Spring has finally sprung, and I thought it would be good to talk about springs as they are a key component in any mechanical systems for pocketqubes and cubesats. I want to share some of the things I learned when I was developing pico satellite systems and what I have learned with my current pocketqube deployer.
In this entry I will focus on compression springs but in future I will talk about torsion springs.
Springs come in a variety of materials and coatings. Since we are talking about cubesats and pocketqubes it is best to go with stainless steel since it has good corrosion resistance.
When I 1st started designing deployers compression springs were the bane of my existence. Mainly because I designed the overall dimensions to fit the satellites, pusher plates and forgot to consider the dimensional requirements of the compression springs I selected.
This is something that I had overlooked (rookie mistake) which caused a lot of pain at the beginning of my space engineering career. While you may want your compression spring to be used as part of a linear mechanism you will need to be careful on its maximum safe travel. The safe travel of a compression spring is less than the distance between the free length (unloaded position) and solid height (fully compressed) of the compression spring. You want to avoid compressing your spring down to its solid height if you want to avoid damaging it which is a good thing. A good rule of thumb that I have been recommended is to avoid the solid height by at least 20% of the total travel range. I overlooked the solid height which meant when I tried to integrate a dummy satellite to a deployer I couldn’t close the door. So, lessons learned remember the maximum deflection of a spring which will affect the size of your spring and actuator.
The outer diameter of your spring will expand when it is compressed which is something that needs to be considered if it is going to be placed in a bore. The spring manufacturer usually provides a recommended hole diameter and the tolerances of the spring.
Also consider the spring rate of a compression spring when preloading it as it will help you determine the travel of the spring. .
You can use hooks law to calculate the preloading of a compression spring.
K= spring rate (spring constant)
After I found a spring that had the desired load, fits inside my assembly like a glove plus I could load the deployer and close the door. Job done! Or so it seemed as when I opened the door the dummy satellites would not deploy as intended. To find out what caused this failed deployment I looked inside the deployer (from a safe distance) and noticed the spring had buckled inside the assembly. I had just learned that compression springs have a slenderness ratio which means that the spring jammed as I was compressing it. Not something you want in your pocketqube or cubesat, especially for a spring loaded deployer.
The slenderness ratio determines whether your spring will buckle, and is easily calculated with the following equation
Slenderness ratio = Free length/mean diameter.
Mean diameter= OD- wire diameter or ID+ wire diameter
If Slenderness ratio >4 the spring is likely to buckle
If Slenderness ratio <4 the spring is unlikely to buckle
If you increase the width, decrease the length or add more coils to your spring then that will reduce the risk of buckling, so will placing it inside a hole of an appropriate diameter or inserting a support rod in the centre.
You can also use wave springs or conical springs which are more stable than compression springs.
This isn’t a fully comprehensive list of things to consider but I thought of mentioning some of the key things I learned about springs (while making a lot of mistakes) that I wished I knew when I was prototyping. I suggest to always consider the design of your springs when you are designing your mechanisms for your pocketqubes or cubesat as this will help avoid problems in the future. Also I will stress you should use off the shelf springs as custom springs are expensive and you may need to wait for a while until you can test your system. Plus, they usually have a large minimum order quantity and the last thing you want is 100 springs you can’t use! Use off the shelf springs for prototyping and once you nailed the design you can go for a custom spring.