The Longman Dictionary defines a jack as a piece of equipment which is used to lift and support a heavy weight, such as a car. Jacks are the primary tool used when changing the tire of a car in a roadside emergency. The jack which is used as original equipment in modern cars is the scissor jack. It is designed to lift the weight off the wheel which needs service, such as changing a flat tire.
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Scissor Jack Elements
U.S. Patent 5975497 for the multi-piece trunnion for a scissor jack lists its elements as a base plate, a load support plate, and a pair of upper channel members, each having a pair of opposed planar sides with an upper and a lower end. The scissor jack also has a pair of lower channel members and each has a pair of opposed planar sides with an upper and a lower end. The upper end of each upper channel member is pivotally attached to the load support plate. The lower end of each lower channel member is pivotally attached to the base plate. A drive screw is mounted between the first and second trunnions to rotate about a longitudinal axis.
Cars which come with scissor jacks can be lifted at special reinforced jack notches on the car's body. These notches were provided for the jacks when the car was built. The notches are the only points where a car can be lifted by the scissor jack because they are reinforced to hold the car's weight. The Reader's Digest Complete Care Manual states that cars which are built with unit-body (car design incorporating the frame and body into one unit) can only be jacked at the jack notches.
The scissor jack is most widely used today because most modern car bumpers cannot support the car's weight. The scissor jack has a wider flat base and a notched or flat top which engages into the jack notches which have been formulated into the car body. Unlike the bumper or side lift jacks or of 1970s era American cars, the scissor jack is placed under the car instead of hooked on the side panel or the bumper.
When the scissor jack is stored in the boot of the car, it is folded so it takes up a minimum amount of space. The usual location of the jack is under the spare tire. The jack is typically bolted to the boot floor.
Most modern cars use a scissor jack because they are safer and easier to use in a roadside emergency. The owners manual and or decals in the boot area of the car will provide instructions about using the scissor jack safely.
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