The 8837 wheeled excavator, released in 1992, is a goofy little set. It is highly functional for its size, but the proportions are wrong somehow. It doesn't look like any excavator I've ever seen.
This model uses one normal sized pneumatic actuator and one of the new small variety. It also uses the new small scoop. Its best feature, in my opinion, is the central axle which controls two functions, steering and slewing. This is the only set to use the central ring gear of the turntable.
Construction of this set is far simpler than the older8851 - Excavator which required an extra arm to complete comfortably.
This model contains a double acting pneumatic system. The components of this system are connected with rubber tubing routed through and around the holes in the beams as well as through some rigid tubes from the flex system.
A single pump is located on the top of the vehicle. Depression of the piston produces pressure. The pressure is split and fed to the inlets of two selector valves (switches) with two outputs each. Selection of the switches in either direction allows pressure to flow to either chamber of the actuators.
Finally, there are two pneumatic actuators which have ports at the head and rod ends to accept input from the switches. Head end pressure extends the piston, while rod end pressure retracts it.
Note that the clear tubing in the photographs is not original. It should be gray for supply and black for output.
The color coding of the tubing in the computer image can be used with the subsequent images to figure out how the pneumatics are routed.
The front (or are they rear?) wheels can be steered using an overhead "Hand of God" control. This control passes through the center of the turntable and mates with a 24 tooth crown gear. The next axle drives a flexible steering rack with a 16 tooth spur gear. The steering itself uses the steering arms which are facing forward in this case. Not too good for caster, better drive it slow. (Actually, there is no caster on these parts regardless of orientation, but I couldn't resist.)
The boom can be luffed pneumatically. An actuator pushes to pivot the boom up. Although retract pressure is also available, the boom can rotate down under its own weight, but the retract pressure would be useful for forcing the bucket into the ground.
The dipper is fixed at one angle and cannot be pivoted (without breaking something). The M.C. Escher-like triangle which accomplishes this angle is kind of fascinating. Stare at the computer image for a while to see what I mean.
The bucket is pneumatically driven with one of the new small actuators. It fits just right here since a full sized until would have looked completely out of place on this jib.
Nothing fancy going on with the mechanism, the bucket just pivots around an axle. The actuator attaches to a link arm which drives the bucket.
The 360 degree rotation of the cab is accomplished quite cleverly. When the steering knob is lifted upward, it disengages from the steering crown gear and meshes with an 8 tooth pinion driving the inner ring gear of the turntable. This causes rotation of the knob to rotate the whole cab in the same direction, but at a different rate. Nice touch.
This set uses 4 white Model Team wheels and tires.