[lyner]

Except that the frequency is, necessarily, very low. This is because there is a large mass which falls under gravity. A rotational system gets over this - which is probably why the jackhammer and beam engine are the only examples I can think of which don't use rotation!

[peppercorn (OP)]

Yes, of course! Return under gravity is the limiting factor here - I should have realised that!
Although slow & steady might not be a bad thing for every application.


Alternatively, like I think you were alluding to previously, using opposing stokes would overcome this:


The yoke is just for timing the two cylinders and is shown massively over-engineered here for illustration. Power is the red arrows (on alternate strokes).
Clearly, this will only work for two-stroke, but a bit more thought could make a 4-stroke equivalent.

[lyner]

Alternatively, like I think you were alluding to previously, using opposing stokes would overcome this:

'fraid not - the hammer can't fall any quicker however many times you lift it up. If you fire very frequently the hammer will merely be suspended at a higher average position. It won't return to the fully compressed position if it isn't given time.
See how clever the use of a rotary system is? The speed is only limited by the breathing and the strength of the components.

[peppercorn (OP)]

Bugger! Really must engage brain!

The original configuration (both pistons in sinc.) would work if material being broken up is forced back up against the hammer (a bit like if the barco was suspended on an A-frame pinned to the ground with a big spring pushing it back down each time). So pulping wood, etc could conceivably utilise a high speed hammer action of this sort.

[lyner]

Yes, a spring would do it.
Though, there would still be the problem /consideration that your system would have a resonant frequency, set by the mass and spring constant.  Sodding great spring and sodding strong A frame needed!
 
I have just thought of a possible advantage in that the stroke of the system would not have to be constant (unlike the stroke of a rotating engine) so it wouldn't 'stall' but just carry on vibrating with smaller amplitudes.

[Geezer]

Regarding efficiency, would it be possible to derive some measure of useful work done from the heat generated in the rock? I imagine there must be quite a lot of friction produced while the rock is getting smooshed. (Not to mention the heat coming out of the operator's mouth when the thing lands on his toe.)  []

Measuring the temperature increase might be a bit tricky, but perhaps not impossible if you had some sort of "calibrated" rock in a well insulated setup.

[nicephotog]

If the rock it is trying to break up doesn't break - or if you try to break up a trampoline (see above) then there is no useful work.

I'm well aware that work-done is an expression of used-levels-of-resource-expended-SUCCESSFULLY.

It's in context of fuel used per strike / per time to achieve the cycle of load - reload in the sequence of moving the hammer,
the fuel yield and gravitational attraction and inertia never will actually change(at least for the fuel grade).

[lyner]

The clue is in the term 'useful work' which is essental to know before you can calculate efficiency.
If you can't specify that then you can't know the efficiency. The only hope of some sort of idea would be amount of flattening of a specific thickness of Tarmac or breaking of a specified sample of stones. Neither of these could easily be related to 'real life' work situations.

[Geezer]

All measurements of efficiency MUST be related to "real life" situations, otherwise they are entirely meaningless. There is a set of "real life" assumptions behind every measure of efficiency. They may be explicit, but they are more commonly implicit.

In the case of the "instrument of dread" in the photo, its main purpose is to compact aggregate and soil. Devising a method of comparing its effectiveness compared to the alternative methods should not challenge your average fifth year pupil too much.

[nicephotog]

sophiecentaur: Neither of these could easily be related to 'real life' work situations.

Would measuring the resultant "mode" of the tarmac p/cm and in a grid do that?
avg.pdf www.nicephotog-jsp.net fortran95 "Plato IDE"
What a thought for MATLAB

[lyner]

Did you mean modulus?
Tarmac is not elastic. It's very much plastic, with a complex modulus. Poor old MATLAB (don't they sell shirts and kitchen utensils?)  it is asked to do some very hard stuff.