machining
How to measure vacuum pump performance How to relate pump performance to holddown force when choosing a vacuum pump for a CNC router table.
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Operating Vacuum
More vacuum Graph 1 shows the vacuum pump’s pumping flow curve superimposed on the router table’s leakage flow curve. The crossing point of the two curves will determine the actual operating vacuum (holddown force) of the system.
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e would all like to maximize the holddown force on our CNC router tables, and traditional thinking for wood machining has always held that the larger the size of the vacuum pump — motor horsepower — the better the holddown force. For the majority of motor-driven equipment used in the industry, such as router motors, performance can be directly related and compared to horsepower. The end vacuum, or the blank off pressure that a pump attains, has also been thought to directly influence holddown. It has become commonplace to specify vacuum pumps achieving end vacuum levels of 29”Hgv in the belief that they provide some superior performance advantage. Unfortunately, both nameplate HP and end vacuum capability have little influence on holddown force. With the expanding use of vacuum in the wood industry, the vacuum pump function deserves closer examination in order to optimize the parts-holding capability of the router table.
Holddown force All vacuum pumps, regardless of design, have a performance curve, which is generally measured in actual CFM (cubic feet/minute) versus vacuum level. Actual CFM (ACFM) is frequently confused with displacement CFM, the latter being a theoretical 60
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value only and therefore not suitable for relevant comparison. Displacement CFM is calculated without the pump running, and as such does not take into account the volumetric losses caused by internal leakage, friction, etc. These losses occur the moment the pump is turned on, and so the actual CFM a pump delivers can be 10 to 25 percent less than the displacement value, depending on the pump type.
The true measure of performance...is determined solely by the amount of delivered capacity. An ACFM pump performance curve will provide a true indication of the performance of a vacuum pump. The curve will show that as the vacuum level of the pump increases, its capacity (CFM) decreases, to a point at the end vacuum or blank off pressure of the pump (i.e. maximum vacuum) where capacity reaches zero. (Note: Always verify the stated accuracy of
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Losses due to volumetric efficiency will vary among pump types, resulting in a range of efficiency levels. For example, it has been well established that a 25 hp rotary vane vacuum pump will provide comparable holddown force to either a 40 hp rotary screw or 40 hp oil sealed liquid ring pump, but at an obviously higher efficiency level.
For a two-shift, five-day-per-week operation, this 15 hp difference can easily amount to $3,500 per year depending on actual electric rates.
End vacuum The end vacuum or blank off pressure of a vacuum pump plays no role continued
Operating vacuum
More vacuum When comparing the two pumps, this illustrates the fact that the ultimate vacuum (no flow vacuum level) doesn’t determine which of the two pumps will create the best operating vacuum on the table. Because of its different curve, Pump 2 has a lower ultimate vacuum, but higher actual operating vacuum.
the pump performance curve with the manufacturer as advertised capacities usually are stated with a certain percentage range.) Spoilboard router tables can also be measured to derive a performance curve, with plotted values for the leak rate (again in ACFM) versus vacuum level. However, unlike a vacuum pump, as the vacuum level increases on a spoilboard table, so does the leak rate (ACFM) through the spoilboard. Therefore, the holddown force any given vacuum pump can provide is equal to the crossover point of both the pump and table curves as shown on page 60.
Pump efficiency It is important to note that different types of vacuum pumps commonly used on CNC routers have varying levels of efficiency in order to provide the same level of holddown force. For any vacuum pump, efficiency is defined as the ratio of CFM/HP at the required operating pressure (or the crossover point). This is why dissimilar styles of vacuum pumps should never be compared by HP alone as performance and efficiency is directly related to actual CFM, not connected HP.
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vacuum pump performance
in determining holddown force. Also referred to as “maximum vacuum,” it is reached only when a zero flow rate condition occurs. On a spoilboard router, this only happens when the control valve to the table is closed (or de-energized), and when no machining is being done. Because the pump ACFM is a criti-
cal function, it is easy to see that on most spoilboard applications, a highefficiency vacuum pump that has an end vacuum of only 23”Hgv will actually achieve a greater holddown force than a comparable low-efficiency vacuum pump that has an end vacuum of 29”Hgv. Re-examination of the pump/table crossover point will help explain this. With its flatter performance curve, the high-efficiency pump crossover point moves up on the router table curve. The shaded area below the router table curve (which also represents a pump’s end vacuum capability) is then irrelevant to holddown force. As the vacuum level increases on a spoilboard router table, so does the actual CFM flow rate. Therefore, the only way to move up the crossover point is to use a vacuum pump that delivers a higher actual CFM flow rate. One additional point to consider
Pump types
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here are a variety of vacuum pump types in use on CNC routers, providing a range of performance options, efficiency levels and cost of ownership
Blowers The two main types of blowers used on CNC routers are rotary lobe and regenerative. These lowpressure units have the benefit of operating without oil in the pumping chamber (in other words, run dry), using noncontacting, and nonwearing internal components. While blowers offer high efficiency and low cost of ownership, they do not safely operate at vacuum levels deeper than 12-13”Hgv, limiting them to large piece holddown, or those parts having minimal porosity.
Oil sealed and oil lubricated There are three main types of oilsealed or oil-lubricated vacuum pumps used on CNC routers, all of which will produce an end vacuum of 29”Hgv. These include oil-sealed liquid ring, and oil-lubricated rotary
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when undertaking a comparison of vacuum pumps is performance data. In the vacuum industry, there are no required national standards (nor common third-party testing agencies) for measuring and/or publishing pump performance curves. This can result in one manufacturer using a different basis, tolerance or standard than another, so two vacuum pumps having very similar performance at a particular operating point may be advertised in a completely different manner. It is always advisable to inquire with the pump manufacturer about the background and traceability of their data.
efficient pump will provide a deeper vacuum based on a fixed leak rate. Most manufacturers should be agreeable to loan a pump for this purpose. It is important to understand that the true measure of performance of any vacuum pump, as represented by its holddown force capability, is determined solely by the amount of deliv-
Information provided by Troy Bridges, Busch Inc. For more information access www.buschpump.com.
ered capacity at the operating pressure. Motor HP, end vacuum capability and displacement CFM are all simply irrelevant. â–˛
Field testing data In lieu of obtaining traceable data, field testing of two or more different vacuum pump types, running on the same setup (a typical setup is best) will provide concrete proof of pump performance, and show that the most screw and rotary vane units. With all oil-sealed or oil-lubricated vacuum pumps, depending on the brand and model, oily/smoky discharges can be present. In addition, wood dust carryover tends to foul the oil, leading to higher-than-normal maintenance costs. Of the three, rotary vane pumps offer the highest efficiency and lowest cost of ownership, followed by rotary screw and oil-sealed liquid ring.
Dry running There are two types of dry-running vacuum pumps used on CNC routers: dry rotary vane and dry rotary claw, both providing 100 percent oil-less compression. Dry-vane pumps have the lowest efficiency level and highest cost of ownership due to their wearing vanes, while rotary claw pumps have the highest efficiency level and lowest cost of ownership (including oil-sealed or oil-lubricated designs) due to their zero-friction pumping chamber. In addition, variable speed control can be used on rotary claw pumps to raise efficiency levels, reliability, and provide precise vacuum control.
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