Compression Springs

Load or force:

The purpose of a Compression spring is to deflect (compress) and that movement is resisted by the material. The resistance is a force with direction opposite of the deflection that is expressed in pounds (lbs.) or kilograms (kg.). Press the spring down the springs pushes up. Press the spring right the spring push back to the left. This force is often expressed as the Load of the spring. Technically if a load is placed on a spring, then the spring deflects until the force of the spring resistance balances the force of the load placed upon it, in this description the Load placed on the spring is also a method of expressing the force of the spring, because they are equivalent.

Rate, factor (k) or spring constant:

As a spring is compressed its force increases. The more deflection the more force. The ratio between Force and Deflection is the rate. The rate always has two units like pounds per Inch (#/” or Lbs./in), Kilograms per mm (kg/mm), or other similar ratio.  In old specifications the rate could be called out simply in pounds, where the default was per inch.  A spring with a 15Lbs./in. rate if deflected 1 inch would have 15# of force.

If the same spring where deflected another inch it would now have 2 total inches of deflection for a resistance force of (1 inch +1 inch) x 15 Lbs./inch for 30 Lbs.

There is some confusion about the use of the word “Rated”.  If someone were to have a spring that could safely hold a force of 30 Lbs. they might use a phrase “The spring is rated for 30 Lbs.” The spring might have a 30 Lbs. spring rate but might also have a 15 Lbs. rate like the earlier example. If the spring was deflected 4 inches to get to the safe load of 30 Lbs. then the actual spring rate would be (30Lbs. ÷ 4 inches = 7.5Lbs./Inch). Because the terminology could be confusing, it is best to say a spring can with stand 30 Lbs. or a spring can safely be loaded to 30 Lbs.

Free Length, Over All Length(OAL), and Spring Length:

This is the length the spring is in an unloaded length like while sitting in your hand, or on a table or in a box. This is where the force on the spring is 'zero', the starting point where the deflection (or movement) is calculated from.

On basic springs where the print or specification does not require actual loads to to held at a fixed postions, then the free length is a point that can be held to give the spring more consistant operation.

Care might be needed on springs with very light rates but quite heavy spring weight. A spring that weighs 3 pounds but has a rate of 10 pounds per inch will measure one length resting on its side horizontal, but measure shorter when standing straight up vertical, because its own weight shortens the spring.

Outside Diameter, Mean Diameter, and Inside Diameter:

Outside Dia. is the most common and easier to measure. Inside Dia. is often listed if a spring must work over a rod or raised 'cap'. Mean Dia. is common on older prints and useful when a spring works where both the inside and outside of the spring are restricted like a hole or rod. It is based to list anything the spring works over or inside of so you spring maunufacture can look for conflicts.

Type of Ends:

4 main types of ends

   Closed Ends Not Ground          Closed Ends Ground


         Open Ends Ground        Open Ends Not Ground

Also a special end used might have extra coils closed 1 1/2 or 2 full closed coils each end. This could be used to help reduce tangling or a structured design requirement. Other type of ends would be combinations where one end may be left open and one end closed, but this is not common.