A clogged return line strainer is one of the most common causes of coolant flooding on deep hole drilling machines. Chips and debris accumulate in the strainer screen, restricting return flow until coolant backs up at the machine — spilling onto the floor, tripping coolant-level alarms, and starving the pump. Cleaning the return line strainer is a simple, fast procedure that prevents these problems.
Strainer Types
Type Comparison
| Strainer Type | Configuration | Mesh Range | Cleaning Method | Best For |
|---|
| Y-strainer | Y-shaped body — screen in angled leg | 20–100 mesh | Remove screen plug — pull and clean screen | Small lines — individual machines |
| Basket strainer | Large cylindrical screen in housing | 10–60 mesh | Open cover — remove basket — empty and clean | High flow — central return lines |
| Duplex strainer | Two strainer chambers with switching valve | 20–100 mesh | Switch to clean chamber — clean offline | Continuous operation — no shutdown needed |
| Inline strainer | Straight body with cylindrical screen | 30–100 mesh | Remove end cap — pull screen | Tight spaces |
| Magnetic strainer | Magnetic rod in flow path | N/A — captures ferrous fines | Remove magnet — wipe clean | Ferrous chip fines |
Strainer Materials
| Material | Application | Chemical Resistance | Mesh Durability |
|---|
| Stainless steel (304/316) | Most coolants — standard | Excellent | High |
| Monel | Corrosive coolants | Excellent — salt water | High |
| Bronze | Non-corrosive coolants | Good | Moderate |
| Nylon / polypropylene | Low-pressure return lines | Good — water-based coolants | Low — replace frequently |
| Steel (carbon) | Oil-based coolants only | Poor — rusts with water-based | Moderate |
Strainer Sizing
| Line Size (NPT) | Strainer Body Size | Typical Mesh | Flow Capacity |
|---|
| 1/2" | 1/2" Y-strainer | 40–60 mesh | Up to 50 L/min |
| 3/4" | 3/4" Y-strainer | 40–60 mesh | Up to 100 L/min |
| 1" | 1" Y-strainer or basket | 20–40 mesh | Up to 200 L/min |
| 1-1/2" | 1-1/2" Y-strainer or basket | 20–40 mesh | Up to 400 L/min |
| 2" | 2" basket strainer | 20–30 mesh | Up to 800 L/min |
| 3" | 3" basket strainer | 10–20 mesh | Up to 2000 L/min |
Clogging Causes and Symptoms
Clogging Causes
| Cause | Description | Frequency | Prevention |
|---|
| Normal chip accumulation | Chips carried by return coolant collect on screen | Most common | Regular cleaning schedule |
| Large chip breakthrough | Chip breaker malfunction — large chips reach strainer | Common | Maintain chip breaker |
| Chip conveyor failure | Chips not removed from coolant — all go to strainer | Moderate | Maintain chip conveyor |
| Machine cleaning | Chips flushed from machine base during cleaning | Periodic | Clean chip pan before flushing |
| Filter failure | Filter element bypass — debris reaches strainer | Rare | Replace filter on schedule |
Symptoms of a Clogged Strainer
| Symptom | Cause | Confirmation |
|---|
| Coolant backup at machine | Return flow restricted — coolant cannot drain | Visual — coolant level rising in machine base |
| Coolant flooding onto floor | Return line completely blocked | Immediate — spill |
| Low coolant level in tank | Coolant trapped in return line — not returning to tank | Check tank level — compare to normal |
| Coolant level alarm | Tank level sensor detects low level | Alarm on HMI |
| Reduced flow at drill | Pump starves from low tank level | Check flow at drill tip |
| Gurgling sound in return line | Partial blockage — air and coolant fighting through | Listen at return line |
| Pressure gauge at pump drops | Pump inlet pressure low — tank level low | Check gauge |
Cleaning Methods
Method Comparison
| Method | Procedure | Time Required | Tools Required | Effectiveness |
|---|
| Manual screen removal | Remove screen — clean with brush | 5–15 minutes | Wrench, brush, bucket | Complete |
| Backflushing | Reverse flow through strainer | 2–5 minutes | Valve (if installed) | Partial — loose debris only |
| Basket dumping | Open cover — lift out basket — dump | 5–10 minutes | Wrench | Complete |
| Magnetic cleaning | Remove magnetic rod — wipe clean | 2 minutes | Rag | Complete — ferrous only |
| Offline cleaning (duplex) | Switch to clean chamber — remove dirty screen | 5 minutes | Wrench | Complete — no shutdown |
Manual Screen Cleaning (Y-Strainer)
| Step | Action | Detail |
|---|
| 1 | Position bucket under strainer | Catch coolant that drains when screen is removed |
| 2 | Close isolation valves before and after strainer | If equipped — prevents coolant flow during cleaning |
| 3 | Loosen and remove screen plug (cap) | Use correct wrench size |
| 4 | Remove screen from strainer body | May need gentle prying |
| 5 | Empty screen contents into bucket | — |
| 6 | Clean screen with stiff brush | Remove all embedded chips |
| 7 | Inspect screen for damage | Holes, tears, corrosion |
| 8 | Rinse screen with clean coolant or water | — |
| 9 | Reinsert screen into strainer body | Fully seated |
| 10 | Apply PTFE tape or sealant to plug threads | — |
| 11 | Reinstall and tighten screen plug | Tighten securely |
| 12 | Open isolation valves slowly | Check for leaks at plug |
Basket Strainer Cleaning
| Step | Action | Detail |
|---|
| 1 | Close isolation valves if equipped | — |
| 2 | Loosen cover bolts or T-handles | Do not remove completely |
| 3 | Lift cover — may need to break seal | Gasket may stick |
| 4 | Lift basket out of housing | Handle carefully — full of chips |
| 5 | Empty basket into waste container | — |
| 6 | Clean basket with brush or pressure wash | Stiff brush — remove all debris |
| 7 | Inspect basket for damage | Tears, holes, corrosion |
| 8 | Clean housing sealing surface | Remove old gasket residue |
| 9 | Reinstall basket in housing | Correct orientation |
| 10 | Install new gasket on cover | If gasket is reusable — inspect carefully |
| 11 | Lower cover onto housing | Align bolts |
| 12 | Tighten cover bolts evenly | Cross-pattern — moderate torque |
| 13 | Open valves slowly | Check for leaks at cover |
Inspection Criteria
Screen Condition
| Condition | Acceptable | Action |
|---|
| Screen holes clear | Yes | Reinstall — continue monitoring |
| Screen partially blocked (< 50%) | Clean only | Clean and reinstall |
| Screen heavily blocked (> 50%) | Clean | Clean — investigate cause of heavy loading |
| Screen has holes or tears | No — replace | Coolant bypasses — chips enter tank |
| Screen corroded | No — replace | Will fail — allow chips to pass |
| Screen deformed | No — replace | Will not seal properly |
Gasket and Seal Condition
| Component | Acceptable | Replace If |
|---|
| Cover gasket (basket strainer) | Pliable — no deformation | Hardened, cracked, deformed |
| Screen plug O-ring (Y-strainer) | Pliable — no cuts | Hardened, cut, flattened |
| Cover sealing surface | Smooth — no scratches | Pitted, corroded, deeply scratched |
Preventive Maintenance
| Task | Frequency | Benefit |
|---|
| Check return line for flow | Daily | Detects developing blockage |
| Clean Y-strainer screen | Weekly (heavy chip load) or monthly | Prevents backup |
| Clean basket strainer | Weekly (heavy) or monthly | Prevents backup |
| Inspect screen for damage | Each cleaning | Catches holes before chips pass |
| Replace gasket | Each basket strainer cleaning | Ensures seal |
| Check strainer for corrosion | Annually | Replace if corroded |
| Verify isolation valve operation | Annually | Prevents valve failure |
FAQ
How do I clean a coolant return line Y-strainer?
Position a bucket under the strainer. Close isolation valves if equipped. Remove the screen plug (cap) from the Y-strainer body. Pull out the cylindrical screen. Empty chips and debris from the screen. Clean the screen with a stiff brush — remove all embedded chips. Rinse with clean coolant or water. Inspect the screen for damage — holes, tears, or corrosion. Reinsert the screen fully into the body. Apply PTFE tape to the plug threads. Reinstall and tighten the plug. Open valves slowly and check for leaks.
How often should I clean the return line strainer?
Clean frequency depends on chip load. For machines with heavy chip generation (BTA drilling), clean weekly. For standard gun drilling, clean monthly. Clean immediately if you see any symptom of clogging: coolant backing up at the machine, low coolant level in the tank, or gurgling sounds in the return line. If the strainer clogs between cleanings, increase cleaning frequency or investigate upstream chip removal.
What happens if the return line strainer clogs?
Coolant return flow is restricted. Coolant backs up in the machine base and work area — flooding onto the floor. The coolant level in the tank drops as coolant is trapped in the return line. The pump may starve, causing reduced flow to the drill and potential pump cavitation. In severe cases, the coolant level alarm trips and the machine stops. A clogged return line strainer is one of the most common causes of coolant-related machine stoppages.
What is the difference between a strainer and a filter?
A strainer is a coarse filtration device (typically 10–100 mesh) installed in the return line to protect the pump and tank from large chips and debris. A filter is a finer filtration device (typically 20–200 µm) installed in the pressure line to remove particles that could damage the drill. Strainers capture large chips that would clog filters. Filters capture fine particles that affect hole quality. Both are needed — strainers protect filters, filters protect the drilling process.
What mesh size should I use for a return line strainer?
20–40 mesh (approximately 400–800 micron openings) is typical for deep hole drilling return lines. This captures large chips and debris while allowing coolant and fine particles to pass through to the filters. Use finer mesh (40–60) for smaller machines with less chip load. Use coarser mesh (10–20) for high-flow central return lines. Too fine a mesh causes the strainer to clog rapidly. Too coarse a mesh allows chips to pass through to the tank and pump.
The return line strainer is the first defense against chips entering the coolant tank. A clogged strainer causes coolant backup, flooding, and pump starvation. Clean the strainer on a regular schedule — weekly for heavy chip loads, monthly for standard operation. Inspect the screen for damage at each cleaning. A few minutes of preventive strainer cleaning prevents hours of cleanup from coolant flooding. This article reflects industry practice as of 2026.
