The Science Behind Scissor Jacks

scissor jackOlder drivers can remember when floor-standing jacks came standard with most cars. These devices used a lever — usually the crow bar — to jack a car’s bumper up inch by inch when changing a tire on the roadside.

Floor-standing jacks went out of vogue about the same time donut spare tires started to become popular. And for the same reason:  Space considerations.

In order meet tougher fuel standards, automakers were challenged with finding ways to make cars lighter and smaller. So things like bulky floor-standing jacks and full-sized spare tires were some of the first things to go.

Scissor Jacks Now the Industry Standard

Not all new cars even come with spare tires anymore. But those that do almost always have scissor jacks.

The benefit of scissor jacks is that they offer the same easy lifting capabilities as floor-standing jacks but they can be twisted down into a very small, compact package that can easily fit inside or under a donut spare.

Scissor jacks are not only lighter and smaller than floor-standing jacks, but they are also less like to tip over in mid-use, making them safer as well.

How Scissor Jacks Work

Scissor jacks are remarkably simple to use … even by Millennial drivers who don’t know how to change a spare tire.

They utilize a concept known as large force amplification. In simple terms, this means that the easy task of turning a nut with a lever is enough to raise an automobile or truck weighing thousands of pounds.

There are two critical parts to the scissor jack. The first is a two-piece mechanism. The second is a self-locking screw. Working in tandem, these two parts allow extremely heavy vehicles to be lifted through an extension of the scissor mechanism, which is held in place by the resistive force of the screw.

If the screw wasn’t able to hold the weight of the vehicle, the jack would instantly collapse, creating a hazardous situation for the driver changing the tire.

Fast, Simple, and Easy

The scissor jack’s central screw has an end-mounted circular ring that is designed to accommodate a large metal arm. Like the standing jack, this is usually the tire iron (although scissor jack tire irons are a lot shorter, smaller, and lighter than old school versions).

When inserted and turned in a clockwise direction, this arm drives the screw through the scissor mechanism’s central pivot point’s threat. The jack elongates and the vehicle lifts easily. Spinning the screw in the opposite direction lowers the vehicle to the ground.