Scissor Jack
A scissor jack is a lifting mechanism made of arms jointed in a diamond shape and a screw that draws them together. As the arms open, the mechanical advantage changes; with the jack fully extended both its capacity and its stability decrease. It is a structure frequently used in RV support legs.
As the screw turns, the two side joints draw together and the mechanism extends vertically. Force transmission depends on the angle the arms make with the horizontal: when the jack is low the arms are close to horizontal, the mechanical advantage is high but the stroke gain is low. As the jack rises, the reverse happens.
- Capacity–height relationship: manufacturer capacity figures are generally valid for a particular working height range. Avoid operating in the fully extended position.
- Lateral strength: a scissor mechanism resists vertical load but is weak against lateral force. If the vehicle slips while on the legs, the mechanism bends.
- Maintenance: the joint pins and the screw need regular grease; corrosion most often starts here.
Practical rule: operate the jack in the middle region of its stroke. If the gap between the ground and the leg is large, instead of extending the jack all the way, put a suitable base plate or block underneath to reduce the working height.
The context in which this term is used: How to choose levelling leg capacity