Hydraulic steering systems are not like other mechanical devices. They have no “dead center.” That is, eight turns to hard over does not mean the rudder will be amidships by turning eight turns back. This nature of hydraulic system comes from the inevitable “blow by” of fluid around the piston seals and, in 1912, the introduction of air into the system both at the pistons and at joints connecting up the piping and tubing. The lack of a true mechanical dead center is why the springs had to be installed.
Four pistons were involved in the total system. Two in the telemotor on the bridge controlled flow of fluid in the system. Two others on the steering motor operated the steam-powered steering engine that actually turned the rudder. When the telemotor was turned say to port, the steering engine would rotate the quadrant gear to the starboard and, thus, cause the rudder to wing to port and the ship would go left. Letting go of the wheel simply stopped the steering engine from rotating the quadrant so that the amount of rudder applied would remain constant.
Rotating the bridge steering wheel back the same number of turns as had been made to port did not return the rudder to exact dead center due to the “slippage” outlined above. As the wheel rotated through the desired dead center a small pump injected fluid into the system to replace any leakage. This pump made an audible “thunk” and told the quartermaster the steering wheel was centered. A small tank for extra fluid remains in position near the top of Titanic’s telemotor and is easily spotted in photographs.
Back aft, the rudder was not necessarily dead center. So, bypass valves in the telemotor allowed fluid to circulate freely in the system. Here’s where the springs performed their primary duty. They provided the power to center the pistons on the steering engine which would, in turn, center up the rudder. Once that happened, all pressure in the hydraulic system effectively disappeared and the rudder remained centered until the steering wheel was turned once again.
While the system depended upon those springs for centering the wheel and rudder, they also provided “live” feedback to the quartermaster. The steering became noticeably more difficult as the wheel neared hard over. Likewise, any resistance to coming back to dead center changed as the system neared center.
As I have said before, rollback as needed for safely steering an automobile would have been anathema in a ship. Vessels need steering systems that do exactly the opposite – allow the steering apparatus to hold a set angle without the “driver” (a.k.a. quartermaster) forced to fight the wheel.
Sometime prior to 1906 a Brow’s telemotor magazine ad appeared extolling the system’s alleged “rollback” characteristics. I suspect is was a lubber of a copywriter who originated this myth. But, it stuck. And it stuck hard, even in the company’s later advertising. I’ve even seen generic telemotor systems described in textbooks with exactly the same wording as in the pre-Titanic advertisements. And, a half century of repetition in otherwise reputable publications has made the myth impossible to overcome. But, I’ve had the privilege of studying up close and in person with hands on a telemotor nearly identical to Titanic’s. The machine does not match the myth.
– David G. Brown
PS -- If you want to see some early 20th century mechanical engineering a its finest, explore the various valves used to open and close the throttles of steam steering engines upon command from the ship's helm.
