### Author Topic: Power, Torque, etc.  (Read 498 times)

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#### splishsplash

• Beginner
• Posts: 1
##### Power, Torque, etc.
« on: January 20, 2014, 06:24:55 PM »
I'm working on a "Swimulator", a device that simulates swimming using a a motorized "rotisserie." It moves two arms through the water mimicking the motion of a person swimming. We want to rotate the "rotisserie" with a radius of about 1ft through a circle where it is submerged about half the time.

That being said, we need the arms to move through the water realistically. They need to hit the water with about the same force they would if they were attached to a person's wrist, and feel the same force as they are pulled through the water.

We're juggling torque, newtons, newton-meters, distance, speed, angular velocity and getting all scrambled.

So, in a nutshell what we are trying to answer is - What equations will we need in order to find the right motor specifications?

How does a figure of "a swimmer's hand hits the water with a force of 350N" relate to the torque of a motor? What is holding torque? If torque decreases with speed, will spinning our "rotisserie" at high speeds mean cause stalls as it pulls through the water? Will it actually hit the water with less force even if its spinning faster?

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: Power, Torque, etc.
« Reply #1 on: January 21, 2014, 03:59:04 PM »
Some textbook on classical mechanics would probably help you with this math!

In brief, torque equals the force exerted times the distance between point of force and center of rotation.
Typically, gearing down a motor to lower speed will increase the delivered torque.
Holding torque is typically the amount of torque a motor can continuously deliver without overheating.
Stall torque is the maximum torque a motor can deliver at rated voltage/current, but will overheat at some point.

If your work is actually something that someone will rely on "for real" (peer reviewed research, human safety, etc) then you may want to include a mechanical engineer into your team to make sure you get this part right.

#### gerard

• Full Member
• Posts: 54
##### Re: Power, Torque, etc.
« Reply #2 on: January 26, 2014, 11:00:08 AM »
"How does a figure of "a swimmer's hand hits the water with a force of 350N" relate to the torque of a motor?"
say the swimmer's hand is 1.5m from his shoulder...
350/1.5=~233Nm
Nm (newton-meters) is a torque rating equivalent to how many newtons force @1m distance from center (the shoulder)
ft-lb in-lb oz-in kgf-cm kg-m etc... are just different types of torque conversions, that can be easily translated using several websites on google
hope this helped a bit

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: Power, Torque, etc.
« Reply #3 on: January 26, 2014, 12:32:54 PM »
Quote
350/1.5=~233Nm

This is wrong -- you MULTIPLY distance by force, not divide!

#### gerard

• Full Member
• Posts: 54
##### Re: Power, Torque, etc.
« Reply #4 on: January 26, 2014, 11:21:37 PM »
Quote
350/1.5=~233Nm

This is wrong -- you MULTIPLY distance by force, not divide!
lol woops
350*1.5=525
Nm/N=m
Nm/m=N