Short version: For most individual CNC machine sumps, a belt skimmer sized to the sump depth is the right answer — and the same design scales up: hanging belt skimmers reach 120" into deep tanks and pits, and 4" and 8" wide belts handle heavy industrial oil loads. Use a tube skimmer when the sump is shallow or the only access is a small opening. Use a disk skimmer when you need higher removal capacity on a larger, open tank with a stable level. Add a coalescer when you want to polish emulsified or fine tramp oil out of circulating coolant rather than just skim the surface. If you run a central coolant system for many machines, you are shopping for a recycling/filtration plant, not a skimmer — see "Central systems" below.

Why tramp oil matters

Tramp oil (way lube, hydraulic oil, spindle oil) floats on water-based coolant, starves it of oxygen, feeds bacteria, causes odor and dermatitis complaints, and shortens coolant life. Removing it continuously is the cheapest coolant-maintenance step there is. Skimmers remove floating oil; they do not fix concentration, pH or biological problems — those need a mixer/proportioner and testing (see Coolant Academy).

The four families and when each fits

Belt skimmers

A continuous belt (poly or stainless) dips into the sump, oil clings to it, wipers scrape it into a discharge. Fits: individual CNC sumps, parts washers, most tanks with at least a few inches of clearance above the fluid. Sizing: by reach (distance from the mounting point to the lowest expected fluid level — 8", 12", 18", 24" and on up to 120"; deep tanks and pits use a hanging pulley assembly) and belt width (1" for typical sumps, 2" for heavier oil loads, 4" and 8" for heavy industrial tanks). Watch for: fluid level swings (choose reach for the lowest level), chip conveyors in the belt path, and belt material (stainless for hot or aggressive fluids). Zebra belt skimmers are made in Solon, Ohio. Free lifetime warranty (product registration required).

Tube skimmers

A continuous loop of tubing runs down into the fluid and back up through a wiper; oil clings to the tube as it passes through the oil layer. There are two designs. Zebra's Sidewinder® (ZVA8) uses a weighted tube: the weight pulls the loop down through the surface into the sump, so the tube runs submerged rather than floating — which is what lets it work in shallow sumps and through a small opening in a machine enclosure where no belt or disk will fit. The other design, from some other makers, is a long floating collector tube that snakes across the surface of a large tank or pit. Fits: shallow sumps, tight enclosures, small access ports — and, with Zebra's long-reach tube models, sumps up to 101" deep; large open tanks (floating-tube type). Watch for: coolant with heavy chip load, and matching tube length to the sump depth (Zebra) or surface area (floating type). Zebra tube skimmers: free lifetime warranty (product registration required).

Disk skimmers

A vertical rotating disk lifts oil to a scraper. Fits: larger open tanks with stable fluid level and higher oil volumes. Watch for: level variation (the disk must stay partly submerged) and space for the disk diameter. Zebra's LH/GS4H series: free lifetime warranty (product registration required).

Coalescers and gravity separators

Coolant is pumped through media that merges fine droplets into a floating layer that is then drawn off. Fits: when the oil is finely dispersed, when you want the coolant returned cleaner than surface skimming alone gives, or as a stage in a recycling loop. Often paired with a skimmer on the separator itself.

Sizing checklist

1. Measure reach at the lowest fluid level, not the fill line (up to 120" is standard). 2. Estimate oil load: a visible sheen = light (1" belt); pooling oil daily = heavy (2" belt, disk, or 4"/8" belt on industrial tanks). 3. Check clearance and mounting (magnetic base, bracket, or lid). 4. Fluid temperature and chemistry (stainless belt for hot or synthetic fluids). 5. Power: 115 V is standard; 220 V and air-driven options exist. 6. Decide where the oil goes: a discharge hose to a container or a coalescer.

Central systems (when a skimmer is not the answer)

If one central sump feeds many machines and you need filtration, chip handling and recycling, look at PRAB, Mayfran, Oberlin Filter, Losma or Transor. Those are capital systems; a sump skimmer is a maintenance tool. Many plants run both.

Where Zebra fits — and where we'll tell you to look elsewhere

Zebra builds from the individual machine sump up: belt skimmers from 1" to 8" wide with reaches to 120" (hanging units for deep tanks), weighted-tube skimmers for shallow sumps (long-reach models to 101"), disk skimmers for open tanks — all made in Solon, Ohio; belt, tube and disk skimmers carry a free lifetime warranty (product registration required). Because we also make the mixers, sump level controls and test tools, we can fix the tramp-oil problem and the concentration problem that usually travels with it, from one vendor.

We are not the right answer for everything. If you need a central coolant-recycling plant or a chip conveyor with integrated filtration, say so when you call and we will tell you — and point you to the kind of equipment that does that job.

Next step

Use the product finder on the belt-skimmer page, or send us a photo of the sump with the fluid level marked and we will size it for you — no charge.