A trolley jack is a hydraulic two-piston vehicle lifting device that converts downward force applied to the handle, or first piston point, into upward force at the second piston point. According to Thomasnet.com, the jack also amplifies the initial force. Trolley jacks work reliably on flat surfaces such as asphalt and concrete. However, they are unsafe to use on angled surfaces, grass, gravel and sand.
Trolley jacks are filled with oil, which lubricates their moving parts and does not oxidize metal. The oil also creates an extremely efficient hydraulic system that intensifies almost all of the initial downward force and transmits it to the second piston.
Trolley jacks have several advantages over other automotive jacks. They offer comparable maximum extension but fold down into a flat unit for convenient storage in small spaces. Small elevator systems often employ trolley jacks as the primary lifting mechanism.
Trolley jacks are not meant to be carried in vehicles for emergency use. Robert Jones, a columnist for HowItWorksDaily, writes that scissor jacks are more effective for this purpose because they are vehicle-specific and safe to use on gravel, dirt roads and on gently sloped surfaces. Advanced models are electric. These plug into vehicle cigarette lighters and perform all lifting functions without manual assistance. As with conventional jacks, safe operation of electric scissor jacks requires proper placement beneath the vehicle chassis.