The History of the Rotary Solenoid

Rotary solenoids are electromechanical devices used for a variety of applications. They have a strong connection to Dayton and are vital for long life operation in challenging environments.

What, Exactly, is a Rotary Solenoid?

A rotary solenoid uses three ball bearings to convert linear motion to rotary motion. When power is applied, it rotates a ratcheting mechanism using an electromagnetic coil. When the solenoid is activated, the core is pulled into the center and the disk rotates. A spring pushes it back to its starting position. The distance the ratcheting mechanism moves and the amount it turns can be controlled. When the solenoid is activated, the core is once again pulled into the center and the disk rotates. A spring pushes it back to its starting position as soon as power is removed. As power is generally not provided continuously, this means most rotary solenoid switches will default to “locked” or “off” if the power goes out. Rotary solenoids are used in some locks, to dispense things from vending machines, as well as in many industrial automation and medical applications.

The “solenoid” part refers to the coil and comes from the French solénoïde, which in turn comes from the Greek sōlēnoeidḗs, which means pipe-shaped. Thus, it relates to a looping of wire that is longer than it is wide, although not necessarily straight. The solenoid is also used to refer to some other devices which convert energy into linear motion.

Rotary solenoids run off of DC power and require a bridge rectifier if being used from an AC power source. Their compact size makes them useful in areas where space is limited. However, they do tend to release some of their energy as heat, which sometimes causes problems if they are under continuous operation.

Who was George H. Leland?

George H. Leland was the founder of the Leland Electric Company and Ledex, Inc. He was born in Dayton, Ohio, in 1887, and died in 1969, at the age of 81. He had seven siblings and four children with his wife, Hazel. His first plan was to be a teacher, and he taught high school for two years before realizing it was not for him. General Electric promptly hired him in Fort Wayne, where he became a designer and taught himself advanced mathematics. He moved on to find the Leland Electric Company, which made fractional horsepower electric motors. He sold his Leland stock after the war to focus on his second company, Leland Development and Engineering Company, later called Ledex, Inc.

However, he is particularly well known for inventing the rotary solenoid. There is a book about Leland’s life, written by his son, Harold, under the title of The Story of George.

When was it invented and Why?

The rotary solenoid was a war-time invention. It was invented in 1944, with the assistance of an engineer named Earl W. Kerman, as a better way of releasing bombs from aircraft. Previously, linear solenoids were being used, but under the heavy shock and vibration, or when under fire, they had a habit of inadvertently releasing, allowing the bomb to fall on an unintended target. Earl Kerman was responsible for the design of the matching bomb release shackle. The design was patented as number 2,496,880.

If you want to see one of these bomb shackles, known as the A-4 bomb release, there’s one at the National Museum of the USAF in Dayton, Ohio, attached to a B-29 aircraft fuselage.

What Happened Next?

After the war, he offered the design to Leland Electric Company, but the board saw little commercial value in it, which led to his resignation as chairman. Instead, the solenoid was made by Ledex, which became a specialist solenoid company that Leland continued to run until Parkinson’s forced him to retire in 1958.

Ledex found a variety of uses for George’s invention. Primarily, during the 1950s, rotary solenoids were used as industrial switches. Their use in locks was also early. Rotary solenoid switching proved to be reliable and long lasting, with an operating life of up to fifty million cycles. It also proved to be very fast, which led to another application: Camera shutters. The controlled high-speed actuators remain useful in imaging applications.

Rotary solenoids became a standard means of switching for high speed and high longevity applications, and they led to the success of the Ledex brand, which also sold linear solenoids, and eventually variations on the rotary solenoid such as the Ultimag and BTA solenoids, the former being capable of even higher speed.

What Happened to Ledex?

Ledex became and today remains the leading brand of rotary solenoids. Ledex brand solenoids were already being licensed internationally in the 1950s. The company was run by Leland himself until 1958, and then by family members. The company was purchased by Lucas Industries, a British owned company, which kept it under family management.

The brand was eventually acquired by Hong Kong-based Johnson Electric, which was founded by Mr. Wang Seng Liang and Mrs. Wang Koo Yik Chun to produce micro motors. The company now manufactures and distributes a wide variety of products including all kinds of solenoids, small motors, switches, etc. Ledex falls under its Industry Products Group.

What are Rotary Solenoids Used for Now?

Rotary solenoids are now used in a wide variety of applications where a long life (and speed) is required, and space is limited. The primary uses remain industrial switches and electronic door locks. They can be used to replace small DC motors or stepper motors if the angular movement is minimal and are generally used when space or power concerns does not allow for a motor. Other uses include camera shutters, valve controls, vending machines, gaming machine and some automotive applications. They were also used in electronic typewriters and dot matrix printers. More recently, they have seen a lot of use in robotics. Solenoids use less power than micro motors and remain essential in applications Mr. Leland would likely never have visualized.

To find out more about the many uses of rotary solenoids and to talk about how Ledex brand rotary solenoids can help with industrial and medical applications, contact Johnson Electric today.


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