Monday, 4 November 2013

Motorisation again

Motorisation of the model is now quite advanced. All 15 of the crawler units have had their motors and gearboxes installed.  The gearing in each consists of 5 steps of reduction, all done with my large-tooth steel gears using pinion/gearwheel clusters bolted together with 3mm bolts and nuts and brass spacers between, as explained in an earlier blog-post, so that large torque transfers are possible.  The first 3 shafts are 4mm rods, the latter 3 shafts being 8mm, since torque increases as one goes done the line of reduction steps.  The final drive to the 4½ inch nylon wheels was explained earlier.

Crawler gearbox from below

Benchtesting a main crawler unit

All 12 main crawler belts installed on crawlers

The crawler belts have now been assembled and fitted.  This presented quite some challenge as my original idea of bolting the treads together with locknutted bolts through holes drilled larger than 4.2mm did not work smoothly.  I had hoped to obtain bolts which were threaded only part way along in order to facilitate the movement through the holes as the belt flexed.  Unfortunately I could only get fully-threaded bolts which tended to jam up.  I therefore had to revert to the tried and trusted method of bolting 1 inch girder cleats to the treads and then locknutting fishplates to the cleats.  This meant an extra 3,600 parts needed to be made, with many more nuts and bolts for the locknutting.  I also needed to increase friction between the nylon wheels and the steel crawler treads.  This was done by attaching a small piece of 1.5mm insertion rubber to the inside of every tread.  The attachment was done using a high-performance rubberising adhesive called Sikaflex 221 as well as two 12mm bolts with locknuts for safety (locknutting to prevent squashing the rubber inserts on tightening).

The result is a smoothly-running belt which develops immense breakout force. (The more vertical load applied the more horizontal force results, according to the well know law of frictional force versus normal reaction) Each of the main crawlers happily carries my weight (85Kg) – this is about what is expected from each when finally assembled.  All 12 of the main crawlers have now been assembled as seen in the photograph.

The 3 small crawlers for the auxiliary unit have also been completed.  These have been fitted and the complete unit tested. Again, it happily carries my weight which is about what is expected of this auxiliary unit. (Note that the wheel/gearing specifications of the smaller crawlers are the same as for the main crawlers – it is only the frame size which is smaller.  Hence, all 15 will travel at the same speed.
Crawlers installed on auxiliary unit

Another view of auxiliary unit
Shown in the accompanying photograph are 4 motor/gearbox units which will be used to slew the upper arms of the machine.  Each employs 6 steps of reduction.  The final gear (48 large toothed) is attached via an aluminium spacer to a 4 inch aluminium wheel as seen.  These wheels will have teeth cut into them to mesh with the large ring gear created using 8mm bolts protruding outwards on a heavy steel strap.  I will leave this operation until the gearboxes have been fitted and I can see the final position taken up.
A slewing motor gearbox

All 4 slewing motor gearboxes
The 4 inch aluminium gear takes 100 seconds per rotation.  This will give a complete slewing time of about 20 minutes.  Although this is a low angular speed the linear speed at which the bucket wheel passes will be reasonable, since the latter sits 16 feet out from the vertical axis of rotation of the machine.
Another set of motor/gearbox units is the set of 3 steering gearboxes shown here. 
A steering thruster unit
All 3 steering thruster units
These drive 12mm threaded rods into small aluminium blocks attached to the crawler frame bridge supports on their steering tillers (and tapped out to 12mm holes).
Shown next are the 4 inch aluminium pulleys at the tops of the 2 upright arms.  These were all cut from an aluminium log 102mm diameter, 1.2m long.  My 455mm abrasive cut-off machine was not up to the job but my friend Dixie Westcott came to the rescue and cut the log into 40 slices on his industrial bandsaw for me. This took 3 days, with numerous breaks to allow the bandsaw to cool!
Some of the 34 pulleys in the upright arms
The day is approaching when I will have to assemble a team of 8 people to help me lift the central unit onto its crawler system.  Shown in the picture here is the wooden cradle I will bolt onto the top of the central unit.
Wooden lifting cradle
Below are shown 2 correct-height trestles on which to rest the cradle which the crawler system is completed.
Main unit working trestles
In the next picture are some components of two 9 foot high scaffolds I will need to set the upright arms in place (one on either side for safety).  Once the cables are in place on the aluminium pulleys the 2 winding drums can be started up so that the machine can at last support itself.

Components of a 9 foot scaffold
A fairly major still to be done is the installation of about 2 kilometres of electrical wiring (including a 10 metre main harness connecting machine to control box. This will have 110 cores, 2 per motor and 2 for lights).

Final delivery arm positioning winch

Wednesday, 3 July 2013

More pictures

Auxiliary unit - frontal view

Rotating yoke on auxiliary unit

Main roller-race and ring gear detail

Counter-balance arm to triangular frame joint

One of 3 fin-like supports

Counterbalance arm detail

View down throat of main frame structure

Main roller-race and ring gear

Service crane and winding drum

Winding drums

Auxiliary unit

Conveyor belt detail inside main frame

Universal suspension of transfer arm


2 motor drives onto winding drum

Wednesday, 19 June 2013

Prototype pictures - Bagger 288

Prototype at work
Main crawlers on prototype

A slight accident with a bulldozer - see yellow bulldozer on top of wheel!
Close view of a bucket

Crossing road to new mine

Meccano model: selection of pictures

12crawler track frames and 3 support bridges

19 foot delivery arm
More work on the auxiliary unit

More work done on auxiliary unit
Winding drums and service crane
14.5 foot bucket wheel arm

Bucket wheel - note protective box for its motors

Driver's cab on bucket wheel arm

Delivery arm

Tuesday, 18 June 2013

Motorisation of the model

I have acquired a 1000 watt transformer/rectifier unit with cooling fan (as shown above), made by Postma and Postma of Port Elizabeth.  Output current is 83 amps at 12 volts (12 x 83 = 996).  Steve Craigie (my electrician) and I tested a single window winder for current drawn.  This was1.8 amps (no load) and 3.5 amps (maximum load).  The 83 amp output will allow the whole model to move – that is, 15 motors.  One should also be able to run the conveyor belts at the same time as their load is very light (10 motors).

I have also done much work on the gearing systems for the motorisation.  All gears and brass fittings have been made.  On the board shown below are about half the total large-tooth gears needed.  In the photograph are two gear clusters, each consisting of a pinion and a gear wheel joined by means of a brass spacer and four 3mm bolts, with nuts.

This allows transfers of very large torques in the reduction gear boxes without the risk of grub screws tearing loose on the axles, which would certainly happen at the high torque needed to move a model of this weight.  I calculate that the final drive wheels in the crawler tracks will rotate at just under 1 rpm.  This gives a linear speed of about 350mm per minute. 

Final drive wheel/gear assembly

Final drive wheel/gear assembly

Components of final drive wheel/gear assembly.

 In the photographs is a final drive wheel/gear assembly.  In the third photo are components of this assembly: two 115mm nylon wheels of 8mm gauge, two 4" face plates (2mm gauge versions of the Meccano face plate), two aluminium spacers with Meccano’s familiar octet of holes and a 48 tooth gear wheel (2mm steel). Eight 65mm stainless steel screws hold the cluster together through the central octet of holes while the nylon wheels each have an octet of countersunk screws fixing them to the heavy face plate.  Countersinking was necessary because clearances between wheels and the crawler track frames are tight. 

To drill the holes in the gears and pinions accurately I have had another drilling jig made by Derek of K.V. Precision Engineering of Uitenhage.  This is shown in the below photograph.

Drilling jig
As you can see it is a beautifully-made tool.  With this I can drill up to 3 concentric octets of standard Meccano holes as well as a small radius quartet of 3mm holes which I use to create the clustered gear wheel/pinion sets shown earlier.  3mm screws were necessary in order to avoid hitting the 8mm shafts or the teeth of the small pinions.