Section 5 Hydraulic cylinders

5.1.1 This Section applies to hydraulic cylinders fitted to lifting or life-saving appliances. A typical example is shown in Figure 9.5.1 Hydraulic cylinder (simplified).

5.1.2 For the purposes of certification or classification of lifting appliances, hydraulic cylinders will be considered for structural adequacy including the effects of hydraulic pressure. Although they are not considered to be pressure vessels, they will nevertheless be required to undergo a pressure load test at the manufacturer’s works. See Ch 9, 5.9 Testing for testing requirements.

5.1.3 Hydraulic cylinders with special features, such as telescopic types (including versions with intermediate supports), self-locking types, with trunnion mounts, with unconventional or no eye plates at one or both ends, etc. shall, as far as possible, be designed in accordance with the requirements outlined in this Section. In such cases, design calculations covering those special features will be required to be submitted.

5.1.4 Where hydraulic cylinders are used in applications covered by specialist Chapters in this Code, requirements given in those Chapters relating to hydraulic cylinders shall also be taken into consideration as applicable.

Figure 9.5.1 Hydraulic cylinder (simplified)

5.2.1 Design pressure

The design pressure, P_D is the maximum internal pressure for which the cylinder has been designed. The design pressure is not to be less than the highest set pressure of any relief valves in the hydraulic system and/or relief valves mounted directly on the hydraulic cylinder ports.

5.2.2 Working pressure

The maximum working pressure, P_w, is the maximum dynamic internal pressure that the cylinder may be subject to in operation which includes internal pressures generated by external forces e.g. due to the SWL enhanced by the hoisting factor, accelerations etc., but excludes short duration pressure peaks (See Ch 9, 5.3 General design requirements 5.3.6). The maximum working pressure is not to exceed the design pressure, P_D.

5.2.3 Relief valve pressure setting

The relief valve setting is the pressure at which the relief valve is set to activate.

5.3.1 Welded cylinder pipes should not generally be used for hydraulic cylinder applications. Proposals to use welded cylinder pipes will be specially considered and need to be agreed with LR. The weld procedure including any heat treatment and non-destructive examination shall be submitted to LR for appraisal.

5.3.2 The cylinder head (or gland) shall have adequate length and capability to guide the piston rod under load including the self-weight of the hydraulic cylinder. The seals are to be adequate to prevent hydraulic leakage, and water and dirt entering the cylinder.

5.3.3 The piston rod shall be suitably finished and coated to ensure adequate sealing capability and corrosion protection.

5.3.4 The eye plates shall be equipped, where possible, with spherical bearings to prevent additional end bending moments being introduced. The design shall not allow free movement of the eye plate between the supporting plates in the direction of the pin. Alternative arrangements will be specially considered.

5.3.5 The relief valve pressure setting shall not exceed the design pressure. This setting may be temporarily increased to allow the crane to be proof load tested, but is to be reset for normal operations. See Ch 9, 5.9 Testing 5.9.5. To prevent unnecessary activation of the relief valve, it is desirable that there should be a margin between the normal pressure at which the hydraulic cylinder operates and the lowest pressure at which any relief valve is set to lift. The relief valve setting may be up to 1,25 times the maximum working pressure. Alternative proposals will be specially considered.

5.3.6 Short duration pressure peaks which exceed the maximum working pressure, P_w, typical of certain offshore crane applications, need not be considered in determination of the maximum working pressure providing the pressure peaks are less than 0,94 x Hydraulic Test Pressure, P_T (see Ch 9, 5.9 Testing 5.9.1) and less than or equal to the Design Pressure, P_D.

5.3.7 The certification requirements for hydraulic cylinders as part of certified lifting and life-saving appliances are detailed in Table 13.2.1 Minimum requirements for the certification of lifting appliances. The classification requirements for hydraulic cylinders as part of classed lifting appliances are detailed in Table 13.3.1 Minimum requirements for the classification of lifting appliances.

5.4.1 Hydraulic cylinders fitted to lifting appliances are to be designed to withstand the load combinations as given in the Chapter relevant to the actual application (see Ch 4, 2.15 Load combinations and Ch 4, 3.5 Load combinations for Shipboard and Offshore cranes, Ch 7, 2.5 Load combinations for Lifts). The factored forces (hoisting and duty factor, as applicable), stowage, test load and any exceptional conditions resulting from the actual application shall be considered.

5.4.2 Hydraulic cylinders fitted to life-saving appliances are to be designed to meet the operational and load test conditions required by SOLAS and the IMO LSA Code as applicable.

5.4.3 Depending on the design of the hydraulic cylinder the following calculations and validations are usually required to be carried out as a minimum:

1. The eye plates shall be designed for the following conditions, see Ch 9, 5.8 Proof of strength and stability 5.8.4:
   • local bending, normal and shear-out stress due to pull forces.
   • bearing stress due to pull and push forces.
2. The following threaded, bolted and/or welded connections (as applicable) are to be calculated as per Ch 9, 5.8 Proof of strength and stability 5.8.1, Ch 4, 2.23 Allowable stress – Joints and connections (as applicable) and/or Ch 9, 5.8 Proof of strength and stability 5.8.2 respectively:
   • connection of cylinder or rod eye plates with the cylinder’s end cap or piston rod.
   • connection of piston with piston rod.
   • connection of cylinder head with cylinder tube.
   • connection of cylinder end cap with cylinder tube.
3. The thickness of the end cap can be calculated as detailed in Ch 9, 5.8 Proof of strength and stability 5.8.2.
4. The hoop and longitudinal stress of the hydraulic cylinder wall shall be calculated as detailed in Ch 9, 5.8 Proof of strength and stability 5.8.5.
5. Diametrical expansion of cylinders with large bores possibly affecting the sealing of the piston is to be considered.
6. Bolted connections are to be designed in compliance with Ch 4, 2.23 Allowable stress – Joints and connections. Tapped holes are to be in compliance with Ch 9, 5.8 Proof of strength and stability 5.8.1. The thread-engaging length is to be at least 1,5 times the major bolt diameter.
7. Axial forces on piston rod.
8. External pressure on hollow piston rods shall be taken into consideration as applicable.
9. The pins supporting the eye plates shall be calculated for shear and bending stresses.
10. Trunnion mounts (if fitted) to the cylinder tube.
11. Additional calculations for other components of the hydraulic cylinder may be required depending on the actual design.

5.4.4 For the evaluation of the required design, working and relief valves pressures, it is not required to take the duty factor into account. For all other design calculations, the duty factor is required to be taken into consideration.

5.5.1 The material requirements (including the Charpy V-notch test requirements) are to be as per the Chapters applicable to the appliance the hydraulic cylinder will be used in. For lifting appliances the materials for hydraulic cylinders are to be selected to meet the requirements detailed in Ch 11 Materials and Fabrication and Ch 4, 2.25 Materials. For life-saving appliances the materials for hydraulic cylinders are to be selected to meet the requirements of Ch 11 Materials and Fabrication and Ch 3, 1.11 Materials.

5.5.2 The materials used for hydraulic cylinder applications are to be sufficiently ductile. Materials that have an elongation of less than 14 per cent shall be specially considered by LR.

5.5.3 Pressure retaining parts of carbon-manganese steel grades are to be delivered in the normalised or hot finished condition, whereas alloy grade steels are to be delivered in the quenched and tempered condition.

5.5.4 The use of stainless steel products in hydraulic cylinder applications is required to be specially considered by LR.

5.5.5 In cases where hydraulic cylinders are fitted to life-saving appliances, attention is drawn to materials where the yield strength is very low compared to the ultimate tensile strength. In such cases, it is particularly important to consider the proof load test case as a design case in order to prevent actual stresses being in excess of the yield strength of the material.

5.5.6 For hydraulic cylinder components subjected to tensile loads (e.g. cylinder end caps on pull cylinders) consideration is to be given to the application of Z-grade material. For the evaluation whether Z-grade materials are to be used in the design the methods and criteria of a recognised National or International Standard can be applied.

5.5.7 The required levels of material certification are given in Table 13.2.1 Minimum requirements for the certification of lifting appliances and Table 13.3.1 Minimum requirements for the classification of lifting appliances for certified and classed appliances respectively.

5.6.1 Hydraulic cylinders are to be equipped with pilot-operated non-return valves at both the inlet and outlet manifolds to ensure that the cylinders remain in position in the event of a hydraulic failure (e.g. hose/tube rupture). The required pilot-operated non-return valves are to be fitted directly to the cylinder ports. The cylinder ports shall either be an integral part of the cylinder wall or shall be connected by means of full penetration welds. Proposals to use valve types other than pilot-operated non-return valves will be specially considered.

5.6.2 The hydraulic system shall be equipped with relief valves to protect the system against overpressure in the hydraulic circuit originating from the lifting appliance itself or the hydraulic system/pump.

5.7.1 The allowable stresses are to be taken from the relevant chapters depending on the nature of the appliance that the hydraulic cylinder will be integral to (see Ch 3, 1.6 Safety and stress factors for life-saving appliances, Ch 4, 2.17 Allowable stress – Elastic failure for cranes, Ch 7, 2.6 Allowable stresses for lifts).

5.7.2 For hydraulic cylinders subject to push loading a second order buckling analysis is to be carried out unless it can be demonstrated that a simplified analysis would give conservative results.

5.7.3 In simplified buckling calculations, the hydraulic cylinder may be idealised using the assumption that the piston rod extends over the complete length of the hydraulic cylinder. The buckling calculations shall then be carried out using the criteria as stated in Ch 4, 2.18 Allowable stress – Compression, torsional and bending members for lifting appliances and Ch 3, 1.6 Safety and stress factors for life-saving appliances respectively.

5.7.5 Alternative proposals to demonstrate adequate safety against column buckling will be specially considered.

5.8.1 Threaded connections (as opposed to bolted connections) shall be calculated as follows:

where

The hydraulic cylinder tube thickness at the threaded section is to be not less than the required thickness for internal pressure measured at the bottom of the internal thread. Alternatively, detailed calculations for the threaded connection as per a recognised National or International Standard (e.g. EN 14359) may be performed and submitted for consideration.

5.8.2 Welded and bolted connections shall be designed based on the principles outlined in Ch 4, 2.23 Allowable stress – Joints and connections.

5.8.3 The hydraulic cylinder end cap may be calculated using the formula below:

where

The above method outlines a simplified approach which does not take into account possible reinforcements of eye plate connected to the end cap. If the end cap is reinforced and the above stress proof fails, alternative means of analysis may be applied.

5.8.4 The eye plates of the hydraulic cylinders may be calculated using the methods of a recognised National or International Standard.

5.8.5 The hoop, longitudinal and radial stresses in the hydraulic cylinder wall can be calculated using the formulae below:

1. Hoop stress, in MPa
   

   where

   σH	=	cylinder hoop stress, in MPa
   p	=	internal pressure, in MPa
   do	=	external cylinder diameter, in mm
   di	=	internal cylinder diameter, in mm
   x	=	actual diameter of the hydraulic cylinder wall stresses are required to be evaluated, in mm
   σa	=	allowable stress as per Ch 9, 5.7 Allowable stresses , in MPa

2. Longitudinal (axial) stress, in MPa
   

   where

   σL

   =

   cylinder longitudinal (axial) stress, in MPa

3. Radial stress, in MPa
   

   where

   σR

   =

   Radial stress, in MPa

5.8.6 Equivalent stress

The equivalent stress shall be calculated taking into account the following stresses:

1. Cylinder tube:
   • Hoop stress (due to internal pressure).
   • Radial stress (due to internal pressure).
   • Axial stress (due to external force and/or internal pressure).
   • Bending stress (due to 2nd order bending, see Ch 9, 5.7 Allowable stresses 5.7.4).
2. Piston rod:
   • Hoop stress (due to external pressure in case of a hollow piston).
   • Radial stress (due to external pressure in case of a hollow piston).
   • Axial stress (due to external force).
   • Bending stress (due to 2nd order bending, see Ch 9, 5.7 Allowable stresses 5.7.4).

5.8.7 Where appropriate, fatigue calculations are to be carried out in accordance with a recognised National or International Standard using load cycles and a load spectrum agreed between the manufacturer and the Owner.

5.8.8 The proof of resistance against column buckling is to be carried out as per Ch 9, 5.7 Allowable stresses 5.7.2.

5.8.9 Cases where the piston is in contact with the end cap or the cylinder head shall be taken into consideration for the design of the hydraulic cylinder. This is of particular importance for cases where the push or pull forces for the contact situation are above those which can be generated by design internal pressures.

5.9.1 The hydraulic cylinder is to be hydraulically tested with the maximum of the following defined test pressures p_T:

1. p_T = 1,5 p_W
2. p_T = 1,3 p_D

where

5.9.2 The minimum hydraulic test pressure for life-saving appliances shall be the pressure in the hydraulic cylinder required for the prototype test of 2,2 times the SWL amplified by an additional safety factor of 1,06. Hydraulic test pressures for life-saving appliances used in offshore applications are often dependant on national administrations’ requirements and will be specially considered.

5.9.3 The hydraulic cylinders are also to be tested as part of the proof load testing of the lifting or life-saving appliances.

5.9.4 The designer shall ensure that the hydraulic cylinder design is able to withstand the hydraulic test pressure as defined in Ch 9, 5.9 Testing 5.9.1 and the proof load testing of the lifting or life-saving appliance. The proof loads for lifting appliances are defined in Ch 12, 1.5 Derricks and derrick cranes. The proof loads for life-saving appliances are defined in Ch 3, 1.12 Testing. The actual stresses due to the applied test pressure and due to proof load testing of the hydraulic cylinders shall not exceed the allowable stresses stated in the relevant chapters applicable to the appliance being considered (see Ch 4, 2.17 Allowable stress – Elastic failure for the test loadcase (case 4) for both lifting and life-saving appliances, Ch 6, 5.2 Load combinations for Ro-Ro access equipment, etc.).

5.9.5 After proof load testing of the lifting or life-saving appliance, the pressure relief valve is to be reset by the manufacturer to the appropriate operational setting to ensure the appliance cannot be overloaded.

5.10.1 Luffing, folding and telescoping motion shall be realised by two independent hydraulic cylinders where each cylinder is able to hold the SWL for handling of personnel. Alternative arrangements using one hydraulic cylinder will be specially considered.