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Posted by: 18. July 2012

Beyond Power and Pressure

 

A waterjet cleaves into 2 ¼-inch titanium at Midwest Precision.
 
Waterjet pumps offer more horsepower and cutting pressure than ever before, but it’s important to avoid getting caught up in any single specification when evaluating this technology. The relationship between cutting speed, power and pressure is more complicated than it might initially appear.
 
Consider this story about Midwest Precision, a fabricating shop in Tulsa, Oklahoma. Generally speaking, higher pressure leads to faster cutting speeds, and that proved to be the case when the shop swapped a 50,000-psi pump for a 90,000-psi model a few years back. However, the shop’s next pump purchase demonstrates that the key phrase here is “generally speaking.” This newest pump provides the same pressure as the old, yet it has enabled faster cutting all the same.
 
The difference is that the new pump’s motor generates 125 hp, more than twice the 60 hp of the old pump’s motor. As a result, the shop can use a larger-diameter nozzle that emits more water—and thus, more of the abrasive garnet that actually removes workpiece material—to provide faster cutting, even at equal pressure. The same principle is at work when shops attempt to boost speed by increasing pressure, as opposed to increasing power like Midwest Precision did. Without additional horsepower, the only way to increase pressure is to use a smaller diameter nozzle. This could partially offset any resulting speed gains because less water and abrasive would reach the workpiece.
 
It follows that it is possible to have a scenario in which a pump with lower pressure actually cuts faster than one with higher pressure. For more on that, click here. It should also be mentioned that not all of the horsepower generated by a hydraulic intensifier pump’s motor makes it to the actual point of cut. Some is diverted for other purposes, such as cooling the systems’ hydraulic oil. Although a full discussion of all the factors that determine waterjet cutting speed is beyond the scope of a blog post, it’s clear that a pump’s raw psi or hp rating shouldn’t be considered in isolation, no matter how impressive those figures may be.
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