07 Main body of Thesis Chap 1 to 3
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CHAPTER 1INTRODUCTION1.1ObjectiveThe objective of this research project is to analyze and to improve the current winch drum design that is used in towing, anchor handling, and various types of winch. Eventually, this project will assist the manufacturers, researchers, and the designers in optimizing the winch drum design (Figure 1.1).Fig. 1.1 Marine Operation winches (courtesy from Plimsoll cooperation Pte Ltd)1.2History of winchA winch is a mechanical device that is used to wind up a rope or cable. In its simplest form, it consists of a spool and attached crank. Historically, winches are used on sailboats for hauling in and tensioning a variety of sail handling, loading and control lines (Figure 1.2). Traditional winches have a crank handle mounted directly to the vertical axis of the drum. More elaborate designs have gear assemblies and can be powered by electric, hydraulic, pneumatic or internal combustion drives. Some may include a solenoid brake and/or a mechanical brake or ratchet that prevents it from unwinding.In the early stage, an improved device provides a pair of cranks rotating about a horizontal axis, using an opposing motion characteristic of bicycle pedalling. This configuration, referred to as a pedestal or “coffee grinder,” is commonly found on larger sailboats. Larger sails lead to higher control line tensions, and larger crews allow some crew members to be dedicated primarily to winch grinding. All the way through history winches have been used and their original source is lost in time. However, the earliest most likely example of a directly coupled winch is the mechanism used at a well-head for lifting water containers.1.3BackgroundUsually, there are more than one layers of rope on the winch drum. Winch drums are subjected to vary high radial forces due to the compressive action of the tension of the rope by pulling or braking load in one or more layers. Despite the higher demands, such an increased payload, not much analytical work has been done on winch drumdesign. Presently, the design criterion is based on one layer of rope on the drum. ForFig. 1.2 Winch in History, Source: Deep Sea Sounding and Dredging (1880)multiple layers of rope on the drum, multiplication factors are used for design calculations.We have studied the following two codes for the design of winches:(a)Det Norske Veritas (DNV) [1], and(b)Standards Association of Australia (SAA) [2]Although the codes for the design of winch are distinct, the assumptions for calculating the hoop stress and thickness are similar. In addition, the assumptions include the uniform tension in the rope throughout the operation, percentage variation of permissible stress from material yield stress and expected rope layers factors for designing. However, the codes do not take into consideration the gradual decrease in tension in the inner wraps on ropes as the number of wraps increases in winch operations. For these assumptions for designing of winch drum, the codes demand a larger thickness to withstand the maximum compressive or hoop stress caused by tension of the rope in pulling or braking of load. Due to the required larger thickness of the drum for designing of winch, it is costly to fabricate.Experimental work conducted by us indicates that the tension in the loaded rope is not uniform throughout the operation in pulling load as well as braking load applications. Therefore, we would like to compute the hoop stress to take into consideration the variation of tension in the loaded rope.1.4Aim of researchThe DNV and SAA design guides specify the rope layer factor or rigidity constant and hoop stress for the drum caused by the uniform tension of the loaded rope to determine the required minimum thickness of the winch drum. However, the above guides provide only empirical formulae for calculating the hoop stress and also their design of the winch drum is based only on the pulling load application.We have conducted experiments to determine the relaxation of stress on the rope and to compute the stress exerted onto the drum so as to estimate the required thickness of the winch drum. It is the objective of this research to provide design guides so that more realistic design for the drum can be carried out based upon the experimental results obtained.1.5ScopeIt is observed that under constant tension, as the number of wraps (each turn of the rope around the full circumference of the drum) on the drum increases, there is a reduction in tension in the inner wraps. So far, no discussion of this phenomenon has been made. Therefore, experimental determination of the relaxation factor will be undertaken, and then used to determine the stress that the rope exerts onto the winch drum. A more realistic determination of the stress on the drum will enable a more efficient design of the drum leading to lower cost of production of the drum.Chapter 2 summarizes the literature review of work done in this topic. Chapter 3 deals with the mathematical calculation of hoop stress for the present winch design. Chapter 4 deals with the experimental set-up and equipment used in our winch drum research project. Chapter 5 discusses the observation and analysis of our research done. Chapter 6 reveals the calculation of required design for determining the required thickness of the drum that comply with our experimental findings. Chapter 7 deals with the conclusion of our research, and Chapter 8 puts forward the recommendation for future improvement work.CHAPTER 2LITERATURE REVIEW2.1OverviewExtensive research on the factors affecting the design of winch drum has been carried out to determine the maximum compressive stress acting on the winch drum, applied by pulling or braking of loads. In this literature survey, we have not only reviewed the literature for the winch drum design but also focused on other relevant aspects such as wire rope guide, crane guide, shaft design and their applications as well as operations.We have also studied the following design codes and permissible stress of the winch drum: Det Norske Veritas {(DNV)[1], Offshore standard DNV-OS-D101 October 2005}, Standard Association of Australia {(SAA), AS 1418, Part 1-1977}[2], British Standard Institution {(BSI), British Standard Marine Series}[3], International Standard {(ISO), Shipbuilding and Marine Structures – Deck Machinery}[4]and others related shells theories.2.1.1Det Norske Veritas (DNV)According to DNV (Det Norske Veritas) standard guide, the drum is to be designed to withstand the surface pressure acting on it based on the maximum number of winding and the rope assumed to be spooled under uniform rope tension. Uniform rope tension means the tension due to safe working load without taking into consideration thedynamic and relaxation effects. According to the guidelines, the maximum permissible hoop stress should not exceed 85% of the material yield stress.2.1.2Standard Association of Australia (SAA)According to SAA (Standard Association of Australia), the drum is to be designed to withstand the permissible compressive stress due to maximum number of winding; the rope is assumed to be spooled under uniform rope tension. According to the guidelines, the maximum permissible hoop stress should not exceed according to their given table correlated with designated materials and diameter of the winch drum.2.1.3British Standard Institution (BSI), British Standard Marine SeriesDesign stress levels: The stress calculations of the mechanical parts are based on:1) the drum load on the winch, in which case the allowable stress of any part of the winch based on the simple elastic theory, shall not exceed 0.4 times the 0.2 % proof stress of the material.2) the maximum torque of the motor corresponding to the most severe working conditions, in which case the allowable stresses shall be within 0.9 of the 0.2 % proof stress of the material.Drum design: A safety factor of 5 shall be applied to the drum load of the winch to establish the required breaking strength of the rope. It is upon this basis that the rope diameter shall be determined and in consequence the minimum drum dimensions established in accordance with the following:1) The drum diameter shall be not less than 14 times the diameter of the steel wire rope. For standard sizes of steel wire rope, see BS 365.2) The drum capacity shall be sufficient to accommodate the complete length of the rope to be stored on the drum.3) During all operating conditions the distance between the top layer of the wire rope when evenly wound on the drum and the outer edge of the drum flanges shall be at least 2.5 times the diameter of the wire rope.2.1.4International Standard (ISO), Shipbuilding and Marine StructuresLoads: The maximum rope tension, in kilonewtons, at the drum exit when the drum just starts to rotate in the opposite direction to the applied driving torque, the prime move being set for limited torque, with a first layer of the rope wound on the drum.Stalling load of the winch: The allowable calculated stresses of any part of the winch based on simple elastic theory shall not e grater than 0.55 times the 0.2% proof stress of the material.Rendering load of the winch: The allowable stresses in the affected parts shall not be greater than 0.85 times the 0.2% proof stress of the material.Holding load of the winch:The allowable calculated stresses of the affected parts shall not be grater than 0.70 times the 0.2% proof stress of the materials.2.2WinchA winch is a device that has a stationary motor-driven or hand-powered machine used for hoisting, hauling or lifting, and also a drum around which is wound a rope or chain attached to the load being moved (Figure 2.1). Most of the winch drum operations can be found in mines and marine applications.Fig. 2.1 Samples of diagrammatic illustration of marine winchesThe term winch describes, basically, the machine which consists of a winch drum, pulley and rope for carrying load and driven by some form of power unit. The consumer can choose their required designated applications that rely on winch specifications which include drum/ rope configuration, drive transmission and type of driven power unit. Definitions used in winch operation are defined below.2.3Definitions [3,5]The following terms are used in the winch design specifications. Figure 2.2 shows the illustration of the following terms:Drum Load: maximum tension measured at the rope exit when the winch is hoisting or hauling in at the nominal speed with the rope wound on the drum.Pulling Load: the loaded rope is winding onto a winch drum in wrap by wrap, starting from the near end to the desired position.Braking load: the unloaded rope is being wound onto a winch drum to the desired number of wraps before the load is being applied.Design stress level: stress calculation of the mechanical parts based on the drum load on the winch, also known as allowable or permissible stress of the winch.No of wrapsFig. 2.2 Schematic illustrations of winchDrum Design: An appropriate design factor shall be applied to the drum load of the winch to withstand the maximum compressive stress from the required braking of the load.Drum Flange: flange height shall be at least 2.5 times the rope diameter beyond the outermost layer when the rope is fully and evenly reeled onto the drum.Brake Design: Winches shall be provided with an automatic or manual braking system for pulling or braking of the load during operation and also for after operation. For the safety requirement, the emergency stop should also be provided.Wire rope: steel wire rope or fibre rope and the ratio between winch drum diameter and rope diameter shall not be less than 14 depending on the type of application. Wraps:each turn of the rope around the full circumference of the drum is called a wrap. (In this case of experimentations, the number of wraps starting from loaded end to the flange end.)Layers: a complete number or wraps extending from flange (near end) to flange (far end) is referred to as a layer.Power sources: winches shall be designed for operation powered by electric, hydraulic, mechanical or steam power depending on the application.However, the main parts of the winch are the drum and flanges. In the earlier period, the manufacturers focused mainly on designing of winch drum which can withstand the maximum compressive stress applied onto the drum by pulling/braking of load with no crushing of the drum. However, with higher load imposed on the winch, compressive analysis of the stress imposed on the drum has to be performed for an economical and viable design.CHAPTER 3MATHEMATICAL CALCULATION OF PRESENT DESIGNIn this chapter, we analyze the present codes and relevant shell methods to determine the hoop stress acting on the winch drum caused by pulling or braking load. Particularly, we want to compare the calculated hoop stress between the present codes and the relevant shells methods. This is to verify the difference in magnitude of hoop stress between them and how conservative the present codes formulae for determining the required drum thicknesses are. In addition, all the calculations are computed based on the assumption of uniform tension of the loaded rope throughout the winch operations.Our analysis is based on one particular design of the winch, which is currently being used, with the following design parameters:Drum diameter = 1000 mmDrum Length = 1775 mmDrum thickness = 70 mmSteel wire rope size (diameter) = 65 mmApplied tension load (pulling) = 200 tons =1.96 MN (1ton = 1000kg = 9810N) Applied tension load (braking) = 300 tons =2.94 MN (1ton = 1000kg = 9810N) Yield Stress for steel (ST 52.3) = 520 MPa3.1DNV standard guideBased on the DNV design codes:The hoop stress in the winch drum, σh (MPa): (3.1)where,σh = hoop stress in winch drum (MPa)T = Load on the rope, Newtond = diameter of the rope, mmt = thickness of winch drum, mmC = 1 for 1 layer.1.75 for more than 1 layer.The assumptions would be made for this design guide before calculation,1.Pitch of the rope is equal to the diameter of the rope.2.Static rope tension is equal to the weight of load.According to the guidelines, the maximum permissible hoop stress σpe should not exceed 85% of the material yield stress (σpe ≤ 0.85 σy). In this case, the material yield stress of ST 52.3 is 520 MPa.For the 1st layer condition with un-groove drum,Hoop stress, σp due to pulling load:Hoop stress, σb due to braking load:Maximum permissible hoop stress, σpe : MPa x y pe 44252085.085.0===σσ where,σpe = Permissible stress (MPa)σy = Material yield stress of the winch drum (MPa) σp = Hoop stress due to pulling load (MPa) σb = Hoop stress due to braking load (MPa)Although the calculated hoop stress for pulling load is within the limit of maximum permissible stress level, the calculated hoop stress for braking load is much greater than the maximum permissible stress as well as material yield stress. On the other hand, the required thicknesses based upon the imposed loads under permissible stress conditions are:For pulling load ,For braking load,Accordingly, the design of the drum is based only on the pulling load condition. Let consider for the case of braking load of 300 tons, the thickness of the drum will be nearly 1.5 times higher than the designed thickness. Also, the hoop stress is much grater than the material yield stress. The comparison between calculated hoop stresses in DNV guide with their permissible stress can be clearly seen in Table 3.1.3.2SAA standard guideBased on the SAA design codes:The hoop stress in the drum σh (3.2) where,σh = hoop stress in winch drum, MPaT = Load on the rope, Newtond = diameter of the rope, mmt = thickness of winch drum, mmC = rope layer factor and rigidity constant of drum shell= 1.0 for single layer= 1.3 for tow layers of rope with wire-rope core (WRC) or wire-stand core (WSC)= 1.4 for two layers of rope with fibre core (FC)= 1.5 for three layers of rope with WRC or WSC= 1.6 for three layers of rope with FC= 1.6 for more than three layers of rope with WRC or WSC= 1.8 for more than three layers of rope with FCThe assumptions would be made for this design guide before calculation,1.Pitch of the rope is equal to the diameter of the rope.2.Static rope tension is equal to the weight of load.The material of winch drum that we used in our design is ST 52.3 and has a yield stress of 520 MPa. For convenience of the calculation, the permissible compressive stress condition should be assumed the same assumption as the DNV guide in this calculation. Based on the SAA design guide:Hoop Stress, σpHoop Stress, σbAccording to these results, the data are similar with the DNV guide for the 1st layer of rope on the drum and also the thickness of the drum for the braking load will be nearly 1.5 times higher than the designed thickness in the permissible stress condition. Also, the hoop stress for braking load is much grater than the material yield stress. The comparison between calculated hoop stresses in DNV and SAA guide with their permissible stress can be clearly seen in Table 3.1.Table 3.1 Comparison between calculated hoop stresses in DNV and SAA guide with their permissible stress3.3Comparison between DNV and SAA standard guideAs mentioned above, the calculated data are similar in both guides; the only difference between these formulae are the rope layer factor or rigidity constant of drum shell and the given permissible hoop stress. (These two guides have slightly differed of their permissible hoop stress). Also, the calculated hoop stress of braking load in both design guide is much higher than the given permissible stress level even in the 1st layer. Thus, the design consideration for winch drum is based on DNV guide’s pulling load condition in our optimizing the winch drum design.Furthermore, we will be mainly emphasized hoop stress onto the winch drum for calculating of required winch drum design rather than the longitudinal or shear stress acting on the winch drum due to the pulling or braking load. This is so as these stresses have been shown to be negligible compared to the hoop stress acting on the winch drum.3.4Calculation of hoop stressIn this section, the hoop stress would be calculated by the appropriate formulae and compared with the above stated winch design codes. We have studied the general shells theories which are appropriate to the winch operation. In the winch operations, the loaded rope is wound onto a winch drum in wrap by wrap starting from the near end flange to the desired position. Also, each successive wrap compresses by radial pressure inwards onto the winch drum. This radial pressure of each respective warp will be used to calculate the hoop stress acting on the winch drum. Also, the hoop stress calculations were used by assuming uniform pressure on the drum. Based on the wire rope users manual [6], the pressure, P between the winch drum and rope is defined as follows:(3.3) where,P = Pressure, MPaT = Load on the rope, NewtonD = Diameter of the drum, mmd = diameter of the rope, mmTherefore, the radial pressure due to pulling load is:Similarly, the radial pressure due to braking load is:3.4.1 Cylindrical shell method [7]As mentioned above, the loaded rope is wound onto a winch drum in wrap by wrap starting from the near end flange to the desired position. Also, each successive wrap compresses by radial pressure inwards onto the winch drum. This pressure caused by each wraps would be assumed to a uniform pressure onto the winch drum. Based on the cylinderical shells of general shape condition, the symmetry of the shells and the loading on an infinite element is as shown in Figure 3.1.a and 3.1.b.The hoop stress, σh takes the form:(3.4)where,R = radius of drum shell, mmFig 3.1.b Element of cylindrical shellFig 3.1.a Cylindrical shell subject touniform pressure by loaded ropeTherefore,Hoop stress, σp due to pulling load isHoop stress, σb due to braking load is3.4.2 Fixed cylindrical shell method [8]The compressive direct stress, σh caused by uniform pressure (different formulae and approach from section 3.4.1) for cylinderical shells with fixed supports (Figure 3.3) takes the form:where,σh = Compressive or hoop stress, MPa N θ = circumferential load, Newton θ = angle of contactFig. 3.3 Illustration of external pressure along the area of contactThe maximum condition is given by θ=90˙. For pulling load, the compressive stress, σp is,For braking load, the compressive stress, σb is,3.4.3Hampe’s solution method [9]These solutions cover many cases of circular cylinders and spheres with various support conditions along the boundaries. Hampe [9] derived a set of general formulae for the stresses and deformation of cylindrical and spherical shells with various boundary conditions. However, the actual winch has fixed boundary conditions on both ends with flanges and the assumed uniform pressure acting on the circular cylindrical shells by loaded rope tension of the pulling and braking of load Figure 3.4. The circumferential load is:(3.6)The derivation of the above equation and the various terms are discussed in Appendix A.For uniform pressure, the hoop stress, σh is,(3.7)The ordinates of deflection line, w p (3.8) The non-dimensional coordinate, ξ with positive direction is,, where 0 ≤ ξ ≤ 1 is shown in Figure 3.4. (3.9).·. Deflection under uniform pressure is,[)]ξξξξξ()()()()(8416315271F S F S F S F S w w p ++++= (3.10)where,E = Young modulus of winch drum, MPa ξ= non-dimensional coordinate with positive directionS i = coefficientL = length of the winch drum in mmThe above Figure 3.4 is the maximum condition of hoop stress that acting on the winch drum by uniform pressure caused by the pulling or braking load.As shown in Appendix A, maximum compressive stress occurs at (ξ=0.5) which is at middle of the winch drum. The uniform pressures acting on the circular cylindrical shells as follows:Fig 3.4 Uniform external pressure acting on the circular cylindrical shellsHoop stress, σp due to the pulling load (200 tons),Maximum hoop stress, σp for uniform pressure calculation: 429=p σ MPa at mid point (ξ=0.5)Hoop stress, σb due to the braking load (300 tons),Maximum hoop stress, σb for uniform pressure calculation: 643=b σ MPa at mid point (ξ=0.5)3.5Analysis of hoop stressThe calculations of hoop stress for design codes and shell methods are under the assumption of uniform tension of the loaded rope. For convenience, comparison of the hoop stress obtained between the shell methods and the design codes are tabulated in Table 3.2. As can be seen in the Table 3.2, the calculated hoop stress based on DNV and SAA design guides are similar with that obtained from the shell methods. However, the calculated hoop stresses for braking load are much larger than the material yield stress.3.5.1Design guideFor drum thickness of 70 mm, the calculated hoop stress is within the permissible stress due to the pulling load. However, the calculated hoop stress due to the braking load exceeds the permissible stress. For the hoop stress to be within the permissible level due to the braking load a drum thickness of 103 mm is required. According to these results, the design codes have been designed the winch drum to meet the pulling load condition only.3.5.2Shell methodsBased on the assumption of uniform tension throughout the loaded rope, the thickness required as computed by the design guides is an agreement with the shell methods.29T a b l e 3.2 C o m p a r i s o n b e t w e e n c a l c u l a t e d H o o p S t r e s s o n D e s i g n C o d e a n d s h e l l s m e t h o d s f o r t h e f i r s t l a y e rP a r t i c u l a rD N V C o d eS A A C o d e M e t h o d I M e t h o d I I M e t h o d I I I1. Y i e l d S t r e s s i n S t e e l (S T 52.3) M P a520520 520 520 5202. P e r m i s s i b l e S t r e s s i n M P a442 442 442 4424423. D u e t o P u l l i n g L o a d (200 t o n s ) w i t h t h i c k n e s s o f 70 m m (σh < σp e )H o o p s t r e s s (M P a )430 430 430 4304294. D u e t o B r a k i n g L o a d (300 t o n s ) w i t h t h i c k n e s s o f 70 m m (σh > σp e )H o o p s t r e s s (M P a )646 646 646646 6435. D u e t o B r a k i n g L o a d (300 t o n s ) w i t h t h i c k n e s s o f 130 m m (σh < σp e )H o o p s t r e s s (M P a )439 439439 439 438。
