Pre-load refers to a state of a spring in a cyclical application. When the spring cycles in the application it goes from its longest length/tallest height to the shortest length/height. The point with the longest length/tallest height in the application will determine the "pre-load". This refers to the loading of the spring at this point. For optimal life the pre-load needs to be matched carefully to the shortest length/height, but in general the spring should not be allowed to return to the free length/height in a cyclical application. This is because the change in stresses between the operating extremes will be significantly increased when using the free state of the spring instead of a "pre-load". Associated Spring RAYMOND engineer's can help minimize stresses to maximize life for your OEM application, simply contact us.