I've always thought that the heavy equipment used in logging operations was pretty interesting. Forestry machines have made appearances in a few of the Technic sets released to date (e.g. 8069 and 8265), but always as the B model. I'd like to see some logging equipment given the proper treatment as the A model in one or more Technic sets.
This model is based loosely on the Caterpillar 545C Wheel Skidder. My goal was to make the model as close to the original in terms of functionality, but, as I have noted below, I had to compromise in a few spots.
The images below were created with LDD. Because LDD is not yet able to bend flex axles, there are two pieces missing from the front (the "roll bars" that extend from the front of the hood to the front of the operator's cabin). Also, LDD does not yet support mini linear actuators, so I used the substitute mini linear actuator created by Zblj (which can be found here: http://www.eurobricks.com/forum/index.php?s=ade9802b23266864529aa74fa359707e&showtopic=52005&st=15) .
Here is a picture of the real thing at work:
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Finally, there is the business end of the skidder -- the arch and grapple tongs. The upper and lower arms of the arch are controlled by two knobs on the left side of the tail section, while the grapple tongs are controlled by a single knob on the right side. The grapple tongs can swing freely in the forward-backward direction. There is also a cable winch with a simple ratcheting mechanism.
A few notes here: On the real 545C, there are a pair of hydraulic rams for each section of the arch. Due to space constraints, I had to use just a single mini linear actuator to control the upper arm. Also, on the real 545C the grapple tongs have a few more degrees of freedom -- they can swing left to right, as well as front to back, and they can rotate. I simply couldn't duplicate all of that complexity here.