A pneumatic rock drill air leg is a controllable telescoping support cylinder. Compressed air moves its piston and tube to extend the leg, support the drill and apply feed thrust toward the rock face. A feed control meters that movement; on a double-acting design, a separate control path directs air for powered retraction. The leg does not create the drill’s percussion or rotation. It positions the machine and supplies the axial force that keeps the bit working against the rock.
The operating details are model-specific. Control positions, safe working angles, compatible leg models, connection dimensions, pressure limits and maintenance procedures must come from the rock-drill and air-leg manuals. This guide explains the buyer-facing working principle and interfaces; it is not a substitute for trained operation, energy isolation or the manufacturer’s procedure.
What is an air leg on a pneumatic rock drill?
An air leg—also called a pusher leg, feed leg or jackleg support—is the articulated pneumatic cylinder beneath or behind an air-leg rock drill. Its foot bears against a stable support point, while a swivel connection allows the drill body to change angle. When air is admitted to the extension side of the piston, the telescoping section moves outward and pushes the drill toward the face.
This arrangement performs two jobs at once: it carries much of the drill’s weight and produces controllable axial feed. That is why an air-leg machine can be practical for horizontal and inclined holes in underground development, tunnelling and similar work where a handheld sinker drill would require the operator to supply more of the support and forward force.
| Component | Primary role | What buyers must verify |
|---|---|---|
| Outer cylinder | Contains the piston and guides the telescoping assembly | Model, bore family, condition and service parts |
| Piston and seals | Convert air pressure into extension or retraction force | Seal kit, piston design and approved lubricant |
| Telescoping tube | Changes the effective leg length | Stroke, straightness, surface damage and contamination protection |
| Foot or stinger | Transfers load into the floor, ladder or approved support | Foot style, wear, retention and suitability for the support method |
| Swivel or leg connection | Joins the leg to the drill while allowing alignment | Mounting interface, pin/retainer arrangement and compatible drill model |
| Feed control | Meters air to control extension and thrust | Control location, neutral behavior and model-specific operation |
| Retract control | Directs air for powered return on double-acting legs | Whether retraction is powered, control logic and hose/passages |
How extension creates feed thrust
Compressed air acting on the piston creates force along the cylinder axis. The useful force at the drill is not determined by pressure alone: piston area, seal friction, leg angle, support geometry, leakage and mechanical condition all affect the result. For that reason, a pressure figure without the exact leg model and test condition is not a reliable compatibility or performance claim.
As the feed control opens, air enters the extension circuit and the telescoping tube advances. The foot reacts against its support point, so the drill is pushed in the opposite direction—toward the bit and rock face. The swivel lets the machine settle into alignment, but it does not correct a poor setup automatically. The foot, leg, drill steel and intended hole axis still need a stable geometry.
Distinguish the hammer-air control from the leg-feed control in the exact equipment manual. Air reaching the percussion mechanism does not establish that the leg is receiving controlled extension or retraction pressure. This functional separation helps frame a complaint such as “the drill runs but the leg does not feed” without assuming that either component has already passed inspection.
How to calculate theoretical air-leg thrust
For a first-pass comparison, theoretical extension force equals air pressure multiplied by the piston’s effective extension area. This is a cylinder calculation—not a promise of usable drill feed. It helps buyers detect mismatched claims and compare like with like before asking for supplier test data.
Use the air pressure at the leg during the relevant operating state, not the compressor receiver gauge, and use the controlled effective piston diameter, not the outside diameter of the air-leg tube.
| Variable | Meaning | Buyer evidence to request |
|---|---|---|
| d | Effective piston diameter on the extension side | Controlled drawing, data sheet or model-specific manual |
| A | Effective extension area: A = πd² ÷ 4 | Calculated from the verified piston diameter |
| P | Air pressure at the leg while the stated condition is being tested | Gauge location, test state and pressure basis |
| Ftheoretical | Ideal extension force: P × A | Calculation with units and assumptions shown |
| Usable feed force | Force available at the drill under the supplier’s defined test condition | Test method, leg position, pressure, stroke point and measurement location |
Metric calculation
If pressure is in megapascals and piston diameter is in millimetres:
Ftheoretical (N) = 0.7854 × P (MPa) × d² (mm²)
The equivalent shortcut when pressure is stated in bar is:
Ftheoretical (N) = 0.07854 × P (bar) × d² (mm²)
These expressions are the same pressure-area relationship with the unit conversion included. Record every unit in the RFQ or comparison sheet; a force value without pressure and piston-area assumptions is not comparable.
Worked example: 70 mm piston at 0.50 MPa
- Effective area = π × (0.070 m)² ÷ 4 = 0.003848 m².
- Theoretical extension force = 500,000 Pa × 0.003848 m² = 1,924 N.
- That is approximately 1.92 kN, or about 196 kgf.
This is an illustrative cylinder calculation, not a PerfoMax model rating and not a recommended operating setting. A different effective diameter or actual leg pressure changes the result. On a powered-retraction circuit, the return-side effective area may be annular because the rod or inner member occupies part of the piston area, so extension and retraction forces should not be assumed equal.
Why actual feed differs from the theoretical value
Usable force at the drill can be lower—or simply defined at a different measurement point—because pressure drops through hoses and controls, seals and guides create friction, leakage increases with wear, side loading adds drag, and the leg angle converts cylinder force into a different force direction at the drill. Foot slip, soft ground, stroke position and support deflection can also prevent the theoretical cylinder force from becoming stable axial feed.
For procurement, treat a theoretical calculation as a consistency check. Treat a supplier-tested feed-force value as comparable only when the pressure, measurement point, leg angle or fixture, stroke position, model configuration and test method are disclosed. Do not apply a generic efficiency percentage to all air legs.
Buyer comparison and release gate
| Check | Accept when | Hold when |
|---|---|---|
| Piston basis | Effective bore or area is tied to the offered model | Only an outer-tube diameter or an unreferenced force figure is supplied |
| Pressure basis | Pressure value, gauge location and operating state are stated | Only compressor pressure is quoted |
| Force basis | Theoretical and tested usable force are clearly distinguished | A single “thrust” value has no method or units |
| Control circuit | Extension, hold and retraction functions match the drill and valve arrangement | The circuit or porting is unknown |
| Mechanical fit | Stroke, swivel or mounting, support geometry and compatible drill models are verified | The order relies on appearance or a family name |
| Operating limit | Maximum permitted pressure comes from controlled model documentation | The calculation is being used to invent a pressure setting |
RFQ record: ask the supplier to state the air-leg model, compatible rock-drill model, effective piston diameter or area, stroke, maximum permitted pressure, test pressure at the leg, theoretical force, tested usable feed force, test method, mounting or swivel interface and parts-document revision. Release the match only when those values agree across the quotation, drawing and manual; otherwise place it on hold for engineering review.
For current commercial options, review the PerfoMax rock drills collection and the FT160-series air-leg model-matching page. The calculation narrows a comparison, but model-controlled compatibility and the manufacturer’s pressure limits remain decisive.
What the feed control actually changes
The feed control is not simply an on/off switch. It meters the leg circuit so the operator can change how quickly the leg extends and how strongly it maintains contact as drilling conditions change. The correct control position cannot be prescribed generically because valve arrangements differ.
Conceptually, the control has three useful states:
- Neutral or hold: the leg should not intentionally advance or retract, although actual behavior depends on valve design, leakage and load.
- Controlled extension: metered air advances the leg and builds feed thrust.
- Retraction: a separate button, lever position or valve path sends air to retract the piston on a double-acting design.
Include the feed-control assembly and its seals in a model-specific inspection, rather than assuming that irregular feed always originates in the cylinder. Order replacement valves only against a confirmed model, part number and drawing revision; appearance alone does not establish control-circuit compatibility.
How powered retraction works
On a double-acting pusher leg, the operator selects the retract function and air is routed to the opposite side of the piston. The pressure differential drives the piston and telescoping section back into the cylinder. Retraction shortens the assembly for repositioning and can assist withdrawal after a hole is complete, subject to the equipment manual and ground conditions.
Some legs, controls and drill families use different circuits or rely partly on external load for return. Never assume that two air legs with similar overall shape have the same porting. Before substitution, confirm whether the candidate is single-acting or double-acting, how exhaust is managed, what the neutral state does, and which control component belongs with it.
Feed thrust is a balance, not a maximum setting
Feed thrust must keep the bit working against the rock without forcing the drill steel out of line or loading the system beyond its intended condition. Too little effective feed can allow bouncing and unstable contact. Too much feed can contribute to stalling, bending, poor collar control or abnormal stress. The correct balance changes during collaring, steady drilling and withdrawal.
This is also why air-leg condition affects apparent drill performance. A worn seal, internal leak, sticking tube, unstable foot or misaligned leg can reduce or fluctuate the thrust reaching the bit even when the rock drill itself hammers normally. Conversely, a healthy leg cannot compensate for insufficient inlet air, a worn chuck, a damaged drill steel, poor flushing or an unsuitable bit.
Air leg, rock drill and drill steel form one system
| Observed behavior | System boundary to check first | Why |
|---|---|---|
| Hammer runs; leg will not extend | Feed control, leg air passage, coupling and seals | Percussion air reaching the drill does not prove the leg circuit is receiving controlled air |
| Leg creeps in neutral | Control valve and internal sealing | Air may be bypassing the intended hold position |
| Leg extends but slips under load | Seals, cylinder/tube condition, leakage and support point | Pressure may not be maintained or the foot may not be reacting against a stable surface |
| Drill stalls as feed increases | Supply capacity, hose loss, bit contact and feed setting | The complete system may be starved or overfed; pressure at the compressor is not the same as pressure at the tool |
| Rod bends or hole starts off line | Leg geometry, collaring method, rod condition and bit contact | Feed force may not be aligned with the intended hole axis |
| Retraction is slow or absent | Retract control, return air path, seals and contamination | Extension and retraction use different valve positions or passages |
For symptom-by-symptom checks, see the published guide on pneumatic air-leg troubleshooting. For the percussion, rotation and flushing mechanisms inside the machine, see pneumatic rock drill anatomy.
Why geometry and support position matter
The air leg can only push along its own axis. Its thrust is transmitted through the swivel, drill body, chuck, drill steel and bit. If the leg is badly placed, some of that force tries to rotate, lift or side-load the drill rather than feed it cleanly into the hole.
A stable setup therefore depends on the approved support method, secure footing, a serviceable foot or stinger, enough stroke for the required position, and alignment with the drill steel. Pinch and crush zones exist around the swivel, telescoping members and support point. Hoses also need routing that does not trap, pull or kink as the leg changes length.
NIOSH research on hand-arm vibration controls for jackleg rock drills describes the jackleg as a pneumatic pusher-leg system combining percussion and rotation for underground drilling. It also reinforces a broader procurement point: machine configuration, support and operator exposure are connected. An air leg should not be evaluated as an isolated cylinder.
Compatibility checks before ordering an air leg
Match the air leg to the exact rock drill and control arrangement. Family names are not enough, and interchangeable-looking joints can differ in dimensions, air passages or retention details.
- Exact rock-drill model and suffix, with nameplate photographs.
- Existing air-leg make, model and controlled part number.
- Swivel or mounting interface drawing, including pin and retainer details.
- Extended length, retracted length and usable stroke from approved data.
- Foot or stinger style and the intended support method.
- Feed-control type, location and neutral/retract behavior.
- Single-acting or double-acting circuit and port arrangement.
- Seal-kit and wear-part part numbers.
- Approved operating pressure and lubricant from the applicable manual.
- Air and water hose routing around the leg through its full movement.
If any of these points is uncertain, treat the proposed leg as unverified. PerfoMax’s active YT28 air-leg pneumatic rock drill page can serve as a commercial route for a model-controlled configuration review; the final match should still be tied to the actual supplier data sheet and drawings.
Inspection points before connecting compressed air
Inspect with the system isolated and depressurized. Check the cylinder and telescoping tube for bends, dents, heavy corrosion and scoring; verify that the foot, pins and retainers are present and serviceable; inspect the swivel and connection for looseness or damage; and confirm that controls return and move as described by the manual. Do not use compressed air to “test” a loose or unsupported leg.
Review the hose, coupling, restraint and air-isolation arrangements at the same time. Confirm positive coupling retention and the required hose-failure controls from the equipment instructions and applicable site requirements before connection. A general checklist does not replace the exact equipment manual or the site’s approved procedure.
Common misconceptions
“More leg pressure always drills faster.”
No. Feed must suit the bit contact, rock, alignment and available air. Increasing it can move the system away from stable drilling rather than improve penetration.
“If the drill hammers, the air leg must be good.”
No. The hammer and leg have related but distinct controls and passages. The drill can run while the leg circuit leaks, sticks or receives no controlled air.
“Any leg that bolts on is compatible.”
No. Mechanical fit does not confirm port alignment, circuit type, stroke, load behavior, control logic or service-part compatibility.
“Retraction is just extension in reverse.”
Only at the highest conceptual level. The actual valve path, piston area, exhaust route and control action are model-specific, and some designs do not provide the same powered return function.
Frequently asked questions
Does the air leg power the rock drill hammer?
The leg supplies support and axial feed. The rock drill’s internal piston, valve and rotation mechanism produce percussion and rotation. They share a compressed-air system, but they are different functions.
What is the difference between feed and thrust?
Feed describes the controlled advance of the drill toward the face. Thrust is the axial force produced and maintained by the leg while doing that work. Movement can stop while thrust remains, depending on the valve state and load.
Why can an air leg extend with no load but slip during drilling?
Possible causes include seal leakage, valve bypass, scored components, an unstable support point or a supply problem under flow. Test and repair it according to the exact manual; do not diagnose solely from unloaded movement.
Can an FT-series air leg be fitted to any YT-series rock drill?
No. The full model and suffix must be confirmed. Check the mounting interface, air passage, control arrangement, dimensions and approved pairing from controlled supplier or OEM data.
What information should be included in an air-leg RFQ?
Provide the rock-drill model and suffix, existing leg identification, interface drawings or measured evidence, required length/stroke, foot style, control type, circuit action, working conditions, quantities and required spare kits. Photographs support identification but do not replace dimensions and part numbers.
Request a model-controlled air-leg match
Send PerfoMax the rock-drill nameplate, existing leg markings, connection photographs, mounting dimensions, required working position and any available parts manual. We can review the machine–leg interface and prepare a quotation around a defined configuration rather than an appearance-based substitution. Review the active YT28 air-leg rock drill configuration with the evidence attached.
Technical reference: NIOSH study on jackleg vibration controls.