E rua ngā momo o te poroporo porowhita:
1) te porowhita porowhita
2) te kakau porotaka kua whakapikohia.
Te pou porowhita tapawhātaputapu I roto i te rakau spline, he whānui te whakamahinga o te rakau spline involute, ā, he nui ngā kawenga e hiahiatia ana, ā, he nui te tika o te pokapū. Me ngā hononga nui ake. Ko ngā rakau spline tapawhā te tikanga e whakamahia ana i roto i ngā waka rererangi, ngā motuka, ngā tarakihana, ngā mahi hanga taputapu mīhini, ngā mīhini ahuwhenua me ngā taputapu tuku mīhini whānui. Nā te mahi maha-niho o te rakau spline tapawhā, he nui te kaha kawe, he pai te koretake me te arataki pai, ā, ka taea e te pakiaka niho papaku te whakaiti i te kukū o te ahotea. Hei tāpiri, he iti ake te ngoikore o te kaha o te rakau me te pokapū o te rakau spline, he watea ake te tukatuka, ā, ka taea te whiwhi i te tika teitei ake mā te huri.
Ka whakamahia ngā rakau spline involute mō ngā hononga me ngā kawenga teitei, te tika o te pokapū, me ngā rahi nui. Ko ōna āhuatanga: he involute te kōtaha niho, ā, he kaha radial kei runga i te niho ina utaina, ka taea te mahi i te mahi pokapū aunoa, kia ōrite te kaha o ia niho, kia kaha te kaha me te roa o te ora, he rite te hangarau tukatuka ki te taputapu, ā, he ngāwari ki te whiwhi i te tino tika me te whakawhiti.