三(san)輥(gun)穿(chuan)(chuan)(chuan)孔的(de)(de)橫斷(duan)面變(bian)形(xing)(xing)(xing)過程(cheng)是(shi)由圓(yuan)形(xing)(xing)(xing)到(dao)(dao)圓(yuan)三(san)角形(xing)(xing)(xing)再(zai)到(dao)(dao)圓(yuan)形(xing)(xing)(xing)的(de)(de)過程(cheng)。這種三(san)角形(xing)(xing)(xing)的(de)(de)變(bian)形(xing)(xing)(xing)特(te)征，使毛管尾部易出現三(san)角形(xing)(xing)(xing)。通常，鋼管外(wai)徑(jing)與(yu)壁厚之比D／S大(da)于9時，會出現尾三(san)角形(xing)(xing)(xing)，D／S值愈(yu)大(da)，尾三(san)角形(xing)(xing)(xing)也愈(yu)大(da)。尾三(san)角的(de)(de)形(xing)(xing)(xing)成，限(xian)制了(le)在三(san)輥(gun)穿(chuan)(chuan)(chuan)孔機上穿(chuan)(chuan)(chuan)制薄壁管，這是(shi)其缺點之一。

  三(san)輥(gun)穿(chuan)孔(kong)(kong)機(ji)軋輥(gun)形(xing)成(cheng)了一個近似封閉的(de)孔(kong)(kong)型，不(bu)需要(yao)像在(zai)(zai)二(er)輥(gun)穿(chuan)孔(kong)(kong)機(ji)上為(wei)保持管坯在(zai)(zai)中心線上和(he)限制(zhi)金(jin)屬(shu)的(de)橫向流動(dong)，必須裝設導(dao)板(ban)(ban)或導(dao)盤。因(yin)此采(cai)用(yong)三(san)輥(gun)穿(chuan)孔(kong)(kong)機(ji)穿(chuan)孔(kong)(kong)就消(xiao)(xiao)(xiao)除(chu)了現(xian)有二(er)輥(gun)穿(chuan)孔(kong)(kong)機(ji)存在(zai)(zai)的(de)缺點。不(bu)僅消(xiao)(xiao)(xiao)除(chu)了不(bu)利導(dao)板(ban)(ban)或導(dao)盤所造成(cheng)的(de)金(jin)屬(shu)應力狀(zhuang)態，也消(xiao)(xiao)(xiao)除(chu)了過大能量(liang)消(xiao)(xiao)(xiao)耗的(de)導(dao)板(ban)(ban)和(he)導(dao)盤，消(xiao)(xiao)(xiao)除(chu)了管坯與(yu)它(ta)們(men)之間的(de)摩擦。降(jiang)低了設備能量(liang)消(xiao)(xiao)(xiao)耗，同時(shi)也消(xiao)(xiao)(xiao)除(chu)了導(dao)板(ban)(ban)、導(dao)盤的(de)工具成(cheng)本，提高(gao)了經濟性。