Direct answer: pulldown, feed force, weight on bit, and pullback describe related but different loads in a drill rig's feed system. Feed force is the general axial force that moves or holds the drill carriage along the feed beam. Pulldown is the force available in the drilling direction. Weight on bit (WOB) is the axial load actually reaching the bit-rock contact. Pullback is the force available in the reverse direction to retract or recover the drill string. A maximum rig rating is a capability limit—not a universal operating setting.
This distinction matters when mines, quarries, contractors, and distributors compare rigs with DTH tools, top-hammer strings, or rotary bits. The number printed in a brochure may refer to cylinder force, carriage force, bit load, or total capacity under specified conditions. Unless the measurement point and operating assumptions are clear, two figures with the same unit may not be directly comparable.
Rock drill pulldown and pullback at a glance
| Term | Direction or location | What it normally describes | Common interpretation error |
|---|---|---|---|
| Feed force | Along the feed beam | General force used to advance or hold the carriage and tool string against the rock | Treating a hydraulic pressure value as force |
| Pulldown | Toward the hole bottom | Forward feed-system capacity available to press the drilling assembly into the hole | Using the maximum rating as the required drilling setpoint |
| Weight on bit (WOB) | At the bit-rock interface | Axial load acting on the bit; on rotary rigs it can include feed-system thrust plus drill-string weight | Assuming WOB always equals the cylinder or carriage rating |
| Pullback | Away from the hole bottom | Reverse force available to retract the carriage, lift the string, or assist recovery | Confusing it with normal forward drilling load |
| Holdback | Controlled reverse restraint | Force used in some systems to prevent overfeeding or to stabilize the drilling contact | Assuming every rig uses the term in the same way |
The safest reading is functional: identify the direction, the point at which force is measured, and whether the value is a maximum, a continuous working rating, or an operator-controlled setting.
What is feed force in rock drilling?
Feed force is the umbrella term for axial force applied through the rig's feed mechanism. Depending on the machine, a hydraulic cylinder, chain, cable, rack, or screw arrangement moves the carriage. The carriage then advances the rock drill, rotary head, or drill string along the mast.
Feed force has two jobs during drilling: keep the tool in controlled contact with the rock and advance the assembly as the hole deepens. It should not be confused with feed speed. Force describes load; speed describes how quickly the carriage travels. A rig may move the carriage quickly during positioning while applying little drilling load, then move slowly under controlled force while drilling.
For a concise explanation of how feed interacts with percussion, rotation, and flushing, see Percussive Rock Drilling Explained.
What does pulldown force mean?
Pulldown is the feed force available in the drilling direction. The name comes from vertical blasthole drilling, but the mast can also be angled or horizontal. In those positions, “down” still means forward along the feed beam toward the hole bottom.
On large rotary blasthole rigs, published specifications often list pulldown capacity separately from pullback capacity. Epiroc, for example, publishes both values for Pit Viper models and also lists a weight-on-bit figure. That separation is useful: it shows that the rig's forward feed capacity, reverse recovery capacity, and bit load are not automatically the same quantity.
A pulldown rating does not tell a buyer how much force should be applied to a particular bit. Required operating load depends on the drilling method, bit design and diameter, rock response, drill-string weight, mast angle, rotation and flushing conditions, and the rig manufacturer's control limits.
Is pulldown the same as weight on bit?
Not necessarily. Weight on bit is the axial load at the bit-rock contact. Pulldown is a feed-system capability or commanded force in the forward direction. Between the actuator and the bit are the carriage, rotary head or rock drill, rods or pipes, joints, guides, friction, gravity, and sometimes reeving or mechanical ratios.
For vertical rotary drilling, the drill string's own weight can add to the load delivered to the bit. For angle or upward drilling, gravity changes its contribution. This is why a rig brochure, operator display, and bit supplier may all use force-related terms while referring to different physical points.
When comparing specifications, ask whether “bit load” or WOB is:
- a calculated value based on hydraulic pressure and geometry;
- a measured value from a load sensor;
- a maximum structural or control-system limit;
- the feed contribution alone; or
- the combined result of feed thrust and drill-string weight.
What is pullback force used for?
Pullback is the force available in the reverse direction. It retracts the carriage, lifts the drill string, helps break joints under controlled procedures, and provides recovery capacity when the string does not move freely. Pullback can therefore be an important procurement parameter for long, heavy strings and difficult hole conditions even though it is not the normal forward drilling load.
TEI's published specifications for portable feed systems, for example, state pullback force together with drill travel and machine weight. This illustrates why the value belongs to the feed/retraction system rather than to the rock-breaking output of the hammer itself.
Pullback capacity is not permission to exceed rod, pipe, thread, chuck, lifting, or mast limits. A stuck string can store substantial energy. Recovery must follow the rig and tool manufacturers' procedures, with personnel clear of the line of fire.
Why feed pressure is not feed force
Hydraulic pressure is commonly shown in bar, MPa, or psi. Force is shown in N, kN, lbf, or sometimes ambiguously as “kg.” Pressure can be used to estimate cylinder force, but pressure alone is not the delivered feed force.
At a simplified level, hydraulic force depends on pressure multiplied by the effective piston area. Actual force at the carriage can then be changed by the rod side of the cylinder, chain or cable reeving, mechanical ratio, friction, seal condition, guide resistance, and control-valve losses. Force at the bit can differ again because of string weight, angle, joints, and contact conditions.
If a supplier lists a force in “kg,” ask whether the intended unit is kilogram-force (kgf) rather than mass. For technical comparison and RFQs, request N or kN and record the measurement point.
How feed requirements change by drilling system
| System | Primary feed job | What too little feed can cause | What excessive feed can cause |
|---|---|---|---|
| DTH | Keep the bit seated and the hammer working against the rock while the piston strikes close to the bit | Unstable contact, inefficient impact use, vibration, or poor control | Restricted rotation, abnormal loading, accelerated bit or hammer wear, or poor response to changing ground |
| Top hammer | Maintain controlled contact so impact energy travels through the rod string while the bit advances | Rod bounce, loose coupling behavior, energy loss, and unstable collaring | Rod bending, thread stress, excessive guide wear, and impaired rotation |
| Rotary tricone | Provide the WOB needed for cutters to crush and indent the formation while torque rotates the bit | Rubbing, low penetration, inefficient cutting action | Cutter or bearing overload, high torque demand, deviation, or accelerated wear |
These are directional effects, not universal setpoints. The correct control values must come from the rig and tool manufacturers for the exact configuration. A DTH rig that is also capable of rotary drilling may show the same maximum pulldown rating for both methods, but the appropriate operating load can be very different.
Why maximum ratings should not become operating setpoints
Maximum pulldown and pullback values help establish the machine's capability envelope. They support structural selection, pipe handling, recovery planning, and application matching. They do not state that full force should be used continuously.
Operating at the maximum without considering bit response can increase torque, vibration, bending, heat, joint stress, deviation, and wear. Conversely, underfeeding can reduce penetration and make the tool bounce or rub. Good drilling uses enough controlled feed for stable energy transfer and cutting, then adjusts within the OEM-approved range as rock and hole conditions change.
Feed also interacts with rotation. Increasing axial load without confirming available torque can stall rotation or overload the string. For the complementary definitions, read Rotation Speed vs Torque in Percussive Rock Drilling.
How to compare two rig specification sheets
| Check | Question to ask | Why it matters |
|---|---|---|
| Force direction | Is the value forward pulldown, reverse pullback, or controlled holdback? | Opposite directions can have different actuator areas and limits. |
| Measurement point | Is force stated at the cylinder, chain/cable, carriage, rotary head, or bit? | Mechanical ratios and losses prevent direct comparison. |
| Rating type | Is it maximum, intermittent, continuous, nominal, or operator-adjustable? | A short-duration capability is not a continuous working load. |
| Mast condition | At what mast angle and carriage position is the rating valid? | Gravity and geometry change the load reaching the bit. |
| String assumptions | Does WOB include pipe or rod weight? | Long strings can make a large contribution in vertical holes. |
| Units | Are values in kN, lbf, kgf, pressure, or mass? | Unit labels must describe the same physical quantity. |
| Control method | How is the value set, limited, and displayed? | Commanded pressure may not equal measured bit load. |
Common specification mistakes
- Comparing pulldown to pullback: they serve opposite directions and may be deliberately unequal.
- Comparing pressure to force: pressure needs actuator area and mechanism details before it can indicate force.
- Ignoring drill-string weight: vertical rotary WOB can include gravity; angled drilling changes that contribution.
- Assuming every “thrust” figure is at the bit: some brochures state actuator or carriage capability.
- Applying rotary WOB logic to DTH or top hammer: the feed system must support a different rock-breaking mechanism.
- Forcing a tool match from one headline number: bit diameter, shank or hammer, rod/pipe, rotation, flushing, and hole conditions must also agree.
Information to confirm before an RFQ
Send enough information for the rig, drill string, and bit to be evaluated as one system:
- rig manufacturer, model, serial range, and feed-system type;
- drilling method: DTH, top hammer, or rotary;
- hole diameter, depth, inclination, and required straightness;
- bit type and diameter, hammer or rock-drill model, and interface;
- rod or pipe diameter, length, thread, quantity, and approximate string mass;
- published pulldown, pullback, WOB, rotation torque, and RPM ranges;
- whether each force is maximum or continuous and where it is measured;
- air or water flushing availability and expected ground conditions;
- the rig OEM's approved feed-control procedure; and
- anticipated recovery or stuck-string risks.
If penetration efficiency is being compared across trials, normalize for hole volume and operating conditions rather than relying on feed force alone. The related guide Specific Energy in Rock Drilling explains why energy per excavated volume can be a more useful comparison.
Frequently asked questions
Is feed force the same as thrust?
Often they refer to the same general axial action, but “thrust” may describe actuator, carriage, or bit load. Confirm direction and measurement point before treating the terms as equivalent.
Why is pullback force different from pulldown force?
The cylinder area, reeving, valve settings, and structural duties can differ by direction. Pullback is sized for retraction and recovery, while pulldown supports forward drilling. Equal ratings are not required.
Should I use the rig's maximum pulldown while drilling?
No universal rule supports that. Maximum pulldown is a capability limit. Use the OEM-approved working range for the exact drilling method, bit, string, hole direction, and rock conditions.
How does mast angle affect weight on bit?
Gravity adds most directly to bit load in vertical downward drilling. As the mast moves away from vertical, only part of the string weight acts along the hole axis; in upward drilling, gravity opposes forward feed.
What unit should a supplier use for feed force?
N or kN are clear SI force units; lbf is common in imperial specifications. If a brochure says only “kg,” ask whether it means kgf and request a confirmed conversion to force.
Match the complete drilling system, not one force rating
Pulldown, feed force, WOB, and pullback are useful only when their direction, measurement point, rating type, and drilling context are known. PerfoMax can review an inquiry against its current rock-drilling tool scope without treating a rig's maximum force as a tool recommendation. Request a quote and include the rig model, feed specifications, drilling method, hole plan, and full string details.
Technical references
- Epiroc: Pit Viper 271 XC E technical data — separately lists WOB, pulldown capacity, and pullback capacity.
- TEI Rock Drills: Portable Drilling Systems — examples of feed travel and pullback specifications.
- Society for Mining, Metallurgy & Exploration / OneTunnel: percussion-assisted rotary blasthole drilling — discusses the role of pulldown in rotary drilling.