Monday, 8 September 2014

Some postscripts

A video clip (about 6 minutes) of the model  has now been made  (by Neil Shaw, an independent film-maker from Cape Town) showing the various motorisations working (traversing back and forth, slewing of upper arms, raising and lowering of bucket when arm rotation of bucket wheel, and all 6 conveyor belts running (including static conveyor of 20 foot).  All the minor movements (accuated with a total of sixteen 6 volt Meccano motors) are also featured (5 service cranes, and up and down movement of front driver’s cabs).  This clip is now owned by AP (Associated Press) and will be sold to about 300 TV stations worldwide.  These stations then have the right to edit the clip for their own use. A short version (about 1 minute) has already appeared on YouTube.
I am happy to say that the final assembly went well with no mishaps.  Stage one was to lift the central unit onto its crawler system.  This was done as follows:  first the wooden lifting cradle was attached to the three upper protruding M20 bolts which go down into the crawler bridge units.  This was then lifted (by 8 people, as the combined unit now weighed over 240kg) onto two strong A-frames, which in turn rested on eight stacks of 2-high bricks.  This allowed enough room under the M20 bolts to slip the bridges on from underneath and to bolt up with M20 locknuts from below (once three 65mm diameter brass spacers had been slipped into place, to get the correct distance between bridges and vertical finlike structures on the outside of the main unit.) These brass fittings had cuplike voids machined out on one side to accommodate the locknuts.  This ensured that flat surfaces on the bridges remained parallel with flat surfaces below the fins.  This had the effect of helping to keep the bridges from buckling forward or backward. Of course, for such buckling to occur, the M20 bolts would have to bend quite seriously. This is not likely, as they are high tensile.
At this stage the whole structure had to be lifted by the 8 lifters and held while the bricks were removed from under the A-frames, a rather tense interval.  When the cradle was replaced on the A-frames there was just enough room to slide the twelve main crawler units sideways onto their M20 rocker axles protruding from the bridges.  Large brass collars then locked them securely in place.
With the crawlers and bridges securely in place the whole central unit weighed more than 500kg.  This was too heavy for even for 8 lifters so the final downward trip of about 1 inch was done using my 2-ton hydraulic car jack and some supporting timber, first at one end of the cradle and then the other.  For the first time the central unit was on its “feet” and mobile.  The 24 cores to the 12 motors were connected to a 12 volt source and the crawler system and found to be satisfactory (before the final lowering onto the ground I had been very careful to check all motor polarities – to have one motor trying to work against the other 11 would be very rough on the works!)
The next big task for the lifting team was performed a week later and involved fixing the main horizontal arms in place.  First the rotating upper toroidal ring had been put in place on its roller race of 65 brass flanged wheels.  A set of 6 locking flanged wheels had been put in place on three strong vertical arms which came up the inside of the upper torus.  These wheels roll on another roller race on the top of the upper torus.  Their purpose is to prevent the whole upper machine from toppling over, should any unforeseen accident occur (the machine will sit on a vertical surface for its lifetime and is well-balanced even if slight tipping happens, having a low centre of gravity in relation to the main support).
The first arm to be attached was the counterbalance. This had already been bolted to the heavy central triangular frame of box girders by means of four M10 bolts into heavy straps (rather than multiple M4 bolts, since the angles were not 90°, as explained elsewhere).  My helpers carried the 140 kg arm over and lifted it into place on some wooden scaffolds (since the winding drum end needed to be over 8 feet off the ground.   There were some anxious moments for me as the four 12mm holes drilled on the bottom of the box girder frame came down on four M12 bolts protruding from the upper ring.  This match had not been tested before, as the separate parts were too heavy for me to move around on my own.  Fortunately the fit was good.  I must admit that I had a 14mm drill bit on standby on my portable drill in case I had to ream a 12mm hole out to 14mm!
Once the counterbalance arm had been bolted in position the bucket wheel arm could be brought over and two M12 bolts slipped into place to act as pivots on which this arm can move (to change the height of digging wheel). Again, the fit had not been tested, due to the enormous weight, but all went together smoothly.  Several of the bolts in critical places on the two components had been replaced by countersunk bolts (into 2mm plate) to avoid bolthead clash as the bucket wheel arm moved.  This turned out to be a wise move.  So ended the day’s work with the horizontal arms resting on trestles at their outer ends.
The next main stage was the upright arm placement.  To be safely accomplished this required some serious scaffolding.  I had made the components ahead of time and now they needed to be erected: two 9 foot high scaffolds were built, one on either side of the model, during the next 2 weeks.
On the day of the fitment my helpers passed each 45kg upright arm up handlers sitting on top of the 9 foot scaffolds. From there they were lowered and M10 bolts put through lugs on the arms and on the waiting model below. All went well except I put an upright on back to front! It had to be turned around and re-bolted. At this stage the upright arms were held in place by wooden struts bridging the gap between scaffolds.
Over the next week or so the 2mm braided steel wires were run onto the large winding drums and around the thirty-four 4 inch aluminium pulleys atop the two upright arms.  The fixed length 8mm threaded rods were slipped into their retaining blocks and lock-nutted.  The winding drums were then started and the cables brought into tension so that the whole model became self-supporting.  By trial and error it was determined that the balance of the model was optimum when 35kg of concrete blocks were placed under the winding drum area.  All motions could now be tested:  traversing, slewing, raising and lowering the bucket wheel, running all 6 conveyor belts, bucket wheel rotation,  all motions of the 5 service cranes, the up and down movement of the 2 drivers’ cabs and the motorised positioning of the final output chute over the static conveyors.  All now works.
Engineering on the models appears to have been adequate.  As a precaution against the finlike structures trying to splay outwards under load (thus deforming the large toroidal frame out of round) I installed 3 heavy straps at the bottoms of the fins and meeting in the central point with a M8 bolt. However, the tension in these has not increased at all after fully loading the structure, indicating that the engineering is adequate (no measureable deformation). 
The mass of the model has now been accurately calculated to be 1335kg.  This calculation was done with the help of my friend Prof Perry Kaye, retired professor of chemistry at Rhodes University.  He accurately weighed each of the most commonly used basic parts on a chemical balance and I was then able to calculate the number of each part used by subtracting what was left over from the total made.
To the best of my knowledge this makes “Bagger 288” the largest Meccano model in the world. I have not been able to find one of greater weight anywhere.  The Ripley’s Museum Ferris wheel (which they claim is the largest Meccano model in the world) has a mass of 544kg.  Purists will claim that my model is not Meccano because of the heavy gauge versions of Meccano parts used, but to the best of my knowledge the Ferris wheel is also not pure Meccano, as, for safety reasons, it had to be re-engineered, with all its Meccano strip spokes replaced by 8mm stainless steel rods.  The fact remains that it is not possible to build a model of this size in Meccano without some heavy gauge adaptation, especially if the model is as mobile and articulated as a bucket wheel excavator.  My philosophy was to rank engineering safety as first priority, followed closely by architectural accuracy.  I wanted to avoid any retrofitting, due to a collapse due to engineering inadequacy.
In conclusion, I must express my grateful thanks to the members of the lifting team. 
Dr Jennifer Williams, Prof John Williams, Dr Kevin Lobb, Andrew Shepherd, myself, Tim Bull, Landman Bester, Charlie O'Donoghue, Jessica Harris, Alexander, Richard Grant, Stefan Just

1 comment:

  1. Fine information, many thanks to the author. It is customwritings review puzzling to me now, but in general, the usefulness and importance is overwhelming. Very much thanks again and best of luck!