Robinson got what he needed with innovations that show promise of finding their way into a variety of industrial hydraulic systems. To control the cylinder's position accurately along its stroke, his team attached a small rack on the cylinder rod, Figure 2. The rack engages a pinion gear to position the sweep of a feedback potentiometer as the rod extends or retracts.

Voltage from the feedback pot passes to the proportional valve, as does voltage from the command pot in the manual controlled. When the voltage of the command pot in the control box equals the voltage on the driven pot, the proportional valve shifts and blocks fluid flow to the cylinder.

The problem of deadband lag was resolved by Ernie Escobar, of SLI Industries, by removing 0.030 in. from the width of the spool lands at the input and output ports, Figure 3. Reworking the valve spool was the key: it reduced the deadband to zero and eliminated any time lag between remote control commands and Kong's responses. When the control operators moved levers to order Kong to caress Dwan, he did so instantly, creating the illusion of a living creature on screen.

Aside from these modifications, Kong's hydraulic system was made with off-the-shelf components, put together in a hurry with products from nearby distributors and manufacturers.

All hydraulic components were rated for 3000 psi because Robinson and his staff wanted to be sure they had all the power they needed. As it turned out, they over-specified, because Kong behaved perfectly operating at pressures to only 1200 psi supplied by pumps delivering flows from 15 to 35 gpm.

"Our Kong is full functional", Robinson said, "He wiggles his arms, turns his head, twitches his ears, rolls his eyes, bends both legs, pulls his mouth back to show his gums, rotates on his hips, thrusts out his legs, and, when he has to, smiles.” All hydraulically.

"He really frightens you, he is so thoroughly alive," reminisces Robinson, "even when you realize he is only eight tons of hydraulic animation, latex skin, horsetail hair hide (1012 lb of it), and 3½ tons of aluminum framework."

Kong preformed with minimal time out to repair frame breakage and fluid leaks. The leaks were often caused by a technician in a hurry, who had not properly tightened a hydraulic connection.

Cost-saving as well as time-saving

What did it cost to animate King Kong with hydraulics? Even with the job completed, Robinson can only estimate.

"But," he told us, "it was only a fraction of the cost of electromechanical animation. You can begin to get some idea from our having Kong ready for the cameras in less than five months, compared to the year-and-one-half estimated for the electromechanical proposal."

"I had told De Laurentiis we probably could do the bare-frame hydraulics and aluminum tube skeleton for around $500,000. Adding two extra arms (for close-up, interior camera shots, see box), and the hydraulic pumps, which were exterior to the studio to avoid noise pickup, and renting cranes to lift the extra arms and erect the big ape, the total possibly went to $800,000 — still several times less than the cost of the electromechanical animation.

"We started with a budged of $1.5 million for all special effects, excluding the animated Kong. Only an auditor's accounting could verify it, but I've been told that we wound up with a budget surplus of about $385,000."

This article was originally published in the April 1977 issue of Hydraulics & Pneumatics. It is reprinted here for its historical and archival value.

Click here to see the theatrical trailer, which shows footage of the electrohydraulic Kong and the electrohydraulic hand that clutches and appears to lift Jessica Lange.

How do you animate a 42-foot, 11½-ton hairy ape?

After Dino De Laurentiis gave the go ahead for Glen Robinson and his crew to build a hydraulically animated ape, they set out to create King Kong. Four-and-one-half months later, Kong was erect, on the set, and ready to perform — a mere half hour before filming. At director Guillermin's commands, the huge Kong instantly and smoothly raised his hands 55 ft into the air; blinked his eyelids; moved his eyes to "focus" on the object of his gaze; and curled or uncurled his 8-in. diameter fingers. Cylinders moved aluminum plates in his chest to make him "breathe".

Hidden within his hairy figure were hydraulic muscles and their commanding nerves — 140 hydraulic cylinders; 84 remotely-controlled proportional valves (stacked 16 high in his belly); two hydraulic motors to turn his wrists and arms; 150 potentiometers; and 3100 ft of hydraulic hose, with as many feet of remote control electrical wiring. To operate the extra pair of arms for detail shots, there were: 50 cylinders; 34 proportional valves; two hydraulic pumps and motors each; potentiometers for each cylinder and motor; and hundreds of feet of hydraulic hose and electrical wiring.

For remote control of each animating cylinder and motor, nearby benches were arrayed with 150 manual control boxes and their actuating levers, manned by as many operators as were needed for a particular scene. Each control box included its own potentiometer to sense control lever position, and to electrically transmit it to the remote proportional valve commanding the related hydraulic cylinder or motor. The photos in this box show some of the stages in construction of the giant King Kong.