Peer into any wind-up toy you had as a child or, later on, that cursed mechanical alarm clock and you’ll find a power spring. The spirally wound hardened metal strip that makes up a power spring is designed to store mechanical energy. Once wound and filled with potential energy, the power spring can release energy to do work quickly, in the case of a tape measure or slowly as they have for hundreds of years with clocks and watches. Most power springs and spring motors are produced by tightly winding spring material on an arbor and attaching the free end to a case. The spring’s natural tendency to expand imparts a momentum to either the arbor or case, producing usable torque. The total torque generated increases as the spring is wound tighter and decreases as it unwinds. Power springs and spring motors work inside a housing and exert this torsional force against either a central shaft (arbor) or the housing. Power springs and spring motors are made from a variety of different materials, depending on their application. Smaller springs can be made from beryllium copper or similar alloys. Larger springs are made from a high-carbon steel called blue clock for its color. The energy storage capacity (ESC) of a spring is determined by bending (deforming) the material in a spring.

As mentioned, the simplest power springs present torque that is proportional to the amount of energy stored or how tightly the spring is wound. A constant-torque power spring or spring motor attaches the free end of a spring to a larger drum. The spring is wound around the drum in either the same direction or in the reverse direction. This configuration allows the spring to unwind presenting a steady force tangential to the coil of the spring.

Features

Power springs are also known as clock springs and have gathered many other aliases over the years such as:

• coil springs
• mainsprings
• hair springs
• wind springs
• spiral springs
• recoil springs
• retractor springs

The performance of the spring is governed by the total number of turns and by the dimensions of the spring material (thickness, width, and length) and the space available (housing and arbor diameters). It is important to realize that as the spring winds and unwinds, contact friction between the concentrically wound spring surfaces will also change.

Applications

Power springs and spring motors can be found in a surprising number of applications. Power springs are the heart of tool counterbalances to keep power tools within easy reach of servicemen and assembly workers. Tape measures are an omnipresent tool that uses a spring motor to retract the tape. When higher torque is needed, power springs can be combined to create high torque spring motors. When constant loading is needed, spring motors act as excellent tensioners for conveyor belts, chart recorders, document sorters, and point of purchase displays. Tool retractors use power springs to pull tools up and out of the way not in use. Torque from a power spring can help lift machine covers up and out of the way while loading and unloading parts. Garage and other lifting doors can use a power spring to help the operator lift a door up and keep it open. Sliding commercial doors like those found on refrigerators in the drink section of the corner store use a power spring to close the door after it has been slid open. The long running capabilities of power springs make them ideal to use to provide rotational power to clocks.

Power spring and spring motors are attractive devices to consider in your application that require charging and then release of rotational kinetic energy. Their compact size and simple design help make them relatively worry free in application. Materials, dimensions, and option combinations of power springs and spring motors make them an attractive consideration for your rotational and torque needs.

Related Information

CR4 Community—Power Spring (Clock Spring) Design

Image credit:

Atlantic Spring, Inc., A Division of MW Industries