Screw Jack Actuator
A screw jack actuator is a drive element that converts the rotary motion of a motor into linear motion through a screw and nut pair. When a trapezoidal screw is used the mechanism is self-locking: it does not back-drive under load even if power is cut. It is the common drive principle of electric levelling legs.
The motor shaft usually turns the screw through a gearbox; since the nut fitted on the screw is prevented from rotating, it travels along the axis. High speed and low torque are thus converted into low speed and high force.
- Self-locking: on trapezoidal screws the friction angle is greater than the helix angle, so the load cannot turn the screw back. That means the leg holds its position all night.
- Efficiency: because of that same friction the efficiency of a trapezoidal screw is low; a ball screw is more efficient but does not self-lock and therefore needs a brake.
- Maintenance: the screw surface must be kept clean and greased. Sand and mud act like an abrasive paste.
In the field this has two practical consequences. First, no continuous power is spent to hold the leg in position; the system draws nothing from the battery as long as it is not moving. Second, instead of forcing a jammed screw you should try cleaning and lubrication; the current protection is already trying to protect the mechanism by cutting excessive torque.
Context in which this term is used: Guide to automatic RV levelling