A scissor lift is a type of platform
that can usually only move vertically. The mechanism to achieve this is the use
of linked, folding supports in a criss-cross "X" pattern. The upward motion is
achieved by the application of pressure to the outside of the lowest set of
supports, elongating the crossing pattern, and propelling the work platform
vertically. The platform may also have an extending "bridge" to allow
closer access to the work area, because of the inherent limits of vertical-only
movement.
The contraction of the scissor action can
be hydraulic, pneumatic or mechanical (via a leadscrew or rack and pinion system). Depending on the power system
employed on the lift, it may require no power to enter "descent"
mode, but rather a simple release of hydraulic or pneumatic pressure. This is
the main reason that these methods of powering the lifts are preferred, as it
allows a fail-safe option
of returning the platform to the ground by release of a manual valve. In conclusion, scissor lifts are very durable & are often available as equipment rentals.
Source: http://en.wikipedia.org/