A combined screw jack and coil spring load limiter mechanism solves a
powered precision closure problem on an industrial dryer
When an industrial machine needs to move a cover or lid on to a dead stop
or towards a sealing face, it must do so precisely and positively, with
contact being made on all dead stops or over the complete sealing face.
One product that is frequently used for this type of operation is a
mechanical linear actuator or screw jack. Conventionally these are best
suited to lowering applications as a simple elongated slot in an
actuator's ram or clevis end allows the cover to be lowered into position
and mate under its own weight by driving the actuator until the lifting
pins are midway in the clevis slot. However, this method cannot be used
to push a cover into position, a problem Spooner Industries was
experiencing on one of its dryer hoods. The company approached Power
Jacks to come up with the solution.
Despite its apparent limitations for this application, a screw jack
remained the best solution, so a little lateral thinking was required. To
ensure that the cover moved precisely into position, Power Jacks designed
a special coil spring load limiter for the end of the jack's lifting
screw. The load limiter consists of a helical coil wire spring with
squared ends, held in place between two mounting plates, one restraining
and the other a moving plate, bolted to the cover structure.
The spring is preloaded between these two plates so that it will not
compress under normal working load when the cover is not in contact with
its dead stops. When the screw jack drives the cover against a dead stop
the spring compresses over a normal working distance of 10mm. Within this
10mm 'window' a limit switch is positioned to signal the machine control
system to halt the screw jack when a positive stop position is reached.
As the rate of compression of the spring is critical to the operation, it
must be selected to suit specific application requirements. Moreover, for
the device to work correctly the spring assembly must not be allowed to
rotate within its fixture. The screw jack was therefore fitted with a
keyed lifting screw to prevent rotation.
Machine safety was of prime concern to Spooner and two extra safety
features were included on the screw jack. These included a rotation
monitor to detect if a jamming condition occurs, and a secondary holding
device for the lifting screw. The monitor is essentially a proximity
sensor that creates a train of pulses from a target ring rotating with
the gear wheel in the screw jack. The machine control system compares
pulse rates to determine a moving or stopped condition.
The secondary holding device was provided by fitting a safety nut in
series with the worm gear. This safety nut is not normally in contact
with the lifting screw threads and is only engaged in the unlikely event
that the trapezoidal screw thread on the gear wheel fails. Prior to
installation, the screw jack's lifting screw was fitted with a bellows
boot cover, while a special flange mount Neeter Drive bevel gearbox was
used to connect the drive system to the screw jack at right angles.
Spooner was very satisfied with the installation and has now used several
variants and models of the design up to 100kN screw jack capacity rating.