In order to continuously maintain a road bridge workmen need access to the complete span of the bridge for inspection and routine maintenance. For work under and on the side of the bridge access needs to be available without the need to erect a special structure for each maintenance operation. One such bridge is the Tay road bridge, which spans the Firth of Tay from Dundee to Newport-on-Tay providing access from the county Angus to Fife. For the Tay road bridge the solution of maintenance access came in the form of two motorised
platforms on the bridge (one each side) that could traverse the bridges span.
Each platform is designed in two sections. The upper section is connected to the bridge and houses the drive mechanism for moving it across the bridge. The lower section is designed to provide access to all areas by being rotated, raised and lowered in relation to the upper section.
On one end of the lower section is an individual work platform on a scissor lift to complete the positioning possibilities for work access.
When designing the platform one of the major problems which had to be solved was how to provide the main lift for raising and lowering the lower work platform. The lift mechanism had to provide a safe and reliable working environment for personnel that would achieve suitable performance while operating in a hostile environment. The solution was to use a Spiracon Roller Screw Jack.
The Spiracon Roller Screw Jack used is an inverted rotating screw configuration with drive motor and auxiliary brake. The gearbox is mounted on the upper platform with the roller nut attached to the lower platform. The screw jack gearbox is a 500kN E-Series single face unit with a 32:1 gear ratio coupled to a 80mm diameter roller screw with a 30mm lead. The screw jack design included the following special features:
• Fail-safe disc brake integrated into the screw jacks shell cap so that it is directly coupled to the roller screw. This provides direct braking control of the roller screw assembly.
• Special motor adapter for a hydraulic motor drive to the worm shaft. The motor is driven by a common hydraulic power pack on the platform.
• Upgraded sealing to protect the unit from the aggressive salt-water atmosphere. This includes bellows boot protection for the roller screw above and below the roller nut.
• Mechanical stop on the end of the roller screw to prevent over travel in emergency conditions. This mechanical stop incorporates a special integral bearing element to prevent the bellows boot located below the roller nut from being rotated and tangled when in operation.
As with all actuator and screw jack applications side loads on the lifting screw or ram are to be avoided at all costs. In this case guide rails were installed either side of the roller screw for its entire travel in order to protect it from side loads.
The roller screw jack was specified for its ability to provide high load rating, good efficiency, reliable operation and robust design. In comparison to hydraulic systems it is able to provide self-locking in any vertical position in the event of power loss and does not suffer from the risk of pressure loss a hydraulic cylinder could incur. Other screw jack types were not selected because:
• Machine screw jack was not suitable due to poor efficiency hence higher power input required, which was not available.
• Ball screw jacks were not suitable due to a ball screw and nut assemblies’ higher risk of failure due to ball track or ball bearing damage or failure due to the harsh environmental conditions.
Since installation the roller screw jacks have worked successfully on the bridges maintenance platform.
About Power Jacks
Power Jacks is a leading manufacturer for precision linear actuation, power transmission and mechanical jacking. The company is based in Scotland where its history for supplying high quality engineered product dates from 1903. The product ranges are detailed on the website www.powerjacks.com and include screw jacks, electric linear actuators, mechanical jacks, bevel gearboxes, planetary roller screws, reduction gearboxes, winches and rotary unions. They can be supplied worldwide singularly or as complete systems, whether in standard or specially engineered designs.
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