r m = ( 1 2 ) O D + I D 2 {\displaystyle r_{m}=({\frac {1}{2}}){\frac {OD+ID}{2}}}
H m = ( N i l m ) {\displaystyle H_{m}=({\frac {Ni}{l_{m}}})}
H m = ( 20 X 2.5 ) {\displaystyle H_{m}=({\frac {20X2.5}{}})}
