How Stuff Works
Trolley jacks
The trolley jack’s origins lie first with the screw jack, which was a type of early mechanical ‘ bottle’ jacking device, consisting of an input shaft and small pinion gear meshed to a larger crown gear barrel, into which were threaded two further layered and threaded sleeves. On turning the pinion, the crown turned more slowly, thus the input effort was multiplied. This then turned out the second and then third sleeves via their square-cut screw threads, so raising the vehicle. This type of mechanical jack was a common car accessory up until the 1960s, when cheaper ‘scissor’ type mechanical jacks became all but ubiquitous.
In the mid-19th century, precision hydraulic engineering started to become more easily achievable. This then led to the production of hydraulic rams, used for many purposes, including ship launching and power presses. These operated by use of a small diameter sealed ‘acting’ piston being moved in a bore containing an oil, connected via a passage to a large diameter ‘reacting’ piston. As the oil could not be compressed, the reacting piston was moved through its bore, in smaller increments to that of the acting piston, but at greatly multiplied forces. For instance, an acting piston face of 1cm surface area would exert a force 50 times greater on a reacting piston face of 50cm surface area.
The logical conclusion was that, sooner or later, smaller versions would be developed, in the form of hydraulic bottle jacks and used as a much easier method for lifting vehicles. Bottle jacks were wonderful devices for lifting and could be stood on load-spreading plates on soft ground but were cumbersome and heavy to use. They were also tricky to insert under low chassis and body members, leading to difficulties in raising ever lower-built motor vehicles. Problems also occurred when a jack needed to be placed quite a distance under the motor vehicle, as this caused difficulty in operating the jack handle in a vertical arc.
The answer lay in the wheeled floor jack, also known as a trolley jack. This type re-oriented the hydraulic operating cylinder from the vertical to the horizontal, the raising action being achieved via a fulcrum, pivot and comparatively long lifting arm. However, there was one further problem to overcome and that was maintaining the lifting face, or ‘pad’ of the jack on a horizontal plane. To do this an additional parallel cantilever was placed below the jacking arm and attached to the pad, keeping it level at all heights through the operating range of the jack.
Trolley jacks with their wheels were more easily slid under lowset vehicles, even more so when lower height versions were cleverly devised, using multiple rams and shaped lifting arms, the latter fabricated from higher grade and stronger materials than had hitherto been the case. The operating handle was also brought to the outside of the vehicle, generally leading to easy jacking.
Perhaps the main drawback of the trolley jack has been the need for it to be situated on a flat, well-set floor, but despite that minor issue, it has become an unsung design classic used by car enthusiasts around the globe.
‘It has become an unsung design classic used by car enthusiasts around the globe’
Why ‘jack’?
Around 400 years ago, the name ‘Jack’ (short for John) became synonymous with a worker of lowly status and so was eventually used for devices which eventually replaced human power.