A screw jack turns a small effort on a handle into a huge lift. Use the view to look inside and trace the load from the platform, down the screw, through the nut and thrust bearing, into the base.
A screw jack is a lifting device built around a threaded screw and nut. Instead of a piston or a pump, it raises a load by turning a thread: the screw climbs as the nut rotates, and whatever sits on top is lifted with it. Because a thread is a wrapped-around ramp, it can turn a modest push on a handle into the force needed to lift many tonnes.
From outside it is a metal column on a base. The interesting part is invisible â how the load passes from the platform, down the screw, across the threads into the nut, and out through the bearing into the ground. A cutaway makes that hidden load path clear.
Everything the jack lifts rests on the platform at the top of the screw. Its weight presses straight down and becomes a compressive force in the screw below.
The screw is strong in compression, so it carries the load down its whole length without bending, holding the platform steady as it rises.
Where the screw meets the nut, their threads are in contact over many turns. Those engaged thread flanks transfer the downward push from the screw into the nut â this is the joint the whole jack depends on.
The nut must turn even while carrying that heavy load, so it rests on a thrust bearing. Rolling elements carry the downward force into the base and let the nut spin with little friction.
Turning the handle drives the gear that rotates the nut. Because the thread is a very gentle ramp, one full turn raises the screw only a little, so a light effort is magnified into a large lifting force.
The thread and the gear act as levers. A long handle turns a small gear that spins the nut many times, and each turn raises the screw only a little, so a gentle push is traded for a big lifting force â the same idea as a lever or a low gear.
It travels a continuous path: from the platform into the screw, across the engaged threads into the nut, through the thrust bearing, and into the base, which passes it to the floor. Every link must carry the same force.
In most jacks the nut turns and the screw is held from rotating, so the screw just rises and falls. That keeps the load steady on the platform instead of spinning with it.
The nut has to turn while a huge downward force presses on it. A thrust bearing supports that force on rolling elements so the nut can still rotate easily, instead of grinding against the base.
A fine thread is naturally self-locking: friction in the threads holds the position when you let go. Many jacks also add a brake or ratchet for safety.