So for the following device: enter image description here

I'm trying to figure out the torque. To do that, I need the area of the gap. In the solution, it says that the gap volume is: $$2gh(r_1+0.5g)\theta$$

But the part of \$ r_1+0.5g \$ is really confusing me. I'm not sure where that comes from. Isn't it supposed to be \$ 2gh(2r_1)\theta \$ ?

  • 4
    \$\begingroup\$ The arc at the rotor is of length r1θ; the arc at the stator is of length (r1+g)θ. Average them. \$\endgroup\$ Apr 24, 2018 at 19:20
  • 2
    \$\begingroup\$ The volume of a cuboid is width*height*depth. The volume under consideration can be approximated to 2 cuboids. \$\endgroup\$ Apr 24, 2018 at 19:36
  • \$\begingroup\$ r1 + g/2 is the half way point of the gap, and gives the best average distance around the arc. \$\endgroup\$ Apr 24, 2018 at 22:15

1 Answer 1


The volume you are trying to calculate is an area between arcs of the same inner angle, but of different radius. It's called an annular sector:

Now, technically, you will need a bit of mathematics to calculate this without any reference but, in the end, the volume is equal to the area of this small region, times the height (which is h in your case).

Practically, according to the shape above, and the picture you posted, you have an area that runs from r to R or, from your shape, it runs from \$r_1\$ to \$r_1 + g\$.

You have two coordinates (these are polar coordinates here), the radius \$r\$ and the angle \$θ\$ (a parameter), which runs from \$0\$ to \$θ\$ (the actual value) in your case. Remember from differential calculus that the area element in polar coordinates is equal to \$rdrdθ\$ (where \$θ\$ is in radians). While you 're at it, check this depiction as well, in which \$Δr\$ represents an infinitesimally small length and \$Δθ\$ an infinitesimally small angle. Because the elements are infinitesimal, this can be considered approximately equal to a rectangle, so its area (in terms of differential calculus) is the product of its sides:

So the area you want to calculate is given by a double integral of the area element over the ranges spanned by the two coordinates in the given area:


Because the function in the inner integral does not contain any \$θ\$, the two integrals can be separated and calculated as multiples:

\$\int_{0}^{θ}dθ(\int_{r_1}^{r_1+g}rdr)=\left[θ\right]_{0}^{θ} \left[\frac{r^2}{2}\right]_{r_1}^{r_1+g} =\frac{1}{2}θ \left[r^2\right]_{r_1}^{r_1+g} = \frac{1}{2}θ \left[ \left(r_1+g \right )^2 - \left(r_1\right)^2 \right] = \frac{θ}{2} \left[ r_1^2 + 2r_1g + g^2 - r_1^2 \right] = \frac{θ}{2} \left[ 2r_1g + g^2 \right] = θg \left(r_1 + \frac{g}{2} \right) \$

This is only one side of the rotor. Twice that is equal to the area of both sides:

\$ A = 2g \left(r_1 + \frac{g}{2} \right)θ \$

This is the area. If you multiply with the height \$h\$ (vertical dimension) you get the volume you are looking for:

\$ V = 2gh\left(r_1 + 0.5g\right)θ\$

  • \$\begingroup\$ +1 and a comment for a well reasoned and clear answer. \$\endgroup\$
    – KalleMP
    Apr 24, 2018 at 21:46

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