Tips & Tricks
Direct Drive Efficiency: Doing More with Less
The muscle behind waterjet technology is a high-pressure pump that converts electric motor horsepower to nozzle hydraulic horsepower. The more nozzle horsepower, the faster you'll cut. However, with two different pump types used in waterjet cutting systems - hydraulic intensifier pumps and crankshaft-driven direct drive pumps - how do you know which type is actually more effective? Here are a few key differences.
Hydraulic intensifier pumps, invented in the 1970s, are complex systems, consisting of an electric motor, hydraulic pump, oil reservoir, manifold, and piston biscuit/plunger. Direct drive crankshaft pumps developed in the 1990s, on the other hand, are far simpler, consisting of the same electric motor directly powering crankshaft-driven plungers that generate the high-pressure water. This is important because direct drive pumps do not suffer from the significant hydraulic losses of an intensifier pump, and are therefore inherently more efficient in converting motor power to nozzle power. With direct drive pumps, a higher percentage of the electrical power that drives the pump actually gets to the cutting nozzle, while less power is wasted as heat.
Let's put some numbers behind this. Intensifier pumps running at 90,000 psi, due to the losses from the hydraulic system, are generally under 60 percent efficient, although some of the more advanced ones can achieve efficiencies around 62 percent. The OMAX direct drive pump running at 60,000 psi, which does not suffer from hydraulic loss, is 85 percent efficient, delivering the maximum hydraulic horsepower possible.
To convert that to practical numbers, let's start at the nozzle. With the abrasive flow rate the same and 42.5 horsepower at the nozzle, an intensifier pump and an OMAX pump will cut at the same speed through most common materials and thicknesses, regardless of pressure. To drive 42.5 horsepower to the nozzle, the OMAX pump needs a 50 horsepower electric motor, while the intensifier pump needs 70 horsepower. The difference in pump efficiencies means that you need 20 extra electric motor horsepower with an intensifier pump just to match the output of an OMAX pump.
Pressure and Velocity
The same nozzle horsepower cutting at the same rate makes sense, but why doesn't the higher pressure make a difference? Well, in some cases it does. With hard, thin materials such as some ceramics, 90,000 psi would cut faster. This is because the pressure affects the velocity of the abrasive, and some ultra-hard materials have a cutting threshold that require that higher velocity to pierce and cut effectively. Most materials, including steels and hard nickel alloys, fall below this cutting threshold, so the extra velocity speed 90,000 psi generates has no added benefit in cutting.
Pressure and Orifice Size
Conversely, as materials get thicker, 60,000 psi becomes the better choice due to the larger orifice associated with it. Again, keeping the nozzle horsepower the same at 42.5 horsepower, the OMAX can drive a 0.016” orifice, while an intensifier pump has 0.012” as its maximum orifice. That larger orifice means that the horsepower of the nozzle has the space to penetrate to the bottom of a thicker workpiece. To match the orifice size would require a significantly larger intensifier pump (at 90,000 psi, you would need a 127 horsepower electric motor generating 79 nozzle horsepower on an intensifier pump to be able to run a 0.016” orifice).
Back to our matched 42.5 nozzle horsepower pumps, the OMAX pump has further advantages. Not only is it using less electrical power (needing only 50 input horsepower versus 70 for the intensifier), but its lack of a hydraulic system means it does not need an extra water cooling system for the hydraulics. Also, intensifier pumps running at 90,000 psi mean higher maintenance, as metal fatigue takes place faster at higher pressures.
Looking at the bottom line, when making a comparison where it matters, at the nozzle, and matching nozzle hydraulic horsepower and abrasive flow rate, the OMAX direct drive pump uses less electricity and less water, while also reducing component fatigue. The pump does this while also running quietly and cleanly, making it the ideal solution for shops that want to do more with less.