Diamond saws often seem to stall easily, they seem to have insufficient torque, why?
The brief answer is: because diamond "cutting" is grinding and not cutting. Speed is more important than torque. Push lightly and maintain speed is the trick. That's the theory anyway ...
The more detailed answer: Too long to list here, but we'll give it anyway :-)
What removes material is the work the cutting tool puts in it. The productivity, or cutting rate, is the measure of material removed per unit of time, i.e. work per unit of time, i.e. power.
In theory, you could calculate the rate of work (power) delivered to the rock by multiplying the blade speed by the force with which the blade rubs against the rock times the energy transfer efficiency.
So much for a nice and simple theory!
In reality, the situation is more complicated than such a simple minded formula.
Firstly, the there is no simple way to evaluate the energy transfer efficiency. That varies wildly with the cutting method, i.e. carbide or diamond, with the mechanical properties of the rock, i.e. hardness, coherence, mineral composition etc.
Secondly (and this is very important) an excavator is not as rigid a machine as one would wish. There is play in the pins and, particularly when offset extensions are used, there is elastic distortion of the excavator members (boom and dipper assemblies) which are not really designed to withstand torque as the prime loading.
To make things worse, the excavator often moves during cutting (for example it stands on rubble). For all of these reasons, the resulting cut is not strictly parallel to the blade. We've estimated that hude proportion of th driving torque on the blade gets wasted on overcoming friction between the blade surface and the surrounding rock.
Thirdly, the saw does not generate power, it utilises the power supplied by the excavator hydraulic system and there are some very practical limitations on the actual make-up of that power.
For example let's take a 25kW pump of a typical 8 tonner. This pump will deliver the ideal 25kW with a combination of 100 L/min and 150 bar.
Now, the flow determines the blade speed and the pressure governs the rubbing force or torque.
If the operator pushes harder on the rock, the blade speed may rapidly drop to half of its value, but to maintain the same power, or productivity, the torque, or pressure, would need to double to compensate. The relief valve in the excavator will not allow that though.
Moreover, there is another complicating factor coming to play: the energy transfer efficiency. This efficiency increases with the speed of blade. It is like sharpening a knife on a grinding wheel: one can push very hard on the blade, but if the wheel does not spin fast enough, nothing much will happen.
Is lowering the pressure to a very low value the answer to great productivity then?
Unfortunately not. In our example, if we loosen the pressure on the blade to require only 50 bar, ie. one third we would need 3x higher flow to keep the power constant. Alas, the excavator pump will not give us 300 L/min. And a good thing too as we would blow the motor to pieces if it did!
The secret is knowing the right pressure-flow combination which gives the highest productivity. This combination vary from one excavator to another and for larger machines is really difficult to maintain manually.
Echidna's Cutting Optimiser is a real solution that offers productivities more than a double of a manual operation.
For more Refer to an article: "speed or torque" in the Knowledge Corner section.