Load limiter adds precision to screw jack
A
Power Jacks product story
Edited by the Engineeringtalk editorial team
Mar 7, 2003
Conventional screw jacks are best suited to lowering applications, but Spooner industries wanted to use the technology push a cover into position on one of its dryer hoods.
When an industrial machine needs to move a cover or lid onto a dead stop or sealing face it must do so precisely and positively with contact 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 actuators 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.
This was the problem that Spooner Industries asked Power Jacks to solve for one of its dryer hoods.
For this particular application a screw jack was the best type of solution.
To push the cover into position precisely Power Jacks designed a special coil spring load limiter for the end of the jacks lifting screw.
The load limiter consists of a helical coil wire spring with squared ends held in place between two mounting plates.
One plate is a restraining plate and the other a moving plate that is bolted to the cover structure.
The spring is preloaded between these two plates so that the spring 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 machines control system to stop the screw jack as a positive stop position is reached.
As the rate of compression of the spring is critical to the operation each spring is designed for each applications specific requirements.
Also for the device to work correctly the spring assembly could not be allowed to rotate in its fixture.
The screw jack was therefore fitted with a keyed lifting screw to prevent rotation.
Safety on the machine was of prime concern and two extra safety features were included on the screw jack.
The first of these is a rotation monitor to detect if a jamming condition occurs.
This monitor consisted of a proximity sensor creating a train of pulses from a target ring that rotates with the gear wheel in the screw jack.
The machines control system can then compare pulse rates to determine a moving or stopped condition.
A secondary holding device for the lifting screw 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.
To complete the unit for system installation the screw jacks lifting screw was fitted with a bellows boot cover and a special flange mount Neeter Drive bevel gearbox was used to connect the drive system to the screw jack at right angles.
Since installation the unit has worked successfully for Spooner Industries which has now used several variants and models of the design up to 100kN screw jack capacity rating.
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