Under a certain condition

the involutes profile replaced by the straight-line profile to internal gear can meet the transmission requirement.The straight-line profile internal gear has a lot of advantages in processing

such as realizing grinding process easily

improving part precision highly and reducing manufacture cost largely.It is difficult to measure the base tangent length of an internal gear

so it is always using pin distance rather than the base tangent length to measure the internal gear in practice.There must be difference in the pin distance formula between the straight-line and involutes profile internal gears.The formula will be deduced with geometric method and approximate empirical formula respectively

and the effect of the backlash on the pin distance was considered.