Load limiter ensures precise positioning

Paul Boughton

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.

For an application such as a paper dryer hood, a screw jack may well be the best type of actuator. To push the cover into position precisely, Power Jacks has designed a special coil spring load limiter for the end of the jack's lifting screw. To complete the unit, the screw jack's lifting screw is fitted with a bellows boot cover, and a Neeter Drive ISK bevel gearbox connects the drive system to the screw jack at right angles.

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 to the machine's 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 application's specific requirements. Also, for the device to work correctly, the spring assembly must not be allowed to rotate in its fixture. The screw jack is therefore fitted with a keyed lifting screw to prevent rotation.

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.

Safety on the machine is of prime concern and two extra safety features are included on the screw jack. First, a rotation monitor detects if a jamming condition occurs. This monitor consists of a proximity sensor creating a train of pulses from a target ring that rotates with the gear wheel in the screw jack. The machine's control system can then compare pulse rates to determine a moving or stopped condition.

Second, a holding device for the lifting screw is 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 event that the trapezoidal screw thread on the gear wheel is operated beyond its normal life.

The original 50kN designed units have worked successfully since installation and have been used in several sites. Load limiters have since been designed for screw jacks based on 25kN, 50kN and 100kN units for both tensile and compressive loads.

For more information, visit www.powerjacks.com

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