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 the concentration calculator).
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
- Measure reach at the lowest fluid level, not the fill line (up to 120" is standard).
- Estimate oil load: a visible sheen = light (1" belt); pooling oil daily = heavy (2" belt, disk, or 4"/8" belt on industrial tanks).
- Check clearance and mounting (magnetic base, bracket, or lid).
- Fluid temperature and chemistry (stainless belt for hot or synthetic fluids).
- Power: 115 V is standard; 220 V and air-driven options exist.
- 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
Send two photos, the opening where the skimmer would mount and the coolant surface, through our Skimmer Sizer and we will tell you which family and which reach fits, no charge. Or read the reach page and pick it yourself.
Three questions we get
Belt or tube for a standard CNC sump?
Belt if there is a few inches of clearance above the fluid and a place to mount it. Tube if the sump is shallow, the only access is a small opening, or you want to move it between machines.
What does reach mean?
For a belt, base of the skimmer to the centerline of the tail pulley. For a tube, base of the mounting plate to the bottom of the tube. Measure the sump depth at the mount and take off an inch or two of clearance.
Will a skimmer fix rancid coolant?
It removes the floating oil that starves the coolant of oxygen, which is the usual cause. It does not fix concentration or a tank that has already gone bad; mix at a set ratio and, if it is far gone, dump, clean, and start again with the skimmer running.