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Custom hydraulic rotary actuators provide 4.3MNm holding torque

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Parker Hannifin has developed custom 25-ton Parker hydraulic rotary actuators for positioning the discharge boom on ships carrying bulk cargo.

Parker Hannifin has worked with EMS-Tech, a designer and supplier of bulk material handling and storage systems, to help the company to develop an integrated unloading system for a fleet of new forebody vessels. 25-ton Parker hydraulic rotary actuators, among the largest the company has ever built, enable the discharge boom on board the ships to be positioned accurately and held in place, allowing cargo to be deposited on shore quickly and safely.

Conveyor and boom systems offer an efficient method of unloading materials from bulk carriers, minimising turnaround times and saving port, crane, and labour costs. Typically a conveyor system carries dry bulk cargo from the ship's hold and elevates it onto a deck-mounted boom conveyor. The end of the boom is then manoeuvred to deposit the cargo into a receiving hopper or onto a stockpile located on shore. Equipped with a self-unloading system, a vessel can discharge its cargo almost anywhere: into an adjacent vessel, on-shore hopper, stockpile, or even onto another conveyor.

Prior to the introduction of the hydraulic slew actuator, wire rope winches were used to position the boom; however, these systems were limited in terms of the levels of positional accuracy and load carrying capacity. To provide a more accurate, reliable and efficient method of control, the latest generation of self-unloaders luff and slew the boom using hydraulic actuators.

Enhanced productivity for minimal cost

When looking for a hydraulic rotary actuator large and powerful enough to position the loaded discharge boom on board a fleet of bulk carriers, EMS-Tech approached Parker. John Elder, Vice President of Marine Systems at EMS-Tech, explains: "Our client, CSL International, was looking to complete a huge fleet renewal programme of its self unloaders. As well as requiring a more efficient and reliable unloading system that would enable them to optimise productivity levels, CSL needed to carry out the project in the shortest time and at the minimum cost. To do this, it was decided that the aft end of existing single-skin tankers would be combined with new forebodies comprising fully integrated self-unloading systems.

"When it came to the design and supply of the boom slew system, we turned to Parker, having had a successful relationship with them for 30 years. Parker was the only company we could think of with the technology to meet the demands of the application and the ability to meet our deadlines."

When designing the actuator for this challenging application, Parker had to consider a wide range of system parameters, including the operating environment, slewing speed, and duty cycle, in addition to factors such as the ship list angle and wind load on the boom, which could greatly influence the torque needed to position the boom effectively. In addition to providing the torque required to slew the discharge boom, the actuator serves to support this very large boom structure and all imposed loads, which can be well in excess of 500 tons; all of this needs to be carried out in harsh environmental conditions.

Following extensive co-ordination between Parker's hydraulics specialists and EMS-Tech's engineers, a design was developed that could meet these demanding requirements. The Parker design took the form of a 25-ton rack-and-pinion hydraulic rotary actuator, coated with a rubberised epoxy paint to protect the system from corrosion. The system is able to control the boom accurately and reliably, providing 4.3MNm of holding torque and 3.5MNm of slewing torque at 275bar. The finished system is capable of discharging a variety of materials, including coal, stone, grain, ore, alumina, and limestone rocks at a rate of 5000 tons per hour.

Construction

The unique actuator features a pinion gear bore that fits over a king pin tied into the structure of the ship. The gear is fixed so that the housing rotates as pressure is applied to the cylinders, causing the actuator to turn at 12deg/minute. To give an idea of the scale of the cylinders, approximately 450 litres of oil are displaced for every 190deg of rotation.

Two trunnion pins project from the sides of the actuator housing to attach the boom to the actuator. These trunnion pins carry the weight of the boom, in addition to transferring the torque from the actuator into the boom to rotate it off the centreline of the ship and into the required position. The actuator is then able to hold the boom steadily against the torque generated by the list of the ship, as well as the wind loads trying to rotate the boom further.

A separate luffing cylinder and a rigid luffing linkage assembly are used to control the boom vertically. This arrangement allows the boom to be positioned along the centreline of the vessel in a storage saddle when it is not in use, and, likewise, to be lifted out of the storage position when it is required.

With the bespoke rotary actuator assembled and connected to a purpose-built hydraulic power unit, complete with directional valves, holding valves, and filtration equipment, Parker was able to help EMS-Tech meet its client's requirements simply, quickly and cost-effectively. John Elder comments: "Parker's support was integral to the success of the renewal programme. The quality and efficiency of the finished solution makes us confident that we made the right decision to work with Parker on the project."

 
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