle running in a cast-iron hole, why not?

Dave

At the risk of hi-jacking Ron's thread – I am in the process of refurbishing a little Faircut table saw (to complement my larger table saws for small scale [G3] modelling purposes). The steel spindle runs directly in a 'head' casting and both casting and spindle have worn – most likely due to lack of proper lubrication. The casting is clearly out-of-round at the back (pulley end) so will need to be bored whatever solution is decided upon.

I've been pondering whether to bore out the casting and lap in a new spindle, or bore the casting and fit oilite bushes or bore seatings and fit ball-races. The casting OD is about 1-1/8" – the spindle (was) 1/2" dia. The ball bearings would probably dictate a much smaller spindle diameter but would run fast enough to let me use 80/85mm (3.3" ) Proxxon saw blades. However, I've also just 'found' half a dozen 5-6" "old tech" carbon steel blades with 1/2" holes that probably date back to my B&D days (e.g. pre-history) but they did cut quite well if I recall correctly (they are all 'branded' saws – not the B&D ones) – so that has muddied the waters a little.

So I cannot make my mind up which way to go but am concerned about sawdust etc with the Oilite approach. Apologies again to Ron for the divert but my question is along the same lines just in a different environment. Generally folk appear to think Oilite superior to Cast-Iron but I've always understood a well lapped cast-iron bearing (which does involve more work I would have thought) to be better… but not sure in this wood-working context though..

Your thoughts please.

Regards,

IanT

Edited for damned 'winkies' !!

Edited By IanT on 27/07/2018 16:45:22

It's not critical unless you're chasing extended life.

Starting from new with a close running fit, the shaft and bed will tend to smooth out metal to metal imperfections, the dirt ending up in the oil and then escaping. (Remember when new cars had 'Running in please pass' signs? ). Thereafter the shaft will ride on the oil until wear enlarges the gap so much that the oil can't fill it. The oil film can be molecule thick, the main problem is to keep enough oil in the bearing to maintain it. Various ways of doing that, between a dripping reservoir, or wick, up to a high-pressure oil pump.

The problem with 'Running in Please Pass' smoothing is that it's hit and miss. To improve the life of a bearing, it's better to micro-finish the surfaces in the factory. The process can be highly elaborate, but for home use lapping is recommended. The bearings I've made don't get enough use to prove whether or not lapping makes any difference. Running small engines on air for a few minutes to impress the kids isn't a good test – I'm not the best person to ask about bearing life as applied to hauling passengers.

Dave

Ian,

My instincts go with ball bearings ..mostly because of simplicity with say a 15x10x4 which if necessary you could stack and or space for greater width and dependng on the spindle ends that could be made from ground rod. Bearing changes would be easy enough in the future and if it doesn't work out then you can go back to the other options still. You also have the options of s/s and shielded or sealed bearings for dust control.

pgk

Pressure fed white metal bearings tend to start life with 0.002" to 0.004" clearance, and would expect a drip fed bearing to run at the lower end. The clearance combined with pressure feed of oil is mainly to provide a flow to cool the heavily loaded bearing, and prevent the constituents of the white metal changing their state. (On main propulsion engines, the temperature of each main bearing is usually monitored). In automotive engines the usual parameter is sump temperature, to control both bearing temperature and to minimise degradation of the oil).

Your cast iron bearings are likely to be relatively lightly loaded, and running at low rubbing speeds, so I would hope that clearances at the smaller end of the above, with felt pad/drip fed lubrication should suffice.

If in doubt, make the bearing longer, rather than larger diameter, to minimise pressure rather than rubbing speed.

On a coupled locomotive, the bearings will be subjected to the static load of the loco, and when running, the inertia loads from the reciprocating parts, (which at speed could exceed the static loading).

Your lubricant could be a high pressure type, such as a gear or hypoid oil, although these tend to be higher viscosity, leading to greater friction.

H T H a little

Howard

For a cast iron bearing , oil it from a hole in the end of the shaft, and form a felt filled well in the bottom half of the bearing.

Ian S C

Several of my Stationary engines have cam shafts running in cast iron bearings and run at 300+ rpm, one engine has a greasing cup to lubricate the bearing and an earlier sibling has just a hole to put lubricating oil onto the shaft, this engine was made in 1923 and its later brother is dated at 1947, they will run for more years than I will

Martin P

Edited By martin perman on 29/07/2018 18:19:42