Cast Iron bearings..?

Is cast iron a good material for motion bearings, any problems with it..?

Ron

Ron,

It is an excellent bearing material for steels in particular. The included carbon in it's structure is an added bonus to help with oil lubrication

Brian

Forgot to say the axles I am thinking of would be from EN8 steel.

Ron

Thanks Brian, thats good to know

Ron

Ron you couldn't do much better.

Ian S C

That's interesting. I am planning to make the rear axleboxes for a clayton lorry from some lumps of Steel I've got kicking around and then make split bronze sleeves to go inside them. Looking at cast iron prices it is cheaper for me to buy some CI bar and machine them than it is for just the cost of the bronze. Looks like time to order some CI bar.

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.

Thanks guys, when I am that stage I will turn myself some C/I bearings.

I did read this morning of someone building a 5 inch steam loco and he argued that cast iron bearings will easily outlast bronze bearings, dont know if that is true but thats what he claimed.

Ron

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

Thanks Dave,

The bearings I will be making will be simple shoulder bushes with a bore size of around 5/8". This leads to another question, to achieve this "thin cushion of oil" what clearance should there be between the axle and bearing.

Ron

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

Good morning Ron,

I've used continually or proof machined cast iron for axle boxes on my locomotives for the last 30 - 40 years and expect to still continue to do the same; my theory being that my old mum's treadle operated sewing machine never wore out and at times when she was going at full tilt the machine was operating at something like 700 strokes per minute, all on plain cast iron bearings with just a drop of Singer sewing machine oil! As a tight fisted old so-n-so i will not pay for some miserable lumps of gunmetal sold as axlebox material, further I can supply my own blowholes and that applies to cylinder castings also.

Regards,

Bob

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

One way to keep the size of the bearing within limits is to use needle roller bearings, ideally the shaft should be hard, but for lighter use, a unhardened shaft should give many hours of use, or you can get a sleeve that fits on the shaft.

Ian S C

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

Don't feed the oil into axle boxes at the top. Yes it's the most convenient place many times, but it's also the point of highest hydro-dynamic pressure. One of the old CMEs set up an experiment with an axle box loaded vertically downwards onto a rotating axle. Oil fed to a felt pad underneath as it should be, and a hole in the top of the box. When the axle was rotated the oil was pumped out of the top hole. Felt pads in the keep are a bit OTT on models, but you can arrange the oilf feed off to one side so it is tangential to the axle hole.

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

Thanks guys for all the advice, its much appreciated and I have learnt a lot from your replies.

When I have finished the little electric 0-4-0 (which is running on oilite bearings) I want to design and build a Class 22 Baby Warship which is a twin bogie / 4 axle loco. Whilst jotting down ideas for the design of the bogies, I realised that if I went with off the shelf oilites they would need to be modified to fit . This is fine but I thought why bother..? better to make my own bearings from scratch that fit the job, hence my interest in going with cast iron.

Thanks again guys.

Regards

Ron

Edited By Ron Laden on 29/07/2018 09:24:53

Edited By Ron Laden on 29/07/2018 09:25:47

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

My own total guess would be that babbit is best because that's what they used in Locomotives

If CI was the biz the loco industry would have jumped at the cost saving