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