Mechanical Design Assignment

Background

You are a newly appointed (and the only) Mechanical Engineer working for a small maritime maintenance company. The company has ambitions to grow and enhance their capability and have just acquired an extra waterside property in Darwin with access to Frances Bay. This property has an existing slipway with flush steel rails originally designed for vessels up to 350 tons. The slipway is in good condition and has a grade of 10% into the sea with an upper section above the high water mark at 2% for a length of 50 metres. A new shed is planned to cover half of this upper section to allow undercover work on client vessels. The slipway rails have a centreline distance of 4m. The shed will be designed by others but will be 50m by 50m with a minimum overhead clearance of 20m. It will have one side open at right angles to the slipway to receive vessels riding on a cradle up the rails into the shed. Three phase power is available. You will need to design and organize construction of a winch and cradle assembly to allow vessels of up to 100 tons to be removed from the sea via the slipway into the shed for maintenance work. The mechanism will need to be reversible to allow the vessel to be returned to the sea. You will need to do some research and make some judgment assumptions about vessel dimensions including draft, overhead clearance, slipway length ( in case it needs modification ), tidal considerations ( to optimize the useability window ) and cradle speeds. Your design choices will need to take into consideration the operating environment and any other relevant conditions. You should investigate the potential users of this facility and the types of vessel which can be accommodated eg the vessel design ( vee mono hull, multi hull, flat bottom barge etc.). You should also investigate how the cradle can be adapted to suit different sizes and designs within the weight limit. The cradle must support the weight of the vessel on its base frame as well as provide an adjustable lateral support at both sides of the vessel to ensure it is both safely captured as well as securely supported prior to being drawn up the slipway. Once in the shed, maintenance people need to be able to get safe access, in, under and all round the vessel. However, adaptability, whilst opening up the facility to wider range of vessels and clients, comes at the cost of complexity and more expense. You may also consider future growth in the cradle and winch system to allow full use of the slipway capacity however you will find it hard to justify any extra cost in this area as your budget is very limited considering the property purchase and a shed still to be built. You must not make arbitrary choices in this assignment as it may cause you serious unforeseen detriment down the line. You must include an appendix with full cost estimates for your project.

I am available as a Specialist Maritime Consultant Engineer to advise with some aspects of this assignment but since I am the Principal of the Consultant Company, my time is charged to your project at $500 per hour or part thereof ( ie 5mins. of my time costs you $500 and 1 hour 5mins costs you $1,000. )

Your Company is working to a very tight deadline on your design proposal and it must be submitted before noon on Monday 25/5/2015. Failure to meet this critical deadline will result in the finance being withdrawn for this project and this will in turn, result in the property finance being withdrawn and your job with it! Submission at least a week before the due time will earn you ( the Engineer ) a generous financial bonus. ( as a student this means an extra 5 marks in 35 ( about 15% ) for a week- early submission. This bonus will only apply to submissions submitted before 12noon on Monday 18/5/2015.

Design Tasks

1. Design a cradle which will safely and securely support vessels up to 100 tons. This cradle should ride on the existing rails with a bed height, upon which the keel may rest, of no more than 600mm above the slipway. This cradle will be pulled up the rails into the shed by a winch mechanism. You need to design the welded/bolted frame, axles, bearings and specify the rotating wheel diameter. You also need to design any threaded fasteners. The attachment for the wire rope also needs to be designed. This cradle will be built on site by your company, under your supervision. Appropriate surface treatments also need to be specified.

2. Design a gear train which has at least a double reduction, spur gear type, winch to allow vessels to be hauled up the slipway via a wire rope. You need to specify the drum size to allow it to hold sufficient rope. All components within this assembly such as shafts, bearings, gears, bolts, welds, frame, wire rope, mounts etc. need to be designed from first principles using appropriate assumptions about modes of failure. You should then choose suitable off the shelf components for bearings, threaded fasteners, wire rope and steel sections. You should also specify how your frame is to be made, how the bearings are mounted etc. You must show ALL your calculations, sketches as well as provide a brief explanation of your design decisions used in your approach. Do not use or submit a spreadsheet to do this.

3. A brake needs to be incorporated in the mechanism to enable the winch to be stopped safely at any point whilst hauling a vessel. The winch is to be driven by an electric motor via a flexible drive. You need to specify the electric motor and quote its speed, mounting and current draw at full rated load to allow the electrical trades to select the appropriate cable, switching and other fittings to enable connection. (When selecting an electric motor, a critical piece of information is the starting torque, rating of the motor.)

4. Provide a plan view layout drawing of major components with key dimensions of your design ( Creo allowed) in A3 size.

5. Off the shelf components must be used where appropriate however the frame will be constructed in house to your design so you will need to accommodate this in your choices.

6. The electric motor can be mounted on a suitable sliding frame to allow for belt tensioning/replacement; a sketch is to be provided as to how you would do this.

7. You must check system inertia to allow correct choice of motor. Provide a specification sheet showing your choice of motor. Your design should be such that the cradle will not "run away" on reversing. This means that system inertia is sufficiently great as to exceed the gravitational pull from the slipway incline at all points of the cradle travel and that the cradle must be driven to allow it to descend the slipway.

8. You may make assumptions about friction levels and other mechanism efficiencies but you must explain your reasoning as well as specify any values you choose as a result of these assumptions.

9. This machinery will be on demand on a 24/7 arrangement ( normal for this industry ) , so you also need to design in to your system a way of providing easy maintenance and regular servicing to ensure reliability and minimize the effects of a marine environment.