Eventually, the coaster train enters the cave of the yeti. Guests catch silhouetted glimpses of the yeti and hear its roars from a distance. However, the tracks bring them in for an up-close and personal encounter.

John Van Oort, Principal Mechanical Engineer, Show Mechanical Group, Walt Disney Imagineering, Glendale, Calif., explained that the power and precise motion of the application clearly called for hydraulics. He revealed that the total thrust from all 19 motion axes generates nearly 260,000 lb of force.

Rather than try to support the yeti by its own legs, the figure is supported through a steel boom attached at the rear of the figure — roughly at the small of its back.

The figure is securely attached to the end of the boom and becomes a load suspended from a cantilevered beam. The boom also provides two major axes of motion: about 2 ft of vertical motion of the torso, and 5 ft of forward-backward thrust. The boom also supports the hoses and control cables going to the yeti.

More than meets the eye

This setup allows the yeti figure to appear to have even more than 19 axes of motion. Van Oort said both feet and the left hand all are attached to structural framework inside the attraction. Legs are free to bend at the hips, knees, and ankles, and the left arm is free to move at the shoulder, elbow, and wrist. None of these joints are considered motion axes, because they are not powered.

Van Oort revealed, "There are no actuators in his legs or left arm; they just go along for the ride." The thrust and vertical motions of the boom are powered by hydraulic cylinders. The structure consists of stationary framework and a sled, which is pushed forward and backward on ways (plane bearings) by hydraulics.

Van Oort explained, "We reverted back to pre-World War II technology in that the sled runs on sliding ways, like in old machine tools. We have a huge cantilevered load, and the only way to the get the PV [combined pressure and velocity] of the bearings down was to put the sled on ways."

Mounted to the sled is a second hydraulic cylinder that pushes and pulls a second sled up and down. The beam that supports the yeti figure is rigidly attached to the framework of the sled that moves vertically. Therefore, this entire assembly generates two axes of motion.

Bringing the yeti to life

Other motion axes include left-right and fore-aft bending and twisting at the waist; turning, tilting and nodding at the neck; plus fully articulated motions of the shoulder, elbow, wrist, hand, and fingers. The remaining motion axes move the mouth and produce facial expressions.

The cylinders that produce these motions become smaller the higher they are located above the waist. This is because cylinders at the waist must move more mass than cylinders in the neck. For example, cylinders in the waist have a bore of 3¼ in., whereas cylinders in the neck have a 1½-in. bore.

All of these motions are accomplished using hydraulics, except those for the eyes. Pneumatic cylinders were chosen to move the eyes because they are lightweight and space was limited. Servo control ensures lifelike movement of the eyes.

The hydraulic system itself is powered by two variable-displacement axial-piston pumps with pressure compensation set at 3000 psi. Each pump is rated for flow of 60 gpm, but only one pump operates at a time; the other is a standby power source. Flow from the pump is used to charge accumulators, which provide the flows needed to perform the show sequence.

Hydraulic fluid flows from either pump to a bank of eight 15-gal accumulators. These accumulators store pressurized fluid until peak flow of 400 gpm is needed. By storing hydraulic energy and releasing it on demand, the accumulators save energy by allowing the system to be driven by a 60-gpm pump instead of a 400-gpm pump.

Van Oort said designers specified standard components wherever they could. However, strength and longevity for the hydraulic cylinder mounts were well beyond what are found on standard JIC cylinders. Van Oort offered, "Standard mounting configurations just don't handle the kind of duty we put them through. So we started with a standard cylinder and specified extended tie rods at the cap end. Then we made our own mounts — even head trunnions — out of aircraft-quality alloys for high strength and light weight.

"Another concern was the rod seals. We have the cylinders manufactured with extra-long glands and double rod seals. We drain the gland volume between the seals with clear tubing and route it to a place where a maintenance person can monitor flow past the inboard seal." By doing this, personnel can replace a rod seal before it fails. If it should fail, the second rod seal acts as a backup.

Management approval

At the opening of the attraction on April 7 earlier this year, Bob Iger, president and CEO of The Walt Disney Company offered, "Expedition Everest is a shining example of what we do best at Disney — create unique, high-quality, and innovative experiences to delight our guests from around the world." Walt Disney Parks & Resorts Chairman Jay Rasulo added, "This extraordinary expedition is another example of the lengths to which Walt Disney Imagineering will go to research subject matter in developing our story lines."

Click here to watch a video explaining more of the hydraulics technology behind the Yeti..

For more information and images covering the technology behind Expedition Everest, click here to view an article published by our sister publication, Machine Design.