Hydraulic Slew Actuator Boosts Productivity

DN Staff

October 18, 2010

6 Min Read
Hydraulic Slew Actuator Boosts Productivity

Hydraulic rotary actuators, each weighing over 25 tons and among the largest ever built by Parker Hannifin, accurately position and lock in place discharge booms on cargo ships to facilitate quick and safe removal of cargo. Parker worked with Canadian-based EMS-Tech, a designer and supplier of bulk material handling and storage systems, to help develop an integrated self-unloading system for a fleet of new forebody vessels.

"The size of the actuator is unique along with the idea that it is being used, even in a marine application, to enhance productivity. The conveyor that the actuator is used on converts the ship to self-unloaders," says Woodie Francis, product marketing manager for Hydraulic Actuators at Parker Hannifin. "It took a lot of manpower to unload these ships in the past, but the newer systems create a fully automated operation. You think of factory automation but it's interesting to see how mechanical devices can assist customers in automating other aspects of life."

Conveyor and boom systems offer an efficient method of unloading materials from a bulk carrier, minimizing turnaround times and saving port, crane and labor 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 maneuvered 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.

"The housing on the actuator forms a part of the structure of the ship, and it is actually the part of the ship that carries the weight of the boom," says Francis. "The boom with the conveyor on it can weigh up to 200 tons. Plus with the loads generated by the boom swinging off the ship, wind loads and the list of the ship become very significant. The actuator housing is providing the support structure for those loads, as well as rotating the boom and holding it against those loads."

All of the controls are hydraulic. Some hydraulic systems are supplied as self-contained units including the valves, the reservoir and pumps. But in this application, the actuator, hydraulic power unit and controls are all part of the ship itself. The power unit was built into the ship's structure.

Prior to the introduction of Parker's hydraulic slew actuators, wire rope winches positioned the boom. But the capabilities of these systems were limited in terms of positional accuracy and load-carrying capacity. The latest generation of self-unloaders luff and slew the boom using hydraulic actuators for a more accurate, reliable and efficient method of control.

EMS-Tech was contracted by CSL International to design and manufacture efficient and reliable self-unloading systems as part of a fleet renewal program to optimize productivity levels. To accelerate the program's timeline and reduce costs, CSL International combined the aft end of existing single-hull tankers that were being phased out 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 a hydraulic rotary actuator large and powerful enough to position the loaded discharge boom on board a fleet of bulk carriers," says John Elder, vice president, Marine Systems at EMS-Tech.

When designing the actuator, Parker had to consider a wide range of system parameters including the operating environment, slewing speed, and duty cycle, as well as 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 and needs to be carried out in tough environmental conditions.

"Most of the actuators used in marine applications are controlled by marine certification agencies like the American Bureau of Ships or Lloyd's of London," says Francis. "The calculations and material certifications have to be presented to these agencies for review.

If the actuators are being made for the first time, there are aspects of manufacturing which require witness review by these agencies as the parts are being made. Comparable to aerospace or NASA, there are agencies which oversee various vital aspects of the design.

Following extensive coordination between hydraulics specialists and EMS-Tech engineers, the team developed a design to meet these demanding requirements. The solution took the form of a 25-ton, rack-and-pinion hydraulic rotary actuator, coated with rubberized epoxy paint at 175-micron dry film thickness to protect the system from corrosion in the hostile working environment.

The purpose-built system is able to control the boom accurately and reliably, providing 4.3 million Nm of holding torque and 3.5 million Nm of slewing torque at 275 bar. The finished system is capable of discharging a variety of materials, including coal, stone, grain, ore, alumina and limestone rocks at a rate of 5,000 tons per hour.

The unique actuator features a pinion gear bore that fits over a kingpin 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 12 degrees per minute. To illustrate the scale of the cylinders, approximately 450l of oil are displaced for every 190 degrees of rotation.

The boom is attached to the actuator by two trunnion pins that project from the sides of the actuator housing. 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 centerline 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.

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