A jack and jack stands are a set of tools used to lift a vehicle or sometimes other heavy objects, and to do so safely. The jack offers the lifting force, usually through a pump handle with a hydraulic cylinder.
Jack stands are designed to hold the weight of the vehicle, or at least a considerable portion of it, and can do so as just a safety item in case the jack fails, or to hold up a portion of the vehicle while the jack is removed.
Considerations When Choosing Jack Stands
Maximum Weight Capacity
There are two common mistakes people make when they see the weight capacity of jack stands. First, it is easy to assume the weight listed on the box is per stand, but it is how much the the pair can hold together. Second, one may assume that the jack stand needs to hold the total weight of the car, and this is not the case. If you lift up one side of a vehicle and support that side by two jack stands, about half of the vehicle's weight is resting on the wheels that are on the ground, and the rest of the weight is split about 50/50 between the two stands, meaning they each hold about 25% of the weight. Cars aren't perfectly balanced, though, so a single jack stand that holds 45% of the vehicle's total weight is a safe bet for all but the most unusual vehicles.
In the case of unusual vehicles, consider how poorly the weight may be balances, as well as how heavily loaded a vehicle is. Work vehicles can be loaded up with a considerable amount of tools, coming close to doubling the weight of the vehicle.