Designing Your First Extension Spring in SCP

Written by: Dr. Richard Dignall, Technical Director, Institute of Spring Technology

Start SCP and click to design a new Extension Spring.

Before doing anything else, check the units, design standard and tolerance standard that are selected. These default to Metric, EN 13906-2 and DIN 2097 (at the time of writing, there is no EN tolerance standard for extension springs). If you need to create your own units to match a drawing, you can do this via Tools > Options. Watch our explainer video here.

Then we work down the left hand side, starting with the Required Data section which deals with the basic inputs that you probably know before starting a design, such as material and end loop type. (Sometimes the material choice will change when you look at the design outputs, but generally you will have a pretty good idea where to start.) You can leave these as they are for now.

Then we come to the Design Parameters region. This is where the basic shape of the spring is entered. We’ll start by leaving the dropdown design option at the top to “Set Initial Tension, Free Length + 3 Other Parameters”.

You can see there are 6 parameters, which together define the spring. Down the left is a column of tick boxes; these tell us which values are specified. You can see that the Initial Tension and Free Length variables are already ticked. This is because the other four variables are related, but the initial tension and free length of the spring are independent, so we must specify them.

Click into the value input box next to Wire Diameter and type 3. As you do this, the tick box is automatically checked to show that we have specified this variable.

Press Enter to move down to the outside diameter box. Enter 20 for this (you could also specify an inside or mean coil diameter by clicking on the underlined label Outside Diameter) and then 25 for the total coils. At this point the spring rate box becomes greyed out and you can’t edit it, and the same with the tick box next to it. This shows that the rate will be the calculated variable.

Type in 50 for the initial tension and 106 for the free length (and press Enter) and lots of numbers will appear all over the screen. This is because we now have enough information to calculate our spring. We haven’t entered any operating data (how the spring is being used), but there’s still plenty of information.

First of all, you can see the spring rate of 6.72 N/mm. Then on the right hand side we have the Calculated Data and Stress Data areas. The Calculated Data is general information about the spring in its free position. You can fully customise what is shown in this grid from Tools > Options > Calculated Data.

If we look in the Stress Data grid, we can see there are 5 grades to the chosen material (EN 10270-1 drawn carbon steel), which are SL, SM, DM, SH and DH. Some materials will have a single grade (e.g. ASTM A228), some several. The tensile strength of each one is shown next to the name. The last column is the initial tension stress, as a percentage of tensile strength. So as the strength increases, the % Tensile value reduces.