Two different oils, two different jobs
On a borewell rig there are two lubricants that are often confused, and mixing them up is a common cause of early hammer wear.
The compressor oil lives inside an oil-injected screw compressor. It cools the compression, seals the rotor clearances and lubricates the bearings, and it is then separated from the air by the air/oil separator and returned to the sump. The aim is that very little of it leaves the compressor with the air.
The hammer oil is a separate rock-drill oil added on purpose to the air going downhole. A DTH hammer is a reciprocating piston working at high speed inside a cylinder, driven by compressed air. Without a film of oil on the piston, cylinder and valve surfaces, metal-to-metal contact causes scoring, heat and rapid wear.
| Item | Compressor oil | Rock-drill (hammer) oil |
|---|---|---|
| Where it works | Inside the compressor airend | Inside the DTH hammer |
| Purpose | Cooling, sealing, bearing lubrication | Lubricating piston, cylinder, valves |
| How it gets there | Injected and recirculated in a closed circuit | Added to the air line by a line oiler or injection pump |
| Ideal amount leaving with the air | As little as possible | The rate the hammer maker recommends |
| Typical properties sought | Thermal stability, good separation | Strong film, tackiness, ability to cling in wet air |
A compressor showing oil at the hose end is not lubricating the hammer properly. It is losing oil through a faulty separator or scavenge line, and that oil is the wrong product for the hammer.
How DTH hammer lubrication works
Hammer oil is injected into the air stream upstream of the drill string. The air carries the oil as a fine mist or film down the inside of the drill pipe to the hammer, where it coats the working surfaces before leaving with the exhaust through the bit.
Two arrangements are common:
- Line oiler (lubricator): a reservoir in the air line, often at the rig or on the compressor outlet, that meters oil into the air flow. Simple and widely used, but it needs regular filling and its metering must be checked.
- Injection pump: a small positive-displacement pump that injects a set quantity of oil into the line. It gives more consistent dosing, especially when water or foam is also being injected.
The hammer manufacturer specifies the oil type and an oil consumption rate, usually related to air flow. We do not give a universal figure here, because hammer models differ; the right rate comes from the hammer manual for the hammer in use. Too little oil lets the piston wear. Too much wastes oil, can make cuttings sticky and adds cost without benefit.
Signs the hammer is not getting enough oil
- No visible oil film on the hammer piston or inside the casing when it is stripped
- Scoring, polishing marks or heat discolouration on the piston and cylinder
- Falling penetration rate with the same compressor pressure and flow
- Hammer running hot or stalling after a pipe change
- Line oiler reservoir level not dropping over a shift, which means oil is not being fed
A simple field check is to hold a clean board briefly near the exhaust at the top of the drill pipe with the hammer off bottom and air flowing. A light oil film suggests oil is reaching the string. No film means the lubricator or pump needs attention.
Moisture in drilling air and why it matters
Compressed air leaves a screw compressor warm and saturated with water vapour. As it cools in the hose and drill pipe, water condenses. In humid monsoon weather, and with large flows, this can amount to a noticeable volume of water every hour.
Water does three things to a hammer. It washes the oil film off the piston and cylinder, it promotes corrosion when the hammer is stored, and it can carry dirt from the hose into the hammer. This is why rock-drill oils are formulated to cling to metal in wet air, and why hammer lubrication matters even more in wet holes or when water injection is used.
For most borewell work, full drying of the air is not needed, and a refrigerated or desiccant dryer is rarely practical on a portable rig. What helps is good aftercooling and moisture separation on the compressor where fitted, draining water from separators and low points, and keeping the oil feed reliable. Hoses should be blown through before connecting to the drill pipe so loose dirt and water do not go straight into the hammer.
How compressor air quality affects hammer life
The compressor’s condition shows up in the hammer. Three faults matter most.
- Oil carryover: a worn or damaged air/oil separator, a blocked scavenge line or overfilled sump sends compressor oil downstream. The hammer is then lubricated by the wrong oil, the compressor loses oil quickly, and in severe cases separator damage can lead to high discharge temperature trips.
- Dirt and debris: a damaged intake filter allows dust into the airend, which wears rotors and contaminates the oil. Dirt in hoses or the drill string goes straight to the hammer and scores the piston. Dusty Deccan sites make intake filter care especially important.
- Heat: a compressor running near its high discharge temperature limit because of blocked coolers or low oil delivers hotter air, thins the hammer oil film and accelerates oil breakdown. A normal oil-injected screw discharge temperature is about 75–95 °C.
Keeping the compressor healthy, with clean coolers, a good separator, the correct oil level and a sound intake filter, is part of looking after the hammer.
Water and foam injection: effect on lubrication
Water or foam is often injected into the air for dust control or hole cleaning. This changes the lubrication picture. Water reaching the hammer strips the oil film faster, so the hammer maker may recommend a higher oil rate or a specific oil when injecting. The injection point and sequence also matter: oil should be added in a way that still coats the hammer, and foam chemicals should be compatible with the hammer oil. These choices should follow the hammer and foam suppliers’ guidance rather than site improvisation.
A practical lubrication and air-quality checklist
- Use the rock-drill oil grade the hammer manual specifies, not compressor oil or used engine oil
- Fill the line oiler at every shift and confirm the level falls as expected
- Check the injection rate when hammer, flow or injection practice changes
- Drain water from separators, receivers and hose low points
- Blow the hose and pipe clear before connecting, and cap pipe ends when stored
- Watch compressor oil consumption; rising top-ups suggest carryover
- Keep the compressor intake filter, coolers and separator in good condition
- Strip, clean and oil the hammer before storage
How Reddy Compressors helps
For borewell rig owners we supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts.
As an independent multi-brand specialist, we sell, hire and service borewell compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, with service teams covering Telangana and Andhra Pradesh from our Tarnaka, Secunderabad office and supply across India.
When a contractor reports short hammer life or oil at the hose end, we check the compressor side: separator condition, scavenge line, oil level and consumption, intake filter, cooler condition and discharge temperature. We can review how the line oiler or injection pump is set up in relation to the compressor and air line, and supply compressor separators, filters, oils and other spares identified from your nameplate. Hammer selection and hammer-specific oil rates remain with the hammer manufacturer’s manual. On our hire jobs, compressors are serviced every 300 running hours, including oil change.
What to send us
- Compressor make, model, nameplate photo and running hours
- How much compressor oil you are topping up per shift or per day
- Hammer make and size, and the oil you are using in the line oiler
- Whether you inject water or foam, and where
- Photos of worn hammer parts or oil at the hose end, if available
Frequently asked questions
Does a DTH hammer need oil?
Yes. A DTH hammer is a fast-moving piston inside a cylinder, and it needs a continuous film of rock-drill oil to prevent scoring and wear. The oil is added to the air line by a line oiler or injection pump at the rig and carried down the drill pipe. The type of oil and the feed rate should follow the manual for the specific hammer being used.
Can I use compressor oil or engine oil in the line oiler?
It is not recommended. Rock-drill oils are formulated to form a strong, tacky film that clings to metal in wet, high-velocity air, which compressor oil and used engine oil are not designed to do. Used engine oil can also carry dirt and acids into the hammer. Use the rock-drill oil grade specified in the hammer manufacturer’s manual for best hammer life.
Why is oil coming out of my compressor with the air?
Oil at the hose end usually means oil carryover from the compressor, often caused by a worn or damaged air/oil separator, a blocked scavenge line, an overfilled sump or running at unusually low pressure. It wastes compressor oil and sends the wrong oil to the hammer. Check oil consumption and have the separator and scavenge system inspected before the problem becomes more serious.
How do I know if my line oiler is working?
Watch the reservoir level: if it does not fall over a shift, oil is not being fed. With air flowing and the hammer off bottom, a clean board held briefly near the exhaust should show a light oil film. When the hammer is stripped, the piston and cylinder should show an oil film rather than dry, polished or scored surfaces.
Does moisture in compressed air damage a DTH hammer?
Water condensing from the compressed air can wash the oil film off the piston and cylinder, promote corrosion during storage and carry dirt into the hammer. It does not usually require a dryer on a portable rig, but good aftercooling, draining separators and hose low points, a reliable oil feed and storing the hammer cleaned and oiled all help protect it.
Do I need more hammer oil when drilling with water or foam?
Often yes. Water or foam reaching the hammer strips the oil film more quickly, so hammer makers may recommend a higher oil rate or a specific oil when injecting. The injection arrangement should also allow the oil to still reach and coat the hammer. Follow the hammer and foam suppliers’ guidance rather than adjusting rates by trial and error on site.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.