Title: Dockside crane
Pages: 57 - 59
Text: Dockside crane: Stothert and Pitt's DD2
Made by Stothert and Pitt Limited, Bath. Designed by the maker's design team; director of research and development, Norman Kerridae. MlMechE.
The DD2 dockside crane was shown to potential buyers for the first time in 1959 with some trepidation, for the docks industry had a reputation for conservatism and the DD2 looked - and was - very different from any previous crane. But the elegance of its simple lines - contrasting with the complicated lattice structure of earlier craneswon immediate interest. And this interest quickly turned into a flood of orders as buyers learnt of the many advantages brought by the thorough reappraisal of crane design and construction which had preceded the building of the prototype. By the end of 1967, 338 DD2s had been supplied in Britain, Ghana, New Zealand and Nigeria, including 185, worth £4 million, to the Port of London Authority. And - partly because of continual improvements in design and construction the DD2 is still reckoned by at least one of its major purchasers to be "the most modern and satisfactory of its type."
The design team set up by Stothert and Pittto produce the DD2 was headed by Norman Kerridge, director of research and development, and included engineers recruited from outside the crane industry. "We set out specifically to simplify operation and maintenance, to reduce weight, and to find a method of construction which would help to keep costs down," says Mr Kerridge.
The first decision was to adopt a large member tubular steel welded construction. By using automatic welding machines labour costs are much reduced compared with the old type of crane. Site erection is reduced from seven or eight weeks to three weeks or less by the use of large-member sections.
The DD2 uses the Toplis level-luffng principle, invented by a Stothert and Pitt engineer in 1914, whereby the geometry of the jib length and its relationship with upper I pulleys at the crane apex (over which the hoist rope is fed to the jib top) is arranged so 'I thatt he load remains at the same height when the jib is luffed in or out. The jib on the DDZ is luffed by an independently powered hydraulic ram which gives smooth acceleration and deceleration and which locks automatically in the event of any failure. This system allows the jib head to be lowered to the ground for inspection and maintenance.
The driver of the DD2 crane has windows on three sides of him. His cab is cantilevered out from the main body of the crane, and there is another window in the cab floor to take ad vantage of this position. The gauge to his left tells him the radius between crane and load which is difficult to judge visually.
The DD2 uses the Mawdomatic hoist control system specially designed for Stothert and Pitt by Mawdsleys of Dursley This system enables the crane driver to utilise the available power fully so that light loads can be hoisted and lowered faster than heavy.
Capital goods awards/Dockside crane
The jib balance weights, below, triangular in section, are of compact design to enable working close to buildings or other cranes.
Lorries can travel between the crane's legs. The driver gets to his cab up the inclined ladder then up a spiral stairway inside the main tube.
The driver has excellent visibility in his cantilevered cab; the top window is hinged so he can put his head out when angles are awkward. The control consists of two joysticks, one for hoisting and slewing,the other for luffing. The slew brake is operated by the driver@s foot. The crane's work capacity is enhanced by excellent facilities for the driver, which both assist accurate and fast working and reduce fatigue. The DD2 was one of the first dockside cranes to provide a correctly designed fully adjustable seat for the driver, vision immediately below the driver being obtained by cantilevering the cab from the body of the crane and putting a window in the floor; all-round vision is equally good. The controls consist of two joysticks, one controlling both hoisting and slowing and the other luffing.. The slew brake (used by the driver to control load swinging) is foot operated. An indicator tells the driver the radius of the load from the crane centre line. The cab is provided with a heater. Entry to the cab is both safer and easier than with previous cranes, exposed vertical ladders having been completely eliminated.
Maintenance aspects have been carefully considered. The machinery house behind the cab is moulded in translucent glass reinforced plastics, thus obviating the need for windows and also cutting both corrosion and painting. Within this housing ample access is provided to all machinery and electrical gear and an internal wall crane car be used to lift out any equipment needing repair or replacement, through removable doors at the back. The extensive use of sealed roller bearings and of graphite-impregnated non-metallic bushes means that lubrication is required only infrequently. Many other points have been considered. The base of the crane is constructed so that I, lorries and fork-lifttrucks can travel between the legs, an important facility on a crowded dockside. The bearings on the hoist gear have been designed to be self-aligning, so reducing site installation problems.
Since launching the crane commercially in 1959 Stothert and Pitt have followed a policy of continual improvements. Most important has been the rationalisation and standardisation of the DD2 range. Before this each docks board tended to demand cranes to its own particular specification. Stothert and Pitt realised that this made economies of scale in production very difficult and therefore extended the original two sizes of DD2 to a range which now covers maximum loads of 3, 5, 6, 7 1/2 and 10 tons and maximum working radii of from 60 to 100 feet. Then, whenever a buyer asked for a crane with non-standard details, the company quoted both for the special crane and for its equivalent in the standard range: the lower price of the standard version usually won the order.
The Vierendeel principle used in the construction of the jib, left, takes full advantage of the strength and resistance to bending stresses of tubular forms and cuts down wind loadings The surface area is reduced, lowering paint costs; tubular construction discourages pools of water collecting and helps prevent corrosion.
A covered spiral staircase leads up to the cab and, top, the machinery house. The house gives plenty of space for maintenance access.
Another advance has been the use of a computer to find the appropriate jib design to suit the various load-radius combinations required. The computer, fed with details of three or four possible design solutions straddling the particular requirements, uses a standard programme to work out the stress calculations for each so that the appropriate solution can be chosen, doing this in an hour where a man would take weeks. Since last year the computer has also been used for structural fatigue calculations A choice of control equipment is now available for the DD2, the latest being the Cranestat static switching system developed by English Electric and first applied to dockside cranes by Stothert and Pitt. This system eliminates the motor generator set and contactors, thus reducing maintenance, and also gives faster hoist acceleration.
Small improvements have also been made. For instance, the platform at the base of the pintle tube used to be covered with non-slip metal panels. These have now been made redundant by incorporating the nonslip surface into the basic structure, thus both reducing cost and avoiding any tendency for corrosion to occur unnoticed beneath the panels.
Stothert and Pitt have throughout followed a policy of co-operating with suppliers to ensure that all bought-in equipment matches their own standards and needs. A good example is the plastics machinery house in which progressive developments by the company in collaboration with the supplier (Longwell Green Coachworks of Bristol) has enabled cost increases to be contained.
The undoubted elegance of the DD2 can of course be largely attributed to the method of construction adopted. But, throughout, the designers aimed at producing a crane of good modern appearance, and this explains the great care which has obviously been taken to avoid impairing the crane's strength and simplicity by bad detailing or unnecessary styling.
Prices: from approximately £25,000 to £40,000 depending on specification.