# Determining the torque needed for a servo motor

I'm looking for a bit of clarity on a design I have in mind. Please bear with me as I have no background in physics, engineering and this is part of my first real DIY project, so the knowledge base upon which I write this question is fairly low.

I'm trying to rotate a hollow bamboo gardening stake (30cm long, 2.5cm diameter, denoted by B in the image below), with a servo motor. I plan on mounting the servo with a rounded mount (denoted by A) that will be printed to fit snugly inside the stake. There will be "fingers" attached to the gardening stake (represented by the light grey cylinders) upon which a PVC cable will rest (represented by the orange line). We will be hanging some light objects (max weight 0.15kg /item) on the cable, between the cylinders.

The goal is to rotate the stake such that the cylinders move from position 1 to 2, and the cable slides off, then rotating the stake back to its original position.

I've done some reading about torque and think the calculation I need to perform is summing the torque required to turn each weight (or force) hanging down on the cable, for its respective distance from the servo. Alas, being a newcomer to the world of physics and engineering, I have no idea if that's right, and prefer to try and be precise than overcompensate by purchasing unnecessarily powerful servo motors.

How would I go about determining the torque required by the servo motor? Or does a simpler design exist to create the desired motion, that I've not thought of?

Welcome to Engineering. Without meaning to scare you, I am afraid there are a lot of unknowns here. For example, are you going to a) support the stake from both sides or is it going to be a cantilever. b) The tension on the PVC cable and its cross-section. c) the size /shape of the mounted things, d) the distance from the rotation axis, e) the mass of the stake, f) the speed on which you need it rotate etc...

Having said that, I will try to give some qualitative pointers.

For starters, if you properly engineering the mounting and you are not too much worried about the rotational speed, a lot of the unknowns become less relevant.

More specifically, I would suggest that you mount the two ends of the gardening stake with two radial bearings, and then mount the motor (this is to stop any bending from the twisting motion).

Additionally, you should use a PVC cable with the minimum tension and cross-section you can afford without compromising other functional specifications. Because, the cable will act as a spring, as you increase tension on the cable or the cross-section, the torque requirements will go up.

• Thank you for taking the time to reply! I'm planning on loosely housing the stake inside a rectangular prism that will be drilled into a wall, thereby allowing the free rotation of the stake. The PVC cable will essentially be that which you find on a clothesline / washing line as this is durable and available. Thanks for the recommendations! You've helped illuminate the unknown unknowns in my scope of knowledge, which gives me a direction to continue learning. I'll start with radial bearings. I'd give you an upvote but it seems I need more experience on the forum (d'oh). – Kieran S Sep 15 '20 at 21:35
• don't worry about the upvote. If you don't mind me asking, so that I get a better idea, is what do you intent to do with this? If we understood better the design intent, someone here might offer an alternative route or better solution... – NMech Sep 15 '20 at 22:17
• Sure, so when completed there will be two of these stakes in parallel, facing one another with "fingers" (the cylinders) inward. The PVC cable will be one long cable that is looped around all of the fingers, and I will hang items by clipping them on either end to the PVC cable. The goal is to simultaneously rotate the stakes so that the cables, and attached items, slide off, into a container (part of a later, separate design), sitting below. Edit: The control for this is going to be built in Arduino, in case that helps anyone. – Kieran S Sep 15 '20 at 23:28