[peppercorn (OP)]

How efficient would this hammer be?



www.gothotrocks.com/barco/barco/barco.htm


With only one moving part & no crank or valve gear it should have a theoretically high efficiency, yes?

[lyner]

Simplicity doesn't necessarily imply efficiency although I can see where you're coming from.

Efficiency is defined as
Useful work out / Energy supplied

It would be difficult to define the useful work got out of it. That would depend upon the amount of actual displacement of the ground surface that it achieves on each impact.
Work  = force times distance
If the ground is springy, then the hammer will bounce up and, eventually, heat will be generated, rather than useful work in deforming the ground permananetly - lowering the efficiency.

The efficiency could be maximised by measuring the characteristics of the ground - density, stiffness and friction  (lumped together as Impedance) then designing a hammer with the appropriate mass, piston area etc. to match to this impedance.

I'll bet they design them by the suck it and see method and that they have no idea of actual efficiency - just whether they 'work' well or not.

[Bored chemist]

On a trampoline its efficiency would be practically zero (except possibly as a means to commit suicide).

[peppercorn (OP)]

Okay, yes - good points.
For arguments sake let's assume we are working on some sort of ideal material.
Say the work we want to do is break-up a very brittle material like individual concrete blocks (on a non-bouncy surface). The hammer is ideally 'tuned' to the task smashing the blocks almost to dust then moving on to the next (during the bounce).
so we're not worried about the external losses, just how the hammer utilises each bang.

For a crank engine at least, I am aware that most I.C. have best efficiency at high revs, but I don't know if that applies for this configuration.
The main difference I can think of is that for slow running pistons engines there is more time for heat to dissipate away from the cylinder walls, but I would have thought this is another 'tunable' issue.

[lyner]

To get an idea of efficiency I think you could only hope for a comparative figure. Take a measured quantity of standard 'big' rocks and see the time taken (multiplied by the respective input powers) of two different jack hammers to reduce the rocks to a specified grit size.
That would tell you which one to buy.

[peppercorn (OP)]

I don't see why the best that could be found is a comparative figure between two jackhammers.
It has been pointed out that efficiency is Useful work out / Energy supplied which means that a true quantitative measurement can be given for any machine.

Modern jackhammers use a crank engine (I imagine for smoother & safer operation [I mean look at that thing!!]), but this means reciprocation to rotation and back to reciprocation again.  So apart from not killing the operator are there any other efficiency gains that only a crank engine will have?

[lyner]

If you could define the conditions of operation so that you could establish the "work got out", then you would have a chance of measuring efficiency. I just think that would be too difficult.

[Don_1]

Do they still use those things, I haven't seen one for years?

[Mazurka]

On the scale shown in the piccy, I guess they have been replaced by hydraulic hammers mounted on excavators.  On small scale jobs hand held pneumatic (or hydrualic) drills are still used - although issues such as vibration white finger are increasingly limiting their use.  The flexibility, ubiquity and work rates of modern excavators on construction sites can often mean it is cheaper to hire in a breaker for a day or two than have all of the faffing about of using pneumatic kit.

For tamping down material such as pipe bedding etc.  vibrating roller or plate machines tend to be used instead of jack hammers.

The picture looks like a health and safety nightmare - the best steel toecaps in the world are not going to stop that hammer...

[nicephotog]

It would be immensley efficient,
-but for fuel yield type and
-full usage of compression ratio
-with the 15:1 parts carburation mix.
Depends what those three points reach.

I wonder if the carburation was effective.
In terms of ware, its efficient.

Amazing they weren't modified over there in the US and turned into motorised pogo sticks.
Did they all get sent to Somalia?
http://www.thenakedscientists.com/forum/index.php?topic=24602.0

[lyner]

It would be immensley efficient,

But how can you say that if you can't identify the work done? However good the  internal combustion bit is, you may still get nothing useful out of it at all. 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.

[peppercorn (OP)]

Okay if its a question of identifying work done (useful or not) then find a large mass of a material that is immune to deformation or cracking at these forces (high grade cast iron?) Stick it and the hammer in a vacuum, then measure how much the worked material heats up. After all where else can the energy go?
One problem: the hammer's engine would need to be cooled, plus some of the heat from it could conduct through the foot to the worked material.

[lyner]

Okay if its a question of identifying work done (useful or not) then find a large mass of a material that is immune to deformation or cracking at these forces (high grade cast iron?) Stick it and the hammer in a vacuum, then measure how much the worked material heats up. After all where else can the energy go?
One problem: the hammer's engine would need to be cooled, plus some of the heat from it could conduct through the foot to the worked material.

Yebbut there would be no 'jackhammer type work' done, in which case the efficiency would be zero.
I can see you are after some measure of output along the same lines as brake horsepower measurement - fair enough. But bhp etc. can be well related to actual motor car performance because the work done is quantifiable.
You could possibly have a defined volume of a defined mix of tarmac which would take a certain amount of energy to compress to a defined percentage of its volume as a standard measure of jackhammer work. 

I guess you could relate a jackhammer output in terms of 'manpower' - how many standard stones a standard man could break into standard small grit in an hour with a standard sledgehammer.
omg, this is really getting daft!

[peppercorn (OP)]

It would be immensely efficient,
-but for fuel yield type and
-full usage of compression ratio
-with the 15:1 parts carburation mix.
Depends what those three points reach.

The biggest loss as with all IC engines is going to be thermal - mainly heat exiting through the exhaust. Like I mentioned earlier the idea of a high reved crank engine having less time to cool between bangs implies a natural gain in efficiency if things are happening fast (although I can't say exactly why or if that applies to a free-piston configuration [like our hammer]). I'm sure modern carburation and lean burn techniques would improve efficiency by some degree also.

[lyner]

The efficiency of the IC iwill certainly impose an upper limit but, as I keep saying, if you can't match the engine to the job, you can have almost zero efficiency.
The OP asked about efficiency of the machine - not just the combustion bit.
Bearing in mind that the material you're working on will be non Newtonian, the time profile of the impact is likely to have a major effect. It may be that a rotary mechanism with a cam, appropriately shaped, would give an improvement  much moe significant than any choice of motor.
As a lad, I used to see compressed air hammers. They would have a very different time profile.

[AllenG]

Do they still use those things, I haven't seen one for years?

I believe that one is used as an earth tamper for compacting soil.
Used when prepping road surfaces for asphalt.

[peppercorn (OP)]

The OP asked about efficiency of the machine - not just the combustion bit.
...
As a lad, I used to see compressed air hammers. They would have a very different time profile.

OK - I want to ask about the 'combustion bit' - how does that compare?

What do you mean by time profile?

Thanks!

[lyner]

By time profile, I meant the way the force they produce varies with time. With a rotary engine, you tend to smooth this out with the flywheel and multiple cylinders but, with a 'hammering' machine, you need an impulsive force. The way the force varies as the machine operates should affect its effectiveness a lot. So, if you let compressed air in through a valve (like the old pneumatic drills) the force may build up slowly but if you ignite the fuel / air mixture, the force may be much more impulsive.

The efficiency of an IC engine can't be much more than 50% (limited by thermodynamics) but, as I keep batting on about but people don't seem to be catching on, that efficiency can only be realised when the piston is allowed to move optimally. If your hammer is pushing against solid, massive concrete, the only work that can be done is in lifting the hammer itself. The time taken to do this will be very different from the time involved in the head of the hammer penetrating some optimally soft road surface and, at the other extreme, when the head is pushing against very soft (no- existent, even) surface. The optimum will depend on what the IC engine designer would want and the mass and footprint of the head, in addition to the surface characteristic.

As with all things, if you can actually ask a defined question, you may get a proper answer. But this question is too undefined to be answered.

[peppercorn (OP)]

Thanks SC.

I quite agree that the question of efficiency is of course inseparable from load matching.  When we think of most uses for mechanical energy the purpose is generally some type of smooth constant movement (like a rotating wheel or a crane hoist), but some jobs can use the concussive shock of a piston engine directly (like a hammer).

Are there any reasons why this configuration could not be used for pulping wood for instance?

The concussive action would be similar to flail-chipper if properly designed.

[lyner]

You could have a point. Every application seems to use rotary engines these days.
btw, I was looking at a huge beam engine the other day - linear in every way!