Compression springs and Extension springs
1) In general, we find it best to have some idea what OD (Outside diameter) you are looking for. Is the spring a quarter inch or two inches in diameter? The OD is the first group of digits in our part numbers.
2) After that, you should have some idea what the length of the spring might be (with no load on it). Is it a quarter of inch long, or 6 inches long? The Free Length is the second group of numbers in our part numbers.
3) And, last but not least you should have some idea of the loads that you want. Usually, it is best to convert this information into a spring rate. The spring rate is how much force (pounds) it takes to compress the spring one inch. The spring rate is the third group of numbers in our part number.
While you may think you don't know this, you probably do have a pretty good idea. Is it going to hold a feather, or an elephant? Does it move 1 inch or one foot when you put the elephant (or the feather) on it?
Once you know how far a spring moves under some load, then all you have to do is divide the force by how far it moved. the answer is the spring rate.
Fortunately, there are a few tricks here that might be helpfull. First, the free length has a zero load. So if you know it, that's half the job. After that,see below.
For compression springs:
a) To get a feel for how much load you want, you can take a bathroom scale and press on it. If you have an old spring or a similar spring, you can put it on the scale and press on the spring. This can also give you an idea of how far the spring moved when you were at the load you wanted.
b) Another way to get there without a scale is to know something about the loads. Lets say the spring will hold a stack of plates that weigh 1 pound, and you want it to compress 2 inches when the plates are placed on the spring. That is 1 pounds divided by 2 inches or 1/2 or .5 pounds per inch spring rate.
For extension springs:
a) To get a feel for how much load you want, you can take a "fish" scale and pull on it. You could also use a hanging grocery scale at to the produce department of most markets. If you have an old spring or a similar spring, you can put one end of the spring on the scale and pull on the other end of the spring. This can also give you an idea of how far the spring stretched when you were at the load you wanted.
b) Another way to get there without a scale is to know something about the loads. Lets say the spring will hold a fish that weighs 1 pound, and you want it to extend 2 inches when the fish is placed on the spring. That is 1 pounds divided by 2 inches or 1/2 or .5 pounds per inch spring rate.
c) Extensions springs can also be "whimpy" or "stiff". Hold the spring sideway by one end and let the other hang. Does the other end sag or stay in place? When you flick the other end with your finger does it snap back or wave around? (We'll ignore the obvious analogies). This is called "initial tension". High initial tensions are stiff, low intial tensions are "whimpy". Although not usually important, initial tensions can have a significant effect on loads.
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Torsion springs
1) In general, we find it best to have some idea what OD (Outside diameter) you are looking for. Is the spring a quarter inch or two inches in diameter? The OD is the first group of digits in our part numbers. If the ID is of concern, just add 2 wire diameters to the ID to get the OD.
2) After that, you should have some idea what the the angle between the legs of the spring will be (with no load on it). If the legs point in the same direction the angle is 0 degrees (or 360 degrees). If the legs point in opposite directions, the angle is 180 degrees. To differentiate betwe 90 degrees and 270 degrees, deflect the spring 90 degrees. If the legs now point in the same direction, the the original angle was 270. If the legs now point in opposite direction then the original angle was 90 degrees.
3) Next, have some idea of the loads that you want. It is usually best to convert this into a spring rate.
Is it going to hold a feather, or an elephant? Does it move 1 degree or 360 degrees when you put the elephant (or the feather) on it? Fortunately, there are a few tricks here that might be helpfull:
a) For torsions springs the load is a "torque", NOT the weight on a scale. It has to be calculated. Torque is usually measured in inch pounds
b) To get a feel for how much load you want, you can still take a bathroom scale and press on it. If you have an old spring or similar spring, you can put it on the scale and press on the spring. This will also give you an idea of how far the spring rotated when you are at the load you want. It may help to put a rod through the center of the coils so that the spring doesn't rotate when you press on it. By the way, make a note of how far it is from the center of the spring to the point at which the spring is touching the scale
To reiterate, we need three things: the weight on the scale, how far the torsion spring rotated. and the distance from the center of the spring to the point where the leg first contacts the scale.
c) The torque is the load you measured times the distance between the center of the spring and the point where the spring is touching the scale. Say the scale measured 2 pounds, and the point that was touching the scale was 1 inch from the center of the spring. The torque is 1 inch times 2 pounds or 2 inch pounds.
d) The spring rate then is the torque divided by the number of degrees that the spring moved. So, if the spring above rotated 30 degrees when you got the 2 pounds on the scale, then the spring rate is 2 inch pounds (as calculated in c above) divided by 30. Or 2/30 = .066666...inch pounds per degree.
e) That number is pretty small and hard to work with, so we multiply it by the number of degrees in one revolutions (360). That gives us a spring rate in inch pounds per revolution. In this example multiplying by 360 (.06666666*360) gives us 24 inch pounds per revolution. This then is the third group of numbers in torsion springs part numbers.
4) Last but not least, torsions spring can be either left hand wound, or right hand wound. Sometimes it makes a difference, sometimes not. Still, you need to order either right or left hand, or often one of each. If you set the spring on a table with the leg on top pointing away from you, then answer one simply question and you'll know. If the top leg is on the right hand side of the spring, then the spring is right hand would. If it is on the left then the spring is left hand wound.
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