The Second Cost Line at a Removal Station: Power, Compressed Air and Vacuum

When a removal station is procured, it is almost always priced on two numbers: unit price and cycle time. Both appear on the quotation, both are easy to compare, and both get challenged in the review meeting. The spending that actually decides whether the machine still makes sense in year three or year five usually appears on no slide at all. It happens at the electricity meter, the compressor and the vacuum generator, settled monthly and accumulated yearly.

Start with power. The intuitive read is that a removal station is a few servo axes plus a small spindle, so how much can it draw. That holds roughly true while running and fails completely while idle. A typical desktop station may draw only a few hundred watts to just over a kilowatt in operation, but its always-on load includes the industrial PC, the monitor, the safety circuit, enabled servo drives, the PLC and remote I/O, and most easily overlooked of all, the vision lighting. Multi-angle ring lights, coaxial lights and bar backlights are modest individually, yet several of them together routinely exceed the spindle itself. They draw from power-on to end of shift regardless of whether a single screw is being removed.

Vacuum is the second hidden load. Compressed air is not a free utility, it is air bought with electricity. Compressing ambient air to 0.6 MPa carries theoretical limits plus drying, filtration and line losses, and the industry rule of thumb is roughly 0.1 kWh per standard cubic metre. A single-stage ejector left energised continuously consumes tens to over a hundred litres per minute, which across eight hours a day and three hundred days a year is a figure that can be traced on the electricity bill. If that vacuum exists to hold one M0.8 screw, the arithmetic looks particularly poor, because the pickup area means it never needed that flow in the first place.

The three ways of making vacuum must be costed separately. A single-stage ejector is cheapest, fastest to respond and nearly maintenance free, at the cost of continuous air consumption. A multi-stage ejector uses cascaded Laval nozzles to extract more vacuum from the same air, cutting consumption noticeably, with slightly lower ultimate vacuum that is still ample for holding a screw. A vacuum pump, whether a central station or small distributed units, does not consume compressed air but consumes power and maintenance instead, including filters, oil mist separation and ageing lines. The deciding factor is not which is more efficient in isolation but duty cycle. The lower the proportion of the cycle spent holding, the more wasteful a continuously energised ejector becomes; a station that holds almost continuously is often better served by a pump.

More important than selection is leakage. The dominant waste in a compressed air system is never what the machine uses, it is what fittings, quick couplers, rotary unions and ageing hose lose. A leak point nobody can hear may lose several cubic metres a day at 0.6 MPa, and a workshop with dozens of them changes the order of magnitude. Removal stations move constantly and route tubing out to moving parts, so hose fatigue and loosened fittings are normal. That belongs on an inspection checklist rather than a purchase order, but if it is not on the checklist it will never be found.

Air quality is another cost that never shows on the electricity bill. Condensation forming inside cylinders and solenoid valves when dew point is out of spec causes slow, stick-slip motion and freeze-ups in winter. Excess oil content carbonises inside ejector nozzles, so extraction capacity falls gradually and the symptom is simply that the screw no longer holds, while troubleshooting starts with the nozzle and the screw. This degradation is slow, and by the time it is obvious there has usually been a period of intermittent dropped screws treated as random. Specifying air supply conditions in the technical agreement, including pressure tolerance, dew point and filtration, is therefore not a formality; it sets the failure rate for the next two years.

One more saving requires almost no investment: idle management. Many shops leave machines powered overnight, leave the air main open around the clock, and do the same at weekends and over holidays. A simple approach works: zone the air supply, add a solenoid master valve per station interlocked with machine state, and cut air and lighting at night while keeping only the safety and communication circuits live. On a new line this costs nothing to include, and retrofitting an existing line is a few valves and a program change, yet it is the fastest payback in the entire energy budget.

Vision power management deserves its own note. Continuous illumination not only draws power, it holds LED junction temperature high, accelerates lumen depreciation, and after two or three years brightness falls to the point of needing compensation, at which point the resulting imaging instability gets misdiagnosed as algorithm drift. Driving the light as a strobe synchronised to camera exposure cuts consumption to a fraction of continuous operation and extends LED life considerably. On a station with an automatic vision screw removal machine this is normally settled together with light selection, because strobing requires a controller coordinated with camera trigger, and retrofitting means opening the electrical cabinet.

Laid out this way, energy and air are part of the station design rather than an afterthought for production. The end effector's vacuum design sets the air consumption baseline, the light drive method sets the always-on load, and the choice of IPC and drives sets idle draw, and all three are locked in at proposal stage by the frame, the cabinet and the cycle time. This is why small screw removal equipment in recent projects increasingly states per-station power draw, air consumption and vacuum level in the technical agreement. The purpose is not to save a little electricity but to turn running cost into a comparable, acceptable parameter instead of a black box discovered three years later.

For buyers, a practical method is to convert quotations into annual running cost before comparing: annual hours times per-station average draw times tariff, plus air consumption times converted air cost, plus annual consumable replacement. After conversion some cheaper units turn out dearer and some apparently expensive designs turn out cheaper. For suppliers, the ones willing to put those numbers into an agreement are usually confident about their own design. Across the M0.8 to M2.0 range, the smaller the screw and the smaller the pickup area, the easier it is to over-engineer vacuum, which is why the M0.8-M2.0 screw removal solution separates air circuit and vacuum configuration by size tier. The point is to match flow to need rather than fit every station with the same oversized arrangement.