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The MV Kigoriak, a 90-m Arctic Class 4 icebreaker, makes use of a unique electrohydraulic steering system that minimizes the effects of its rudder impacting hard ice. |
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Fig.1. Icebreaker's steering system features two AutoNav-built 6-in. bore, 30-in. stroke cylinders, which rotate the tiller to 45° each side. |
AutoNav Marine Systems, Inc., of Coquitlam, B.C., specializes in designing steering systems for ships as long as 800 ft — something they've been doing for more than 60 years. Their new Type P electrohydraulic steering system uses a pair of double-acting cylinders coupled to one or more tillers to develop the torque necessary to move the ship's rudder. Engine-driven or electrically driven pumpsets provide the hydraulic flow and pressure to move the rudder and hold it against rudder loads. A range of control systems generates the steering commands and monitors the system operation.
Lower pressures = fewer problems
The U.S. Coast Guard conducted a survey of hydraulic steering failures and determined that systems designed and operated below 2000 psi have more than 8 times the reliability of those over 2000 psi. As system cleanliness on ships rarely approaches aerospace standards, contamination is a major reason for failure. Thus, AutoNav's manual hydraulic systems used on smaller vessels have a nominal working pressure of 1000 psi — the maximum a person can develop with reasonable efficiency using axial piston pumps. Power steering systems use a nominal working pressure of 1450 psi. This value was chosen as a compromise between high-pressure systems, which use expensive components that require tight filtration, and low-pressure systems, which are less expensive and more reliable, but can be bulky and heavy.
Custom-designed components
Tillers are the bell crank and hub that connect steering cylinders to the rudder shaft, Figure 1. A variety of regulations sets criteria for their dimensions. The tiller sizes are related to developed torque, and their dimensions form the basis for selection of bores and strokes. Paul Wagner, AutoNav's executive chairman and lead designer, explains that the company's design emphasizes the use of the largest bore with the shortest stroke that will not cause mechanical interference between the oscillating cylinders and the tiller boss or arms. This results in the most compact actuator and the strongest piston rod (to resist buckling from excessive loads).
AutoNav designs and manufactures their own cylinders with bores up to 3-in. They focus on preventing corrosion: bronze and brass cylinders and trunnions and stainless steel piston rods. Self-aligning and self-lubricating cylinder mounts are also used to eliminate the need for lubrication and allow less critical installation alignment. Teflon-based hydraulic seals minimize friction — critical in the manually operated systems. Larger cylinders are built from steel, because larger vessels have relatively dry steering compartments. The smaller range of this series also uses stainless steel piston rods for corrosion resistance of the most critical exposed working part.
International steering gear regulations require stops to prevent the steering cylinders from bottoming out at extreme strokes and possibly becoming damaged. The use of standard commercial cylinders requires that stops be designed, fabricated, and installed to impact on the tiller before the cylinder bottoms. This is not only expensive, but wastes valuable space and results in an unbalanced stopping couple — which causes higher stresses in rudder stocks and their support bearings. AutoNav developed a design using adjustable stop collars on the piston rods, which can be adjusted on installation, so that both cylinders share stopping forces equally with no side loads.
AutoNav specially designs and manufactures manual helm pumps, a multi-piston, swash-plate pump. The pump shaft is directly rotated by the steering wheel. The helmsman directly develops the hydraulic flow and pressure, which may be required to move the rudder actuator. Commercial piston pumps run at high speed and require relatively loose working clearances to avoid seizing. If used as a manual helm pump driven directly by the steering wheel, the resulting internal slippage would allow the wheel to turn without delivering much flow. AutoNav builds their pumps with extremely close tolerances, so internal slippage is minimal at normal wheel rotational speeds of around 1 rev/sec or less. A reservoir is incorporated into the helm pump for fluid expansion and make-up.
Valves on ice
All steering gears require a crossport relief valve or equivalent to relieve excessive force in case of rudder overload. On icebreakers, such as the MV Kigoriak (pictured on top) the rudder impacts on heavy ice, and the rate of impact can be too high for the response time of a typical relief valve. Under these conditions, pressure will rise too quickly, and mechanical damage will occur to the steering.
AutoNav's fast-acting relief valve (FARV) has been designed to fully open within 10 msec and dump the cylinder oil in 0.1 sec. On large icebreakers, this can result in instantaneous flows of over 20,000 gpm. According to Wagner, "Not only must the valve operate very quickly, but the flow paths must be specially designed or the oil cannot escape once relieved."
The FARV also includes a specially designed receiver tank, which absorbs the discharge energy and refills the steering cylinder once the impact is over. Prior technology used a rupture disc, which had serious-limitations. The disc could allow debris to enter the system. When the disc would rupture, an engineer had to enter the steering compartment, remove the disc, and install a new one. In the time to do this, the vessel had no steering, and could become frozen into the ice.
Another option: rotary actuators
An option often chosen for military vessels achieves rudder movement via a direct rotating actuator. This actuator provides higher efficiency, greater compactness, improved strength, and reduced maintenance.
Some rotary actuators use a rackand-pinion or other type of lineartorotational translation linkage. These typically have a load-holding problem, due to inherently high internal leakage (slippage). The higher the operating pressure, the worse the problem. European technology has used low-pressure systems to minimize this problem. Unfortunately, low pressures result in bulky systems and large piping sizes. In addition, the sealing techniques are not very effective or long lasting. After hundreds of sealing tests, AutoNav developed a seal machined from UHMW polyurethane, which has extreme wear resistance, low friction, low creep, and long life. Users have reported more than 40,000 hours of service with seals still looking like new. Interestingly, the seals actually improve over the first several thousand hours, as they burnish into the working surfaces. Rack-and-pinion actuators also require more space than a rotary vane, have lower efficiency (due to gears), and develop excessive wear around the midships or rudder center position where the steering works about 90% of the time. This slack build-up degrades steering accuracy.
Most industrial rotary actuators can only develop torque and cannot withstand large radial and axial loads. A ship steering system must resist radial loads for the rudder reaction, as well as axial loads due to carrying the rudder and rudder shaft weight. Wagner builds integral selflubricating bearings into the housing to accept these loads. Pure rotation and low friction bearings result in a measured mechanical efficiency in excess of 95%. "The synthetic bearing liners are of such high efficiency, low wear, and high bearing load capability, that not one has worn out in over 20 years of service," he claims.
The AutoNav actuator has recently found acceptance for use on frigates and other large naval vessels for several reasons:
Conclusion
AutoNav has developed steering systems that succeed for several reasons: