Bearing clearances

[Martin CottrellParticipant @martincottrell21329]

Hi all,

I'm currently building a 4" scale Little Samson traction engine and about to embark on construction of the bronze bearings for the crankshaft, intermediate gear shaft and rear axle shaft. I read somewhere a while ago that if traction engine bearings are machined to too close a tolerance for their respective shafts, they can become over tight or even seize once the engine is in steam due to expansion of components at different rates and directions. The advise was to machine the bearings " a little loose" on the shafts. My question is how loose? The shaft diameters are 1.000" for the crankshaft, 1.125" for the intermediate & 1.375" for the rear axle.

Any thoughts would be appreciated!

Regards Martin.

[Martin CottrellParticipant @martincottrell21329]

[John FieldingParticipant @johnfielding34086]

A rule of thumb is for bearings that are well supported and force fed with oil, like car crankshafts etc, then 0.5 to 1 thou per inch of diameter is recommended. For drip fed bearings and ones where some misalignment can occur due to flexing etc, then 2 thou per inch in diameter is the minimum and often a lot more is required. On locomotive crank pins and axle boxes for example the bearing clearance is often as much as 1/16-inch to 1/8-inch total, that is on the full size jobs of course. The problem is you cannot scale nature so if a shaft needs 5-thou on the full size engine the model will need about the same!

The rear wheel bearings will need the most clearance and the crankshaft a little less, what the exact "ideal clearance" is is a moot point. You have to go with the best thumb suck and see what happens. If things bind then give it more clearance.

[mechman48Participant @mechman48]

When I served my apprenticeship, a long time ago, the general 'rule of thumb' was accepted as …1 thou' per inch shaft diameter, & over the years I very rarely deviated from that.

George.

[John FieldingParticipant @johnfielding34086]

Martin you might like to reconsider your choice of bearing material!

I have long ago ditched the use of bronze on two counts. The first is the cost, it has gone through the roof recently and when I priced it recently I nearly needed CPR, the cost was astronomical! Today my preference is to use cast iron and Doug Hewson also advocates its use as it has some very good benefits over bronze.

Cast iron contains flakes or nodules of graphite, which is in itself an excellent dry lubricant. It also has a surface when machined which is porous, similar to phosphor bronze, so it has millions of little inclusions that hold oil. Bronze in the harder varieties has a surface which is smooth and doesn't hold oil to any great extent. So if the bearing is starved of oil it tends to pick-up steel splinters and these grind the bearing away. Cast iron is marginally softer and can accommodate tiny metal debris better, like white metal linings do.

But it really comes down to ease of machining and cost in my book. Cast iron of the continuously poured or spun cast and chilled variety after the continuous pouring promotes the maximum growth of the graphite particles and it is a fairly "soft" material to machine. When the bearing is run it has the ability to form a glass hard surface, which still has tiny oil pockets and graphite. Good cast iron is like machining chalk, and a good HSS or carbide tipped tool will make light work of it and the surface finish straight off the machine is more than good enough to use right away without any honing. Honing does give a better initial finish but just running will do the same thing after a while if the fitting is acceptable.

Cast iron is an interesting material from a metalalogy view point. Unlike other metals it has no plastic region when heated or cooled. The difference between the liquid state for pouring and the solid state is only a few degrees, so it "sets" almost instantly. Steel and other materials go through a plastic state as they cool from the liquid state. Hence, they can be forged or squeezed into shape when hot, cast iron doesn't have that capability.

Another good material to use is Vesconite, which is a self lubricating plastic.  Fitted as a sleeve into a steel or cast iron housing it offers some give and it can tolerate minimum oiling and accept a fair bit of debris.  It is extensively used in the mining industry where the underground conditions have high dust in the air and shackle pin bushes on tipper trucks, front end diggers etc use it widely.  Take a look on their website and see the steam locomotive page where it is used to refurbish the valve gear on the South African locos at Reef-Steamers.  I can vouch for its efficacy as I also use it in some jobs. Vesconite is one of the hidden materials but it is made in South Africa and they have a plant in the USA also making it under license and NASA use it as well, so it is well respected!

Edited By John Fielding on 14/03/2016 09:37:34

[Martin CottrellParticipant @martincottrell21329]

John & George,

Thanks both for the helpful replies. I machined the bearings over the weekend & aimed for +0.002" on the smaller shaft sets & +0.003 on the rear axle set. The engine won't be finished for at least another couple of years so I'm afraid I can't provide any immediate feedback!

I appreciate the time you took to explain the benefits of substituting the bronze with cast iron for the bearing material John. I agree the price of bronze is somewhat eye watering however I was fortunate enough to "barter" my needs with a friend who exchanged some suitable lumps in exchange for some assistance with his workshop re-location. When the need arises in future for a similar bearing requirement I will look at using cast iron as a alternative to bronze both on cost and suitability.

Regards Martin.

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