Oil Skimmers for Parts Washers: How to Keep Tramp Oil Out of Your Wash Tank
Last updated: November 13, 2026
A parts washer takes oil off your parts. That oil does not disappear. It ends up in the wash tank, and if nobody removes it, it can end up back on the parts. This guide explains in plain English where the oil goes, how oil skimmers and coalescing filters take it out, and how to tell when your washer has an oil problem.
Who this is for: plant managers, maintenance teams and buyers who run or plan a production parts washer.
Quick Answer: What Does an Oil Skimmer Do in a Parts Washer?
An oil skimmer lifts floating oil off the surface of the wash solution in a parts washer tank. Removing that oil keeps it from sticking back onto clean parts and makes the wash solution last longer. Skimmers work best when the tank is calm. For heavy oil or washers that run around the clock, a coalescing filter cleans the solution continuously instead.
Key Observations
- Oil washed off parts collects in the tank. Left alone, it gets redeposited on parts.
- A skimmer only removes oil that floats. Oil that mixes into the water stays in the bath.
- Skimmers work best when the tank is still, such as overnight or at shift change.
- For heavy oil or 24-hour running, Inline recommends an oil coalescing filter.
- Filters remove solids such as metal chips, not oil.
Oil control options at a glance:
| Option | What it removes | Works best when |
|---|---|---|
| Belt oil skimmer | Oil floating on the surface | The tank is calm, such as off-shift |
| Oil coalescing filter | Oil in solution pumped out of the tank, cleaned and returned | Heavy oil loads or 24-hour operation |
| Bag filters and chip basket | Solids such as chips and dirt, not oil | All the time, as part of normal filtration |
| Bath dump and refill | Everything, including oil mixed into the water | When soluble oil builds up in the wash tank |
Dealing with oily parts or a bath that does not last? Inline Cleaning Systems reviews your parts and process and recommends the right oil control. Get a Free Quote or call +1 574-722-3714.
In this guide
- Quick answer
- Where the oil comes from
- What tramp oil is
- Why floating oil is a problem
- Floating oil vs mixed-in oil
- How a belt skimmer works
- Coalescing filters
- Skimmer or coalescer?
- Can filters remove oil?
- Oil and wastewater
- Signs of an oil problem
- Checklist
- FAQ
- Next step
Where Does the Oil in a Parts Washer Come From?
The oil in a parts washer comes from the parts themselves. Manufactured parts arrive covered in oil, grease, dirt and metal chips from processes like machining, grinding and stamping. The washer sprays hot water and detergent onto the parts, the oil comes off, and the used solution drains back into the tank to be filtered and used again.
That reuse is a good thing. Inline Cleaning Systems builds its washers around a closed-loop system that recirculates and filters the solution, which saves water and detergent. But it also means every bit of oil taken off the parts is now in the tank.
What Is Tramp Oil?
Tramp oil is a common name for unwanted oil that gets into a fluid it does not belong in. In a parts washer, it is the oil washed off the parts that collects in the wash tank. Some of it floats on top of the solution, and some of it mixes into the water.
Why Is Floating Oil a Problem in a Parts Washer?
Floating oil is a problem because the washer can pick it up and spray it back onto clean parts. Inline Cleaning Systems says that without oil skimming, oil builds up in the bath and is redeposited onto parts, which undermines cleanliness.
- Dirtier parts: oil can end up back on the parts you just cleaned.
- Shorter bath life: the wash solution has to be dumped and replaced sooner.
- Harder disposal: oily wastewater usually needs treatment before it can be discharged.
Floating Oil vs Mixed-In Oil: Which Can a Skimmer Remove?
A skimmer can only remove oil that floats to the surface. Inline’s belt skimmer removes insoluble oils, meaning oils that do not mix with water and rise to the top of the tank.
Some oils dissolve into the wash solution. These are called soluble oils, and a skimmer cannot pick them up. They build up in the bath over time, and that build-up is what eventually forces a bath dump, where the used solution is drained and replaced with fresh solution.
How often that happens depends on your parts and your quality requirements. A washer cleaning parts with a lot of soluble oil, such as parts coming off a stamping press, may need fresh water every day. Other baths last 3 to 6 months or more. The right interval is the one your production results support.
| Type of oil | What it does in the tank | How it is removed |
|---|---|---|
| Floating (insoluble) oil | Rises to the surface | Oil skimmer or coalescing filter |
| Mixed-in (soluble) oil | Stays mixed in the solution and builds up | Periodic bath dump and refill |
How Does a Belt Oil Skimmer Work?
A belt oil skimmer works by running a belt down into the wash tank and back up again. Oil floating on the surface sticks to the belt, the belt carries it up out of the tank, and the oil is scraped off into a container. The cleaner solution stays in the tank.
The key thing to know is timing. Floating oil needs calm water to rise and settle on the surface. While the washer is running, the pump keeps the solution moving, so the oil stays mixed around. That is why Inline says its belt-type skimmer works best during an off-shift, when oil can settle to the surface. Shift changes and overnight are good times to skim.
Disc skimmers and belt skimmers are the two most common types that reach into a wash sump. Inline used to fit disc skimmers, but it has now standardized on belt skimmers.
What Is an Oil Coalescing Filter?
An oil coalescing filter is a separate unit that pulls solution out of the washer tank, separates the oil from it, and sends the cleaned solution back to the tank. It runs continuously, so it does not have to wait for the tank to go calm.
Inline Cleaning Systems recommends an oil coalescing filter for washers with heavy oil contamination or 24-hour operation. Coalescing systems are needed when a washer runs continuously or removes a high volume of oil.
Belt Skimmer or Coalescing Filter: Which Do You Need?
Choose based on how much oil comes off your parts and how many hours the washer runs.
| Question | Belt skimmer | Oil coalescing filter |
|---|---|---|
| How it works | A belt lifts floating oil off the surface | Solution is pumped out, cleaned and returned |
| When it works best | When the tank is calm, such as off-shift | All the time, while the washer runs |
| Best for | Moderate oil and washers with downtime between shifts | Heavy oil loads and 24-hour operation |
A simple rule of thumb from Inline’s engineers: if the belt skimmer runs continuously and oil still builds up on the tank surface, the skimmer is not enough, and it is time to look at a coalescing filter.
If you are not sure, Inline can recommend skimming, filtration and oil-water separation equipment based on your parts and process.
Can Filters Remove Oil From a Parts Washer?
No. Filtration removes solids such as metal chips and dirt, not oil. On an Inline washer, a removable chip basket catches larger debris before it reaches the tank, and bag filters catch fine particles before the solution is sprayed onto parts. Oil needs a skimmer, a coalescing filter or a bath dump.
How Does Oil Removal Help With Wastewater?
Oil removal helps because oily wash water is harder to get rid of. Used wash and rinse solution is dumped and replaced from time to time, and the disposal options depend on local rules and how dirty the bath is.
- Many water-based detergents can go to the sanitary sewer where local codes allow it.
- Solution with heavy oil usually needs pre-treatment, such as skimming, coalescing or filtration, before it is discharged.
- Where on-site discharge is not allowed, a licensed waste handler collects the used solution.
Good oil control extends bath life, which means fewer dumps and less waste to dispose of.
What Are the Signs of an Oil Problem in a Parts Washer?
The most common sign is oil on parts that should be clean. This table shows what to check, based on Inline’s troubleshooting guidance.
| What you see | Possible cause | What to check |
|---|---|---|
| Oil on cleaned parts | Floating oil not being removed | Is the skimmer running? Is it running when the water is calm? |
| Oil on cleaned parts | Too much soluble oil in the bath | Time since the last bath dump |
| Oil on cleaned parts | Detergent or temperature not right for the oil | Detergent concentration, detergent type, wash temperature setting |
| A layer of oil on the tank surface | More oil coming in than the skimmer can handle | Whether a coalescing filter is needed |
Temperature matters too. Most water-based washers run between 140 F and 160 F (60 C to 70 C). For heavy oils, Inline often runs the wash solution hotter and adds oil skimming so removed oil does not build up in the tank.
Simple Oil Control Checklist
- Run the skimmer when the tank is calm, such as at shift change or overnight.
- Look at the tank surface. A visible oil layer means oil is coming in faster than it is removed.
- Keep the chip basket and bag filters clean so solids do not build up.
- Check detergent concentration and pH on schedule.
- Log every bath dump so you know how long your bath really lasts.
- If oil is heavy or the washer runs 24 hours, ask about an oil coalescing filter.
Oil Skimmer FAQ
An oil skimmer lifts floating oil off the surface of the tank and works best when the tank is calm. An oil coalescing filter pumps solution out of the tank, cleans it and returns it, so it can run continuously. Inline recommends coalescing filters for heavy oil or 24-hour operation.
While the washer runs, the pump keeps the solution moving, so oil does not get a chance to rise to the surface. Skimmers work best when the solution is still, such as during shift changes or overnight.
No. A skimmer only removes oil that floats. Oil that mixes into the water (soluble oil) stays in the bath and builds up until the solution is dumped and replaced.
It depends on the oil coming off your parts. A washer with a lot of soluble oil, for example from a stamping press, may need fresh water every day, while other baths last 3 to 6 months or more. Set the interval from your actual production results.
Yes. Higher temperatures can help dissolve stubborn oils, though they use more energy and make more steam. For heavy oils, Inline often runs hotter wash solution and adds oil skimming.
In many cases, an existing washer can be upgraded. Inline’s engineering team can assess your machine and recommend the most cost-effective path, whether that is an upgrade or a replacement.
Next Step: Get Oil Control Sized for Your Parts
The right oil control depends on your parts, the oil on them and how many hours your line runs. Send Inline Cleaning Systems your part details, the type of oil or coolant on them and your daily running hours. The team will recommend a washer with the right skimming and filtration.
Get a Free Quote, call +1 574-722-3714, or see the oil skimmer and coalescing filter options on the parts washer features page. You can also read about Inline’s heated aqueous parts washers.
Related reading: aqueous parts washer guide, parts washer maintenance and air knife drying.