# Strength of springs - length of travel vs tension weight

• R_Rose
In summary, the conversation revolves around designing crossbows and trebuchets using springs and pulleys. The main concern is how to calculate the energy obtained from different types of springs and their impact on the effectiveness of the weapons. The use of bungees is also mentioned as a potential source of energy. The speaker acknowledges the limitations of mathematical models and emphasizes the importance of experimentation in this type of hobby.
R_Rose
I'm trying to design some novel cross bow designs and also trebuchet's with the trebuchet (similar to catapults to those who don't know) using springs or a combination of weight and springs. I'm looking into using linear springs and pullies for the crossbow depending on whether they can be efficient or not.

What I need to know most is how to calculate the energy that can be obtained from a spring and which aspect gives the greatest increase of energy - such as pull weight vs expansion distance. Also, is there a difference in using a compression spring vs tension - can they both give the same results? I was looking at using a number of heavy duty coil truck springs and a block and tackle which can allow for greater distance of movement of the attached rope to compensate for the great compression strength vs short movement of the spring, so the 8" travel of expansion of the spring will translate to 16" or 24" depending upon what pulley system I use. I need to to know how I can calculate expected energy from the different springs that I use.

Another idea that I've had is to use tension springs and pulleys as needed to make for appropriate travel distances of bolt carraige or throwing arm.

Finally we come to bungees which I've come to respect greatly as they can store an awful lot of energy and for their size and weight can be very useful. I have no idea how to calculate energy available in these cords.

If anyone can help me out, I'd greatly appreciate it! Thanks

R_Rose said:
cross bow designs and also trebuchet's

I suggest that your hobby is of the kind where empirical science (experimentation) is more applicable than calculations. Since you want to push things to the limit, that means past the edge where simple mathematical models are valid. The nonlinear properties of materials come into play, and there is no simple formula for those.

## 1. What is the relationship between the length of travel and tension weight of a spring?

The length of travel and tension weight of a spring have an inverse relationship. This means that as the length of travel increases, the tension weight decreases, and vice versa.

## 2. How does the length of travel affect the strength of a spring?

The length of travel directly impacts the strength of a spring. The longer the length of travel, the weaker the spring becomes, and the shorter the length of travel, the stronger the spring becomes.

## 3. Can a spring have a longer length of travel without losing its strength?

No, a spring's strength is directly related to its length of travel. Increasing the length of travel will result in decreased strength, and vice versa.

## 4. How does tension weight affect the durability of a spring?

The tension weight of a spring is directly related to its durability. The higher the tension weight, the more force the spring can withstand without breaking. However, a spring with a higher tension weight may have a shorter lifespan due to increased stress and wear.

## 5. Is there an ideal length of travel and tension weight for a spring?

The ideal length of travel and tension weight for a spring will depend on its intended use. For example, a spring used in heavy-duty applications may require a shorter length of travel and higher tension weight, while a spring used in lighter applications may benefit from a longer length of travel and lower tension weight. It is essential to consider the specific needs and requirements before determining the ideal length of travel and tension weight for a spring.

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