Ọpa alajerun jẹ paati pataki ninu apoti jia aran, eyiti o jẹ iru apoti jia ti o ni jia aran (eyiti a tun mọ ni kẹkẹ alajerun) ati dabaru alajerun. Ọpa alajerun jẹ ọpá iyipo lori eyiti a gbe dabaru alajerun naa. Ni igbagbogbo o ni okun helical (awọn alajerun dabaru) ge sinu oju rẹ.
Awọn ọpa alajerun nigbagbogbo jẹ awọn ohun elo bii irin, irin alagbara, tabi idẹ, da lori awọn ibeere ohun elo fun agbara, agbara, ati resistance lati wọ. Wọn ti ṣe ẹrọ ni deede lati rii daju iṣiṣẹ dan ati gbigbe agbara daradara laarin apoti jia.