Ram-Type Steering Gear

What sets ram-type gear apart from rotary vane

Rotary vane steering gear is compact and needs no tiller, but the vane seals set a practical ceiling on torque and pressure. Ram-type gear moves the rudder stock through a tiller arm connected to one, two or four hydraulic rams, so torque is increased simply by using longer rams, a longer tiller arm or higher system pressure, without asking a rotating seal to hold that pressure. This is why almost every large tanker, bulk carrier and container ship above roughly 15,000 to 20,000 dwt uses a two-ram or four-ram gear rather than a vane unit.

Main components

Rams and tiller

Each ram is a double-acting hydraulic cylinder pinned at one end to the ship's structure and at the other to the tiller, which is keyed or shrunk onto the rudder stock. Four-ram arrangements give full torque even if one hydraulic circuit is lost, satisfying the duplication requirement for the main steering gear.

Hydraulic power units

Two independent power units, each with its own pump, are the norm. Variable-displacement axial piston pumps are common because they allow the rudder angle to be controlled by varying pump swash plate angle rather than throttling flow, which keeps losses low across the full angle range.

Rudder angle feedback and buffer system

A mechanical or electronic feedback linkage from the tiller reports actual rudder angle back to the control system, and hydraulic relief or buffer stops absorb the shock load if the rudder strikes ice, a mooring line or a grounding obstruction before the mechanical stops are reached.

Selection and sizing

The gear is sized to the rudder torque calculated from rudder area, ship speed and the required hardover time, not chosen off a general capacity table:

• Maximum design torque, which must cover the worst case of full rudder angle at maximum service speed astern as well as ahead.
• Hardover time from 35 degrees one side to 30 degrees the other, which class rules require within 28 seconds with the main steering gear in operation.
• Working pressure of the hydraulic circuit, typically in the 150 to 300 bar range depending on manufacturer and ram bore.
• Number of rams and power units, driven by the duplication requirement rather than torque alone.

Regulations and class

SOLAS Chapter II-1 requires a main steering gear capable of the 35/30 degree hardover in 28 seconds and an auxiliary steering gear able to move the rudder from 15 degrees on one side to 15 degrees on the other within 60 seconds, either as a separate system or as one of the two power units of the main gear used alone. IACS Unified Requirements set the structural design load cases for the rams, tiller and rudder stock coupling. Class surveys include an annual function test of both power units and a full stroke test of the rudder from hardover to hardover, witnessed and logged.

Typical faults

What to look for in a supplier

• Genuine or class-approved replacement seals matched to the ram bore and rod finish, since generic seal kits often leak within months on high-pressure gear.
• Experience with the specific manufacturer, as Rolls-Royce, Kawasaki and other steering gear specialists differ enough in valve block design that a generalist hydraulic shop can misdiagnose a fault.
• Ability to supply and pressure test replacement rams or tiller assemblies against the original class-approved drawing, not a look-alike part.
• On-board commissioning support for the annual steering gear trial, since a rushed test after repair can miss a fault that only shows up at full rudder angle.

Log the actual hardover time at every steering gear trial, not just a pass or fail; a gear that still meets 28 seconds but has crept up from 20 over two years is telling you a pump or ram is wearing before it fails outright.