Direct answer: when a pneumatic rock-drill air leg will not extend, slips under drilling load, leaks continuously, moves in a jerky way, or cannot maintain feed force, do not replace the leg immediately. First separate the fault into three groups: air supply and hose problems, setup/alignment problems, or an internal air-leg problem such as leakage, control restriction, seal wear, or mechanical damage. The fastest diagnosis starts upstream, checks the leg unloaded and then under controlled load, and only then moves to internal service.
This guide is for underground mines, quarry crews, tunneling contractors, distributors, and maintenance teams using air-leg pneumatic rock drills. It focuses on the pusher leg itself rather than percussion-system troubleshooting.
1. What the Air Leg Actually Does
A pneumatic air leg is a pusher cylinder that supports the rock drill and applies controlled thrust toward the face. It does not create the drill’s percussion energy. That distinction matters because low penetration can come from the compressor, hose, rock drill, drill steel, bit, flushing, operating settings, or the air leg.
The current PerfoMax product page presents FT160A, FT160B, FT160C and FT170 as available configurations. FT160BC, FT160BD, FT160S or any other suffix found in historical or external documents requires document-based verification and is not currently presented as an available configuration. Confirm the exact model, manufacturer, pressure range, mounting interface, travel and approved rock-drill pairing in the controlled product documents and written quotation; no interchangeability is implied.
2. Symptom-to-Cause Troubleshooting Table
| Symptom | First checks | What it may indicate | When to stop and service |
|---|---|---|---|
| Will not extend | Air isolation, hose/coupling, working pressure, control movement | No supply, restriction, blocked control, stuck piston, severe internal leakage | Supply is confirmed but the leg still will not move, or movement is unsafe/abnormal |
| Extends very slowly | Pressure drop, hose restriction, couplings, contamination | Insufficient delivered air, restricted flow, internal bypass, sticking | Speed remains abnormal after supply and external restrictions are corrected |
| Extends unloaded but slips or collapses while drilling | Dynamic pressure at the tool, leaks, footing, angle, feed demand | Pressure loss, internal leakage, unstable setup, excessive demand | The leg cannot safely hold controlled thrust at verified supply conditions |
| Continuous air hiss | Hose, fittings, connections, control area, cylinder/rod area | External leak or internal seal/control leakage | Leak location points inside the leg or cannot be safely isolated externally |
| Jerky or sticky travel | Air quality, moisture, contamination, alignment, visible damage | Contamination, corrosion, damaged sliding surfaces, seal drag, misalignment | Rod/cylinder damage, binding, or unpredictable movement is present |
| Leg feels normal but drilling is slow | Percussion, drill steel, bit, flushing, compressor capacity | The air leg may not be the root cause | Diagnose the rock drill and drill string instead of repeatedly adjusting the leg |
3. Check the Air Supply Before Blaming the Air Leg
Pneumatic cylinder force and speed depend on the air actually delivered during operation. A compressor gauge can look acceptable while pressure at the drill falls because of long hoses, undersized passages, restrictive couplings, leakage, water, or simultaneous demand from other equipment.
Use this upstream-first sequence:
- Confirm the exact air-leg and rock-drill model. Record the leg suffix, drill model, hose arrangement, and any recent component changes.
- Inspect the hose route. Look for kinks, crushed sections, loose couplings, damaged fittings, and obvious external leakage.
- Check air quality. Excessive water and contamination can affect valves, seals, lubrication, and smooth actuator movement.
- Measure working pressure close to the tool under actual flow. Do not rely only on static pressure at the compressor receiver.
- Compare unloaded and loaded behavior. A leg that extends normally with no drilling load but loses thrust under load often points to delivered-air loss, leakage, or excessive demand rather than a fully seized cylinder.
For a PerfoMax FT160-series unit, compare the observed working condition with the exact product/model documentation. Do not apply one pressure number to a different pusher-leg design simply because the cylinder looks similar.
4. Won’t Extend: Diagnostic Sequence
If the leg does not extend at all, the main question is whether air reaches the leg and whether the control path can pass that air.
- Isolate and depressurize the system before touching or disconnecting hoses.
- Verify that the correct hose is connected and that the supply path is open.
- Inspect couplings and hoses for obstruction, collapse, or damage.
- Restore air according to the site procedure and verify delivered working pressure.
- Operate the leg control through its normal range without forcing it.
- If supply is present but there is no movement, remove the leg from service for control, piston, cylinder, or seal inspection using the correct model procedure.
Do not bypass a control or improvise a pressure connection to “prove” the cylinder can move. Unexpected extension can create a crush hazard.
5. Extends Unloaded but Slips Under Drilling Load
This symptom is especially useful because it shows that the piston can move, but the system cannot maintain stable thrust. Work through these possibilities in order:
- Dynamic pressure loss: the leg has enough air to extend freely but not enough delivered pressure under combined drill demand.
- External leakage: a hose, fitting, or connection leaks more severely as demand rises.
- Internal leakage: worn or damaged sealing surfaces allow air to bypass the piston instead of maintaining force.
- Unstable footing: the leg foot moves, sinks, or skids and feels like pneumatic “slip.”
- Poor angle or alignment: side loading can make extension feel sticky and can reduce stable support.
- Excessive feed demand: using the air leg to compensate for poor percussion, a dull bit, or another drilling problem can overload the support system.
6. Air Leak: External Connection or Internal Seal?
A continuous hiss does not automatically prove a failed piston seal. First localize the leak with the site’s approved leak-detection procedure and PPE. Check the hose, quick connection, threaded fitting, and control area before opening the cylinder.
If external connections are sound but the leg cannot hold thrust and leakage appears to be internal, seal wear becomes a stronger suspect. Pneumatic-cylinder manufacturers commonly treat piston seals as service components, and insufficient piston sealing is a recognized cause of internal leakage. The correct repair still depends on the exact air-leg model and parts specification.
Do not mix seals, pistons, rods, or control parts between FT160 suffixes without verified compatibility. Similar external dimensions do not prove interchangeability.
7. Jerky, Sticky, or Uneven Extension
Jerky movement usually requires a broader check than “add more oil.” Moisture and contamination in compressed-air systems can damage sealing surfaces, promote corrosion, disturb lubrication, restrict valves, and make actuator response unstable. Misalignment or visible damage can create a mechanical bind even when the air system is healthy.
Check:
- water and contamination in the air line;
- lubrication condition required by the drill/leg system;
- visible rod, cylinder, foot, or mounting damage;
- side loading caused by poor setup angle;
- whether the symptom changes unloaded versus under load;
- whether the problem began after a hose, control, seal, or drill-model change.
If travel is unpredictable, the leg binds, or structural damage is visible, remove it from service rather than forcing movement.
8. Air Leg or Rock Drill? Separate the Two Faults
A healthy air leg can hold the drill correctly while penetration is still poor. If the leg extends smoothly, holds controlled thrust, and shows no abnormal leakage, move the diagnosis to the drilling system:
| Observation | More likely next diagnostic area |
|---|---|
| Stable leg, weak percussion sound or hard starting | Rock drill, lubrication, compressor delivery |
| Stable leg, slow drilling with worn bit | Bit condition and rock match |
| Stable leg, poor cuttings removal | Flushing/water system |
| Stable leg, rod bends or collaring is unstable | Feed setting, alignment, rod condition, operating technique |
| Leg loses position or thrust while drill otherwise runs normally | Air leg, supply, footing, seals, controls |
For model-specific drill troubleshooting, see the related YT28 rock drill troubleshooting guide.
9. FT160 Suffix Matching Matters During Repair
PerfoMax's current product page lists FT160A, FT160B, FT160C and FT170 and treats exact model matching as a compatibility requirement. FT160BC, FT160BD, FT160S or any other suffix found in historical or external documents requires verification against the intended rock-drill model; the FT160 family name alone does not establish a pairing or interchangeability.
That affects troubleshooting because a “replacement FT160” that does not match the intended suffix can create a second problem after the first one is repaired. Before ordering a complete leg or internal parts, send the exact suffix and paired rock-drill model.
10. Eight-Step Field Troubleshooting Workflow
- Identify: record air-leg suffix, rock-drill model, hose configuration, and recent changes.
- Make safe: isolate and depressurize before inspection or disconnection.
- Inspect externally: hose, couplings, fittings, mounting, foot, rod/cylinder surface, and visible damage.
- Check air quality: drain/condition the system according to site procedure and investigate unusual water or contamination.
- Verify delivered air: check working pressure near the tool under operating flow.
- Test unloaded movement: confirm smooth extension and control response without forcing the leg.
- Test controlled loaded support: determine whether thrust is stable or collapses as drilling demand rises.
- Localize and service: if the supply/setup is correct but leakage, sticking, or force loss remains, remove the leg from service and inspect it with the correct model procedure and parts.
11. Common Troubleshooting Mistakes
- Replacing the air leg before checking pressure at the tool under flow.
- Using compressor receiver pressure as proof that the leg receives adequate air.
- Increasing feed aggressively to compensate for a dull bit or weak percussion.
- Calling every leg “FT160” and ignoring the suffix.
- Treating a slipping foot or poor drilling angle as an internal seal failure.
- Continuing to operate with contaminated or water-laden compressed air.
- Installing unverified seals or internal parts because they appear dimensionally similar.
- Disconnecting or opening a pressurized leg.
12. Pre-Shift Air-Leg Checklist
- Confirm hose and coupling condition.
- Check for obvious external leakage.
- Inspect the leg foot, mounting, rod/cylinder surface, and visible damage.
- Confirm smooth control and extension response.
- Check that the paired rock drill and air-leg suffix are correct.
- Confirm the compressed-air system is drained/conditioned and lubricated according to the equipment procedure.
- Stop if the leg binds, collapses unexpectedly, or shows unsafe structural damage.
13. What to Send in a Service or Replacement RFQ
To shorten diagnosis and avoid an incorrect replacement, send:
- rock-drill make and model;
- exact air-leg model and suffix;
- clear nameplate and full-leg photos;
- whether the problem occurs unloaded, under drilling load, or both;
- working pressure measured near the tool during operation;
- hose length, internal diameter if known, and coupling arrangement;
- observed leak location or sound;
- air quality/moisture history and recent maintenance;
- photos of damaged seals, rod, piston, control parts, or fittings after safe disassembly;
- quantity and whether you need a complete leg or verified spare parts.
For current supply options, review the PerfoMax FT160-Series Pneumatic Air Legs, browse Rock Drills, or send an RFQ for model and suffix review.
FAQ
Why will my pneumatic air leg not extend?
Start with the supply path: isolation, hose, coupling, working pressure, and control movement. If delivered air is confirmed but the leg still does not move, investigate control restriction, piston sticking, internal leakage, or mechanical damage using the correct service procedure.
Why does the air leg extend normally but slip while drilling?
This often points to a problem that appears only under load: dynamic pressure loss, external or internal leakage, unstable footing, poor alignment, or excessive feed demand. Measure conditions under drilling flow rather than diagnosing from an unloaded test alone.
Does a constant air leak always mean the piston seal is bad?
No. A hose, fitting, coupling, control component, or another external joint may be leaking. Localize the leak first. Internal seal leakage becomes more likely when external joints are sound and the leg cannot maintain thrust.
Can FT160A, FT160B, FT160C, FT160BC, and FT160BD be interchanged?
Do not assume interchangeability. The current PerfoMax product page presents FT160A, FT160B, FT160C and FT170 as available configurations; FT160BC, FT160BD and FT160S are not currently presented as available. Confirm the exact nameplate, manufacturer, suffix, dimensions, mounting interface, travel and approved rock-drill pairing from controlled documents before ordering a replacement or internal parts.
How can I tell whether poor drilling is caused by the air leg or the rock drill?
If the leg extends smoothly, maintains controlled thrust, and has no abnormal leakage, investigate percussion, drill steel, bit condition, flushing, lubrication, and compressor delivery. If the leg itself loses position, collapses, leaks, or binds, troubleshoot the pusher system first.