I am trying to design something for a friend, but am struggling to come up with the mechanics. Can you help?

So imagine a long extendable pole, like a bow staff. Imagine after pulling out a "peg" the bow staff extends using springs. Now say I wanted to automatically close this extended pole. What would be a good way of doing this?

My idea is a wire/string inside the staff that runs from the top to the bottom. Pulling this wire can compress the staff. In my head, some kind of motor can wrap up the string to compress the staff. My worry is that the motor wouldn't be able to pull against the strength of the springs. Can this be solved through a gear mechanism allowing the motor to be safe from the resistance of the springs? If so, how?

My second idea is to eliminate the motor. Instead, the wire is simply pulled from the bottom to compress the staff. But I wouldn't want to pull the string the entirety of the bow staff. Potentially through some pully or gear system, a semi-short pull of the string can compress the entire staff. Like the string would only have to be pulled out half or a quarter of the distance of the bow staff, but still, compress the entire thing.

Which method is more doable? Is there a better way to automate the compression of the staff?

This is only in the planning stages so springs are not chosen, though their strength will be similar to that of the springs used inside an umbrella. Here is a picture that should provide some basic dimensions. Fully extended, each section is about 13cm. The inner diameters are 9.5mm, 6.5mm, and 3.5mm. basic pic I look forward to hearing your answers. I don't have a lot of experience with gears which is why I am asking. Thanks!

  • 2
    $\begingroup$ Rough dimensions of what you're thinking of would help. Compressing a spring 4 inches vs 4 feet is a world of difference. $\endgroup$
    – jko
    Jan 19, 2021 at 19:59
  • $\begingroup$ Great point thank you, I updated the question with very rough dimensions. Let me know what you think or if you need any other details! $\endgroup$
    – Max
    Jan 23, 2021 at 1:02
  • 1
    $\begingroup$ Older cars used to have motor driven extending antennas. A similar mechanism could work here. $\endgroup$
    – Eric S
    Jan 23, 2021 at 2:56

1 Answer 1


Both of these ideas are doable, however if you go with the second idea that is by simply pulling the string with some kind of pulley or lever mechanism, you still would need some kind of actuator to pull the string. It maybe a piston or a solenoid but still an actuator. plus you would need a mechanism to pull the whole staff with just one forth pull of string and that would be a bit difficult to manage it in such a small space.

with the first method that is by using a motor and a brake wire, according to the type of the spring you mentioned that is one which is used in a umbrella, I don't think that compressing that spring would be such a problem. A small dc motor with brake wire attached to its shaft would do it. This won't need much space it will be having very low weight and can be done in very low cost.

To calculate the torque required to compress the spring, multiply the stiffness of string by distance by which it needs to be compressed. That will be your force which is needed to be applied by the motor. Force = stiffness * compression distance. Now you can calculate the torque required for the compression by using simple formula Torque = force * perpendicular distance. Now that you have required torque you can select a motor with given rated torque and other specifications like rpm, power etc.

if you have concern that motor will take the load or it will damage the motor then you can go for simple gear arrangement to pull the string. This way the load will not be directly applied on the motor shaft and you can also select a appropriate gear ratio which will be able to overcome the force given by springs. And you can also provide your motor with a simple bearing housing so that the load coming on to the shaft will be taken by the bearing and motor will be safe.

If you are deciding to automate this process you will need a feedback of the rotation of the motor so that you can check how much the staff has compressed. You can either use a rotary feedback that is a encoder or a Potentiometer they will give the angle by which motor has rotated which can be converted to the distance traveled or compressed. or you can use a linear feedback like limit switch or a proximity sensor. according to me a limit switch or a proximity would be easier to install on to mechanism, would be easy to program on an Arduino and also be cost effective.

so you can go for both the ideas, both will work but i am not sure about how second idea is going to work as i think putting a pulley system into the mechanism is bit difficult. where as first idea is quite sure that it will work, you will find the motor of desired specs and it would be easier to automate the mechanism if it has a motor and a some kind of sensor. So i would suggest that you go for the first idea..


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