Friday, September 2, 2016

Churchill bridge layer

Powertrain: 2 XL motors
Main actuator: L motor
locking mechanism: servo motor
controlled by Sbrick

The Churchill AVLB (armored vehicle-launched bridge) is one of my most interesting and challenging models that I have recently built. The mechanism utilized to lift the bridge required a lot of development to ensure that it is structurally strong enough to lift and hold the bridge in mid air.
The operation of deploying a bridge is a rather interesting process, most modern AVLBs utilize a scissor bridge to accomplish this task, the Churchill AVLB is rather different, as it uses a singe hydraulic cylinder to accomplish the task. The laying system consists of three major components, the main arm, lower carriage, and hydraulic actuator. Though in this case a brick built linear actuator is used with a 1:24 reduction coupled to a L motor, giving it 155 ncm of torque. The sequence for deploying the bridge first involves the linear actuator retracting fully, to lift the bridge at 90°, then a servo motor engages with catches on the lower carriage. This is used to stabilize the vehicle, and prevent suspension buckling from the weight of the bridge being supported by only the first couple road wheels. Then the linear actuator extends again, this lays the bridge to its final position. To lift the bridge, the process is repeated in reverse order.

The biggest difficulty of building this model is ensuring that the design of the linear actuator and supporting structure of the main arm were anchored to the suspension assemblies  to prevent separation or failure during the process of laying the bridge. There is an immense amount of force (for lego pieces) applied on the actuator, when the bridge is in the final stage, as the position of the main arm is at a rather shallow angle, and the leverage on the actuator. The lightweight ≈1kg bridge effectively weighs ≈9kg, because of the position of the lifting point on the main arm. The next challenge was to ensure that the locking mechanism was strong enough to retain the bridge in the upright position. There was two variants of the locking mechanism, the first involved an auto locking mechanism, by utilizing contacts on the back of the carriage to move the catches into position. This mechanism worked most of the time, but wasn't robust enough for my liking. So it was converted to a simple locking mechanism that was more durable and reliable.

This model is my second rendition of the AVLB variant, the first version was attempted in 2014, though the model was abandoned, as I wanted to use two linear actuators to lift the bridge, but they weren't strong enough to lift the weight of the bridge. I wasn't too fond of the suspension on the Churchill either, as it was incorrectly spaced, resulting in a model that was proportionately too long. To create a proper suspension, standard intervals wouldn't work. I decided to use axles and technic perpendicular axle/pin connectors to create 2.5 stud spacing between the road wheels. As the Churchill utilizes 11 road wheels per side, this resulted in a large amount of parts being used for the suspension alone. The first and eleventh suspension units were rather difficult to replicate, as they are both angled, this allows the churchill to cross trenches and go over obstacles. The eleventh unit was more difficult than the first one, as it was in close proximity to the drive sprocket. The upper rails of the sponsons are faithful to the Churchill, as it lacked return rollers. One interesting fact about the suspension, is that several road wheels/suspension units could be damaged, and the churchill could still be combat operational.

This variant of the AVLB was my primary choice, as I wanted to replicate something unique. There are a variety of different combat engineering vehicles built on the Churchill hull. (ARK, Crocodile, AVRE, Toad, Kangaroo) I originally intended to build the AVRE (armored vehicle royal engineers) with the small box girder bridge, and shooting spigot mortar. Then reuse the hull to build the AVLB. I first started building the hull with a four speed transmission, but the transmission proved to be too large, and had lots of issues with gears slipping. I decided to abandon the transmission and use a simple direct drive system.Though as summer was busy, and college was approaching soon. I decided to build this variant, as I had failed previously, and this has always been a vehicle I have wanted to replicate. I am rather pleased with the aesthetics of the model, as it used nearly all of my light bluish gray tiles and 1x plates in the bridge and hull details. I utilized a 1/35 Churchill VII model as a reference for most of the sponson details. Though my model is based on the Churchill IV hull, the only major difference that is noticeable is the crew hatches on the sponsons are square, as opposed to the round hatches in later variants.

a couple photos of the real vehicle in operation

A couple photos of the old, abandoned version from 2014


More photos

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