torque for dummies

Yeh, I'm a torque-dork. Trying to get my mind around what a torque rating actually means in the real world without getting all greeky about it...

So a motor is rated at 10 pound-feet of torque. Please check my assumptions:

1. At 1 foot distance from the center of the motor shaft, 10 lbs of force are generated.

2. At ten feet from the center of the shaft, 1 lb of force is generated.

3. At 6 inches from the center of the shaft, 20 lb of force is generated.

I'm not speccing something specific here, I'm just looking for a rule of thumb, so I can go on to develop more intelligent questions.

Thanks,

TM

Sounds like, it's many years since static class. Wiki, as usual, got some good stuff:

http://en.wikipedia.org/wiki/Torque_and_speed_of_a_DC_motor

http://en.wikipedia.org/wiki/Torque

You have the basic understanding of 10 lb*ft. of torque.

Of course, when it comes to motors, there is a little more to it as the torque output will vary with the speed and with the way the drive operates if there is one.

What type of motor are you talking about?

TimothyMoulder said:

Yeh, I'm a torque-dork. Trying to get my mind around what a torque rating actually means in the real world without getting all greeky about it...

So a motor is rated at 10 pound-feet of torque. Please check my assumptions:

1. At 1 foot distance from the center of the motor shaft, 10 lbs of force are generated.

2. At ten feet from the center of the shaft, 1 lb of force is generated.

3. At 6 inches from the center of the shaft, 20 lb of force is generated.

I'm not speccing something specific here, I'm just looking for a rule of thumb, so I can go on to develop more intelligent questions.

Thanks, TM

Click to expand...

Non geeky response - you got it. So far

Now for the parts that threw me for a loop for several weeks
1. Constant torque load (conveyers cranes etc). Assuming the LOAD is constant (ie pounds on crane hook) how can torque be constant with increasing speed?
2. IF The motor torque is greater than the load torque the motor can accelerate the load. IF less the motor will slow.

Two was pretty obvious but I had XXXX of a time getting my mind around #1. Still have to think on it sometimes. Sure will be nice when it is automatic.

Dan Bentler

leitmotif said:

1. Constant torque load (conveyers cranes etc). Assuming the LOAD is constant (ie pounds on crane hook) how can torque be constant with increasing speed?

Click to expand...

Two important concepts.

First, a motor's rating defines its capability. It doesn't define its output - the load does that. The example I like to use: If you put the blade from your bathroom vent fan on a 300 hp motor, will the motor output 300 hp? Of course not!

Second, hp is a funtion of time and speed. In US customary units hp = lb-ft_Torque x rpm / 5252

So, with a conveyor or a crane you have a fixed load pulled over a fixed radius winch or headshaft pulley. Since T = F x r you have a constant torque. (Assuming the torque is within the motor's ratings, of course.)

If you increase the speed the load and the pulley radius don't change, so of course the torque can't change. Remember, the load determines the torque, not the motor rating! However, intuitively you know that at a faster speed something has to change, and you are right! Although torque is the same, power increases at the higher rpm.

From Chapman handbook:
The torque on an object is defined as the product of the force applied to the object and the smallest distance between the line of action of the force and the object's axis of rotation. If r is a vector pointing from the axis of rotation to the point of applicalion of the force, and if F is the applied force, then the torque can be described as:
Torque = (force applied)(perpendicular distance)
= (F) (r sin (a))
= rF sin (a)
where (a) is the angle between the vector r and the vector F. The direction of the torque is clockwise if it would tend to cause a clockwise rotation and counterclockwise if it would tend to cause a counterclockwise rotalion.
The units of torque are newton-meters in SI units and pound-feet in lhe English system.

widelto said:

From Chapman handbook:
The torque on an object is defined as the product of the force applied to the object and the smallest distance between the line of action of the force and the object's axis of rotation. If r is a vector pointing from the axis of rotation to the point of applicalion of the force, and if F is the applied force, then the torque can be described as:
Torque = (force applied)(perpendicular distance)
= (F) (r sin (a))
= rF sin (a)
where (a) is the angle between the vector r and the vector F. The direction of the torque is clockwise if it would tend to cause a clockwise rotation and counterclockwise if it would tend to cause a counterclockwise rotalion.
The units of torque are newton-meters in SI units and pound-feet in lhe English system.

Click to expand...

geek version

I like the geek version. It is important in motion control. In many applications the geek version has made a significant difference to the performance of the project.

Clay B.:
LOL, sorry I thought it was going to be an easy explanation.

widelto said:

Clay B.:
LOL, sorry I thought it was going to be an easy explanation.

Click to expand...

No problem... I just saw the funny in it... As Peter said when you get to motion control the geek version is important.

The non-geek version works when your doing conveyors and other basic motor funtions...

FYI...Peter invented the geek version...