Additional Math Pages & Resources

Thursday, April 1, 2010

Pounds per foot, or foots per pound?

This topic follows the one on bolts, because it's all about Torque. Torque is a force applied to rotate an object around its axis. Knowing a bit of math helps you use a torque wrench.

Torque is commonly used to describe how tight a bolt should be, or how much twisting/pulling power is generated by a car engine.

(A full discussion of this would take us way out of the range of elementary math and into zones that require an engineering degree.)

But our bolt blog reminded me that the degree of tightness you apply to a fastener was once described in ft/lbs or lbs/ft. Now we are more likely to use a Metric unit called Newton meters or N•m. In either case, it's a measurement of force applied to a lever of a given length.

Remember the blog on rate? We discussed heart beats per minute and respirations per minute? This is the same kind of thing - one Newton of force per meter, or one pound of force per foot.

Here's a typical torque wrench. Yes, it's one foot long.
  • The head is on the left, with a switch that enables it to work in either direction
  • The head is capable of accepting sockets that drive fasteners or nuts. 
  • The handle is on the right, with a feature that lets you adjust the torque to be applied. 
  • A CLICK / SNAP feature tells you when the fastener is tight.
The sound and feel mean you don't have to be looking at the wrench when it hits the proper tightness. You can be groaning with your eyes closed and still notice that it's tight enough. SNAP. CLICK. Stop pulling now is what those indications mean.
An older design uses a needle and scale, so you can see the force you are applying, and stop yourself at the proper tightness. But you have to watch what you are doing.

What happens if the bolt or nut is too tight for your torque wrench? If you pull or push too much, you will break the wrench. If you need extra force, use a wrench with a higher rating and longer handle.


You can also make an extension and lengthen the arm of the wrench so you will have more leverage.

CAUTION - the extension MUST be at the head of the wrench, not the handle. If you put a pipe on the handle to extend it, you can break the wrench or bolt. Yes, I do know this from personal experience!
Not like the the picture above! You have to do it like the photo below!



In this example, the total length of the extended wrench is now 5 feet, so 10 pounds of force on the handle will give you 50 pounds of tightening force on the wheel nut.

If you weight 120 pounds and climb onto the handle, what will the force be at the wheel nut?

That's right. 600 ft/lbs.