That's a good point, but I think for an approximation, we can assume its constant. It gets up to speed very fast and then stops equally fast. Presumably this is

That's a good point, but I think for an approximation, we can assume its constant. It gets up to speed very fast and then stops equally fast. Presumably this is because of the constant-torque spring they're using? In any case, by observation it does seem that for most of the time, the speed isn't changing.