Hello,

I am currently in high school taking a class in mechanical engineering. My teacher has assigned the class a project, that being to build an underwater ROV that must perform two tasks: It must have the ability to pick up various types of objects, and it must turn two valves shut in order to stop a leaking air (simulated oil) line; all of this must be accomplished while only viewing our actions through an onboard camera. The first task was easy to find a solution to (scooping/hooking), as for the second solution, my team members and I devised a plan to take a "high-torque" motor, attach a plexiglass plate perpendicular to the shaft, with skinny nails fixed with epoxy in order to turn the valves. Now, after spending many headache-filled class periods watching/playing around with CAD as my "expert in AP Physics" team member tried to find the right motor we would need to get, I have decided to research torque and perform the calculations myself. Unfortunately, this has proven to be fairly difficult as I have a limited background with physics and will not be taking AP Calculus until next year. We have already been given small DC motors to use as thrusters, however, when I tested these, they were easily stopped with the smallest of applied forces, and therefore would not be ideal for my application. The valve in question is a "standard" outdoor hose/spigot valve; metal, with semi-triangular cutouts running through the thickness of the valve. My instructor told me that once I figure out how much torque I needed, he might be willing to loan me, or buy me a motor to fit our needs. The motor does not have to have a high RPM, as the valves fully open/close with 1.57 full revolutions, and, as the RPM rate increases, so does the price of the motor - I have noticed. I am not necessarily looking for a spit-back answer, but more a demonstration/explanation of how to figure out the information I need, as I do recognize that I must learn the material and not just "copy and paste" the answer of someone else. If someone could please help me figure out how the equations etc. work that would be most appreciated.

As you probably know the motor needs to provide a certain amount of torque to turn off the spigot. So first order of busness is to measure the amont of torque your mechanism needs to apply. Perhaps you can rig up an experiment using weights to do that. Then the next step is to design the gear train that will connect the output of your motor to the valve. If you hook them directly together then you will definitely need a low RPM motor with signiifcant torque. But if you can build a gear set using a couple of gears you can probably devise a system with a motor that has less torque but higher RPM. For example a 10:1 gear ratio allows you to use a motor with 1/10 the torque rating, but it needs to turn at 10 times the RPM. If the motor output is specifed in terms of RPM and torque, the product of those two figures is power; if you introduce a gear train that reduces RPM then to maintain constant power the torque that you get goes up. This is how the gears on a bicycle work - when going up hill you select a low gear that reduces RPM at the wheel but increases torque.

One other thought - you seem to have settled on a plain old spigot, such as used for an outside faucet. Have you considered other types of valves, which may be smaller and easier to turn?

Hello ebaines,

Thank you for replying. First off, we are forced to use that spigot in question; part of the challenge for this project was to design our own idea for actually turning the valve (for instance, one group is 3d-printing a custom housing that will (hopefully) fit over the octagonal shape of the valve and thus turn it). I completely understand what you are talking about with gear ratios (I learned about gears and gear systems in my previous class - pre-engineering). What I don't understand is the mechanics and the equations for finding out the torque. I need to find out what torque motor I need to obtain in order to turn the valve…

As Ebaines noted, you just don't just find out Torque without some mechanical work. You will never know the motor torque needed without knowing what torque is needed to turn the valve! e.g.-How many ft. lbs. or inch lbs. are needed to open or close the valve? If it takes 5 ft. lbs. to turn the valve shaft, then you obviously need more than 5 lbs. torque. A finely finished ball bearing shaft needs less torque than a shaft with a packing box. Mount the valve on a board and start hanging weights on the handle, at 90 degrees, until the weight will turn the handle. I do a lot of mechanical work. Some valves open easily, others very hard, some turn by hand, others need pipe wrenches.