Flash Overs

[ban-all-sheds]

I had a fabulous chain hoist once (my Dad got it - dunno where).

It was seriously heavy - almost needed a chain hoist itself to get it mounted.

But as a guide to how much advantage it gave, once in place you only needed to grip the chain between finger and thumb to be able to transmit enough force to lift an "A" Series engine.

 

[SimonH2]

If you really want to get weird in the rotational mechanical domain, how and why does a crankshaft harmonic balancer work?

Oooh lovely question.

Short answer, the movement of the piston/conrod assembly isn't sinusoidal - its close but the geometry means there's some second harmonic present in the accelerations. Work out the crank positions for the piston to be mid stroke, and you'll find that the crank pin is a little above the main journals. So it takes slightly less than half a cycle going over the top, and slightly more than half a cycle going round the bottom.
These second harmonics add up, while in a 4 cylinder engine (with conventional crank layout), the fundamentals cancel out. The result is an engine that bobs up and down at twice the frequency that it's running at.
A dynamic balancer is a balance shaft (or ideally a pair of 2 shafts, contra rotating) running at twice the crank speed, with offset weights phased so as to counteract this second harmonic.

Better still is to have a V8 engine with banks set 90˚ apart. The 2nd harmonics for each pair of pistons resolve to a horizontal acceleration. but the real beauty is that if the crank is phased correctly, all these horizontal accelerations cancel each other out - they try and bend the block, but there's nothing left at the engine mounts. One of the reasons V8 engines are known as smooth engines - I do find 4 pot engines very rough, much prefer the V8

EDIT: A 60˚ V6 also gets some gain - the harmonics resolve to an effect that tries to rock the engine around a vertical axis.

 

[JohnW2]

But as a guide to how much advantage it gave, once in place you only needed to grip the chain between finger and thumb to be able to transmit enough force to lift an "A" Series engine.

Indeed, but if the engine had weighed 7.36 tons, I don't think you'd have got it to rise a foot in 30 seconds, or anything like that!

Kind Regards, John

 

[ban-all-sheds]

Better still is to have a V8 engine with banks set 90˚ apart. The 2nd harmonics for each pair of pistons resolve to a horizontal acceleration. but the real beauty is that if the crank is phased correctly, all these horizontal accelerations cancel each other out

Depends on the design of the crankshaft. Basically you get to choose between one which requires counterbalance weights on the shaft to cancel out vibrations, which makes the engine less responsive, or one which still needs counterbalance shafts but is lighter.

 

[SimonH2]

Depends on the design of the crankshaft. Basically you get to choose between one which requires counterbalance weights on the shaft to cancel out vibrations, which makes the engine less responsive, or one which still needs counterbalance shafts but is lighter.

Go and re-read. The second harmonic component cannot be balanced with crank weights.

For a 4 pot (inline) engine, the second harmonics sum to a net up-down motion (roughly, there are other components) - and this needs a pair of balance shafts running at twice the crank speed to balance. In theory it should be possible to rearrange the crank pin phasing, but that would throw other stuff so far out as to make the engine impractical.
Hence certain manufacturers having had dynamic balance shafts over the years where they've wanted a very smooth 4 pot engine.

The difference with a 90˚ V8 is that it's possible to arrange the crank to keep the second harmonics within the engine block.

EDIT: Ahh, I see what you are getting at now. Within a V8, there's a choice between having a crank without balance weights where the primary forces are balanced (as in a 4 cyl inline) but the 2nd harmonics aren't, or a heavier cross plane crank where the 2nd harmonics are balanced but crank weights are required to balance the primary forces.

 

[ban-all-sheds]

I wonder what this is like:

 

[SimonH2]

I imagine ... verrrrry smoooooth

Incidentally, some years ago I got to go round the Aston Marton factory at Newport Pagnell. Very nice.
Don't know if it's still the same, but then each engine was built by one person - and took a week. Parts were carefully selected - pistons and conrods selected for weight match for example. And when finished, the builder's last task was attach a plate with the date and builder's name.

 

[Porque223]

The answer I got from a guy dead several years now is [was?]:
-Systems have analogies in the electrical, hydraulic, acoustic, pneumatic and mechanical domains. People who design loudspeakers, e.g., need to know these analogies and must be able to write the equations in either or both domains.
-Steel crankshafts and bells have a high Q [much higher than that of a lead crankshaft or a bell made of lead ] which means oscillations can build up and so some of the engine power will go into destroying the engine.
The elec. analogy of a crankshaft is a tuned LC circuit being pulsed and a resistor is used to reduce the Q and damp out oscillations in such a circuit.
The analog of a electrical resistor in a mechanical rotational system is a viscous coupling hooked to a mass. If this coupling gets hot, just like a resistor, it must be damping out unwanted oscillations.

BTW, helicopters have certain speed/altitude areas that people are not supposed to be in due to more and more of the engine power being diverted into destroying the craft.
One widow sued a manufacturer for her husband's wrongful death alleging that the data the company could have obtained from this dangerous experiment was not worth her husband's life.
I don't know if she prevailed.

 

[westie101]

This just circulated at work

A fatal accident has occurred in another Distribution Network Operator (DNO) area. A Jointer received a fatal electric shock while undertaking a metal clad cut-out change. Investigations are currently being carried out by the Health and Safety Executive, the contractor organisation and the DNO to determine what went wrong during this routine jointing activity.

The Jointer had removed the metal clad cut-out and was about to fix the new plastic cut-out when he came into contact with the live conductor, the reason this happened has not yet been established.

No other info availabhle to us