Lifting Gear and Weak Link

Lifting Sling/Strap Material

The lifting Slings we intend to use are made out of a material called high tenacity polyester yarn which is renowned for its hardwearing properties. Widely used through out industry and construction due to their low weight, flexibility and ease of use. These lifting slings are also relatively cheap with an example of lifting capabilities and prices from lift safe solutions below;

For our Gantry crane, it is recommended that we supply our product with straps capable of lifting 2 tonne to ensure durability through use and be at least 6m in total length to ensure it can deal with awkward loads.

Source: www.liftsafesolutions.co.uk

Bow Shackle Material

When deciding which material would be best suited to use for our bow shackles, it came down to two options, high tensile or stainless steel. High Tensile steel would offer us extreme durability and strength at a relatively low cost; however from our design specification, it is clear that the crane will be used in disaster areas, it is therefore accepted that the crane must be durable in all weather conditions. This therefore lead us to turn to using stainless steel Bow Shackles, which still offer great strength yet also offer good corrosive resistance. An example of the load capacity and price for an ideal Bow shackle can be seen below.

Source: www.s3i.co.uk

D ring limits to fail before 1.2 tonnes to prevent damage to crane.

With our D-rings also being manufactured from stainless steel it is accepted that it is more likely that the crane will fail before any of the lifting gear. As a result of this it has been seen as a good idea to implement a safety device within the design to ensure the protection of the main crane assembly, this will ensure longer product life and serviceability, key for our design success. To this end, we have proposed that all our D-rings, which connect the load to the pulley system be designed to fail when it is loaded with more than 1.2 tonnes, a safe loading limit with enough leeway either side of 1 tonne to ensure safe operation of the crane. 

Winching System Selection

It was decided that and electrical winching system would be more unreliable than a pure mechanical system. Another disadvantage is the fact that it needs power to operate; this would mean that a power source will need to be present. This could only be done in a couple of ways either connecting it to the Landover’s power source or having a generator. Therefore, it was decided that the western differential pulley system would be the best system to use for this task because of its simplicity and ability to get large mechanical advantages easily.

Aluminium Type Decision

From research into materials, 7075 aluminium has been chosen as the type of aluminium which will be used for the crane. This is due to its high strength and corrosion resistance which means it does not require any form of coating. It is also one of the easiest aluminium alloys to weld, most commonly and effectively welded using resistance welding, which is a very quick form of welding. It has a greater springback capability than most forms of aluminium which is also useful in this application where it may suffer a large amount of abuse in the situations it will be required.

http://www.suppliersonline.com/propertypages/7075.asp

Bearing Decision

 

From the research which has been presented roller bearings have been chosen for use in the crane because of the load that they will experience in this application. This is because they will experience a  high radial load with a negligible thrust load, which is what this type of bearing is designed to cope with. They have also been chosen as they are available as a sealed cartridge unit which requires no maintenance in it's lifecycle. This makes them preferable to plain bearings because although they have a greater resistance to axial load, they require good lubrication to work and maintenance to keep them in working order.

Winching System Types

Block and tackle pulley system:

A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. The pulleys are assembled together to form blocks so that one is fixed and one moves with the load. The rope is threaded, or reeved, through the pulleys to provide mechanical advantage that amplifies that force applied to the rope.

Western Differential Pulley:

A differential pulley, sometimes called a "chain hoist," or sometimes colloquially called a "chain fall," is used to manually lift very heavy objects like car engines. It is operated by pulling upon the slack section of a continuous chain that wraps around pulleys. The relative sizes of two top pulleys determine the maximum weight that can be lifted by hand.

Electrical Winch:

A winch is a mechanical device that is used to pull in (wind up) or let out (wind out). Winches stand at the heart of machines as diverse as tow trucks, steam shovels and elevators. The winch drums have gear assemblies which are powered by electric drives. Some may include a solenoid brake and/or a mechanical brake or ratchet and pawl device that prevents it from unwinding unless the pawl is retracted.

Electric Chain Hoist:

An electric chain hoist will can lift heavy loads easily and efficiently. Some electric chain hoist models offer hook suspensions, fully enclosed non-ventilated systems that protect the motor from being contaminated by outside environmental elements, quick connect chains to prevent chains from slacking, and push button operation. 

Assembly pins and retaining pins

In order to make the crane practical, it must be able to be constructed and adjusted quickly and easily, without the need for tools. To make this possible, the sections which it breaks into for transport will be assembled using pins which will fit into joints, and be prevented from moving in place by the use of retaining pins or 'R Clips'. The inspiration for this has been taken from the way in which a medium girder bridge (MGB), made by WFEL Ltd and in service with the british army, is assembled and held together. Pictures of these pins will be uploaded when we have obtained them.

The 'R' clips are shown below, and it is anticipated that they will be purchased from an external supplier because they are avaliable mass produced.

The pins which will be used in the crane have been modelled in CAD, and will be made from steel.

15mm diameter general connecting pin

20mm diameter top beam connecting pin (showing R clip hole)

Material Properties

Material	Yield Strength/	Ultimate Strength/	Density/
2800 Maraging Steel	2617	2693	8000
Steel AISI 4130	951	1110	7850
Aluminium Alloy 2014-T6	414	483	2800
Brass	200	550	5300
Carbon steel 1090	250	841	7580
Copper	70	220	8920
High Density Polyethylene	30	37	950
Steel AISI A11	5171	5205	7450
Stainless Steel AISI 302	520	860	8190
Steel API 5L X65	448	531	7800
Steel ASTM A514	690	760	7800
Steel ASTM A36	250	400	7800
Titanium 11	940	1040	4500
UHMWPE	3447	6894	970

Current crane products

GANTRY:

This crane works by winching the weight up and sliding it along the top boom. It would be broken down and moved to the problem area and reassembled. Early possible problems I would like to investigate include; when the top boom is over four metres long will it deflect under a weight of 1000kg, are the feet stable enough and would it be better to have more legs on each end.  When at an end, would the supports be strong enough and would it cause the other end to be lifted?

SPIDER:

The spider crane mechanically moves to the required position, it then winches the load up and lifts it, it then carries it to the required area. Initial areas to investigate with this is how much this would cost, this could be a massive weakness with this design. Mechanically my worries include; not being heavy enough on the base causing it to tip over when lifting things with the crane head, would this design be able to get over serious obsticles associated with rubble covered areas.

JIB        

                                                             

The jib crane uses a fixed body, with a swinging arm to move what it has lifted it up. If used in this situation it would have to be carried to the desired point and the body would have to be dug into a solid foundation. Main concerns which I will have to investigate include; the crane being to heavy, not being able to dig the base into the ground because of the rubble and whether or not it would be pulled over when dug into the ground by the 1000kg weight being lifted up.

Calculations required to ensure quality:

Stress:  σ/y= M/I= E/R

Buckling: P_critical = 4π2EI/L^2 

Bending deflection: -EI(d^2 v)/〖dz〗^2 

Shear force: dM/dx

Important to ensure that at every point do the maximum stress equation. This will prove whether or not the material chosen will be successful. It is important to me that these equations are all considered to ensure that the crane will avoid failure.

Research - Bearing Types and Applications

Research - Types of Crane

Slide showing different types of existing crane

Maximum Crane Specifications - Size and Weight

Maximum Crane specifications – Size and Weight

The brief we have been given states that the crane must:

·         Be able to be disassembled and carried manually over 100m of rough ground.

·         Be able to be carried by a standard Land Rover size 4x4.

From research, it has been found that the average man can be expected to carry 25kg at knuckle height, meaning that any single piece of the crane should not weigh more than 100kg to allow it to be carried by four men.

From http://www.healthyworkinglives.com/advice/work-equipment/manual-handling.aspx

Research into the capacity of a Landrover Defender 130, as recommended by Landrover for this sort of application, shows that the maximum load capacity is 1243kg, which when accounting for occupants (based on being 4 x 80kg workers) means that the maximum weight for the crane is 923kg.

The overall length of the vehicle is 5130mm which would be the maximum length for any part to be carried on the roof, assuming that the vehicle will be fitted with a full length roof rack. The dimensions of the load space of the vehicle are 1640 x 1060 x 1160mm, so will be the maximum size for any item to be carried in the load bed of the vehicle.

From http://www.carfolio.com/specifications/models/car/?car=27676 and http://www.landrover.com/gb/en/lr/defender/explore/double-cab-high-capacity-pick-up/