A useful Steam Engine

Having proudly made my first steam engine (though yet to be powered by Steam .. a boiler is an upcoming challenge ...), I thought about the next step.

The first step was a small oscillator with a 12 mm bore and perhaps a 20mm travel.

I want to make something bigger (with valves and stuff), but I'd like it (in theory) to be practically useful.

I've set my thoughts on something that could drive a lathe. Which means somewhere around a horsepower. My current lathe has a 550W motor which is about 3/4 horsepower.

My naive internet research has found a horse power calculator which reads HP = PLAN / 33000

P = Pressure ( I assume lb/sq in)

L = stroke length

A = piston area

N = RPM

So (ignoring losses), an engine with one piston, 2 square inch area (1.6 inch diameter), 2 inch stroke and 360 rpm (6 rps) and 15lbs pressure will generate about 0.65 HP.

My first thought was, 'Nah - that's far too little'. Then I thought of 30lbs of force pushing 6 times a second and thought that might be about right.

So. Is my technical understanding correct?

Can I really build a steam powered motor for my lathe in less space than the electric one takes up?

I imagine that, realistically, a single piston engine with a 2 inch stroke and 1/6 inch bore is not the best configuration in practice.

Given all that, are there any plans for an engine of around 1 HP which I could have a bash at? Ideally free and not using castings. Oh, and to be awkward, in metric, by preference.

Iain

The figure of 33,000 is foot pounds per minute, (equals 1 HP. )

So your dimensions need to be in feet.

It'late, hope I've got it right!

If you look around at the engines used for steam launches (not models) there are many designs which are working engines of around that power and upwards.

Steampunk lathe? I like the sound of this idea!

Stuart Models give some power outputs for many of their models. E.G. the 5A a claimed 1.5HP with a 2.25" bore and 2" stroke - considerably more power than my little Myford's electric motor. Go for it!

Actually, look into it in a lot more detail first, no doubt it's more complex than that.

Don't forget to include power losses for the transmission, belts/pulleys etc.

Edited By Brian Sweeting on 09/10/2017 23:25:45

Correct, but pressure should also be pounds per sq ft, and you need a 2 in the top line as it will presumably be double acting

and the stroke length in feet too?

if I plug in some numbers (and forgive me, it's 5:30 am before I set off for a conference...)

P (16 lbs/sq in) = 2160 (lbs/sq ft)

L (2 in) = 0.1666 feet

A (2 sq in) = 0.0139

N = 460

which Excel calculates out to be 0.0000846 HP

which wouldn't turn a lathe for an ant.

What am I getting wrong?

Iain

The engine may be about the same size as your motor but what about the rest of the steam plant.

The 5A will develop (0.47kw) but at 1000rpm and working at 80psi (100max)  not what was stated above which is quite a volume of steam, so that needs a boiler with large heating surface. That steam comming out of the boiler needs replacing with water so you will need to add a pump and water storage tank. What about all that wasted half used steam, put it through an oil separator and a condensor for better performance, then you need to add an air pump.................................

Look at some steam launches with that size engine and then look at the size of steam plant and you may be put off the idea

Using 15psi, 0.333ft stroke, 2sq in area and 360 rpm / 33000 I get 0.109 IHP 

Above units taken from ME Handbook

J

PS for those playing with numbers do remember most steam engines are double acting so stroke should be 2 x length!

Edited By JasonB on 10/10/2017 08:29:07

Also, the PLAN formula gives the theoretical internal power of the engine which is much more than will available at the output. Lots of things eat away at the actual power output, for example:

• 'Wire Drawing' (ie friction) in the steam feed piping and valves reduces steam pressure before it gets into the cylinder. Likewise, back pressure on the exhaust side piping reduces the effective pressure on the piston.
• Leaks past the piston, valve, and glands reduce pressure
• Heat losses due to radiation and convection reduce pressure inside the engine
• Friction in the piston, glands, bearings, eccentric and valve all consume power that won't be available at the output
• Power is consumed moving the mass of all the moving parts - piston, conrod, crank, axle, eccentric, valve rod, governor and valve etc.
• Any condensation in the engine has multiple bad effects on efficiency

Many inefficiencies are much worse in small engines than they would be in a big one. For example heat loss from a cylinder is proportional to the square of it's dimensions whereas the amount of heat contained inside is proportional to the cube. Thus big cylinders are always more efficient than small ones, even before insulation.

Driving a lathe requires the engine to produce an output power. The dimensions of an engine can be calculated from PLAN by assuming typical efficiencies. As a starting point I'd assume the output of a small double acting steam engine to be about 2% of PLAN.

I've not the time needed crunched the numbers but L and A will need to be much bigger.

The HP from PLAN formula is an example of inconsistent Imperial Measure causing confusion. A jumble of Inches, Feet, and Minutes in a calculation where the answer is in feet, pounds and seconds tweaked to allow comparison with a mythical average horse.

Indicated Horse Power = PLAN / 33000

Where:

• P is Pressure in Pounds per Square Inch *
• L is Piston stroke in Feet
• A is Piston Area in Square Inches
• N is Revolutions per Minute

The PSI/Feet/Inches/Minute conversions are hidden inside the rarely explained magic number '33000'. Yuk!

Dave

* Actually the Mean Effective Pressure, which is less than the input pressure.

Edited By SillyOldDuffer on 10/10/2017 10:49:37

Edited By SillyOldDuffer on 10/10/2017 11:24:44

But in the real world, losses of 80 per cent and upwards will give a much different result.

I like the idea of looking at small steam launch engines for a starting point. Seems they, and the associated steam plant, would be about the right size. To do things properly you will of course need to hire an apprentice to shovel coal into the boiler as you turn.

For double acting steam engines the formula is 2x PLAN / 33000

Roy

People used to use steam engines of that size to run lathes, so the empirical answer is yes.

You will need a decent sized boiler good supply of coal.

Of course if you boil the water using electricity you will probably need to upgrade your workshop's electricity supply...

Neil

It might be interesting to build a modern design gas-fired flash steam generator. You could even run a moderate amount of superheat and get some good efficiency out of it. Then send the exhaust steam to a condensor and recycle the condensate as nice hot feedwater.

Model Engineer Magazine, 11 February 1969, Vol. 122, No. 3057, cover & pp.159 & 160.

Cover picture is of a 10" Southbend Lathe powered by a 'Neptune' steam engine.

The 'Neptune' is a twin cylinder, double acting, diagonal paddle engine, 3/4" bore x 1 1/2" stroke, designed by Edgar T Westbury. (Apparently.)

Description of Gilbert G Emerson's version on pages 159 & 160. He attached it to his lathe seemingly to test its' power rather than to carry out turning work.

Quote:

"That it [The Neptune Engine] has ample power was proved when it drove my toolroom Southbend 10" lathe at around 200rpm with 80psi initial pressure in the air tank. I forgot to release the belt tension, but the Neptune started the 6 in. No. 18 chuck, as well as the GE 3/4 hp floor motor. It will be seen that the safety checks on the cylinders blew at this load, but she only hesitated a moment."

Earlier in the text Mr Emerson had written:

"The safety checks were made of 1/8 in. stainless steel balls seated in 3/32 in. holes, with Schrader valve springs, which pop at 80psi."

Regards,

Phil

Edited By Weary on 10/10/2017 12:48:27

Edited By Weary on 10/10/2017 12:49:15

I think it's finger trouble on your calculator. Using your figures gives 0.069 HP, but 16 psi is 2304 pounds per sq ft (perhaps you meant 15 psi), and you've forgotten the 2 for double acting, so the real answer using your figures is 0.149 HP. However, your pressure is very low for a working engine, but you need to allow for cut-of and compression as well as mechanical efficiency. I think Hopper's 80% losses is a bit pessimistic, I think Bill Hall got much better than this on his tests

This all goes to show that mixing feet and inches in formulae is not good practrice. Yes it works, as it cancels out, but it makes it much easier to make errors. It makes more sense to change the conversion factor 33000 ft.lb/HP to 396000 in.lb/HP and then use inches throughout.

Whilst this would be a fun project, managing a steam engine and boiler at the same time as concentrating on using the lathe would be a bit much for my old brain.

You'll need a working governor in order to get full power from the engine under load without the engine racing away off load. Ideally the governor woild control valve cutoff rather than throttling the steam supply, as the latter inherently result in a speed change for a change of load.

Andrew

Would it be easier to build one of the Stuart lathes or similar to drive with a modest size engine?

One of the guys over on MEM did just that.

Thanks for posting the video Jason. Enjoyed watching it during my coffee break.

Dave

Wow, that is quite a project. He certainly has a lot more patience including dexterous fingers than I've got. One thing does puzzle me, anybody got an idea what the loose rings running on the line shifting are for? These appear at about 1min 10secs on the video, I can remember putting old bits of pyjama cord around the hubs of bicycles when I was a kid to try and keep the hub clean and oil free but would anyone bother on line shifting?

Phil