A Hydraulic Pulling Jack is a single- or double-acting hydraulic cylinder configured to retract under load, generating a tensile pulling force along a rod, cable, or strand instead of pushing like a standard jack. Bridge builders and rigging crews rely on these jacks daily — most post-tensioning of concrete strands worldwide happens through hollow-bore pulling jacks. Oil pumped into the rod-side chamber retracts the piston, dragging the anchored load toward the cylinder body. A typical 60-tonne stressing jack pulls 600 kN through a 200 mm stroke and locks the strand with wedge anchors at the back end.
Hydraulic Pulling Jack Interactive Calculator
Vary bore, rod diameter, and hydraulic pressure to see annulus pulling force compared with full-bore pushing force.
Equation Used
The pulling jack uses the annulus area on the rod side of the piston, so the rod cross-section is subtracted from the full bore area. Pressure in bar and diameters in millimeters are converted internally to SI units. Tonnes are shown using the article convention of approximately 10 kN per tonne.
- Annulus-side pressure acts uniformly on the rod-side piston area.
- Rod diameter is clipped to the bore diameter if the slider combination is invalid.
- Tonnes use the article convention of about 10 kN per tonne.
How the Hydraulic Pulling Jack Actually Works
A Hydraulic Pulling Jack flips the geometry of a normal hydraulic jack. Instead of extending a piston rod to push something, you pressurise the annulus side — the ring-shaped area on the rod side of the piston — so the rod retracts and pulls. The load attaches either to the front nose of the rod through a clevis or hook, or in the case of a hollow ram jack, the load passes straight through the centre of the cylinder and anchors at the rear face with a wedge plate or strand grip. Either way, the force vector points back into the cylinder body, and the body itself reacts against a fixed bearing plate or structural member.
The pulling force you get is lower than the pushing force of the same cylinder, because the effective piston area is the full piston area minus the rod cross-section. On a 100 mm bore, 60 mm rod cylinder at 700 bar, the push force is around 55 tonnes but the pull force is only around 35 tonnes. You have to size for the smaller annulus area or you will undershoot your tension target. Get this wrong on a post-tensioning job and you will lock off strands at 70% of the design tension instead of the specified 75% — which on a precast bridge girder means cracking under live load within months.
Seals are the other thing that bites you. A pulling jack runs the rod seal under pressure constantly during stressing, not the piston seal, so rod seal selection drives jack life. Polyurethane U-cups handle 700 bar continuous; nitrile lip seals will extrude through the seal gland clearance above about 450 bar and you will see oil weeping down the rod within ten cycles. The seal gland clearance must be 0.05 to 0.10 mm for a 60 mm rod — tighter and the seal galls, looser and it extrudes. Bypass past a worn rod seal is the single most common failure mode on stressing jacks, and it shows up as the gauge pressure climbing while the strand elongation stalls.
Key Components
- Hollow Cylinder Body: Houses the piston and provides the reaction surface that pushes against the bearing plate when the strand pulls. Wall thickness is sized for hoop stress at full pressure plus a 1.5 safety factor — on a 700 bar jack with a 100 mm bore, you need at least 18 mm of 4140 steel wall.
- Hollow Piston and Rod: The bored-through piston lets the strand pass through the centre of the jack. The rod is typically chrome-plated to 25 µm minimum thickness with surface finish Ra ≤ 0.4 µm, otherwise the rod seal wears out in a few hundred cycles.
- Rod Seal Stack: Polyurethane U-cup primary seal plus a back-up ring and a wiper. This stack is what carries the working pressure on a pulling jack. Replace the wiper at any sign of grit ingress — one cycle with debris on the rod will cut the U-cup.
- Wedge Anchor Plate (Stressing Jacks): Two- or three-piece tapered wedges that bite into the strand at the rear of the jack. The wedge taper is typically 7° and the seating draw-in is 6 mm — you must subtract this from your target elongation or you lock off short.
- Pressure Gauge and Calibration Cell: Stressing jacks need calibrated load measurement, not just gauge pressure. A 0.5% class load cell in series with the rod tells you actual force; gauge pressure alone drifts by 3-5% as seal friction changes with temperature.
- Hydraulic Pump: Single-acting pulling jacks use a hand pump or air-driven pump rated to 700 bar. Flow rate sets stressing speed — a 1 L/min pump pulls a 200 mm stroke through a 100/60 mm jack in roughly 50 seconds.
Real-World Applications of the Hydraulic Pulling Jack
You see Hydraulic Pulling Jacks anywhere a controlled tensile force has to be applied through a fixed anchor point. The hollow-ram pulling jack is the standard tool for post-tensioning concrete and ground anchors. Solid-rod pulling jacks show up in rigging, salvage, structural pull-back work, and cable installation. The reason you reach for a pulling jack instead of a winch or come-along is precise force control — you can hold a calibrated 50 tonne pull indefinitely, which a wire rope winch cannot do without slipping or overheating the brake.
- Civil Engineering: Post-tensioning of precast concrete bridge girders using VSL or Freyssinet hollow stressing jacks, typically 200-tonne units pulling 7-strand or 19-strand tendons to 75% UTS.
- Geotechnical: Stressing of ground anchors and rock bolts on slope stabilisation projects — a Dywidag DSI 60-tonne hollow jack tensions a 32 mm threadbar anchor to design lock-off load.
- Marine Salvage: Pulling parted mooring chains and grounded vessel anchors on North Sea recovery jobs using twin Enerpac RCH-606 hollow rams with pulling adaptors, working through a chain stopper.
- Structural Steel: Pull-back rigging on warped truss chords during heritage restoration — a Holmatro HJ-30 pulling jack draws a deflected member back to plumb before the splice plates go on.
- Wire Rope and Cable: Tensioning of suspension bridge stay cables and overhead transmission line conductors — a Tensa or BBR stressing jack pulls a single strand to 200 kN per the cable schedule.
- Heavy Lift Rigging: Strand-jack lifting systems on refinery vessel installations — VSL or Mammoet strand jacks act as continuous pulling jacks, gripping and releasing strand bundles in a step-climb cycle to lift loads of up to 5,000 tonnes.
The Formula Behind the Hydraulic Pulling Jack
The pulling force of a hydraulic pulling jack is the working pressure times the annulus area — the piston area minus the rod cross-section area. Practitioners get caught out here because they size the jack on the bore alone and discover the pull force is 30-40% less than the equivalent push force. At the low end of the typical 350 bar working pressure range, you are leaving capacity on the table for any job that needs short-stroke high-force pulls. At the nominal 700 bar most stressing jacks run at, you hit the design sweet spot — seals last, gauges read accurately, and the cylinder body is not stressed near yield. Push above 800 bar to chase more force and you accelerate seal extrusion, gauge drift, and rod plating fatigue dramatically.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fpull | Pulling force at the rod end | N | lbf |
| p | Hydraulic working pressure on the rod side (annulus) | Pa (or bar) | psi |
| Dbore | Cylinder bore diameter | m | in |
| Drod | Piston rod diameter | m | in |
Worked Example: Hydraulic Pulling Jack in a mine shaft headframe guy-rope tensioning job
You are sizing a hollow pulling jack for tensioning four 28 mm spiral-strand guy ropes on a refurbished mine shaft headframe at a potash operation in Saskatchewan. The lock-off tension per rope is 320 kN. You have a Holmatro HJ-Series hollow jack catalogue with a 110 mm bore, 65 mm rod option. You need to confirm it pulls hard enough at the typical operating pressures and identify where in the pressure range you should set the pump.
Given
- Dbore = 0.110 m
- Drod = 0.065 m
- pnom = 700 bar (70 MPa)
- Frequired = 320 kN
Solution
Step 1 — calculate the effective annulus area, which is what carries pressure on a pulling stroke:
Step 2 — at the nominal 700 bar (70 MPa), compute the pulling force:
That gives you 35% headroom over the 320 kN lock-off — comfortable, not wasteful, and exactly where you want a stressing jack to sit on a critical-tension job.
Step 3 — at the low end of the typical operating range (350 bar, half pressure), the jack only delivers:
That is below your 320 kN requirement, so a pump set at 350 bar will not finish the job — the strand will stop elongating and the operator will assume the jack has failed when in fact the pump pressure is just too low. You need to set the pump regulator above roughly 520 bar to clear the lock-off load with a small margin.
Step 4 — at the high end (800 bar, the catalogue maximum for this jack):
You can pull 495 kN if you really need it, but at 800 bar you are within 100 bar of the rod seal extrusion threshold for most polyurethane U-cups. Run there occasionally for over-pull tests, not as your everyday setting.
Result
The Holmatro 110/65 hollow jack pulls 433 kN at the nominal 700 bar setting — solid headroom over the 320 kN lock-off requirement. Across the operating range you get 216 kN at 350 bar (insufficient), 433 kN at the 700 bar sweet spot, and 495 kN at 800 bar maximum, so set the pump regulator at 700 bar and live there. If your gauge reads 700 bar but the strand stops elongating before the target draw, suspect three things: rod seal bypass leaking oil from the annulus to the piston side (you will hear the pump cycle without rod movement), wedge anchor draw-in larger than the assumed 6 mm because the wedges are reused and rounded over, or a partially closed return line restricting the rod-side flow path. Confirm seal bypass by isolating the cylinder and watching if the piston creeps back under load — any creep over 30 seconds means the rod seal is gone.
Choosing the Hydraulic Pulling Jack: Pros and Cons
A pulling jack is not the only way to apply a tensile load. Wire rope winches, chain hoists, and turnbuckles all do the same job in some contexts. The choice comes down to force level, force precision, hold time, and stroke length. Here is how the hydraulic pulling jack stacks up against the two alternatives you are most likely to consider for the same job.
| Property | Hydraulic Pulling Jack | Wire Rope Winch | Chain Hoist (Lever or Electric) |
|---|---|---|---|
| Typical pull force range | 50 kN to 10,000 kN (strand-jack systems) | 10 kN to 500 kN | 5 kN to 250 kN |
| Force accuracy / precision | ±1-2% with calibrated load cell | ±10-15% (drum friction, brake slip) | ±15-20% |
| Stroke per cycle | 100-1000 mm (single stroke), unlimited via strand-jack step-climb | Continuous (limited by drum capacity) | Continuous |
| Hold time at full load | Indefinite (mechanical wedge lock-off) | Hours (brake heat-limited) | Indefinite (pawl) |
| Capital cost (60-tonne class) | £8,000-£20,000 jack + pump | £3,000-£6,000 | £1,000-£3,000 |
| Reliability / failure mode | Rod seal bypass after 500-1000 cycles | Wire rope fatigue, brake glaze | Pawl wear, chain stretch |
| Best application fit | Post-tensioning, stressing, calibrated pulls | Long continuous pulls, recovery | Light rigging, intermittent pulls |
| Setup complexity | High — pump, hoses, anchors, calibration | Medium — drum mounting, fairlead | Low — hook and go |
Frequently Asked Questions About Hydraulic Pulling Jack
Because the rod takes up part of the piston face on the pull side. When you pressurise the annulus, the effective area is π/4 × (Dbore2 − Drod2), not the full bore area. On a typical 2:1 area ratio cylinder (rod diameter about 70% of bore), the pull force is roughly 50% of the push force; on a 60% rod-to-bore ratio it works out closer to 65-70%. Sizing on bore alone is the single most common mistake on pulling jack jobs.
Mechanically yes, you can plumb pressure into the rod port of a standard cylinder and it will pull. But standard push cylinders have rod seals sized for occasional retract pressure, not continuous high-pressure stressing duty. The rod seal on a typical Parker or Hercules push cylinder is rated for 350 bar dynamic; pulling jacks running at 700 bar will extrude that seal within a few dozen cycles. Use a cylinder spec'd as a pull cylinder or a stressing jack — Enerpac, Holmatro, VSL, Freyssinet, and Dywidag all sell these as catalogue items with the right rod seal stack.
That is seal friction, and it is normal on pulling jacks. The pressure gauge reads cylinder pressure, but a fraction of that pressure is consumed pushing the rod seal through its gland, and that fraction grows as the seal ages, as oil viscosity rises in cold weather, and as the rod accumulates wear. A 5-10% discrepancy between gauge-derived force and load-cell-derived force is expected. This is exactly why post-tensioning standards require calibrated load cells, not gauge pressure, for verifying lock-off tension on critical structures.
Choose hollow-ram whenever the load is a strand, threadbar, cable, or anything you can pass through the centre of the jack and anchor at the rear face. The advantage is that the load line stays straight and the jack just slides along the strand — no off-axis bending, no clevis pin to fail. Solid-rod pulling jacks make sense when you cannot route the load through the cylinder, for example pulling a structural member back to plumb where the attachment is a lug welded to the steelwork. Off-axis loading on a solid-rod pulling jack is the killer here — anything more than 2-3° of misalignment will side-load the rod seal and you will lose the seal in tens of cycles, not hundreds.
You add the seating draw-in to your target elongation. Standard 7° taper wedges on a 15.7 mm seven-wire strand pull 6 mm into the anchor head when the jack releases — that 6 mm comes off your strand elongation, so the strand ends up at lower tension than the gauge said when you locked off. If your design elongation is 180 mm, you stress to 186 mm at the jack and let it draw back 6 mm on release. Reused wedges that have rounded teeth can pull in 8-10 mm instead of 6, which is why the wedge specification on every post-tensioning job calls for new wedges every cycle on critical strands.
Stick-slip on a pulling jack is almost always seal friction overcoming piston momentum at low flow rates. The rod seal grips, pressure builds until the static friction breaks, the piston jumps forward, pressure drops, and the cycle repeats. You see this most often on hand-pump jobs where the operator pumps slowly. Three fixes: increase pump flow rate so the seal stays in dynamic friction, switch from nitrile to PTFE-faced rod seals which have a much smaller difference between static and dynamic coefficient, or warm the oil up — cold oil viscosity makes seal friction much worse below 5 °C. If the chatter only started recently on a previously smooth jack, the rod plating has likely worn through to the base steel in spots, and you are dragging the seal across bare steel.
Not safely on a single strand — load sharing is the problem. Two jacks fed from one pump will not pull equally because of small differences in seal friction, hose length, and bleed-down. One jack will reach its target first and lock off, and the second jack will then take the entire remaining load alone, potentially overstressing the strand or the second jack. If you need more force than one jack can deliver, use a single larger jack or a strand-jack system designed for parallel operation with active load-balancing. Parallel jacking on separate strands of a multi-strand tendon is fine and routine — that is how multi-strand stressing jacks work internally.
References & Further Reading
- Wikipedia contributors. Prestressed concrete. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
