What Are the Functions of a Screw Jack?

Renaissance scientists identified six categories of simple mechanical machines: the inclined plane, the wheel and axle, the wedge, the pulley, the lever and the jackscrew. The screw jack, or jackscrew, affords tremendous mechanical advantage. Some manual jackscrews can support tons of load.

Function

The function of a jackscrew is to afford a large mechanical advantage to lift a heavy load, such as a car or the corner of a house, a short height. A jackscrew has a handle that is used to turn a screw. The handle end travels much farther than the screw elevates.

The mechanical advantage is the ratio of the force gotten out of a machine to the force put into the machine. By the law of conservation of energy, (force out)/(force in) equals (distance in)/(distance out).

The mechanical advantage of a jackscrew can therefore be measured by determining the distance a handle must travel in one revolution, then performing enough revolutions to make the jack rise a measurable distance. So (handle turns) --- (distance in one turn / elevation) gives the mechanical advantage.

Carjack

Use as a carjack is the most familiar use of a jackscrew. The car needs to be lifted a short distance to replace a tire. By turning a crank several revolutions, a heavy vehicle can be lifted manually.

Leveling a Pool Table

A jackscrew can also be used to raise something that needs leveling. For example, a heavy pool table may need thin supports added under a leg. With a jackscrew, only one person is needed to complete the job, supporting the table while supports are slid into place. In a similar way, a jackscrew can be used to level a house.

Unload a Boat on Land

A jackscrew can be used to unload a boat on dry land. For example, the bow (front) of a motorboat on a trailer can be lowered with a jackscrew in order to raise the stern (back). A support called a "jackstand" can then be placed under the elevated stern. Then the jackscrew can be used to raise the bow so that a jackstand can be slid under to support it. This allows one to treat the entire underside of a boat--scraping off marine growth, repainting, etc.

Replacing Beams and Posts

Suppose the columns or posts of a porch or similar structure need replacement. A jackscrew can support the weight of the porch while the post is removed. Though the porch would be high up, it need be lifted only a short distance. The jack can press a long 4x4 against the porch underside just next to the post being replaced.

Airplane

Commercial airplanes use jackscrews to power wing flaps, the rudder and the landing gear. Jackscrews are considered more reliable than hydraulic pumps. The jackscrews in planes are operated electronically, of course, unlike the above examples. Instead of lifting a heavy load a short distance, they extend an object and support them against tremendous frontal forces.

References

• Open PRT Specification Design

Resources

• William Ballantyne Anderson; Physics for Technical Students; 1914
• Screwjack Wear in a Plane