How Does a Car Jack Work? | Lifting Force Explained

A car jack lifts a vehicle by multiplying a small input force into a much larger lifting force using mechanical advantage, either through hydraulic fluid pressure or a threaded screw mechanism.

When a flat tire leaves you stranded, the car jack becomes the most important tool you didn’t think about until that moment. Whether it’s the scissor jack that came with your trunk or a hydraulic floor jack in a garage, the physics behind it is the same: take your modest arm strength and turn it into enough force to raise a two-ton vehicle. Here’s exactly how that happens inside the metal and fluid.

The Two Core Mechanisms: Hydraulic vs. Screw

Every car jack relies on one of two methods to multiply force, and knowing which one you’re using changes how you operate it safely.

Hydraulic jacks (the floor/trolley type common in shops) use Pascal’s law: pressure applied to a confined fluid transmits equally in all directions. Pumping the handle moves hydraulic oil from a small reservoir into a cylinder, where the pressurized fluid pushes a much larger piston upward. The force multiplication comes from the ratio of piston areas — a small pump piston can generate enough pressure to raise a large lifting piston holding the vehicle. Per mechanical engineering fundamentals, this is the same principle that powers hydraulic brakes and heavy equipment.

Scissor and screw jacks use a threaded screw and linked arms. Turning the handle rotates the screw, which pulls the linkage together and converts rotational motion into vertical lift. These are simpler, slower, and typically included as emergency jacks in vehicles because they’re compact and require no fluid.

A Step-by-Step Look Inside a Hydraulic Jack

A hydraulic car jack contains five main parts: a pump piston, hydraulic fluid (usually oil), a reservoir, a check valve, and a main lifting cylinder. Here’s the sequence that makes it lift:

1. Close the release valve to seal the hydraulic circuit so fluid can’t escape.
2. Pump the handle, which forces oil from the reservoir past the check valve into the pump chamber and then into the main cylinder.
3. Pressure builds with each stroke, acting on the large lifting piston. The force ratio means one pump stroke can lift hundreds of pounds.
4. The lift arm rises as the piston extends, raising the vehicle at the jacking point.
5. To lower, open the release valve slowly so fluid returns to the reservoir. Rushing this step causes a sudden drop.

Haynes Manuals emphasizes that the jack is a lifting device only — it is not designed to support a vehicle for work underneath it.

How to Use a Car Jack Safely

The procedure is straightforward, but the safety rules are not optional. Home Depot and Car and Driver both stress that most jack-related injuries come from skipping the preparation steps.

• Park on level, solid ground — gravel, turf, or soft asphalt can cause the jack to tip or sink.
• Chock the wheels that stay on the ground, set the transmission to Park, and engage the parking brake.
• Find the correct jacking point in your owner’s manual. Lifting from the wrong spot can bend the frame or puncture the floor pan.
• Keep the jack vertical and straight during the entire lift. A leaning jack is unstable.
• Use jack stands before any work under the vehicle. Place the stands at the vehicle’s reinforced lifting points, lower the vehicle onto them, and confirm the weight is fully supported before crawling underneath.
• Choose a jack rated for your vehicle.

If you’re shopping for a high-capacity unit, check out our tested roundup of the best 5-ton electric car jacks for models that handle heavy trucks and SUVs with power-assist lifting.

Common Mistakes That Turn a Simple Job Dangerous

Even experienced drivers make these errors. The most frequent involve ground condition, jack placement, and assuming the jack alone is enough support.

• Lifting on soft or uneven ground. A jack needs a solid, level surface to stay stable. Asphalt on a hot day can deform under the base, allowing the vehicle to shift.
• Using the wrong jacking point. The pinch weld or reinforced frame section is the only safe spot — never lift by the floor pan, control arms, or axle tubes unless the manual says otherwise.
• Skipping wheel chocks and the parking brake. The vehicle can roll forward or backward during the lift, knocking the jack out of position.
• Opening the release valve too quickly. The vehicle drops hard, potentially crushing anything — or anyone — beneath it.
• Using a jack with inadequate capacity. Overloading a jack can cause internal failure, especially in hydraulic models where seals blow under excess pressure.