Section 5 Design and construction

5.1 General

5.1.1 Rudder actuators are to be designed in accordance with the relevant requirements of Vol 2, Pt 8, Ch 2 Other Pressure Vessels for Class I pressure vessels (notwithstanding any exemptions for hydraulic cylinders).

5.1.2 Accumulators, if fitted, are to comply with the relevant requirements of Vol 2, Pt 8, Ch 2 Other Pressure Vessels.

5.1.3 The welding details and welding procedures are to be approved. All welded joints within the pressure boundary of a rudder actuator or connecting parts transmitting mechanical loads are to be full penetration type or of equivalent strength.

5.1.4 The construction is to be such as to minimise local concentrations of stress.

5.1.5 The design pressure for calculations to determine the scantlings of piping and other steering gear components subjected to internal hydraulic pressure shall be at least 1,25 times the maximum working pressure to be expected under the operational conditions specified in Vol 2, Pt 6, Ch 1, 4.2 Performance requirements for rudder-type steering systems 4.2.1.(a), taking into account any pressure which may exist in the low pressure side of the system. Fatigue criteria may be applied for the design of piping and components, taking into account pulsating pressures due to dynamic loads.

5.1.6 For the rudder actuator, the permissible primary general membrane stress is not to exceed the lower of the following values: or

where

σ_B

specified minimum tensile strength of material at ambient temperature

σ_y

specified minimum yield stress or 0,2 per cent proof stress of the material, at ambient temperature

A and B are given by the following Table:

	Wrought steel	Cast steel	Nodular cast iron
A	3,5	4	5
B	1,7	2	3

5.2 Rudder, rudder stock, tiller and quadrant

5.2.1 For the requirements of rudder and rudder stock, see Vol 1, Pt 3, Ch 3, 2 Rudders.

5.2.2 For the requirements of tillers and quadrants including the tiller to stock connection, see Table 1.5.1 Connection of tiller to stock.

Table 1.5.1 Connection of tiller to stock

Item

Requirements

(1)

Dry fit – tiller to stock

(a) For keyed connection, factor of safety against slippage, S = 1,0 The maximum stress in the fillet radius of the tiller keyway should not exceed the yield stress

(b) For keyless connection, factor of safety against slippage, S = 2,0 The maximum equivalent von Mises stress should not exceed the yield stress

For conical sections, the cone taper should be ≤1:15

(d) Grip stress not to be less than 20 N/mm²

(2)

Hydraulic fit - tiller to stock

(a) For keyed connection, factor of safety against slippage, S = 1,0 The maximum stress in the fillet radius of the tiller keyway should not exceed the yield stress

For conical sections, the cone taper should be ≤1:10

(b) For keyless connection, factor of safety against slippage, S = 2,0 The maximum equivalent von Mises stress should not exceed the yield stress

For conical sections, the cone taper should be ≤1:15

(d) Grip stress not to be less than 20 N/mm²

(3)

Ring locking assemblies fit – tiller to stock

(a) Factor of safety against slippage, S = 2,0
The maximum equivalent von Mises stress should not exceed the yield stress

(b) Coefficient of friction = 0,12

(4)

Bolted tiller and quadrant

(this arrangement could be accepted provided the proposed rudder stock diameter in way of tiller does not exceed 350 mm diameter) (see Symbols)

Shim to be fitted between two halves before machining to take rudder stock, then removed prior to fitting

Minimum thickness of shim,

For 4 connecting bolts: t _s = 0,0014 δ_su mm

For 6 connecting bolts: t _s = 0,0012 δ_su mm

Key(s) to be fitted

Diameter of bolts,

A predetermined setting-up load equivalent to a stress of approximately 0,7 of the yield strength of the bolt material should be applied to each bolt on assembly. A lower stress may be accepted provided that two keys, complying with item (5), are fitted.

Distance from centre of stock to centre of bolts should generally be equal to

Thickness of flange on each half of the bolted tiller

(5)

Key/keyway

(see Symbols)

Effective sectional area of key in shear ≥0,25 δ_su ² mm²

Key thickness ≤ 0,17 δ_su mm

Keyway is to extend over full depth of tiller and is to have a rounded end. Keyway root fillets are to be provided with suitable radii to avoid high local stress

(6)

Section modulus – tiller arm (at any point within its length about vertical axis)
(see Symbols)

To be not less than the greater of:

(a)

(b)

If more than one arm is fitted, combined modulus is to be not less than the greater of (a) or (b)

For solid tillers, the breadth to depth ratio is not to exceed 2

(7)

Boss

(see Symbols)

Depth of boss ≥ δ_su

Thickness of boss in way of tiller ≥ 0,4δ_su

Symbols

b _s

distance between the section of the tiller arm under consideration and the centre of the rudder stock, in mm

NOTE: b _T and b _s are to be measured with zero rudder angle

b _T

distance from the point of application of the load on the tiller to the centre of the rudder stock, in mm

n _T

number of bolts in the connection flanges, but generally not to be taken greater than six

t _s

thickness of shim for machining bolted tillers and quadrants, in mm

Z _TA

section modulus of tiller arm, in cm³

δ_su

Rule rudderstock diameter in way of tiller (see Vol 1, Pt 3, Ch 3 Ship Control Systems)

δ_T

diameter of bolts securing bolted tillers and quadrants, in mm

5.2.3 The factor of safety against slippage, S (i.e. for torque transmission by friction) is generally based on

where M is the maximum torque at the relief valve pressure which is generally equal to the design torque as specified by the steering gear manufacturer.

5.2.4 For conical sections, S is based on the following equation:

where

interfacial surface area, in mm²

weight of rudder and stock, if applicable, when tending to separate the fit, in N

Q	=	shear force = in N
Q	=	where d _m in mm is the mean contact diameter of tiller/stock interface and M, in Nmm is defined in Vol 2, Pt 6, Ch 1, 5.2 Rudder, rudder stock, tiller and quadrant 5.2.3

cone taper half angle in radians (e.g. for cone taper 1:10, θ = 0,05)

coefficient of friction

σ_r

radial interfacial pressure or grip stress, in N/mm².

5.3 Components

5.3.1 Special consideration is to be given to the suitability of any component necessary for the operation of the steering gear which is not duplicated. Any such component shall, where appropriate, utilise anti-friction bearings such as ball bearings, roller bearings or sleeve bearings which shall be permanently lubricated or provided with lubrication fittings.

5.3.2 All steering gear components transmitting mechanical forces to the rudder stock, which are not protected against overload by structural rudder stops or mechanical buffers, are to have a strength at least equivalent to that of the rudder stock in way of the tiller.

5.3.3 Actuator oil seals between non-moving parts, forming part of the external pressure boundary, are to be of the metal upon metal type or of an equivalent type.

5.3.4 Actuator oil seals between moving parts, forming part of the external pressure boundary, are to be duplicated, so that the failure of one seal does not render the actuator inoperative. Alternative arrangements providing equivalent protection against leakage may be accepted.

5.3.5 Piping, joints, valves, flanges and other fittings are to comply within the requirements of Vol 2, Pt 7, Ch 1 Piping Design Requirements for Class I piping systems components. The design pressure is to be in accordance with Vol 2, Pt 6, Ch 1, 5.1 General 5.1.5

5.3.6 Hydraulic power operated steering gear is to be provided with the following:

Arrangements to maintain the cleanliness of the hydraulic fluid, taking into consideration the type and design of the hydraulic system.
A fixed storage tank having sufficient capacity to recharge at least one power actuating system including the reservoir, where the main steering gear is required to be power operated. The storage tank is to be permanently connected by piping in such a manner that the hydraulic systems can be readily recharged from a position within the steering gear compartment and provided with a contents gauge.

5.4 Valve and relief valve arrangements

5.4.1 For vessels with non-duplicated actuators, isolating valves are to be fitted at the connection of pipes to the actuator, and are to be directly fitted on the actuator.

5.4.2 Arrangements for bleeding air from the hydraulic system are to be provided, where necessary.

5.4.3 Relief valves are to be fitted to any part of the hydraulic system which can be isolated and in which pressure can be generated from the power source or from external forces. The settings of the relief valves is not to exceed the design pressure. The valves are to be of adequate size and so arranged as to avoid an undue rise in pressure above the design pressure.

5.4.4 Relief valves for protecting any part of the hydraulic system which can be isolated, as required by Vol 2, Pt 6, Ch 1, 5.4 Valve and relief valve arrangements 5.4.3, are to comply with the following:

The setting pressure is not to be less than 1,25 times the maximum working pressure.
the minimum discharge capacity of the relief valve(s) is not to be less than 110 per cent of the total capacity of the pumps which can deliver through it (them). Under such conditions the rise in pressure is not to exceed 10 per cent of the setting pressure. In this regard, due consideration is to be given to extreme foreseen ambient conditions in respect of oil viscosity.

5.5 Flexible hoses

5.5.1 Hose assemblies approved by Clasifications Register LR may be installed between two points where flexibility is required but are not to be subjected to torsional deflection (twisting) under normal operating conditions. In general, the hose should be limited to the length necessary to provide for flexibility and for proper operation of machinery, see also Vol 2, Pt 7, Ch 1, 13 Flexible hoses.

5.5.2 Hoses should be high pressure hydraulic hoses according to recognised Standards and suitable for the fluids, pressures, temperatures and ambient conditions in question.

5.5.3 Burst pressure of hoses is to be not less than four times the design pressure.

5.6 Noise and vibration

5.6.1 The reduction of airborne noise, and the structural vibration caused by the steering equipment, is to be regarded as an essential part of the design. The noise and vibration acceptance levels are normally specified for each ship. The techniques to be employed to achieve this are:

Reduction of noise and vibration at source.
Control of noise and vibration transmission paths by use of vibration mounting systems, structural damping, hydraulic silencers and flexible pipes.

5.6.2 All possible noise and vibration transmission paths are to be considered to eliminate noise shorts.