A hydraulic jack is a hand pump and a ram in one body, with a check valve to hold the load between strokes and a release valve to lower it, and it lifts but must never be trusted to hold. A 5 tonne-force jack at 20 MPa needs a ram of about 55.9 mm and, from 15 kgf of handle force on a 10:1 lever, a pump piston of about 9.7 mm that raises the load only 0.6 mm a stroke, so a 150 mm lift takes about 250 strokes.
Part of Hydraulic Systems
A hydraulic jack lifts a heavy load a short distance with a hand lever. It is the simplest complete hydraulic system and the model for the press.
Size the Ram, Then the Pump
Step 1: Ram. For a load of 5 tonnes-force (49,050 N) at a working pressure of 20 MPa:
- Ram area = 49,050 / 20,000,000 = 2.4525 x 10⁻³ m² (24.5 cm²)
- Ram diameter = √(4 x 2.4525 x 10⁻³ / π) = 55.9 mm
Step 2: Pump piston. With 15 kgf (147.2 N) of handle force on a 10:1 lever:
- Force on the pump piston = 1,472 N
- Pump area = 1,472 / 20,000,000 = 7.36 x 10⁻⁵ m² (0.736 cm²), a diameter of 9.68 mm, about 9.7 mm
- Area ratio = 24.5 / 0.736 = 33.3, so the total gain is 10 x 33.3 = 333, and 147.2 N becomes 49,050 N, as required
Step 3: Lift per stroke. With a 20 mm pump stroke the oil per stroke is 0.736 x 2 = 1.47 mL, and that raises the ram 1.47 / 24.5 cm² = 0.6 mm. To lift 150 mm: 150 / 0.6 = 250 strokes.
That is a lot of strokes, which is why bought jacks use a larger pump piston at low load and a second, smaller stage for the final lift. Choosing a lower pressure also helps, since a larger pump piston moves more oil per stroke; see the pressure trade in Practical Systems.
The Parts
- A ram in a steel cylinder body, sealed with a cup seal or packing (Seal Systems) and guided so it cannot tilt.
- A pump piston in a small bore cut into the same body, with a lever and a link.
- Two check valves: an inlet from the reservoir and an outlet to the ram, per Check Valves. The outlet valve is what holds the load between strokes.
- A release valve (a needle valve) that opens the ram to the reservoir, so the load can be lowered slowly; see Flow Control Valves.
- A reservoir big enough to hold the oil the ram displaces when fully extended.
- A relief valve set from the lowest-rated part (Pressure Relief Valves), so overloading cannot burst the body.
- A base wide enough to spread the load.
The Base Must Spread the Load
A 100 mm square base under 49,050 N puts 49,050 / 0.01 m² = 4.9 MPa on the ground, a figure that most soil cannot bear (the corpus gives no ground figure here, so treat soil as weak). Stand the jack on a wide plank or a steel plate and keep it level; a jack that tilts under load pushes sideways and kicks out.
Common Mistakes
- Treating the jack as a support. A seal or valve leak lowers the load, slowly or at once.
- Lifting off-centre. The jack tips.
- Extending the ram to its limit. Fit a stop that keeps the ram’s guide length inside the body.
- Releasing the valve fast. The load drops; open it a little at a time.
- Pumping with the release valve slightly open. No pressure builds and the load stays.
Safety
Never get under a load held by a jack
A hydraulic jack holds a load through two small seats and a set of seals; any of them can leak, grit can lift a valve, and a jack can tip. Lift the load only high enough to put a stand, a block or a crib under it, lower the load onto that, and keep every part of your body out from under it. Oil under pressure from a pinhole in the body injects through skin: never search for a leak with a hand, pass card or wood across the fitting, and treat any suspected injection as a surgical emergency. Relieve the pressure before moving or servicing the jack.
What’s Next
- Learn next: Crane Design — other ways to lift heavy loads
- Combine with: Hydraulic Press — a jack in a frame