be synchronised to work together, and
that their screw operation can be used
to amplify force. Essentially there are two
types of screw jack: translating, in which
the lifting spindle is driven axially through
the gearbox, and rotating (or travelling
nut), where the spindle is axially fixed and
rotates to drive a travelling nut fixed to
the load. The former is generally the more
economic option, although with longer
strokes the spindle may protrude through
the bottom of the gearbox when retracted
which can be a disadvantage.
Unless fixed securely to the load, the
spindle may rotate under friction when
lifting, therefore an anti-rotation measure
may be necessary. The latter, rotating
type, ensures there is no protrusion below
the gearbox - though the free end of the
spindle will normally need securing by
means of a flanged bearing.
Depending on the duty cycle or lifting
speed required, a trapezoidal spindle
or ball screw spindle can be selected.
Trapezoidal spindles have the great
advantages of being simple, robust,
reliable, low cost and inherently selflocking. However, the greater expense of a
ball screw can be justified.
Because frictional losses are
reduced significantly, they can
operate at greater speeds and
are highly efficient, so will require
smaller motor drives.
Ball screw jacks tend to be specified when
frequent actuation is expected while
trapezoidal screw jacks are favoured
for systems that will be used less often.
However as ball screws are not normally
self-locking, a brake motor must be used in
all cases to prevent back driving.
Selection of a screw jack for a given
application may require some thought
and Drive Lines will help identify the best
choice. We will also help calculate the
optimum size, typically a unit that is big
enough to cope with likely shock loads but
not so big that it requires excessive power.
We will often sit down with the client
and discuss the optimum solution – for
example if travel speed is not important
then a smaller motor may be specified,
saving cost for the client.
Screw jacks are usually driven by a
standard industrial AC motor, to which
it is coupled by a shaft, couplings and
gearheads, as appropriate. However,
other types of motors could also be
used, such as: DC in remote locations
where an alternating current supply is
unavailable; compact servo in spacelimited applications; air or hydraulic motor
in potentially explosive atmospheres.
The configuration of the mechanical drive
elements is often dictated by the physical
layout of the system. Drive Lines always
likes to carefully scrutinise this element of
the design project; noting that simplicity is
always better than complexity because it
aids efficiency and reliability.
It also looks for ways to design out possible
failure modes: for instance an overhead
system that pulls the load up is preferable
to one that pushes from below because
it is in tension rather than compression,
therefore preventing buckling of the
spindle if overloaded.
Issue 23 PECM
99