Bronze, Phosphor Bronze, Cast Iron or Graphite for LTD Stirling bearing

Following on from my other question....

The plans call for bronze or graphite for the displacer rod bearing. Looking at various suppliers 'bronze' seems to cover regular bronze (the coppery coloured stuff that I'd call bronze), phosphor bronze and gunmetal (can't believe how pricey this stuff is these days, maybe I should take up woodwork!)

As the part is going to be glued in place I thought cast iron, brass or even aluminium might do the job until I came across this interesting page:

**LINK**

which, as the rod through the bearing is steel, shows that graphite is miles out in front in the slipperiness department!

Trouble is I can't see it for sale anywhere and have no idea how to machine it (some web forum I read said it was very dusty). Does it usually go under a trade name or something? Will it be possible to make an accurate 2mm hole through it with a D bit? Should I just use brass (which according to that page has a lower coefficient of friction than cast iron when put with steel)?

TIA

Paul

Machining graphite is horrendous. I spent many a hour removing graphite dust from motors and electrical gear caused by operators not using extraction equipment after machining electrodes for spark erosion. The last thing I would use is graphite it gets everywhere and is abrasive and a good conductor of electricity.

David

I would have thought there was also an explosion risk. Any dust particles in correct concentration in an enclosed space may expose spontaineously.

Amorphous carbon might be abrasive, but graphite is used as a lubricant (think graphite chain grease? )

, so cannot be an abrasive hazard!

Motor brushes (don't wear very quickly) are often of quite soft graphite (DC brushes rather than AC).

We use graphite marvers when forming glass beads, so it is available. Not cheap, mind.

I doubt there is going to be a great loss of energy whichever recommended material option you take, for this application.

I spent 3 years as an electrician in the toolroom, although none of it was very clean as most of the machining was cast iron the edm room was a special kind of hell, liberal amounts of carbon dust mixed with the foul smelling oil used to submerge the workpiece usually meant an overall change after a visit to fix something.

Mike

Noggin End sell small amounts of graphite.

SA660 is bearing bronze which would be the one to look for. Or LG2 if you want to get that from Noggin too

Right, I'm giving graphite a wide berth and going for the easy solution - brass.

I work for a specialist graphite machining company. so see the stuff (in various grades - and there are many, with widely differing properties) being machine daily. Most grades are easy enough to machine with normal metalworking tools - HSS, carbide inserts etc. For short run special profiles, form tools made from silver steel & gauge plate used unhardened also work. It is abrasive to the tools - a bit like cast iron. It is also very dirty, but (according to HSE investigations conducted at my employer's previous company), poses no particular health hazards and, in particular, poses no explosion risk as airborn dust as it requires the continous application of heat to cause it to burn. Motor brushes are mainly carbon IIRC, not graphite (we don't make motor brushes).

If you PM me the dimensions of the billet you require, I will see if I can get you a suitable piece of fine grained Isostatic graphite from the oddments bin. My employer is usually OK with this & only requires that photos or videoes of the finished item / machine are sent so he has the option of using them for publicity purposes. This grade machines to a good finish & parts can be made to close tolerances (we hold sizes to better than 0.01mm) - it is a bit dirty, mind !

Nigel B

It would appear thaf there is some confusion, among the responders, re the allotropes (same element but different physical forms/structures) of carbon. Graphite is down there close to talc on the Mohs hardness scale (talc is designated as 1) and diamond (another allotrope) is designated as 10, at the other end of the scale.

Amorphous carbon will be somewhere between these two extremes. At the atomic scale, graphite is made up of hexagonal plates of atoms which easily slide over one another, amorphous carbon is simply bundles of atoms joined/held together haphazardly and diamond has a very fixed structure where the atoms are regularly spaced and very securely fixed in position - to the point that diamond will scratch/abrade any other known mineral.

Graphite has a very low electrical resistance, amorphous carbon is of variable resistance, but rather more than graphite. Diamond is an electrical insulator.

Motor brushes are made of a chosen mixture, depending on application. Binders are incorporated in the production stages, to maintain form, and then later removed by heating to high temperature.

We used to melt metals such as rhodium in carbon crucibles using induction furnaces. As an aside, carbon does not melt - it sublimes between 3 and 4 thousand degrees Celsius...

The graphite we machine is man made from, as I understand it, a mixture of coke and pitch. The coke is ground to specific grain sizes according to the grade of graphite required. Other compounds may also be added, like iron filings or sulphur to vary the properties. The resultant thick paste is either extruded or isostatically pressed into blanks, then baked to form a carbon block. The carbon blocks are stacked, covered in deep layer of coke, then heated to very high temperture by passing an electric current through the stack - this converts the carbon to graphite. Improperly graphitised blocks have a higher resistance & are harder to machine & I understand that the graphitisation process is "one chance" process - if it is not done properly first time around it can't be re-visited..

The properties of the blocks vary depending on the mix & how it is processed - some are soft enough to be able to take chunks out with a teaspoon, while others are almost like ceramic & can only be worked with diamond tools - with many variations in between. My understanding is that these man made graphites are different to the natural deposits that are used for graphite lubricating powders - but my understanding may be flawed. When we are asked to supply graphite powders, these come from different suppliers & are physically different to the talc-fine dust that comes out of our dust extractors from machining extruded & isostatic blocks.

Nigel B

I have use all the above except graphite, but I have used Graphite impregnated Teflon, all work well.

You can't see too much of it, but this motor has a brass bush, bore 3/16", 3/4" long.

Ian S C

Hi PaulR,

Last year NigelB was good enough to send me some graphite, from which I have machined some bearings for Stirling Engine. Unfortunately, I have not made any further progress with it yet due to health problems for both me and my wife. It's a bit messy to machine, but if you cover the oily bits of your lathe, it cleans up easily with a vacuum cleaner.

I used Noggin's End Graphite rod to make the pistons of a pair of LTD Stirlings with great success. It machined well, the only downside being the mess. In small quantities this is easily managed. One advantage for use as a piston is that Graphite does not corrode: I had trouble with steel pistons, possibly due to acetic acid released by the silicon sealant.

Reducing friction is key to getting these engines to run. I used ball rather than graphite bearings and found that expensive ones worked much better than the cheapo variety. A slippery graphite piston helped enormously too. Even so, 'running in' the engine took a long time.

You do need to be careful buying 'graphite'. Depending on the intended use, it's often mixed with something else. For example, 9H pencil leads are mostly baked clay, whilst 9B pencil leads are almost pure. Dry battery anodes are something else again. The differences may explain why some people have had bad experiences with 'graphite'.

Dave

Hello,

I have had some graphite which was in my materials cupboard for many years. I had reason to use it a year or two back. Yes it was very messy, but machined very well indeed (super finishes). So my experience says go for it! The only snag is that I don't know what grade it was. I was told it was pure graphite whatever that means.

Andrew.

I can't remember the specs for the graphite used for pistons, foundries use the graphite that is required, the same stuff is ok for the bearings on LTD motor crankshafts, pointed end resting on the graphite.

Ian S C

I made a graphite piston for my sterling engine. To avoid some of the mess i taped the vacuum cleaner hose to the toolpost. As per soem advice here I polished the end result with plain copy paper. It was from whatever quality of graphite rod it was that was for sale on ebay. It drilled easily and held a tapped thread. I'd guess that for an accurate small hole one might pilot drill then make up a simple pin reamer (three flats to a point)

To make bushes from Carbon Impregnated Teflon, you can machine it from solid, but having turned a few you have a whole pile of swarf, now this stuff is expensive, so I made a mould, this gets stuffed with Teflon, and packed down, next the mould is heated to blue (steel mould), when it is cool the core gets pushed out along with the bush. These bushes will out last most of the rest of the motor.

The first motor I used one of these bushes (1992) still has the original bush. After 1000hrs of running, I had to make a new crankshaft with larger crank pins (3/8" instead of 1/4", a couple of months later I went to reassemble the motor, a BETA type and found that the Teflon had swollen, and the displacer rod would not fit, I had to ream it to make it fit. I think the bush had got some oil on it during the previous year of fairly heavy use. After all these years the motor now needs a new hot end.

Ian S C