Coil spring load limiter gives precise movements

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.

Problem: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, since 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. But this method cannot be used to push a cover into position. This was the very same problem that UK-based firm Spooner Industries asked precision screw jack manufacturer Power Jacks to solve for one of its dryer hoods.

Solution: To push the cover into position precisely, 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 plate is a restraining plate and the other a moving plate that is bolted to the cover structure. The spring is pre-loaded 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 machine’s control system to stop the screw jack as the positive stop position is reached. Since the rate of compression of the spring is critical to the operation, each spring is designed for each application’s specific requirements. And, for the device to work correctly, the spring assembly cannot 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 also important and so a rotation monitor was incorporated into the design, to detect when jamming conditions occur. This monitor consists of a proximity sensor which creates 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.

And an extra 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 jack’s lifting screw was fitted with a bellows boot cover and a special flange mount ‘Neeter’ drive bevel gearbox, which was used to connect the drive system to the screw jack at right angles.

Applications: Any application where a machine designer or machine tool builder needs a cover or lid to move precisely into position, such as onto a dead stop or sealing face. Spooner has already used several variants of the screw jack design, up to 100kN capacity rating.

Power Jacks

Tom Shelley

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